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The Cellular Level of Organization
A cell is the basic, living, structural and functional unit of the body. Cytology is the study of cells.
Functions of the cell:
1. Basic unit of life
2. Protection and support
3. Movement
4. Communication
5. Cell metabolism and energy release
PLASMA MEMBRANE
Plasma membrane = cell membrane = cytoplasmic membrane is the outer, limiting membrane separating
the cell's internal parts from the extracellular fluid and external environment. (extracellular = outside the cell;
intracellular = inside the cell; intercellular = between cells).
Thickness of plasma membrane = 6.5 to 10 nanometers. It is composed of phospholipids and proteins
with smaller amounts of cholesterol, glycolipids, water, carbohydrates and ions.
Fig 3.2 p 70: fluid mosaic model.
The phospholipids are arranged in a double layer (phospholipid bilayer). The proteins associated with the
phospholipid bilayer are classified into 2 categories: integral proteins and peripheral proteins. Integral proteins
are embedded in the phospholipid bilayer. [Note glycoproteins in fig 3.2 p 70. Glycolipids and glycoproteins
are cell identity markers that enable cells to recognize other chemicals
and potentially dangerous foreign cells.] Integral proteins are mobile. Some integral proteins form channels
through which substances can be transported (e.g. water). Others provide receptor sites that enable a cell to
recognize other cells of their own type to form tissues; to recognize and respond to foreign cells; and to
recognize and attach to hormones, nutrients and other chemicals.
Peripheral proteins are loosely bound to the membrane surface and easily separated from it. Less is
known about their function. However, enzymes catalyze reactions, and it is believed that some of these
proteins serve as enzymes.
Functions of the plasma membrane:
1. Provides boundary to enclose cellular contents—channel
2. Mediates the passage of materials—transporter
3. Provides receptors for chemicals—receptor
4. Catalyzes reaction inside or outside of cell—enzyme
5. Distinguishes your cells from anyone else’s (exception: identical twin)—cell identity
6. Facilitates contact with other cells—attachment sites
The plasma membrane allows some substances to enter or leave the cell and restricts the passage
of other substances. For this reason, the plasma membrane is called selectively permeable,
differentially permeable or semipermeable. Permeability is the ability to cross a membrane.
Impermeable or non-permeable means a substance cannot cross a membrane.
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Objective: Describe the processes by which substances are transported across the cell
membrane.
CELLULAR MOVEMENTS
1. Diffusion is the movement of molecules from an area of greater to an area of lesser
concentration. (e.g. movement of oxygen, carbon dioxide across the cell membrane.)
Other gradients across a cell membrane: Concentration gradient is the difference between chemicals on one
side of a plasma membrane and the other. Extracellular Na and intracellular K . Intracellularly there are also
+ +
negatively charged proteins. More negatives inside than outside. Electrical gradient is the difference in the
distribution of positively and negatively charged ions between one side of a plasma membrane and the other.
Because ions are influenced by both a concentration gradient and an electrical gradient, the combined effect is
called the ion’s electrochemical gradient.
2. Osmosis is the diffusion of water molecules from an area of greater to an area of lesser concentration
across a selectively permeable membrane (plasma membrane). The water molecules pass through
channels formed by integral proteins in the membrane. Fig. 3.16 p 93.
Osmotic pressure is the pressure required to prevent the movement of pure water into a
solution containing solutes when a selectively permeable membrane separates the solutions. The
greater the solute concentration, the greater its osmotic pressure.
Isotonic solution (same as) = equal osmotic pressure between 2 different solutions.
Hypotonic solution (less than)= having an osmotic pressure lower than that of a solution with which it is
compared.
Hypertonic solution (more than) = having an osmotic pressure greater than that of a solution with
which it is compared.
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Crenation—the shrinkage of red blood cells, due to water loss
Equilibrium—balanced water distribution across the membrane of red blood cells.
Plasmolysis (Hemolysis)—lysis of the membrane of a red blood cell and the release of
hemoglobin. Fig 3.17 p 95
3. Facilitated transport requires integral proteins to serve as protein carriers (fig 3.18 p 96).
a. passive transport is the movement of molecules from an area of greater to an area of lesser concentration
with the assistance of protein carriers. It does not require energy. Some substances are large molecules,
insoluble in lipids and cannot pass through the cell membrane unassisted. Example: glucose or C .
6H12O6
It moves from the more concentrated side of the membrane to the less concentrated side of the membrane
or along the concentration gradient.
The movement of ions can be regulated by opening or closing the gate. It can be used to create an
electrical gradient. Gated membrane channel is a protein channel that can act as a gate. The protein can
change shape to open or close the pore. K , Cl , Na , Ca are the most common ions that move
+ — + +2
through channels passively.
b. active transport is the movement of molecules from an area of lesser to an area of greater
concentration with the assistance of protein carriers. It requires energy in the form of ATP
(adenosine triphosphate). The breaking of phosphate bonds releases energy. fig 3.19 p 97 is the
ATP molecule.
ATPADP + P (releases energy)
ADPAMP + P (releases energy)
AMP adenosine + P (releases energy)
C O6H12 6 + O CO + H O releases energy, and this released energy if used to form ATP.2 2 2
Example: moving glucose from the digestive tract into the bloodstream even though the blood concentration
may be higher. Amino acids and monosaccharides are actively transported. Solutes that are actively transported
include K , H , Na , Ca , I , and Cl , amino acids and monosaccharides.
+ + + +2 — —
One of the most important transport systems is the sodium-potassium pump in nerve and muscle
cells: Na is expelled from the cell and K is pumped into the cell. Fig 3.19 p 97. Even as Na
+ + +
increases outside, still more is pumped out (against the concentration gradient). Likewise, as K +
increasing inside (against the concentration gradient), more is pumped in.
Endocytosis is the uptake into a cell of large molecules and particles in which a segment of plasma membrane
surrounds the substance, encloses it and brings it in within a vesicle. A vesicle is a small, spherical membranous sac
formed by budding off from an existing membrane. Fig 3.21 p 99 diagrams endocytosis. Basic types include
phagocytosis ("cell eating") and pinocytosis ("cell drinking").
Phagocytosis engulfs solids. Pinocytosis engulfs extracellular liquids. Receptor-mediated
endocytosis is the selective uptake of large molecules and particles by cells.
Terms: WBC, pseudopodium, bacterium, lysosome, phagocytic vesicle.
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Receptor-mediated endocytosis is demonstrated in fig. 3.22 p 101. It is more specific than
pinocytosis in that it literally selects what can enter the cell. A ligand is a chemical substance that binds
to a specific receptor. It is the receptors that identify the substance or ligand that may or may not be
transported across the membrane. Recall: selective permeability.
Exocytosis is a process of discharging cellular products too large to go through the membrane. Reverse of
endocytosis (fig. 3.22 p 101). Examples: neurotransmitters, secretory cells (e.g., from pancreas). Secretion is
also the process of discharging cellular products to the outside of a cell.
Secretory cells include sweat glands and oil glands in the skin. Summary: Figure 3.22 p 100-101.
CYTOPLASM
Cytosol is the fluid portion of the cytoplasm. Cytoplasm is the substance inside the plasma
membrane and external to the nucleus. It is the matrix or ground substance in which cellular
components (organelles) are found. Chemically, the cell is 75-90% water. Contains dissolved
substances (e.g., salts, ions, glucose), making it a solution; suspended substances (amino acids, fatty
acids, lipids, proteins), making it a suspension; and organelles. Since mostly water, it is the solvent in
which some chemical reactions occur.
Objective: Describe the structure and function of the parts of the cell.
ORGANELLES
Organelles are membrane-bound structures with a variety of functions. They assume specific roles
in growth, maintenance, repair and control.
The nucleus (fig 3.10 p 83)
1. spherical or oval
2. largest organelle
3. contains hereditary factors (i.e., genes)
4. controls cellular structure and directs cellular activities
5. usually 1 nucleus/cell. However, RBCs contain none, and skeletal muscle cells contain several
nuclei.
6. surrounded by a double membrane called nuclear membrane or nuclear envelope.
a. space between double membranes called perinuclear cisterna.
b. nuclear membrane contains nuclear pores that allow for communication with the cytoplasm
via another organelle called the endoplasmic reticulum.
7. Structures within the nucleus:
a. karyolymph or nucleoplasm is a gel-like fluid
b. nucleoli - one or more spherical bodies composed of protein, DNA and RNA. Site for the
synthesis of ribosomal RNA. Disappear during cell division (mitosis) and reappear
afterwards.
c. genetic material - composed of DNA. A thread-like mass called chromatin when the cell is
not actively dividing. Shortens and thickens into rod-like chromosomes during cell division
(3.36 p 86).
Ribosomes
1. Granular structures
2. Composed of ribosomal RNA (rRNA) and some specific ribosomal proteins.
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3. Structurally, a ribosome consists of 2 subunits.
4. Functionally, ribosomes are the site of protein synthesis (they receive genetic instructions
from DNA and use it to produce proteins).
5. Free ribosomes are scattered throughout the cytoplasm, either singly or in clusters. They
synthesize proteins for intracellular use. Membrane-bound ribosomes attached to the
endoplasmic reticulum synthesize protein for extracellular use and for insertion into the
plasma membrane.
Endoplasmic Reticulum (ER)
1. A membranous system of tubular canals that begin at the nuclear membrane and branch
out through the cytoplasm (fig 3.5 p 76)
2. Two types:
a. granular (rough) e.r. - studded with ribosomes. They synthesize proteins for secretion
phopholipids and forms new membranes for cellular structures.
b. agranular (smooth) e.r. - free of ribosomes. They synthesize phospholipids, fats, and
steroids.
3. Functions:
a. contributes to the mechanical support and distribution of the cytoplasm
b. involved with intracellular exchange of materials within the cytoplasm
c. provides surface area for chemical reactions
d. storage area for synthesized products
e. together with the Golgi complex, the ER is involved with packaging and shipping of products
outside of the cell.
Golgi complex (fig. 3.6 p 77)
1. consists of 4-8 flattened membranous sacs stacked on each other. The ends of the sacs are
expanded. The stacks of sacs are called cisternae, and designated cis- (entry), medial- and trans-
(exit) based on their function.
2. Function: to process sort and deliver proteins to various parts of the cell. See fig. 3.7 p 78-79. (Protein
synthesis, vesicle formation; lysosome formation for digestion and molecules taken up
by phagocytosis; exocytosis or secretory granule containing extracellular enzymes).
When the vesicle exits the face cistern, it becomes a secretory vesicle for extracellular export, a
membrane vesicle that merges with the membrane, or storage vesicle (e.g., lysosome).
Mitochondria
1. spherical, rod-shaped or filamentous structures scattered throughout the cytoplasm.
2. composed of 2 membranes: outer and inner. The inner membrane is convoluted into projections
called cristae. Filled with a matrix.
a. the inner membrane provides a large surface area for chemical reactions (fig 3.9 p 81).
b. enzymes involved in energy-releasing reactions that result in the formation of ATP
are located on the cristae (cellular respiration).
c. "powerhouse of the cell"
3. large numbers of mitochondria are found in active cells that expend a lot of energy (muscle,
liver, kidney tubular cells).
4. mitochondria are self-replicative. They have their own DNA. Replicate as the need for ATP
increase.
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Lysosomes
1. membrane-enclosed vesicles formed from the Golgi complex (fig 3.8 p 80).
2. contain powerful digestive enzymes
3. autolysis (self-digestion) - In a healthy cell, lysosomes cannot move into the cytoplasm. In a
damaged cell, lysosomes release their enzymes and break down cell constituents. Autophagy is
the process by which worn-out organelles are digested.
Peroxisomes
1. similar to lysosomes, but smaller.
2. abundant in liver
3. contain catalase among other enzymes; enzymes that oxidize (remove H atoms from substrates)
oxidize alcohol.
catalase
H O
2 2 H2O + O2
Cytoskeleton is a network of several different kinds of protein filaments that extend throughout the
cytosol. There are several kinds of filaments: microfilaments, microtubules, intermediate filaments (fig.
3.3 p 73).
Microfilaments
1. rod-like structures 3-12 nm in diameter
2. variable arrangements (bundles, meshwork, scattered)
3. some contain protein actin and some contain proteins myosin
4. function in contraction in muscle cells or provide support, shape and cell movement in
non-muscle cells.
Microtubules
1. straight, slender, cylindrical structures 18-30 nm in diameter
2. provide support and shape in cell
3. assist in movement of pseudopodia
4. form the structure of flagella, cilia, centrioles and the mitotic spindle
5. protein tubulin
Intermediate filaments
1. 7-11 nm in diameter
2. proteins vary
3. scattered or bundled
4. appear to provide structural reinforcement.
Centrosome and centrioles:
Centrosome = dense mass near the nucleus. Consists of a pair of centrioles. Fig 3.4 p 75. Centriole
= triplet clusters of microtubules arranged in a circular pattern (9+0). The pair is arranged at right
angles to one another.
Cells contain DNA for their own self-replication. They function in cell division. Cells lacking
centrioles (e.g. nerve cells) cannot reproduce. (Exception: plant cells lack centrioles but can reproduce.
Centrioles are only found in animal cells).
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Flagella and cilia: movement (fig 3.4 p 75)
There are no structural differences between flagella and cilia. Flagella = long, slender, few in number. Have
a whip- like motion. Move the cell along. Example: sperm. Cilia = short, numerous over the surface of the cell.
Move like oars. Used for moving substances along the surface of the cell. 9+2 pattern = nine pairs of
microtubules in an outer ring and two single microtubules in the center
Terms: radial spoke, inner single microtubule, outer double microtubule, central sheath
INCLUSIONS
Cell inclusions = a lifeless, often temporary, cellular constituent, as opposed to an organelle.
Examples: melanin - found in skin, hair and eye cells. Protects the body from harmful ultraviolet rays.
glycogen - stored polysaccharide. When the body needs energy, it converts glycogen to glucose. The
digestion of glucose releases ATP.
lipids - stored in fat cells (adipocytes). Can be decomposed for producing energy.
mucus - produced by cells that line organs. Function is lubrication and protection.
Objective: Describe the sequence of events (processes) of cell transcription, translation,
replication and mitosis.
REPLICATION, TRANSLATION AND MITOSIS (3.24 p 106)
Both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids composed of
nucleotides. Every is composed of 3 united subunits: phosphoric acid (phosphate), a pentose nucleotide
sugar and a nitrogenous base, either a purine or a pyrimidine.. The purines are adenine and guanine . The
pyrimidines are thymine, cytosine and uracil . They are called nitrogenous bases or bases because they have
basic characteristics which raise the pH of a solution. The differences between DNA and RNA lie in their
sugars and pyrimidines:
1. DNA = deoxyribose sugar and RNA = ribose sugar (fig 2.24 p 58).
2. DNA contains pyrimidines cytosine and thymine while RNA contains pyrimidines cytosine
and uracil. Guanine and adenine can be found in both DNA and RNA.
When nucleotides join together, they form a linear macromolecule in which the backbone is made up
of alternating phosphates and sugars with the bases to one side of the backbone.
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P
S
P
S
P
S
P
S
RNA is a single-stranded. The order of nucleotides and the order of the bases differ for any
particular RNA or DNA.
P
S-U
P
S-G
P Notice: no thymine
S-A
P
S-C
DNA is a double-stranded structure and is represented as 2 chains of nucleotides, connected to one another by
hydrogen bonding. Adenine is always bonded to thymine by 2 hydrogen bonds. Guanine is bonded to cytosine by 3
hydrogen bonds. This is called complementary base pairing. Even though the order of the bases in DNA may vary,
the number of purines always equals the number of pyrimidines.
P P
S-T=A-S
P P
S-GC-S
PPNotice: no uracil
S-A=T-S
P P
S-CG-S
Once the two strands have paired, the structure resembles a ladder. Finally, this ladder twists to form a
helix. Thus, DNA is called a double-stranded helix. It resembles a spiral staircase.
DNA is the genetic material of which chromosomes are made. It is found primarily within the
nucleus of the cell. This genetic material is capable of replication so that each daughter cell, during
mitosis, receives a copy. It controls the metabolism of the cell, producing the characteristics of the
individual.
Each strand of the double stranded structure of DNA can serve as a template (mold) for the
formation of a complementary strand. Replication requires the following steps:
1. The 2 strands of DNA "unzip" by breaking their weak H bonds. The enzyme that regulates the
“unzipping” is deoxyribonuclease or DNAase.
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2. New complementary nucleotides, always present in the nucleus, move into place by the process
of complementary base pairing.
3. The complementary nucleotides become joined together.
4. When the process is finished, 2 complete DNA molecules are present, identical to the original.
The replication process is called because each double strand contains one old andsemiconservative
one new strand. Research shows that replication begins somewhere along the length of the DNA
molecule and proceeds towards each end.
DNA controls protein synthesis. Messenger (mRNA) serves as the go-between for DNA in the
nucleus and the ribosomes in the cytoplasm. One strand of the DNA can serve as a template for the
production of a complementary strand of RNA, as well as a template for another strand of DNA. The
mRNA sequence then contains a sequence of nucleotides that are complementary to those of a single
gene. This process is called (fig 3.13 p 88).transcription
The mRNA moves from the nucleus to the ribosomes where it dictates the sequence of amino acids
that will become a protein.
DNA
mRNA
protein
transcription translation =
protein synthesis
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Translation (protein synthesis) (Fig. 3.31, p 108-109)
1. Initiation begins when 40S subunit of ribosome binds to AUG of mRNA
2. Elongation - tRNA attaches to the 60S ribosome and amino acids join together to produce
the protein
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3. Termination - ribosome encounters UAA, UAG, or UGA (nonsense codons). Ribosome
separates into subunits and detaches from mRNA.
MITOSIS
Mitosis is cell division in which daughter cells retain the same number and kinds of chromosomes as
the mother cell. The mother cell is the cell that divides and produces 2 daughter cells. Replication is the
process by which DNA makes a copy of itself. Because of replication, each chromosomes in the mother
cell contains duplicate chromatids, called a chromatid pair or chromosome. Each chromosome is
composed of 2 chromatids held together at the centromere. The 2 chromatids contain identical genes, the
units of heredity that control the cell, because they contain 2 identical pieces of double- stranded DNA.
(When counting chromosomes during cell division, it is necessary to count only the number of
centromeres).
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Since different genes are found on different chromosomes, each daughter cell must receive a copy
of each chromosomes in order to have a full complement of genes. This process is used to replace dead
cells, injured cells, and for embryonic development.
Mitosis can be broken down into 4 phases: prophase, metaphase, anaphase, and telophase. In
between cell division, the cell is in interphase.
During interphase:
1. the nuclear membrane and nucleoli are visible.
2. the chromosomal material is present as chromatin - fine threads dispersed through
the nucleoplasm.
3. the centrioles are present just outside the nucleus 2 pairs are present in animal cells.
4. DNA is replicating so that as mitosis begins each chromosome consists of duplicate chromatids
5. normal cell operations, such as protein synthesis, are taking place.
6. centrioles begin replication in G and complete replication by the end of G phase.1 2
During prophase:
1. the chromatin material shortens and thickens so that the chromosomes are readily visible.
2. by the end of prophase, each pair of centrioles has migrated to an opposite side of the nucleus.
3. spindle fibers appear between the separating pairs of centrioles. As the spindle appears, the nuclear
membrane and nucleolus disappear. Spindle fibers are microtubules.
During metaphase:
1. the spindle is fully formed and stretches between the centrioles located at the poles of the
spindle. Spindle fibers are bundles of microtubules. The spindle fibers stretch from the pole to
the middle of the spindle. Some fibers are attached to the centromere.
2. Aster rays are present in animal cells. They are microtubules that radiate out from the centrioles.
3. Chromosomes (ditrads) are arranged across the center or equator of the cell.
During anaphase:
1. The centromeres divide and the duplicate chromatids separate, each moving toward an opposite pole of the
spindle. The spindle fibers attached between centrioles and centromeres appear to shorten and spindle fibers
between centromeres appear to lengthen. This is called .karyokinesis
During telophase:
1. the spindle apparatus disappears as the daughter nuclei appear
2. nuclear membrane forms around the chromosomes in each daughter cell
3. within the nucleus, the chromosomes become indistinct chromatin again as the nucleoli reappear
4. cytoplasmic division, cytokinesis, occurs. A cleavage furrow forms on opposite sides of the cell
membrane. Furrowing is complete when each daughter cell has a complete cell membrane
enclosing it.
5. In each daughter cell, an immature centriole forms at a right angle to each mature centriole.
Each cell now has 2 pairs of centrioles.
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Somatic cell = one body cell that undergoes mitosis.
What regulates cell division?
A key signal that induces cell division is maturation promoting factor (MPF). One component of
MPF is a group of enzymes called cdc2 proteins (cell division cycle). Another component of MPF is
cyclin. Cyclin builds up during interphase, activates cdc2, which activates MPF. The result is that the
cell undergoes mitosis.
Growth = anabolism > catabolism
Aging, death = catabolism > anabolism
What regulates cell death?
Apoptosis (“falling off”) is genetically programmed death. “Cell suicide” genes produce enzymes that
damage the cell (e.g., disrupt cytoskeleton and nucleus). The cell shrinks, pulls away from neighboring cells; the
DNA fragments; the cytoplasm shrinks. Phagocytes will ingest the dying cell. No inflammatory response.
Apoptosis removes unneeded cells during fetal and embryonic development. It regulates cell number after birth
and eliminates potentially dangerous cancer cells.
Necrosis is a pathological death that results from tissue injury. Adjacent cells swell, burst and release their
cytoplasm into interstitial fluid. This cellular debris stimulates an inflammatory response.
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