Cell Membranes & Transport Flashcards
What is the permeability of the membrane affected by?
- Temperature
- Organic solvents eg ethanol
How does temperature affect membrane permeability?
Increased temperature increases membrane permeability as the molecules moving through the membrane have more kinetic energy and will diffuse more quickly. If temp gets too high, proteins may denature.
How do organic solvents affect membrane permeability?
Ethanol dissolves/emulsifies the phospholipids and may denature the proteins, resulting in gaps or holes within the cell membrane. The cell membrane becomes more porous.
What are the 6 methods of transport across the membrane?
- Diffusion
- Facilitated diffusion
- Co-transport
- Active transport
- Bulk transport
- Osmosis
Which methods of transport require ATP?
Active transport and bulk transport (endo/exocytosis)
What is diffusion?
The passive movement of a molecule or ion down a concentration gradient, from a region of higher concentration to a region of lower concentration
What are the 6 factors affecting the rate of diffusion?
- Concentration gradient
- Thickness of the surface
- Surface area of the membrane
- Size of the diffusing molecule
- Nature of the diffusing molecule (polar/non-polar)
- Temperature
What is the equation for rate of diffusion?
Rate of diffusion = surface area x difference in concentration / length of diffusion path
What is facilitated diffusion?
The passive transfer of polar molecules or charged ions (insoluble in phospholipids) down a concentration gradient, across a membrane, by channel or carrier proteins in the membrane
What is the rate of facilitated diffusion through a membrane affected by?
- The number of carrier/channel proteins present in the membrane - once they are all filled the rate will plateau
- The steepness of the concentration gradient
What are channel proteins and what are their features?
Protein molecules filled with hydrophilic pores
- Ions (water soluble) pass through as the pores are hydrophilic
- Each channel is specific for one type of ion
- Channels open and close depending on needs of the cell
What do carrier proteins do and how?
They allow for diffusion of larger polar molecules eg glucose
- Polar molecule attaches to a binding site on the carrier protein, causing the protein to change shape releasing the molecule through to the other side of the membrane
What is co-transport?
Type of facilitated diffusion that brings molecules and ions into cells together on the same protein transport molecule. PASSIVE
Describe sodium-glucose co-transport
- There’s a concentration gradient for sodium ions to move into the cell
- Sodium ions and glucose bind to the carrier protein
- The carrier protein changes shape, and the sodium ions and glucose are transported to the other side of the membrane
What is active transport?
Transport of ions and molecules against the concentration gradient. Uses energy from the hydrolysis of ATP, made by the cell in respiration. If respiration is inhibited, eg by cyanide poison, then active transport is too
What kind of particles are actively transported?
Only charged particles, eg ions and polar molecule eg glucose that are insoluble in lipid.
Describe active transport
- The molecule or ion binds to the carrier protein on outside of cell membrane
- ATP transfers a phosphate group to the carrier protein
- The carrier protein changes shape and carries the ion or molecule across the membrane
- The molecule or ion is released into the cytoplasm of the cell
- The carrier protein returns to its original shape
What are the 2 types of bulk transport?
Endocytosis
Exocytosis
What is exocytosis?
The process by which substances may leave the cell having been transported through the cytoplasm in a vesicle, which fuses with the cell membrane and secretes its contents to the outside of the cell
What is endocytosis?
The cell membrane folds around the particle. The folding closes off the link to the outside of the cell so now the particle is fully trapped inside the cell in a vesicle or vacuole.
What are the 2 types of endocytosis?
- Phagocytosis (uptake of solids)
- Phinocytosis (uptake of liquids)
What is osmosis?
Osmosis is the diffusion of water from an area of higher water potential to an area of lower water potential across a selectively permeable membrane. It’s a passive process so doesn’t require ATP.
What is water potential?
The tendency for water to leave a solution or cell by osmosis. Measured in kPa
Water potential equation
Water potential = solute potential + pressure potential
What happens if a plant cell is placed in a hypotonic solution?
Water will move into the cell by osmosis causing the plant to become TURGID
What happens if a plant cell is placed in a hypertonic solution?
Water will move out of the cell and into the surrounding solution by osmosis. The vacuole shrinks and the cytoplasm and cell membrane will withdraw from the cell wall causing the cell to become plasmolysed.
What is incipient plasmolysis?
Theoretically: the point at which the cell membrane is just about to come away from the cell wall.
Experimentally: the point where 50% of the cells in a sample are plasmolysed
At incipient plasmolysis, what would the pressure potential be?
Pressure potential = 0
So water potential = solute potential
What do animal cells become when water moves out of them?
Crenated
