Bio ch 7

Bi-lipid layer

Contains phospholipids (phosphate, glycerol and two fatty acids)

• Amphipathic (single molecule that is both hydrophilic and hydrophobic (phospholipid))

Semi-permeable

allows only certain molecules through

•small uncharged

Fluid mosaic of plasma membrane

Mosaic of various proteins in bi-lipid layer

• Also contains glycolipids and sterols

Membrane proteins

a collage of different proteins, often clustered in groups, embedded in the fluid matrix of the lipid bilayer

• This structure resembles a tile mosaic

Fluidity in membranes

Not static, constantly moving

• Held together via hydrophobic interactions

• Weaker than covalent, allow movement

• Both lipids and some proteins can move

Factors affecting fluidity

Temperature

• Lower temperature, slow down

Nature of the fatty acids

• Saturated improve viscosity (less fluid)

• Unsaturated enhance fluidity

The role of cholesterol in membrane fluidity

At lower temperatures, prevents tight packing of phospholipids

Too fluid

•Prevents proper protein function

• Membrane could rupture

Too solid

• Changes permeability

• Doesn’t allow proteins to move around

Organisms will change their membrane structure to

Suit the environment they observe most of

Two major types of membrane proteins

Peripheral proteins

Integral proteins

Peripheral proteins

bound to the surface of the membrane

Integral proteins

penetrate the hydrophobic core

Transmembrane proteins

integral proteins that span the membrane

Hydrophobic regions of an integral protein consist of

nonpolar amino acids, often coiled into α helices

Cell-surface membranes can carry out several functions

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix (ECM)

HIV cannot enter the cells of resistant individuals who lack

CCR5

Cells recognize each other by binding to molecules

often containing carbohydrates, on the extracellular surface of the plasma membrane

Carbohydrates on the extracellular side of the plasma membrane vary among

species, individuals, and even cell types in an individual

Membrane carbohydrates may be covalently

Bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins)

Transport proteins

allow passage of hydrophilic substances across the membrane

Channel proteins

have a hydrophilic channel that certain molecules or ions can use as a tunnel

Aquaporins

greatly facilitate the passage of water molecules

Diffusion

the tendency for molecules to spread out evenly into the available space

Although each molecule moves randomly,

diffusion of a population of molecules may be directional

Dynamic equilibrium

many molecules cross the membrane in one direction as in the other

Concentration gradient

the region along which the density of a chemical substance increases or decreases

•Substances diffuse down this

Passive transport

no energy is expended by the cell to make it happen

•diffusion of a substance across a biological membrane

Ex: tea

Osmosis

the diffusion of water across a selectively permeable membrane

Water diffuses across a membrane from

the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

Membrane is

Semi-permeable (solutes can’t move through, water can)

Tonicity

the ability of a surrounding solution to cause a cell to gain or lose water

Isotonic

Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Hypertonic

Solute concentration is greater outside than inside the cell; cell loses water

Hypotonic

Solute concentration is less outside than inside the cell; cell gains water

Cells without cell walls will shrivel in

hypertonic solution and lyse (burst) in a hypotonic solution

Osmoregulation

control of solute concentration and water balance

Facilitated Diffusion

•Assist in diffusion of substance that would normally

• Diffuse very slowly across the membrane

• Never diffuse across the membrane

• No energy is consumed

Ion channels

Facilitate the transport of ions

Active transport

requires energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradients

•Carrier proteins

Exocytosis

transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell

Endocytosis

the cell takes in macromolecules by forming vesicles from the plasma membrane

Three types of endocytosis

• Phagocytosis (“cellular eating”)

• Pinocytosis (“cellular drinking”)

• Receptor-mediated endocytosis