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Flashcards in Membranes Deck (14):

What are membranes?

Fluid bilayers of phospholipids studded with a Mosaic of proteins - provide boundaries in cellular organisation. Selective permeability allows chemical gradients to be maintain across membranes, and the membrane surface is often the location of complex reactions.


How are surface areas increased with membranes?

They are often highly folded, such as in the case of the cell membrane of certain prokaryotes. Eukaryotic cells are distinguished by the presence of membranes within the cell itself. These membranes separate regions of differing biochemistry, and allow for the specialist functioning of the various components of the endomembrane system - the endoplasmic reticula, Golgi apparatus, vesicles, vacuoles and microbodies. Some organelles, such as the nucleus, chloroplast and mitochodria, are bound by two membranes.


What is the phospholipid bilayer?

They are composed of glycerol, two fatty acids and a phosphate; other groups, such as choline, can be attached to the phosphate 'head'. The phosphate head is charged and therefore hydrophilic; lipid 'tails' are non-polar and therefore hydrophobic. In aqueous environments, hydrophobic interactions cause phospholipids to form a bilayer, with the hydrophobic tails In the middle of the bilayer away from water and hydrophilic heads in contact with aqueous solutions. The hydrophobic centre of the phospholipid bilayer forms a barrier to the passage of polar molecules and ions.


How does cholesterol control membrane fluidity?

The phospholipids in a bilayer are fluid and constantly change position. The fluidity of membranes is modified through the addition of cholesterol. Cholesterol has a hydrophilic hydroxyl 'head' (OH) and a rigid hydrophobic 'tail' (the four-ring steroid structure and non-polar side chain). The structure of cholesterol reduces membrane fluidity while also preventing lipid crystallisation at low temperatures.


What is a peripheral (or extrinsic) protein?

Are those are easily removed from membranes in the laboratory using ionic washes. Peripheral proteins are only held in place at the surface of the membrane by charged or polar amino acid R groups or by a small number of hydrophobic interactions.


What are integral (or intrinsic) proteins?

Are those that, in the laboratory, cannot be washed from the membrane. The integral proteins are held firmly in place within the membrane by strong hydrophobic interactions with lipid tails. Integral proteins are either transmembrane (spanning the membrane and held in place by hydrophobic alpha-helices), or they are embedded in one side of the bilayer only.


Why do some proteins and phospholipids have carbohydrate chains added to them?

So they become glycoproteins and glycolipids. The carbohydrate portions of these molecules are on the outside of the membrane.


What are the functions of membrane proteins?

Passive / active transport proteins, enzymes, receptor/ attachments proteins and cell-cell recognition.


What are passive transport proteins?

Are transmembrane proteins that transport molecules across membranes down a concentration gradient. Channel proteins provide a pore that can facilitate or speed up diffusion. Each channel protein is specific for one particular ion or molecule; for example, aquaporin facilitates the diffusion of water across membranes. Transport can also be achieved by carrier proteins which bind to a specific molecule to allow its passage. Some of these transport proteins are 'gated' - binding of one molecule is required to allow the passage of another.


What are active transport proteins?

They pump ions and molecules against the concentration gradient. The hydrolysis of ATP usually provides the energy source for the phosphorylation and conformational change of protein pumpss. The sodium-potassium pump is a good example.


What are enzymes in membrane proteins?

They allow the location of catalysis to be carefully controlled within a cell.


What are receptor proteins?

Hydrophilic signalling molecules are unable to cross the hydrophobic region of the membrane. Specific receptors proteins are found in the plasma membrane of the relevant target cells. When signalled, these transmembrane receptor proteins stimulate a response within the cell, such as the phosphorylation of a key enzyme.


What are attachment proteins?

They are proteins that provide cytoskeleton attachment points within the membrane for the structural support of the cell. Other proteins attach to the extra-cellular matrix to hold the cell in place. In multicellular organism, proteins form intracellular junctions that provide anchorage to other cells and hold cells together in tissues.


What is the function of cell-cell recognition?

In multicellular organisms, cell-cell recognition is achieved by membrane glycoproteins. For example, the ABO blood grouping system in humans is based on different carbohydrate chains presented by the glycoproteins on the membrane suraces of the RBCs.