Movement of Substances Into & Out of Cells (Edexcel IGCSE Science (Double Award): Biology): Flashcards
Exam code: 4SD0
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Define diffusion.
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Define diffusion.
The movement of particles from a region of higher concentration to a region of lower concentration — down a concentration gradient.
Why is diffusion described as a passive process?
Because it needs no energy from the cell.
The diffusing particles have their own kinetic energy and spread out randomly from high to low concentration.
What does it mean that the cell membrane is partially permeable?
It allows some gases and molecules to cross, but not others.
True or False?
Starch can diffuse freely across the cell membrane.
False.
Small molecules such as oxygen can diffuse freely across the membrane, but larger molecules like starch cannot.
Give an example of diffusion in a photosynthesising leaf.
The concentration of carbon dioxide in the chloroplasts is lower than in the surrounding atmosphere, so carbon dioxide diffuses into the leaf cells.
Give an example of diffusion in the lungs.
The concentration of oxygen inside the alveoli is higher than in the blood of the capillaries, so oxygen diffuses into the blood to be transported round the body.
Give an example of diffusion in the liver.
The concentration of urea, a waste product, is higher in the liver cells than in the blood flowing through, so urea diffuses out into the blood.
Define osmosis.
The movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), through a partially permeable membrane.
Osmosis is the diffusion of water molecules across a ＿＿＿＿＿＿＿＿ membrane.
Osmosis is the diffusion of water molecules across a partially permeable membrane.
How does dissolved solute affect the direction of osmosis?
The more solute a solution contains, the lower its water concentration.
Water therefore moves by osmosis from the more dilute solution to the more concentrated one.
What happens to an animal cell in a strong sugar solution?
It loses water by osmosis and becomes crenated (shrivelled).
What happens to an animal cell placed in distilled water?
It gains water by osmosis and eventually bursts, because it has no cell wall to maintain its structure.
What happens to a plant cell in a strong sugar solution?
It loses water, the vacuole shrinks, and the cell membrane pulls away from the cell wall.
The cell becomes flaccid or plasmolysed.
What happens to a plant cell placed in distilled water?
It gains water, the vacuole expands, and the membrane pushes against the cell wall, making the cell turgid.
The cell wall stops the cell bursting.
Why are turgid cells important to a plant?
They provide structural support and prevent wilting.
Define active transport.
The movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration, requiring energy released by respiration.
Why does active transport require energy?
Because the particles are moved against a concentration gradient.
The energy is released during cellular respiration.
Give two examples of active transport in living organisms.
Absorption of the products of digestion into the bloodstream from the small intestine
Absorption of mineral ions from the soil into the roots of a plant
True or False?
Osmosis requires energy released by respiration.
False.
Osmosis is passive and needs no energy. It is active transport that requires energy released by respiration.
Name the four factors that affect the rate of diffusion.
Surface area to volume ratio
Diffusion distance
Temperature
Concentration gradient
What happens to surface area to volume ratio as a cell gets bigger?
It decreases — volume increases faster than surface area.
There is then less surface area per unit of volume, so diffusion alone becomes less efficient.
How do larger organisms overcome a low surface area to volume ratio?
They have specialised exchange surfaces, such as the alveoli in the lungs and the villi in the small intestine.
Name three features of a specialised exchange surface.
A large surface area
Thin walls
A good blood supply, which maintains a steep concentration gradient
Why can a unicellular organism rely on diffusion alone?
It has a large surface area to volume ratio and a short diffusion distance, so substances reach every part of the cell quickly enough.
How does the small intestine increase its surface area?
Its inner surface is highly folded, and the cells lining the ileum have an increased surface area for absorbing the products of digestion.
Why are alveoli and capillary walls only one cell thick?
To minimise the diffusion distance, so oxygen and carbon dioxide can diffuse rapidly between the air in the alveoli and the blood in the capillaries.
How does temperature affect the rate of diffusion?
The higher the temperature, the more kinetic energy the molecules have, so they move faster.
This causes more collisions with the cell membrane and a faster rate of movement across it.
How does the concentration gradient affect the rate of diffusion?
The greater the difference in concentration either side of the membrane, the faster the movement across it, because more random collisions occur on the higher-concentration side.
The shorter the diffusion ＿＿＿＿＿＿＿＿, the faster diffusion can occur.
The shorter the diffusion distance, the faster diffusion can occur.
As a cell increases in size, its surface area to volume ratio ＿＿＿＿＿＿＿＿.
As a cell increases in size, its surface area to volume ratio decreases.
True or False?
A large cube has a higher surface area to volume ratio than a small one.
False.
As size increases, volume increases faster than surface area, so the ratio decreases. Small cubes have the higher ratio.
True or False?
Lowering the temperature slows the rate of diffusion.
True.
The molecules have less kinetic energy, so they move more slowly and collide with the membrane less often.
How do you calculate the surface area of a cube?
Area of one side × number of sides.
How do you calculate the volume of a cube?
Length × width × height.
How are the pink agar blocks made?
Molten agar is mixed with sodium hydroxide (an alkali) and phenolphthalein indicator, which turns the agar pink.
Once cooled, the solid pink agar is cut into blocks.
What colour is phenolphthalein in acidic and alkaline solutions?
Colourless below pH 8.3
Pink in alkaline solutions
Why does a pink agar block turn colourless in dilute acid?
The acid diffuses into the block and a neutralisation reaction occurs with the alkaline sodium hydroxide, so the phenolphthalein turns colourless.
Outline the agar-cube diffusion method.
Cut cubes of pink agar to different side lengths, such as 0.5 cm, 1 cm and 2 cm.
Pour 50 ml of dilute hydrochloric acid into a boiling tube.
Using forceps, add one cube and start a timer.
Stop the timer when the cube has turned completely colourless, and record the time.
Repeat at least twice more per size, then repeat for the other cube sizes.
Why do larger agar cubes take longer to lose their colour?
They have a smaller surface area to volume ratio, so less surface area per unit volume, and the diffusion distance to the centre is longer.
Smaller agar cubes have a ＿＿＿＿＿＿＿＿ surface area to volume ratio than larger cubes.
Smaller agar cubes have a larger surface area to volume ratio than larger cubes.
True or False?
The rate of diffusion through the agar changes with cube size.
False.
The rate of diffusion through the agar stays constant, since concentration gradient and temperature are unchanged. It is the total rate of exchange that depends on the cube's SA:V ratio.
What does the agar-cube investigation model?
How small organisms and cells with a high SA:V ratio can rely on diffusion alone to exchange substances such as oxygen and carbon dioxide, while larger organisms cannot.
Give one limitation of the agar-cube practical and an improvement.
Judging exactly when the cube has turned completely colourless is subjective and introduces human error.
Instead, measure the distance in mm the acid has diffused into each cube after a set time.
Outline the potato-cylinder osmosis method.
Prepare sucrose solutions from 0 to 1 mol/dm3 and set up 6 labelled test tubes with 10 cm3 of each.
Cut 6 equally sized potato cylinders with a cork borer and ruler.
Blot each with a paper towel and weigh it.
Put one cylinder in each solution, and after 4 hours remove, blot and reweigh them.
What is calculated from the potato cylinder masses?
The percentage change in mass for each cylinder.
Which potato cylinder gains the most mass, and why?
The one in distilled water.
There is the highest concentration gradient, so water moves into the cells by osmosis, increasing turgor pressure and making the potato firm.
Which potato cylinder loses the most mass, and why?
The one in the strongest sucrose solution.
Water moves out of the cells by osmosis, so they become flaccid and plasmolysed, and the potato feels soft.
What does no change in a cylinder's mass tell you?
There was no net movement of water into or out of the cells.
The external solution was the same concentration as the cytoplasm, so there was no concentration gradient.
A potato cylinder placed in distilled water will ＿＿＿＿＿＿＿＿ mass.
A potato cylinder placed in distilled water will gain mass.
Why use several potato cylinders at each concentration?
Slight differences between cylinders make results unreliable. Repeats allow anomalous results to be identified and ignored when a mean is calculated.
Which variables must be controlled in the osmosis practical?
The volume of sucrose solution used
The dimensions of the potato cylinders
Blotting each cylinder before every weighing
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