Plant Biology — IB Biology HL Study Guide (HL Extension)

For: IB Biology HL candidates sitting IB Biology HL.

Covers: IB Topic 9 (HL only) — water and mineral transport in xylem (transpiration, cohesion-tension theory), phloem translocation (pressure-flow), plant growth (auxin, gibberellin, abscisic acid), reproduction in flowering plants, photoperiodism.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology HL style for educational use. They are not reproductions of past IBO papers.

1. Why Plant Biology Matters in HL

Topic 9 (HL only) covers the unique physiology of land plants: how they move water 100+ m up a tree, how they use sugar produced in leaves at sites elsewhere, how hormones coordinate growth. About 4-7% of HL Paper 1+2.

2. Water transport in xylem

Pathway: roots → xylem → stem → leaves → atmosphere (through stomata).

Cohesion-tension theory:

1. Water evaporates from mesophyll cells in leaves through stomata = transpiration.
2. This creates a negative pressure (tension) in the xylem.
3. Water molecules cohere (via H-bonds) → tension pulls a continuous water column up.
4. Adhesion between water and xylem walls helps support against gravity.

Xylem is dead at maturity (no cytoplasm) — just hollow tubes of lignified cell walls. This minimises resistance to flow.

Factors affecting transpiration: light (opens stomata), temperature (raises evaporation), humidity (high humidity slows it), wind (removes humid boundary layer at leaf surface).

3. Mineral uptake

Roots absorb minerals from soil via:

• Active transport through root hair plasma membrane (e.g. K⁺ pumped in against gradient using ATP).
• Mass flow of dissolved ions in the water moving inward.
• Mycorrhizal fungi symbiosis: fungal hyphae extend into soil and exchange phosphorus + nitrogen for plant sugars.

Root structure: epidermis with root hairs (large surface area) → cortex → endodermis (Casparian strip controls entry to vascular tissue) → xylem.

4. Phloem translocation — pressure-flow

Pathway: source (leaves making sugar) → phloem sieve tubes → sink (roots, fruits, growing tissues).

Pressure-flow mechanism:

1. Companion cells at source actively load sucrose into sieve tubes (lowers water potential).
2. Water enters from xylem by osmosis → high pressure at source.
3. Sucrose unloaded at sink (consumed or stored) → water leaves → low pressure at sink.
4. Pressure difference drives mass flow source → sink.

Phloem is alive at maturity (sieve tubes have cytoplasm but no nucleus; companion cells have full machinery).

5. Plant hormones

Auxin (IAA): produced in shoot apical meristem; promotes cell elongation. Asymmetric distribution → phototropism (bend toward light) and gravitropism. Used commercially as rooting hormone and selective herbicide (mimics 2,4-D).

Gibberellin: stimulates seed germination (mobilising starch reserves), stem elongation. Triggers bolting in long-day plants.

Cytokinin: promotes cell division (cytokinesis); delays leaf senescence.

Abscisic acid (ABA): stress hormone. Closes stomata when plant is water-stressed. Promotes seed dormancy.

Ethylene: gas hormone. Triggers fruit ripening, leaf abscission, flower senescence.

6. Reproduction in flowering plants

Flowers are the reproductive organs. Key parts:

• Stamens: anther (produces pollen) + filament. Male.
• Carpels (pistil): stigma → style → ovary (with ovules). Female.
• Petals: attract pollinators (insects, birds, wind).

Pollination: pollen transfer from anther to stigma. Self (same plant) or cross (between plants).

Fertilisation: pollen tube grows down style → delivers sperm to ovule. Two sperm: one fertilises egg (→ embryo), other fertilises polar nuclei (→ endosperm). Double fertilisation is unique to angiosperms.

Seed and fruit: ovule → seed; ovary wall → fruit. Fruit aids dispersal (wind, animal, water).

7. Photoperiodism and flowering

Plants sense day length using phytochrome, a pigment.

Two forms:

• $P_r$ (red-absorbing) — inactive form.
• $P_{fr}$ (far-red-absorbing) — active form.

Daylight (red wavelengths) converts $P_r\to P_{fr}$. At night, $P_{fr}$ slowly reverts to $P_r$.

Long-day plants (e.g. spinach, lettuce) flower when $P_{fr}$ persists long into night → only when night is short.

Short-day plants (e.g. chrysanthemum, soybean) flower when $P_{fr}$ has had time to revert — only when night is long.

Day-neutral plants flower regardless of day length.

8. Worked Example

A small willow tree is enclosed in a sealed plastic chamber on a sunny day.

(a) Predict what happens to xylem flow rate after 2 hours. (b) Predict what happens to phloem flow rate. (c) Why does the rate of photosynthesis decrease over time?

Solution.

(a) The chamber traps water vapour, raising humidity. Transpiration slows, so xylem flow slows.

(b) Phloem flow continues for a while (sucrose still being made and used) but slows as photosynthesis declines (less sugar available at source).

(c) CO₂ in the chamber depletes as photosynthesis consumes it. CO₂ is the substrate for the Calvin cycle; without enough CO₂, the cycle slows.

9. Common Pitfalls

• Xylem is dead, phloem is alive: easy to flip these. Xylem cells are reinforced with lignin and have no cytoplasm; phloem sieve tubes have cytoplasm and live companion cells.
• Source and sink can change: a developing fruit is a sink; a mature fruit storing sugars can later become a source.
• Auxin direction: auxin moves down from shoot apex to roots in the stem. Higher concentration near shoot apex.
• Phytochrome and night length: plants actually respond to night length, not day length. Interrupting the dark period with a flash of red light disrupts flowering in short-day plants.

10. Practice Questions

1. Explain why a leaf with stomata only on its upper surface would transpire less than a normal leaf.
2. Why are mycorrhizal fungi essential for many plants growing in nutrient-poor soils?
3. A long-day plant is given 16 hr light + 8 hr dark; flowering occurs. The same plant is given 8 hr light + 16 hr dark; no flowering. A flash of red light during the dark period is added to the 16 hr dark version. Predict outcome.

11. Quick Reference Cheatsheet

• Xylem: dead, transports water by transpiration pull (cohesion-tension).
• Phloem: alive, transports sucrose by pressure-flow source-to-sink.
• Mycorrhizae: fungal-plant symbiosis for P, N uptake.
• Auxin: cell elongation, phototropism. Gibberellin: germination. ABA: stress.
• Double fertilisation: unique to angiosperms; one sperm + egg, second + polar nuclei.
• Phytochrome: $P_r\leftrightarrow P_{fr}$. Night length controls flowering.

12. What's Next

Plant Biology connects to Topic 8 (Metabolism) for photosynthesis and to Topic 4 (Ecology) for plant-environment interactions. Use Ollie for any specific plant physiology problem.