1.4 Membrane transport Flashcards
what are vesicles and how do they function in endocytosis / exocytosis
- Vesicles are fluid filled membranes, present in eukaryotes.
- Vesicles are formed by a part of the plasma membrane pulling away from the rest of the membrane and pinching off, proteins in the membrane carry out this process, this requires ATP
what is endocytosis and how does it work
- part of plasma membrane is pulled inwards; a droplet of fluid becomes enclosed when a vesicle is pinched off.
- Mass movement of molecules into a cell by formation of a vesicle
- E.G: Phagocytosis (cell eating) + Pinocytosis (cell drinking)
- requires energy
- A vesicle is formed on the inside of the plasma membrane and contains materials that were outside the cell. This is called endocytosis
- Vesicles used in endocytosis often hold water or solutes and often are used to transport larger molecules that could otherwise not pass through the plasma membrane.
vesicle movement in cells
-Vesicles move contents around in cells
-Materials in vesicles need to be moved
-Proteins in vesicle membranes need to be moved
Example: Protein is synthesised by ribosomes in the rER, vesicles the proteins bud off the rER and carry them to the Golgi apparatus where they fuse with the Golgi. Protein undergoes final processing and then is put back into vesicles which bud off and carry proteins to the cell membrane where they can be secreted by exocytosis.
Protein synthesis
- Proteins are synthesised by ribosomes in the rER
- Vesicles bud off from the rER and carry the proteins to the Golgi apparatus
- The Golgi apparatus modifies the proteins
- Vesicles bud off from the golgi apparatus and carry modified proteins to the plasma membrane
Exocytosis
Vesicles fuse with the plasma membrane, the contents of the vesicles are expelled, the membrane then flattens out again.
-Mass movement of molecules out of a cell by the merging of a vesicle with the plasma membrane
-Hydrophobic tails are flexible, this allows the membrane to be able to bend, break and reform
-Requires ENERGY
Example: Digestive enzymes are released from glands by exocytosis. Exocytosis can also be used to expel waste products or unwanted materials.
Simple diffusion
- Passive movement of particles from an area of higher concentration to an area of lower concentration
- Net movement of particles from higher to lower down a concentration gradient
- Passive process
- Non-polar particles can diffuse through easily
- The centre of the membrane is hydrophobic so ions or with positive or negative charges cannot pass easily through
- Polar molecules with partial positive or negative charge can diffuse at low rates
- Small polar molecules can pass through easily
- Greater the concentration gradient = faster the diffusion
facilitated diffusion
- Ions or other particles (large or charged) that cannot diffuse through phospholipids can pass into or out of cells if there are channels for them through the plasma membrane
- Requires the use of specific protein channels in the membrane
- The walls of these channels consist of protein
- The diameter and chemical properties of channels ensures that only one type of particle passes through, for example, sodium ions, potassium ions, not both.
- High to low concentration through a protein channel = facilitated diffusion
- Passive
Osmosis
-The passive movement of water molecules across a partially permeable membrane, from a region of low solute concentration to a region of high solute concentration.
-Water is able to move in and out of most cells freely
-Net movement of water through a semi-permeable membrane from an area of low solute concentration to an area of higher solute concentration.
-Regions with higher solute content have lower water content
-Passive, no energy required
-Water can diffuse through membranes easily however some cells have ‘aquaporin’s’ which greatly increase membrane permeability to water
Examples: kidneys and root hair cells which both absorb water
Active transport
- Allows cells to pump substances through specific protein channels
- Can move substances from areas low concentration to areas of high concentration
- Against the concentration gradient
- Cells sometimes take in substances even though there is a higher concentration inside than outside, cells also sometimes pump substances out even though there is a higher concentration outside
- Active transport requires ATP
- Active transport is carried out by globular proteins in membranes, often called pumps.
- A molecule or ion enters the central chamber of the pump protein, using ATP the protein changes shape. After this the molecule or ion can pass through to the opposite side of the membrane
Example of active transport: sodium and potassium in axons
- The interior of the pump is open to the inside of the axon, three sodium ions enter the pump and attach to their binding sites
- ATP transfers a phosphate group from itself to the pump, this causes the pump to change shape and the interior is then closed
- The interior of the pump opens to the outside of the axon and three sodium ions are released
- Two potassium ions from outside can then enter and attach to their binding sites
- Binding of potassium causes release of the phosphate group, this causes the pump to change shape again so that it is again only open to the inside of the axon
- The interior of the pump opens to the inside of the axon and the two potassium ions are released, sodium ions can then enter and bind to the pump again…
Example of facilitated diffusion: potassium in axons
- Potassium channels in axons are voltage gated. If there are more positive charges outside the cell, then the axon will be closed.
- When there are more positive charges inside this causes the potassium channel to open and let potassium ions close and then quickly close.
- This is due to the ‘ball and chain’ which block the channel shut.
Example of endocytosis (placenta proteins)
Example: in placenta proteins from the mother’s blood stream are absorbed into the foetus by endocytosis, some cells take in large undigested food particles by endocytosis
Preventing osmosis in excised tissues and organs
- tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis because animals cells can be damaged by osmosis
- in a hypertonic solution, water leaves the cell by osmosis, so the cytoplasm shrinks in volume. the area of plasma membrane does not change so it develops indentations
- in a hypotonic solution the cells take in water by osmosis and swell up, they may eventually burst.
- in a solution with the same osmolarity as the cells (isotonic solution) water molecules enter and leave the cell at the same rate so they remain healthy
- normal saline is used in many medical procedures and can be used for –> used as a basis for eye drops + used to rinse wounds
