biological membranes

what are the functions of membranes?

• keep cellular components inside the cell

• allow select molecules in and out of the cell

• site for biochemical reactions

• allow cell to change shape

• isolate organelles to allow cellular processes to occur separately

what does the fluid mosaic model suggest?

proteins are found within the phospholipid bilayer

structure of phospholipids :

phosphate group polar hydrophilic head with two fatty acid chains as non polar hydrophobic tail

what are the two types of phospholipid structures?

micelle (vesicle) and bilayer (membrane)

what is the role of phospholipid bilayers?

act as a barrier to control what enters and exists the cell

how do molecules pass through membranes?

• small non polar molecules like oxygen rapidly diffuse

• small polar molecules like water diffuse slowly

• charged particle ions are unlikely to cross a membrane

what are intrinsic proteins?

transmembrane proteins that embedded in both layers of the membrane with hydrophobic amino acids holding them in place

channel proteins :

hydrophilic channel allowing passive movement across membrane down the concentration gradient

carrier proteins :

allow passive and active transport involving the protein shape changing

glycoproteins :

intrinsic proteins with an attached carbohydrate involved in cell adhesion and signalling

glycolipids :

lipids attached with a carbohydrate chain acting as antigens

extrinsic proteins :

proteins on the extracellular side acting as receptors involved in cell recognition which can dissociate into the cytoplasm

structure of cholesterol :

type of lipid with molecular formula C27H46O

function of cholesterol :

regulates membrane fluidity meaning more cholesterol means less fluid and less permeable

why does a membrane need to be fluid?

• for diffusion across membrane

• for membranes to fuse

• to move and change shape e.g. during phagocytosis

effect of heat on phospholipid bilayer :

as temperature increases, kinetic energy increases creating gaps therefore the permeability increases

effect of solvents on phospholipid bilayer :

water is a polar molecule important in maintaining membrane stability but non polar substances like alcohol can disrupt this

what happens when the phospholipid bilayer is heated above 45c?

bilayer starts to melt and proteins denature increasing permeability

rate of diffusion is proportional to :

surface area x difference in concentration / length of diffusion path (membrane)

what is facilitated diffusion?

polar or large molecules cannot pass through the bilayer therefore carrier and channel proteins allow this although it is still passive

how do slightly polar molecules pass through the membrane?

they diffuse through hydrophilic channels in channel proteins

what is the process of carrier protein transport?

• high concentration of glucose inside the cell

• carrier proteins take in molecules from outside the cell

• glucose molecules bind to carrier

• ATP binds to inside the cell

• binding of glucose molecules change the shape of the protein

• carrier protein changes shape allowing molecules through

examples of active transport :

• uptake of amino acids and glucose in small intestine

• mineral ions in plant roots

• extortion of hydrogen ions and urea

what is bulk transport?

movement of large substances through endocytosis or exocytosis

endocytosis :

• phagocytosis

• pinocytosis

• receptor mediated endocytosis

• bulk transport into the cell

exocytosis :

bulk transport out of the cell

what is the highest water potential?

pure water at 0kPa

why do solutes decrease water potential?

because water molecules bind to solute molecules therefore less water molecules are free to diffuse

water potential =

solute potential + pressure potential

what does high osmotic concentration mean?

a high concentration of dissolved solutes resulting in low water potential

what does low osmotic pressure mean?

dilute concentration of dissolved solutes resulting in high water potential

isotonic solutions :

• solutions have equal osmotic pressure than cytoplasm

• equal solute concentrations

• no effect on cells

hypotonic solutions :

• solutions have lower osmotic pressures than cytoplasm

• low solute concentration

• causes cells to swell

hypertonic solutions :

• solutions have higher osmotic pressure than cytoplasm

• higher solute concentration

• causes cells to shrink