Download cell membranes

• Nuclei (yellow) and actin (red)
Figure 4.6x
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THE CYTOSKELETON
The cell’s internal skeleton helps organize its
structure and activities
• network of protein fibers
Figure 4.17A
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• Microfilaments of actin enable cells to change
shape and move
• Intermediate filaments reinforce the cell and
anchor certain organelles
• Microtubules
– give the cell rigidity
– provide anchors for organelles
– act as tracks for organelle movement
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Actin subunit
Tubulin
subunit
Fibrous subunits
25 nm
7 nm
MICROFILAMENT
10 nm
INTERMEDIATE
FILAMENT
Figure 4.17B
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MICROTUBULE
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How do cilia and flagella move?
• A cilia or flagellum is composed of a core of
microtubules wrapped in plasma membrane
• Eukaryotes have “9+2” structure
Cilia and flagella move when microtubules bend
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FLAGELLUM
Electron micrograph
of sections:
Outer microtubule
doublet
Plasma
membrane
Flagellum
Central
microtubules
Outer microtubule
doublet
Plasma
membrane
Figure 4.18A
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Basal body
Basal body
(structurally identical to centriole)
Slide 43
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polar
head
P
–
hydrophobic molecules
nonpolar
tails
Phospholipid bilayer
hydrophilic molecules
cytosol
Cell surfaces protect, support, and join cells
Surfaces allow exchange of signals and molecules.
• Plant cells connect by plasmodesmata
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Walls of two
adjacent
plant cells
Vacuole
PLASMODESMATA
Layers of one
plant cell wall
Cytoplasm
Plasma membrane
Figure 4.19A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Animal cells - extracellular matrix
– sticky layer of glycoproteins
– binds cells together in tissues
– can also protect and support cells
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• Tight junctions can bind cells together into
leakproof sheets
• Anchoring
junctions link
animal cells
TIGHT
JUNCTION
ANCHORING
JUNCTION
• Gap junctions
allow substances
to flow from cell
to cell
GAP
JUNCTION
Plasma
membranes of
adjacent cells
Figure 4.19B
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Extracellular
matrix
Eukaryotic organelles fall into 4 functional groups
• 1. Manufacture and transport –
dependent on network of membranes
- Nucleus
- Ribosomes
- Rough ER
- Smooth ER
- Golgi apparatus
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2. Breakdown – all single-membrane sacs
• Lysosomes (in animals, some protists)
• Peroxisomes
• Vacuoles (plants)
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3. Energy Processing – involves extensive
membranes embedded with enzymes
• Chloroplasts
• Mitochondria
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4. Support, Movement, Communication
• Cytoskeleton – includes cilia, flagella,
filaments, microtubules
• Cell walls
• Extracellular matrix
• Cell junctions
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What do these have in common?
•
•
•
•
•
•
HIV infection
Transplanted organs
Communication between neurons
Drug addiction
Cystic fibrosis
hypercholesteremia
Membranes organize the chemical activities of cells
• selectively permeable
• hold teams of enzymes


Cytoplasm
Figure 5.10
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Plasma membrane
• Contact between cell and environment
• Keeps useful materials inside and
harmful stuff outside
• Allows transport, communication in
both directions
Plasma membrane components
 Phospholipid bilayer
 Cholesterol
 Proteins
 Glycocalyx
polar
head
P
–
hydrophobic molecules
nonpolar
tails
Phospholipid bilayer
hydrophilic molecules
cytosol
THE PLASMA MEMBRANE
phospholipids
cholesterol
cytoskeleton
peripheral
protein
integral
protein
Cholesterol blocks some small molecules, adds fluidity
• Membrane Proteins
– span entire membrane or lie on
either side
– Purposes
• Structural Support
• Recognition
• Communication
• Transport
• Glycocalyx
– Composed of sugars protruding from
lipids and proteins
– Functions
• Binding sites for proteins
• Lubricate cells.
• Stick cells down.
• Many membrane proteins are enzymes
• Some proteins function as receptors for
chemical messages from other cells
– The binding of a messenger to a receptor may
trigger signal transduction
Messenger molecule
Receptor
Activated
molecule
Figure 5.13
Enzyme activity
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Signal transduction
• The plasma membrane of an animal cell
Glycoprotein
Carbohydrate
(of
glycoprotein)
Fibers of the
extracellular
matrix
Glycolipid
Phospholipid
Cholesterol
Microfilaments
of the
cytoskeleton
Proteins
CYTOPLASM
Figure 5.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Diffusion and Gradients
– Diffusion = movement of molecules
from region of higher to lower
concentration.
– Osmosis = diffusion of water across a
membrane
• In passive
transport, substances
diffuse through
membranes without
work by the cell
Molecule
of dye
Membrane
EQUILIBRIUM
EQUILIBRIUM
Figure 5.14A & B
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(a)
selectively
permeable
membrane
H2O
free water
molecule: can
fit through pore
sugar
bound water molecules
clustered around sugar:
cannot fit through pore
pore
(b)
selectively permeable
membrane
sugar molecule
water molecule
pure water
bag
bursts
Osmosis = diffusion of water across a membrane
• water travels from
an area of higher
concentration to
an area of lower
water
concentration
Hypotonic
solution
Hypertonic
solution
Selectively
permeable
membrane
Solute
molecule
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
Water
molecule
Selectively
permeable
membrane
Solute molecule with
cluster of water molecules
NET FLOW OF WATER
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Figure 5.15
Water balance between cells and their
surroundings is crucial to organisms
osmoregulation = control of water balance
• Osmosis causes cells to shrink in a hypertonic
solution and swell in a hypotonic solution
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10 microns
isotonic solution
equal movement of water
into and out of cells
hypertonic solution
net water movement
out of cells
hypotonic solution
net water movement
into cells
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Passive transport = diffusion across membranes
• Small nonpolar molecules - simple diffusion
• Many molecules pass through protein pores by
facilitated diffusion
Solute
molecule
Transport
protein
Figure 5.17
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Active transport
• transport proteins needed
• against a concentration gradient
• requires energy (ATP)
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• Active
transport in
two solutes
across a
membrane
FLUID
OUTSIDE
CELL
Transport
protein
First
solute
1
• Na+/K+
pump
Phosphorylated
transport protein
First solute,
inside cell,
binds to protein
2
ATP transfers
phosphate to
protein
3
Protein releases
solute outside
cell
5
Phosphate
detaches from
protein
6
Protein releases
second solute
into cell
Second
solute
• Protein
shape
change
4
Second solute
binds to protein
Figure 5.18
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Exocytosis and endocytosis transport large
molecules
exocytosis = vesicle fuses with the membrane
and expels its contents
FLUID OUTSIDE CELL
Figure 5.19A
CYTOPLASM
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b
– or the membrane may fold inward, trapping
material from the outside (endocytosis)
Figure 5.19B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Phagocytosis, “cell eating”
—How the human immune system ingests whole
bacteria or one-celled creatures eat.
phagocytosis
food particle
1
2
3
particle
enclosed in vesicle
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pinocytosis
2
vesicle containing
extracellular
fluid
(cytoplasm)
extracellular fluid
cytosol
receptors
vesicle
captured
molecules
coated
pit
vesicle
bacterium
pseudopodium
vesicle
Receptor-mediated endocytosis
• Cholesterol can accumulate in the blood if
membranes lack cholesterol receptors
LDL PARTICLE
Phospholipid
outer layer
Receptor protein
Protein
Cholesterol
Plasma membrane
Vesicle
CYTOPLASM
Figure 5.20
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What do these have in common?
•
•
•
•
•
•
HIV infection
Transplanted organs
Communication between neurons
Drug addiction
Cystic fibrosis
hypercholesteremia