10/16/2012
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Cell Structure and Function
Cell Size
Most cells are relatively small because as
size increases, volume increases much
more rapidly.
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Visualizing Cells
Resolution - minimum distance two points can
be apart and still be distinguished as two separate points
Human eye has resolution of 100 m
Compound microscopes - magnify in stages using
multiple lenses
Transmission electron microscope - electrons
transmitted through specimen
Scanning electron microscope - electrons beamed
onto surface of the specimen
Surface to Volume Ratio limits size of cells.
Large cells require more raw materials.
V = cm3 S.A. = cm2 Restrictions on
size and shape
Cells compartmentalize to increase SA/Vol,
specialize rxn within, localize reactions where needed.
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Cell Theory
All organisms are composed of one or
more cells.
Cells are the smallest living units of all
living organisms.
Cells arise only by division of a previously
Basic Aspects of Cell
Structure and Function
Plasma membrane
Lipid bilayer Proteins
• Channels, transport, pumps, receptors
DNA-containing region
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Cell Characteristics
Genetic material
single circular molecule in prokaryotes near nucleoid double helix (DNA) located in nucleus in eukaryotes;
encased in nuclear envelope
Cytoplasm fills cell interior
Contains sugars, amino acids, proteins Eukaryotes have organelles
Plasma membrane encloses the cell
Phospholipid membrane with proteins
Cells of Living Things
Prokaryotic
Usually single celled. Can form colonies. No nucleus or
membrane-bound organelles. Genetic material localized (nucleoid) Ex. Bacteria, Archaebacteria Eukaryotic Kingdoms: Protista, Fungi, Plants, Animals. Nuclear membrane encloses DNA.
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Prokaryotic Cells - features
Simplest organisms (single-celled)
Small
Cytoplasm is surrounded by plasma
membrane
Rigid cell wall composed of peptidoglycan
No distinct interior compartments
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Prokaryotic Cells - roles
Harvest light via photosynthesis
Break down dead organisms & recycle
components
Cause disease
Prokaryotes - types
Archaebacteria & Bacteria
Archaebacteria don’t have peptidoglycan
in cell wall; bacteria do
Classified by differences in cell walls:
gram-positive (single-layered cell wall)
gram-negative (multilayered cell wall)
Susceptibility of bacteria to antibiotics depends on
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Defining Structures of
Eukaryotic Cells
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Plant Cells – Cell Walls
Cell wall – in plant, fungi, and many
protists
Support and protection
Composed of cellulose (plants)
Plant Cells – Cell Wall
Primary walls – constructed during cellulargrowth; adjacent to middle lamella
Middle lamella – sticky substance in
between cells, keeping them adhered together
Secondary walls – deposited inside the
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Animal Cells
Animal cells lack cell walls.
Form extracellular matrix (glycoproteins)
Provides support, strength, and resilience
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Major Cellular Components
Nucleus
Ribosomes
Endoplasmic reticulum
Smooth and Rough
Golgi body
Various vesicles Mitochondria
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Components of the Nucleus
Nuclear envelope - Surrounds nucleus
Chromosome - One DNA molecule and
associated proteins. Organized DNA.
Chromatin - DNA molecules and histone
proteins. Condenses to form DNA.
Nucleolus - RNA and proteins that will be
Nucleus
Repository for genetic material
Nucleolus - region of intensive ribosomal
RNA synthesis
Surface of nucleus bound by two
phospholipid bilayer membranes
nuclear membrane with pores
Allows proteins to come in the nucleus
Allows RNA & protein-RNA complexes to leave
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Chromosomes
DNA of eukaryotes is divided
into linear chromosomes.
exist as strands of chromatin,
except during cell division
Allows proteins to attach to
nucleotide sequences to regulate gene expression
associated with packaging
histones, packaging proteins
The Nuclear Envelope
Double - membrane system
Two lipid bilayers. 20-40 nm thick. Surrounds chromatin/nucleoplasm
Pores allow exchange. Composed of
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Ribosomes
Smallest, most numerous
organelle.
Composed of rRNA and
proteins. Synthesized by nucleolus.
Large and small subunits. Found free and bound to
E.R. Differ only in what they are making.
Catalyzes formation of
The Endomembrane System
Organelles in which lipids are assembled
and proteins are produced and modified
Are in direct contact or send vesicles
(membrane-bound sacs).
Occupy ½ of cell volume.
Nuclear envelope, endoplasmic reticulum,
The Endoplasmic Reticulum
Network of tubes and sacs that are
continuous with nuclear membrane. Most extensive mem. Sys.
Rough (ribosome studded) and Smooth.
Rough: production of secretory proteins. Signal sequence on polypeptide instructs ribosome to attach to ER.
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Golgi Bodies
Enzymatic finishes on
proteins and lipids, and packaging in vesicles.
Polarity of cisternae. Forms glycolipids,
glycoproteins,
Products of Golgi leave as
vessicles. From one cisternae to another or
out of cell. Cis (forming) face
Lysosomes
Membrane-bound
organelle thatcontains hydrolytic enzymes
responsible for the digestion of macromolecules, autolysis,
intracellular digestion.
Dead cells no longer able to
maintain H+ gradient (use H+
pump) so organelle breaks down releasing contents.
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Lysosome
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Vacuoles
Storage of water or ions,
pigments, hold food, pump out water.
Are larger than vesicles
formed from golgi/E.R.
In plants is enclosed by
Peroxisomes
Contain enzymes
(catalase) that
break down H2O2
formed during metabolism of alcohols, F.A.’s.
Specialized forms
[glyoxysome] found in seeds and
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Mitochondria
Production of ATP
Double-membrane system Two distinct compartments
Have their own DNA.
Maternal in origin.
Divide on their own,
independent of cell.
Have ribosomes, produce
Chloroplast
Found in photosynthetic eukaryotes Two outer membranes
Semifluid stroma; site of carbon fixation.
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Endosymbiosis
Endosymbiotic theory
suggests engulfed
prokaryotes provided hosts
with advantages associated
with specialized metabolic
activities.
Mitochondria – oxidative
metabolism
Evidence for Endosymbiosis
1. Double membrane of organelles – 1 from host, 1 from engulfed bacteria
2. Mitochondria and prokaryotes same size
3. Mitochondrial cristae resembles bacterial membrane folds
4. Mitochondrial ribosomes resemble those of prokaryotes
5. Circular DNA in mitochondria & chloroplasts as in prokaryotes
6. Mitochondria divide by fission as do prokaryotes
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Cytoskeleton
Protein fibers that support and give shape to a
cell, involved in organelle movement throughout cell, chromosome movement during cell division and large cell movements (cell motility and
cytokinesis)
3 Groups of Fibers classified according to size:
Mircrotubules (thickest)
Components of the
Cytoskeleton
Microtubules
and Tubulin subunits; form hollow
tube.
Provide framework for cell, organized
by centrosome from which they usu. originate.
“Rail” system for organelle transport. Component of Centriole.
Replicated prior to mitosis.
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Cilia and Flagella and the
Structural Basis of Cell Motility
Surrounded by
plasma membrane.
Motor proteins
(dynein) on
microtubules use ATP to change shape and “ratchet” past one
another.
Movement causes
Components of the
Cytoskeleton
Microfilaments (aka
actin filaments)
Solid “rope”of two actin proteins
Thinner and more flexible than microtubules
Principle component of muscle fibers.
Provide mechanism to support cell shape. Found just inside the c. mem.
Enable cell movement,
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Components of the Cytoskeleton
Intermediate Filaments Tough and durable;
made of keratin.
Mechanically
strengthen/reinforce cells or cell parts that are under stresses.
Provide structure to
long cells.
Found in desmosomes. Give nucleus shape
Centrioles
Occur in pairs near the nuclear membrane
Assist in assembling the microtubules in
animal cells
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Cell-to Cell Junctions
Plants
Plasmodesmata
Perforations in cell wall that allow passage for
water/solutes to adjacent cells.
Animals
Tight Junctions. Prevent leakage between cells (ie.
Stomach)
Desmosomes. Mechanically attach cells to each other.
Serve as anchoring sites for inter. filaments in cell.
Gap Junctions. Analogous to plasmodesma. Fxn as
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Plant Cell Walls
Protect plants, allow for shape and prevent excess H2O uptake.
Composed of cellulose Plasmodesmata
connect neighboring cells.
Secondary cell wall
Plant Cell Wall
Cell secretions form pectin (polysaccharide glue) which acts as
adhesive. Laid down in
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Extracellular matrix (ECM)
Intricate network of proteins and
polysaccharides that are organized into a meshwork on the outside of cells.
Large polysaccharides and proteoglygans form a
“gel-like” material that resist compression.
Proteins like collagen (most abundant protein in animals as part of bone and skin) and elastin (stretch and recoil)
provide structure and strength.
Adhesive-like proteins (fibronectins and laminin)
