CB6 — Plant Structures and Their Functions
Photosynthesis — CP4
Photosynthesis converts light energy into chemical energy stored in glucose:
$$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$
Occurs in chloroplasts. Chlorophyll (green pigment in thylakoid membranes) absorbs light energy (mainly red and blue wavelengths).
Two stages:
- Light-dependent reactions (thylakoids): light splits water (photolysis) → releases O₂ + produces ATP and reduced NADP.
- Light-independent reactions / Calvin cycle (stroma): CO₂ fixed using ATP + reduced NADP → produces G3P → glucose.
Factors limiting photosynthesis:
- Light intensity: rate increases with light intensity until another factor limits.
- CO₂ concentration: rate increases with [CO₂] until another factor limits.
- Temperature: rate increases up to optimum (~25°C for most plants); above optimum, enzymes denature.
- Water availability: stomata close to prevent water loss if water scarce → CO₂ entry blocked → rate falls.
CP4 — Photosynthesis rate (pondweed/Elodea): place pondweed in sodium hydrogencarbonate solution (provides CO₂) under a lamp. Count oxygen bubbles per minute at different distances. Closer lamp = higher light intensity = faster rate. Variables to control: CO₂ concentration (use buffer), temperature (water bath), same plant.
Uses of glucose in plants: respiration (energy), cellulose (cell walls), starch (storage), sucrose (transport in phloem), amino acids (protein synthesis — needs nitrates absorbed from soil), lipids (seed storage).
Mineral ions in plants
Plants absorb mineral ions from soil via root hair cells (by active transport):
| Mineral | Use | Deficiency symptom |
|---|---|---|
| Nitrates (NO₃⁻) | Protein/amino acid/DNA synthesis | Stunted growth, pale/yellow leaves |
| Magnesium (Mg²⁺) | Chlorophyll synthesis | Yellowing leaves (chlorosis) |
| Phosphates (PO₄³⁻) | ATP, DNA, cell membrane | Poor root growth |
| Potassium (K⁺) | Opening/closing stomata, enzyme function | Yellowing/brown leaf edges |
Transpiration and CP6
Transpiration: the evaporation of water vapour from the leaf surface (mainly through stomata) and its loss to the atmosphere. Water moves up the plant via the xylem (dead, hollow, lignified vessels) from roots → stems → leaves.
Mechanism: water evaporates from mesophyll cells → reduces water potential in leaf → water pulled up xylem by tension (transpiration pull / cohesion-tension mechanism). Root hair cells absorb water by osmosis from soil.
Factors affecting transpiration rate:
- Light: stomata open in light (guard cells photosynthesise → take up K⁺ → water enters by osmosis → guard cells become turgid → stomata open). Higher light = more transpiration.
- Temperature: warmer = faster evaporation = more transpiration.
- Humidity: drier air = steeper water potential gradient = more transpiration.
- Wind: removes water vapour from around stomata = more transpiration.
CP6 — Potometer: measures water uptake (approximates transpiration rate). Shoot placed in capillary tube. Air bubble moves along scale as water is absorbed. Measure distance moved per unit time in different conditions (fan on/off, heat lamp, Vaseline on leaves/abaxial or adaxial surface).
Translocation: movement of sugars (sucrose) through phloem from source (leaves) to sinks (roots, fruits, growing tips). Living tissue; requires ATP.
Stomata and guard cells
Stomata are pores in leaf epidermis, mostly on underside (abaxial surface). Guard cells control stomatal opening:
- Open in light: guard cells take up K⁺ (active transport) → water enters by osmosis → cells become turgid → stomata open (allows CO₂ in for photosynthesis and O₂ out).
- Close in dark or when water stressed: K⁺ leaves → water leaves → guard cells become flaccid → stomata close (reduces water loss).
Plant hormones
Auxins (IAA): produced in shoot tips. Promote cell elongation. Unequal distribution causes phototropism: light hits one side → IAA moves to shaded side → cells on shaded side elongate more → shoot bends toward light.
Gibberellins: promote seed germination, stem elongation, fruit development without fertilisation (parthenocarpy). Used commercially to produce larger seedless grapes.
Ethene: gas. Promotes fruit ripening. Used commercially to ripen bananas during transport (introduced just before sale).
Abscisic acid ABA: stress hormone. Promotes stomatal closure when plant is water-stressed; promotes seed dormancy.
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