Exam questions · Biology
Supplying the Cell
- 66 exam questions
- 180 marks
- 65 quick checks
Diffusion
Just this lesson-
1 Name [1 mark]
Name the waste product that diffuses from cells into the blood plasma, to be excreted by the kidneys.
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Model answer
Urea.
Mark scheme
- Urea — 1 mark
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2 Describe [2 marks]
Describe what is meant by diffusion.
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Model answer
The spreading out of particles (of a gas or of a substance in solution), resulting in a net movement from an area of higher concentration to an area of lower concentration.
Mark scheme
- Particles spread out / move randomly — 1 mark
- Net movement from higher to lower concentration — 1 mark
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3 Name [3 marks]
Figure 1 shows particles on both sides of a cell membrane. Name the parts labelled A, B and C.
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Model answer
A is the region of higher concentration. B is the region of lower concentration. C is the cell membrane.
Mark scheme
- A: higher concentration — 1 mark
- B: lower concentration — 1 mark
- C: (cell) membrane — 1 mark
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4 Give [2 marks]
Give two substances that move into or out of cells by diffusion.
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Model answer
Any two from: oxygen, carbon dioxide, urea.
Mark scheme
- Oxygen — 1 mark
- Carbon dioxide — 1 mark
- Urea — 1 mark
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5 State [3 marks]
State three factors that affect the rate of diffusion.
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Model answer
The concentration gradient (difference in concentration); the temperature; the surface area of the membrane.
Mark scheme
- Concentration gradient — 1 mark
- Temperature — 1 mark
- Surface area (of the membrane) — 1 mark
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6 Explain [2 marks]
Explain why diffusion is faster at a higher temperature.
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Model answer
At a higher temperature the particles have more energy, so they move faster and spread out more quickly.
Mark scheme
- Particles have more energy — 1 mark
- So they move faster / spread out faster — 1 mark
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7 Calculate [2 marks]
A cell takes in 36 micrograms of a substance by diffusion in 9 minutes. Calculate the mean rate of uptake, in micrograms per minute.
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Model answer
36 ÷ 9 = 4 micrograms per minute.
Mark scheme
- 36 ÷ 9 — 1 mark
- 4 micrograms per minute (unit needed) — 1 mark
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8 Explain [3 marks]
Explain why oxygen diffuses from the air in the alveoli into the blood.
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Model answer
The concentration of oxygen is higher in the air in the alveoli than in the blood. So there is a net movement of oxygen from the alveoli into the blood, down the concentration gradient.
Mark scheme
- Higher concentration of oxygen in the alveoli — 1 mark
- Lower concentration of oxygen in the blood — 1 mark
- Net movement down the concentration gradient — 1 mark
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9 Explain [6 marks]
Explain how the concentration gradient, the temperature and the surface area of the membrane each affect the rate of diffusion.
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Model answer
The greater the concentration difference between the two sides, the faster the rate of diffusion, because more particles move down the gradient than back against it. The higher the temperature, the faster the rate of diffusion, because the particles have more energy and move faster. The larger the surface area of the membrane, the faster the rate of diffusion, because more particles can cross at the same time.
Mark scheme
- Bigger concentration difference gives faster diffusion — 1 mark
- Because more particles move down the gradient — 1 mark
- Higher temperature gives faster diffusion — 1 mark
- Because particles have more energy / move faster — 1 mark
- Larger surface area gives faster diffusion — 1 mark
- Because more particles can cross at once — 1 mark
Quick check
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1
Which statement describes diffusion?
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C: Net movement of particles from high to low concentration
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
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2
Which of these moves into cells by diffusion?
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B: Oxygen
Oxygen diffuses into respiring cells. Urea and carbon dioxide diffuse out of them.
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3
What happens to the rate of diffusion if the temperature is increased?
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A: It increases
The particles have more energy and move faster, so diffusion is faster.
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4
Which change would make diffusion across a membrane slower?
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D: A smaller membrane
A smaller surface area lets fewer particles cross at once, so the rate falls.
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5
A cell takes in 50 units of a substance in 10 minutes. What is the rate of uptake?
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B: 5 units per minute
Rate = 50 ÷ 10 = 5 units per minute.
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6
Where does urea diffuse to from the cells?
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C: Into the blood plasma
Urea diffuses from the cells into the blood plasma, and the kidneys then excrete it.
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7
Does diffusion need energy from respiration?
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A: No
No. The particles move on their own, so the cell does not have to use energy.
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8
Why does carbon dioxide diffuse out of respiring cells?
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D: Its concentration is higher inside the cell
Respiration makes carbon dioxide, so its concentration is higher inside the cell than outside, and it moves down the gradient.
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9
What does a steeper concentration gradient do to the rate of diffusion?
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B: It makes it faster
A bigger difference in concentration means more particles move down the gradient, so diffusion is faster.
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10
Once the particles are evenly spread out, what is true?
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C: There is no net movement
The particles are still moving, but as many move one way as the other, so there is no net movement.
Osmosis
Just this lesson-
1 Name [1 mark]
Name the type of membrane through which osmosis takes place.
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Model answer
A partially permeable membrane.
Mark scheme
- Partially permeable (membrane) — 1 mark
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2 State [2 marks]
State what is meant by osmosis.
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Model answer
The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Mark scheme
- Diffusion of water from a dilute to a concentrated solution — 1 mark
- Through a partially permeable membrane — 1 mark
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3 Name [3 marks]
Figure 1 shows two solutions separated by a membrane. Name the parts labelled A, B and C.
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Model answer
A is the dilute solution. B is the concentrated solution. C is the partially permeable membrane.
Mark scheme
- A: dilute solution — 1 mark
- B: concentrated solution — 1 mark
- C: partially permeable membrane — 1 mark
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4 Explain [2 marks]
In Figure 1, the sugar molecules do not move across the membrane. Explain why.
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Model answer
The membrane is only partially permeable. The sugar molecules are too large to pass through its tiny holes, but the small water molecules can.
Mark scheme
- Membrane is partially permeable — 1 mark
- Sugar molecules are too big to pass through — 1 mark
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5 Describe [2 marks]
A plant cell is placed in pure water. Describe what happens to the cell.
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Model answer
Water enters the cell by osmosis, and the cell becomes turgid (swollen and firm).
Mark scheme
- Water enters by osmosis — 1 mark
- Cell becomes turgid / swollen and firm — 1 mark
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6 Explain [3 marks]
Explain why a red blood cell placed in pure water may burst, but a plant cell does not.
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Model answer
Water enters both cells by osmosis. A red blood cell has no cell wall, so nothing stops it swelling, and it may burst. A plant cell has a strong cell wall, which stops it bursting.
Mark scheme
- Water enters by osmosis — 1 mark
- Red blood cell has no cell wall, so it may burst — 1 mark
- Plant cell wall prevents bursting — 1 mark
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7 Explain [3 marks]
A plant that has not been watered for several days wilts. Use ideas about osmosis to explain why.
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Model answer
The soil water becomes more concentrated, so water leaves the plant cells by osmosis. The cells lose water and go flaccid, so they can no longer hold the leaves and stems up and the plant wilts.
Mark scheme
- Water leaves the cells by osmosis — 1 mark
- Cells become flaccid / lose firmness — 1 mark
- So stems and leaves are no longer supported (wilting) — 1 mark
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8 Explain [2 marks]
Solution P has a concentration of 0.2 mol/dm³ and solution Q has a concentration of 0.6 mol/dm³. They are separated by a partially permeable membrane. State the direction in which water moves and explain why.
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Model answer
Water moves from P to Q. P is the more dilute solution, and water moves from a dilute solution to a concentrated solution by osmosis.
Mark scheme
- From P to Q (into the more concentrated solution) — 1 mark
- Water moves from dilute to concentrated by osmosis — 1 mark
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9 Explain [4 marks]
A student puts a red blood cell into a very concentrated salt solution. Describe and explain what happens to the cell.
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Model answer
The salt solution is more concentrated than the cytoplasm of the red blood cell. Water leaves the cell by osmosis, moving from the more dilute cytoplasm to the more concentrated solution. The cell loses water, so it shrinks and shrivels.
Mark scheme
- Solution is more concentrated than the cell — 1 mark
- Water leaves the cell by osmosis — 1 mark
- From dilute (cell) to concentrated (outside) — 1 mark
- Cell shrinks / shrivels — 1 mark
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10 Describe and explain [6 marks]
Describe and explain what happens to plant cells when they are placed in (a) a dilute solution and (b) a concentrated solution.
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Model answer
In a dilute solution, the solution outside is more dilute than the cell sap, so water enters the cells by osmosis. The vacuole swells and the cell becomes turgid. The cell wall stops it bursting. In a concentrated solution, the solution outside is more concentrated than the cell sap, so water leaves the cells by osmosis. The vacuole shrinks and the cell becomes flaccid. If a lot of water is lost, the membrane pulls away from the cell wall.
Mark scheme
- Dilute: water enters by osmosis — 1 mark
- Cell becomes turgid / swollen and firm — 1 mark
- Cell wall prevents bursting — 1 mark
- Concentrated: water leaves by osmosis — 1 mark
- Cell becomes flaccid / soft — 1 mark
- Reason: outside solution is more concentrated than the cell sap — 1 mark
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11 Explain [2 marks]
A potato cell is placed in a concentrated sugar solution. Use the term water potential to explain the direction in which water moves.
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Model answer
The sugar solution has a lower water potential than the cell sap. Water moves by osmosis from the cell, which has the higher water potential, into the solution.
Mark scheme
- The solution has a lower water potential than the cell — 1 mark
- Water moves from higher to lower water potential, out of the cell — 1 mark
Quick check
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1
Osmosis is the movement of which substance?
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A: Water
Osmosis is the diffusion of water through a partially permeable membrane.
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2
In which direction does water move by osmosis?
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C: From a dilute solution to a concentrated solution
Water moves from a dilute solution to a concentrated solution through a partially permeable membrane.
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3
A plant cell is placed in a very dilute solution. What happens?
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B: It becomes turgid
Water enters by osmosis and the cell becomes turgid. The cell wall stops it bursting.
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4
What does flaccid mean?
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D: Soft and limp from losing water
A flaccid cell has lost water and is soft and limp.
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5
Why does a red blood cell burst in pure water, but a plant cell does not?
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C: A red blood cell has no cell wall
A red blood cell has no cell wall, so it cannot resist the pressure as water enters. A plant cell has a strong cell wall.
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6
Two solutions of 0.1 and 0.5 mol/dm³ are separated by a partially permeable membrane. Which way does water move?
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A: From 0.1 into 0.5
Water moves from the more dilute solution (0.1) to the more concentrated one (0.5).
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7
A cell is in a solution of the same concentration as its contents. What happens to the water?
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B: There is no net movement
There is no net movement of water. Water molecules still move both ways, but equally.
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8
Why does a plant wilt when it is short of water?
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D: Its cells become flaccid
Its cells lose water by osmosis and become flaccid, so they no longer hold the plant up.
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9
What does a partially permeable membrane do?
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A: Lets some substances through but not others
It lets small molecules such as water through but stops larger molecules.
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10
A student says: "Water moves by osmosis to where there is more water." What is wrong with this?
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C: It is the wrong way round: water moves from more water to less
Water moves from where there is more water (a dilute solution) to where there is less (a concentrated solution).
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11
Water moves by osmosis from a region of
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C: Higher to lower water potential
Water moves down a water potential gradient, from a higher to a lower water potential.
Investigating Osmosis in Plant Tissue
Just this lesson-
1 State [2 marks]
State two variables that should be controlled in the investigation.
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Model answer
Any two from: the length (and width) of the potato cylinders; the volume of the solutions; the time the cylinders are left in the solutions; the temperature; the type of potato.
Mark scheme
- Size / length of cylinder — 1 mark
- Volume of solution — 1 mark
- Time in solution — 1 mark
- Temperature / type of potato — 1 mark
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2 Explain [2 marks]
Explain why the potato cylinders are blotted dry before their final mass is measured.
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Model answer
To remove solution from the surface of the cylinder. Any left on the surface would add mass, so the result would not be due only to osmosis.
Mark scheme
- To remove liquid from the surface — 1 mark
- Because it would add extra mass / give an inaccurate result — 1 mark
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3 Calculate [2 marks]
A potato cylinder has a mass of 2.50 g at the start and 2.85 g at the end. Calculate the percentage change in mass.
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Model answer
(2.85 − 2.50) ÷ 2.50 × 100 = +14%.
Mark scheme
- 0.35 ÷ 2.50 — 1 mark
- +14 (%) — 1 mark
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4 Calculate [3 marks]
A potato cylinder has a mass of 3.20 g at the start and 2.72 g at the end. Calculate the percentage change in mass.
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Model answer
(2.72 − 3.20) ÷ 3.20 × 100 = −15%.
Mark scheme
- Change = −0.48 g — 1 mark
- −0.48 ÷ 3.20 × 100 — 1 mark
- −15 (%) — 1 mark
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5 Explain [3 marks]
Figure 1 shows the results of an investigation into the effect of sugar concentration on potato cylinders. Estimate the concentration at which there is no change in mass, and explain what this tells you.
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Model answer
About 0.33 mol/dm³ (accept 0.30 to 0.35), where the line crosses 0% change. At this concentration the solution is the same as the concentration of the potato cells, so there is no net movement of water.
Mark scheme
- 0.30 to 0.35 mol/dm³ — 1 mark
- Same concentration as the potato cells — 1 mark
- So no net movement of water by osmosis — 1 mark
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6 Use the graph [2 marks]
Use Figure 1 to give the percentage change in mass in the solution of concentration 0.6 mol/dm³, and say what this shows about the movement of water.
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Model answer
−12% (accept −11 to −13). A loss of mass shows that water moved out of the potato by osmosis.
Mark scheme
- −12% — 1 mark
- Water left the potato (by osmosis) — 1 mark
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7 Explain [2 marks]
The potato cylinders in the most concentrated solutions lost mass. Explain why.
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Model answer
The solution was more concentrated than the cell sap in the potato. Water moved out of the potato cells into the solution by osmosis, so the mass fell.
Mark scheme
- Solution more concentrated than the potato cell sap — 1 mark
- Water left the cells by osmosis — 1 mark
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8 Explain [2 marks]
A student calculates the percentage change in mass for each cylinder instead of just the change in mass. Explain why.
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Model answer
The cylinders do not all start with exactly the same mass, so a percentage of the starting mass allows a fair comparison.
Mark scheme
- Cylinders have different starting masses — 1 mark
- Percentage change allows a fair comparison — 1 mark
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9 Suggest [2 marks]
Suggest two ways in which the student could make the results more reliable.
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Model answer
Repeat the investigation with several cylinders in each solution and calculate a mean; check for anomalous results; keep the control variables the same each time.
Mark scheme
- Repeat and calculate a mean — 1 mark
- Spot and deal with any anomalous results / control variables carefully — 1 mark
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10 Describe [6 marks]
Describe how you would carry out an investigation into the effect of sugar concentration on the mass of potato cylinders.
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Model answer
Cut cylinders of potato of the same length with a cork borer. Measure the mass of each cylinder. Put the same volume of different concentrations of sugar solution into test tubes, including pure water. Put one cylinder in each tube and leave them for the same length of time at the same temperature. Take the cylinders out, blot them dry and measure the mass of each again. Calculate the percentage change in mass for each, and repeat the investigation to calculate a mean.
Mark scheme
- Cut identical cylinders of potato — 1 mark
- Measure the starting mass — 1 mark
- Same volume of different concentrations of solution — 1 mark
- Same time (and temperature) in the solutions — 1 mark
- Blot dry and measure the final mass — 1 mark
- Calculate the percentage change / repeat and take a mean — 1 mark
Quick check
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1
What is the independent variable in the potato investigation?
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B: The concentration of the solution
The independent variable is the one you change: the concentration of the solution.
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2
What is the dependent variable?
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A: The percentage change in mass
The dependent variable is what you measure: the percentage change in mass of the potato.
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3
A potato cylinder gains mass in a solution. What does this show?
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C: The solution was more dilute than the cell sap
The potato gained water by osmosis, so the solution was more dilute than the potato cell sap.
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4
A cylinder goes from 2.00 g to 1.80 g. What is the percentage change in mass?
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D: −10%
The change is −0.20 g. Dividing by the start mass gives −0.1, so the change is −10%.
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5
Why is the surface of each cylinder blotted dry?
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B: To remove liquid that would add extra mass
Solution left on the surface would add mass and affect the result.
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6
On the graph, where does the line cross zero change?
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A: At the concentration of the potato cell sap
This is the concentration at which the solution matches the potato cell sap, so there is no net movement of water.
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7
Which would make the results more reliable?
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C: Repeating and calculating a mean
Repeating the test and calculating a mean reduces the effect of random errors and anomalies.
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8
Which of these is a control variable?
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D: The volume of the solution
The volume of the solution has to be the same in every tube. The concentration is what is changed, and the mass is what is measured.
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9
Why is pure water used as one of the solutions?
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A: It is the most dilute solution and gives the zero concentration
Pure water is the most dilute solution, so it shows the biggest gain in mass, and gives a zero point on the concentration scale.
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10
A student finds the line crosses zero at 0.3 mol/dm³ and wants a more accurate value. What should they do?
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B: Use extra solutions with concentrations close to 0.3
Using more solutions close to 0.3, such as 0.25, 0.30 and 0.35 mol/dm³, narrows down where the line crosses zero.
Active Transport
Just this lesson-
1 State [1 mark]
State what is needed for active transport.
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Model answer
Energy (from respiration).
Mark scheme
- Energy from respiration — 1 mark
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2 Describe [2 marks]
Describe what is meant by active transport.
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Model answer
The movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient), using energy from respiration.
Mark scheme
- Movement from dilute to concentrated / against a concentration gradient — 1 mark
- Needs energy from respiration — 1 mark
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3 Name [4 marks]
Figure 1 shows ions being taken into a cell. A is a carrier protein. Name the structure B, name the process shown, and explain how you know it is that process.
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Model answer
B is a mitochondrion. The process is active transport. The ions move from a lower to a higher concentration, which is against the concentration gradient, and the mitochondrion supplies energy from respiration, which diffusion would not need.
Mark scheme
- B: mitochondrion — 1 mark
- Active transport — 1 mark
- Ions move from lower to higher concentration / against the gradient — 1 mark
- Needs energy from respiration — 1 mark
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4 Give [1 mark]
Give one example of active transport in a plant.
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Model answer
Mineral ions being taken into root hair cells from the soil.
Mark scheme
- Mineral ions into root hair cells — 1 mark
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5 Explain [3 marks]
Explain why root hair cells take up mineral ions by active transport and not by diffusion.
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Model answer
The concentration of mineral ions in the soil solution is lower than in the root hair cell. Diffusion only moves substances from a higher to a lower concentration. Active transport can move ions against the concentration gradient, using energy.
Mark scheme
- Lower concentration of ions in the soil than in the cell — 1 mark
- Diffusion would not move ions into the cell / only goes down the gradient — 1 mark
- Active transport moves them against the gradient using energy — 1 mark
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6 Explain [2 marks]
Sugar is absorbed from the gut into the blood by active transport. Explain why the sugar must be absorbed.
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Model answer
Sugar molecules are used for cell respiration, so the body needs to absorb them.
Mark scheme
- Sugar is used for respiration — 1 mark
- To release energy for the body's cells — 1 mark
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7 Explain [3 marks]
Root hair cells contain large numbers of mitochondria. Explain why.
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Model answer
Mitochondria release energy by respiration. Root hair cells take up mineral ions by active transport, which needs energy. So they need many mitochondria to supply it.
Mark scheme
- Mitochondria release energy / respiration — 1 mark
- Active transport needs energy — 1 mark
- Root hair cells take in ions by active transport — 1 mark
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8 Explain [3 marks]
A farmer notices that plants in waterlogged soil take up fewer mineral ions, although the soil contains plenty. Use ideas about active transport to explain why.
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Model answer
Waterlogged soil contains little oxygen, so the root cells respire less. They release less energy, and active transport of mineral ions needs energy. So fewer ions are taken up.
Mark scheme
- Little oxygen in the waterlogged soil, so less respiration — 1 mark
- Less energy released — 1 mark
- Active transport of ions needs energy, so less uptake — 1 mark
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9 Compare [3 marks]
Give three differences between diffusion and active transport.
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Model answer
Diffusion moves substances from a higher to a lower concentration, but active transport moves them from a lower to a higher concentration. Diffusion does not need energy, but active transport needs energy from respiration. Diffusion goes down the concentration gradient, but active transport goes against it.
Mark scheme
- Direction: high to low (diffusion) compared with low to high (active transport) — 1 mark
- Diffusion needs no energy, active transport does — 1 mark
- Down the gradient compared with against the gradient — 1 mark
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10 Compare [6 marks]
Compare diffusion, osmosis and active transport.
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Model answer
Diffusion is the net movement of particles from a higher to a lower concentration, down the concentration gradient, and needs no energy. Osmosis is the diffusion of water only, from a dilute solution to a concentrated solution through a partially permeable membrane, and needs no energy. Active transport moves substances from a more dilute to a more concentrated solution, against a concentration gradient, and needs energy from respiration.
Mark scheme
- Diffusion: particles from high to low concentration — 1 mark
- Diffusion needs no energy — 1 mark
- Osmosis: water only — 1 mark
- Osmosis: dilute to concentrated through a partially permeable membrane — 1 mark
- Active transport: dilute to concentrated (against the gradient) — 1 mark
- Active transport needs energy from respiration — 1 mark
Quick check
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1
What does active transport need?
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C: Energy from respiration
Active transport needs energy from respiration, because it moves substances against a concentration gradient.
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2
In which direction does active transport move substances?
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A: From low concentration to high concentration
Active transport moves substances from a more dilute to a more concentrated solution, against the concentration gradient.
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3
Which structures supply the energy for active transport?
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D: Mitochondria
Mitochondria release energy from respiration, so cells that do a lot of active transport have many of them.
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4
Which of these is an example of active transport?
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B: Mineral ions moving into root hair cells
Root hair cells take in mineral ions from the very dilute soil solution by active transport.
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5
Why do plants need mineral ions?
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A: For healthy growth
Plants need ions for healthy growth, for example nitrate to make proteins and magnesium to make chlorophyll.
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6
Sugar concentration is lower in the gut than in the blood. How does sugar move into the blood?
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C: By active transport
It has to move against the concentration gradient, so it is absorbed by active transport.
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7
Which process needs no energy from the cell?
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D: Diffusion and osmosis
Diffusion and osmosis are passive processes, which move things down a gradient without energy from the cell.
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8
What happens to active transport if respiration in a cell stops?
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B: It stops or slows down
Active transport needs energy from respiration, so it stops or slows down.
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9
Which statement about osmosis is correct?
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A: It needs no energy and moves only water
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. It needs no energy.
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10
A cell takes in a substance from a place where it is at a lower concentration. Which is the most likely process?
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C: Active transport
Moving a substance from a lower to a higher concentration is against the gradient, so it must be active transport.
The Cell Cycle and Mitosis
Just this lesson-
1 Name [1 mark]
Name the type of cell division that produces two genetically identical cells for growth and repair.
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Model answer
Mitosis.
Mark scheme
- Mitosis — 1 mark
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2 State [2 marks]
State two reasons why mitosis is important in a human.
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Model answer
Any two from: growth (and development); repair of damaged tissue; replacement of worn-out cells.
Mark scheme
- Growth / development — 1 mark
- Repair of damaged tissue — 1 mark
- Replacement of cells — 1 mark
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3 Name [3 marks]
Figure 1 shows the three stages of the cell cycle. Name the stages A, B and C.
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Model answer
A is growth and DNA replication (accept interphase). B is mitosis. C is the division of the cytoplasm and cell membrane (accept cytokinesis).
Mark scheme
- A: growth and DNA replication — 1 mark
- B: mitosis — 1 mark
- C: the cytoplasm (and cell membrane) divides — 1 mark
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4 Describe [3 marks]
Describe what happens in a cell during the first stage of the cell cycle, before mitosis.
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Model answer
The cell grows. The number of sub-cellular structures, such as ribosomes and mitochondria, increases. The DNA replicates to form two copies of each chromosome.
Mark scheme
- The cell grows — 1 mark
- More ribosomes and mitochondria / sub-cellular structures — 1 mark
- DNA replicates, giving two copies of each chromosome — 1 mark
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5 Explain [2 marks]
A human skin cell has 46 chromosomes. It divides by mitosis. State the number of chromosomes in each new cell, and explain your answer.
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Model answer
46 in each new cell. The DNA is copied before the cell divides, and one copy of each chromosome is pulled to each end, so each new cell gets a full set.
Mark scheme
- 46 — 1 mark
- DNA replicated first / one copy of every chromosome goes to each new cell — 1 mark
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6 Put in order [2 marks]
Figure 2 shows six stages of mitosis in the wrong order. Write the letters in the correct order, starting with the cell before it divides.
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Model answer
B, E, D, A, F, C.
Mark scheme
- B first and C last — 1 mark
- E, D, A, F in the correct order between them — 1 mark
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7 Calculate [2 marks]
A cell divides by mitosis. Every new cell then divides, and this happens 5 times in total. Calculate the number of cells at the end.
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Model answer
\(2^{5} = 32\) cells.
Mark scheme
- Doubles each time / \(2^{5}\) — 1 mark
- 32 — 1 mark
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8 Explain [4 marks]
Explain how mitosis helps a cut in the skin to heal.
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Model answer
Cells at the edge of the cut divide by mitosis. This makes new cells, which fill the gap and replace the damaged cells. The new cells are genetically identical to the original skin cells, so they have the same genes and do the same job, and the skin is repaired.
Mark scheme
- Cells next to the cut divide by mitosis — 1 mark
- New cells are made — 1 mark
- They fill the gap / replace damaged cells — 1 mark
- New cells are genetically identical, so they do the same job — 1 mark
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9 Describe and explain [6 marks]
Describe the stages of the cell cycle, and explain why mitosis produces two genetically identical cells.
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Model answer
In the first stage the cell grows and the numbers of ribosomes and mitochondria increase. The DNA replicates, giving two identical copies of each chromosome. In mitosis, one set of chromosomes is pulled to each end of the cell and the nucleus divides. Finally the cytoplasm and cell membrane divide, forming two cells. The two cells are genetically identical because the DNA was copied exactly before division and each new cell receives one copy of every chromosome, so each has the same genes and the same number of chromosomes.
Mark scheme
- The cell grows and makes more ribosomes and mitochondria — 1 mark
- DNA replicates / two copies of each chromosome — 1 mark
- In mitosis one set of chromosomes is pulled to each end — 1 mark
- Nucleus divides, then cytoplasm and membrane divide — 1 mark
- Each new cell gets one copy of every chromosome — 1 mark
- So cells have the same genes / are genetically identical — 1 mark
Quick check
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1
How many cells does one cell make when it divides by mitosis?
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B: Two
Mitosis, followed by division of the cytoplasm, makes two cells.
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2
What happens to the DNA before mitosis?
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C: It is copied
The DNA replicates, giving two identical copies of each chromosome, so that each new cell can have a full set.
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3
What happens to the chromosomes in mitosis?
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A: One set is pulled to each end of the cell
One set of chromosomes is pulled to each end of the cell and the nucleus divides.
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4
A human cell with 46 chromosomes divides by mitosis. How many chromosomes are in each new cell?
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D: 46
Mitosis makes genetically identical cells with the same number of chromosomes as the parent cell.
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5
Which of these is NOT a job of mitosis?
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C: Making egg cells with half the chromosomes
Mitosis is used for growth, repair and replacement. Making egg and sperm cells uses a different type of cell division, which you meet later in the course.
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6
A cell divides by mitosis 3 times. Every new cell also divides. How many cells are there at the end?
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B: 8
The number doubles each time: 1, 2, 4, 8. That is 2 to the power 3, which is 8.
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7
What are the two new cells formed by mitosis called?
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A: Daughter cells
The two cells formed when a cell divides are called daughter cells.
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8
A student says: "The two new cells are identical to the parent cell because they are the same size." What is wrong with this?
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D: Identical means the same genes, and it is the DNA copying that makes this so
Cells being genetically identical means they have the same genes. It is the copying of the DNA before division that makes them identical, not their size or shape.
Cell Specialisation and Differentiation
Just this lesson-
1 Give [2 marks]
Give two ways in which a sperm cell is adapted to its function.
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Model answer
Any two from: a tail to swim; many mitochondria to release energy for swimming; an acrosome containing enzymes to digest the egg's outer layer.
Mark scheme
- Tail, to swim to the egg — 1 mark
- Many mitochondria, for energy — 1 mark
- Acrosome with enzymes to digest the egg — 1 mark (maximum 2 marks)
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2 Name [3 marks]
Figure 1 shows a sperm cell. Name the structures labelled A, B and C.
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Model answer
A is the acrosome. B is the mitochondria (the middle section). C is the tail.
Mark scheme
- A: acrosome — 1 mark
- B: mitochondria — 1 mark
- C: tail — 1 mark
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3 Explain [2 marks]
Muscle cells contain many mitochondria. Explain why this adaptation helps them to do their job.
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Model answer
Muscle cells contract, and contraction needs energy. Mitochondria release that energy in respiration.
Mark scheme
- Mitochondria release energy / respiration — 1 mark
- Muscle cells need energy to contract — 1 mark
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4 Describe [3 marks]
Describe how a root hair cell is adapted to its function.
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Model answer
The root hair cell has a long, thin projection, which gives a large surface area for absorbing water and mineral ions. It has a large permanent vacuole, which speeds up the movement of water into the cell. It has many mitochondria, which release energy for the active transport of mineral ions.
Mark scheme
- Long projection / root hair, giving a large surface area — 1 mark
- Large permanent vacuole speeds up water uptake — 1 mark
- Many mitochondria release energy for active transport — 1 mark
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5 Describe [3 marks]
Describe how xylem vessels are adapted to carry water up a plant.
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Model answer
Xylem vessels are hollow tubes because the cells are dead and have no end walls, so water can flow through easily. Their walls are strengthened with rings of lignin, which stops them collapsing.
Mark scheme
- Hollow / no cytoplasm / no end walls — 1 mark
- Cells are dead — 1 mark
- Walls strengthened with lignin — 1 mark
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6 Explain [4 marks]
Explain how the structure of a nerve cell is related to its function.
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Model answer
A nerve cell carries electrical impulses from one part of the body to another. The long axon allows the impulse to travel a long distance. The myelin sheath insulates the axon and speeds up the impulse. The branched dendrites and axon terminals connect the cell to many other nerve cells.
Mark scheme
- Function: carries electrical impulses — 1 mark
- Long axon carries impulses over long distances — 1 mark
- Myelin sheath insulates / speeds up the impulse — 1 mark
- Branched ends connect to other nerve cells — 1 mark
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7 Explain [3 marks]
Explain why differentiation is important in a multicellular organism.
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Model answer
Differentiation produces cells with different structures. Each type of cell can carry out its own particular function efficiently, such as contracting or carrying impulses. Different types of cell working together makes it possible for a complex organism to survive.
Mark scheme
- Cells become specialised / have different structures — 1 mark
- Each can do a particular job efficiently — 1 mark
- Specialised cells work together to keep the organism alive — 1 mark
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8 Compare [2 marks]
Compare differentiation in mature animals with differentiation in plants.
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Model answer
Most animal cells differentiate early on, and cell division in mature animals is mainly for repair and replacement. Many plant cells keep the ability to differentiate throughout life.
Mark scheme
- Most animal cells differentiate early / division for repair and replacement — 1 mark
- Many plant cells can differentiate throughout life — 1 mark
Quick check
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1
What is differentiation?
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C: A cell changing to become specialised
Differentiation is the process by which a cell changes to become specialised for a particular job.
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2
Which part of a sperm cell contains enzymes to digest the egg's outer layer?
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B: The acrosome
The acrosome at the front of the sperm head holds the digestive enzymes.
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3
Why does a nerve cell have a long axon?
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A: To carry impulses over long distances
A long axon lets the cell carry electrical impulses from one part of the body to another.
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4
Which cell contains special protein fibres that slide over each other?
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D: Muscle cell
Muscle cells contain protein fibres that slide together, shortening the cell so that it contracts.
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5
How does the shape of a root hair cell help it?
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B: It gives a large surface area for absorption
The long projection increases the surface area, so more water and mineral ions can be absorbed.
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6
Which tissue carries dissolved sugars around a plant?
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C: Phloem
Phloem carries dissolved sugars. Xylem carries water and mineral ions.
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7
Why is xylem strengthened with lignin?
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A: To stop the tube collapsing
Lignin rings make the walls strong so the hollow tube does not collapse as water is pulled up.
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8
Which of these is the correct order of organisation, smallest first?
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D: Cell, tissue, organ, organ system
Specialised cells form tissues, tissues form organs, and organs make up organ systems.
Stem Cells
Just this lesson-
1 Name [1 mark]
Name the plant tissue that contains stem cells.
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Model answer
Meristem.
Mark scheme
- Meristem — 1 mark
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2 Describe [2 marks]
Describe what is meant by a stem cell.
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Model answer
An undifferentiated cell that can divide to make more cells, and that can differentiate into specialised cells.
Mark scheme
- An undifferentiated cell — 1 mark
- Can divide / produce more cells and differentiate into other cell types — 1 mark
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3 Name [3 marks]
Figure 1 shows three places where stem cells are found. Name the places A, B and C.
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Model answer
A is an embryo. B is (adult) bone marrow. C is a meristem (in a plant).
Mark scheme
- A: embryo — 1 mark
- B: bone marrow — 1 mark
- C: meristem / plant shoot tip — 1 mark
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4 State [2 marks]
State two places in the human body where stem cells are found.
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Model answer
In the embryo and in the (adult) bone marrow.
Mark scheme
- Embryo — 1 mark
- (Adult) bone marrow — 1 mark
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5 Describe [2 marks]
Describe one difference between stem cells from human embryos and stem cells from adult bone marrow.
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Model answer
Embryonic stem cells can become most types of human cell, but adult bone marrow stem cells can become fewer types, such as blood cells.
Mark scheme
- Embryonic stem cells can form most types of cell — 1 mark
- Bone marrow stem cells can form fewer types / mainly blood cells — 1 mark
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6 Explain [3 marks]
Figure 2 shows a young plant. Name the tissue found at A and at B, and explain why the plant can keep growing throughout its life.
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Model answer
A and B are both meristem tissue (meristems). The plant can keep growing because meristems contain stem cells that keep dividing and differentiating throughout the life of the plant.
Mark scheme
- A and B are meristems — 1 mark
- Stem cells in meristems keep dividing — 1 mark
- And differentiate into any type of plant cell, throughout its life — 1 mark
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7 Explain [3 marks]
Explain how stem cells help a human embryo to develop into a baby.
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Model answer
Stem cells in the embryo divide to make many more cells. The cells then differentiate into the different types of specialised cell, such as nerve, muscle and blood cells. The specialised cells form the tissues and organs of the baby.
Mark scheme
- Stem cells divide to make more cells — 1 mark
- Cells differentiate into different specialised cells — 1 mark
- Specialised cells form the tissues and organs — 1 mark
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8 Explain [3 marks]
Stem cells from bone marrow can be used to replace blood cells but not brain cells. Explain why.
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Model answer
Adult bone marrow stem cells can only differentiate into a limited range of cell types. They can form blood cells, but they cannot differentiate into most other types of cell, such as brain cells.
Mark scheme
- Adult stem cells differentiate into fewer types of cell — 1 mark
- They can form blood cells — 1 mark
- They cannot form most other types, such as brain cells — 1 mark
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9 Compare [6 marks]
Compare stem cells in human embryos, in adult bone marrow and in plant meristems.
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Model answer
All three are stem cells, which means they are undifferentiated, they can divide, and they can differentiate. Embryonic stem cells are found in human embryos and can become most types of human cell. Stem cells in adult bone marrow can become fewer types of cell, including blood cells. In plants, meristem tissue is found at the tips of shoots and roots and can become any type of plant cell, throughout the life of the plant. So the difference is in the range of cells each can make and the length of time they are active.
Mark scheme
- All are undifferentiated cells that divide and can differentiate — 1 mark
- Embryos: most types of human cell — 1 mark
- Bone marrow: fewer types, such as blood cells — 1 mark
- Meristems: any type of plant cell — 1 mark
- Meristems are in the tips of shoots and roots — 1 mark
- Meristems stay active throughout the plant's life / embryonic cells more types than adult — 1 mark
Quick check
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1
What does undifferentiated mean?
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C: Not specialised for a particular job
An undifferentiated cell has not yet specialised for a particular job.
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2
Which cells can differentiate into most types of human cell?
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B: Embryonic stem cells
Stem cells from human embryos can be made to differentiate into most types of human cell.
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3
What can stem cells from adult bone marrow form?
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A: Many types of cell, including blood cells
Adult bone marrow stem cells can form many types of cell, including blood cells.
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4
Where are meristems found?
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D: At the tips of the shoots and roots
Meristems are found at the tips of shoots and roots, where the plant grows.
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5
What can meristem cells become?
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C: Any type of plant cell
Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.
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6
Why can a plant keep growing throughout its life?
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B: Meristems keep producing new cells
Its meristems contain stem cells that keep dividing and differentiating for the whole life of the plant.
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7
Why is a red blood cell NOT a stem cell?
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A: It is specialised and cannot become other types of cell
A red blood cell is specialised for carrying oxygen. It has differentiated and does not divide to make other cell types.
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8
Which statement about stem cells is correct?
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D: Embryonic stem cells can become more types of cell than adult stem cells
A stem cell can make more cells like itself and some of the cells it makes can differentiate. Embryonic stem cells can become more types of cell than adult stem cells.