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CHAPTER 4
CELL STRUCTURE AND FUNCTION
Animal Cell
I. METHODS FOR STUDYING CELL STRUCTURE -“How do we look at cells?”
Microscopy:
Purpose: To study the detail of the microscopic
structures of whole tissues and cells.
a) Light Microscopes -- use visible light to view
microscopic organisms.
1) Resolution – the ability to distinguish between
two points. The resolving power of a microscope
is essential for distinguishing detail (cannot be
better than approximately the wavelength of light
==> 4000 - 6000 Å (0.4 - 0.6 µ); can be much
worse depending upon quality of optics).
2. Magnification is the ability to enlarge the image of the
object; this is unrelated to the resolution.
3. Contrast in Light MicroscopyPurpose: to allow for the more easier viewing of the
cellular substructures.
• Stains -- can be specific for different substances
associated with different cellular subunits; e.g. Gram’s
Stain for bacterial identification.
b) Phase contrast microscopy- microscopy based
upon the differential densities of cellular
substructures.
c) Electron Microscope -- electrons moving at high
speeds have characteristics of a waves.
• Resolving power is related to the inverse of the wavelength of
electron beams. Theoretical resolving power is 0.1 nm; actual
resolving power is 2 nm.
1. TEM (transmission electron microscope) - analogous to light microscope (has
objective lens, condenser lens system, lenses for magnification): increase contrast
with stains and by creating a type of phase contrast.
2. SEM (scanning electron microscope) - move a small (20 - 50 Å diameter)
beam of electrons across specimen and measure how many electrons are emitted
at each point; display on TV. Shows specimen surfaces with great depth of focus
but with lower resolution than TEM.
NOTE:
TEM specimen preparation - most common is to remove water and embed in plastic and
slice very thin section (specimens must be thin for TEM).
SEM specimen preparation - remove water (critical point dry) and coat with gold or
platinum to make surface electrically conductive.
d) Cell Fractionation: to study isolated cell components
1. Lyse Cells (chemically or physically) --> release
organelles, macromolecules etc.
2. Separate components --> differential centrifugation
separates cell components based upon differences in density.
Scanning Electron Micrograph: Bacterial Spores
Scanning Electron Micrograph: Celery Shoot Apical Meristem
Transmission Electron Micrograph: Chloroplasts
Transmission Electron Micrograph: Neurospora crassa
Ultracentrifugation: Separation of blood components
II. Cell Theory
• The cell theory is based upon the experimental discoveries
and innovations of Leewonhoeuk, Hooke, Schleiden,
Schwann, and Virchow.
a) Cells are the basic structure of life.
b) Cells perform the basic functions of life.
c) Cells arise from pre-existing cells.
Cell Theory Exceptions:
• Viruses: do not have the ability to reproduce on their own.
• Mitochondria and Chloroplasts: have their own DNA and can
self-replicate.
III. Limitation to Cell Size
• The limitation in the size of cells entails an
understanding of the changes in surface area to volume
ratio as cells increase in size.
• In addition, the relationship between cell size and rates of
diffusion must be established.
a) As the size of a given cell increases, there is a decrease in the
surface area to volume ratio.
b) This means that the rate of molecule diffusion will occur more
slowly. D:\ICIB.exe
c) Cells that are too large may not obtain the materials
needed to maintain homeostasis.
IV. Cell types
a) Prokaryotes:
1. are unicellular.
2. have no nucleus or membrane-bound organelles.
3. are small in size (1-10 mm).
4. have DNA that is concentrated in a region called the
nucleiod.
5. exist in colonies and a variety of shapes (rods, spheres, helices).
6. have cell walls made of peptidoglycans that supports, protects, and
maintains the shape of cells.
7. have sticky capsules and pili that help in adhering to surfaces.
8. include Monerans (bacteria and archae).
b) Eukaryotes-nucleus and organelles bounded by internal membranes.
Examples include protists, fungus, plants, and animals.
V. Eukaryotic Cell Structure
a) Endomembrane system
1. Nucleus
• nuclear membrane: double-membrane lined by nuclear lamina
and nuclear pores.
•DNA and histones (structural proteins) make up the
chromosomes.
•Nucleolus – site of rRNA synthesis (rRNA is an essential
component to ribosomes structure.
ELECTRON MICROGRAPH OF A NUCLEUS
•2. Endoplasmic Reticulum: - double-membrane systems
of transport tubes that are continuous with nuclear
membrane.
• Rough ER contains bound ribosomes.
•Smooth ER no ribosomes; lipid synthesis; detoxification of drugs and
poisons; carbohydrate metabolism.
3. Golgi Apparatus – flattened discs; used to
package, modify, and secrete lipids and proteins;
derived from the endoplasmic reticulum.
4. Lysosomes:
• membrane-bound
sac.
• contain enzymes that are used to digest
macromolecules from golgi bodies and
pinocytotic vessicles.
5. Vacuoles - store water, food, and metabolic wastes in cells.
6. Plasma (Cell) Membrane
• composed of a double layer of phospholipids and integral and
peripheral proteins.
• controls what enters/exits cells.
• aids in maintaining the shape of cells.
• contain identification markers for intercellular
recognition and signaling.
The Fluid Mosaic Model of the Cell Membrane
7. Ribosomes:
• found attached to the rough ER or floating freely in the
cytoplasm.
• site of protein synthesis (translation).
Ribosome with mRNA and growing polypeptide chain.
Polypeptide
mRNA
Ribosome
b) Other Membranous Organelles
1. Mitochondria
• Site of cellular respiration.
• can exist as single entities or as
numerous organelles within cells
based upon the cell’s energy
requirements.
• Mitochondria are double-membrane organelles.
• The membranes are made of phospholipids and proteins.
• The outer membrane is smooth while the inner membrane
(cristae) is highly folded.
• Mitochondria have distinct regions where
cellular respiration occurs.
Mitochondria Structure
• The mitochondrial matrix is the site of the Krebs Cycle.
• The cristae is the site of the electron transport chain.
• The highly convoluted cristae enables the mitochondria to
efficiently generate ATP by providing a large surface area
for the oxidation-reduction reactions of the electrontransport chain.
2. Chloroplasts:
• Site of photosynthesis in certain autotrophic
organisms (plants and algae).
• Belong to a family of organelles called plastids.
• Contain the green, light capturing pigment
called chlorophyll.
Chloroplasts
GRANA
• Consist of a double membrane.
• Contain inner membrane system arranged into flattened discs
called thylakoids which when stacked are collectively known as
grana.
• The grana are the site of the light reactions of
photosynthesis.
• The grana are surrounded by a fluid called the stroma.
3. Peroxisomes:
• contain specialized enzymes that function in breaking
down fatty acids for use in cellular respiration.
• convert toxic substances (i.e.: alcohol) into less harmful
metabolites.
• are bound by a single membrane.
4. Cytoskeleton
• Composed of a network of protein fibers that help
maintain cellular structure and function.
• Anchors many organelles into fixed positions.
• Aid in cell motility.
• Makes up cilia and flagella.
• Examples include microtubules, microfilaments,
and intermediate filaments.
Microtubules: hollow tubes of tubulin (13); large diameter;
maintain cell shape, cell motility (cilia/flagella), chromosome
movement, organelle movement.
Microfilaments: intertwined strands of actin (2); small
diameter; cell shape, muscle contraction, pseudopodia
movement; cyclosis; mitosis.
Intermediate Filaments: super coiled fibrous proteins; small
diameter; cell shape; anchors nucleus; example (Keratin).
Cytoskeleton
Microtubule Cross-Section – “9 + 2” Pattern
Cilia of the Respiratory System
Centrioles and Centrosomes:
• Centrosomes, a region located near the nucleus, are
the organelles to which microtubules grow out from.
• In animal cells, centrioles aid in
microtubules assembly. However, they are
not essential for this function in all
eukaryotes.
• Centrioles have a nine sets of microtubule
triplets arranged in a ring.
5. Cell Wall:
• Found in plant cell.
• Aids in support and protection.
• Made of the polysaccharide, cellulose.
• Porous.
Plant Cell Wall
Note: Cell Walls are also found in prokaryotes, fungi,
and some protists.