5

Life’s Border:The Plasma Membrane

I. Introduction (Sections 5.1 and 5.2)A. Importance of the plasma membrane (seen

in malfunctioning transport in cystic fibrosis)

B. Overview of major functions: 1.Regulates what goes into and out of a

cell.

2.Communication with other cells

II.Four Components of the Plasma Membrane (Section 5.3)A. Phospholipid Bilayer

1.Phospholipids—two fatty-acid chains and a polar phosphate group attached to glycerol: Figure 5.1a

2. Arrangement of phospholipids in water (two layers, heads pointed out, tails pointed in): Figure 5.1b

3. Permeability of bilayer—lipid center is a barrier to passage of large hydrophilic molecules, but it allows nonpolar, hydrophobic molecules to pass. (Interactive Activity 1)

B. Cholesterol (prevents passage of some small molecules and adds fluidity): Figure 5.2

C. Proteins—integral (span entire membrane) and peripheral (lie on either side) with diverse range of functions: Figure 5.21. Structural support—attach to

cytoskeleton

2. Recognition—helps immune system determine self from foreign (that’s why we reject transplants

3. Communication—receptors and binding sites (Interactive Activity 2)

4. Transport—allow molecules to pass

D. Glycocalyx Figure 5.2—sugar components protruding from lipids and proteins, functions:

1.Binding sites for proteins in communication, and recognition

2.Lubricate cells3.Stick cells down

III. Moving Materials In and Out: Diffusion and Gradients (Section 5.4)A. Random Movement and Diffusion: Figures

5.3 and 5.4

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.

Diffusion

Random

Movement

B. Diffusion through Membranes:

C. Permeability verses semi-permeabilityD. Osmosis = net movement of water across

a semi-permeable membrane from an area of lower solute concentration to higher solute concentration: Figure 5.5 animation

E. Importance of osmosis to membrane function in animals (drinking sea water) and plants (turgid pressure): Figure 5.6

OSMOSIS

Hypertonic Solution – More water inside cell than outside

Plasmolysis

Plasmolysis

Cells shrink and die

Iso-osmotic Solution – same concentration of water inside and outside cell

Animal cell

Plant cell

No net gain or loss of water

Hypotonic Solution – more water outside cell than inside

Cytolysis – cell swells and bursts

Builds up turgor pressure – cell becomes stiff, keeping plant upright

IV. Two Main Types of Transport across Cell Membranes (Section 5.5)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: Figure 5.7a

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): Figure 5.7b

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): Figure 5.7c

Na-K pump Figure 5.8

V. Getting the Big Stuff In and Out (Section 5.6)

A. Exocytosis: Figure 5.9—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

Pinocytotic vessicle

1. Pinocytosis, “cell drinking”—water and solvents are enclosed in invaginating vesicle, used in digestive tract: Figure 5.10a

D. Receptor-mediated endocytosis—more specific with receptor capturing ligand and concentrating into an invaginating pit: Figure 5.10b

E. Phagocytosis, “cell eating”—How the human immune system ingests whole bacteria or one-celled creatures eat: pseudopodia, Figure 5.10c (Interactive Activity 4 & 5)