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Membrane Structure and Function
Membranes
 are of fundamental importance to life
 have selective permeability - some substances cross the membrane more easily than others
 the hydrophobic region allows passage of non-polar molecules [O2 , CO2
(Note: carbon dioxide is not polar because of the linear arrangement of the two C=O double bonds)] and restricts passage of polar molecules and ions
(i.e., hydrophilic compounds).
 Water is polar and consequently diffuses very slowly across the membrane despite its small size; other hydrophilic molecules avoid contact with the hydrophobic lipid bilayer molecules by interacting with transport proteins.
These proteins are very specific and selective in what they will transport.
I. Membrane Structure
 8 nm thick (recall resolving power of microscopes)
 first seen with an electron microscope in the 1950s
Fluid mosaic model
 currently accepted membrane model
 modification of the original Davson-Danielli sandwich (1935) by S.J. Singer and
G. Nicolson (1972)
 Membrane components are arranged in a phospholipid bilayer with proteins embedded in or associated with embedded proteins.
 This model is supported by electron microscopy (freeze fracture experiments)
The membrane structure
 is dynamic and changing
 varies in composition - mitochondrial membranes have a higher protein content than the plasma membrane
 inner and outer surfaces are distinct and vary in composition - asymmetrical
bifacial quality is determined by the ER
 Must remain fluid to function - if it solidifies then the permeability changes. This affects membrane function as well as the activities of protein associated with the membrane
Factors contributing to fluidity
 weak hydrophobic interactions between each of the layers in the lipid bilayer
 the presence of unstaturated fatty acids in the phospholipids

 cholesterol (reduces fluidity at warm temperatures by restraining phospholipid movement; increases fluidity at lower temperatures by preventing tight packing of phospholipids)
 movement of proteins (more restricted – in some cases membrane proteins are immobilized due to attachments to cytoskeleton)
II. Membrane Components
1. Lipids
 basic fabric of the membrane – phospholipids are amphipathic molecules that contain both hydrophobic and hydrophilic portions
 phospholipids and other lipids (e.g., cholesterol) stabilizes membrane fluidity by making it less fluid at higher temperatures by restraining movement of phospholipids but more fluid at lower temperatures because cholesterol prevents the tight packing of phospholipids hydrophobic tails
2. Proteins
 proteins (greater than fifty proteins are found in red blood cells) determine
the specific membrane functions
 integral proteins - completely (transmembrane) or partially span lipid bilayer - hydrophobic regions are surrounded by hydrophobic tails and hydrophilic regions exposed to aqueous cytoplasm or environment
 peripheral proteins - associated with membrane surface e.g., fibronectin
Functions
 Transport
 Enzymatic activity
 Receptors – signal transduction
 Intracellular junctions
 Cell to cell recognition - sorting of cells - tissue formation, foreign cells - immune system
 Attachment to cytoskeleton and ECM
3. Carbohydrates
 branched oligosaccharides – usually < 15 monomers
 covalently bonded to lipids (glycolipids) or proteins (glycoproteins)
 highly variable in composition - varies from species to species
 important in cell to cell recognition – (e.g., blood type - A, B, AB, O is due to variations in oligosaccharides on the red blood cell surface)
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III Mechanisms of Membrane Transport
1 . Passive Transport - diffusion of a substance across a biological membrane a) Diffusion
 Diffusion is movement from an area of high concentration to low concentration
 Any substance will diffuse down its own concentration gradient irrespective of other substances
 A concentration gradient represents potential energy  drives diffusion so that there is a net movement from an area of high concentration to that of low concentration. Diffusion results from kinetic energy (thermal motion in this case). The system moves towards increasing entropy.
 Diffusion is a spontaneous process that decreases the free energy of the system, which will eventually reach a dynamic equilibrium.
 Many substances move across the membrane by diffusion e.g., There is an O2 gradient in respiring cells. The consumption of O2 maintains the gradient. b) Osmosis
 Osmosis is a form of passive transport defined by the movement of water across a selectively permeable membrane
 Water moves from an area of higher free water concentration (i.e., lower solute concentration) to an area of lower free water concentration (i.e., higher solute concentration)
 Movement of water across a membrane is from the hypotonic solution (lower solute concentration) to the hypertonic solution (higher solute concentration).
 An equilibrium is established when solutions on both sides of the membrane are of equal solute concentrations or isotonic. c) Water Balance in Cells i) Cells without walls
 can't tolerate excess shrinkage or swelling due to loss or gain of water and therefore they live in isotonic environments
 osmoregulation – control of water balance. Many organisms without rigid cell walls have modified membranes with reduced water permeability and organelles, such as the contractile vacuole in Paramecium, are used to pump water out of the cell.
 ii) Cells with cell walls (e.g., prokaryotes, plants, fungi and some protists)
 Hypertonic solutions - plasma membrane shrinks away from the cell wall resulting in a condition called plasmolysis, which is usually lethal.
 Hypotonic solutions - wall functions in water balance - permits only limited swelling

d). Facilitated Diffusion
 Transport proteins aid the movement of specific polar and ionic substances across the membrane.
 Some transport proteins form channels (channel proteins, e.g., aquaporins) that facilitate the passage through the membrane
 Other transport proteins (carrier proteins) undergo conformational changes
(i.e., changing shape) in order to shuttle molecules across membranes
 Movement is still down a concentration gradient but is more efficient than diffusion
Transport proteins
 Are very specific (i.e., like enzymes) and have binding sites akin to an enzyme’s active site. These sites can be saturated – There is a limited number of transport proteins in a membrane
 can be inhibited by mimics
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 function by either i) producing a specific corridor or channel (channel proteins) or ii) undergoing a conformational change that somehow facilitates transport
(carrier proteins). Some channels are gated and open or close according to a stimulus.
2. Active Transport
 Cellular energy (i.e., ATP) is expended to move substances across the membrane against their concentration gradient.
 Allows maintenance of concentration gradients of certain small molecules.
 Specific carrier proteins embedded in the membrane are involved in this process a) Electrogenic Pumps
 Animal cells maintain steep gradients of K+ (higher inside cell) and Na+ (higher outside cell) with the aid of a sodium – potassium pump.
 ATP is used to phosphorylate a transport protein inducing conformational changes that facilitate translocation of ions across the membrane.
 The sodium – potassium pump transports 3 Na + out of the cell and two K + into the cell. A membrane potential is established as a consequence of the unequal distribution of ions.
Membrane potential is a voltage across a membrane resulting from unequal charge distribution. This ranges from -50 to -200 mV in cells (a negative membrane potential means that the inside of the cell is negative relative to the external environment). This favours the passive transport of cations into the cell and anions out of the cell.

Ion gradients are also known as electrochemical gradients
There are two forces driving the movement of ions across the membrane. i) electrical force - membrane potential ii) chemical force - concentration gradient
 a transport protein that generates a membrane potential is called an electrogenic pump. The sodium-potassium pump is the major electrogenic pump in animal cells. The main electrogenic pump in bacteria, plants and fungi is the
Proton pump which pumps H + out of the cells.
 Electrogenic pumps store energy that can be tapped for cellular work. b) Cotransport
 energy stored in a gradient created through an active transport pump can be coupled to facilitate the movement of a second molecule in the direction of the electrochemical gradient.
 A second protein couples the energy in the electrochemical gradient to move a second substance against its concentration gradient e.g., plant cells couple the energy stored in a proton gradient to transport amino acids, sugars and other nutrients against their concentration gradients c) Transport of large molecules exocytosis - secretion of large molecules by vesicles fusing with the membrane endocytosis - cell brings in macromolecules and particulate matter by forming vesicles from the plasma membrane. There are three types of endocytosis
i) phagocytosis (cellular eating) - cell develops pseudopodia which surround and engulf particles in a membrane bound sac large enough to be called a vacuole - fuses with lysosomes
ii) pinocytosis (cellular drinking) - gulps droplets of extracellular fluid - unspecific
iii) receptor mediated endocytosis – very specific receptors bind extracellular substances called ligands
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