Worked Solutions
Module 2: Organisation of Living Things — Worked Solutions (Preliminary Biology)
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Worked examples for Preliminary Biology Module 2 — Organisation of Living Things. Each shows where the marks are awarded, the key idea, and a full model answer in your choice of tutor — Stella, Ella or Cassie.
How to use these
Attempt each question under exam conditions first, then check your response against the model answers. Use the tutor tabs to read the solution in the style that suits you: Stella is direct and to the point, Ella is warm and explains the why, and Cassie is concise and uses bullet points.
Example 1 — Adaptations of the alveoli
Question
Explain how the structure of the alveoli in the human lungs is adapted for efficient gas exchange. (4 marks)
Solution
Gas exchange is diffusion of oxygen and carbon dioxide across the alveolar surface, so the alveoli are adapted to make diffusion fast.
They are numerous and rounded, giving a very large total surface area for exchange. Their walls are one cell thick, giving a short diffusion distance.
They are surrounded by a dense network of capillaries, which maintains a steep concentration gradient by constantly delivering deoxygenated blood and removing oxygenated blood. The surface is also moist, allowing gases to dissolve before diffusing.
Each feature maps onto a factor that speeds diffusion — surface area, distance and gradient. Name the feature and the effect to get full marks.
Gas exchange in the lungs is just diffusion — oxygen moving into the blood and carbon dioxide moving out — so everything about the alveoli is designed to make that diffusion as fast as possible.
First, there are millions of alveoli, each a tiny rounded sac. Together they create an enormous total surface area, so lots of gas can be exchanged at once.
Second, the alveolar walls are only one cell thick, and so are the capillary walls beside them. That gives a very short diffusion distance, so gases cross quickly.
Third, each alveolus is wrapped in capillaries. Blood flowing through constantly brings in deoxygenated blood and carries away oxygenated blood, which keeps a steep concentration gradient — and a steep gradient means faster diffusion. The moist lining also lets gases dissolve first. All three factors — surface area, distance and gradient — work together for efficient exchange.
- Large total surface area (many rounded alveoli) → more gas exchanged at once
- Walls one cell thick → short diffusion distance → faster diffusion
- Dense capillary network → maintains steep concentration gradient (delivers deoxygenated, removes oxygenated blood)
- Moist surface → gases dissolve before diffusing
Where the marks go
- 1 mark: Links large surface area (many alveoli) to increased gas exchange
- 1 mark: Links thin (one-cell-thick) walls to a short diffusion distance
- 1 mark: Links the capillary network to maintaining a steep concentration gradient
- 1 mark: Identifies a further adaptation (e.g. moist surface) with its effect
Key idea
The alveoli maximise the rate of diffusion by combining a large surface area, a short diffusion distance and a steep concentration gradient maintained by surrounding capillaries.
Example 2 — Xylem and phloem
Question
Compare the structure and function of xylem and phloem in a flowering plant. (5 marks)
Solution
Both are vascular (transport) tissues that move substances through the plant, and both run continuously from roots to leaves.
Xylem transports water and dissolved minerals in one direction only — upwards from the roots to the leaves. It is made of dead, hollow cells with no end walls, forming continuous tubes strengthened with lignin, which also provides support.
Phloem transports the products of photosynthesis (sugars) — this is translocation — and can move them in both directions, to wherever they are needed. It is made of living cells (sieve tube elements) connected by sieve plates, supported by companion cells.
So: both transport, but xylem is dead, lignified and carries water up only, while phloem is living and carries sugars both ways. Give a similarity plus structure-and-function for each.
Let's begin with what they share. Xylem and phloem are both vascular tissues — the plant's "plumbing" — and both form continuous transport pathways running from the roots up to the leaves.
Now xylem. Its job is to carry water and dissolved minerals upward from the roots to the rest of the plant. To do this it's made of dead, hollow cells joined end to end with their end walls gone, forming long open tubes. Their walls are reinforced with lignin, which both waterproofs them and gives the plant support. Flow is one-way: up.
Phloem does something different — it carries the sugars made in photosynthesis to wherever the plant needs them (growing tips, roots, fruit). This is called translocation, and because "where it's needed" changes, phloem can move substances in both directions. It's made of living cells called sieve tube elements, joined by perforated sieve plates, and helped by neighbouring companion cells that keep them alive.
So the comparison comes down to this: both transport, but xylem is dead and lignified and moves water upward, while phloem is living and moves sugars in either direction.
Similarities:
- Both vascular (transport) tissues
- Both form continuous pathways from roots to leaves
Xylem:
- Dead, hollow cells; no end walls; lignified walls (also gives support)
- Transports water and minerals; upward (one direction) only
Phloem:
- Living cells (sieve tube elements) with sieve plates; companion cells
- Transports sugars (translocation); both directions
Where the marks go
- 1 mark: Identifies a valid similarity (both vascular/transport tissues forming continuous pathways)
- 1 mark: Describes xylem structure (dead, hollow, lignified cells)
- 1 mark: States xylem transports water and minerals upward (one direction)
- 1 mark: Describes phloem structure (living sieve tube elements with sieve plates / companion cells)
- 1 mark: States phloem transports sugars (translocation) in both directions
Key idea
Both are transport tissues, but xylem is dead and lignified and carries water and minerals upward, while phloem is living and translocates sugars in both directions.