B5 : Photosynthesis

Question 1

Uses of glucose

Answer 1

1. The glucose produced by photosynthesis is mainly used for respiration in plants.
2. However, it can also be converted into other products such as protein, fats and oils, cellulose, and starch

Question 2

Uses of protein

Answer 2

1. Proteins are needed for cell growth and repair.
2. A source of nitrogen is also needed for glucose to be converted into proteins.
3. Plants get this nitrogen by absorbing nitrate ions (minerals) from soil.

Question 3

Uses of fats and oil

Answer 3

1. Fats and oils are stored in structures such as seeds.

2. For example, sunflower seeds are a key ingredient in cooking oils.

Question 4

Uses of cellulose

Answer 4

Cellulose is used to strengthen cell walls. It is particularly important for fast-growing plants.

Question 5

Uses of starch

Answer 5

Starch is insoluble (cannot be dissolved) and is stored in stems, leaves or roots.

Question 6

Ion Deficiencies

Answer 6

1. Plants need many mineral ions (minerals) for healthy growth.
2. They extract most of these mineral ions from the soil. If the soil quality is poor, plants can suffer from ion deficiency conditions (disorders).
3. Ion deficiencies include Nitrate deficiency and Magnesium deficiency

Question 7

Nitrate deficiencies

Answer 7

1. Nitrates are needed for the production and synthesis (creation) of amino acids/proteins.
2. Amino acids are needed for protein synthesis
3. Without proteins, plants cannot grow. This means that a nitrate deficiency causes stunted growth.

Question 8

Magnesium deficiency

Answer 8

1. Magnesium is required for the production of chlorophyll.
2. Magnesium deficiency causes chlorosis: The plant cannot photosynthesise well. This means that it cannot produce the glucose needed for healthy growth.
3. When magnesium is absent, plant leaves are yellow.

Question 9

key factors affecting the rate of photosynthesis

Answer 9

1. Temperature
2. Carbon dioxide concentration
3. Light intensity
4. Chlorophyll Concentration

Question 10

Light Intensity

Answer 10

1. Increasing light intensity increases the rate of photosynthesis because more energy is provided.
2. However, if the light intensity is increased above a certain threshold, the rate of photosynthesis can no longer be further increased by lights intensity because another factor (such as temperature) is limiting the rate of the reaction.

Question 11

Chlorophyll Concentration

Answer 11

High chlorophyll concentration gives a high rate of photosynthesis.

Question 12

Carbon Dioxide Concentration

Answer 12

1. Increasing the carbon dioxide concentration increases the rate of photosynthesis.
2. However, above a certain threshold, the rate of photosynthesis cannot be further increased by carbon dioxide concentration because another factor (such as light intensity) is limiting the rate of reaction.

Question 13

Temperature

Answer 13

1. Increasing the temperature increases the rate of photosynthesis because more energy is provided.
2. However, if the temperature is increased to above about 45°C, the enzymes that catalyse (speed-up) the reaction begin to denature (not work anymore).
3. This causes the rate of the reaction to drop sharply until it stops altogether.

Question 14

Photosynthesis Word Equation

Answer 14

Carbon Dioxide + water (arrow) Glucose + Oxygen

Question 15

Photosynthesis Chemical Equation

Answer 15

6CO2 + 6H20 → C6H12O6 + 6O2

Question 16

Limiting Factors

Answer 16

1. A limiting factor is a factor or resource that’s in such short supply, the rate of photosynthesis is limited by it.
2. The identity of the limiting factor in photosynthesis depends on the conditions faced by the plant
3. Situations where limiting factors may occur include winter, night, warm and bright, and mineral-deficient soil

Question 17

Winter

Answer 17

In winter, temperature is usually the limiting factor.

Question 18

Night

Answer 18

At night, light intensity is usually the limiting factor.

Question 19

Warm and Bright

Answer 19

In warm and bright conditions, carbon dioxide concentration is usually the limiting factor.

Question 20

Mineral-deficient Soil

Answer 20

1. If plants are grown in mineral-deficient (lacking minerals) soil, they may not absorb enough minerals to produce lots of chlorophyll.
2. In this case, chlorophyll concentration can be the limiting factor.

Question 21

Testing the rate of photosynthesis

Answer 21

1. You can easily investigate the effect of light intensity on the rate of photosynthesis by using an aquatic (lives in water) plant like pondweed.
2. To do this, change the distance between the lamp and the pondweed and count the number of bubbles (oxygen) produced.
3. In this experiment, light intensity is the independent variable (and limiting factor) and the number of bubbles is the dependent variable.

Question 22

Photosynthesis

Answer 22

1. Photosynthesis is the process by which plants make carbohydrates from raw materials, using energy from light.
2. Green plants make the carbohydrate glucose from carbon dioxide and water. At the same time oxygen is produced. Plants need energy of sunlight for this. This reaction is called photosynthesis

Question 23

Process of photosynthesis

Answer 23

1. light energy is absorbed by chlorophyll - a green substance found in chloroplasts in green plant cells and algae
2. absorbed light energy is used to convert carbon dioxide (from the air) and water (from the soil) into a sugar called glucose
3. oxygen is released as a by-product

Question 24

Variegated leaves

Answer 24

Some leaves have white patches where there is no chlorophyll. These are called variegated leaves.

Question 25

Testing a leaf for starch

Answer 25

1. Boil the leaf to kill it and stop photosynthesis.
2. Place it into alcohol and boil to remove chlorophyll.
3. Place into water to soften the leaf and remove excess alcohol.
4. Place onto a petri dish and add iodine.
5. If the iodine changes from orange/brown to blue/black starch is present.

Question 26

Chlorophyll

Answer 26

1. This is where photosynthesis occurs.
2. Chloroplasts contain chlorophyll which allows light energy to be absorbed and converted to chemical energy.
3. pigment that makes plants look green
4. when sunlight falls on chlorophyll molecule, some of the energy in the light is absorbed, and the molecule then releases energy to make carbon dioxide combine with water, with the help of enzymes inside the chloroplasts to convert light energy into chemical energy