(Credits to Mustafa Asif)
What is photosynthesis?
It is the process by which green plants trap light energy using chlorophyll and convert it to chemical energy for the production of glucose from water and carbon dioxide.
Oxygen is released as a by-product.
Adaptations of the leaf
Leaf is an organ specialised for photosynthesis.
Its broad and thin lamina provides and large surface area for maximum absorption of light.
The thin lamina allows carbon dioxide to enter/diffuse into the cells quickly.
The palisade cells contains numerous chloroplast and are packed closely together at the upper surface of the leaf.
This enables them to absorb light energy efficiently.
The spongy mesophyll cells are loosely packed so that large intercellular spaces are present for rapid diffusion of gaseous exchange.
The numerous stomata at the lower epidermis allows rapid gaseous exchange between the leaf and the air.
The extensive vain system of xylem and phloem help to provide rapid transport of water to, and manufactured food from the leaf.
Effects of varying light intensity on photosynthesis.
Light energy is trapped by chlorophyll to split water molecules during photosynthesis.
Increasing light energy will speed up photolysis, hence increasing photosythesis.
Effects of varying wavelights on photosynthesis.
Red and blue wavelights highly absorbed.
Green and yellow reflected.
More light absorbed will increase rate of photosynthesis.
Effects of varying carbon dioxide on photosynthesis
Increased CO2, increases rate of glucose produced, resulting in increase in rate of photosynthesis.
Effects of varying temperature on photosynthesis
Temperature affects rate of enzyme-catalysed reaction in photosynthesis.
Low temperature: enzymes inactive
High temperature: enzymes denature and no photosynthesis takes place.
1. The word equation which best describes photosynthesis is .
a. carbon dioxide + energy → glucose + oxygen + water
b. glucose + oxygen → water + carbon dioxide + energy
c. glucose + oxygen → water + carbon dioxide + energy
d. water + carbon dioxide + energy → glucose + oxygen
2. Which of the following cells of a leaf do not have chloroplasts?
a. Guard cells
b. Palisade mesophyll
c. Spongy mesophyll
d. Upper epidermis
3. How does most of the carbon dioxide enter the photosynthesising cells of a leaf?
a. Diffusion through the epidermis of the leaf
b. Diffusion through the stomata of the leaf
c. Diffusion through the xylem in the leaf
d. Diffusion through the phloem in the leaf
4. Which of the following statements is the most accurate?
a. Food can travel up the stem in the phloem.
b. Food can travel up or down the stem in the xylem
c. Food can travel up or down the stem in the phloem.
d. Food can travel down the stem in the phloem.
5. Which of the following conditions is least likely to increase the rate of transpiration in a plant?
a. a rise in temperature
b. an increase in humidity
c. increased air movement.
d. increased sunlight
Structured Question Worked Solutions
1. The equation for photosynthesis is usually given as:
a. Which cell organelle does photosynthesis occur in?
b. Comment on the accuracy of the given equation in describing the process of
b. The equation is not very accurate because
it does not show that photosynthesis is a two-step process
only one step is light-dependent and the other is light-independent
water is formed during the light-independent stage.
2. The graph below shows the effect of light intensity on a plant’s rate of photosynthesis at two different temperatures. Curve A was obtained with the plant at low temperature while curve B was obtained at a higher temperature.
a. What factor(s) was (were) affecting the rate of photosynthesis at each of the regions, 1, 2 and 3, on the graph? Give reasons for your answers.
b. Suggest two factors that might have been limiting the rate of photosynthesis at point 4 on curve B.
Region 1: Light intensity is the limiting factor because when light intensity is increased, the rate of photosynthesis increases.
Region 2: Light intensity is not the limiting factor. Temperature or concentration of carbon dioxide could be the limiting factor because increasing light intensity does not increase the rate of photosynthesis.
Region 3: Light intensity is the limiting factor because as light intensity increases, the rate of photosynthesis increases.
b. Concentration of carbon dioxide and temperature
3. Photosynthesis takes place mostly in the leaves of plants. The leaves of most plants are wide
and thin, as shown in the diagram.
a. Name the cells in the leaf where most photosynthesis takes place.
b. Which tissue in the leaf supplies water to the cells for photosynthesis?
c. Explain how being wide helps a leaf to carry out photosynthesis.
d. Explain how being thin helps a leaf to carry out photosynthesis.
e. Most leaves are covered with a transparent, waterproof cuticle. How does this feature aid in photosynthesis?
a. Mesophyll cells
c. Being wide means that the leaf has more surface area for light absorption. Light energy is needed for photosynthesis.
Being thin means that gases such as carbon dioxide, which is one of the raw materials for photosynthesis, can easily diffuse into the mesophyll cells, since there is a shorter distance to travel.
It is also easier for light to penetrate the leaf to reach all photosynthetic cells.
e. The waterproof cuticle helps prevent excessive water loss by evaporation from the leaf surface. This
ensures enough water for photosynthesis. It also helps to focus the light rays onto the mesophyll layer.
4. Discuss the major adaptations of the leaf to photosynthesis.
The petiole holds the leaf lamina away from the stem so that the lamina can absorb suffi cient sunlight and air.
The leaf lamina has a large fl at surface compared to its volume. This enables the leaf to absorb the maximum amount of sunlight for photosynthesis.
A thin expanded lamina ensures that carbon dioxide, the raw material for photosynthesis, can rapidly reach the inner cells of the leaf.
A network of veins extends throughout the leaf. Veins are made of the vascular tissues, xylem and phloem. Xylem transports water and mineral salts to the leaf cells. Phloem transports sugars away from the leaf cells to other parts of the plant. This maintains a concentration gradient so that the photosynthetic reactions proceed in the forward direction.
A waxy cuticle on the leaf epidermis protects the leaf by reducing evaporation of water. Water is an essential raw material for photosynthesis. Lack of water would also make the leaf wilt, reducing surface area for light absorption.
Stomata on the lower epidermis open in sunlight, allowing carbon dioxide to diffuse in and oxygen to diffuse out.
Mesophyll cells of the leaf contain numerous chloroplasts. Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis.
Mesophyll cells in the upper regions of the leaf (palisade mesophyll) contain more chloroplasts since more light energy can be absorbed near the leaf surface.
An interconnecting system of air spaces in the lower regions of the leaf (spongy mesophyll) allow rapid diffusion of carbon dioxide into mesophyll cells.
5. Discuss why the following statement is true: “Temperature will only increase the rate of photosynthesis to a certain extent. At very high temperatures, the rate of photosynthesis will decrease.”
Photosynthesis is an enzyme-catalysed reaction.
There will be increased collision of enzyme and substrate as temperature increases.
This results in more enzyme-substrate complex being formed and thus a greater rate of reaction. However, at very high temperatures, enzymes being protein in nature, will be denatured.
The substrate no longer fits into the active site and an enzyme-substrate complex cannot be formed.
Thus the rate of photosynthesis will decrease.