Cell Membranes and Transport Flashcards
How do lipid soluble substances move through the cell membrane?
SMALL UNCHARGED MOLECULES: dissolve in the hydrophobic tails of the phospholipids and diffuse across the membrane (oxygen, carbon dioxide)
NON-POLAR SUBSTANCES: dissolve in phospholipids and diffuse through cell membranes
How do water soluble substances move across the cell membrane?
- polar molecules such as glucose/amino acids AND charged ions cannot easily diffuse through the phospholipids as they are repelled by the hydrophobic fatty acid tails of the bilayer
- they will instead pass through specific intrinsic carrier or channel proteins spanning the membrane, as they cannot diffuse directly across
What affects the permeability of the cell membrane?
- TEMPERATURE: increased temperature increases membrane permeability as the molecules moving through the membrane will have more kinetic energy and so will diffuse more quickly, will also cause globular proteins in the membrane to dissolve
- ORGANIC SOLVENTS: ethanol will dissolve/emulsify the phospholipids and denature the proteins, leaving gaps or holes in the cell membrane (more porous)
What are the methods of transport across the membrane?
PASSIVE
- diffusion
- facilitated diffusion
- co transport
- osmosis
ACTIVE
- active transport
- bulk transport (endo/exocytosis)
What is diffusion?
The passive movement of a molecule or ion down the concentration gradient from a region of higher concentration to a region of lower concentration
If there is equilibrium, then there is no net movement
What factors affect the rate of diffusion?
- concentration gradient
- thickness of the diffusion surface
- surface area of the membrane
- size of the diffusion molecule
- nature of the diffusion molecule
- temperature
How does concentration gradient affect the rate of diffusion?
The greater the difference in concentration of molecules in two areas, the more molecules will diffuse in a given time
Why does the thickness of the diffusion surface affect rate of diffusion?
The shorter the distance that diffusion takes place across, the more molecules will diffuse in a given time
How does the surface area of the diffusion surface affect the rate of diffusion?
The larger the area, the higher the number of molecules that will diffuse in a given time
How does the size of the diffusing molecule affect the rate of diffusion?
Smaller molecules diffuse faster than larger molecules due to having more kinetic energy
How does the nature of the diffusing molecule affect rate of diffusion?
molecules that are soluble in phospholipids (non-polar) diffuse faster than water soluble molecules (polar) as they cannot diffuse diffuse anywhere in the membrane
How does temperature affect the rate of diffusion?
Increased temperature increases the rate of diffusion as the molecules or ions have more kinetic energy
What happens to the rate of diffusion of different sized molecules through a membrane, as the solubility in lipid increases?
- as solubility of molecules in lipids increases, the rate of diffusion through the membrane increases
- smaller diffuse at a greater rate than larger molecules with the same degree of lipid solubility
- molecules that are more lipid soluble dissolve in the hydrophobic fatty acid interior more easily and so pass through the membrane quicker
- dissolved small molecules move between the phospholipids molecules move more easily than the larger molecules - their rate of diffusion is quicker
What is facilitated diffusion?
- passive transfer of polar molecules or charged ion down a concentration gradient across a membrane by channel or carrier proteins in the cell membrane
- used to transport ions and large polar molecules such as Na_, glucose and amino acids (all relatively insoluble in phospholipids)
- more rapid than simple diffusion as going through a protein is easier than going through a bilayer
What affects the rate of facilitated diffusion?
- number of channel/carrier proteins present in the membrane (once all filled, rate will plateau
- steepness of the concentration gradient
- size of the molecule
- temperature (level of kinetic energy)
What are channel proteins?
- protein molecules with water filled hydrophilic pores
- ions (water soluble) can pass through because the pores are hydrophilic
- each channel is specific for one type of ion
- channels open and close depending on the needs of the cell
The higher the concentration of ions, the more collisions they have with the binding sites on the channel protein so more ions will pass through the channel proteins
What are carrier proteins?
- allow diffusion of larger polar molecules, such as glucose and amino acids
- polar molecule attaches to a binding site on the carrier protein, causing the protein to change shape, releasing the molecule though to the other side of the membrane
The higher the concentration of polar molecules, the more they will have with the binding sites of the carrier proteins, more molecules will pass through the carrier protein
What are gated protein channels?
Can be opened and closed to allow ions through (Na+ ion gated channels that open during the initiation of the nerve impulse)
What is co-transport?
A type of facilitated diffusion that brings molecules and ions into cells together on the same protein molecule
SECONDARY ACTIVE TRANSPORT
Describe the co-transport of sodium and glucose
- there is a concentration 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
- this can move glucose against its concentration gradient without the use of ATP
What are 2 processes which require sodium-glucose co-transport?
- absorption in the small intestine (ileum)
- reabsorption of solutes in the nephron
What is active transport?
The transport of ions and molecules against the concentration gradient using energy from the hydrolysis of ATP, made by the cell in respiration
Is inhibited when respiration is inhibited
Requires an intrinsic carrier protein, because the transport is against the concentration gradient so the carrier acts as a pump
Only charged particles that are insoluble in lipids can be actively transported
How does active transport work?
- the molecule or ion binds to the carrier protein on the outside of the cell
- 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 processes require active transport?
- muscle contraction
- nerve impulse transmission
- reabsorption of glucose in the nephron
- mineral ions uptake in plant roots
