Chapter 3
I. Cells
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A. Functional Anatomy of Cells
• 1. Cell structures

a. Plasma membrane—separates the cell from its
surrounding environment

b. Cytoplasm—thick gel-like substance inside of
the cell composed of numerous organelles
suspended in watery cytosol;

c. Nucleus—large membranous structure near the
center of the cell
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B. Cell Membranes
• 1. Fluid mosaic model—theory explaining
how cell membranes are constructed
• 2. Chemical attractions are the forces that
hold membranes together

a. Groupings of membrane molecules form rafts,
each of which float as a unit in the membrane

b. Rafts may pinch inward, bringing material into
the cell or organelle
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• 3. Primary structure of a cell membrane is a double layer
of phospholipid molecules

a. Heads are hydrophilic (water-loving)

b. Tails are hydrophobic (water-fearing)

c. Molecules arrange themselves in bilayers in water

d. Cholesterol molecules are scattered among the
phospholipids to allow the membrane to function
properly at body temperature

e. Most of the bilayer is hydrophobic
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II. Cytoplasm and Organelles
• A. Cytoplasm—gel-like internal substance of
cells that includes many organelles
suspended in watery intracellular fluid called
cytosol
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• B. Two major groups of organelles:

1. Membranous organelles

2. Nonmembranous organelles
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C. Organelles
• 1. literally means “little organ”
• 2. 100’s to 1000’s in each cell
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3. Endoplasmic reticulum

a. Made of canals with membranous walls and flat,
curving sacs arranged in parallel rows throughout
the cytoplasm; extend from the plasma membrane
to the nucleus

b. Proteins move through the canals
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
c. Two types of endoplasmic reticulum:
• 1. Rough endoplasmic reticulum

Ribosomes dot the outer surface of the
membranous walls

Ribosomes synthesize proteins, which
move toward the Golgi apparatus and then
eventually leave the cell

Function in protein synthesis and
intracellular transportation
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• 2. Smooth endoplasmic reticulum

No ribosomes border membranous wall

Functions are less well established and
probably more varied than for rough
endoplasmic reticulum

Synthesizes certain lipids and
carbohydrates and creates membranes for
use throughout cell

Removes and stores Ca++ from cell’s interior.
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4. Ribosomes

a. Many are attached to the rough endoplasmic
reticulum and many lie free, scattered through the
cytoplasm

b. Each ribosome is a nonmembranous structure
made of two pieces, a large subunit and a small
subunit; each subunit is composed of rRNA

c. Ribosomes in the endoplasmic reticulum make
proteins for “export” or to be embedded in the
plasma membrane; free ribosomes make proteins
for the cell’s domestic use
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5. Golgi apparatus

a. Membranous organelle consisting of cisternae
stacked on one another and located near the
nucleus

b. Processes protein molecules from the
endoplasmic reticulum

c. Processed proteins leave the final cisterna in a
vesicle; contents may then be secreted to outside
the cell
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6. Lysosomes

a. Made of microscopic membranous sacs that
have “pinched off” from Golgi apparatus

b. The cell’s own digestive system; enzymes in
lysosomes digest the protein structures of
defective cell parts, including plasma membrane
proteins, and particles that have become trapped
in the cell
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7. Proteasomes

a. Hollow, protein cylinders found throughout the
cytoplasm

b. Break down abnormal/misfolded proteins and
normal proteins no longer needed by the cell

c. Break down protein molecules one at a time by
tagging each one with a chain of ubiquitin
molecules and unfolding it as it enters the
proteasome, then breaking apart peptide bonds
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8. Peroxisomes

a. Small membranous sacs containing enzymes
that detoxify harmful substances that enter the
cells

b. Often seen in kidney and liver cells
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9. Mitochondria

a. Made up of microscopic sacs; wall composed of
inner and outer membranes separated by fluid;
thousands of particles make up enzyme molecules
attached to both membranes

b. The “power plants” of cells; mitochondrial
enzymes catalyze series of oxidation reactions
that provide about 95% of cell’s energy supply

c. Each mitochondrion has a DNA molecule,
allowing it to produce its own enzymes and
replicate copies of itself
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10. Nucleus
• a. Definition—spherical body in center of cell;
enclosed by an envelope with many pores
• b. Structure- Consists of nuclear envelope
(composed of two membranes each with
essentially the same molecular structure as
plasma membrane) surrounding
nucleoplasm; nuclear envelope has holes
called nuclear pores
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
c. Contains DNA (heredity molecules), which
appear as the following:
• Chromatin threads or granules in nondividing
cells
• Chromosomes in early stages of cell division
• Functions of nucleus are functions of DNA
molecules; DNA determines both structure and
function of cells and heredity
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11. Cytoskeleton
• a. The cell’s internal supporting framework
made up of rigid, rodlike pieces that provide
support and allow movement and
mechanisms that can move the cell or its
parts
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• b. Centrosome

An area of the cytoplasm near the nucleus that
coordinates the building and breaking of
microtubules in the cell

Plays an important role during cell division

The general location of the centrosome is
identified by the centrioles
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• c. Cell extensions

Cytoskeleton forms projections that extend the
plasma membrane outward to form tiny, fingerlike
processes

Microvilli—found in epithelial cells that line the
intestines and other areas where absorption is
important; they help to increase the surface area
manyfold

Cilia and flagella—cell processes that have
cylinders made of microtubules at their core; cilia
are shorter and more numerous than flagella;
flagella are found only on human sperm cells
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III. Cell Connections
• A. Gap junctions—membrane channels of
adjacent plasma membranes adhere to each
other; have two effects:

1. Form gaps or “tunnels” that join the
cytoplasm of two cells

2. Fuse two plasma membranes into a single
structure
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• B. Tight junctions

1. Occur in cells that are joined by “collars” of
tightly fused material

2. Molecules cannot permeate the cracks of tight
junctions

3. Occur in the lining of the intestines and other
parts of the body, where it is important to control
what gets through a sheet of cells
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