Module 1
Science and Chemistry of Life
a. Critical Thinking in Science and Life
Have you ever tried a new or “improved” product and been disappointed when it
didn’t work as expected? Everyone learns, sometimes the hard way, how useful it can be
to cast a skeptical eye on advertising claims or get an unbiased evaluation of, say, a used
car you are considering buying. This objective evaluation of information is called
evidence-based or critical thinking.
Scientists use critical thinking in their own work and to review findings reported
by others. Anyone can make a mistake, and there is always a chance that pride or bias
will creep in. Critical thinking is a smart practice in everyday life, too, because so many
decisions we face involve scientific information. Will an herbal food supplement really
boost your immune system? Is it safe to eat irradiated food?
An easy way to begin evaluating information is to notice where it is coming from
and how it is presented. Here are two simple strategies for assessing sources. Let credible
scientific evidence, not opinions or hearsay, do the convincing For instance, if you are
concerned about reports that heavy use of a cell phone might cause brain cancer,
information on the website of the American Cancer Society is more likely to be reliable
than something cousin Fred heard at work. Informal information may be correct, but you
can’t know for sure without investigating further.
Question credentials and motives For example, if an advertisement is designed to
look like a news story, or a product is touted on TV or a blog by someone being paid for
the job, your critical thinking antennae should go up. Is the promoter simply trying to sell
a product with the help of “scientific” window dressing? Can any facts presented be
checked out? Responsible scientists try to be cautious and accurate in discussing their
findings and are willing to supply the evidence to back up their statements.
Even if information seems authoritative and unbiased, it is important to be aware
of the difference between the cause of an event or phenomenon and factors that may only
be correlated with it. For example, studies show that recirculation of air in an airplane’s
passenger cabin increases travelers’ exposure to germs coughed or sneezed out by others.
An “airplane cold,” however, is caused directly by infection by a virus.
Understanding the distinction between facts, opinions, and speculation is essential
for critical thinking and discerning the validity of information in various contexts. While
facts represent verifiable information grounded in evidence, opinions involve subjective
judgments or evaluations, and speculation entails predictions or hypotheses based on
incomplete information or conjecture.
Facts, as the foundation of empirical knowledge, provide a solid basis for
understanding the world around us. They are objective, measurable, and independent of
individual perspectives or biases. For example, the price of a loaf of bread at a specific
store can be verified by checking the receipt or consulting the store's pricing information.
Similarly, historical events, scientific measurements, and statistical data are examples of
factual information that can be corroborated through observation, experimentation, or
documentation.
In contrast, opinions reflect personal beliefs, preferences, or interpretations of
facts, and are inherently subjective in nature. While opinions may be informed by facts or
evidence, they are ultimately shaped by individual perspectives, experiences, and values.
For instance, one person may enjoy the taste of a particular brand of bread, while another
may find it unpalatable. Such judgments of taste are subjective and cannot be objectively
verified, as they depend on individual sensory perceptions and preferences.
Speculation involves making predictions or hypotheses based on limited
information or uncertain outcomes. Unlike facts, which are grounded in empirical
evidence, speculation entails conjecture or educated guesses about future events or
trends. For example, a marketer may speculate that a new brand of bread will be favored
by consumers based on market research, consumer trends, or promotional strategies.
However, until there are concrete statistics or sales data to support this prediction, it
remains speculative rather than factual.
Furthermore, it is important to recognize that the distinction between facts,
opinions, and speculation is not always clear-cut, and may vary depending on context and
interpretation. In some cases, what may initially be considered a fact may later be subject
to revision or reinterpretation based on new evidence or perspectives. Similarly, opinions
and speculation may be informed by factual information or expert analysis, adding
nuance to their validity and credibility.
In navigating information in today's complex and interconnected world, critical
thinking skills are paramount in discerning between facts, opinions, and speculation. By
evaluating sources, considering multiple perspectives, and seeking corroborating
evidence, individuals can make informed judgments and decisions based on a foundation
of reliable information and reasoned analysis.
b. Science in Perspective
The practice of science indeed stands as a cornerstone of human knowledge,
offering powerful insights and explanations into the natural world and the universe at
large. From the theory of evolution to the laws of thermodynamics, science provides a
framework for understanding the underlying principles governing the cosmos, life, and
everything in between. However, it is important to recognize that science represents just
one facet of the human experience, albeit a crucial and transformative one.
Beyond the realm of science lies a rich tapestry of human endeavors,
encompassing art, philosophy, spirituality, culture, and personal experience. Each of
these domains offers its own unique perspectives, insights, and modes of understanding
the world and our place within it. While science provides empirical evidence and testable
hypotheses, art invokes emotion, creativity, and subjective interpretation. Philosophy
delves into the realms of ethics, metaphysics, and epistemology, probing the nature of
existence and knowledge. Spirituality and religion offer frameworks for grappling with
existential questions and seeking meaning and purpose in life. Culture shapes our
identities, values, and traditions, reflecting the diversity and complexity of human
societies. And personal experience, shaped by individual perspectives, memories, and
emotions, provides a deeply subjective lens through which we navigate the world.
Furthermore, the interplay between science and other facets of human experience
is dynamic and multifaceted. Science often draws inspiration from art, philosophy, and
spirituality, enriching its inquiry with broader perspectives and deeper insights.
Conversely, scientific discoveries and advancements can influence and inform cultural,
ethical, and philosophical discourse, challenging existing paradigms and shaping societal
attitudes and values. Indeed, the synthesis of diverse perspectives and disciplines fosters
interdisciplinary collaboration and holistic understanding, enriching our collective
knowledge and appreciation of the world.
Moreover, it is essential to recognize the limitations of science and the inherent
uncertainty that accompanies the pursuit of knowledge. While science strives for
objectivity and empirical evidence, it is inherently provisional, subject to revision and
refinement in light of new discoveries and perspectives. Additionally, science cannot
address certain existential questions or aspects of human experience that lie beyond the
scope of empirical investigation. In these instances, other modes of inquiry, such as
introspection, intuition, and contemplation, may offer valuable insights and
understanding.
In essence, the practice of science represents a powerful tool for exploring and
explaining the natural world, but it is just one part of the broader tapestry of human
experience. Embracing the diversity of perspectives and disciplines enriches our
understanding of ourselves, our world, and our place within it, fostering a more holistic
and nuanced appreciation of the complexities of existence.
You’ve probably said, “I’ve got a theory about that!” This expression usually
means that you have an untested idea about something. A scientific theory is the
opposite: It is an explanation of a broad range of related natural events and observations
that is based on repeated, careful testing of hypotheses. Lists some major scientific
theories related to biology. Before scientific research established one of them, the germ
theory of disease, some people tried to appease malevolent spirits they blamed for
outbreaks of infectious disease.
The process of scientific inquiry involves the formulation, testing, and refinement
of hypotheses and theories based on empirical evidence and logical reasoning.
Hypotheses are proposed explanations for specific phenomena or observations, while
theories are comprehensive explanations that integrate multiple hypotheses and have
withstood rigorous testing and scrutiny over time.
After years of testing and validation by numerous scientists through
experimentation, observation, and peer review, hypotheses that have not been disproved
gain credibility and may be elevated to the status of theories. This confidence in the
validity of a theory arises from the accumulation of consistent and reproducible evidence
from diverse sources, which supports the explanatory power and predictive accuracy of
the theory.
The theory of evolution, proposed by Charles Darwin in the 19th century, is a
quintessential example of a scientific theory that has been extensively tested and
corroborated by tens of thousands of scientific observations. The theory of evolution
provides a comprehensive framework for understanding the diversity of life on Earth, the
mechanisms of adaptation and speciation, and the interconnectedness of all living
organisms through common ancestry.
Over the past century and a half, the theory of evolution has been subject to
intense scrutiny and investigation across multiple scientific disciplines, including
genetics, paleontology, ecology, and comparative anatomy. Through the accumulation of
evidence from fossil records, molecular biology, biogeography, and experimental studies,
the theory of evolution has withstood the test of time and emerged as one of the most
robust and widely accepted theories in modern science.
The evidence supporting the theory of evolution is vast and diverse,
encompassing fossil transitions, anatomical similarities and differences among species,
genetic and molecular homologies, observed instances of natural selection and adaptation
in natural populations, and the predictive power of evolutionary principles in fields such
as medicine, agriculture, and conservation biology.
Moreover, the theory of evolution serves as a unifying principle in biology,
providing a coherent explanation for the observed patterns and processes of life, from the
origin of species to the emergence of complex ecosystems. By elucidating the
mechanisms of descent with modification, genetic variation, and reproductive success,
the theory of evolution has revolutionized our understanding of the natural world and
continues to inspire new avenues of scientific inquiry and discovery.
In conclusion, the theory of evolution stands as a testament to the power of
scientific inquiry and evidence-based reasoning in elucidating the fundamental principles
of life and the natural world. Through decades of rigorous testing and validation, the
theory of evolution has earned its place as a cornerstone of modern biology, shaping our
understanding of biodiversity, ecological interactions, and the origins of life itself.
Science demands critical thinking, so a theory can be modified, and even rejected,
if results of new scientific tests call it into question. It’s the same with other scientific
ideas. Today, for instance, advances in technology are giving us a new perspective on
subjects such as the links between emotions and health. Some “facts” in this textbook one
day will likely be revised as we learn more about various processes. This willingness to
reconsider ideas as new information comes to light is a major strength of science.
Because science requires an objective mindset, scientists can only do certain kinds
of studies. No experiment can explain the “meaning of life,” for example, or why each of
us dies at a certain moment. Such questions have subjective answers that are shaped by
our experiences and beliefs. Every culture and society has its own standards of morality
and esthetics, and there are probably thousands of different sets of religious beliefs. All
guide their members in deciding what is important and morally good and what is not. By
contrast, the external world, rather than internal conviction, is the only testing ground for
scientific views.
Because science does not involve value judgments, it sometimes has been or can
be used in controversial ways. For example, some people worry about issues such as the
use of animals in scientific research and possible negative consequences of genetic
modification of food plants. There has been great debate over the causes of global climate
change and the use of “industrial” fishing methods on the high seas. Meanwhile, whole
ecosystems are being altered by technologies that each year allow millions of a forest’s
trees to be cut and hundreds of millions of fishes to be taken from the sea. The scientific
community alone can’t resolve these issues. That responsibility also belongs to us.
c. Atoms and Elements
Like all else on Earth, your body consists of chemicals. Some of them are solids,
others liquid, still others gases. Each of these chemicals consists of one or more elements.
An element is a pure substance: that is, it cannot be broken down to another substance by
ordinary physical or chemical techniques. There are more than ninety natural elements on
Earth, and scientists have created many other artificial ones.
Overall, organisms consist mostly of four elements: oxygen, carbon, hydrogen,
and nitrogen. The human body also contains some calcium, phosphorus, potassium,
sulfur, sodium, and chlorine, plus many different trace element. A trace element is one
that makes up less than 0.01 percent of body weight. Trace elements are extremely
important, however. For example, without the trace element iron, your red blood cells
can’t carry oxygen. The body’s chemical makeup is finely tuned. Many trace elements
found in our tissues—such as arsenic, selenium, and fluorine—are toxic in amounts
larger than normal. Atoms of the same or different elements can combine into molecules
—the first step in biological organization. Molecules in turn can combine to form larger
structures, as described shortly.
An atom is the smallest unit that has the properties of a given element. A million
could fit on the period at the end of this sentence. In spite of their tiny size, however, all
atoms are composed of more than one hundred kinds of subatomic particles. The ones we
are concerned with in this book are protons, electrons, and neutrons.
All atoms have one or more protons. These particles carry a positive charge,
marked by a plus sign (p1). Atoms also have one or more neutrons, which have no
charge. Neutrons and protons make up the atom’s core, the atomic nucleus. Electrons
move around the nucleus, in the space that occupies 99.99 percent of the atom’s volume.
Electrons have a negative charge, which we write as e2. An atom usually has equal
numbers of electrons and protons.
Each element is assigned an “atomic number,” which is the number of protons in
its atoms. Elements also have a “mass number”—the sum of the protons and neutrons in
the nucleus of their atoms. Appendix II of this textbook has charts of the elements and of
the atomic numbers of the common elements in living things.
Atoms of a given element have exactly the same number of protons, but they may
have different numbers of neutrons. When an atom of an element has more or fewer
neutrons than the most common number, it is called an isotope (eye-so-tope). For
instance, while a “standard” carbon atom has six protons and six neutrons, the isotope
called carbon 14 has six protons and eight neutrons. These two forms of carbon atoms
also can be written as 12C and 14C. The prefix iso- means “same,” and all isotopes of an
element interact with other atoms in the same way. Most elements have at least two
isotopes. Cells can use any isotope of an element for their metabolic activities, because
the isotopes behave the same as the standard form of the atom in chemical reactions.
Have you heard of radioactive isotopes? They are isotopes of elements such as
uranium, which emit energy. This unexpected chemical behavior is what we call
radioactivity. The nucleus of a radioisotope is unstable, but it stabilizes itself by emitting
energy and certain types of particles. This process, called radioactive decay, occurs
spontaneously, and it transforms a radioisotope into an atom of a different element. The
decay process happens at a known rate. For instance, over a predictable time span,
potassium 40 becomes argon 40. Scientists can use radioactive decay rates to determine
the age of very old substances, such as ancient rocks and fossils. Radioisotopes also have
important uses in medicine.
d. Science Comes to Life: PET Scanning—Using Radioisotopes in Medicine
Emissions from radioisotopes can reveal the activity of body cells. As a result,
they are useful tools in medicine because they permit physicians to diagnose disease, or
track its course, without doing surgery. The technology called PET (short for Positron
Emission Tomography) is a prime example.
A PET (Positron Emission Tomography) scan is a powerful imaging technique
used in the diagnosis and monitoring of various medical conditions, including cancer. In
the context of cancer diagnosis, PET scans play a crucial role in detecting and visualizing
tumors by exploiting the metabolic differences between cancerous and healthy tissues.
During a PET scan, a patient is injected with a tracer, a molecule in which
radioisotopes have been substituted for some atoms. These radioisotopes emit positrons,
which are positively charged particles, as they undergo radioactive decay. The tracer
molecule is typically designed to mimic a biological compound that is metabolically
active within the body, such as glucose or amino acids.
In the case of cancer, the cells within a tumor exhibit heightened metabolic
activity compared to normal body cells. This increased metabolic activity is a hallmark of
cancerous growth, driven by the rapid proliferation and energy demands of cancer cells.
As a result, cancer cells take up the tracer molecule at a faster rate than surrounding
healthy tissues.
After the tracer is injected, the patient undergoes imaging with a PET scanner,
which detects the positrons emitted by the radioisotopes within the tracer molecules.
When a positron collides with an electron within the body, they annihilate each other,
producing two gamma rays that travel in opposite directions. The PET scanner detects
these gamma rays and uses the information to reconstruct three-dimensional images of
the distribution of radioactivity within the body.
In a PET scan of a cancer patient, the areas of increased radioactivity correspond
to regions of heightened metabolic activity, typically indicating the presence of tumors or
other abnormalities. By visualizing these areas of increased tracer uptake, physicians can
identify the location, size, and extent of tumors, as well as assess their response to
treatment over time.
The information obtained from PET scans is invaluable in guiding treatment
decisions and monitoring the progress of cancer therapy. For example, PET scans can
help determine the stage of cancer, evaluate the effectiveness of chemotherapy or
radiation therapy, and detect cancer recurrence or metastasis.
Moreover, PET imaging can be combined with other imaging modalities, such as
CT (Computed Tomography) or MRI (Magnetic Resonance Imaging), to provide
complementary information about the structure and function of tissues. This multimodal
approach, known as PET/CT or PET/MRI fusion imaging, offers enhanced diagnostic
accuracy and spatial resolution, enabling more precise localization and characterization
of tumors.
In summary, PET scans represent a sophisticated and non-invasive tool for the
detection and characterization of cancerous tumors. By exploiting the metabolic
differences between cancerous and healthy tissues, PET imaging provides valuable
insights into the biology of cancer and aids in the diagnosis, staging, and management of
the disease. Continued advancements in PET technology and tracer development hold
promise for further improving the accuracy and utility of this imaging modality in cancer
care.
Sarah sat nervously in the waiting room of the hospital, her mind swirling with a
mixture of hope and apprehension. As a leukemia patient, she had been through countless
tests and treatments, each one a step closer to the ultimate goal: a bone marrow transplant
that could potentially save her life. However, before she could undergo such a procedure,
there was one final hurdle to clear – a scan of her heart.
The thought of having her heart examined filled Sarah with both anticipation and
anxiety. It wasn't just a routine check-up; this scan would determine whether her heart
was strong enough to withstand the rigors of surgery. A radioactive tracer would be
injected into her bloodstream, allowing the scanner to create video images of her heart in
action. It was a crucial step in assessing her overall health and fitness for the transplant.
As she waited, Sarah couldn't help but reflect on the journey that had led her to
this moment. The diagnosis of leukemia had turned her world upside down, forcing her to
confront mortality in a way she never imagined. But she refused to let it defeat her. With
the support of her family, friends, and medical team, she had fought tooth and nail,
clinging to hope even in the darkest moments.
Now, as she sat in the hospital, she couldn't help but feel a sense of gratitude for
the advances in medical technology that made procedures like this possible. The ability to
peer inside her body and assess its inner workings with such precision was nothing short
of miraculous. And yet, for all the marvels of modern science, there was still an element
of uncertainty – a lingering fear that her body might not be up to the challenge.
But Sarah was determined not to dwell on the negative. Instead, she focused on
the positives – the prospect of a new lease on life, free from the shadow of illness that
had haunted her for so long. She thought about all the things she wanted to do once she
was healthy again – the places she wanted to visit, the dreams she wanted to pursue.
Finally, the nurse called her name, jolting her out of her reverie. With a deep
breath, Sarah rose from her seat and followed the nurse down the hallway, her heart
pounding in her chest. As she lay down on the scanner bed, she closed her eyes and
whispered a silent prayer, hoping against hope that the images the machine produced
would be favorable.
Minutes stretched into hours as Sarah lay perfectly still, the rhythmic thumping of
the scanner providing a backdrop to her thoughts. And then, finally, it was over. The
nurse helped her sit up, a sympathetic smile on her face as she handed Sarah a tissue to
wipe away the tears that had welled up in her eyes.
"Well, how did it go?" Sarah asked, her voice trembling with emotion.
The nurse smiled reassuringly. "Everything looks good," she said. "Your heart is
strong, Sarah. You're ready for the next step."
Relief flooded through Sarah like a tidal wave, washing away the fear and
uncertainty that had plagued her for so long. With a grateful smile, she thanked the nurse
and made her way back to the waiting room, her heart brimming with hope for the future.
As she stepped out into the sunlight, Sarah couldn't help but feel a sense of
wonder at the resilience of the human spirit. Despite the challenges she had faced, she
refused to give up hope – and now, that hope had been rewarded. With renewed
determination, she set her sights on the road ahead, ready to face whatever obstacles lay
in her path with courage and grace.
Radioisotopes have revolutionized the field of cancer treatment, offering new
avenues for combating the disease with targeted precision. Among the myriad of
methods, one particularly innovative approach involves the use of implanted "seeds"
containing radioactive isotopes such as iodine or palladium. These seeds serve as
localized sources of radiation, strategically placed within the body to target and destroy
cancerous cells, especially in cases like prostate cancer.
For patients like John, who was diagnosed with prostate cancer, the concept of
radioactive seed implantation offered a glimmer of hope amidst the uncertainty of his
diagnosis. After consulting with his oncologist and weighing the risks and benefits, John
opted for this cutting-edge treatment. The procedure involved the insertion of tiny seeds,
each containing a carefully measured dose of a radioisotope, directly into the prostate
gland. These seeds emitted radiation that targeted the cancer cells while minimizing
damage to surrounding healthy tissue.
The use of radioisotopes in cancer treatment underscores the importance of safety
protocols and rigorous monitoring. While the radiation emitted by these seeds is powerful
enough to destroy cancer cells, it is also crucially important that it decays quickly into
more stable elements to ensure the safety of the patient. This rapid decay allows for the
effective treatment of cancer while minimizing the risk of long-term radiation exposure.
Throughout John's treatment journey, he experienced the remarkable synergy
between cutting-edge medical technology and compassionate care. The oncology team
meticulously planned each step of his treatment, ensuring that the dosage and placement
of the radioactive seeds were optimized for maximum effectiveness. Regular follow-up
appointments and imaging studies provided reassurance that the treatment was achieving
its intended goal – targeting the cancer while sparing healthy tissue.
Beyond the medical aspects, John also found solace in the supportive community
of fellow cancer patients and survivors. Sharing experiences, exchanging advice, and
offering words of encouragement became an integral part of his healing process.
Together, they navigated the ups and downs of cancer treatment, finding strength in unity
and resilience in the face of adversity.
As time passed, John's journey with prostate cancer took on a new dimension. The
initial fear and uncertainty gradually gave way to a sense of empowerment and hope.
With each passing day, he regained a sense of normalcy, cherishing the moments of joy
and gratitude that punctuated his life. And through it all, the radioisotope seeds continued
their silent work, battling the cancer from within, one decay at a time.
In the broader context of cancer treatment, the use of radioisotopes represents a
shining example of the marriage between scientific innovation and compassionate care. It
is a testament to the tireless efforts of researchers, clinicians, and patients alike, who
refuse to relent in the face of a formidable adversary. And as John's story illustrates, the
journey towards healing is not just about eradicating cancer – it's about reclaiming life,
one precious moment at a time.
e. Chemical Bonds: How Atoms Interact
By way of their electrons, atoms of many elements interact with other atoms.
Electrons may be shared, one atom may donate one or more electrons to another atom, or
an atom may receive electrons from other atoms. Which of these events takes place
depends on how many electrons a given atom has and how the electrons are arranged.
You’ve probably heard that like charges (11 or 22) repel each other and unlike
charges (12) attract. Electrons carry a negative charge, so they are attracted to the positive
charge of protons. On the other hand, electrons repel each other. In an atom, electrons
respond to these pushes and pulls by moving around the atomic nucleus in “shells”. A
shell has three dimensions, like the space inside a balloon, and the electron or electrons
inside it travel in “orbitals.” Each orbital is like a room that can hold no more than two
occupants. This means that in an atom, a maximum of two electrons can occupy an
orbital. Recall from Section 2.1 that atoms of different elements differ in how many
electrons they have. They also differ in how many of their “rooms” are filled.
Hydrogen, the most fundamental element in the universe, serves as the
cornerstone of atomic structure, embodying simplicity and elegance in its composition.
With just one proton and one electron, hydrogen stands as the epitome of minimalism in
the realm of atoms. Its lone electron orbits the nucleus in a single shell, its energy level
delicately balanced within the confines of this elemental unit.
In contrast to hydrogen's solitary electron, atoms of other elements follow a
different blueprint, adhering to the rules of atomic structure laid out by quantum
mechanics. The first shell of these atoms is capable of holding a maximum of two
electrons, forming a stable configuration reminiscent of the hydrogen atom's simplicity.
This shell, often referred to as the K shell, serves as the foundation upon which the
intricate architecture of atomic structure is built.
As elements grow in complexity, their atomic structure reflects this evolution,
with additional electrons populating shells farther away from the nucleus. These outer
shells, known as the L, M, N, and so forth, accommodate increasing numbers of
electrons, each shell representing a higher energy level than the one preceding it. This
hierarchical arrangement of electrons around the nucleus forms the basis of the periodic
table, with elements organized according to their atomic number and electron
configuration.
The distribution of electrons within an atom's shells plays a critical role in
determining its chemical properties and reactivity. Elements strive to achieve a state of
stability by filling their outermost electron shells, either through gaining, losing, or
sharing electrons with other atoms. This process, known as chemical bonding, underpins
the vast array of chemical reactions and molecular formations observed in nature.
The concept of electron shells and their role in atomic structure not only provides
insight into the behavior of individual atoms but also serves as a foundation for
understanding the behavior of matter on a macroscopic scale. From the formation of
molecules to the properties of materials, the intricate dance of electrons within their
respective shells governs the physical and chemical properties of substances, shaping the
world as we know it.
The shells around an atom’s nucleus are equivalent to energy levels. The shell
closest to the nucleus is at the lowest energy level. Each shell farther out from the nucleus
is at a progressively higher energy level. Because the atoms of different elements have
different numbers of electrons, they also have different numbers of shells that electrons
can occupy. A shell can have up to eight electrons, but not more. This means that larger
atoms, which have more electrons than smaller ones do, also have more shells.
Bonds form because an atom is most stable when its outer shell is filled. For
atoms that have too few electrons to fill their outer shell, chemical bonding with other
atoms can provide stability. Hydrogen and helium atoms have a single shell. The shell is
full when it contains two electrons. Some other kinds of atoms that have unfilled outer
shells tend to form chemical bonds that fill vacant “slots” in their outer shell so that it has
a full set of eight electrons. Atoms of oxygen, carbon, hydrogen, and nitrogen—the most
abundant elements in the body—are in this category. Look for electron vacancies in an
atom’s outer shell and you will always have a clue as to whether the atom will bond with
others.
Many molecules contain atoms of only one element. Molecular nitrogen (N2 ),
with its two nitrogen atoms, is an example. Many other molecules are compounds—they
combine two or more elements in proportions that never vary. For example, water is a
compound. No matter where water molecules are—in a lake or your bathtub—each one
always has one oxygen atom bonded to two hydrogen atoms.
In a mixture, two or more kinds of molecules simply mingle. The proportions may
or may not be the same. For example, the sugar sucrose is a compound of carbon,
hydrogen, and oxygen. If you swirl together molecules of sucrose and water, you’ll get a
mixture—sugar-sweetened water. If you keep the same amount of water but add more
sucrose, you will still have a mixture—just an extremely sweet one, such as syrup.
f. Important Bonds in Biological Molecules
Overall, an atom carries no charge because it has as many electrons as protons.
That balance can change if an atom has a vacancy—an unfilled orbital—in its outer shell.
For example, a chlorine atom has one vacancy and therefore can gain one electron. A
sodium atom, on the other hand, has a single electron in its outer shell, and that electron
can be knocked out or pulled away. When an atom gains or loses an electron, the balance
between its protons and its electrons shifts so that it has a positive or negative charge. An
atom or other particle that has a charge is called an ion.
It’s common for neighboring atoms to accept or donate electrons among one
another. When one atom loses an electron and one gains, both become ionized.
Depending on conditions inside the cell, the ions may separate, or they may stay together
as a result of the mutual attraction of their opposite charges. An association of two ions
that have opposite charges is called an ionic bond. The process in which an atom or
molecule loses one or more electrons to another atom or molecule is known as oxidation.
It’s what causes a match to burn and an iron nail to rust, and it is part of all kinds of
important metabolic events in body cells.
In a covalent bond, atoms share two electrons. The bond forms when two atoms
each have a lone electron in their outer shell and each atom’s attractive force “pulls” on
the other’s unpaired electron. The tug is not strong enough to pull an electron away
completely, so the two electrons share an orbital. Covalent bonds are extremely strong
and stable. Structural formulas a single line between two atoms means they share a single
covalent bond. Molecular hydrogen, a molecule that consists of two hydrogen atoms, has
this kind of bond and can be written as H}H. In a double covalent bond, two atoms share
two electron pairs, as in an oxygen molecule (O O). In a triple covalent bond, two atoms
share three pairs of electrons. A nitrogen molecule (N;N) is this way. All three examples
are gases. When you breathe, you inhale H2 , O2 , and N2 molecules.
In a nonpolar covalent bond, the two atoms pull equally on electrons and so share
them equally. The term “nonpolar” means there is no difference in charge at the two ends
(“poles”) of the bond. Molecular hydrogen is a simple example. Its two hydrogen atoms,
each with one proton, attract the shared electrons equally.
In a polar covalent bond, two atoms don’t share electrons equally. The atoms are
of different elements, and one has more protons than the other. The one with the most
protons pulls more, so its end of the bond ends up with a slight negative charge. We say it
is “electronegative.” The atom at the other end of the bond ends up with a slight positive
charge. For instance, a water molecule (H—O—H) has two polar covalent bonds. The
oxygen atom carries a slight negative charge, and each of the two hydrogen atoms has a
slight positive charge.
A hydrogen bond is a weak link that has formed between a covalently bonded
hydrogen atom and another atom taking part in a separate covalent bond. The dotted lines
in the diagram represent this link. Individual hydrogen bonds are weak, so they form and
break easily. Despite this property, hydrogen bonds are vital in biological molecules—
molecules that contain carbon and that are formed in living things. For example, the
genetic material DNA is built of two long strands of chemical units that hydrogen bonds
hold together.
g. Water: Necessary for Life
Life on Earth probably began in water, and for all life forms it is indispensable.
Human blood is more than 90 percent water, and water helps maintain the shape and
internal structure of our cells. As described next, three properties of water suit it for its
key roles in the body.
Water is a liquid at body temperature. As a result, our watery blood flows and our
cells have the fluid they need to function properly. What keeps water liquid? You may
recall that while a water molecule has no net charge, it does carry charges that are
distributed unevenly. The water molecule’s oxygen end is slightly negative and its
hydrogen end is a bit positive. This uneven distribution of charges makes water
molecules “polar.” Because they are polar, the molecules can attract other water
molecules and form hydrogen bonds with them. Collectively, the bonds are so strong that
hey hold the water molecules close together . This effect of hydrogen bonds is why water
is a liquid unless its temperature falls to freezing or rises to the boiling point.
Water attracts and hydrogen-bonds with other polar substances. Because polar
molecules are attracted to water, they are said to be hydrophilic, or “water loving.” Water
repels nonpolar substances, such as oils. Hence nonpolar molecules are hydrophobic, or
“water fearing.” We will return to these concepts when we look at the structure.
Water’s hydrogen bonds give it a high heat capacity—the ability to absorb a great
deal of heat energy before water warms significantly or evaporates. This is because it
takes a large amount of heat to break the many hydrogen bonds in a quantity of water.
Water’s ability to absorb a lot of heat before becoming hot is the reason it was used to
cool automobile engines in the days before alcohol-based coolants became available. In a
similar way, water helps stabilize the temperature inside cells, which are mostly water.
The chemical reactions in cells produce heat, yet cells must stay fairly cool in order for
their proteins to function properly.
When water absorbs enough heat energy, hydrogen bonds between water
molecules break apart. Then liquid water evaporates: Molecules at its surface begin to
escape into the air. Heat is lost when a large number of water molecules evaporate. This
is why sweating helps cool you off on a hot, dry day. Your sweat is 99 percent water.
When it evaporates from the millions of sweat glands in your skin, heat leaves with it.
Water also is a superb solvent, which means that ions and polar molecules easily
dissolve in it. In chemical terms a dissolved substance is called a solute (sahl-yoot).
When a substance dissolves, water molecules cluster around its individual molecules or
ions and form “spheres of hydration.” This is what happens to solutes in blood and other
body fluids. Most chemical reactions in the body occur in water-based solutions.
h. Acids, Bases, and Buffers: Body Fluids in Flux
As you know, a water molecule, H2 O, consists of two hydrogen atoms and one
oxygen atom. Depending on chemical conditions, a water molecule can naturally separate
into two ions—a proton, also called a hydrogen ion, or H+—and a hydroxide ion (OH2).
These ions are the basis for the pH scale. This numerical scale represents the
concentration (relative amount) of H1 in water, blood, and other fluids. There are huge
numbers of hydrogen ions in the body and they can have major effects on body functions.
Pure water, often considered the universal solvent, possesses a remarkable
equilibrium between its constituent ions, hydrogen ions (H ) and hydroxide ions (OH ),⁺ ⁻
resulting in a state of neutrality. In this delicate balance, the number of hydrogen ions is
equal to the number of hydroxide ions, giving rise to a pH value of 7 on the pH scale.
This equilibrium is a fundamental characteristic of water chemistry, playing a pivotal role
in numerous biological, chemical, and environmental processes.
The concept of pH, originally introduced by Danish chemist Søren Sørensen in
the early 20th century, serves as a quantitative measure of acidity or alkalinity in aqueous
solutions. The pH scale ranges from 0 to 14, with values below 7 indicating acidity,
values above 7 indicating alkalinity, and a pH of 7 representing neutrality. In the case of
pure water, the equal concentrations of H and OH ions confer a pH of 7, signifying a⁺ ⁻
perfectly balanced solution.
However, even minor deviations from this equilibrium can have profound
implications for the properties and behavior of water. For example, the addition of an
acid, such as hydrochloric acid (HCl), increases the concentration of H ions, shifting the⁺
pH below 7 and rendering the solution acidic. Conversely, the addition of a base, such as
sodium hydroxide (NaOH), increases the concentration of OH ions, raising the pH⁻
above 7 and imparting alkaline properties to the solution.
The logarithmic nature of the pH scale underscores the sensitivity of aqueous
systems to changes in acidity or alkalinity. Each unit of change on the pH scale represents
a tenfold difference in the concentration of hydrogen ions. For instance, a solution with a
pH of 6 is ten times more acidic than a solution with a pH of 7, while a solution with a
pH of 8 is ten times less acidic than a solution with a pH of 7. This logarithmic
relationship highlights the exponential nature of proton concentration changes and their
impact on chemical reactivity and biological processes.
Beyond its significance in chemistry, the equilibrium between H and OH ions⁺ ⁻
in water holds profound implications for environmental sustainability and human health.
Maintaining proper pH levels in natural water bodies is essential for supporting aquatic
ecosystems and preserving biodiversity. Additionally, in biological systems, the pH of
bodily fluids plays a critical role in enzyme function, cellular metabolism, and overall
physiological balance.
In conclusion, the equilibrium between H and OH ions in pure water represents⁺ ⁻
a cornerstone of aqueous chemistry, manifesting as a state of neutrality with a pH of 7.
Understanding the principles of pH and its relationship to proton concentration provides
valuable insights into the behavior of aqueous solutions and their broader implications for
diverse fields ranging from biology to environmental science.
The watery fluid inside most body cells is about 7 on the pH scale. Blood and the
watery fluids outside cells usually have a slightly higher pH, ranging between 7.3 and
7.5. These facts are relevant because proteins and many other biological molecules can
function properly only within a narrow pH range. Even small changes in pH can
drastically affect life processes.
An acid donates protons (as H1) to other solutes or to water molecules when it
dissolves in water. A base accepts H1 when it dissolves in water. When either an acid or
a base dissolves, OH2 then forms in the solution as well. Acidic solutions, such as black
coffee and lemon juice, release more H1 than OH2; their pH is below 7. Basic solutions,
such as household bleach and dissolved baking soda, release more OH2 than H1. Basic
solutions are also called alkaline fluids; their pH is above 7.
Most acids are classed as either weak or strong. Weak acids, such as acetic acid,
don’t readily donate H1. Depending on the pH, they just as easily accept H1 as give it up,
so they alternate between acting as an acid and acting as a base. On the other hand, strong
acids totally give up H1 when they dissociate in water. The hydrochloric acid (HCl) in
your stomach and sulfuric acid (H2 SO4 ) are examples.
High concentrations of strong acids or strong bases can be helpful in the stomach.
For instance, when you eat, cells in your stomach secrete HCl, which separates into H1
and Cl2 in water. The H1 ions make stomach fluid more acidic, and the increased acidity
switches on enzymes that can chemically break down food. The acid also helps kill
harmful bacteria. Eating too much of certain kinds of foods can lead to “acid stomach.”
Antacids are strong bases. For example, milk of magnesia releases magnesium ions and
OH2, which combines with excess H1 in your stomach fluid. This chemical reaction
raises the fluid’s pH, and your acid stomach goes away. Strong acids or bases can also be
harmful. For example, many drain cleaners and other household products can cause
severe chemical burns. So can sulfuric acid in car batteries. Smoke from fossil fuels and
motor vehicle exhaust releases strong acids that alter the pH of rain.
Salts are compounds that release ions other than H1 and OH2 in solutions. Salts
and water often form when a strong acid and a strong base interact. Depending on a
solution’s pH value, salts can form and dissolve easily. Many salts dissolve into ions that
have key functionsin cells. For example, nerve impulses depend on ions of sodium,
potassium, and calcium.
Because shifts in pH can seriously disrupt body functions, there must be
homeostatic mechanisms to counteract them. Fortunately, body fluids usually stay at a
consistent pH because they are stabilized by buffers—substances that can compensate for
pH changes by donating or accepting H1. Pairs of buffers, often a weak acid or a base
and its salt, operate as a balancing system that can keep the pH of a solution stable.
For example, when a base is added to a fluid, OH2 is released. However, if the
fluid is buffered, the weak acid partner gives up H1. The H1 combines with the OH2,
forming a small amount of water that does not affect pH. So, a buffered fluid’s pH stays
constant even when a base is added to the fluid.
A key point to remember is that the action of a buffer can’t make new hydrogen
ions or eliminate those that already are present. It can only bind or release them. Carbon
dioxide forms in many reactions in the body and it takes part in an important buffer
system in the blood. In this system it combines with water to form the compounds
carbonic acid and bicarbonate. When the acidity of blood starts to drop (that is, its pH
starts to rise) due to other factors, the carbonic acid neutralizes the excess OH2 by
releasing H1.
Together these reactions usually keep the blood pH slightly basic, beween 7.3 and
7.5, but a buffer system can neutralize only so many ions. Even slightly more than that
limit causes the pH to swing widely. A buffer system failure in the body can be disastrous
for homeostasis. If blood’s pH (7.3–7.5) declines to even 7, a person will fall into the
deep state of unconsciousness called a coma. In acidosis, carbon dioxide builds up in the
blood, too much carbonic acid forms, and blood pH plummets. The condition called
alkalosis is an abnormal increase in blood pH. Untreated, acidosis or alkalosis can cause
death.
i. Molecules of Life
You may remember from Section 2.4 that substances containing the element
carbon and formed in living things are called biological molecules. The four main kinds
of biological molecules are carbohydrates, lipids, proteins, and nucleic acids. All
biological molecules are organic compounds—that is, they contain carbon and at least
one hydrogen atom. An inorganic compound is one that doesn’t have both carbon and
hydrogen. Water (H2 O) is an example.
Carbon’s importance to life starts with its versatile bonding behavior. As you can
see in the sketch at left, each carbon atom can share pairs of electrons with as many as
four other atoms. The covalent bonds are fairly stable, because the carbon atoms share
pairs of electrons equally. This type of bond links carbon atoms together in chains. The
chains form a backbone to which atoms of hydrogen, oxygen, and other elements can
attach. The angles of the covalent bonds help produce the shapes of organic compounds.
A chain of carbon atoms, bonded covalently one after another, forms a backbone from
which other atoms can project:
Functional groups, the molecular building blocks of biological molecules, are
indispensable components that contribute to the diverse array of properties exhibited by
organic compounds. These groups consist of specific atoms or clusters of atoms that are
covalently bonded to carbon atoms, forming distinct structural motifs within larger
molecules. The kind, number, and arrangement of these functional groups play a pivotal
role in shaping the chemical and biological characteristics of molecules, including their
polarity, acidity, reactivity, and overall functionality.
The versatility of functional groups stems from their ability to modify the
chemical behavior and structural architecture of organic molecules. By imparting unique
chemical properties, functional groups confer distinct functionalities upon biomolecules,
thereby enabling them to fulfill specific roles in biological systems. For example,
hydroxyl (-OH) groups, commonly found in alcohols and carbohydrates, contribute to the
polar nature of molecules and facilitate hydrogen bonding interactions with other
molecules, influencing solubility and reactivity.
Similarly, carbonyl (-C=O) groups, present in compounds such as ketones and
aldehydes, confer varying degrees of polarity and reactivity, depending on their precise
location and molecular context. The presence of carboxyl (-COOH) groups, characteristic
of organic acids such as amino acids and fatty acids, imparts acidity to molecules,
influencing their behavior in aqueous environments and participating in chemical
reactions through proton donation.
Moreover, amino (-NH2) groups, prevalent in amino acids and amines, contribute
to the basicity of molecules, enabling them to accept protons and form hydrogen bonds
with other molecules. Phosphate (-PO4H2) groups, abundant in nucleic acids and
phospholipids, play essential roles in cellular signaling, energy transfer, and membrane
structure through their involvement in phosphorylation reactions and lipid bilayer
formation.
The structural diversity of functional groups extends beyond these examples,
encompassing a wide range of motifs such as sulfhydryl (-SH), methyl (-CH3), and ether
(-O-) groups, each conferring distinct properties and functionalities to organic molecules.
The spatial arrangement of functional groups within a molecule further influences its
three-dimensional structure, molecular interactions, and biological activity, underscoring
the intricate relationship between structure and function in biochemistry.
In the context of biomolecules, such as proteins, carbohydrates, lipids, and nucleic
acids, functional groups serve as key determinants of their physiological roles and
biochemical properties. The specific arrangement of functional groups within these
macromolecules governs their ability to bind substrates, catalyze reactions, transmit
signals, and participate in cellular processes, ultimately contributing to the dynamic
complexity of living organisms.
In summary, functional groups represent fundamental units of chemical diversity
and biological functionality in organic molecules. Their presence, arrangement, and
chemical properties dictate the behavior and physiological significance of biomolecules,
illuminating the intricate interplay between structure, function, and molecular diversity in
the realm of biochemistry and biology.
Small differences in a biological molecule’s functional groups can result in major
differences in the body. Human sex hormones are an example. In females, estrogen
guides the development of outward sexual traits such as the “filling out” of breasts. In
males, testosterone produces outward traits such as beard growth. Chemically the
hormones are almost identical, except that estrogen has a hydroxl group in the location
where testosterone has an oxygen atom, and testosterone has a methyl group that estrogen
lacks.
How do cells make the organic compounds they need for their structure and
functioning? To begin with, whatever happens in a cell requires energy, which is
provided by a compound called ATP that you will learn more about shortly. Chemical
reactions in cells also require a class of proteins called enzymes, which make reactions
take place faster than they would on their own.
Condensation reactions As a cell builds or changes organic compounds, a
common step is the condensation reaction. Often in this kind of reaction, enzymes
remove a hydroxyl group from one molecule and an H atom from another, then speed the
formation of a covalent bond between the two molecules. The discarded hydrogen and
oxygen atoms may combine to form a molecule of water (H2 O). Because this kind of
reaction often forms water as a by-product, condensation is sometimes called dehydration
(“un-watering”) synthesis. Cells can use condensation reactions to assemble polymers.
Poly- means “many,” and a polymer is a large molecule built of three to millions of
subunits. The subunits, called monomers, may be the same or different.
Hydrolysis reactions Hydrolysis is the reverse of condensation. In a first step,
enzymes that act on particular functional groups split molecules into two or more parts.
Then they attach an —OH group and a hydrogen atom from a molecule of water to the
exposed sites. With hydrolysis, cells can break apart large polymers into smaller units
when these are required for building blocks or energy.
In addition to condensation and hydrolysis, cells may obtain needed organic
compounds by relocating functional groups. One molecule gives up a functional group
that immediately is attached to another molecule. The transfer changes the structure and
function of both molecules involved. Electrons may also move from one molecule to
another.
j. Carbohydrates: Plentiful and Varied
Saccharide comes from a Greek word for sugar. A monosaccharide, meaning “one
monomer of sugar,” is the simplest carbohydrate. It has at least two —OH groups joined
to the carbon backbone plus an aldehyde or a ketone group. Monosaccharides usually
taste sweet and dissolve easily in water. The most common ones have a backbone of five
or six carbons; for example, there are five carbon atoms in deoxyribose, the sugar in
DNA. The simple sugar glucose is the main energy source for body cells. Each glucose
molecule (below) has six carbons, twelve hydrogens, and six oxygens. (Notice how it
meets the 1:2:1 ratio noted above.) Glucose is a building block for larger carbohydrates.
It also is the parent molecule (precursor) for many compounds, such as vitamin C, which
are derived from sugar monomers.
Unlike the simple sugars, an oligosaccharide is a short chain of two or more sugar
monomers that are joined by dehydration synthesis. (Oligo- means “a few.”) The type
known as disaccharides consists of just two sugar units. Lactose, sucrose, and maltose are
examples. Lactose (a glucose and a galactose unit) is a milk sugar. Sucrose, the most
plentiful sugar in nature, consists of one glucose and one fructose unit. You consume
sucrose when you eat fruit, among other plant foods. Table sugar is sucrose crystallized
from sugar cane and sugar beets.
The “complex” carbohydrates, or polysaccharides, are straight or branched chains
of sugar monomers. Often thousands are joined by dehydration synthesis. The many
chemical bonds in polysaccharides store a great deal of energy. That energy is released to
cells when the digestive system breaks these sugars down. Polysaccharides make up most
of the carbohydrates humans eat. The most common ones—glycogen, starch, and
cellulose—consist only of glucose.
The storage of glucose in the form of cellulose represents a vital aspect of plant
biology, serving as a structural component in the cell walls of plants. Cellulose, a
complex polysaccharide composed of repeating glucose units linked together by beta-1,4
glycosidic bonds, provides strength, rigidity, and support to plant cells and tissues. This
abundant carbohydrate polymer forms the basis of plant structural integrity, allowing
them to withstand mechanical stress and environmental pressures.
While cellulose serves as a primary energy reserve in plants, its structural
complexity presents a challenge for digestion in animals, including humans. Unlike some
herbivores and microorganisms, humans lack the necessary digestive enzymes, such as
cellulase, to break down cellulose into its constituent glucose molecules. As a result,
cellulose passes through the human digestive system largely undigested, earning the
designation of "dietary fiber."
Despite its indigestibility, dietary fiber plays a crucial role in human health and
digestion. As it moves through the gastrointestinal tract, fiber absorbs water and swells,
adding bulk to the fecal matter and promoting regular bowel movements. This increased
bulk and improved intestinal transit time aid in the prevention of constipation and
promote overall digestive health.
Furthermore, dietary fiber offers a range of additional health benefits beyond its
role in digestion. Soluble fibers, such as those found in oats, legumes, and fruits, have
been shown to lower cholesterol levels by binding to bile acids and promoting their
excretion. Additionally, fiber-rich diets have been associated with reduced risk factors for
various chronic diseases, including cardiovascular disease, type 2 diabetes, and certain
cancers.
The fermentation of undigested fiber by beneficial bacteria in the colon also
yields short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, which
serve as a source of energy for colonocytes and exert various physiological effects.
Butyrate, in particular, has been implicated in maintaining colonic health, regulating
inflammation, and supporting immune function.
Furthermore, dietary fiber can modulate the composition and activity of the gut
microbiota, promoting the growth of beneficial bacteria and inhibiting the proliferation of
pathogenic organisms. This prebiotic effect contributes to the maintenance of a healthy
microbial ecosystem within the gut, which in turn influences numerous aspects of human
health, including immune function, metabolism, and nutrient absorption.
In conclusion, while humans lack the ability to digest cellulose found in plant
tissues, dietary fiber derived from cellulose-rich sources plays a critical role in promoting
digestive health, regulating bowel function, and reducing the risk of chronic diseases. By
embracing a diet rich in fiber-containing foods, such as whole grains, vegetables, fruits,
and legumes, individuals can harness the myriad health benefits associated with this
essential dietary component.
Many plant-derived foods are rich in starch, which is one form in which plants
store glucose. In starch the glucose subunits form a string, as with the starch amylose.
The polysaccharide glycogen is one form in which animals store sugar, most notably in
muscles and the liver. When a person’s blood sugar level falls, liver cells break down
glycogen and release glucose to the blood. When you exercise, your muscle cells tap into
their glycogen stores as a quick source of energy.
k. Lipids: Fats and Their Chemical Relatives
Oil and water don’t mix. Why? Oils are a type of lipid, and a lipid is a nonpolar
hydrocarbon. A lipid’s large nonpolar region makes it hydrophobic, so it does not
dissolve easily in water. Lipids do easily dissolve in other nonpolar substances. For
example, you can dissolve melted butter in olive oil. Here we are interested first in fats
and phospholipids, both of which have chemical “tails” called fatty acids. We will also
consider sterols, which have a backbone of four carbon rings.
The lipids called fats have as many as three fatty acids, all attached to glycerol.
Each fatty acid has a backbone of up to thirty-six carbons and a carboxyl group (—
COOH) at one end. Hydrogen atoms occupy most or all of the remaining bonding sites. A
fatty acid typically stretches out like a flexible tail. In saturated fats, the fatty acid
backbones have only single covalent bonds. Animal fats are saturated and solid at room
temperature.
Examples are butter, lard, or chicken fat. The fatty acid tails of unsaturated fats
have one or more double covalent bonds. Such strong bonds make rigid kinks that
prevent unsaturated fats from packing tightly. Most vegetable oils such as canola oil,
peanut oil, corn oil, and olive oil are unsaturated. They stay liquid at room temperature.
Butter, lard, oils, and other dietary fats consist mostly of triglycerides. These fats
have three fatty acid tails attached to a glycerol backbone. Triglycerides are the most
common lipids in the body as well as its richest source of energy. Compared to complex
carbohydrates, they yield more than twice as much energy when they are broken down.
This is because triglycerides have more removable electrons than do carbohydrates—and
energy is released when electrons are removed. In the body, cells of fat-storing tissues
stockpile triglycerides as fat droplets.
Some unsaturated fats are unhealthy. Those known as trans fats are an example. A
double bond in “cis” fatty acids keeps them kinked, but in trans fatty acids a double bond
keeps them straight. Some trans fatty acids occur naturally in beef and lamb, but they can
also form in a manufacturing process (called hydrogenation) that for many years was
used to solidify vegetable oils for solid margarines and shortenings used in many
commercially prepared foods. A diet high in trans fatty acids increases the risk of heart
disease, which is why government regulations now require they be phased out as
ingredients in a wide range of food products.
A phospholipid has a glycerol backbone, two fatty acid tails, and a hydrophilic
“head” with a phosphate group— a phosphorus atom bonded to four oxygen atoms—and
another polar group. Phospholipids are the main materials of cell membranes, which have
two layers of lipids. The heads of one layer are dissolved in the cell’s fluid interior, while
the heads of the other layer are dissolved in the surroundings. Sandwiched between the
two are all the fatty acid tails, which are hydrophobic.
Sterols are among the lipids that have no fatty acid tails. Sterols differ in the
number, position, and type of their functional groups, but they all have a rigid backbone
of four fused-together carbon rings. Many people associate the sterol cholesterol with
heart disease. However, normal amounts of this sterol are essential in the body. For
instance, the sterol cholesterol is a vital component of membranes of every cell in your
body.
.Cholesterol, often associated with negative connotations due to its role in
cardiovascular health, actually serves as a precursor molecule for a diverse array of vital
biological compounds essential for human health and function. Among the most
significant derivatives of cholesterol are vitamin D, bile salts, and steroid hormones, each
playing distinct roles in various physiological processes.
Vitamin D: Cholesterol serves as the precursor for the synthesis of vitamin D, a
group of fat-soluble secosteroids responsible for regulating calcium and phosphate
metabolism in the body. When exposed to ultraviolet (UV) radiation from sunlight, a
chemical reaction occurs in the skin that converts a form of cholesterol into vitamin D3
(cholecalciferol), which is then further metabolized in the liver and kidneys to produce
the active hormone, calcitriol. Calcitriol plays a crucial role in calcium absorption from
the intestines, bone mineralization, immune function, and modulation of cell growth and
differentiation.
Bile Salts: Cholesterol is also a precursor for the synthesis of bile salts,
amphipathic molecules synthesized in the liver and released into the small intestine to aid
in the digestion and absorption of dietary fats. Bile salts emulsify dietary lipids, breaking
them down into smaller droplets and facilitating their digestion by pancreatic lipases.
Additionally, bile salts play a role in the absorption of fat-soluble vitamins (A, D, E, and
K) and cholesterol itself. After aiding in fat digestion, bile salts are reabsorbed in the
terminal ileum and recycled back to the liver in a process known as enterohepatic
circulation.
Steroid Hormones: Cholesterol serves as the precursor for the synthesis of steroid
hormones, a class of lipophilic hormones that regulate a wide range of physiological
processes, including reproduction, development, metabolism, and immune function.
These hormones include glucocorticoids (e.g., cortisol), mineralocorticoids (e.g.,
aldosterone), and sex hormones such as estrogen, progesterone, and testosterone.
Estrogen and progesterone are involved in the regulation of the menstrual cycle,
pregnancy, and secondary sexual characteristics, while testosterone plays a key role in
male reproductive function, muscle development, and bone density.
The synthesis of steroid hormones begins with the conversion of cholesterol into
pregnenolone, which serves as a precursor for the biosynthesis of all steroid hormones.
Pregnenolone is then metabolized through various enzymatic pathways in specialized
endocrine glands such as the adrenal cortex, gonads, and placenta to produce specific
steroid hormones tailored to the body's physiological needs.
In later, we will delve deeper into the functions and mechanisms of action of
steroid hormones, exploring their roles in reproduction, development, growth,
metabolism, and homeostasis. Understanding the intricate interplay between cholesterol
and its derivatives provides valuable insights into the complexity of human physiology
and the interconnectedness of biochemical pathways that govern health and disease.
