transport across cell membranes Flashcards
what are the 4 functions of cell membranes ( plasma membranes)
1) control the movement of substances into and out of cells
2) to keep the cells contents together and seperate from other cells
3) to allow communication between cells
4) membranes allow cellular compartments to have different conditions
describe the cell membrane
cell membranes are complex structures made up of two layers of lipids called a phospholipid bilayer. the phospholipid bilayer consists of phospholipids , cholesterol, glycolipids , glycoproteins and proteins scattered throughout
what is the head of the phospholipid
hydrophillic head - polar and soluble
what is the tail of the phospholipid
hydrophobic , non polar and insoluble
explain why phospholipids form a bilayer in plasma membrane
phospholipids have a phosphate group which are hydrophillic and will face the solution . the fatty acid tails which are hydrophobic will move away from aqeuous enviroment . phospholipids will form two layers with the hydrophobic fatty acid tails facing inwards and phosphate groups outward towards the solution
what is the cell surface membrane
the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the enviroment
what are the functions of phospholipids in the plasma membrane
allow lipid - soluble substances to enter and leave the cell
prevent water soluble substances leaving and entering the cell
make the membrane flexible and self healing.
what is the fluid mosaic model
fluid - moving , phospholipids can move around within each layer so membrane is flexible
mosaic - made of pieces eg proteins
where is the model from
the agreed structure is based upon experimental and chemical evidence and so is classed as a model
what are the two types of proteins in the membrane
extrinsic or intrinsic
what are the functions of the proteins in the cell membrane
provide structural support , act as channels supporting water soluble substances across the membrane , allow active transport across the membrane through carrier proteins , form cell- surface receptors for identifying cells , help cells adhere together , act as receptors for example for hormones
what does the cholesterol do
cholesterol molecules occur within the phospholipid bilayer of the cell - surface membrane . they add strength to the membranes . cholesterol molecules are very hydrophobic and therefore play an important role in preventing loss of water and dissolved ions from the cell . furthermore cholesterol pull together the fatty acid tails limiting their movement without making the membranes as a whole too rigid
describe glycolipids
glycolipids are made up of a carbohydrate covalently bonded with a lipid . it extends from the bilayer into the watery enviroment where it acts as a cell surface receptor for specific chemicals their functions are too
act as recognition sites
help maintain the stability of the membrane
help cells attach to one another and so form tissues
describe glycoprotiens
carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane . these glycoproteins also act as cell - surface receptors more specifically for hormones and neutrotransmitters .
the function of glycoproteins are too
act as recognition sites
help cells to attach to one another and so form tissues
allows cells to recognise one another , for example lymphocytes can recognise organisms own cells
in general why do most molecules not freely diffuse across the membrane
because many are
not soluble in lipids and therefore cannot pass through the phospholipid bilayer
too large to pass through the channels in the membrane
of the same charge as the charge on the protein channels and so even if they are small enough to pass through , they are repelled