biological membrane structure

membrane components

lipid bilayer: structural backbone

phospholipids (most abundant

• glycolipids

• cholesterol

proteins are:

• transporters

• enzymes

• signal transductors

carbohydrates attached to proteins and lipids

lipid bilayer

polar head group of phospholipids has an affinity for water

their hydrocarbon tails avoid water, favored structure is a biomolecular sheet

formation is a self assembly

hydrophobic interactions are the major driving forces

electrostatic and hydrogen bonding between polar head and water

self sealing

lipids in a membrane

majority are phospholipids

they are amphipathic - hydrophilic and hydrophobic components

these lipids spontaneously form bilayers in aqueous solution:

• vesicles

• micelles

have cholesterol

integral proteins in the cell membrane

span the cell membrane, sometimes once, others more

either terminus may be inside or outside the cell

peripheral membrane proteins

these proteins adhere sometimes temporarily to the biological membrane

they do not enter into the hydrophobic space within the cell membrane but can push into the peripheral regions of the lipid bilayer

they attach to integral membrane proteins

membrane carbohydrates

components of complex molecules on the cell membrane

• glycoproteins

• glycolipids

• proteoglycans

carbohydrates forms a layer around the cell on the outside of the membrane only called the glycocalyx

this layer functions as a barrier between a cell and its surrounding, also a mediator for cell-cell interactions. It protects a cell membrane from the direct action of physical forces and stresses allowing the membrane to maintain its integrity

functions of the cell (plasma) membrane

it is a sheet like structure that forms closed boundaries

physical isolation, acts as a barrier

holds molecules inside the cell

allows the regulation of exchange into and out of the membrane

fluid mosaic model

structure of cell membranes as a dynamic and flexible arrangement of various components

small amounts of lipids interact specifically with particular membrane proteins

proteins can diffuse laterally but are not free to rotate

passive diffusion

unaided spontaneous movement of solute molecules down their concentration gradient, from high to low, until solutes equilibrate across the bilayer

entropically driven

maximum entropy at equilibrium

facilitated diffusion

carrier mediated diffusion (uses an integral protein)

occurs down concentration gradient

dependent on integral proteins

does not used energy

active transport

movement against a concentration gradient

does not use energy

2 forms: primary active transport, secondary active transport

primary active transport

removes 3 Na⁺ ions from the cell cycle in exchange from 2 K⁺ ions.

movement of Na is against the concentration gradient. this means the process needs energy

The ATPase binds to ATP and 3 Na ions. ATP phosphorylates the ATPase and induces conformational change. The Na ions are released into the ECF. 2 K ions bind from the ECF and the ATPase is dephosphorylated and reverts to its previous conformation. K ions are release into the cell

secondary active transport

carrier protein that moves a specific substance against its concentration gradient

carrier protein can also move another substance at the same time

the concentration gradient for one substance provides the driving force for the carrier protein

second substance gets a free ride

endocytosis and exocytosis

brings proteins/solid particles into and out of the cell

• phagocytosis (cell eating)

• pinocytosis (cell drinking)

• receptor mediated endocytosis