NDA Biology · Plant Biology

Plant Processes — Gas Exchange, Transpiration and Tropisms

Photosynthesis and respiration are linked by oxygen; transpiration is water loss mostly through the lower-surface stomata; and tropisms are directional growth responses (shoots up = negative geotropism).

All Plant Biology notes NDA Biology strategy

Why this matters

3 PYQs, but high quality — including the chapter's one HARD question (the vaseline-on-leaf transpiration experiment, 2021). These are 'Apply' questions: you reason about a sequence (photosynthesis makes O₂ → respiration uses it) or a direction (shoots grow against gravity, water leaves through lower stomata), not a single recalled fact.

Concept 1 of 3

The oxygen link — photosynthesis feeds respiration

Intuition

Two opposite plant processes hand a gas back and forth. Photosynthesis MAKES oxygen as a by-product. Respiration — the energy-releasing process in every living cell — USES that oxygen. So the gaseous product of one process is the requirement of the other.

Definition

Photosynthesis produces oxygen as a gaseous by-product. Respiration is the vital, energy-RELEASING process that uses oxygen (and glucose) to make ATP. So oxygen links them: photosynthesis (product = O₂) → respiration (requires O₂).

Worked example

Name a plant process whose gaseous PRODUCT is required by another energy-releasing process. Give both the process and the gas.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 1Plant BiologyEASY

The gaseous product of a process in plants is a requirement for another vital process that releases energy. Given below are four combinations of the process and product. Identify the correct answer.

[Q73 · Apr · 2021]

Photosynthesis + Oxygen is the linked pair

The question pairs a process with its gaseous product that another vital process needs. The right pair is photosynthesis and oxygen — not 'respiration and nitric oxide' or 'germination and CO₂'. Photosynthesis makes O₂; respiration uses it.

Concept 2 of 3

Tropisms — directional growth responses

Intuition

A tropism is growth that bends in response to a direction in the environment. Name it by the stimulus (geo = gravity, photo = light, hydro = water, chemo = chemical) and the sign (positive = towards, negative = away). A shoot grows UP, AWAY from gravity — that's NEGATIVE geotropism. A root grows DOWN, towards gravity — positive geotropism.

Definition

A tropism is a directional growth response to a stimulus:

Geotropism (gravitropism) — response to gravity. Shoots = negatively geotropic (grow UP, away from gravity); roots = positively geotropic (grow DOWN, towards gravity).
Phototropism — response to light. Shoots are positively phototropic (bend towards light).
Hydrotropism — towards water; chemotropism — towards a chemical.

Naming rule: stimulus + sign (positive = towards, negative = away).

Worked example

A bean shoot grows straight upward, against the pull of gravity. Name this response precisely (stimulus + sign).

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Plant BiologyEASY

Shoots of plant show upward movement and it can be designated to be

[Q107 · Sep · 2021]

Shoot up = NEGATIVE geotropism (not negative phototropism)

A shoot's upward growth against gravity is negatively geotropic. The distractor 'negatively phototropic' is wrong — shoots are POSITIVELY phototropic (towards light). The upward-against-gravity fact is specifically negative geotropism.

Concept 3 of 3

Transpiration — water loss through lower-surface stomata

Intuition

Transpiration is the evaporation of water from a plant, mostly through tiny pores called stomata. In a typical dicot leaf, MOST stomata are on the LOWER surface. So if you want to slow water loss the most, you seal the lower surface — that leaf keeps its water longest.

Definition

Transpiration is loss of water vapour from the plant, chiefly through the stomata on the leaf. In most dicot leaves, stomata are concentrated on the LOWER surface, so most transpiration happens there.

Coating the lower surface (where most stomata are) blocks the MOST transpiration → that leaf dries up last.
Coating the upper surface blocks little (few stomata there) → that leaf still loses water fast.

Worked example

Vaseline is smeared on the LOWER surface of leaf X and the UPPER surface of leaf Y on the same unwatered dicot plant; leaf Z is untouched. Which leaf stays moist the longest?

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 3Plant BiologyHARD

In a dicot pot herb, vaseline/vegetable oil was applied on the upper surface of one leaf (Experimental leaf 1) and on the lower surface of another leaf (Experimental leaf 2). Vaseline/Vegetable oil was not applied on the control leaf. The plant was deliberately not watered for several days. Which leaf will dry up last ?

[Q74 · Apr · 2021]

Lower surface coated = dries up LAST

Because most dicot stomata are on the lower surface, sealing the LOWER surface cuts water loss the most, so that leaf dries up last. The trap is to pick the upper-coated leaf — but sealing the stomata-poor upper surface barely helps.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Watch out for (3)

Photosynthesis + Oxygen is the linked pair
→ The oxygen link — photosynthesis feeds respiration
Shoot up = NEGATIVE geotropism (not negative phototropism)
→ Tropisms — directional growth responses
Lower surface coated = dries up LAST
→ Transpiration — water loss through lower-surface stomata

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