Cell Membrane & Transport Across Membrane Flashcards
what are the five functions of biological membranes in cells?
definition of cell’s boundaries: separates interior of cell from surrounding environment, selectively permeable phospholipid bilayer allows only desirable substances entry
organisation and localisation: molecules or structures with specific functions embedded in membranes or localised within organelles (eg. hydrolytic enzymes in lysosomes), to organise and compartmentalise functions within cells
regulation of cell’s contents: proteins and other components help regulate transport of substances (to take up useful stuff, remove metabolic waste products, confine chemicals)
signal transduction: specific protein receptors on plasma membrane detect specific signals and triggering specific responses
cell-to-cell communication: membrane proteins for adhesion and communication btw adjacent cells
what is the fluid mosaic model of the cell membrane?
the membrane is viewed as:
a mosaic or collage of proteins randomly distributed in or loosely attached to
a fluid phospholipid bilayer which is free to move about laterally
what are the characteristics of the fluid mosaic model?
the fluid layer is asymmetrical, and may differ in composition and arrangement of proteins and lipids (phospholipids, glycolipids, cholesterol)
it is fluid or mobile, with possible lateral movement
unit membrane is a dynamic structure, with some embedded proteins moving freely due to weak hydrophobic interactions or fixed in positions by microfilaments on cytoplasmic face
membranes are amphipathic, made up of hydrophilic phosphate head and hydrophobic hydrocarbon tails (the hydrophobic core)
how did they prove the mosaic structure of the phospholipid bilayer?
use of electron microscopy and freeze fracturing with a knife, and the fracture plane follows the hydrophobic interior of a membrane, splitting the bilayer while membrane proteins go wholly with one layer
what is the structure of a phospholipid, and what role does it play in the cell membrane?
made of: a glycerol backbone with three hydroxyl (-OH) groups, two fatty acid chains, a negatively charged phosphate group, additional small charged molecules which may be linked to the phosphate group. amphipathic, since hydrophobic fatty acids as tail, hydrophilic head.
two layers of phospholipids form the phospholipid bilayer, so membrane has hydrophilic border and hydrophobic core (effective hydrophobic barrier against polar and charged solutes, around 8nm thick which is thicker than glycerol head)
what is membrane fluidity, and what enables it?
the ability of phospholipid molecules to move within membrane
membrane’s phospholipid molecules are held together primarily by hydrophobic interactions between the hydrophobic fatty acid tails (weak interactions), so molecules can move freely and laterally (in rare cases, they can flip-flop)
how does temperature affect membrane fluidity, and why?
directly related
at low temperatures, low KE and tighter packing of hydrocarbon chains, more hydrophobic interactions between phospholipid molecules, restricting their motion, semisolid
at high temperatures, KE and motion of hydrocarbon chains increases, more lateral movements of individual molecules to overcome hydrophobic interactions and increased space between molecules, fluid state
how does the length of phospholipid fatty acid chains affect membrane fluidity, and why?
inversely related
the longer the hydrocarbon chains, the more hydrophobic interactions between chains, so the higher the melting point
how does the degree of unsaturation affect membrane fluidity, and why?
directly related
as degree of unsaturation increases, there are less saturated lipids with long, straight hydrocarbon chains which allow for close packing and enhance membrane solidification
unsaturated lipids with kinks prevent the chains from packing closely together, enhancing membrane fluidity
how does cholesterol affect membrane stability, membrane fluidity, and membrane permeability?
stability: increases, fluidity: dual effects, permeability: decreases
[stability = fluidity + permeability]
cholesterol molecules are intercalated into lipid monolayers, so its rigid steroid ring interferes with the motions of the hydrocarbon chains of phospholipids, enhancing mechanical stability
at high temperatures, cholesterol restrains phospholipid movements by interfering with hydrocarbon chain motion, decreasing fluidity
at low temperatures, cholesterol prevents hydrocarbon chains from packing closely, increasing fluidity
cholesterol fills in spaces between hydrocarbon chains, plugging transient gaps and decreasing membrane permeability
compare the location, structure, solubility, and how they can be released from membranes for integral (intrinsic) and peripheral (extrinsic) proteins
location: integrals are deeply embedded (unilateral or transmembrane), peripheral are loosely bound (on both cytoplasmic and exterior sides)
structure: integral are amphipathic, held in place by extensive hydrophobic interactions with hydrocarbon tails of phospholipids. peripheral are rich in hydrophilic aa to interact with surrounding water and polar surface of bilayer
solubility in aqueous media: integral usually insoluble, peripheral usually soluble
integral released through detergents or non-polar solvents, peripheral released by mild treatment (eg. adjustment of ionic strength or pH)
state the six different functions of membrane proteins
anchorage, transport, enzymatic activity, signal transduction, cell-to-cell recognition, intercellular joining
how do membrane proteins function in anchorage?
they attach the cell membrane to other substances, stabilising position of cell membrane and maintaining cell shape
on inner cytoplasmic side, bound to microfilaments of cytoskeleton
on exterior side, attach the cell to fibres of extracellular matrix
how do membrane proteins function in transport? compare carrier and channel proteins
carrier proteins: bind solutes to its binding site and transport them across the membrane, involving conformational change of protein, may require energy in the form of ATP
channel proteins: some integral proteins have a water-filled central pore that acts as a HYDROPHILIC PORE, passageway that permits movement of hydrophilic, polar, charged particles down conc gradient across cell membrane
- leak channels permit the movement of substances at all time
- gated channels can open and close to regulate ion passage (eg. voltage-gated Na+ or K+ channels
how do membrane proteins function in enzymatic activity and signal transduction?
enzymes embedded in the membrane catalyse reactions in extracellular fluid or within the cytosol (depends on active site location)
proteins with very specific 3D conformations are ideal as receptor molecules for chemical signalling between cells (works by binding a ligand to receptor protein, triggering changes in cell). type and number of receptor proteins differ
how do membrane proteins function in cell-to-cell recognition and intercellular joining?
recognition proteins are usually glycoproteins, many possible shapes to the carbohydrate side chains, so each cell can have its own specific markers, allowing cells to recognise eo
membrane proteins of adjacent cells may adhere together in various kinds of intercellular junctions (gap or tight junctions)
what are the structural features of membrane carbohydrates, how are they bonded, and what can they form?
short, branched chains of fewer than 15 sugar units, covalently bonded to polar ends of phospholipid molecules in outer lipid layer (glycolipids) or covalently bonded to membrane proteins (glycoproteins)
what is the function of carbohydrates in the cell membrane?
maintaining the orientation of glycoproteins and glycolipids within membrane, since carbohydrates are highly hydrophilic and must be kept in contact with external aqueous environment
recognition components, involved in: sorting of cells into tissues binding extracellular signal molecules in antibody-antigen reactors intercellular adhesion to form tissues cell-to-cell recognition
what is transport across membranes important?
maintain suitable pH and ionic concentration for enzyme activity
obtain food supplies for energy and raw materials
excrete toxic substances
secrete useful substances
generate ionic gradients essential for nervous and muscular activity
compare the main differences between passive and active processes for transport across membranes (concentration gradient, energy requirements)
passive processes occur down / along conc gradient, active processes occur up / against a conc gradient
passive processes do not require cellular energy expenditure, active processes require cellular energy expenditure in the form of ATP
define diffusion
diffusion is the net movement of molecules from an area of higher concentration to a region of lower concentration, down a concentration gradient
(energy is involved in the form of intrinsic thermal energy, and diffusion continues until dynamic equilibrium is reached)
compare simple and facilitated diffusion
simple diffusion is for molecules with a small molecular weight and/or are hydrophobic and non-polar and uncharged, and can cross the bilayer(‘s hydrophobic core) directly
facilitated diffusion is for molecules with a large molecular weight and/or are hydrophilic and polar and charged, and need transport proteins to enhance / increase the rate of transport of the substance across the membrane
what are the factors that affect the rate of diffusion?
concentration gradient: the steeper, the faster
diffusion distance: the shorter, the faster
surface area: the larger, the faster
type of structure: presence of transient gaps in cell membrane enhances diffusion
size of molecule: the smaller, the faster
temperature: the higher, the faster
define osmosis
osmosis is the net movement of water molecules from a region of less negative water potential to a region of more negative water potential across a partially permeable membrane
