membranes Flashcards
LOs (j) explain the fluid mosaic model and the roles of the constituent biomolecules (including phospholipids, proteins, glycolipids, glycoproteins and cholesterol) in cell membranes (k) outline the functions of membranes at the surface of cells and membranes within the cell (l) explain how and why different substances move across membranes through simple diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis
define fluid mosaic model
fluid: membrane is a dynamic structure where phospholipids and proteins are able to move
mosaic: random arrangement of proteins which are embedded in phospholipid bilayer
what does a membrane consist of
phospholipid bilayer
cholesterol
proteins
glycoproteins
glycolipids
function of p/lipid bilayer
function of cholesterol
Cholesterol regulates membrane fluidity*
1. preventing the membrane from being
overly fluid at warmer temperatures as cholesterol’s rigidity restricts phospholipids’
lateral movement;
2. prevents membrane from being overly firm at lower temperatures = cholesterol
prevents the close packing of phospholipids and hence prevents its solidification/
crystallization;
how are proteins held in membranes
hydrophobic interactions formed between nonpolar hydrophobic hydrocarbon tails of bilayer and non polar R groups of amino acids on exterior surface of proteins
AND
interactions between the charged phosphate head of phospholipid bilayer and charged/polar R groups of amino acids on exterior surface of proteins
function of proteins in membranes
- function as channels/carriers for facilitated diffusion or active transport
- function as enzymes
- function as receptor proteins
- function to stabilise membrane structure: proteins are non-covalently bound to cytoskeleton on cytoplasmic side and extracellular matrix on extracellular side
how do channel proteins work
have a hydrophilic pore/channel for direct diffusion of ions (charged molecules) across the membrane from a high to low solute concentration
how do carrier proteins work
bind to the solute on one side of the membrane (binding site) = protein undergoes conformational change = solute is released and able to access opposite side of membrane
direction of flow is dependent on concentration of solutes across the membrane (bidirectional)
functions of glycoproteins and glycolipids
- function as markers/recognition sites in cell-cell recognition and adhesion
- function as receptors
location: always projecting out onto extracellular matrix
functions of membranes
- regulate movement of substances: membranes act as selectively permeable barriers/boundaries between in and out the cell, between organelle and cytoplasm, and between compartments within an organelle
2a. allow for compartmentalisation which allows: unique enviroments for highly specialised activities, spatial separation of biochemical processes and their sequential operation within a cell, and accumulation of ions to high concentrations - act as a surface for chemical reactions to occur in a sequential mamner (functionally realted proteins are grouped together on the membrane)
- increase surface area for reactions
(refer to page 12 for notes for point 5 and 6)
define simple diffusion
net movement of molecules/ions from a region of high concentation to region of low concenration down a concentration gradient
no ATP nor transport protein required
define facilitated diffusion
net movement of molecules/ions from a region of high concentation to region of low concenration down a concentration gradient through a transport protein
no ATP nor transport protein required
how does a charged/polar molecule pass through a membrane
- polar and charged molecules are hydrophilic
- The hydrophobic core* of the phospholipid bilayer would repel/impermeable to these hydrophilic solutes;
- transmembrane transport proteins can be a channel or carrier that provides a
hydrophilic* channel/pore through the membrane for the passage of the solutes by faciliated diffusion - because a transport protein is specific* to its own solute, different transport proteins are
needed for different solutes;
define active transport
energy-consuming transport of ,olecules/ions across a membrane through transmembrane carrier proteins called pumps againist a concentration gradient
ATP required
one direction only
how does a pump work (tutorial qn 3bii)
- sodium from cytoplasm binds to binding site of sodium potassium pump (carrier protein)
- binding of sodium stimulates phosphorylation by ATP (adding phosphate group)
- phosphorylation causes conformational change in pump = expels sodium to opposite sid
- 2 extracellular potassium binds to protein, triggering release of phosphate group
- pump returns to original conformation releasing potassium into cell (moves from outside to inside cell)
- binding aite for sodium become receptive again
define osmosis
net movement of water molecules from a region of high water potential to a region of low water potential down a water potential gradient through a selectively permeable membrane
no ATP not transport protein required
movement is down a gradient
describe how water can pass through a membrane by osmosis
- phospholipids are fluid = membrane laterally shifts
- creates transient pores in the membrane
- water is a small and polar moecule
- water passes through the membrane through the transient pores
describe how water can pass through a membrane by facilitated diffusion
- water is a polar molecule = will be repelled by the hydrophobic core of the phospholipid bilayer
- exterior of aquaporin channels are made up of amino acids with hydrophobic R
groups that are able to interact with the hydrophobic core region of the phospholipid
bilayer = penetrate the length of the membrane - interior of aquaphorin channels lined with amino acids with hydrophilic R groups = creates a hydrophilic pore
- hydrophilic pore allows passage of polar water molecules to other side of membrane
define bulk transport
is an active process as ATP is used to mobilise the movement of the membranes
not considered active transport: does not involve transporting molecules by a transmembrane carrier protein – no changing in conformation of a carrier protein in the membrane unlike active transport
describe exocytosis
secretion of macromolecules to the exterior of the cell by fusion of vesicle membrane with the plasma membrane
describe endocytosis (phagocytosis)
- phagocytosis is for the intake of macromolecules
- pseudopodia* are formed and
extended outwards to engulf the large insoluble macromolecule/solids; - the ends of the pseudopodia fuse = forms a vesicle / vacuole containing the solid matter
- vesicle/vacuole pinches off and enters into the cytoplasm;
describe endocytosis (pinocytosis)
- pinocytosis is for the uptake of substances that are non-specific and in liquid form
- a small area
of the plasma membrane invaginates - forms tiny vesicles containing liquid from the surrounding = released into cytoplasm of cell
describe receptor medicated endocytosis
- function; For transport to acquire specific molecules in large quantity against concentration gradient (may not be in very high concentration in extracellular fluid)
2.protein receptors are embedded in membranes that are exposed to the extracellular fluid - extracellular substances that bind to the recptors = ligands
- receptor mediated* endocytosis occurs when specific ligands bind to receptor proteins on the
membrane = membrane invaginates - membrane has coat proteins = help to deepen the pit to form vesicles
- forms vesicles that are released into the cytosol
what is ATP used for in bulk transport
energy from ATP is used to extend the membranes around the materials to: form vesicles that are needed, move vesicles within the cell or invagination of cell membrane
how is the thickness of the membrane maintained by different secretion processes
- exocytosis involves removal of part of CSM when secretory vesicles pinch off from CSM
- secretion of substances in the cell involves fusing of the transport vesicle membrane with CSM which adds to CSM
- rate of removal of CSM is same as the rate of addition of the CSM = CSM area is kept constant
how do membrane proteins get embedded into plasma membrane
- membrane proteins are synthesised in rough endoplasmic reticulum
- the proteins are glycosylated in the cisternal soace = forms glycoproteins
- glycoproteins get packed into vesicles = sent to golgi apparatus = fuse with cis face of GA = glycoproteins are released
- glycoproteins undergo futherr carbohydrate modification and lipids are glycosylated into glycolipids in the GA
- the membrane glycoproteins and glycolipids are transported in lipids to the plasma membrane
- vesicles fuse with membrane = membrane glycolipids and glycoproteins are embedded on the outside of the plasma membrane
define endocytosis
(in general)
uptake of substances into cell/organelle by infolding or extension of cell surface membrane to form a vesicle or vacuole = allows cell to acquire macromolecules and particulate matter respectively
how are transient pores formed
qn 10ci of tut
- phospholipid bilayer contains unsaturated hydrocarbon chains are bent/have kinks;
- they prevent the close packing of phospholipids in the membrane = allow the
phospholipids to move more easily in the membrane; - resulting in fewer hydrophobic interactions/van der Waals forces compared to
- when the bilayer is more fluid, forms transient pores
