Biological Membranes

Plasma Membranes Structural Support

• protein in cell membrane holds the interior structural scaffolding (cytoskeleton) in place to maintain cell shape

• membrane proteins can form adhesion between neighbouring cells and cell to extracellular matrix to form the structure of tissues

Plasma Membranes Physical Isolation

• separates its internal contents from the external environment

• it’s selectively permeable because of protein within plasma membrane

Plasma Membranes Regulation of Exchange

• as a barrier that can regulate flow of materials into and out of the cell

Plasma Membranes Communication

• establishes communication between cells and its external environment by some of the membrane proteins

Fluid Mosaic Model

• phospholipids (barrier)

• proteins (communication/functions)

• configuration can change at any time

• plasma is bilayer structure with thickness of ~8 nm

• two layers of phospholipid with protein molecules and a small amount of carbohydrates inserted in or adhered to the structure

Types of Lipids

Phospholipids

• glycerol backbone with two fatty acid chains

• most abundant

Sphingolipids

• heads may be phospholipids or glycolipids

• slightly longer

• more cholesterol

• placental alkaline phosphatases that protrude above glycolipids and associated with lipid raft

Cholesterol

• inserted between heads of phospholipids

• helps make membranes impermeable to water and keeps them flexible

Lipid Anchored Proteins

• insert themselves into either side of the cell membrane

• covalently bounded to lipid tail within lipid bilayer

Transmembrane Proteins

• span entire width and protrude into both cellular spaces

• function as channel for transportation

Peripheral Membrane Proteins

• loosely attached to either the polar head or other membrane proteins

• don’t form bonds

Functions of Membrane Proteins

Ion channels- form pores that allow specific ion to pass through

Receptors- specific cellular recognition site sends message to cells

Carriers/Transporters- more specific than ion channels, selectively moving a molecule or ion

Enzymes- catalyze specific biological reactions

Linkers- anchor plasma membrane of neighbouring cells

Cell-identity Markers- identify a cell whether it is one of its own or foreign

Transport Mechanisms

Diffusion- movement of solute down concentration gradient

Osmosis- movement of solvent, requires water channels (aquaporins)

Facilitated Diffusion- carrier proteins, higher to lower concentration

Active Transport- move against concentration gradient, requires ATP

Symport- carries 2 solutes in same direction (secondary active transport)

Antiport- transported molecules moving in opposite direction