9.2Chapter 9: Plants and Photosynthesis

Photosynthesis — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 9 2 quizzes

📚 Alhussan Biology - Stage 9 Hub / Topical Revision

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9.2 Photosynthesis — Chloroplasts, Light Energy and Word Equations — Alhussan International Academy — Biology (Stages 7–9) notes: Describe photosynthesis, write its word and symbol equations, and explain the role of chloroplasts and how plants use glucose.Photosynthesis = the pro…

Gas movement
Chlorophyll
Aerobic respiration
Starch store
Building other molecules
Net effect

Section 1 of 8What photosynthesis is

Photosynthesis

Photosynthesis the process by which green plants, algae and some bacteria make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll.

Light energy is transferred to chemical energy stored in glucose.
It makes the biomass of plants, from wood to cereal crops to flowers, and is the source of nearly all food and energy in ecosystems.

Biomass

Biomass animals must eat plants to get biomass, but plants make their own from sunlight, air and water.

Oxygen

Oxygen it is a product of photosynthesis and is used for aerobic respiration in most living things.

Section 2 of 8Equations

Reactants

Reactants carbon dioxide from the air, which is also made in aerobic respiration, and water from the soil.

Energy

Energy light, usually from the Sun, captured by chlorophyll.

Word equation: carbon dioxide + water → glucose + oxygen (in light, using chlorophyll).

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Gas movement

Gas movement carbon dioxide goes into the leaf and oxygen comes out through the stomata.

Section 3 of 8Where it happens: chloroplasts

Location

Location photosynthesis happens in chloroplasts, found mostly in palisade cells in leaves.

Chlorophyll

Chlorophyll the green pigment in chloroplasts absorbs red and blue light and reflects green, so leaves look green.

Reactions

Reactions light energy is used to split water molecules and to build glucose from carbon dioxide, and most reactions take place inside chloroplasts.

Evidence

Evidence a cell without chloroplasts cannot photosynthesise, and the number of chloroplasts shows how much photosynthesis a cell carries out.

Section 4 of 8What happens to the glucose

Aerobic respiration

Aerobic respiration about half of the glucose is used to release energy for all the other reactions in the cells.

Starch store

Starch store small, soluble glucose cannot be stored, so molecules join to form large starch molecules.
Starch is stored in leaves and lasts several days without light, so cells can respire in the dark.
Starch is also stored in root tubers such as potatoes.

Building other molecules

Building other molecules glucose is the building block for other carbohydrates, proteins, lipids and chlorophyll.
It makes cellulose for cell walls and sucrose for transport in the phloem.
It combines with nitrate to make amino acids and then proteins.
It is converted to lipids (oils) in seeds.

Section 5 of 8Leaf adaptations

Feature	How it helps
Broad, flat shape	Large surface area to absorb light
Thin	Short diffusion distance for gases and light reaches all cells
Palisade cells near top, many chloroplasts	Absorb most light
Air spaces in spongy layer	Allow carbon dioxide to diffuse to cells
Stomata, with guard cells	Gas exchange; open in light, closed to save water
Veins (xylem and phloem)	Bring water, remove sugars
Transparent upper epidermis and waxy cuticle	Lets light through; reduces water loss

Section 6 of 8Photosynthesis compared with respiration

Photosynthesis

Photosynthesis it stores energy and takes in carbon dioxide.

Respiration

Respiration it releases energy and produces carbon dioxide, and goes on all the time.

Net effect

Net effect in daylight a healthy plant photosynthesises faster than it respires, so there is a net release of oxygen.

Section 7 of 8Key terms

Term	Meaning
Photosynthesis	Making glucose from carbon dioxide and water using light energy.
Chlorophyll	The green pigment that absorbs light for photosynthesis.
Chloroplast	The organelle where photosynthesis takes place.
Glucose	A simple carbohydrate made in photosynthesis.
Starch	An insoluble carbohydrate used as a storage molecule in plants.
Biomass	The mass of living material.