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Chapter 3
Anatomy of Cells
Functional Anatomy of Cells
• The typical cell (Figure 3-4)
– Also called composite cell
– Varies in size; all are microscopic (Table 3-1)
– Varies in structure and function (Table 3-2)
Functional Anatomy of Cells
• 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 (Table 3-2)
– Nucleus
• large membranous structure near the center of the cell
Cell Membranes
• Each cell contains a variety of membranes:
– Plasma membrane (Figure 3-5)
– Membranous organelles
• sacs and canals made of the same material as the plasma
membrane
Cell Membranes
• 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
Cell Membranes
• Primary structure of a cell membrane is a double
layer of phospholipid molecules (Figure 2-25)
– Heads are hydrophilic (water-loving)
– Tails are hydrophobic (water-fearing)
– Molecules arrange themselves in bilayers in water
– 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
Cell Membranes
• Membrane proteins (Table 3-3)
– 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
Cell Membranes
• Structure
– Sheet (bilayer) of
phospholipids stabilized by
cholesterol
• Function
– Maintain wholeness
(integrity) of a cell or
membranous organelle
Cell Membranes
• Structure
– Membrane proteins that act
as channels or carriers of
molecules
• Function
– Controlled transport of
water-soluble molecules
from one compartment to
another
Cell Membranes
• Structure
– Receptor molecules that
trigger metabolic changes in
membrane
• Function
– Sensitivity to hormones and
other regulatory chemicals;
involved in signal
transduction
Cell Membranes
• Structure
– Enzyme molecules that
catalyze specific chemical
reactions
• Function
– Regulation of metabolic
reactions
Cell Membranes
• Structure
– Enzyme molecules that
catalyze specific chemical
reactions
• Function
– Regulation of metabolic
reactions
Cell Membranes
• Structure
– Membrane proteins that
bind to molecules outside
the cell
• Function
– Form connections between
cells and other structures
such as tissue fibers or
other cells
Cell Membranes
• Structure
– Membrane proteins that
bind to support filaments
within the cytoplasm
• Function
– Support and maintain the
shape of a cell or
membranous organelle; cell
movement
Cell Membranes
• Structure
– Glycoproteins or proteins in
the membrane that act as
markers
• Function
– Recognition of cells or
organelles
Cytoplasm and Organelles
• Cytoplasm
– gel-like internal substance of cells
– includes many organelles suspended in watery
intracellular fluid called cytosol
Cytoplasm and Organelles
• Two major groups of organelles (Table 3-2)
– Membranous organelles
• specialized sacs or canals made of cell membranes
– Nonmembranous organelles
• made of microscopic filaments or other nonmembranous
materials
Cytoplasm and Organelles
• Endoplasmic reticulum
– 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
Cytoplasm and Organelles
• Two types of endoplasmic reticulum:
– 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
Cytoplasm and Organelles
• 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
• Removes and stores Ca++ from cell’s interior.
Cytoplasm and Organelles
• Ribosomes (Figure 3-4)
– 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, each composed of rRNA
• large subunit
• small subunit
– 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
Cytoplasm and Organelles
• 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
Cytoplasm and Organelles
• Lysosomes
– 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
• plasma membrane proteins
• particles that have become trapped in the cell
Cytoplasm and Organelles
• Peroxisomes
– Small membranous sacs containing enzymes that
detoxify harmful substances that enter the cells
– Often seen in kidney and liver cells
Cytoplasm and Organelles
• Mitochondria (Figure 3-9)
– 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
Cytoskeleton
• The cell’s internal supporting framework
–
–
–
–
made up of rigid, rodlike pieces
provide support
allow movement
Include mechanisms that can move the cell or its
parts (Figure 3-10)
Cytoskeleton
• 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.)
– Smallest cell fibers are microfilaments
(Figure 3-11)
• “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
Cytoskeleton
• Cell fibers (cont.)
– Intermediate filaments
• twisted protein strands slightly thicker than microfilaments
• form much of the supporting framework in many types of
cells
– Microtubules
• tiny, hollow tubes that are the thickest of the cell fibers
• made of protein subunits arranged in a spiral fashion
• function is to move things around in the cell
Cytoskeleton
• Centrosome
– 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
Cytoskeleton
• Cell extensions
– Cytoskeleton forms projections that extend the plasma
membrane outward to form tiny, fingerlike processes
– Three types (Figure 3-12):
• Microvilli
– found in epithelial cells that line the intestines and other areas where
absorption is important
– 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
Nucleus
• Definition—spherical body in center of cell;
enclosed by an envelope with many pores
Nucleus
• Structure
– Nuclear envelope surrounding nucleoplasm
– Nuclear pores
– DNA
•
•
•
•
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
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 3-14)
Cell Connections
• Desmosome
– 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
Cell Connections
• Gap junctions
– Membrane channels of adjacent plasma membranes
adhere to each other
• Form gaps or “tunnels” that join the cytoplasm of two cells
• Fuse two plasma membranes into a single structure
Cell Connections
• Tight junctions
– 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