Movement into & out of Cells (Cambridge (CIE) AS Biology): Flashcards
Exam code: 9700
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- FrontDiffusion
Define diffusion.
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Define diffusion.
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Is diffusion active or passive? Explain.
Passive — it does not require ATP from the cell.
Molecules move using their own kinetic energy.
Define facilitated diffusion.
Facilitated diffusion is the diffusion of molecules or ions across a membrane through channel or carrier proteins, down a concentration gradient.
Why do some substances need facilitated diffusion to cross the membrane?
Large or polar (water-soluble) molecules and ions cannot pass through the hydrophobic phospholipid bilayer.
They need transport proteins instead.
State three factors that affect the rate of diffusion.
Concentration gradient — steeper gives a faster rate.
Temperature — higher gives a faster rate.
Surface area and diffusion distance (membrane thickness).
Facilitated diffusion moves substances through channel and proteins.
Facilitated diffusion moves substances through channel and carrier proteins.
True or False?
Facilitated diffusion requires ATP.
False.
Facilitated diffusion is passive; substances move down their concentration gradient without using ATP.
Define osmosis.
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Define water potential.
Water potential is the tendency of water molecules to move from one region to another; it is measured in kilopascals (kPa).
What is the water potential of pure water?
Zero (0 kPa).
This is the highest possible water potential.
How does adding solute affect the water potential of a solution?
It lowers the water potential, making it more negative.
The more concentrated the solution, the lower its water potential.
In which direction does water move during osmosis?
From a region of higher water potential (dilute) to a region of lower water potential (concentrated).
This is down a water potential gradient.
Osmosis is the movement of water across a permeable membrane.
Osmosis is the movement of water across a partially permeable membrane.
True or False?
A concentrated sugar solution has a higher water potential than pure water.
False.
Adding solute lowers water potential, so a concentrated solution has a lower (more negative) water potential than pure water (0 kPa).
Define active transport.
Active transport is the movement of molecules or ions across a membrane against their concentration gradient (low to high), using ATP and carrier proteins.
Why does active transport require ATP?
It moves substances against their concentration gradient (from low to high concentration).
This needs energy, released from ATP.
Describe the role of carrier proteins in active transport.
The specific molecule binds to the carrier protein.
Using energy from ATP, the protein changes shape to move the molecule across the membrane.
Give one difference between active transport and facilitated diffusion.
Active transport uses ATP and moves substances against the gradient.
Facilitated diffusion is passive and moves substances down the gradient.
Active transport moves substances their concentration gradient.
Active transport moves substances against their concentration gradient.
The energy for active transport is provided by .
The energy for active transport is provided by ATP.
True or False?
Active transport can move substances from a low to a high concentration.
True.
It uses ATP to move substances against their concentration gradient, from low to high concentration.
Define endocytosis.
Endocytosis is the bulk transport of materials into a cell by engulfing them in a section of the cell surface membrane to form a vesicle. It requires ATP.
Define exocytosis.
Exocytosis is the bulk transport of materials out of a cell, when a vesicle fuses with the cell surface membrane to release its contents. It requires ATP.
Why are endocytosis and exocytosis described as bulk transport?
They move large quantities of material, or molecules too large to cross the membrane by diffusion or through transport proteins.
Do endocytosis and exocytosis require energy?
Yes — both require ATP.
Energy is needed to move vesicles and to change the shape of the membrane.
Give one example of exocytosis in cells.
The secretion of substances such as enzymes, hormones or neurotransmitters from a cell.
During exocytosis, a fuses with the cell surface membrane to release its contents.
During exocytosis, a vesicle fuses with the cell surface membrane to release its contents.
True or False?
Endocytosis brings materials into the cell.
True.
Endocytosis engulfs material in the membrane to form a vesicle, bringing it into the cell.
How can osmosis be investigated using potato tissue?
Cut potato cylinders of equal size and measure their initial mass.
Place them in sucrose solutions of different concentrations, then measure the change in mass.
Why is the change in mass measured in a potato osmosis experiment?
Water entering or leaving the cells by osmosis changes the tissue's mass.
Mass increases if water enters and decreases if water leaves.
Why is percentage change in mass calculated rather than the actual change?
The potato pieces may have slightly different starting masses.
Percentage change allows a fair comparison between samples.
State two variables that should be controlled in this experiment.
Temperature.
Volume of solution, size of tissue pieces and time left in the solution.
In a potato osmosis experiment, the of each piece is measured before and after.
In a potato osmosis experiment, the mass of each piece is measured before and after.
True or False?
A potato cylinder placed in pure water will decrease in mass.
False.
Pure water has a higher water potential, so water moves into the cells by osmosis and the mass increases.
How can Visking (dialysis) tubing be used to model diffusion across a membrane?
Visking tubing is partially permeable.
Small molecules (e.g. glucose) diffuse through it, but large molecules (e.g. starch) cannot.
In a Visking tubing experiment, how do you show that glucose has diffused out but starch has not?
Test the surrounding water with Benedict's solution — it turns orange/red, showing glucose diffused out.
Test with iodine — it stays orange-brown, showing starch stayed inside.
How can agar be used to investigate diffusion?
A coloured substance (e.g. an acid or dye) diffuses through blocks of agar jelly.
The distance travelled or the time taken is measured.
Why are non-living materials like Visking tubing useful for modelling diffusion?
They provide a partially permeable barrier without any living processes.
Results are due to diffusion alone, not active transport or respiration.
tubing is partially permeable and is used to model a cell membrane.
Visking tubing is partially permeable and is used to model a cell membrane.
True or False?
Starch molecules can diffuse through Visking tubing.
False.
Starch molecules are too large to pass through the partially permeable Visking tubing; only small molecules such as glucose can.
Define surface area to volume ratio (SA:V).
The surface area to volume ratio compares an object's surface area to its volume, showing how much surface is available per unit of volume.
How does the SA:V ratio change as an object gets larger?
The SA:V ratio decreases.
Volume increases faster than surface area as size increases.
How do you calculate the surface area and volume of a cube?
Surface area = 6 × (length × length).
Volume = length × length × length.
A cube has sides of 2 cm. Calculate its SA:V ratio.
Surface area = 6 × (2 × 2) = 24 cm^2^.
Volume = 2 × 2 × 2 = 8 cm^3^.
SA:V = 24 : 8 = 3 : 1.
Why is a high SA:V ratio important for exchange?
A larger surface area per unit of volume allows faster exchange of substances by diffusion, relative to the cell's needs.
As an object increases in size, its surface area to volume ratio .
As an object increases in size, its surface area to volume ratio decreases.
True or False?
A small cell has a larger surface area to volume ratio than a large cell.
True.
Smaller objects have a greater SA:V ratio, giving more surface area per unit of volume for exchange.
How can agar blocks be used to investigate the effect of SA:V ratio on diffusion?
Cut agar containing an indicator into cubes of different sizes.
Place them in a coloured acid or alkali and time how long the colour change takes to reach the centre.
In the agar block experiment, what is the independent variable and what is measured?
Independent variable = the size (SA:V ratio) of the agar cubes.
Measured = the time for the colour to diffuse fully to the centre.
Why does a smaller agar cube change colour fully faster?
It has a larger SA:V ratio.
The diffusion distance to the centre is shorter, so the substance reaches the centre more quickly.
State two variables that must be controlled in the agar cube experiment.
Temperature and concentration of the solution.
Volume of solution and type of agar used.
Smaller agar cubes change colour faster because they have a larger area to volume ratio.
Smaller agar cubes change colour faster because they have a larger surface area to volume ratio.
True or False?
Larger agar cubes take longer for a substance to diffuse to their centre.
True.
Larger cubes have a smaller SA:V ratio and a greater diffusion distance, so it takes longer.
How can you estimate the water potential of plant tissue such as potato?
Place equal pieces of tissue in a range of sucrose solutions of known water potential.
Measure the % change in mass and find the solution where there is no change.
At the point of zero change in mass, what does this tell you about the water potential?
The water potential of the solution equals the water potential of the plant tissue.
There is no net osmosis.
How do you find the concentration that gives zero % change in mass?
Plot % change in mass (y-axis) against solution concentration (x-axis).
Read off the value where the line crosses the x-axis (0% change).
Why is percentage change in mass used rather than the actual mass change?
The tissue samples have slightly different starting masses.
Percentage change allows a valid comparison between samples.
The tissue's water potential equals the solution's water potential when there is net change in mass.
The tissue's water potential equals the solution's water potential when there is no net change in mass.
True or False?
A graph can be used to find the concentration at which the tissue neither gains nor loses mass.
True.
The point where the line crosses 0% change gives the concentration with a water potential equal to the tissue.
What happens to a plant cell placed in a solution with a higher water potential (e.g. pure water)?
Water moves into the cell by osmosis.
The cell becomes turgid (swollen and firm).
Define turgid.
A turgid cell is firm and swollen because it has taken in water by osmosis, pushing the membrane against the cell wall.
Why does a plant cell not burst when placed in pure water?
The strong, inelastic cell wall resists further expansion.
This prevents the cell from bursting when it fills with water.
What happens to a plant cell placed in a solution with a lower water potential (concentrated)?
Water moves out of the cell by osmosis.
The cell becomes plasmolysed — the membrane pulls away from the cell wall.
Define plasmolysis.
Plasmolysis is when a plant cell loses so much water by osmosis that the cell membrane pulls away from the cell wall.
A plant cell that has taken in water and become firm is described as .
A plant cell that has taken in water and become firm is described as turgid.
True or False?
A plant cell in a concentrated sugar solution will become turgid.
False.
Water moves out of the cell, so it becomes plasmolysed (flaccid), not turgid.
What happens to an animal cell (e.g. red blood cell) in a solution with a higher water potential (pure water)?
Water moves into the cell by osmosis.
The cell swells and bursts (lysis).
Define lysis.
Lysis is the bursting of an animal cell when too much water enters it by osmosis and the cell membrane breaks.
Why does an animal cell burst in pure water but a plant cell does not?
Animal cells have no cell wall to resist expansion.
Plant cells have a strong cell wall that prevents bursting.
What happens to a red blood cell in a solution with a lower water potential (concentrated)?
Water moves out of the cell by osmosis.
The cell shrinks and shrivels (crenation).
Define crenation.
Crenation is the shrinking and shrivelling of an animal cell when water leaves it by osmosis into a more concentrated solution.
An animal cell placed in pure water will swell and .
An animal cell placed in pure water will swell and burst.
True or False?
Animal cells can burst in pure water because they have no cell wall.
True.
With no cell wall to resist expansion, water entering by osmosis makes the cell swell and burst.
Why do plant and animal cells respond differently to being placed in pure water?
Plant cells have a cell wall that stops them bursting, so they become turgid.
Animal cells have no cell wall, so they burst (lyse).
Compare what happens to a plant cell and an animal cell in a concentrated (low water potential) solution.
Plant cell: water leaves and the cell becomes plasmolysed (membrane pulls from the wall).
Animal cell: water leaves and the cell shrinks (crenates).
What terms describe a plant cell and an animal cell that have gained water in pure water?
Plant cell = turgid (firm).
Animal cell = lysed (burst).
In a concentrated solution a plant cell becomes plasmolysed, while an animal cell becomes .
In a concentrated solution a plant cell becomes plasmolysed, while an animal cell becomes crenated.
True or False?
Both plant and animal cells burst when placed in pure water.
False.
Animal cells burst, but plant cells do not — their cell wall stops them bursting, so they become turgid.
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