Plant Life: Nutrition, Photosynthesis and Transport
Parts of a plant
| Part | Function |
|---|---|
| Root | Anchors; absorbs water and minerals. Tap root (dicots), fibrous root (monocots). Root hairs absorb. Modified roots: carrot, radish, turnip (tap); sweet potato; banyan prop roots; pneumatophores (mangroves) |
| Stem | Supports; conducts. Modified stems: potato (tuber), ginger (rhizome), onion (bulb), sugarcane, runners |
| Leaf | Photosynthesis, transpiration. Venation parallel or reticulate |
| Flower | Reproduction |
| Fruit and seed | Dispersal and new plants |
Photosynthesis
Plants (and algae, some bacteria) make food using sunlight, water, and carbon dioxide with chlorophyll:
6CO2 + 6H2O + sunlight -> C6H12O6 + 6O2
- Site: chloroplasts (in the leaf). Chlorophyll absorbs red and blue light and reflects green.
- Water is taken through roots (xylem); CO2 enters through stomata.
- Products: glucose (stored as starch) and oxygen. Oxygen released comes from water.
- Starch test: iodine turns blue-black with starch (used to show photosynthesis in the leaf).
- Light and dark reactions: light reactions give ATP and release O2; Calvin cycle (dark reaction) makes glucose.
- Conditions: light, chlorophyll, CO2, water.
- Plants that lack chlorophyll (fungi, Cuscuta (amar bel)) cannot do photosynthesis.
Modes of nutrition
| Mode | Meaning | Examples |
|---|---|---|
| Autotrophic | Make their own food | Green plants, algae |
| Heterotrophic | Depend on others | Animals, fungi |
| Parasitic | Feed on a living host | Cuscuta, tapeworm, lice, Plasmodium, Ascaris |
| Saprotrophic | Feed on dead and decaying matter | Fungi (bread mould), bacteria, mushrooms |
| Symbiotic | Both benefit | Lichens (algae + fungus), Rhizobium in legumes |
| Insectivorous | Plants that trap insects for nitrogen | Pitcher plant, venus flytrap, sundew, bladderwort |
| Holozoic | Animals take in solid food (digest internally) | Humans, amoeba |
- Nitrogen cycle and nitrogen fixation: Rhizobium (in root nodules of pulses, e.g. gram, pea), Azotobacter, blue-green algae and lightning.
- Minerals needed: macro (N, P, K, Ca, Mg, S) and micro (Fe, Zn, Cu, Mn, B, Mo). Magnesium is a part of chlorophyll; nitrogen in proteins and chlorophyll. Deficiency of nitrogen causes yellowing (chlorosis).
- Hydroponics: growing plants in a nutrient solution without soil.
Transport in plants
- Water and minerals go up through xylem: driven by transpiration pull and root pressure.
- Transpiration: loss of water vapour from leaves through stomata. It cools the plant and pulls water up. Stomata close at night (in most plants).
- Phloem carries sucrose, amino acids from leaves to other parts in both directions: translocation.
- Wilting: loss of water makes the plant limp.
Respiration in plants and photosynthesis compared
- Plants respire all the time (day and night); they photosynthesise only in light. During the day, photosynthesis exceeds respiration so net O2 is released.
| Photosynthesis | Respiration |
|---|---|
| Makes food (glucose) | Breaks down food to release energy |
| Takes in CO2 and releases O2 | Takes in O2 and releases CO2 |
| Only in chlorophyll-containing cells, in light | All living cells, all the time |
| Stores energy | Releases energy (ATP) |
Plant hormones and movement
| Hormone | Function |
|---|---|
| Auxin | Cell elongation, phototropism; promotes growth |
| Gibberellin | Stem growth, seed germination |
| Cytokinin | Cell division; delays ageing |
| Abscisic acid (ABA) | Inhibits growth; closes stomata; wilting and seed dormancy |
| Ethylene | Ripening of fruit (gas) |
- Tropism: growth movement in response to a stimulus: phototropism (light; stem bends towards light), geotropism (gravity; roots grow down), hydrotropism (water), chemotropism (pollen tube growth), thigmotropism (touch; tendrils).
- Nastic movement is not directional: touch-me-not (Mimosa pudica) folds its leaves on touch.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.