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Chapter 3
Anatomy of Cells
Introduction
• In the 1830s, two German scientists, Matthais Schleiden and Theodore
Schwann, advanced one of the most important and unifying concepts in
biology- the cell theory
• It states simply that the cell is the fundamental organizational unit of life
• They were the first to suggest that all living things are composed of cells
Functional Anatomy of Cells
• The typical cell (Figure 3-1)
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Also called composite cell
Varies in size; all are microscopic
Varies in structure and function
Functional Anatomy of Cells
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Cell structures
 Plasma membrane—separates the cell from its surrounding environment
 Cytoplasm—thick gel-like substance inside of the cell composed of
numerous organelles suspended in watery cytosol; each type of organelle
is suited to perform particular functions (Figure 3-2)
 Nucleus—large membranous structure near the center of the cell
Cell Membranes
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Each cell contains a variety of membranes:
 Plasma membrane (Figure 3-3), functions:
• Selective barrier, maintains cells integrity
• Serves as markers that self-identify each individual cell
• Molecule receptor site for certain hormones and other molecules
• Transport mechinism
 Membranous organelles—sacs and canals made of the same material as
the plasma membrane
• Fluid mosaic model—theory explaining how cell membranes are constructed
 Molecules of the cell membrane are arranged in a sheet
 The mosaic of molecules is fluid; that is, the molecules are able to float
around slowly
 This model illustrates that the molecules of the cell membrane form a
continuous sheet
• Primary structure of a cell membrane is a double layer of phospholipid
molecules
 Heads are hydrophilic (water-loving)
 Tails are hydrophobic (water-fearing)
 Molecules arrange themselves in bilayers in water
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Cholesterol molecules are scattered among the phospholipids to allow the
membrane to function properly at body temperature
Most of the bilayer is hydrophobic; therefore water or water-soluble
molecules do not pass through easily
Chemical attractions are the forces that hold membranes together
 Groupings of membrane molecules form rafts, each of which float as a unit
in the membrane (Figure 3-4)
 Rafts help organize the various components of a membrane
 Rafts may pinch inward, bringing material into the cell or organelle
• Membrane proteins (Table 3-4)
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A cell controls what moves through the membrane by means of membrane
proteins embedded in the phospholipid bilayer
Some membrane proteins have carbohydrates attached to them, forming
glycoproteins that act as identification markers
Some membrane proteins are receptors that react to specific chemicals,
sometimes permitting a process called signal transduction
Cytoplasm and Organelles
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Cytoplasm—gel-like internal substance of cells that includes many organelles
suspended in watery intracellular fluid called cytosol
Two major groups of organelles (Table 3-3):
 Membranous organelles are specialized sacs or canals made of cell
membranes
 Nonmembranous organelles are made of microscopic filaments or other
nonmembranous materials
Endoplasmic reticulum (Figure 3-5)
 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
 Proteins move through the canals
 Two types of endoplasmic reticulum:
• Rough endoplasmic reticulum
 Ribosomes dot the outer surface of the membranous walls giving it
the “rough” appearance
 Ribosomes synthesize proteins, which move toward the Golgi
apparatus and then eventually leave the cell
 Function in protein synthesis and intracellular transportation
Cytoplasm and Organelles
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Two types of endoplasmic reticulum (cont.)
• 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
++ (calcium ions) from cell’s interior.
 Removes and stores Ca
• Ribosomes (Figure 3-6)
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Many are attached to the rough endoplasmic reticulum and many lie free,
scattered through the cytoplasm
Each ribosome is a nonmembranous structure made of two pieces, a large
subunit and a small subunit; each subunit is composed of rRNA (ribosomal
RNA)
Their function is protein synthesis
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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Golgi apparatus
 Membranous organelle consisting of cisternae stacked on one another and
located near the nucleus (Figure 3-7)
 Processes protein molecules from the endoplasmic reticulum (Figure 3-8)
 Processed proteins leave the final cisterna in a vesicle; contents may then
be secreted to outside the cell
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Lysosomes (Figure 3-9)
 Made of microscopic membranous sacs that have “pinched off” from Golgi
apparatus
 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
• Proteasomes (Figure 3-10)
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Hollow, protein cylinders found throughout the cytoplasm
Break down abnormal/misfolded proteins and normal proteins no longer
needed by the cell
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
Cytoplasm and Organelles
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Peroxisomes
 Small membranous sacs containing enzymes that detoxify harmful
substances that enter the cells
 Often seen in kidney and liver cells
• Mitochondria (Figure 3-11)
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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
The “power plants” of cells; mitochondrial enzymes catalyze series of
oxidation reactions that provide about 95% of cell’s energy supply
Each mitochondrion has a DNA molecule, allowing it to produce its own
enzymes and replicate copies of itself
Nucleus
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Definition—spherical body in center of cell; enclosed by an envelope with
many pores
One of the largest cell structures
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
(Figure 3-12)
 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
Cytoskeleton
• The cell’s internal supporting framework made up of rigid, rod-like pieces that
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provide support and allow movement and mechanisms that can move the cell
or its parts (Figure 3-13)
Cell fibers
 Intricately arranged fibers of varying lengths that form a three-dimensional,
irregularly shaped lattice
 Fibers appear to support the endoplasmic reticulum, mitochondria, and
“free” ribosomes
Cytoskeleton
• Cell fibers (cont.)
Microfilaments (Figure 3-14)
• Smallest cell fibers
• Often serve as “Cellular muscles”
• Made of thin, twisted strands of protein molecules that lie parallel to the
long axis of the cell
• Microfilaments can slide past each other, causing shortening of the cell;
occurs most in muscle cells
 Intermediate filaments
• Slightly thicker than microfilaments
• Form much of the supporting framework in many types of cells; such as
the outer layer of skin
• Microtubules
 Thickest of cell fibers
 Made up of protein subunits arranged in a spiral fashion
 Called “engines” because they move things around in a cell or even the
entire cell
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• Centrosome
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An area of the cytoplasm near the nucleus that coordinates the building and
breaking of microtubules in the cell
Nonmembranous structure also called the microtubule-organizing center
(MTOC)
Plays an important role during cell division
The general location of the centrosome is identified by the centrioles
Cell extensions
 Cytoskeleton forms projections that extend the plasma membrane outward
to form tiny, fingerlike processes
 There are three types of these processes; each has specific functions
(Figure 3-15):
• 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
Cell Connections
• Cells are held together by fibrous nets that surround groups of cells (e.g.,
muscle cells), or cells have direct connections to each other
• There are three types of direct cell connections (Figure 3-16)
• Desmosome
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Fibers on the outer surface of each desmosome interlock with each other;
anchored internally by intermediate filaments of the cytoskeleton
Spot desmosomes, connecting adjacent membranes, are like “spot welds”
at various points
Belt desmosomes encircle the entire cell like a collar
• Gap junctions
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Membrane channels of adjacent plasma membranes adhere to each other;
have two effects:
Form gaps or “tunnels” that join the cytoplasm of two cells
Fuse two plasma membranes into a single structure
• Tight junctions
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Occur in cells that are joined by “collars” of tightly fused material
Molecules cannot permeate the cracks of tight junctions
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