CELL MEMBRANE STRUCTURE AND FUNCTION

FLUID MOSAICS OF LIPIDS AND PROTEINS

COMPOSITION

Phospholipids Give membrane hydrophilic and

hydrophobic properties Phosphate – hydrophilic, face aqueous exterior

or interior Fatty acid – hydrophobic, face each other inside

the membrane

Proteins Cholesterol, Glycolipids, and

Glycoproteins

COMPOSITION

Phospholipids Proteins: many different roles

Hydrophilic Have charged and polar side groups

Hydrophobic Have nonpolar side groups

Cholesterol, Glycolipids, and Glycoproteins

COMPOSITION

Phospholipids Proteins Cholesterol, Glycolipids, and

Glycoproteins Cholesterol: resists changes in membrane

fluidity caused by changes in temperature Glycolipids & Glycoproteins: have a

carbohydrate attached; cell-cell recognition

SELECTIVE PERMEABILITY

THE PURPOSE: TO SEPARATE THE INTERNAL FROM THE EXTERNAL ENVIRONMENT

Can Pass Through: Small, Uncharged polar Small Nonpolar

Can NOT Pass Through (without assistance): Hydrophilic substances Large Polar Ions Water

Note: Plant Cell Walls are made of cellulose and are external to the cell membrane. They are also found in Prokaryotes and Fungi.

PASSIVE AND ACTIVE TRANSPORT

PASSIVE TRANSPORT

Does not require the input of energy Net movement of molecules from high to

low concentration Used to import resources and export wastes Water moves across the membrane through

proteins called “aquaporins.” Osmosis: the diffusion of water across a

selectively permeable membrane. Tonicity: the ability of a solution to cause a

cell to gain or lose water

PASSIVE TRANSPORT

Solutions outside a cell may be… Isotonic – same concentration of solutes as the interior of the cell; water moves across membrane at the same rate in both directions

KEY: term – exterior solution; effect – direction of water flow

PASSIVE TRANSPORT

Solutions outside a cell may be… Hypotonic – lower concentration of solute than the interior of the cell; water moves across membrane into the cell faster than it flows out, causing it to swell and lyse (burst)

KEY: term – exterior solution; effect – direction of water flow

PASSIVE TRANSPORT

Solutions outside a cell may be… Hypertonic - higher concentration of solute than the interior of the cell; water moves across membrane out of the cell faster than it flows in, causing it to shrivel and possibly die

KEY: term – exterior solution; effect – direction of water flow

ACTIVE TRANSPORT

Requires energy to move molecules from low to high concentration

Free energy often from ATP Uses membrane proteins

Example: Sodium-potassium pump

EXOCYTOSIS AND ENDOCYTOSIS

MOVING LARGE MOLECULES BETWEEN THE INTERNAL AND EXTERNAL ENVIRONMENT

Exocytosis – internal vesicles fuse with the plasma membrane to secrete macromolecules out of the cell

Endocytosis – the cell takes in macromolecules and other particles by forming vesicles from the plasma membrane