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South Tuen Mun Government Secondary School
Biology Revision Note 16
The importance of photosynthesis
 Green plant is the only living thing that can change light energy into chemical energy in the food.
 Green plants change inorganic substance into organic food. The food is used by animals and other non-green
organisms.
 Photosynthesis is the only process that takes in carbon dioxide and gives out oxygen. It balances other
processes such as respiration and combustion that gives out carbon dioxide and take in oxygen.
Chemical equation of photosynthesis:
Light intensity, concentration of carbon dioxide in air and temperature are environmental factors that affect the rate
of photosynthesis.
When light intensity increases, the rate of photosynthesis increases because more light energy is used for
photosynthesis. When the concentration of carbon dioxide increases, the rate of photosynthesis increases because
carbon dioxide is the raw material for photosynthesis.
From 0 – 37℃, when the temperature increases, the rate of photosynthesis increases because the enzyme becomes
more active. Above 37℃, when the temperature increases, the rate of photosynthesis decreases because enzymes
are denatured.
The concept of limiting factor: when a chemical process is controlled by a number of factors, the one nearest its
minimum value (most unfavourable) will control the rate of the process.
At low light intensity, when light intensity increases, the rate of photosynthesis increases because the amount of
light energy controls the rate of photosynthesis. This shows that light intensity is the limiting factor.
At high light intensity, when light intensity increases, the rate of photosynthesis remains unchanged because the
rate of photosynthesis is controlled by other factors e.g. concentration of carbon dioxide.
If concentration of carbon dioxide is the limiting factor, the rate of photosynthesis can be increased by an increased
concentration of carbon dioxide as shown in the diagram above.
In a greenhouse, the rate of photosynthesis can be maximized with a higher light intensity, a higher concentration
of carbon dioxide, optimum temperature, supply of water. When the rate of photosynthesis is maximized, the
photosynthetic rate of the plant increases and thus the crop yield can be maximized.
Adaptation of the green leaf for photosynthesis
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The leaf blade is broad and flat to increase the surface area for absorbing more sunlight.
The leaf blade is thin, so that all parts of the leaf is close to the surface, to allow rapid diffusion of gases to the
mesophyll cells.
The upper and lower epidermis are made up of living cells that are transparent to allow the entry of light.
The impermeable cuticle reduces water loss.
Guard cells control the size of the stomata to regulate the rates of gaseous exchange and transpiration.
Palisade mesophyll cells are closely packed, with the highest density of chloroplasts. It is found near the upper
surface to absorb most light for photosynthesis.
Spongy cells are loosely packed, with less density of chloroplasts. It has many air spaces for diffusion of
gases.
Midrib contains vascular bundle with xylem and phloem. Xylem transports water and minerals from the roots
to the leaf. Phloem transports organic food (sucrose) from leaf to other parts of the plant.
Midrib and veins contains xylem that supports the leaf so that the flat surface of the leaf faces the sun.
Biochemical pathway of photosynthesis:
(I)
Photochemical reactions / light reactions – light is required.
 Light absorption / photoactivation of chlorophyll
light energy / solar energy / sun
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chlorophyll

excited electrons

Electron transport chain  energy
Photophosphorylation – the formation of ATP (chemical energy) using light energy
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(II)
Photolysis of water – some energy is used to split water to form hydrogen and oxygen. Oxygen is
given out as a by-product. Hydrogen is accepted by NADP (nicotinamide adenine dinucleotide
phosphate) to form NADPH (reduced NADP).
ATP and NADPH are used in dark reactions.
Dark reactions / carbon fixation / Calvin cycle – light is not required.
5-C compound reacts with carbon
dioxide to form 2 x 3-C compounds.
ATP and NADPH are used to change
the 3-C compound into triose
phosphate. Triose phosphate is used
to make glucose (6-C compound) and
regenerate the 5C compound.
The site of photosynthesis – chloroplast
Outer membrane – control the passage of substances in and out of the chloroplast.
The inner membrane is highly folded to form the thylakoid. Thylakoid increases the surface area to pack more
chlorophyll and the electron transport chain. Chlorophyll absorbs light energy for carrying out the light reactions.
Stroma contains enzymes for the dark reaction / carbon fixation / Calvin cycle and stores excess food in the form
of starch.
The fate of the photosynthetic products:
The fate of glucose:
(a) Glucose is broken down to release energy by respiration.
(b) Glucose is converted to starch for storage.
(c) Starch is changed to sucrose for the transport to root, stem and other storage organs by the phloem.
(d) Glucose is used to make cellulose which makes the cell wall.
Intermediates are used to make fatty acids and glycerol. Fatty acids and glycerol combines to make lipid.
Intermediates combine with nitrates to make amino acids. Amino acids are used to make protein.