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CIE iGCSE Co-Ordinated Science B3.3 Active transport Exam Style Questions Paper 4

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Question

(a) Active transport is used in the uptake of mineral ions from the soil into the root hair cells.
(i) Define active transport.
(ii) Magnesium ions and nitrate ions are two mineral ions taken up by active transport.
Explain the importance of magnesium ions and nitrate ions in plants.
(b) A student investigates the effect of different concentrations of salt solution on plant tissue.
The student cuts pieces of carrot to the same size and mass.
The pieces of carrot are immersed in different concentrations of salt solution for two hours.
After two hours, the student calculates the percentage change in mass of each piece of carrot.
The student’s results are shown in Table 2.1.
(i) Explain the result for the piece of carrot immersed in the 0.0 mol per dm³ salt solution (pure water).
Use ideas about water potential in your answer.
(ii) Describe the appearance of carrot cells immersed in the 0.8 mol per dm³ salt solution after two hours.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B3.3 — Active transport (Part (a)(i))
• Topic B6.1 — Photosynthesis / Mineral ions (Part (a)(ii))
• Topic B3.2 — Osmosis (Part (b)(i) & (b)(ii))

▶️ Answer/Explanation

(a)(i) Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient), using energy from respiration.
This process requires energy because substances are moved in the opposite direction to diffusion, from where they are less concentrated to where they are more concentrated.

(a)(ii) Magnesium ions: Required for the production of chlorophyll.
Nitrate ions: Required for the production of amino acids (and proteins / DNA / RNA).
Magnesium is a central component of the chlorophyll molecule, while nitrate ions provide nitrogen needed to build amino acids, the building blocks of proteins.

(b)(i) Water enters the carrot cells by osmosis, from a region of higher water potential (pure water outside) to a region of lower water potential (inside the cells), through a partially permeable membrane.
The +20% change in mass shows water has moved into the cells, making them heavier and more turgid because the pure water outside has a higher water potential than the cell sap.

(b)(ii) The cells become flaccid / smaller with a smaller vacuole; they are plasmolysed, meaning the cell membrane has pulled away from the cell wall.
In the concentrated salt solution (0.8 mol/dm³), water leaves the cells by osmosis, causing the protoplast to shrink and pull away from the rigid cell wall.

Question

Fig. 1.1 is a diagram of a root hair cell.
(a) Using letters A – E in Fig. 1.1, identify the part of the cell:
(i) where protein synthesis takes place
(ii) made from cellulose
(b) Amino acids are used in protein synthesis. State the name of the process used to transport amino acids from sources to sinks in plants.
(c) Root hair cells are specialised for absorption. Complete Table 1.1 about substances absorbed by root hair cells.
(d) Fig. 1.2 shows two of the same plant cells, one immersed in pure water and the other immersed in concentrated salt solution.
Complete Fig. 1.2 to show the contents of the plant cell immersed in the concentrated salt solution.
(e) Explain the effects of high winds on transpiration rate in plants.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B2.1 — Cell structure (Part (a))
• Topic B8.4 — Translocation (Part (b))
• Topic B3.3 — Active transport / B8.2 — Water uptake (Part (c))
• Topic B3.2 — Osmosis (Part (d))
• Topic B8.3 — Transpiration (Part (e))

▶️ Answer/Explanation

(a)(i) D
The part labelled D represents the ribosomes, which are the sites of protein synthesis in the cell. Ribosomes translate mRNA into polypeptide chains during translation.

(a)(ii) A
The part labelled A represents the cell wall, which is made of cellulose (a structural polysaccharide) in plant cells. Cellulose provides structural support and protection.

(b) Translocation
Translocation is the process by which sucrose and amino acids are transported in the phloem from sources (where they are produced or stored) to sinks (where they are used or stored).

(c) Table completion:

(d) Plant cell in concentrated salt solution:
The plant cell undergoes plasmolysis. The cell membrane pulls away from the cell wall as water leaves the cell by osmosis. The vacuole shrinks and becomes smaller. The cell becomes flaccid because water potential outside the cell is lower than inside the cell.

(e) High winds increase the transpiration rate.
Wind blows water vapour away from the leaf surface, maintaining a steep concentration gradient between the inside of the leaf and the external air. This increases the rate of evaporation of water from the mesophyll cells and diffusion of water vapour out through the stomata.

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