Biological membranes Flashcards
selectively permeable membranes
able to control the movement of substances into and out of cells, regulating the composition of fluid within individual cells, provides a barrier between the inside and outside of a cell, control the flow of information between cells either by recognizing signal molecules received from other cells, or by sending chemical or electrical signals to other cells, made of 3 major components: lipids, proteins and carbohydrates
lipids, proteins and carbohydrates structure and bonds
have a common general structure - two layered sheets of lipid molecules which have proteins embedded in them, lipid and protein molecules can move about on the plane of the membrane, they are held together by non-covalent interactions, carbohydrates are attached by covalent bonds to some of the lipid and protein molecules
phospholipids
made up of two fatty acid chains joined to a glycerol molecule and a polar phosphate containing head, the head is hydrophilic and the tails are hydrophobic, the heads are polar and tails are non-polar, makes membrane fluid and permits the passage of lipid soluble molecules
cholesterol
cholesterol is a steroid, majority of molecule is hydrophobic and so is attracted to the fatty acids of the phospholipids in the membrane, OH group is hydrophilic and so orientates itself nearer the hydrophilic phosphate heads, it’s role is to regulate the fluidity of the membrane
glycolipids
glycolipids have a sugar such as glucose or galactose instead of a phosphate containing head, glycolipids are found on the outer surface of the plasma membrane with their sugar exposed at the cell surface, recognition sites for substances such as insulin and act as antigens
glycolipids and phospholipids similarities
both have a hydrophilic head and two hydrophobic tails, if submerged in water, these molecules will spontaneously associate to form bilayers, with the tails on the inside and the heads facing outwards
why are bilayers formed?
bilayers formed because the tails have a strong tendency to stay away from water and so are squeezed together by water molecules, some bilayers will close in on themselves to form sealed compartments called liposomes which eliminates the edges where tails would be in contact with water
membrane proteins
some proteins extend across the phospholipid bilayer and are often referred to as intrinsic, integral or transmembrane proteins, other proteins are only partly embedded in the membrane and are referred to as extrinsic or peripheral proteins, certain substances will need to be allowed into and out of the membrane and this is what membranes do
other functions include:
- provide channels for passive transport of materials
- act as active transport pumps to move substances
- act as hormone binding sites (hormone receptors)
- are immobilised enzymes
- help with cell adhesion forming junctions between neighbouring cells
cell signalling
cells communicate with each other through chemical messengers such as hormones, these molecules will be produced by one cell and will travel to other cells in the body, these molecules will only affect some cells - will have an appropriate cell membrane receptor, the receptors will detect the messenger molecule and changes will occur in the cell, receptors in the plasma membrane that are involved in cell signalling are usually glycoproteins but glycolipids may also be involved, a cell with a receptor molecule is referred to as a target cell
what happens when receptors are activated by a messenger molecule?
- may cause the release of a second messenger in the cell which brings out a series of changes
- may result in the opening of a protein channel that was previously closed allowing specific molecules to enter and exit the cell
- may activate an enzyme in the cell or the receptor may be an enzyme itself
what is the thickness of a cell membrane?
7nm - 10nm
intrinsic proteins
form channels or carrier proteins for water soluble molecules
extrinsic proteins
act as enzymes
glycoproteins
helps with cell to cell adhesion
diffusion
few substances can directly diffuse throw lipid bilayer and the only molecules that can do this are lipid-soluble or non-polar molecules or very small polar molecules or uncharged molecules, it is a passive process meaning there is no energy involved and substances can only move down the concentration gradient, net movement of a substance from concentration of high to low
facilitated diffusion
transport of substances across the membrane by a trans-membrane protein, the transport proteins are specific to one molecule so substances can only cross the membrane when there is an appropriate protein, also a passive process so no energy involved and substances can only move down the concentration gradient
what are the two types of transport proteins?
channel proteins - form a channel in the membrane which will allow charged substances (ions) to diffuse across membranes, most channels can be gated which will allow the cell to control the entry and exit of ions
carrier proteins - used for larger molecules, have a binding site for a specific solute, constantly flips between two states so that the site is alternately open to opposite sides of the membrane, substance will bind to the side where it is at high concentration and be released where it is at low concentration
active transport
the pumping of substances across a membrane by a trans-membrane carrier protein molecule which are highly specific meaning there is a different carrier for each molecule to be transported, the protein carriers are also ATPase enzymes which catalyse ATP => ADP + phosphate and the energy is released to change shape and pump the molecule across the membrane, it is an active process, it is the only transport mechanism that can transport substances up/against the concentration gradient
endocytosis
process by which cells absorb molecules by engulfing them, there are two forms of endocytosis: phagocytosis and pinocytosis
it is an active process and requires ATP
pinocytosis
process by which small particles and fluids are taken in, plasma membrane invaginates inwards and then the membranes fuse around the molecule to form a vesicle
phagocytosis
protrusions called pseudopodia extend from the cell and wrap themselves around a large molecule (a phagocyte engulfing a pathogen)
exocytosis
the process whereby substances are secreted from a cell, it is an active process and requires ATP
osmosis
describes movement of water across the membrane, net movement of water molecules from an area of low concentration to an area of low concentration of a substance to an area of high concentration of a substance
water potential
the tendency of water to diffuse from one area to another, used to describe the concentration of water
aquaporins
special protein channels which allow water molecules through
turgid
when a cell is swollen and hard
plasmolysed
process in which cells lose water in a hypertonic solution
