BIO 120 Biology (Chapter: Photosynthesis and Cell Cycle)
The process through which solar energy is transformed into chemical energy is known as
photosynthesis. The sugar and other organic compounds contain the energy. In the end, it is how
living creatures obtain practically all of the energy required to survive, either directly or
indirectly.
Function of Photosynthesis Each creature obtains energy using either an autotrophic or a
heterotrophic nutritional system. Autotrophs are living things that can feed themselves by using
photosynthesis or, in certain circumstances, other mechanisms that transform energy and
inorganic molecules into substances necessary for existence. The majority of ecosystems are
known to use autotrophs as producers, including algae and green plants.
Heterotrophs are creatures that obtain the nutrients they require from substances produced by
other species. They are referred to as consumers inside food chains as a result. Animals, for
instance, eat producers or other creatures that have previously eaten producers to get their
nourishment.
Photosynthesis in plants The organelle in charge of photosynthesis is the chloroplast. There is an
exterior membrane and an inner membrane that enclose the stroma, a viscous fluid that contains
enzymes and is located inside the chloroplast. Thylakoids are an intricate network of flattened
sacs produced by the stromal inner membrane system. Collections of thylakoids are known as
thylakoids. The membrane of thylakoids contains the pigment chlorophyll, which is necessary
for photosynthesis.
The chemical equation can be written as follows:Although both cellular respiration and
photosynthesis involve reduction/oxidation reactions, photosynthesis involves the splitting of
water and the transfer of electrons through a process that eventually reduces CO2 to sugar
molecules with the addition of hydrogen ions. Water is present on both sides of the equation. In
contrast to the electron transport chain used in cellular respiration, where electrons cascade down
an energy gradient to combine with oxygen to generate water, in photosynthesis, electrons are
moved up an energy gradient with the help of solar energy.
The light responses and the Calvin cycle are two steps that make up photosynthesis. With the
production of NADPH and ATP, the light reactions transform light energy into chemical energy
in the thylakoid membranes. The stroma, where enzymes carry out the Calvin cycle to produce
sugar, receives these energy molecules next. The process of photosynthesis is depicted in the
diagram below. Click image to expand Light Responses In photosynthesis, the light reactions'
main function is to absorb light energy and generate ATP and NADPH.
Light has a wave nature. It's crucial to grasp certain fundamental aspects of light in order to
comprehend this initial phase of photosynthesis. One type of electromagnetic radiation, visible
light, is energy that moves through waves made of vibrating electric and magnetic fields. The
wavelength is the separation between two adjacent places on succeeding waves, such as two
crests or two troughs. The electromagnetic spectrum is the collection of wavelengths that make
up electromagnetic energy. Visible light is the region of the spectrum that can be seen by the
human eye and spans from violet light at shorter wavelengths to red light at higher energies.
Most of the characteristics of light may be explained by the wave model. Yet, certain
characteristics can only be described by a paradigm in which light is viewed as having a particle
nature. A photon is an electromagnetic radiation particle. A photon's discrete quantity of energy
is inversely proportional to the wavelength of light. Biological pigments. Light can be reflected,
transmitted, or absorbed as it interacts with materials. Visible light is absorbed by chemicals
called photosynthetic pigments. Chlorophyll a and Chlorophyll b are two different types of
photosynthetic pigments. The center of the spectrum is not absorbed by either chlorophyll or b,
although they both absorb violet-blue and red light. This portion of the spectrum, which also
contains green light, is reflected instead. Plants seem green because the eye reflects approval.
Photosystems. As components of photosystems in the thylakoid membrane, chlorophyll
molecules and proteins are referred to as light-harvesting complexes. In photosystem II,
chlorophyll absorbs solar energy and stores it for later use when electrons are stimulated to
higher energy levels. These energized electrons then go to a protein reaction hub, where
chlorophyll molecules are found. Because of how well chlorophyll absorbs red light at 680 nm,
these molecules are known as P680. Similar to this, chlorophyll in the reaction center of
photosystem I is referred to as P700 because it best absorbs red light at 700 nm.
Flow of Noncyclic Electrons. In the first phase of photosynthesis, there are two types of electron
transport pathways: noncyclic and cyclic. A photon of light is absorbed by chlorophyll in
photosystem II, and the energy from the photon is transferred to an electron in the P680
chlorophyll at the reaction center, energizing it to an excited state. This is the non cyclic energy
flow. The principal electron acceptor is then a molecule that receives this electron. Moreover, a
water molecule is broken by an enzyme in the light-harvesting complex, releasing the oxygen
atom that will later combine with two protons to create O2. A P680 molecule also receives the
electrons from this mechanism.
The principal electron acceptor transports electrons to the photosystem I reaction center through
an electron transport chain. Enough energy is released during this phase to power ATP
production. Photosystem I is also capable of absorbing light during this process. The P700
chlorophyll an is located in the reaction center, which receives the energy that was absorbed.
Collectively, these procedures enable the photosystem I reaction center to send an electron to an
additional cytochrome complex-based electron transport chain. By consuming roughly the same
amount of energy as ATP, this process releases the energy needed to drive the synthesis of
NADPH from NADP+.
Electron Flow in Cycles Cyclic electron flow, which only includes photosystem I under specific
circumstances, is the second route. The cytochrome complexes receive electrons released by
photosystem I's principal electron acceptor, which are subsequently cycled back into the
chlorophyll P700 at the reaction center. Just ATP is synthesized as a result of this cycle. The
chloroplast may control the synthesis of NADPH in this fashion by changing the electron flow
from a noncyclic to cyclic.
Chemiosmosis. Chemiosmosis is the mechanism that uses protons transported across a
membrane during an electron transport chain to power the synthesis of ATP from the
phosphorylation of ADP by ATP synthase in both mitochondria and chloroplasts. In this
instance, the stroma receives the release of ATP while the protons are discharged into the
thylakoids. The name of this synthetic pathway, photophosphorylation, refers to the fact that light
eventually acts as the driving force for this process.
Clause Cycle The Calvin cycle, which uses energy obtained from the initial stage of
photosynthesis to synthesize sugar from carbon dioxide (CO2), is analogous to the Krebs cycle
in cellular respiration. Via stomata, which are apertures in the leaves, CO2 is delivered to the
chloroplast. The Calvin cycle is anabolic in contrast to the catabolic Krebs cycle. The cycle uses
ATP as an energy source and NADPH reduces the ability to convert CO2 into a precursor to
glucose. This is glyceraldehyde-3-phosphate as a precursor (G3P).
Carbon fixation, the first stage of the cycle, entails the sequential integration of three different
CO2 molecules. This fixation takes place when an enzyme called rubisco attaches a CO2
molecule to the five-carbon molecule ribulose bisphosphate (Rupp). The resultant molecule then
divides into two 3-phosphoglycerate molecules. Reduction is the cycle's second stage.
glyceraldehyde-3-phosphate is created when the 3-phosphoglycerate is phosphorylated by ATP
and reduced by NADPH. The RuBP required for the subsequent cycle of carbon fixation is
renewed at the Calvin cycle's last stage. This requires both the use of ATP and a complicated
chain of processes.
Nine molecules of ATP and six molecules of NADPH are needed for the whole cycle to produce
one glyceraldehyde-3-phosphate from three molecules of CO2, and both are produced by the
light reactions of photosynthesis. The other anabolic pathways in the cell will use this
three-carbon sugar as a starting point to generate other sugars. The stages of a cell's existence
from the beginning to the end are described by the cell cycle. Two principles of cell theory must
be recalled in order to explain this cycle. The first is that all cells are created by cell division,
which is just a fancy way of saying that cells divide to create new cells. In other words, no cell
spontaneously forms and must always emerge from another cell. For new cells to develop, cells
must divide. Cell division serves as a means of reproduction in several species. Cell
differentiation is necessary in some cases to create the new cells needed for both growth and
repair.
Overview of Cell Division
To create new cells, a cell cannot simply divide in half. Instead, identical genetic material is
given to two daughter cells throughout the cell division process. The cell must first replicate its
DNA, then split into two different halves and divide. The genome is the collection of a cell's
genetic material. Prokaryotic cells generally have a single DNA molecule in their nucleoid that
serves as their genome. DNA is packed into chromosomes in the nucleus of eukaryotic cells,
which also houses the genome. Each chromosome is made up of a single, delicate thread of
chromatin, which is made up of both DNA and proteins.
The eukaryote species has a certain number of chromosomes. For instance, the somatic cells
found in humans have 46 chromosomes. The gametes, which are the sperm and egg cells in
humans, have half as many chromosomes—23. The whole set of 46 chromosomes is produced
during sexual reproduction when the two gametes come together. In eukaryotic cells, cellular
division can occur in one of two ways. The first route is mitosis, a division of the nucleus that
results in the production of two identical daughter cells. The second route, known as meiosis,
entails division in which the chromosomes are evenly split between two gametes.
Cycle of Mitotic Cells There are several distinct stages in the cell cycle. The interphase makes up
the majority of the cell cycle. 90% of the cell cycle occurs during this phase, which includes all
of the activities necessary to produce the proteins and organelles that keep the cell alive. The G1,
S, and G2 phases of interphase can also be separated. The G1 and G2 stages are largely ones of
growth. G1 and G2 are hence also referred to as the "first gap" and the "second gap,"
respectively. The "synthesis" in the S phase refers to the process of duplicating the
chromosomes. The mitotic (M) phase, which includes both mitosis and cytokinesis, is the last
stage of the cell cycle. Prophase, prometaphase, metaphase, anaphase, and telophase are the
further divisions of the M phase.
Metaphase. Nucleosome fibers are more securely wound into distinct chromosomes during
prophase. Two identical double helix molecules are present on each chromosome; these are
referred to as chromatids and sister chromatids, respectively. A link between these sister
chromatids is known as a centromere. Animal cells divide during the G2 phase, with the
centrosome acting as the cell's microtubule hub. The centrosome is made up of centrioles and an
aster, which is a collection of a few short microtubules. The mitotic spindle, which is made up of
microtubules and related proteins, will be coordinated by centrosomes. Centrosomes separate
from one another during the prophase.
Prometaphase. The nuclear envelope starts to detach during prometaphase. On both sides of the
centromere of sister chromatids, protein complexes termed kinetochores are involved in this
phase. The linked chromosomes migrate toward the pole from which the microtubule extends
when some of the spindle microtubules bind to the kinetochores. Metaphase. The centrosomes
have been pushed to opposing ends of the cell during metaphase, the longest phase of mitosis.
Along an imagined metaphase plate, the chromosomes are arranged in the cell's center. The
sibling chromatids in each chromosome are joined to totally different spindle fibers. Anaphase.
Anaphase, which only lasts a few minutes, happens when the two sister chromatids are
separated, resulting in an unzipping that starts at cellular centromeres. These freed chromosomes
go to the cell's extreme ends. The cell grows until its two ends are filled with chromosomes and
all the components required for life.
Telophase. Daughter cells start to develop in telophase. This happens when the metaphase plate's
former position, along the plasma membrane, begins to divide the cell's ends into two. When
chromosomes unfold to generate looser chromatin structures, nucleoli emerge. Cytokinesis. Cell
division is finished in animal cells during cytokinesis. The cleavage furrow, which is controlled
by proteins, is where the cleavage site is located. Cell walls in plant cells prevent cleavage
furrows from developing. Instead, vesicles gather in the center of the cell to create a cell plate
with the help of microtubules. Ingredients for constructing the cell wall accumulate in the cell
plate, where they finally converge to create the two daughter cells.
Control of the Cell Cycle A certain amount of regulation of the cell cycle is necessary to avoid a
scenario where cells reproduce uncontrollably. At particular checkpoints in the cell cycle, a
stop-and-go mechanism serves as a switch, and this control takes place. The "restricted point" is
the first checkpoint, which happens during the G1 phase. When a cell receives the "go" signal
during this phase, it often signifies that it will continue through the S and G2 phases and divide
into two cells. The cycle will enter the G0 phase, a non-dividing route, if it does not get a "go"
signal. The M checkpoint is one of the two additional checkpoints, which also happen at the
conclusion of the G2 phase and during mitosis. kinases, which are regulatory proteins that
phosphorylate other proteins, at the G1 and G2 checkpoints, are responsible for starting the "go"
signal. Before they are joined to a protein known as a cyclin, these kinases become active. These
combinations are known as calmodulin phosphatase, and although the precise mechanism by
which they are activated within the cell is still under investigation, it is known to entail both
internal and external stimuli.
Growth factors, which are proteins that encourage cell proliferation in other cells, are one type of
internal stimulus. The existence of additional cells is another type of cue. Density-dependent
inhibition is the process through which cells stop dividing when they get too crowded. A surface,
such as the extracellular matrix of tissue, is required for animal cells to divide. We refer to this
regulation as anchorage dependency. Cancer cells divide excessively and even invade other
tissues inside an organism because they are unable to recognize the stimuli that control cellular
division. A tumor develops when cells proliferate and layer themselves. A benign tumor is one
that doesn't spread from its initial location and may be surgically removed. On the other hand, a
malignant tumor affects an organ's ability to function normally by spreading cancerous cells to
other parts of the body.
Malignancy is the procedure through which cancerous cells spread to a new site and develop a
tumor. Cancer cells have the ability to cease dividing, but they do so outside of the normal
cellular boundaries.
