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5
Life’s Border
The Plasma Membrane
5.1 The Importance of Activity at the Cell’s Periphery
5.2 Why Do We Need the Plasma Membrane?
5.3 Four Components of the Plasma Membrane
5.4 Moving Materials In and Out: Diffusions and Gradients
5.5 How Do Materials Get In and Out of the Cell?
5.6 Getting the Big Stuff In and Out
Essay How Did We Learn? The Fluid-Mosaic Model of the Plasma Membrane
LECTURE OUTLINE
I.
Introduction
A. Importance of the plasma membrane (seen in malfunctioning transport in cystic fibrosis)
B. Overview of major functions:
1. Keeping the goods concentrated, while keeping harmful materials out, requires
transport in two directions
2. Communication with other cells
II.
Four Components of the Plasma Membrane
A. Phospholipid Bilayer
1. Phospholipids: two fatty-acid chains and a polar phosphate group attached to glycerol:
2. Arrangement of phospholipids in water (two layers, heads pointed out, tails pointed
in):
3. Permeability of bilayer: lipid center is a barrier to passage of large hydrophilic
molecules, but it allows nonpolar, hydrophobic molecules to pass.
B. Cholesterol (prevents passage of some small molecules and adds fluidity):
C. Proteins: integral (span entire membrane) and peripheral (lie on either side) with diverse
range of functions:
1. Structural support—attach to cytoskeleton
2. Recognition—helps immune system determine self from foreign (that’s why we reject
transplants; good figures to show this in Scientific American July 1997 article on
xenotransplantation).
3. Communication—receptors and binding sites
4. Transport—allow molecules to pass
D. Glycocalyx—sugar components protruding from lipids and proteins, functions:
1. Binding sites for proteins in communication and recognition
2. Lubricate cells
3. Stick cells down
III. Moving Materials In and Out: Diffusion and Gradients
A. Random Movement and Diffusion:
1. Diffusion = movement of molecules from region of higher to lower concentration
2. Concentration gradient = difference between the highest and lowest concentration of a
solute; like bike coasting downhill, the tendency is for molecules to travel from high to
low concentration.
B. Diffusion through Membranes
C. Permeability verses semipermeability. Why certain molecules pass through membranes.
D. Osmosis = net movement of water across a semipermeable membrane from an area of
lower solute concentration to higher solute concentration:
E. Importance of osmosis to membrane function in animals (drinking seawater) and plants
(turgid pressure):
IV. Two Main Types of Transport across Cell Membranes
A. Passive Transport
1. Simple diffusion (water, gases, fat soluble) membrane is permeable, so they travel down
concentration gradient and enter without energy output by cell:
2. Facilitated diffusion (larger polar molecules) membrane is impermeable, so even if they
want to travel down the concentration gradient, they can’t without help from a
membrane channel (transport protein):
B. Active Transport: if molecules have to pass across the membrane up their concentration
gradient, they cannot use the energy of diffusion, but must expend energy (ATP):
1. Na-K pump:
2. All three types of transport through membranes are compared in an animation
V.
Getting the Big Stuff In and Out
A. Exocytosis: movement of materials out of the cell by fusion of vesicles with the plasma
membrane (export or removal of wastes in single-celled organisms)
B. Endocytosis: infolding of the plasma membrane to bring large materials into the cell
C. Pinocytosis: “cell drinking”—water and solvents are enclosed in invaginating vesicle, used
in digestive tract:
D. Receptor-mediated endocytosis: more specific with receptor capturing ligand and
concentrating into an invaginating pit:
E. Phagocytosis: “cell eating”—how the human immune system ingests whole bacteria, or
one-celled creatures eat using pseudopodia,