Transport across cell membranes Flashcards
Fluid-mosaic model of membrane structure
Molecules within membrane can move laterally (fluid) eg phospholipids
Mixture of phospholipids, proteins, glycoproteins and glycolipids
Structure of a cell membrane
Phospholipid bilayer
Embedded proteins (intrinsic or extrinsic)
Glycolipids (lipids and attached polysaccharide chains) and glycoproteins (proteins with polysaccharide chain attached)
Cholesterol (binds to phospholipid hydrophobic fatty acid tails)
Phospholipid bilayer
Phosphate heads are hydrophilic so they are attracted to water - orients to the aqueous environment on either side of the cell
Fatty acid tails are hydrophobic so they are repelled by water - orients to the interior of the membrane
How the fluid-mosaic model explains how molecules enter/leave a cell
Phospholipid bilayer allows the movement of non-polar small/lipid-soluble molecules down a concentration gradient. Restricts movement of larger/polar molecules
Channel proteins (some are gated) and carrier proteins allow the movement of water-soluble/polar molecules/ions down a concentration gradient (facilitated diffusion)
Carrier proteins allow the movement of molecules against a concentration gradient using ATP (active transport)
Features and functions of the plasma membrane
Phospholipid bilayer maintains a different environment on each side of the cell or compartmentalisation of cell.
The phospholipid bilayer is fluid so it can bend to take different shapes for phagocytosis/to form vesicles
Surface proteins/extrinsic/glycoproteins/glycolipids for cell recognition/act as antigens/receptors
Cholesterol regulates fluidity/increases stability
The role of cholesterol
Makes the membrane more rigid/stable/less flexible by restricting lateral movement of molecules making up the membrane eg phospholipids (bind to fatty acid tails causing them to pack more closely together)
Not present in bacterial cell membranes
Movement across membranes by simple diffusion and factors affecting rate
Net movement of small, non-polar molecules (eg oxygen or carbon dioxide) across a selectively permeable membrane down a concentration gradient
Passive, no ATP or energy required
Factors affecting rate: surface area, concentration gradient, thickness of surface/diffusion distance
Movement across membranes by facilitated diffusion, factors affecting rate
Net movement of larger/polar molecules (eg glucose) across a selectively permeable membrane, down a concentration gradient
Through a channel/carrier protein
Passive, no ATP or energy required
Factors affecting rate – surface area, concentration gradient (until the number of proteins is the limiting factor as all are in use / saturated), number of channel/carrier proteins
Role of carrier/channel proteins during facilitated diffusion
Carrier proteins transport large molecules, the protein changes shape when the molecule attaches
Channel proteins transport charged/polar molecules through its pore (some are gated so that it can open/close).
Different channel and carrier proteins facilitate the diffusion of different molecules
Movement across membranes by active transport and factors affecting rate
Net movement of molecules/ions against a concentration gradient using carrier proteins.
Uses energy from the hydrolysis of ATP to change the shape of the tertiary structure and push substances through
Factors affecting rate – pH/temp (tertiary structure of carrier protein), speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)
Movement across membranes by co-transport, illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum
Sodium ions actively transported out of epithelial cells lining the ileum into the blood by the sodium-potassium pump. This creates a concentration gradient of sodium (higher concentration of sodium in lumen than epithelial cell)
Sodium ions and glucose move by facilitated diffusion into the epithelial cell from the lumen, via a co-transporter protein
Creating a concentration gradient of glucose - higher concentration of glucose in epithelial cell than blood.
Glucose moves out of the cell into blood by facilitated diffusion through a protein channel
Movement across membranes by osmosis and factors affecting rate
Net movement of water across a selectively permeable membrane down a water potential gradient.
Passive
Factors affecting rate – surface area, water potential gradient, thickness of exchange surface/diffusion distance
What is water potential
The likelihood (potential) of water molecules to diffuse out of or into a solution; pure water has the highest water potential and adding solutes to a solution lowers the water potential (more negative)
How might cells be adapted for transport across their internal or external membranes
By an increase in surface area
Increase in number of protein channels/carriers
