1.4 Membrane Transport Flashcards
How does the process of endocytosis work?
A vesicle is formed by pinching off a small piece of the plasma membrane of cells, this is done by using ATP. A vesicle is formed on the inside of the membrane and contains material from outside of the cell.
It might contain water or solutes, or larger molecules needed by the cell that cannot pass through the plasma membrane for example, in the placenta proteins from the mothers blood are absorbed by the foetus this way.
What allows endocytosis and exocytosis?
The fluidity of the membrane allows vesicles to be formed.
How are vesicles formed in endo and exocytosis?
A small region of membrane is pulled from the rest of the membrane and is pinched off. Proteins in the membrane carry out this process using energy from ATP.
What are vesicles used for?
- Endo and exocytosis.
- To move materials around in the cell, for example moving proteins synthesised in the rER and then fuse to the golgi apparatus, then vesicles carry the proteins to the membrane and expel them from the cell.
- In a growing cell when the membrane needs to increase in size. Phospholipids are synthesised and then brought to the membrane to increase its size.
How does the process of exocytosis work?
A vesicle will approach the membrane from inside the cell, it may be full of proteins or digestive enzymes that need to be released to the rest of the organism. The vesicle fuses with the main membrane releasing its contents.
Which membrane transport is passive and which is active?
PASSIVE
- Simple diffusion
- Facilitated diffusion
- Osmosis
ACTIVE
- Endocytosis
- Exocytosis
- Active transport
How does facilitated diffusion work?
Ions and other particles that cannot pass between the phospholipids can pass into or out of the cells if there are channels for them in the plasma membrane. These channels are holes with a very thin diameter. The diameter and chemical properties of the channel ensure that only one type of particle passes through.
Cells can control which types of channels are synthesised and placed in the plasma membrane and in this way they can control which substances diffuse in and out.
How does simple diffusion work?
Diffusion is the spreading out of particles in liquids and gases that happens because the particles are in continuous random motion. More particles move from an area of high concentration to an area of low concentration than move in the opposite direction. There is therefore a net movement from the higher to the lower concentration - a movement down the concentration gradient.
Simple diffusion in the membrane can only happen if the phospholipid bilayer is permeable to the particles, non-polar molecules such as oxygen diffuse through easily. However ions with positive or negative charges cannot just diffuse through because the centre of the membrane is hydrophobic, but polar molecules can at a slow rate. Smaller particles pass through more easily than larger ones.
How does osmosis work?
Water molecules, despite being polar, can pass through the membrane because they are so tiny.
Osmosis happens because where it is more concentrated with solutes, they dissolve and form intermolecular bonds with the water molecules, then this prevents the water molecules from moving as easily but where there is less solutes they can move more easily so the next movement is from high solute to low solute concentration.
How does active transport work?
Active transport is carried out by globular proteins in membranes, usually called pump proteins. The molecule or ion enters the pump and can reach as far as the central chamber. A conformational change to the protein takes place using ATP and after this the molecule can pass to the opposite side of the membrane and the pump protein returns to its original conformation.
What is the structure and function of sodium potassium pumps in axons?
A nerve impulse involves rapid movements of sodium and then potassium ions across the axon membrane. These movements occur by facilitated diffusion through sodium and potassium channels. They occur because of concentration gradients between the inside and outside of the axon. The concentration gradients are built up by active transport, carried out by a sodium-potassium pump protein. This process described below restores the normal of the axon after an impulse has been transmitted.
Basically what the pump does is take 3 sodium out of the axon and put potassium back in.
1) 3 sodium molecules attach to the pump from the inside of the axon.
2) ATP transfers a phosphate which changes the shape of the protein. The pump opens to the outside and the three sodiums are released.
3) Two potassiums then enter from the outside
4) The binding causes the release of the phosphate group and so the protein changes shape back.
5) The 2 potassiums are then released inside the axon.
Explain the facilitated diffusion of sodium-potassium pumps in axons?
A nerve impulse involves rapid movements of sodium and then potassium ions across the axon membrane.
This is how potassium leaves.
Each potassium channel consists of four protein subunits with a narrow pore between them that allows potassium ions to pass in either direction. The pore is 0.3nm wide,. Potassium ions are slightly smaller than that but when they dissolve in water a net is formed around them of water molecules, the bonds between the water molecules breaks and bonds form between the ion and the series of amino acids in the channel. Then the potassium ion passes through. Other molecules are too large or too small to form bonds with the amino acids. POTASSIUM LEAVES THIS WAY.
Potassium channels in axons are voltage gated. Voltages across membranes are due to an imbalance of positive and negative charges across the membrane. If an axon has relatively more positive charges outside than inside, potassium channels are closed. At one stage during a nerve impulse there are relatively more positive charges inside. This causes potassium channels to open allowing potassium to go out. Then it rapidly closes again as an extra globular protein like a ball attached by a chain of amino acids fits inside the pore. The ball remains until it returns to its normal closed state.
What is osmolarity?
The osmolarity of a solution is the total concentration of osmotically active solutes. A hypertonic solution has a higher osmolarity and a hypotonic solution has a lower osmolarity.
How can animal and plant cells be damaged by osmosis?
Plant cells want to be turgid. They want to be in a hypotonic solution so that water keeps coming into the cell. If they are in a hypertonic solution then water leaves and they become plasmolised.
Animal cells in a hypotonic solution will burst. In a hypertonic solution, water will leave the cell, so the cytoplasm shrinks in volume but the cell wall stays the same so it develops indentations which are sometimes called crenellations.
So you need to keep organs in isotonic solutions.
