2.3 Transport across cell membranes Flashcards
What is the fluid-mosaic model of membrane structure?
- Molecules within membrane can move laterally (fluid) eg/phospholipids
- Mixture of phospholipids, proteins, glycoproteins and glycolipids
Describe the structure of a cell membrane:
Phospholipid bilayer
- Phosphate heads are hydrophilic so attracted to water - orientate to the aqueous environment either side of the membrane
- Fatty acid tails are hydrophobic so repelled by water - orientate to the inside/interior of the membrane
Embedded proteins
- Channel and carrier proteins
- Glycolipids and glycoproteins
- Cholesterol
Explain how the phospholipid bilayer allows molecules to enter/leave a cell?
- Allows movement of non-polar small/lipid-soluble molecules eg/ oxygen or water, down a concentration gradient (simple diffusion)
- Restricts the movement of larger/polar molecules
Explain how the channel proteins and carrier proteins allows molecules to enter/leave a cell?
- Allow movement of water-soluble/polar molecules/ions, down a concentration gradient (facilitated diffusion)
Explain how the carrier proteins allows molecules to enter/leave a cell?
- Allows the movement of molecules against a concentration gradient using ATP (active transport)
What are the features of the plasma membrane and how is it adapted for its other functions?
Phospholipid bilayer
- Maintains a different environment on each side of the cell or compartmentalisation of cell
Phospholipid bilayer is fluid
- Can bend to take up different shapes for phagocytosis / to form vesicles - Surface proteins / extrinsic / glycoproteins / glycolipids
- Cell recognition / act as antigens / receptors
Cholesterol
- Regulates fluidity / increases stability
What is the role of cholesterol?
- Makes the membrane more rigid / stable / less flexible, by restricting lateral movement of molecules making up membrane e.g. phospholipids (binds to fatty acid tails causing them to pack more closely together)
Note: not present in bacterial cell membranes
What moves across the membrane when simple diffusion takes place and what factors affect rate?
- Net movement of small, non-polar molecules e.g. oxygen or carbon dioxide, across a selectively permeable membrane, down a concentration gradient
- Passive / no ATP / energy required
- Factors affecting rate – surface area, concentration gradient, thickness of surface / diffusion distance
What moves across the membrane when facilitated diffusion takes place and what factors affect rate?
- Net movement of larger/polar molecules e.g. glucose, across a selectively permeable membrane, down a concentration gradient
- Through a channel/carrier protein
- Passive /no ATP/energy required
- Factors affecting rate – surface area, concentration gradients (until the number of proteins is the limiting factor as all are in use / saturated), number of channel/carrier proteins
What are the roles of carrier/channel proteins in facilitated diffusion?
Carrier proteins transport large molecules, the protein changes shape when molecule attaches
Channel proteins transport charged/polar molecules through its pore (some are gated so can open/close)
Different carrier and channel proteins facilitate the diffusion of different specific molecules
What moves across the membrane when active transport takes place and what factors affect rate?
- Net movement of molecules/ions against a concentration gradient
- Using carrier proteins
- Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push the substances though
Factors affecting rate – pH/temp (tertiary structure of carrier protein), speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)
Describe the sodium-potassium pump:
- The concentration of sodium ions (Na+) is higher outside the cell than inside
- The concentration of potassium ions (K+) is higher inside the cell than outside
- Three Na+ and one molecule of ATP bind to the pump protein
- The ATP is hydrolysed and ADP is released. The phosphate groups remains bound to the pump protein. As a result the shape of the pump protein changes and the three Na+ pass from the cell against the concentration gradient of Na+ and are released
- Two K+ bind to the pump protein
- The phosphate group bound to the pump protein is released. As a result the structure of the pump protein changes back to its original shape and the two K+ pass into the cell against the concentration gradient for K+ and are released
How does movement across the membrane take place by co-transport?
- Sodium ions actively transported out of epithelial cells lining the ileum, into the blood, by the sodium-potassium pump. Creating a concentration gradient of sodium (higher conc. 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 conc. of glucose in epithelial cell than blood
- Glucose moves out of cell into blood by facilitated diffusion through a protein channel
What is exocytosis?
A vesicle containing molecules of substance fuses with the inside of the cell surface membrane and the molecules are secreted from the cell
What is endocytosis?
The cell surface membrane binds to molecules: a vesicle sac forms. The sac enters the cell
