Diffusion,osmosis and Active Transport Flashcards
What is Diffusion
Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration
Molecules move down a concentration gradient, as a result of their random movement
Factors that Influence Diffusion
Surface area to volume ratio
The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
Many cells which are adapted for diffusion have increased surface area in some way - eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)
Distance
The smaller the distance molecules have to travel the faster transport will occur
This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible
Temperature
The higher the temperature, the faster molecules move as they have more energy
This results in more collisions against the cell membrane and therefore a faster rate of movement across them
Concentration Gradient
The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
This is because on the side with the higher concentration, more random collisions against the membrane will occur
Define Osmosis
Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
Osmosis in Plant Tissues
When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall
Water entering the cell by osmosis makes the cell rigid and firm
This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight
The pressure created by the cell wall stops too much water entering and prevents the cell from bursting
If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts
What happens to plant cells when its placed in a concentrated solution?
When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells) water molecules will move out of the plant cells by osmosis, making them flaccid
If plant cells become flaccid it can negatively affect the plant’s ability to support itself
If looked at underneath the microscope, the plant cells might be plasmolysed, meaning the cell membrane has pulled away from the cell wall
What happens to plant cell when its placed in high water potential
When plant cells are placed in a solution that has a higher water potential (dilute solution) than inside the cells (e.g. distilled water) then water moves into the plant cells via osmosis
These water molecules push the cell membrane against the cell wall, increasing the turgor pressure in the cells which makes them turgid
Animal cells in solutions of different concentrations
Animal cells also lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall, the results on the cell are more severe
If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)
If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts
Define Active Transport
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
Importance of Active Transport
Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)
Active transport is vital process for the movement of molecules or ions across membranes
Including:
uptake of glucose by epithelial cells in the villi of the small intestine and by kidney tubules in the nephron
uptake of ions from soil water by root hair cells in plants
Explain the process of Protein Carriers
Active transport works by using carrier proteins embedded in the cell membrane to pick up specific molecules and take them through the cell membrane against their concentration gradient:
Substance combines with carrier protein molecule in the cell membrane
Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane
Substance released into cell
