Plant Biology: Structure and Photosynthesis
Green Organisms: Masters of Photosynthesis
Plant Structure
Vascular plants have three main organs:
- Roots: anchor, absorb water and minerals, store food; root hairs maximise surface area; contain xylem + phloem
- Stems: support, transport via xylem (water/minerals upward via transpiration pull) and phloem (sugars bidirectionally via pressure flow)
- Leaves: primary photosynthesis organ; mesophyll cells (palisade + spongy) contain chloroplasts; stomata (pores guarded by guard cells) regulate gas exchange
Non-vascular plants (mosses, liverworts) lack xylem/phloem and are limited in size.
Photosynthesis
<br>Photosynthesis occurs in chloroplasts in two stages:
- Light-dependent reactions (thylakoid membranes): light energy splits water (photolysis, releasing O₂), energises electrons → produces ATP and NADPH, reducing equivalents
- Light-independent reactions / Calvin cycle (stroma): uses ATP + NADPH to fix CO₂ into G3P → glucose; enzyme RuBisCO catalyses CO₂ fixation
Chlorophyll mainly absorbs red and blue light; reflects green (hence plant colour).
Why Leaves Are Green
Chlorophyll absorbs mostly red (~680 nm) and blue (~450 nm) wavelengths. Green light (~530 nm) is poorly absorbed and reflected — which is why leaves appear green. Carotenoids absorb additional blue-green light and protect against photooxidative damage.
Where does the Calvin cycle (light-independent reactions) occur within the chloroplast?
The Calvin cycle uses ATP and NADPH to fix CO₂ into carbohydrates and occurs in the soluble stroma of the chloroplast.
Correct answer: Stroma
What is the immediate product of CO₂ fixation by RuBisCO in the Calvin cycle?
RuBisCO combines CO₂ with RuBP (5-carbon) to produce two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound — this is the first stable product.
Correct answer: 3-phosphoglycerate (3-PGA)
Explain the role of stomata in balancing photosynthesis and water conservation in a plant.
Stomata regulate the gas exchange vs water loss trade-off.
Correct answer: Stomata open to allow CO₂ in (necessary for photosynthesis) and O₂ out, but opening also allows water to evaporate (transpiration). Plants face a trade-off: more open stomata = more photosynthesis but more water loss. Guard cells regulate pore size based on light, CO₂ levels, and water status. During drought, stomata close to conserve water — reducing photosynthesis as a cost.