Photosynthesis, Nutrients and Hormones

Photosynthesis (NCERT Class 10 and 11):

• Overall: $6\,\mathrm{CO_2} + 12\,\mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} + 6\,\mathrm{H_2O}$, in the presence of sunlight and chlorophyll. It makes glucose; respiration breaks glucose down.
• It takes place in the chloroplast, the only part of a leaf cell that holds chlorophyll.
• Light reactions (in the grana, the thylakoids): chlorophyll absorbs light, water is split (photolysis), oxygen is released, and ATP and NADPH are made. The splitting of water by light is the Hill reaction.
• Light-independent (dark) reactions in the stroma: the Calvin cycle uses that ATP and NADPH to reduce carbon dioxide to sugar.
• So the oxygen given off comes from water, and the carbon in sugar comes from carbon dioxide.
• Extra glucose is stored as starch. Starch test: boil the leaf in water (kills the cells), then in alcohol (removes the chlorophyll), then add iodine: starch turns blue-black.

Leaf pigments (NCERT Class 11):

• Chlorophyll is held in the chloroplasts. Magnesium is the metal at the centre of its molecule.
• Chlorophyll a is the chief pigment: it forms the reaction centre where light energy is turned into chemical energy.
• Chlorophyll b and the carotenoids are accessory pigments: they catch more light and pass the energy to chlorophyll a.
• Carotenoids: carotene (orange) and xanthophyll (yellow). They protect chlorophyll from photo-oxidation and give the yellow and orange colours of autumn leaves.
• In the human eye, carotenoids such as lutein also shield the retina from strong light.

Essential mineral elements (NCERT Class 11, Mineral Nutrition):

• Macronutrients (needed in large amounts): carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium.
• Micronutrients (needed in traces): iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
• Magnesium is part of chlorophyll; nitrogen is part of every protein and of chlorophyll; iron is needed to make chlorophyll.
• A shortage that turns leaves yellow is chlorosis; it follows a lack of nitrogen, magnesium or iron, among others.

The five plant hormones (NCERT Class 10 and 11):

• Auxin: cell elongation; makes shoots bend towards light (phototropism); used to make cuttings root.
• Gibberellins: stem elongation, bolting, breaking seed dormancy; used to make grapes bigger.
• Cytokinins: promote cell division; delay the ageing of leaves. Most plentiful where cells divide fast, as in fruits and seeds.
• Ethylene: a gas that ripens fruit; it can promote or inhibit, but is largely an inhibitor of growth.
• Abscisic acid (ABA): the growth inhibitor; causes dormancy, closes stomata in drought, and the fall of leaves (abscission). It is called the stress hormone.
• Thyroxine is an animal hormone, made by the thyroid gland. It is not a plant hormone.