Chapter 3
Structure and Function of the Cell
Part A: Cell and Plasma Membrane
I.
Functions of the Cell
Function
Basic unit of life
Protection and support
Movement
Communication
Cell metabolism and energy release
Inheritance
II.
Example
N/A
Bone cells produce bone
Muscle cells produce movement
Cells produce and receive chemical and electrical signals
Chemical reactions
Each cell contains copy of genetic information
Plasma (Cell) Membrane Structure
A. General
1. Definitions
a. Intracellular = inside cell
b. Extracellular = outside cell
c. Intercellular = between cells
2. Fluid mosaic model
a. Plasma membrane is neither rigid nor static in structure
b. Membrane is highly flexible and can change shape/composition
c. Membrane is selectively permeable
d. Phospholipid bilayer with “floating” cholesterol and proteins
B. Membrane lipids
1. Phospholipid bilayer
a. Nonpolar tails are directed toward center = hydrophobic
b. Polar heads are directed toward surfaces = hydrophilic
2. Cholesterol = important in determining fluid nature of membrane
C. Membrane proteins
1. Structural classification
a. Integral/intrinsic proteins = extend from one surface to other
b. Peripheral/extrinsic proteins = attached to one surface (inner/outer)
2. Functional classification
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Membrane Proteins
Description
Used for cell recognition by immune system, recognition of oocyte by
sperm cell, Intercellular communication
Marker molecules
Attachment sites
Proteins attach cells to other cells
Integral proteins form tiny channel thru which ions/small molecules pass
Nongated ion channel = always open so ions pass freely
Ligand-gated ion channel = ligand opens gate to allow ions to pass
Voltage-gated ion channel = change in charge opens gate
Form receptor site on outer cell surface
Ligand-gated ion channel, Channel opens or closes
Changes permeability of cell to some substances
2 messenger system: 1st messenger is hormone or neurotransmitter and
2nd messenger released from membrane protein complex
Catalyze chemical reactions in plasma membrane
Move ions/molecules across membrane by changing shape
Channel proteins
Receptor molecules
Receptors linked to
channel proteins
Receptors linked to a
mediator
Enzymes
Carrier proteins
III.
Movement through the Plasma Membrane
A. Movement directly across membrane
1. Diffusion
a. Passive transport
b. Substances move from ↑ concentration to ↓ concentration
c. Substances move down the concentration gradient
d. Rate influenced by four factors
Factor
Size concentration gradient
Temperature
Size of diffusing molecules
Viscosity of solvent
Influence on rate
Larger concentration difference = faster diffusion
Higher temperature = faster diffusion
Smaller particle size = faster diffusion
Less viscous solution = faster diffusion (faster in water than oil)
e. Example: oxygen moves from lungs into blood
2. Osmosis
a. Passive transport
b. Water moves from ↑ to ↓ concentration across selectively
permeable membrane
c. Water moves down the concentration gradient
d. Osmotic pressure (P) = P to prevent water movement by osmosis
Classification
Isosmotic
Hyperosmotic
Hyposmotic
Description
Two solutions with same osmotic pressure
One solution has greater solute concentration/osmotic pressure than another
One solution has less solute concentration/osmotic pressure than another
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e. Cell response
Classification
Isotonic
Hypertonic
Hypotonic
Description
Solution in which cell neither shrinks nor swells
Solution in which cell shrinks or crenates
Solution in which cell swells and can burst (lysis)
f. Example: water moves into/out of kidney tubules
3. Filtration
a. Passive transport
b. Pressure difference moves particles from ↑ pressure to ↓ pressure
across a partition with holes (sieve)
c. Example: wastes move from blood into kidney glomerulus
B. Mediated transport
1. Characteristics of mediated transport
a. Uses carrier protein that has specificity for specific type molecule
b. Competition for carrier protein by similar molecules can interfere
c. Saturation = carrier proteins transport molecules at maximum rate
2. Facilitated diffusion
a. Passive transport
b. Substances move from ↑ to ↓ concentration thru a carrier protein
c. Substances move down the concentration gradient
d. Example: glucose moves into/out of cells
3. Active transport
a. Active transport = requires energy in form of ATP
b. Substances are pumped (carrier protein) from ↓ to ↑ concentration
c. Substances move down concentration gradient
d. Example: Na+-K+ pump moves 3 Na+ into and 2 K+ out of cells
4. Secondary active transport
a. Active transport = requires energy in form of ATP
b. Ion pumped up the concentration gradient across cell membrane so
when it moves back down the concentration gradient thru a carrier
protein it brings with it (cotransport or symport) another substance
c. Countertransport/antiport = molecules move in opposite directions
d. Example: Na+-K+ exchange pump linked to glucose pump
C. Mass transport
1. Endocytosis
a. Active transport = requires energy in form of ATP
b. Cell internalizes substances by enclosing them in a vesicle
c. Phagocytosis = solid particles are ingested by cell
d. Pinocytosis = molecules dissolved in liquid ingested by cell
2. Exocytosis
a. Active transport = requires energy in form of ATP
b. Cell expels substances by enclosing them in a vesicle that merges
with plasma membrane
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Part B: Cytoplasm and Cell Organelles
I.
Cytoplasm = between nuclear membrane and plasma membrane
A. Cytosol = fluid solution containing dissolved ions
B. Cytoskeleton
Subdivision
Structure
Function
Ambeoid movement (change shape)
Globular proteins = twisted,
Microfilaments
double chain of actin
Cell contraction
molecules
Mechanical support for microvilli
Reinforcing rods = bearing tension
Intermediate
Fibrous proteins
filaments
Anchors organelles
Provides rigidity and shape for cell
Straight, hollow tubes
Microtubules
composed of globular proteins Anchors organelles
called tubulin
Forms centrioles, cilia, and flagella
II.
Organelles = small structures within cell specialized for specific function
A. Nuclear organelles
Organelle
Nucleus
Structure
Function
Large membrane-bound structure near
Contains genetic information of cell
center of cell
Separates nucleus from cytoplasm and
Nuclear
Double membrane with pores that
allows passage of small molecules thru
envelope
surrounds nucleus
pores
Nucleoplasm Fluid-filled region within nucleus
Solvent for chemical reactions/transport
Chromatin
Dispersed DNA + proteins
Ultimately determines protein structure
Chromosomes
Allows separation of genetic
Condensed DNA
information during cell division
Nucleolus
Rounded dense region in nucleus
Produces ribosomal RNA (rRNA)
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B. Cytoplasmic organelles
Organelle
Structure
Function
Divide and produce
spindle fiber during cell
division
Move material across
surface of cell via power
and recovery strokes
Movement of the cell in
wavelike fashion
Centrioles
Cilia
Flagella
Microvilli
Ribosomes
Rough
endoplasmic
reticulum
Extensions of the plasma
membrane
Large and small subunits
composed of rRNA
Interconnected canals
studded with ribosomes;
Increase cell surface area
Synthesis of proteins for
use within cell
Produce secretory proteins
Produce membranes
Produce lipids
Smooth
endoplasmic
reticulum
Interconnected canals
studded with enzymes
Golgi
apparatus
Flattened membranous
sacs containing cisternae
Membrane-bound sacs
produced by Golgi
apparatus
Membrane-bound sacs
containing hydrolytic
Lysosome
enzymes that pinch off
from Golgi apparatus
Inner (cristae) and outer
Mitochondria membrane separated by
intermembranous space;
matrix within cristae
Secretory
vesicles
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Detoxification
Store calcium
Packages proteins and
lipids made by RER/SER
into secretory vesicles
Stores protein and lipids
products produced by cell
Digest foreign particles
taken into cell or structures
of the cell no longer
functional (autophagia)
Provide energy for the cell
Part C: Cell Life Cycle and Mitosis
I.
Cell Life Cycle
A. Changes in cell from formation until it divides to produce two cells
1. All cells in body except sex cells formed by mitosis
2. Contain 46 chromosomes (23 pair) in humans = diploid number (2n)
a. 22 pair of autosomes
1) Each pair look alike structurally = homologous
2) In each pair one is from mother and one is from father
b. 1 pair of sex chromosomes
1) XX = female and XY = male
2) Mother contributes X and father contributes either X or Y
II.
Interphase
A. Phase between cell division (90%)
B. Three subphases
1. G1 phase = first gap phase
a. Cell performs metabolism/normal function
b. Organelles duplicated
c. Increase in cell size
2. S phase = synthesis phase = DNA replication
a. Cell begins with diploid (2n) number of chromosomes (46)
b. DNA double strands separate
c. DNA polymerase adds complementary nucleotides to each strand
d. End Product
1) Two new DNA molecules each with one old strand and one
new strand = semi-conservative replication
2) Chromosome number = 2n duplicated
3. G2 phase = second gap phase = cell performs metabolism/normal
function
a. Cell performs metabolism/normal function
b. Centrioles duplicate
c. Increase in cell size
4. G0 = cells do not undergo division
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III.
Cell Division
A. Division of nucleus = mitosis
1. Prophase
a. Chromatin strands condense to form chromosomes
1) Chromosome = two identical strands called chromatids
2) Chromatids connected at centromere
3) Chromosome number = 2n duplicated
b. One pair of centrioles move to opposite pole
1) Spindle fibers = project from centriole to centromere
c. Nuclear envelope disappears
d. Nucleolus disappears
2. Metaphase = chromosomes line up along equator
3. Anaphase
a. Chromatids separate and now each one called chromosome
b. Chromosomes move toward opposite poles
4. Telophase
a. Chromosomes finish migration
b. Spindle fiber disappears
c. Nuclear envelope reappears
d. Nucelolus reappears
B. Division of cytoplasm = cytokinesis
1. Begins in anaphase = cleavage furrow or puckering of plasma membrane
2. Actin filaments pull plasma membrane inward dividing cell in half
3. End result = two cells with diploid (2n) number of chromosomes
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Stage of
Cell Cycle
Chromosome
Description
#
46 (diploid) Increase in cell size
Duplication of organelles
G1 Phase
DNA replication (each chromosome now
46 doubled
Interphase
S Phase
composed of two chromatids)
46 doubled
Increase in cell size
Duplication of organelles
G2 Phase
Chromatin condenses to form chromosomes
Nuclear membrane disappears
Nucleolus disappears
Centrioles
Spindle fibers – centriole to
46 doubled
Prophase
duplicate and kinetochore of centromere
produce
Astral fibers – centriole to
mitotic
cytoplasm
apparatus
Chromosomal centromeres align along equatorial
Mitosis1
46 doubled
Metaphase
plane
Centromere divides forming two chromatids
46 doubled
Anaphase
Chromatids move toward opposite poles
Chromosomes reach poles of cell; elongation of
chromosomes into chromatin
46
Nuclear membrane reappears
Telophase
(two nuclei)
Nucleolus reappears
Mitotic apparatus disassembles
Begin in anaphase with cleavage furrow =
constriction of cell membrane
46
2
Cytokinesis
Actin filaments form contractile ring that pulls
(two cells)
plasma membrane inward, which completely
separates cells at end of telophase
1
Division of nucleus into two nuclei
2
Division of cytoplasm into two cells
Subdivision
Part D: Protein Synthesis
I.
II.
DNA
A. DNA controls production of proteins by sequence of nucleotides
B. Nucleotides are read as triplets = sequence of three
C. Gene = all triplets needed to code for a protein
Transcription
A. Formation of pre-mRNA
1. Double-stranded DNA separates
2. RNA polymerase adds complementary nucleotides
3. Codon = sequence of three nucleotides in mRNA
1. 64 possible codons code for 20 amino acids
2. Start codon = AUG = methionine
3. Stop codon = UAA or UGA or UAG = no amino acid
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III.
IV.
B. mRNA moves through nuclear pores to cytoplasm
Translation
A. tRNA
1. Composed of RNA
2. Has anticodon (three nitrogenous bases) on one end
3. Has amino acid on opposite end
B. Formation of proprotein or proenzyme
1. P site on ribosome binds to start codon of mRNA
2. Anticodon attached to tRNA connects to codon of mRNA in P site
3. Anticodon attached to tRNA connects to codon of mRNA in A site
4. Ribosomal enzyme catalyzes peptide bond between adjacent amino
acids
5. Amino acid released from tRNA in P site
6. tRNA in P site is released
7. tRNA in A site jumps to P site
8. Steps 2-7 repeat until stop codon is reached
9. Ribosome releases protein and mRNA at stop codon
C. Polyribosomes
1. Each mRNA has multiple ribosomes that attach for translation
2. Each mRNA can therefore result in many proteins/enzymes
Regulation of Protein Synthesis
A. Proteins in nucleus associated with DNA control transcription
Some hormones can affect rate of transcription such as thyroid hormone
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