Active Transport, Bulk Transport & Osmosis Flashcards
What is the definition of active transport?
The movement of molecules or ions into or out of a cell from an area of low concentration to an area of high concentration (against a concentration gradient) across a partially permeable membrane.
What does active transport require?
- energy (ATP from respiration). It is an active process.
- transport / carrier protein in the cell surface membrane - acts as a “pump” and is complementary in shape to the molecule they transport. Molecules are pumped in one direction across the membrane.
What is active transport important for?
- reabsorption of useful molecules and ions into the blood after filtration into kidney tubules
- absorption of some digestive products from digestive tract
- loading of sugar from photosynthetic cells into phloem tissue
- loading of inorganic ions from soil into root hairs
How do molecules move by active transport?
- Molecule / ion binds to receptors with a complementary shape on the carrier protein.
- ATP binds to the carrier protein on inside of the cell. It is hydrolysed to ADP + P with the release of energy. The phosphate molecule remains bound to the protein.
- The energy provided allows the carrier protein to change shape (conformational change).
- Molecule / ion released inside cell.
- Phosphate is released from the carrier protein and recombines with ADP.
- Carrier protein returns to it’s original shape.
What factors affect the rate of active transport?
- speed of individual carrier proteins
- number of carrier proteins
- rate of respiration and release of ATP molecules
What is bulk transport?
Another form of active transport - moves substances that are too large to move through channel or carrier proteins.
- exocytosis
- endocytosis
What is exocytosis?
Bulk transport of material out of a cell using a vesicle which fuses with the cell surface and the contents are released.
What is endocytosis?
Bulk transport of material into a cell by the fusing of a vesicle with the cell surface membrane.
- phagocytosis (solids)
- pinocytosis (liquids)
Why is energy required in bulk transport?
- transporting vesicles along the cytoskeleton
- fusing vesicles to the membrane
What is water potential?
The pressure exerted by water molecules as they collide with a membrane or container. It is measured in Pa or kPa.
What is the definition of osmosis?
Osmosis is the net random movement of water molecules from a region of high water potential to a region of low water potential (down a concentration gradient) across a a partially permeable membrane.
Osmosis is a passive process.
How does water potential work in pure water and solutions?
Pure water has a water potential of 0. Solutions have a value below 0.
Which factors affect the rate of osmosis?
- water potential gradient
- thickness of exchange surface
- SA of exchange surface
What is meant by hypotonic solution?
A solution with more water in it than solute, compared to another solution.
What is meant by hypertonic solution?
A solution with more solute in it than water, compared to another solution.
What is meant by isotonic solution?
Two solutions / cells that have the same water potential.
How does osmosis work in plant cells?
- Plasmolysed - hypertonic - lower water potential outside cell
- Flaccid - isotonic - equal water potential inside and out
- Turgid - hypotonic - higher water potential outside cell
How does osmosis work in animal cells?
- Crenated - hypertonic - lower water potential outside cell
- Normal - isotonic - equal water potential inside and out
- Cytolysis - hypotonic - higher water potential outside cell
What happens to animal cells in a hypotonic solution?
- Outside environment has a higher water potential, water enters the cell by osmosis
- Hydrostatic pressure inside the cell increases
- Cell surface membrane cannot withstand pressure increase, animal cell bursts (cytolysis)
What happens to animal cells in a hypertonic solution?
- Outside environment has a lower water potential, water leaves the cell by osmosis
- Hydrostatic pressure inside the cell decreases
- Cell shrinks and shrivels (crenation)
What happens to plant cells in a hypotonic solution?
- Outside environment has a higher water potential, water enters the cell by osmosis
- Hydrostatic pressure inside the cell increases
- Cell swells (turgidity) but the cellulose cell wall prevents bursting - cytoplasm pulled towards from cell wall
What happens to plant cells in a hypertonic solution?
- Outside environment has a lower water potential, water leaves the cell by osmosis
- Hydrostatic pressure inside the cell decreases
- Cell shrinks and shrivels - cytoplasm pulled away from cell wall (plasmolysis)
