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Chapter 7
Cell Structure: A Tour of the Cell
Cell:
A basic unit of living matter separated from
its environment by a plasma membrane.
The smallest structural unit of life.
Cell Theory: Developed in late 1800s.
1. All living organisms are made up of one or
more cells.
2. The smallest living organisms are single
cells, and cells are the functional units of
multicellular organisms.
3. All cells arise from preexisting cells.
Microscope Features
Magnification:
 Increase in apparent size of an object.
 Ratio of image size to specimen size.
Resolving power: Measures clarity of image.
 Ability to see fine detail.
 Ability to distinguish two objects as separate.
 Minimum distance between 2 points at which
they can be distinguished as separate and
distinct.
Microscopes
Light Microscopes: Earliest microscopes
used.
Lenses pass visible light through a specimen.
 Magnification:
Highest possible from 1000 X to
2000 X.
 Resolving
mm).
power: Up to 0.2 mm (1 mm = 1/1000
Types of Microscope
Electron Microscopes: Developed in 1950s.
Electron beam passes through specimen.
 Magnification:
Up to 200,000 X.
 Resolving power: Up to 0.2 nm (1nm =
1/1’000,000 mm).
Two types of electron microscopes:
1. Scanning Electron Microscope: Used to study
cell or virus surfaces.
2. Transmission Electron Microscope: Used to
study internal cell structures.
Components of All Cells:
1. Plasma membrane: Separates cell contents
from outside environment. Made up of
phospholipid bilayers and proteins.
2. Cytoplasm: Liquid, jelly-like material inside
cell.
3. Ribosomes: Necessary for protein synthesis.
Procaryotic versus Eucaryotic Cells
Feature
Procaryotic
Eucaryotic
Organisms
Bacteria
All others (animals, plants,
fungi, and protozoa)
Nucleus
Absent
Present
DNA
One chromosome
Multiple chromosomes
Size
Small (1-10 um)
Large (10 or more um)
Membrane
Bound
Organelles
Absent
Present (mitochondria,
golgi, chloroplasts, etc.)
Division
Rapid process
(Binary fission)
Complex process
(Mitosis)
Relative Sizes of Structures
1 nanometer (10-9 m)
water molecule
10 nanomters (10-8 m)
small protein
100 nanometers (10-7 m)
HIV virus
1 micron (10-6 m)
cell vacuole
10 microns (10-5 m)
bacterium
100 microns (10-4 m)
large plant cell
1 millimeter (10-3 m)
single cell embryo
Relative Sizes of Procaryotic and
Eucaryotic Cells and Viruses
Relative Sizes of Cells and Other Objects
Prokaryotic Cells
 Bacteria
 Small
and blue-green algae.
size: Range from 1- 10 micrometers in length.
About one tenth of eukaryotic cell.
 No
nucleus: DNA in cytoplasm or nucleoid region.
 Ribosomes
 Cell
are used to make proteins
wall: Hard shell around membrane
 Other
structures that may be present:
• Capsule: Protective, outer sticky layer. May be used for
attachment or to evade immune system.
• Pili: Hair-like projections (attachment)
• Flagellum: Longer whip-like projection (movement)
Procaryotic Cells: Lack a Nucleus and
other Membrane Bound Organelles
Eucaryotic Cells
 Include
protist, fungi, plant, and animal cells.
 Nucleus:
Protects and houses DNA
 Membrane-bound
Organelles: Internal
structures with specific functions.
 Separate
and store compounds
 Store energy
 Work surfaces
 Maintain concentration gradients
Membrane-Bound Organelles of Eucaryotic
Cells
 Nucleus
 Rough
Endoplasmic Reticulum (RER)
 Smooth
 Golgi
Endoplasmic Reticulum (SER)
Apparatus
 Lysosomes
 Vacuoles
 Chloroplasts
 Mitochondria
Eucaryotic Cells: Typical Animal Cell
Eucaryotic Cells: Typical Plant Cell
Nucleus
Structure
 Double
nuclear membrane (envelope)
 Large
nuclear pores
 DNA (genetic material) is combined with histones
and exists in two forms:
• Chromatin (Loose, threadlike DNA, most of cell life)
• Chromosomes (Tightly packaged DNA. Found only
during cell division)
 Nucleolus:
Dense region where ribosomes are made
Functions
 House
and protect cell’s genetic information (DNA)
 Ribosome synthesis
Structure of Cell Nucleus
Endoplasmic Reticulum (ER)
 “Network
within the cell”
 Extensive maze of membranes that branches
throughout cytoplasm.
 ER is continuous with plasma membrane and
outer nucleus membrane.
 Two types of ER:
 Rough Endoplasmic Reticulum (RER)
 Smooth Endoplasmic Reticulum (SER)
Rough Endoplasmic Reticulum (RER)
 Flat,
interconnected, rough membrane sacs
 “Rough”: Outer walls are covered with
ribosomes.
Protein making “machines”.
May exist free in cytoplasm or attached to ER.
 Ribosomes:
 RER
Functions:
 Synthesis
of cell and organelle membranes.
 Synthesis and modification of proteins.
 Packaging, and transport of proteins that are
secreted from the cell.
• Example: Antibodies
Rough Endoplasmic Reticulum (RER)
Smooth Endoplasmic Reticulum (SER)
 Network
of interconnected tubular smooth
membranes.
 “Smooth”:
 SER
No ribosomes
Functions:
 Synthesis
of phospholipids, fatty acids, and
steroids (sex hormones).
 Breakdown of toxic compounds (drugs, alcohol,
amphetamines, sedatives, antibiotics, etc.).
 Helps develop tolerance to drugs and alcohol.
 Regulates levels of sugar released from liver into
the blood
 Calcium storage for cell and muscle contraction.
Smooth Endoplasmic Reticulum (SER)
Golgi Apparatus
 Stacks
of flattened membrane sacs that may be
distended in certain regions. Sacs are not
interconnected.
 First described in 1898 by Camillo Golgi (Italy).
 Works closely with the ER to secrete proteins.
 Golgi
Functions:
 Receiving
side receives proteins in transport vesicles
from ER.
 Modifies proteins into final shape, sorts, and labels
proteins for proper transport.
 Shipping side packages and sends proteins to cell
membrane for export or to other parts of the cell.
 Packages digestive enzymes in lysosomes.
The Golgi Apparatus: Receiving,
Processing, and Shipping of Proteins
Lysosomes
 Small
vesicles released from Golgi containing at
least 40 different digestive enzymes, which can
break down carbohydrates, proteins, lipids, and
nucleic acids.
 Optimal pH for enzymes is about 5
 Found mainly in animal cells.
 Lysosome Functions:
 Molecular
garbage dump and recycler of
macromolecules (e.g.: proteins).
 Destruction of foreign material, bacteria, viruses,
and old or damaged cell components.
 Digestion of food particles taken in by cell.
 After cell dies, lysosomal membrane breaks down,
causing rapid self-destruction.
Lysosomes: Intracellular Digestion
Lysosomes, Aging, and Disease
 As
we get older, our lysosomes become leaky,
releasing enzymes which cause tissue damage and
inflammation.

Example: Cartilage damage in arthritis.
 Steroids
or cortisone-like anti-inflammatory agents
stabilize lysosomal membranes, but have other
undesirable effects (affect immune function).
 Diseases
from “mutant” lysosome enzymes are
usually fatal:
 Pompe’s
disease: Defective glycogen breakdown in liver.
 Tay-Sachs disease: Defective lipid breakdown in brain.
Common genetic disorder among Jewish people.
Vacuoles
 Membrane
bound sac.
 Different sizes, shapes, and functions:
 Central
vacuole: In plant cells. Store starch, water,
pigments, poisons, and wastes. May occupy up to
90% of cell volume.
 Contractile
vacuole: Regulate water balance, by
removing excess water from cell. Found in many
aquatic protists.
 Food or Digestion Vacuole: Engulf nutrients in
many protozoa (protists). Fuse with lysosomes to
digest food particles.
Central Vacuole in a Plant Cell
Interactions Between Membrane
Bound Organelles of Eucaryotic Cells
Chloroplasts
 Site
of photosynthesis in plants and algae.
CO2 + H2O + Sun Light -----> Sugar + O2
 Number
may range from 1 to over 100 per
cell.
 Disc shaped structure with three different
membrane systems:
1. Outer membrane: Covers chloroplast surface.
2. Inner membrane: Contains enzymes needed to
make glucose during photosynthesis. Encloses
stroma (liquid) and thylakoid membranes.
3. Thylakoid membranes: Contain chlorophyll,
green pigment that traps solar energy. Organized
in stacks called grana.
Chloroplasts Trap Solar Energy and
Convert it to Chemical Energy
Chloroplasts
Contain their own DNA, ribosomes, and
make some proteins.
 Can divide to form daughter chloroplasts.
 Type of plastid: Organelle that produces and
stores food in plant and algae cells.
Other plastids include:

 Leukoplasts:
Store starch.
 Chromoplasts: Store other pigments that give
plants and flowers color.
Mitochondria (Sing. Mitochondrion)

Site of cellular respiration:
Food (sugar) + O2 -----> CO2 + H2O + ATP
Change chemical energy of molecules into the
useable energy of the ATP molecule.
 Oval or sausage shaped.
 Contain their own DNA, ribosomes, and
make some proteins.
 Can divide to form daughter mitochondria.
 Structure:


Inner and outer membranes.

Intermembrane space

Cristae (inner membrane extensions)

Matrix (inner liquid)
Mitochondria Harvest Chemical Energy
From Food
Origin of Eucaryotic Cells
 Endosymbiont
Theory: Belief that
chloroplasts and mitochondria were at one
point independent cells that entered and
remained inside a larger cell.
 Both
organelles contain their own DNA
 Have their own ribosomes and make their own
proteins.
 Replicate independently from cell, by binary
fission.

Symbiotic relationship
 Larger

cell obtains energy or nutrients
Smaller cell is protected by larger cell.
The Cytoskeleton
Complex network of thread-like and tubelike structures.
Functions: Movement, structure, and structural
support.
Three Cytoskeleton Components:
1. Microfilaments: Smallest cytoskeleton fibers.
Important for:
 Muscle
contraction: Actin & myosin fibers in
muscle cells
 “Amoeboid
motion” of white blood cells
Components of the Cytoskeleton are
Important for Structure and Movement
Three Cytoskeleton Components:
2. Intermediate filaments: Medium sized fibers
 Anchor
organelles (nucleus) and hold cytoskeleton
in place.
 Abundant
in cells with high mechanical stress.
3. Microtubules: Largest cytoskeleton fibers.
Found in:
 Centrioles:
A pair of structures that help move
chromosomes during cell division (mitosis and
meiosis).
Found in animal cells, but not plant cells.
 Movement of flagella and cilia.
Typical Animal Cell
Cilia and Flagella
Projections used for locomotion or to move
substances along cell surface.
 Enclosed by plasma membrane and contain
cytoplasm.
 Consist of 9 pairs of microtubules surrounding
two single microtubules (9 + 2 arrangement).

Flagella: Large whip-like projections.
Move in wavelike manner, used for locomotion.
 Example:
Sperm cell
Cilia: Short hair-like projections.
 Example:
Human respiratory system uses cilia to
remove harmful objects from bronchial tubes and
trachea.
Structure of Eucaryotic Flagellum
Cell Surfaces
A. Cell wall: Much thicker than cell membrane,
(10 to 100 X thicker).
Provides support and protects cell from lysis.



Plant and algae cell wall: Cellulose
Fungi and bacteria have other polysaccharides.
Not present in animal cells or protozoa.
Plasmodesmata: Channels between adjacent plant
cells form a circulatory and communication system
between cells.

Sharing of nutrients, water, and chemical messages.
Plasmodesmata: Communication
Between Adjacent Plant Cells
Cell Surfaces
B. Extracellular matrix: Sticky layer of glycoproteins
found in animal cells.
Important for attachment, support, protection, and
response to environmental stimuli.
Junctions Between Animal Cells:
 Tight
Junctions: Bind cells tightly, forming a leakproof
sheet. Example: Between epithelial cells in stomach lining.

Anchoring Junctions: Rivet cells together, but still allow
material to pass through spaces between cells.

Communicating Junctions: Similar to plasmodesmata in
plants. Allow water and other small molecules to flow
between neighboring cells.
Different Animal Cell Junctions
Important Differences Between
Plant and Animal Cells
Plant cells
Cell wall
Animal cells
None (Extracellular matrix)
Chloroplasts
No chloroplasts
Large central vacuole
No central vacuole
Flagella rare
Flagella more usual
No Lysosomes
Lysosomes present
No Centrioles
Centrioles present
Differences Between Plant and Animal Cells
Animal Cell
Plant Cell
Typical Plant Cell
Summary of Eucaryotic Organelles
Function: Manufacture
 Nucleus
 Ribosomes
 Rough
ER
 Smooth ER
 Golgi Apparatus
Function: Breakdown
 Lysosomes
 Vacuoles
Summary of Eucaryotic Organelles
Function: Energy Processing
 Chloroplasts
(Plants and algae)
 Mitochondria
Function: Support, Movement, Communication
 Cytoskeleton
(Cilia, flagella, and centrioles)
 Cell walls (Plants, fungi, bacteria, and some
protists)
 Extracellular matrix (Animals)
 Cell junctions
The Cell Membrane and Cell
Transport
Functions of Cell Membranes
1. Separate cell from nonliving environment. Form
most organelles and partition cell into discrete
compartments.
2. Regulate passage of materials in and out of the cell
and organelles. Membrane is selectively permeable.
3. Receive information that permits cell to sense and
respond to environmental changes.
 Hormones
 Growth factors
 Neurotransmitters
4. Communication with other cells and the organism as
a whole. Surface proteins allow cells to recognize
each other, adhere, and exchange materials.
I. Fluid Mosaic Model of the Membrane
1. Phospholipid bilayer: Major component
is a phospholipid bilayer.
 Hydrophobic
 Hydrophilic
tails face inward
heads face water
2. Mosaic of proteins: Proteins “float” in the
phospholipid bilayer.
3. Cholesterol: Maintains proper
membrane fluidity.
The outer and inner membrane surfaces
are different.
Membrane
Phospholipids
Form a
Bilayer
The Membrane is a Fluid Mosaic of
Phospholipids and Proteins
Notice that inner and outer surfaces are different
A. Fluid Quality of Plasma
Membranes
 In
a living cell, membrane has same fluidity as salad
oil.
 Unsaturated hydrocarbon tails INCREASE
membrane fluidity
 Phospholipids and proteins drift laterally.
 Phospholipids move very rapidly
 Proteins drift in membrane more slowly
 Cholesterol: Alters fluidity of the membrane
 Decreases fluidity at warmer temperatures (>
37oC)
 Increases fluidity at lower temperatures (< 37oC)
B. Membranes Contain Two Types of
Proteins
1. Integral membrane proteins:
Inserted into the membrane.
Hydrophobic region is adjacent to hydrocarbon tails.
2. Peripheral membrane proteins:
Attached to either the inner or outer membrane surface.
Functions of Membrane Proteins:
1. Transport of materials across membrane
2. Enzymes
3. Receptors of chemical messengers
4. Identification: Cell-cell recognition
5. Attachment:
 Membrane to cytoskeleton
 Intercellular junctions
Membrane Proteins Have Diverse Functions
C. Membrane Carbohydrates and Cell-Cell
Recognition
 Found
on outside surface of membrane.
 Important
for Cell-cell recognition: Ability of one cell to
“recognize” other cells.
 Allows
immune system to recognize self/non-self
 Include:
• Glycolipids: Lipids with sugars
• Glycoproteins: Proteins with sugars
• Major histocompatibility proteins (MHC or
transplantation antigens).
 Vary
greatly among individuals and species.
 Organ transplants require matching of cell markers
and/or immune suppression.
The cell plasma membrane is Selectively
Permeable
A. Permeability of the Lipid Bilayer
1. Non-polar (Hydrophobic) Molecules
• Dissolve into the membrane and cross with ease
• The smaller the molecule, the easier it can cross
• Examples: O2 , hydrocarbons, steroids
2. Polar (Hydrophilic) Molecules
• Small polar uncharged molecules can pass through
easily (e.g.: H2O , CO2)
• Large polar uncharged molecules pass with
difficulty (e.g.: glucose)
3. Ionic (Hydrophilic) Molecules
• Charged ions or particles cannot get through
(e.g.: ions such as Na+ , K+ , Cl- )
Transport Proteins in the membrane: Integral
membrane proteins that allow for the
transport of specific molecules across the
phospholipid bilayer of the plasma
membrane.
How do they work?
 May provide a “hydrophilic tunnel”
(channel)
 May bind to molecule and physically move
it
 Are specific for the atom/molecule
transported
III. Passive transport: Diffusion of
molecules across the plasma membrane
A. Diffusion: The net movement of a
substance from an area of high
concentration to area of low
concentration.
Does not require energy.
B. Passive transport: The diffusion of
substance across a biological membrane.
Only
substances which can cross bilayer
by themselves or with the aid of a protein
Does not require the cell’s energy
Passive Transport: Diffusion Across a
Membrane Does Not Require Energy
IV. Osmosis:
The diffusion of water across a semipermeable membrane.
Through osmosis water will move from an
area with higher water concentration to
an area with lower water concentration.
Solutes can’t move across the semipermeable membrane.
Osmotic Pressure: Ability of a solution to take up water through
osmosis.
Example: The cytoplasm of a cell has a certain osmotic pressure
caused by the solutes it contains.
There are three different types of solution when compared to the
interior (cytoplasm) of a cell:
1. Hypertonic solution: Higher osmotic pressure than cell due to:
Higher solute concentration than cell or
Lower water concentration than cell.
2. Hypotonic solution: Lower osmotic pressure than cell due to:
Lower solute concentration than cell or
Higher water concentration than cell.
3. Isotonic solution: Same osmotic pressure than cell.
Equal concentration of solute(s) and water than cell.
V. Cells depend on proper water
balance
Animal Cells:
Do best in isotonic solutions.
Examples:

0.9% NaCl (Saline)

5% Glucose
If solution is not isotonic, cell will be affected:
 Hypertonic
solution: Cell undergoes crenation. Cell
“shrivels” or shrinks.
 Example:
5% NaCl or 10% glucose
 Hypotonic
solution: Cell undergoes lysis. Cell swells
and eventually bursts.
 Example:
Pure water.
V. Cells depend on proper water
balance
Plant Cells: Do best in hypotonic solutions, because the
cell wall protects from excessive uptake of water.
 Hypertonic
solution: Cell undergoes plasmolysis. Cell
membrane shrivels inside cell wall.
 Isotonic
solution: Cell becomes flaccid or wilts.
 Hypotonic
solution: Turgor. Increased firmness of
cells due to osmotic pressure.
 This
is the reason why supermarkets spray fruits
and vegetables with pure water, making them look
firm and fresh.
VI. Facilitated Diffusion:
Some substances cannot cross the membrane
by themselves due to their size or charge.
Membrane proteins facilitate the transport of
solutes down their concentration gradient.
No cell energy is required.
Transport Proteins
Specific
: Only transport very specific
molecules (binding site)
 Glucose
 Specific
ions (Na+, K+, Cl- )
Facilitated Diffusion Uses a Membrane
Transport Protein
VI. Active Transport:
 Proteins use energy from ATP to actively
“pump” solutes across the membrane
 Solutes are moved against a concentration
gradient.
 Energy is required.
Example:
The Na+-K+ ATPase pump:
Energy of ATP hydrolysis is used to
move Na+ out of the cell and K+ into
the cell
Endocytosis:
Moving materials into cell with vesicles.
Requires use of cell energy.
1. Pinocytosis (“Cell drinking”): Small droplets of liquid
are taken into the cell through tiny vesicles.
Not a specific process, all solutes in droplets are taken in.
2. Phagocytosis (“Cell eating”): Large solid particles are
taken in by cell.
Example: Amoebas take in food particles by surrounding
them with cytoplasmic extensions called pseudopods.
Particles are surrounded by a vacuole.
Vacuole later fuses with the lysosome and contents are
digested.
Endocytosis Uses Vesicles to Move
Substances into the Cell
Endocytosis:
3. Receptor mediated endocytosis: Highly specific.
Materials moved into cell must bind to specific receptors
first.
Example: Low density lipoproteins (LDL):
 Main form of cholesterol in blood.
 Globule of cholesterol surrounded by single layer of
phospholipids with embedded proteins.
 Liver cell receptors bind to LDL proteins and remove
LDLs from blood through receptor mediated
endocytosis.
 Familial hypercholesterolemia: Genetic disorder in
which gene for the LDL receptor is mutated.
Disorder found in 1 in 500 human babies worldwide.
Results in unusually high levels of blood cholesterol.
Blood Cholesterol is Taken Up by Liver Cells
through Receptor Mediated Endocytosis
Exocytosis:
Used to export materials out of cell.
Materials in vesicles fuse with cell membrane and are
released to outside.
 Tear glands export salty solution.
 Pancreas uses exocytosis to secrete insulin.