membrane structure and transport Flashcards
explain why charged ions and polar molecules are unable to pass through cell membrane (3)
- non polar hydrophobic fatty acid tails make up the hydrophobic core of the cell membrance
- membrane is impermeable to charged ions and polar molecules, as they are repelled by the hydrophobic core of the cell membrane
- therefore, it require channel/carrier proteins which allow facilitated diffusion
explain why membrane is called fluid mosaic
- “fluid” means that the phospholipids and proteins are free to move within the membrane
- the non-polar fatty acid tails of phospholipids allow phospholipids to be held together by weak hydrophobic interactions. thus they can move laterally by diffusion within their own layer
- most protein molecules are embeddded in the phospholipid bilayer in a random manner
describe the different functions of proteins in cell surface membranes (7)
- act as enzymes to catalyse reactions
- act as receptors that have a specific binding site with a specific shape that is complementary to the shape of a chemical messenger (e.g. a hormone)
- act as receptors which detect specific signals on the extracellular surface of the cell and relay the message to the inside of the cell, triggering a specific cellular response within the cell
- as transport proteins such as channel proteins / carrier proteins that help to regulate the movement of polar molecules or charged ions across membranes, via facilitated diffusion or active transport
- involved in cell-cell recognition where the carbohydrate chain of glycoproteins serves as markers, that distinguishes one cell from another
- involved in cell-cell adhesion, binding cells together into tissues ;
- can attach to the cytoskeleton and extracellular matrix (ECM) and coordinate extracellular and intracellular changes
describe the different functions of glycolipids and glycoproteins (3)
- are involved in cell-cell recognition where the carbohydrate chain functions as a marker that distinguishes one cell from another
- are involved in cell-cell adhesion by binding cells together into tissues
- act as receptor sites for chemical signals such as hormones
describe the different functions of cholesterol (3)
- maintains membrane fluidity / increases the stability and flexibility of membranes
- by restraining the movement of phospholipids at relatively warm temperatures to decrease fluidity / hindering the close packing of phospholipids at low temperatures to increase fluidity
- acts like a plug, reducing the escape or entry of charged ions and small polar
molecules through the membrane
describe the functions of cell surface membrane
(10)
(OUTSIDE OF CELL)
1. Compartmentalisation – the cell surface membrane separates the cytoplasm from the extracellular fluid, thus keeping desirable substances within the cell and undesirable substances out ;
2. Signal transduction (cell signalling) – specific signals on the extracellular surface of the cell are detected and relayed to the inside of the cell, triggering a specific cellular response within the cell
(WITHIN CELL)
1. Compartmentalisation – membranes surrounding cell organelles enable separate compartments to be formed inside the cell, thereby allowing specialised metabolic pathways to take place
2. (Providing optimal conditions) Hydrolytic enzymes in lysosomes require lower pH from the rest of the cell to function optimally
3. (Allowing formation of ion gradient) The outer and inner mitochondrial membrane enclose the intermembrane space in mitochondria to allow the formation of a proton gradient which is used for the chemiosmotic synthesis of ATP
4. (Allowing different processes & functions) Different reactions could take place e.g. Krebs cycle in the mitochondrial matrix / Calvin cycle in the stroma of chloroplast, as reactants and enzymes of a metabolic pathway could be kept together
5. Selective barrier – the hydrophobic core of membranes acts as a selective barrier, restricting the movement of charged ions or polar molecules across the membrane into the organelles
6. Transport – transport proteins embedded in membranes regulate the movement of substances into and out of the organelles
7. Membranes are the site where enzymes may be embedded and ordered as a team that carries out sequential steps of a metabolic pathway
8. Membranes allow for transport proteins/enzymes/receptors to be embedded, leading to an increase in surface area for metabolic reactions to occur efficiently
why is transport across membranes important for various reasons (5)
- to obtain nutrients
- to excrete waste substances/toxic substances
- to secrete useful substances
- to generate the ionic gradient essential for nervous and muscular activity
- to maintain a suitable ph and ionic concentration within the cell for enzyme activity
definition of diffusion
diffusion is the net movement of molecules from a region of higher concentration of that molecule to a region of lower concentration of the same molecule down a concentration gradient
diffusion is a ____ process which _______ energy and continues until a dynamic _______ is reached
diffusion is a passive process which does not require energy and continues until a dynamic equilibrium is reached
membranes are most permeable to _____________ molecules
membranes are most permeable to hydrophobic non-polar and uncharged molecules
which molecules diffs
use faster? fat-soluble molecules or water-soluble molecules
fat-soluble molecules
definition of osmosis
it is the net movement of water molecules from a region of higher water potential to a region of lower water molecules across a selectively permeable membrane
osmosis is a ____ process which _______ energy
osmosis is a passive process which does not require energy
definition of facilitated diffusion
it is the passive movement of polar molecules or charged ions from a region of higher concentration of that molecule/ion to region of a lower concentration of the same molecule/ion down a concentration gradient, across a selectively permeable membrane via the aid of specific transport proteins
how does channel protein allow water-soluble molecules to pass through
amino acid residues with polar or charged R groups make up the inner lining of the hydrophobic channel, allowing water soluble molecules to pass through it easily
