(a) A student investigated how surface area affected the rate of diffusion. The student started with four agar cubes that each had a volume of \(1cm^3\). The agar cubes were dyed with a red indicator. The red indicator turned yellow when exposed to acid. The student cut the cubes into different numbers of blocks to increase the surface area. The blocks were placed in acid, as shown in Fig. 3.1. The student recorded the time taken for all of the agar blocks in each test‑tube to turn yellow. The student repeated the investigation twice and calculated a mean.
(i) Calculate the mean time for all the blocks to turn yellow in test‑tube C. Give your answer to the nearest whole number. Space for working.
(ii) Using the results shown in Table 3.1, describe the effect of surface area on diffusion.
(iii) State one factor, other than surface area, that could affect diffusion in this investigation.
(iv) Complete the sentences to describe how the acid particles diffuse to cause the colour change. The acid particles move from the acid solution into the ………. , down a …………. gradient. The energy for this process comes from the ………….. energy of random movement of the acid particles.
(b) Describe one way that osmosis differs from other types of diffusion.
(c) Substances needed for respiration diffuse into cells.
(i) Circle the two substances needed for aerobic respiration in humans.
carbon dioxide glucose glycogen oxygen protein urea water
(ii) State where aerobic respiration occurs in cells.
(iii) State the name of the part of all cells that substances pass through to enter the cell.
▶️ Answer/Explanation
Ans:
(a)(i) 82
The mean time for test-tube C is calculated as \(\frac{80 + 85 + 81}{3} = 82\) (rounded to the nearest whole number).
(a)(ii)
Increasing the surface area increases the rate of diffusion, as seen by the shorter time taken for the blocks to turn yellow.
(a)(iii)
Temperature or concentration of acid could also affect the rate of diffusion.
(a)(iv)
The acid particles move from the acid solution into the agar, down a concentration gradient. The energy comes from the kinetic energy of the particles.
(b)
Osmosis differs from diffusion as it specifically involves the movement of water molecules across a partially permeable membrane.
(c)(i)
The two substances needed for aerobic respiration are glucose and oxygen.
(c)(ii)
Aerobic respiration occurs in the mitochondria of cells.
(c)(iii)
Substances pass through the cell membrane to enter the cell.
(a) Fig. 1.1 is a diagram showing the diffusion of oxygen molecules across a cell membrane.
The arrow shows the direction of the net movement of oxygen molecules through the membrane.
(i) Using the information in Fig. 1.1, explain why the net movement of oxygen molecules is from the outside of the cell to the inside of the cell.
(ii) State where the energy for diffusion comes from.
(b) (i) State the name of the cells that transport oxygen in the human body.
(ii) State the name of the molecule that oxygen binds to when it is transported around the body.
(c) Table 1.1 shows some factors that may affect the rate of diffusion of oxygen molecules.
Complete Table 1.1 by stating whether each factor increases, decreases or has no effect on the rate of diffusion of oxygen molecules.
(d) Some molecules are transported by active transport rather than by diffusion.
Describe what is meant by the term active transport.
▶️ Answer/Explanation
(a)(i)
The net movement of oxygen is from outside to inside the cell because there is a higher concentration of oxygen molecules outside. Diffusion occurs down the concentration gradient, from high to low concentration.
(a)(ii)
The energy for diffusion comes from the kinetic energy of the molecules, which causes their random movement.
(b)(i)
Oxygen is transported by red blood cells in the human body.
(ii) Oxygen binds to haemoglobin, forming oxyhaemoglobin for transport.
(c)
• Increasing temperature increases diffusion rate (more kinetic energy).
• Decreasing surface area decreases diffusion rate (less space for molecules to pass).
• Increasing concentration gradient increases diffusion rate (steeper gradient drives faster movement).
• Thicker membrane decreases diffusion rate (longer distance for molecules to travel).
(d)
Active transport is the movement of molecules against their concentration gradient (from low to high concentration), requiring energy (ATP) from respiration. Unlike diffusion, it is an energy-dependent process.
Final Answers:
(a)(i) Net movement occurs due to higher oxygen concentration outside the cell.
(a)(ii) Kinetic energy of molecules.
(b)(i) Red blood cells.
(b)(ii) Haemoglobin.
(c) See completed table above.
(d) Movement against gradient using energy from respiration.