Photosynthesis — the autotrophic nutrition of plants (spec 2.17-2.18)
Plants make glucose from CO₂ and water using light absorbed by chlorophyll.
Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll. Glucose is the starting point for all the organic molecules in the plant (and ultimately for all heterotrophs that eat plants).
The equation.
Word: carbon dioxide + water glucose + oxygen
Balanced symbol:
Light and chlorophyll are conditions — they are NOT used up in the reaction. They sit ABOVE the arrow.
Where it happens. In chloroplasts, found mainly in the palisade mesophyll cells just below the upper surface of the leaf. Chloroplasts contain the green pigment chlorophyll which absorbs red and blue light from sunlight.
Sources of the reactants:
- CO₂ — diffuses in through stomata (mainly on the lower epidermis of the leaf).
- H₂O — absorbed from the soil by root hair cells, transported up the stem in xylem.
Fates of the products:
- Glucose — used immediately in respiration to release ATP; converted to starch for storage (insoluble; does not affect water potential); built into cellulose for cell walls; built into sucrose for transport in the phloem; combined with nitrate to make amino acids (then proteins).
- Oxygen — by-product. Used in respiration or released through stomata.
Demonstrating the requirements of photosynthesis:
- Light is needed. Destarch a plant by leaving it in the dark for 48 h. Cover part of a leaf with foil for 24 h in the light. Test the leaf for starch: only the uncovered region turns blue-black.
- Chlorophyll is needed. Use a variegated leaf (with green and white patches). After destarching and exposing to light, test for starch: only the green regions turn blue-black; the white (chlorophyll-free) regions do not.
- CO₂ is needed. Place a destarched plant in a sealed jar with soda lime (absorbs CO₂). After exposure to light, test the leaf: NO starch produced. A control with no soda lime DOES produce starch.
Required practical — rate of photosynthesis in pondweed (e.g. Cabomba or Elodea). Place pondweed in water with sodium hydrogencarbonate (NaHCO₃ — provides CO₂). Shine a lamp at known distance d. Count oxygen bubbles per minute OR collect O₂ in an inverted measuring cylinder. Vary the IV (light intensity = 1/d², distance from lamp). Rate = bubbles per minute OR cm³ O₂ per minute.
- Equation: .
- Site: chloroplasts (in palisade mesophyll cells).
- Conditions ABOVE the arrow: light + chlorophyll.
- Three classic 'need' experiments: light, chlorophyll (variegated leaf), CO₂ (soda lime).
- RP: pondweed bubbles to measure rate.