movement in and out of cells Flashcards
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
the cell is surrounded by … which can restrict the free movement of the molecules
cell membrane
The cell membrane is a
a partially permeable membrane – this means it allows some molecules to cross easily, but others with difficulty or not at all
Diffusion helps living organisms to
obtain many of their requirements
get rid of many of their waste products
carry out gas exchange for respiration
Brownian Motion
All particles move randomly at all times
This is known as Brownian motion
The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions
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)
Diagram showing the large surface area: volume of a root hair cell
The highly folded surface of the small intestine increases its surface area
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 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
Water can move in and out of cells by
osmosis
Osmosis is
the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
The cell membrane is partially permeable which means
it allows small molecules (like water) through but not larger molecules (like solute molecules)
a diluted solution has a
high water potential
a concentrated solution has a
low water potential
Osmosis Experiment potato
The most common osmosis practical involves cutting cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration
The potato cylinders are weighed before placing into the solutions
They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed
The potato cylinder in the distilled water will have increased its mass the most as there is a greater concentration gradient in this tube between the distilled water (high water concentration) and the potato cells (lower water concentration)
This means more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid – the potato cylinders will feel hard
A turgid plant cell
The potato cylinder in the strongest sucrose concentration will have decreased its mass the most as there is a greater concentration gradient in this tube between the potato cells (higher water concentration) and the sucrose solution (lower water concentration)
This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder – the potato cylinders will feel floppy
If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall
A plasmolysed plant cell
If there is a potato cylinder that has not increased or decreased in mass, it means there was no overall movement of water into or out of the potato cells
This is because the solution that cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient
A turgid plant cell
more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid – the potato cylinders will feel hard
A plasmolysed plant cell
This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder – the potato cylinders will feel floppy
If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall
Importance of Osmosis in 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
Osmosis in plant tissues
Plant cells that are turgid are full of water and contain a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)
This pressure prevents any more water entering the cell by osmosis, even if it is in a solution that has a higher water potential than inside the cytoplasm of the cells
This prevents the plant cells from taking in too much water and bursting
Plant roots are surrounded by soil water and the cytoplasm of root cells has a lower water potential than the soil water
This means water will move across the cell membrane of root hair cells into the root by osmosis
The water moves across the root from cell to cell by osmosis until it reaches the xylem
Once they enter the xylem they are transported away from the root by the transpiration stream, helping to maintain a concentration gradient between the root cells and the xylem vessels
osmosis in animal tissues
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
What is 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
what is needed and why in 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)
Examples of active transport include
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
How Protein Molecules Move Particles
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
