Chapter 4
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.
Microscopy
First observations of cells were made with
light microscopes:
Robert Hooke (1665): Used primitive microscope to
observe cork (dead plant cells). Coined the word
cell.
Anton van Leeuenhoeck (1670s): Made single lens
microscopes. First person to observe live cells
under microscope: “animalcules” (protists) in
water, red blood cells, sperm, bacteria, and insect
eggs.
Theodor Schwann (1830s): Observed harder to view
animal cells. Called cells “elementary particles” of
both plants and animals.
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
Endoplasmic Reticulum (SER)
 Golgi 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