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Basic Cell Structure
The Cell Theory
“Cell Doctrine”
Into the Cell
All organisms are constructed of one
or more cells.
The cell is the basic unit of life.
1.
2.
¸ Minimum level of complexity that
exhibits all characteristics of life.
All cells derive from previous cells.
3.
What does a cell need?
Isolated activity compartments
Selective isolation from environment
(plasma membrane)
n Energy (ATP)
n Instructions (DNA)
n Machinery to carry out instructions and
regulate processes (proteins)
n Compartmentalization of incompatible or
specialized activities (organelles)
n
Cell & Organelle Membranes:
More than just a phospholipid bilayer
• Cell membrane
•Plasmalemma
• Organelle membranes
Cell Size Varies
with Function
n
n
n
n
n
n
n
Heyer
Human nerve cell:
> 1 meter
Frog egg: 2 mm
Average animal cell:
50 µm
Human red blood cell:
~8 um
Bacteria: 1–2 µm
Organelle: 1 µm
Virus: 50 nm
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Basic Cell Structure
Why Must Cells
Be Small?
Smaller cells have more total surface area
n
Increased surface area makes it easier for gasses,
nutrients, etc. to enter a cell
‹ Same volume fi
Why Must Cells
Be Small?
So, Why Can’t Cells
Be Even Smaller?
Upper limits on cell
size are set by
diffusion distance.
Molecules must
diffuse through cell.
vThey must be big
enough to hold all
their parts!
vMacromolecules
cannot be shrunk.
v Volume controlled
by nucleus.
Early views of cells
n
n
n
Plasma membrane
“Nucleus” ( “center”)
– filled with “chromatin”
(“colored stuff”)
“Cytoplasm” (“cell fluid”)
Modern views of cells
n
Better microscopes and
stains
n
“Cytoplasm” and
“chromatin” much more
complicated, structured,
and dynamic than
previously appreciated.
unstained
human cheek cell
– Electron microscope.
50µm
stained
5 µm
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Basic Cell Structure
Two major types of cells
EUKARYOTIC CELL
Membrane
Cell-Type
Systematics
PROKARYOTIC CELL
Five-Kingdom Model
DNA
(no nucleus)
Membrane
Cytoplasm
Contrasting
eukaryotic and
prokaryotic cells in
size and complexity
I. Prokaryotic — Bacteria
• No membranous organelles
II. Eukaryotic — Protists
• Single celled or simple colonial
III. Eukaryotic / multicellular— Plant
• Organelles present, including chloroplasts
• Cellulose cell wall around plasma membrane
IV. Eukaryotic / multicellular—Fungi
• No chloroplasts
• Chitinous cell wall
V. Eukaryotic / multicellular— Animal
• No chloroplasts nor cell wall
• Varied cell morphology
Organelles
Nucleus (contains DNA)
1 µm
Prokaryotes lack a true
endomembrane system
Prokaryotic Cell Structure
n
n
n
n
n
Single-celled
organisms
No nucleus
nor other
membranous
organelles
No cytoskeleton
Most with
cell wall
outside plasma
membrane
Some with capsule
outside cell wall
v The plasma
membrane is
the only
membrane.
Photosynthetic prokaryote
Eukaryotic Cell Structure
Cytoplasm
—organelles & cytoskeleton
n
Fills cell; contains
– cytosol
• fluids and more
• gel & sol state
– membrane-bound organelles
• concentrate specific
enzymatic activities
• isolate incompatible reactions
or toxic products
– cytoskeleton
• maintain and alter cell shape
• hold and move organelles, etc.
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Basic Cell Structure
Organelles & Cytosol
Cellular Organelles
n
Nucleus
– Nuclear
envelope
– Nucleolus
• Nucleus of nucleus
• Transcribes rRNA for
ribosomes
– Chromatin/
chromosomes
– Nuclear lamina
n
• Protein scaffolding
underlying the
envelope and
supporting the
chromatin
Cytosol: Dense, semisolid aqueous gel containing a
tremendous variety of solutes and macromolecular machines
Nuclear envelope: 2 membranes
Nucleus
n
n
n
Nuclear
envelope
surface
Double-membrane
with pores
Contains DNA of
chromosomes
Controls cell
Pore
complexes
– structure
– function
n
Blueprints for new cells
Nuclear
lamina
Nuclear envelope: 2 membranes
Cellular Organelles
n
Endoplasmic
Reticulum
Smooth E.R.
– New membrane synthesis
– Lipid synthesis &
processing
– Steroid synthesis
– Lipophilic detoxification
– Intra-cellular Ca ++ store
– Usually tubular appearance
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Basic Cell Structure
Cellular Organelles
n
Endoplasmic
Reticulum
Cellular Organelles
n
Ribosomes
– Free and RER-bound
Rough E.R.
– Ribosome attachment sites
– Synthesis of membrane proteins
– Synthesis of proteins for
secretion or intra-organelle
storage
– Flattened beaded stacks;
Membrane often continuous
with nuclear envelope
RER to Golgi
Cellular Organelles
n
Golgi Complex
Transporting proteins in vesicles
– Further processing and trafficking of proteins from RER
– Receives vesicles from ER; buds off vesicles to plasma membrane
for export
Progression of membrane & proteins
Cellular Organelles
n
Vesicles
– Shuttle
between
organelles
and plasma
membrane
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Basic Cell Structure
Cellular Organelles
n
Lysosomes are digestive compartments
Lysosome
– Contains
hydrolytic
enzymes
– Digest food,
bacteria, old
cell parts
– Apoptosis
(cellular
self-destruct)?
Vacuoles
Lysosomes are digestive compartments
Lysosomes
Central vacuole
in a plant cell
Large storage vessels
150 µm
Fat vacuoles
in adipose cells
Contractile vacuole
Endomembrane System
n
Continuous exchange of membrane and
membranous contents
Paramecium lives in
fresh water
— takes on water
by osmosis.
Pumps out water
with the contractile
vacuole.
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Basic Cell Structure
Endomembrane System
Peroxisomes
n
n
May form from ER or autonomously
Both produces and removes hydrogen peroxide
– Detoxifies organic compounds, e.g. ethanol
– Destroy bacteria
w Nuclear envelope
w Endoplasmic reticulum
n
w Golgi apparatus
n
w Lysosomes
ß-oxidation of
fatty acids
Phospholipid
synthesis
— esp. myelin
Chloroplast
Peroxisome
Mitochondrion
w Vacuoles & vesicles
Figure 6.19
Cellular Organelles: bioenergetics
n
Mitochondria
1 µm
Cellular Organelles: bioenergetics
n
Chloroplast
– Triple-membrane
– Photosynthesis
– Double-membrane
– Aerobic respiration
• uses sunlight to construct organic molecules (CO 2Æ sugar)
• Powerhouse of cell
• Converts chemical
energy from
catabolism into ATP
– Also have some of their own DNA
– Plants
& some
protists
– Have some of own
DNA to maintain
activity when
nucleus is
unavailable
Cytoskeleton
Cytoskeleton
protein framework that
extends throughout the
cytoplasm
Organizing
the
organelles
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Basic Cell Structure
Cell Support Structures
n
Cytoskeletal structures
Cytoskeleton
structural
protein
motor
protein
Cytoskeleton
microtubules
microfilaments
intermediate
filaments
Tubulin
(dimer)
Actin
Keratin
(and others)
Dynein
(and others)
Myosin
∅
Cytoskeleton
Tensegrity:
A balance of
tension &
compression
Cytoskeleton
Cellular Structure:
Centrioles
Microtubule organizing centers
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Basic Cell Structure
The nucleus & the cytoskeleton
Vesicle transport along
microtubule “rails”
ATP energy is
used to pull
vesicles along
microtubules.
n
Intermediate filaments penetrate envelope to
connect with nuclear lamina
n
Microtubule organizing center adjacent to nucleus
Cell Mobility Structures
flagella & cilia
Eukaryotic versions
n
Flagella
n
Cilia
HOW
HOW DO
DO CILIA
CILIA AND
AND FLAGELLA
FLAGELLA MOVE
MOVE ??
n
Dynein arms cause
microtubules to bend
Molecular
motors:
structure of
cilia &
flagella
Fig. 4.19
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Basic Cell Structure
Myosin cross-bridges pull
actin filaments along
Cytoskeletal support of
cell membrane
Crowded network
of cross-linked
microfilaments
(red) underlying
the plasma
membrane (blue) .
“cytoplasmic cortex ”
Cytoskeleton and pseudopodia
Cytoplasmic Streaming
n
n
Motor proteins pull
cytoplasmic
macromolecular
complexes and
organelles along the
microfilament
meshwork
Many Variations in Theme
5 µm
The Cell: A Living Unit greater than the sum of Its parts
n Cells rely on the integration of structures and organelles
in order to function
Selective growth
of the
microfilament
meshwork
produces a bulge
(pseudopodium)
in cell membrane
for amoeboid
movement or
phagocytosis.
Figure 6.32
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