20: Photosynthesis

Question 1

State the word equation for the overall process of photosynthesis.

Answer 1

carbon dioxide + water – (light energy) (chlorophyll) –> carbohydrates + oxygen

Question 2

Explain how photosynthesis allows a continuous input of energy to ecosystems.

Answer 2

During photosynthesis, light energy from the sun is captured by plants and algae. It is then converted to chemical energy that is stored in their bodies. The chemical energy is transferred along food chains and is eventually lost as heat to the surroundings.

Question 3

Explain how photosynthesis maintains the oxygen balance in the atmosphere.

Answer 3

Respiration of organisms consumes oxygen. The oxygen consumed is replaced by the oxygen released from photosynthesis.

Question 4

Explain how photosynthesis maintains the carbon dioxide balance in the atmosphere.

Answer 4

Respiration of organisms releases carbon dioxide. The carbon dioxide is consumed in photosynthesis, balancing the increase in carbon dioxide concentration in the atmosphere due to respiration.

Question 5

List 3 significances of photosynthesis.

Answer 5

1. It provides the basic food source for most organisms.
2. It maintains energy flow in ecosystems.
3. It maintains the balance of oxygen and carbon dioxide in the atmosphere.

Question 6

List 2 ways to test is photosynthesis has occurred.

Answer 6

1. Detecting the release of oxygen: oxygen is a by-product of photosynthesis and can be tested using a glowing splint. Oxygen is present if the splint relights.
2. Detecting the presence of starch: iodine test can be carried out. Starch is present if the iodine solution turns from brown to blue-black.

Question 7

Explain why destarching the plant is necessary before an investigation on the requirements of photosynthesis.

Answer 7

If the presence of starch is used as evidence that photosynthesis has occurred, any starch that is present before the investigation in the leaves must be removed. This ensures that the starch detected at the end of the investigation was made during the investigation.

Question 8

State the procedures of destarching.

Answer 8

The plant is put in the dark for at least 24 hours. In darkness, the starch in the leaves is broken down to glucose, which is used as an energy source or combined with fructose to form sucrose.

Question 9

State the main site of photosynthesis in plants.

Answer 9

Leaves

Question 10

List 4 advantages of the adaptations of leaves for photosynthesis.

Answer 10

1. The light absorption is maximised.
2. Gas exchange is facilitated.
3. Transport of materials is facilitated.
4. Water loss is reduced.

Question 11

List and explain 3 ways in which the shape and arrangement of leaves are well adapted for photosynthesis.

Answer 11

1. The leaf blade is often broad and flat to provide a large surface area for absorbing sunlight.
2. The leaf is thin to allow gases and light to reach the photosynthetic cells easily.
3. The leaves are arranged in a way that there is little overlapping among the leaves, allowing each leaf to expose to maximum amount of light.

Question 12

State the properties and functions of the palisade mesophyll in terrestrial dicotyledonous plants.

Answer 12

It consists of cylindrical, tightly packed cells that contain many chloroplasts. It is located on the upper side of the leaf, which is exposed directly to sunlight. These allow effective absorption of sunlight.

Question 13

State the properties and functions of the spongy mesophyll in terrestrial dicotyledonous plants.

Answer 13

It consists of loosely packed cells of irregular shapes with many air spaces. The air space allow gases to diffuse freely inside the leaf. The cells contain chloroplasts for the absorption of sunlight.

Question 14

List 3 differences between the palisade mesophyll and the spongy mesophyll in terrestrial dicotyledonous plants.

Answer 14

1. Arrangement of cells: the palisade mesophyll cells are cylindrical and closely packed, while the spongy mesophyll cells are irregular in shape and loosely packed.
2. Air spaces: The air spaces among palisade mesophyll cells are narrower while there are large air spaces among spongy mesophyll cells.
3. Amount of chloroplast: palisade mesophyll cells contain more chloroplasts and spongy mesophyll cells contain fewer chloroplasts.

Question 15

State the properties and functions of the cuticle in terrestrial dicotyledonous plants.

Answer 15

It is a thin waxy layer covering the upper and lower epidermis. It prevents excessive water loss by evaporation from the leaf. This ensures that water is kept inside for photosynthesis, for keeping the cells turgid and supporting the leaf blade.

Question 16

State the properties and functions of the upper and lower epidermis in terrestrial dicotyledonous plants.

Answer 16

They are covered by a thin layer of waxy cuticle. The epidermis is the outermost layer of cells covering the upper and lower surfaces of leaf, which protects the inner layers of cells. In most terrestrial dicotyledonous plants, there are more stomata in the lower epidermis than in the upper epidermis. Epidermal cells have no chloroplasts except the guard cells.

Question 17

State the properties and functions of the stomata in terrestrial dicotyledonous plants.

Answer 17

There are more stomata (stoma) in the lower epidermis than in the upper epidermis. Each stoma is surrounded by a pair of guard cells that contain chloroplasts. Stoma open when the conditions are favourable for photosynthesis, and close when the conditions are less favourable. This regulates the passage of gases and water vapour into and out of the leaf.

Question 18

State the properties and functions of the vascular bundles in terrestrial dicotyledonous plants.

Answer 18

The midrib and veins contain vascular bundles which are made up of xylem and phloem. These allow efficient transport of materials into and away from the leaf. They also support and spread the leaf blade.

Question 19

State the structural features of chloroplasts.

Answer 19

A chloroplast is bound by a double membrane and is filled with a jelly-like fluid called stroma. A network of disc-like membranous sacs called thylakoids is suspended in the stroma. They are arranged in stacks called grana (granum). Chlorophyll is located on the thylakoid membranes.

Question 20

State three substances that are found in the stroma.

Answer 20

In the stroma, there are enzymes that catalyse photosynthetic reactions, starch grains, and other photosynthetic products like lipid droplets.

Question 21

List and explain 2 ways in which the features of a chloroplast are well adapted for photosynthesis.

Answer 21

1. A chloroplast has about 50 grana, each consisting of about 50 thylakoids. This provides a large surface area to pack more chlorophyll for absorbing light.
2. The grana are interconnected by extensions from the thylakoids. This allows efficient transport of photosynthetic products within the chloroplast.

Question 22

What are the two main stages of photosynthesis?

Answer 22

Photochemical reactions and carbon fixation (Calvin cycle).

Question 23

Explain the meaning of photochemical reactions.

Answer 23

They are a series of reactions occurring in the thylakoids of chloroplasts, which require light. It involves excitation of electrons in chlorophyll and the splitting of the water molecule, leading to the production of oxygen.

Question 24

Explain the meaning of carbon fixation.

Answer 24

It is a series of reactions occurring in the stroma of chloroplasts, which do not require light. It involves the synthesis of various molecules from carbon dioxide, leading to the production of carbohydrates.

Question 25

Explain the difference between carbon fixation and carbon dioxide fixation.

Answer 25

Carbon dioxide fixation is a stage **within** carbon fixation, where carbon dioxide gas is fixed with a 5-C compound to form 2 3-C compounds. Carbon fixation refer to the whole **Calvin cycle**.

Question 26

Name the 4 main stages of photochemical reactions.

Answer 26

1. Light absorption and electron emission 2. Electron transport 3. Formation of ATP 4. Photolysis of water

Question 27

Explain the process of light absorption and electron emission in photochemical reactions.

Answer 27

Chlorophyll molecules in the thylakoid membrane **capture light energy**. Some electrons of the chlorophyll molecule are **raised to a higher energy level** and are emitted from the chlorophyll molecule.

Question 28

Explain the process of electron transport in photochemical reactions.

Answer 28

The excited electrons pass through an **electron transport chain**, which consists of a series of **electron carriers** of **decreasing energy levels** in the thylakoid membrane. Energy is released in a step-wise manner.

Question 29

Explain the process of the formation of ATP in photochemical reactions.

Answer 29

Some energy released by the electrons in the electron transport chain is used to form **ATP** by combining a phosphate with ADP, which is known as **photophosphorylation**. Light energy captured by the chlorophyll molecules is converted to chemical energy **stored in ATP**, which acts as an **energy carrier**.

Question 30

Explain the process of photolysis of water in photochemical reactions.

Answer 30

The light energy captured by chlorophyll molecules is also used to **split water molecules** to form **hydrogen and oxygen**. The hydrogen is accepted by **NADP** which is then reduced to **NADPH**, while **oxygen** is released as a **by-product** of the reaction, as a gas to the atmosphere.

Question 31

Name the 3 main stages of carbon fixation.

Answer 31

1. Carbon dioxide fixation and formation of 3-C compound 2. Reduction of 3-C compound and formation of glucose 3. Regeneration of the carbon dioxide acceptor

Question 32

Explain the process of carbon dioxide fixation in carbon fixation.

Answer 32

Carbon dioxide diffuses from the air spaces in leaves into the photosynthetic cell and then into the **stroma** of the chloroplast. Under the action of **enzymes**, one molecule of carbon dioxide combines with one molecule of a **5-C compound** to form **2 molecules of a 3-C compound.**

Question 33

Name the 3-C compound being reduced in carbon fixation.

Answer 33

G3P (Glycerate-3-phosphate)

Question 34

Name the enzyme which fixes carbon dioxide in carbon fixation.

Answer 34

RUBISCO

Question 35

Explain the process of reduction of the 3-C compound in carbon fixation.

Answer 35

Using energy from the breakdown of ATP into ADP and phosphate, and the hydrogen from NADPH, the 3-C compound is **reduced to triose phosphate**. ADP and NADP are formed and will be **reused** in photochemical reactions. Two molecules of triose phosphate combine to form one molecule of **glucose**.

Question 36

Explain the process of regeneration of the carbon dioxide acceptor in carbon fixation.

Answer 36

Most of the triose phosphate formed is used to regenerate the **original 5-C carbon dioxide acceptor**, so the cycle repeats and forms more glucose. This requires **energy from ATP**.

Question 37

State the balanced chemical equation of photosynthesis.

Answer 37

6CO₂ + 6H₂O --(light captured by chlorophyll) --> C₆H₁₂O₆ + 6O₂

Question 38

Name the primary product of photosynthesis.

Answer 38

Triose phosphate

Question 39

State the fate of triose phosphate right after it is produced.

Answer 39

Most triose phosphate produced is used to **regenerate the 5-C carbon dioxide acceptor**. The rest of triose phosphate is quickly synthesised into **glucose or fructose**. Some glucose is used up in photosynthesising cells as an **energy source**.

Question 40

State how carbohydrates used for transport are synthesised from triose phosphate.

Answer 40

Glucose and fructose, which are synthesised from triose phosphate, are combines to form **sucrose** for transport to non-photosynthetic tissues.

Question 41

Explain why sucrose is a suitable form for transport in plants.

Answer 41

It is **soluble but relatively inactive**.

Question 42

State how carbohydrates used for storage are synthesised from triose phosphate.

Answer 42

Excess glucose, which is synthesised from triose phosphate, is quickly built into starch and stored in cells.

Question 43

Explain why starch is a suitable form for storage in plants.

Answer 43

It is **insoluble** in water. It can be stored in leaves without affecting the water potential of the cells.

Question 44

State 5 possible forms of carbohydrates which are synthesised from triose phosphate.

Answer 44

Glucose, fructose, sucrose, starch, cellulose

Question 45

State three forms of carbohydrates which starch can be converted into.

Answer 45

It can be broken down to form **glucose** and used as an **energy source**. The glucose can also be used to make **cellulose**. Starch may also be converted into **sucrose** for transport.

Question 46

State how lipids are synthesised from triose phosphate.

Answer 46

Glucose may be broken down in different reactions. The **metabolic intermediates** can be used to synthesise **glycerol and fatty acids**. Glycerol and fatty acids can combine to form lipids, and fatty acids can also be used to form **cell membranes**.

Question 47

State two applications of lipids in plants.

Answer 47

Fatty acids can be used to form **cell membranes**. Lipids may be stored as **energy reserve** in some plants (eg. maize).

Question 48

State how proteins are synthesised from triose phosphate.

Answer 48

Intermediates of glucose breakdown may combine with **inorganic ions** to form **amino acids**, which are the building blocks of proteins.

Question 49

State three applications of proteins in plants.

Answer 49

They are important for **growth and repair**, and for the synthesis of **enzymes**. They are also the components of **cell membranes**.

Question 50

Explain the effect of light intensity on the rate of photosynthesis at low to moderate light intensity.

Answer 50

At low to moderate light intensity, the rate of photosynthesis **increases proportionately** with increasing light intensity. This is because **more energy is supplied** to the photochemical reactions.

Question 51

Explain the effect of light intensity on the rate of photosynthesis at the optimum light intensity.

Answer 51

The **saturation point** is reached and the rate of photosynthesis just starts to level off.

Question 52

Explain the effect of light intensity on the rate of photosynthesis at very high light intensity.

Answer 52

At very high light intensities, chlorophyll and other photosynthetic components may be **damaged** and the rate of photosynthesis drops.

Question 53

Explain the meaning of limiting factor.

Answer 53

The limiting factor is the factor that is **furthest below the optimal level**. It plays a determining role on the rate and limits the rate from increasing further.

Question 54

State the limiting factors of the rate of photosynthesis of a plant in low light intensity and optimum light intensity respectively.

Answer 54

Low light intensity: light intensity is the limiting factor Optimum light intensity: Other factors other than light intensity (eg. carbon dioxide concentration, temperature, etc.)

Question 55

Explain the effect of carbon dioxide on the rate of photosynthesis at low light intensity.

Answer 55

At low light intensity, the rate of photosynthesis **increases proportionately** with carbon dioxide concentration. This is because **most substrates** are supplied to the Calvin cycle. Carbon dioxide concentration is the limiting factor.

Question 56

Explain the effect of carbon dioxide on the rate of photosynthesis at the optimum carbon dioxide concentration at low light intensity.

Answer 56

As the carbon dioxide concentration increases beyond the optimum value, the rate of photosynthesis **levels off** because it is limited by other factors, such as light intensity or temperature.

Question 57

Explain the effect of carbon dioxide on the rate of photosynthesis of a plant in high and low light intensities respectively, when the carbon dioxide concentration is between the optimum carbon dioxide concentrations at low light intensity and high light intensity.

Answer 57

The rate of photosynthesis of the plant in high light intensity **continues to increase**, but the rate for a plant in low light intensity **levels off**. This shows that **light intensity is the limiting factor** at this range of carbon dioxide concentrations in the curve showing the rate of photosynthesis of the plant in low light intensity.

Question 58

Explain the effect of temperature on the rate of photosynthesis of a plant in high light intensity.

Answer 58

At high light intensities, the effect of temperature on rate of photosynthesis is similar to typical **enzymatic reactions**, since photosynthesis is a process that requires many enzymes. The rate of photosynthesis drops above the optimum temperature of the enzymes involved in the reactions.

Question 59

State and explain 6 constructions in greenhouses to improve productivity.

Answer 59

1. **Heater**, to increase the temperature or supply extra carbon dioxide by burning **paraffin**. 2. **Transparent walls**, to trap warm air and allow sunlight to shine on the crops. 3. **Water sprays**, to moisten the air by spraying mist. 4. **Artificial lighting**, to provide light on cloudy days and at night 5. **Exhaust fans and windows** for ventilation 6. **Sensors** for humidity and pH, to monitor the conditions for corp growth.

Question 60

State three advantages of growing crops in greenhouses.

Answer 60

1. It **reduces damage** by pests, pollution, and bad weather. 2. It improves quality of crops. 3. Crops can be produced **all year round**, hence the yield is improved.