Plants are autotrophs — they make their own food using sunlight. They carry out several vital life processes: photosynthesis, respiration, transportation, and excretion. Understanding these processes explains how a tiny seed can grow into a large tree.
Key Concepts and Definitions
Photosynthesis is the process by which green plants make their own food using sunlight, carbon dioxide (CO2), and water. It takes place in the chloroplasts of leaf cells, which contain chlorophyll (the green pigment that traps light energy).
Word equation:
Carbon dioxide + Water --[light, chlorophyll]-→ Glucose + Oxygen
Oxygen is released as a by-product — this is the source of oxygen in our atmosphere.
Leaves are the main organs of photosynthesis. They have a large, flat surface to absorb maximum light and contain tiny pores called stomata (singular: stoma) that allow gas exchange (CO2 in, O2 out).
Respiration in plants: Like all living organisms, plants respire 24 hours a day, breaking down glucose to release energy. During the day, photosynthesis produces more oxygen than respiration uses, so the net exchange is CO2 in, O2 out. At night, only respiration occurs, so CO2 is released.
Transportation in plants: Water and minerals are transported from roots to leaves through xylem tissue (upward flow). Manufactured food (glucose/sucrose) is transported from leaves to all parts through phloem tissue (in all directions).
Root pressure and transpiration pull drive water up through the xylem. Transpiration is the loss of water vapour through stomata; it creates a suction that pulls water up from roots.
Excretion in plants: Plants produce fewer waste products than animals. They store waste in leaves (which fall off), resins, gums, and latex. CO2 from respiration and O2 from photosynthesis are also released through stomata.
Worked Examples
Why are leaves broad and flat?
A broad, flat surface provides maximum area to trap sunlight for photosynthesis. More surface area also allows more stomata and thus better gas exchange of CO2 and O2.
What would happen to a plant kept in complete darkness?
Without light, photosynthesis cannot occur. The plant cannot make food (glucose). It will use up stored food through respiration and eventually wilt and die.
Why does the rate of transpiration increase on a hot, dry, windy day?
High temperature increases the kinetic energy of water molecules, causing faster evaporation. Dryness (low humidity) increases the concentration gradient between the leaf and air. Wind removes water vapour near stomata, maintaining the gradient. All three factors increase transpiration.
Starch test on a variegated (partially green, partially white) leaf. Which part will show starch?
Only the green parts contain chlorophyll and can photosynthesise to make glucose, which is stored as starch. The white parts lack chlorophyll and will not show starch (blue-black colour with iodine).
Why are xylem and phloem called vascular tissues?
They form the vascular (transport) system of plants. Xylem vessels are hollow, dead cells that carry water and dissolved minerals upward. Phloem cells are living and carry dissolved sugars and other organic compounds to all growing and storage organs.
A plant is placed in a sealed glass box. Initially CO2 decreases and O2 increases (daylight). What happens at night?
At night photosynthesis stops. The plant continues to respire, consuming O2 and releasing CO2. So CO2 in the box will increase and O2 will decrease overnight.
Why do plants need minerals from soil even though they make their own food?
Photosynthesis produces carbohydrates (glucose). Plants also need proteins for growth and chlorophyll for photosynthesis. Proteins contain nitrogen, and chlorophyll contains magnesium — both obtained as mineral salts from the soil through roots.
- Key Formulas / Equations
- Photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 (glucose) + 6O2
- Respiration (aerobic): C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Common mistakes
Many students think plants only photosynthesise and do not respire. In fact, plants respire all the time (day and night). Photosynthesis only happens when light is available. Also, do not say "plants breathe in CO2" — plants exchange gases through stomata by diffusion, not breathing.
Summary
Plants make food by photosynthesis using light, CO2, and water, releasing oxygen as a by-product. They respire continuously to release energy from glucose. Water and minerals move up through xylem; food moves through phloem. Transpiration drives water movement and helps cool the plant. Plants excrete waste products through fallen leaves and resins.