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Chapter 4
Membrane Structure and Function
Plasma Membrane Structure and Function
• Regulates the entrance and exit of molecules into and out of the cell
• Phospholipid bilayer with embedded proteins – Hydrophilic (water-loving) polar heads – Hydrophobic (water-fearing) nonpolar tails – Cholesterol (animal cells)
Outside
Inside
plasma membrane
glycolipid glycoprotein
integral protein cholesterol
peripheral protein
filaments of cytoskeleton
hydrophobic tails
hydrophilic heads
phospholipid bilayer
carbohydrate chain
The Plasma Membrane Plasma Membrane Proteins
• 5 Membrane Protein Functions
Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely . Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl–) channel; a thick mucus collects in airways and in pancreatic and liver ducts.
Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body . Different carriers respond differently to blood levels of glucose.
b. a.
c.
Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity.
d. e.
Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.
Receptor Protein Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone.
Permeability of the Plasma Membrane
• Differentially permeable
• Factors that determine how a substance may be transported across a plasma membrane: – Size – Nature of molecule – polarity, charge
Permeability of the Plasma Membrane
• Concentration gradient – More of a substance on one side of the
membrane – Going “down” a concentration gradient
• From an area of higher to lower concentration – Going “up” a concentration gradient
• From an area of lower to higher concentration • Requires input of energy
macromolecule
H2O
protein
+
+ -
- charged molecules and ions
phospholipid molecule
noncharged molecules
• Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction
water molecules (solvent)
dye molecules (solute)
a. Crystal of dye is placed in water
b. Diffusion of water and dye molecules
c. Equal distribution of molecules results
– Movement of molecules from an area of higher to lower concentration
– Solution contains a solute (solid) and a solvent (liquid)
Diffusion • Gases can
diffuse through a membrane
• Oxygen and carbon dioxide enter and exit this way
capillary alveolus
bronchiole
oxygen
O2
O2 O2
O2
O2
O2
O2 O2
O2
O2
O2
O2
Diffusion
Permeability of the Plasma Membrane
• Several factors influence the rate of diffusion – Temperature
• As temperature increases, the rate of diffusion increases
– Pressure – Electrical currents – Molecular size
• Membrane is not permeable to solute
a.
10%
5%
< 10%
> 5%
solute water
b.
c.
beaker
less water (higher percentage of solute)
more water (lower percentage of solute)
more water (lower percentage of solute)
less water (higher percentage of solute)
differentially permeable membrane
thistle tube
• Diffusion of water across a differentially permeable membrane
Osmosis
Isotonic No net gain or loss of water 0.9% NaCl
Hypotonic Cell gains water Cytolysis – hemolysis
Hypertonic Cell loses water Crenation
nucleus 6.6 µm
Animal cells
plasma membrane
In an isotonic solution, there is no net movement of water .
6.6 µm In a hypotonic solution, water enters the cell, which may burst (lysis).
6.6 µm In a hypertonic solution, water leaves the cell, which shrivels (crenation).
Tonicity Isotonic
No net gain or loss of water
Hypotonic Cell gains water Turgor pressure keeps plant erect cell wall keeps cell shape
Hypertonic Cell loses water Plasmolysis
chloroplast
nucleus
25 µm In an isotonic solution, there is no net movement of water.
central vacuole
Plant cells
25 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.
cell wall
40 µm In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell.
plasma membrane
Tonicity
Inside plasma
membrane carrier protein
solute Outside
– Small molecules that are not lipid-soluble – Molecules follow the concentration gradient – No Energy is required
Passive Transport: Facilitated Transport
Sodium-Potassium Pump
K+
Na+ Inside
carrier protein
Outside K+
K+
K+
Na+
Na+ Na+
Na+
1. Carrier has a shape that allows it to take up 3 Na+.
Active Transport – Energy is required
K+
P
Na+
ADP ATP
K+ K+
K+
Na+
Na+
2. ATP is split, and phosphate group attaches to carrier.
K+
K+ K+
K+
P
Na+ Na+
3. Change in shape results and causes carrier to release 3 Na+
outside the cell.
K+
K+
K+
K+
P
Na+ Na+
4. Carrier has a shape that allows it to take up 2 K+.
K+
K+
K+
K+
P
Na+
Na+
5. Phosphate group is released from carrier.
K+
K+
K+ K+
Na+
Na+
6. Change in shape results and causes carrier to release 2 K+
inside the cell.
paramecium
solute
solute
a. Phagocytosis
b. Pinocytosis
vacuole
coated vesicle
plasma membrane
coated pit c. Receptor-mediated endocytosis
399.9 µm
vesicle
vacuole forming
pseudopod of amoeba
0.5 µm
vesicles forming
coated vesicle
coated pit
receptor protein
Endocytosis
plasma membrane
Inside
Outside
secretory vesicle
Exocytosis