Transport across cell membranes Flashcards
What is the fluid-mosaic model of membrane structure?
- molecules are free to move laterally in phospholipid bilayer
- there are many components within it e.g. phospholipids,, proteins, glycoproteins & glycolipids
What is the arrangement of the components of a cell membrane?
- phospholipids form bilayer - phosphate heads pointing outwards & fatty acid tails pointing inwards
- intrinsic proteins span over whole bilayer e.g. channel & carrier proteins
- extrinsic proteins are on the surface of membrane e.g. receptors
- glycolipids & glycoproteins found on exterior surface
- cholesterol (sometimes present) bonds to fatty acid tails
What is the arrangement of phospholipids in a cell membrane?
- bilayer with water present on either side
- hydrophobic fatty acid tails repel water so point away from it
- hydrophilic phosphate heads attract water so point towards it
What is the role of cholesterol in cell membranes when it is present?
restricts movement of other molecules that make up membrane so decreases fluidity
How are cell membranes adapted for other functions?
- phospholipid bilayer is fluid so membrane can bend for vesicle formation or phagocytosis
- glycoproteins/glycolipids act as receptors/antigens which are involved in cell recognition
How does movement across membranes occur by simple diffusion?
- lipid-soluble or very small substances e.g. oxygen move from area of high to low conc. down the concentration gradient across the phospholipid bilayer
- is a passive process (doesn’t require energy from respiration)
What are the limitations imposed by the nature of the phospholipid bilayer?
restricts movement of water soluble & larger substances e.g. Na+ / glucose due to hydrophobic fatty acid tails
How does movement across membranes occur by facilitated diffusion?
- water soluble substances move down a conc gradient through specific carrier/channel proteins
- passive process so doesn’t require energy from ATP
What determines what substances move by proteins?
the shape/charge of proteins
What is the role of channel proteins in facilitated diffusion?
- they facilitate diffusion of water-soluble substances
- hydrophilic pore filled with water which may be gated (can open & close)
What is the role of carrier proteins in facilitated diffusion?
- they facilitate diffusion of larger substances
- complementary substances attach to binding site of protein which causes it to change shape to transport substance
How does movement across the membrane occur by osmosis?
- water diffuses from an area of high to low water potential down the gradient through a partially permeable membrane
- passive process so doesn’t require energy from ATP
What is water potential?
- a measure of how likely water molecules are to move out of a solution
- distilled water has max water potential
How does movement across membranes occur by active transport?
- substances move from area of low to high conc against conc gradient
- requires hydrolysis of ATP and specific carrier proteins
What is the role of carrier proteins & the importance of hydrolysis of ATP in active transport?
- complementary substances bind to specific carrier protein
- ATP binds hydrolysed into ADP + Pi releasing energy
- carrier protein changes shape releasing substances on side of high conc
- Pi is released so protein returns to original shape
How does the movement across membranes occur by co-transport?
- 2 different substances bind to & move at the same time by a co-transporter protein (type of carrier protein)
- movement of one substance against its conc gradient is often coupled with the movement of another down its conc gradient
What is an example that illustrates co-transport?
absorption of sodium ions & glucose (or amino acids) by cells lining mammalian ileum:
- Na+ actively transported from epithelial cells to blood establishing conc gradient of Na+ higher in lumen than epithelial cell
- Na+ enters epithelial cell down its conc gradient with glucose against its conc gradient by a co-transporter protein
- glucose moves down a conc gradient into blood by facilitated diffusion
What are factors that affect the rate of movement across cell membranes?
- increasing surface area of membrane increases rate of movement
- increasing no. of channel/carrier proteins increases rate of facilitated diffusion/active transport
- increasing conc gradient increases rate of simple/facilitated diffusion & osmosis until no. of proteins becomes limiting factor as they’re all in use
- increasing water potential gradient increases rate of osmosis
What are the adaptations of specialised cells in relation to the rate of transport across their internal & external membranes?
- folded membrane increases surface area e.g. microvilli in ileum
- more protein channels/carriers for facilitated diffusion
- large no. of mitochondria to make more ATP by aerobic respiration for active transport
