E3 Introduction to cell membrane structure and function Flashcards
what does amphipathic mean?
comprising hydrophilic and hydrophobic regions
what are cytoplasmic membranes formed by?
- lipid bilayers
- the bilayer is made up from 2 leaflets
what are the 3 major types of membrane lipids?
- glycerophospholipids
- sphingolipids
- sterols
describe the structure of glycerophospholipids
- phosphorylated head group, 3-carbon glycerol backbone and 2 hydrocarbon fatty acid chains / tails
- amphipathic
- various derivatives present in varying amounts in membranes of all cells
- fatty acids are around 20 carbons long
what do the polar group of a glycerophospholipid include?
choline or serine
what can the fatty acid tails in glycerophospholipids be?
- saturated or unsaturated
what does it mean for a fatty acid to be unsaturated?
- 1 or more carbon double bond within the chain
- creates kinks in the tail and lessens the melting and boiling points as they can’t pack as close together
describe the structure of sphingolipids
- phosphorylated head group, sphingosine backbone and 2 hydrocarbon fatty acid chains (1 of the fatty acid chains is contributed by sphingosine)
- amphipathic
- larger than glycerophospholipids
what does the polar group of a sphingolipid include / what can it be?
includes choline or can be a sugar
where are sphingolipids most abundant?
myelin sheath surrounding nerve cells
describe sterols in different organisms
- cholesterol in animals
- ergosterol in fungi
- hopanoids in bacteria
what does more cholesterol do to cell membranes?
more rigid / stiff
what feature of cell membranes can be associated with cell signalling and transport across mmebranes?
clustering of lipid molecules to give domains or rafts
what can the composition of a membrane influence?
- thickness, curvature and lateral freedom of movement of lipids in the bilayer
- i.e. the fluidity
why are membranes self-sealing?
- because they are dynamic structures
- can self-seal due to amphipathic properties (ensure cell remains in tact - not damaged or killed)
what properties are the self-sealing property of membranes important for?
- cell division
- endocytosis and exocytosis
what can cell membranes help to regulate and maintain?
- pH and ionic concentration
- cell volume
- ion gradients for excitable tissues (muscles and nerves)
- concentration of metabolites and extrusion of waste substances and toxins
state some functions of membrane proteins
- channels
- pores
- transporters
- receptors
- adhesion molecules
describe the functions that channels, pores and transporters can perform
- passive transport
- diffusion
- facilitated diffusion
- active transport
what do receptor and adhesion molecules allow for in membranes?
- bind extracellular molecules without necessarily transporting them across the membrane
- allow cells to sense their environment and to adhere to their tissue
what main feature do membranes have?
selective permeability
what molecules can pass through the bilayer unaided?
- urea
- water
- gases
(however, rate of water movement may be insufficient for cell function)
what are the 2 major classes of integral membrane proteins?
- channels (pores)
- transporters
what is diffusion driven by?
- differences on either side of a membrane
- differences can be chemical or electrical
what is diffusion rate influenced by?
- steepness of concentration gradient
- temperature
- size or mass of the diffusing substance
- surface area
- diffusion distance
- number of channels / transporters
what are some examples of transporters?
- uniporters
- symporters
- antiporters
- ATP-binding cassette (ABC) system
state 3 examples of gated ion channels
- ligand-gated
- voltage-gated
- mechanosensitive
what are gap junctions?
- channels that connect directly with neighbouring cells
what ratio of sodium and potassium ions are moved and how much ATP is required for this in active transport?
- 3 sodium ions out of the cell
- 2 potassium ions into the cell
- 1 ATP hydrolysed for this above ratio
what is the difference between a symport and antiport in co-transport?
symport:
- transports 2 different molecules in the same direction
- eg. sodium ion and glucose in active co-transport in the ileum
antiport:
- transports 2 different molecules in opposite directions (one into the cell and one out)
describe the ATP-binding cassette transporter
- over 200 different ABC systems
- present in eukaryotes and prokaryotes
- transport is best studied in Gram negative bacteria
- in human cells, ABC transporters are involved in cystic fibrosis and multi-drug resistance
- requires energy released by hydrolysis of ATP
describe exocytosis
- secretion of proteins out across the cytoplasmic membrane
- packaged into secretory vesicles by Golgi and targeted to cytoplasmic membrane
- vesicles fuse with cytoplasmic membranes and release their contents extracellularly
what is the name for the dedicated transporter systems in bacteria? why are they necessary?
- translocases
- constraints of the cell wall means exocytosis and endocytosis might not be possible like in animal cells
describe endocytosis in general
- uptake of macromolecules from extracellular space across the cytoplasmic membrane
- occurs continuously in animal cells
- small portion of cytoplasmic membrane envelopes the molecule, invaginated and pinches off as an intracellular vesicle
what are the 3 distinct mechanisms of endocytosis?
- pinocytosis
- phagocytosis
- receptor-mediated endocytosis
describe pinocytosis
- bring small amounts of extracellular fluid in
- a constitutive and continuous process involves uptakes of extracellular fluid via small membrane vesicles
describe phagocytosis
- endocytosis by macrophages and neutrophils to ingest bacteria and cell debris in a phagosome / phagolysosome
- enzymes and reactive oxygen species like superoxide and nitric oxide are pumped in to help destroy the engulfed bacteria or debris
describe receptor-mediated endocytosis
- involves the protein ‘clathrin’ forming clathrin-coated pits and vesicles
- ligand binds to receptor than causes pit to form vesicle
- protein has a distinctive 3-legged structure called a triskelion which assemble to form a basket-like structure to stabilise the endocytic vesicle
