Topic 10 - Plants and Food (Unit 3)

Question 1

What is the main storage carbohydrate made by plants?

Answer 1

Starch is the main storage carbohydrate made by plants.

Question 2

What are plants?

Answer 2

Plants are autotrophs, which means they can make complex molecules from simple molecules, and also producers, so that they can make their own food.

Question 3

How do you test a leaf for starch?

Answer 3

Remove the outer waxy layer by placing the leaf in boiling ethanol: place the plant in boiling water for 30 seconds, turn off the Bunsen burner, place the leaf in a boiling tube of ethanol and then place the tube in a beaker of hot water - most of the chlorophyll is removed. The leaf is then placed in cold water to soften it before being spread out on a tile and covered with iodine. Any blue-black areas contains starch.

Question 4

Why is starch an effective way of storing carbohydrate?

Answer 4

Starch is an effective way of storing carbohydrate because it is insoluble, compact and can be broken down easily.

Question 5

How do you test for starch in food?

Answer 5

Add a few drops of yellow-brown iodine solution. If the food contains starch, a blue-black colour is produced.

Question 6

In what part of plants is starch made?

Answer 6

Starch is only made in the parts of plants that contain chlorophyll. You can show this by testing a leaf from a variegated plant, which has green and white areas to its leaves. The white parts of the leaf will give a negative starch test.

Question 7

What three things are required for starch production by leaves?

Answer 7

Carbon dioxide from the air, light and chlorophyll in the leaves.

Question 8

How do you investigate the need for light in photosynthesis?

Answer 8

Destarch the plant, by placing it in a dark cupboard for 24 hours, in order to ensure that any starch already present in the leaves will be used up and will not affect the results of the experiment. Following this process, partially cover a leaf of the plant with aluminium foil and place the plant in sunlight for a day. Remove the covered leaf and test for starch using iodine.

Question 9

How do you investigate the need for carbon dioxide in photosynthesis?

Answer 9

Destarch the plant, by placing it in a dark cupboard for 24 hours. Then, enclose one leaf with a conical flask containing potassium hydroxide, which will absorb any carbon dioxide. Enclose another leaf with a conical flask containing no potassium hydroxide (as the control). Place the plant in bright light for several hours before testing both leaves for starch using iodine.

Question 10

How do you investigate the need for chlorophyll in photosynthesis?

Answer 10

Drop the leaf in boiling water to kill the tissue and break down the cell walls. Transfer the leaf into hot ethanol in a boiling tube seven minutes - this removes the chlorophyll so colour changes from iodine can be seen more clearly. Dip the leaf in boiling water so that the leaf tissue can be softened. Spread the leaf out on a white tile and cover it with iodine solution.

Question 11

What is starch composed of?

Answer 11

Starch is composed of long chains of glucose. A plant does not make starch directly, but first produces glucose, which is then joined together in chains to form starch molecules. A carbohydrate made of many sugar sub-units is called a polysaccharide.

Question 12

What is the chemical formula for glucose?

Answer 12

The chemical formula for glucose is C₆H₁₂O₆.

Question 13

What is photosynthesis?

Answer 13

Photosynthesis is the process by which plants use the simple inorganic molecules, carbon dioxide and water, in the presence of chlorophyll and light, to make glucose and oxygen.

Question 14

What is the word equation for photosynthesis?

Answer 14

The word equation for photosynthesis is:

carbon dioxide + water > glucose + oxygen.

Question 15

What is the chemical equation for photosynthesis?

Answer 15

The process of photosynthesis is commonly written as:

6CO₂ + 6H₂O > C₆H₁₂O₆ + 6O₂.

Question 16

What is the role of chlorphyll?

Answer 16

The role of chlorophyll is to absorb the light energy needed for the reaction to take place.

Question 17

What are the products of photosynthesis?

Answer 17

Glucose and oxygen are the products of photosynthesis.

Question 18

How are leaves adapted to be able to photosynthesise efficiently?

Answer 18

To be able to photosynthesise, efficiently, leaves need to have a large surface area to absorb light, many chloroplasts containing chlorophyll, a supply of water and carbon dioxide and a system for carrying away the products of photosynthesis to other parts of the plant. Most leaves are supported by a leaf stalk which can grow to allow the blade of the leaf to be angled to receive the maximum amount of sunlight.

Question 19

What is the function of the upper and lower epidermis?

Answer 19

The two outer layers of cells have few chloroplasts and are covered by a thin layer of a waxy material called cuticle. This reduces water loss by evaporation, and acts as a barrier to the entry of disease-causing microorganisms such as bacteria and fungi.

Question 20

How is the lower epidermis adapted to carry out its function?

Answer 20

The lower epidermis has many pores (called stomata), which allow carbon dioxide to diffuse into the leaf and reach the photosynthetic tissues. They also allow oxygen and water vapour to diffuse out.

Question 21

How is each stoma formed?

Answer 21

Each stoma is formed as a gap between two highly specialised cells called guard cells, which can change their shape to open or close the stoma.

Question 22

What is the palisade mesophyll layer made of?

Answer 22

The palisade mesophyll layer is a tissue made of long, narrow cells, each containing hundreds of chloroplasts, and is the main site of photosynthesis. Since the palisade cells are close to the source of light, light is able to pass through to the enormous numbers of chloroplasts which lie below.

Question 23

What is the spongy mesophyll layer made of?

Answer 23

The spongy mesophyll layer is a tissue of more rounded, loosely packed cells, with air spaces between them; these cells also photosynthesise, but have fewer chloroplasts than the palisade cells. They form the main gas exchange surface of the leaf, absorbing carbon dioxide and releasing oxygen and water vapour. The air spaces allow these gases to diffuse in and out of the mesophyll.

Question 24

What supplies water and mineral ions to the leaf?

Answer 24

Water and mineral ions are supplied to the leaf by vessels in a tissue called xylem, which forms a continuous transport system throughout the plant. Water is absorbed by the roots and passes up through the stem and through the veins in the leaves in the transpiration system. In the leaves, the water leaves the xylem and supplies the mesophyll cells.

Question 25

What carries away the products of photosynthesis?

Answer 25

The products of photosynthesis, such as sugars, are carried away from the mesophyll cells by phloem, which supplies all other parts of the plant, so that tissues and organs that cannot make their own food receive it. The veins in the leaf contain both xylem and phloem tissue, and branch again and gain to supply all parts of the leaf.

Question 26

How is the structure of the leaf adapted for gas exchange?

Answer 26

Leaves are thin, which gives a short diffusion distance for the gases; they are flat, providing a large surface area to volume ratio; they also have many stomata which allow movement of gases in and out of the air spaces inside the leaf to maintain a steep concentration gradient.

Question 27

How is the internal structure of a leaf adapted to maximise gas exchange?

Answer 27

Air spaces allow gas movement around the loosely packed mesophyll cells. Many stomata in the lower epidermis open in sunlight to allow gas movement. Thin cell walls allow gases to move into the cells easily. Moist air which gases can dissolve into for easier movement into and out of cells. The close contact between the cells and the air spaces allows for efficient gas exchange.

Question 28

What are stomata?

Answer 28

Stomata are spaces found between two guard cells predominantly on the lower epidermis of the leaf.

Question 29

What are the guard cells responsible for?

Answer 29

The guard cells are responsible for the opening and closing of the stomatal pore which controls gas exchange and water loss.

Question 30

How do stomata and diffusion play a role in a leaf’s gas exchange?

Answer 30

Stomata open when water moves into the guard cells causing them to become turgid. This allows gases to diffuse in and out of the leaf through the stomatal pore. Stomata close when the guard cells lose water to the neighbouring epidermal cells and they become flaccid, preventing any diffusion into our out of the leaf.

Question 31

How does the exchange of carbon dioxide and oxygen depend on the balance between respiration and photosynthesis?

Answer 31

When light intensity is high, a plant carries out photosynthesis at a much higher rate than it respires. So, in bright light, there is an overall uptake of carbon dioxide from the air around a plant’s leaves and a surplus production of oxygen. A plant only produces more carbon dioxide than it uses up in dim light.

Question 32

How does the concentration of carbon dioxide in the air around plants change throughout the day?

Answer 32

The air contains the least carbon dioxide in the afternoon, when photosynthesis happens at its highest rate. At night, when there is no photosynthesis, the level of carbon dioxide rises due to less carbon dioxide being absorbed by plants, while carbon dioxide is still being added to the air from the respiration of other organisms.

Question 33

What is a limiting factor?

Answer 33

The component of a reaction that is in shortest supply so that it prevents the rate of reaction from increasing.

Question 34

How does light intensity affect the rate of photosynthesis?

Answer 34

The more light that the plant receives, the faster the rate of photosynthesis. This will continue until a limiting factor prevents the rate from increasing.

Question 35

What are the three main limiting factors of photosynthesis?

Answer 35

The three main limiting factors are: temperature, carbon dioxide intensity and water (and mineral ions).

Question 36

How does light intensity affect photosynthesis?

Answer 36

The more light that the plant receives, the faster the rate of photosynthesis. This will continue until a limiting factor prevents the rate from further increasing.

Question 37

How does temperature affect the rate of photosynthesis?

Answer 37

The lower the temperature, the less kinetic energy particles have, resulting in fewer successful collisions occurring over a period of time. Increasing temperature increases the kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in the formation of products. At higher temperatures, however, enzymes that control the processes of photosynthesis can be denatured (where the active site changes shape and is no longer complementary to its substrate) – this reduces the overall rate of photosynthesis.

Question 38

How does carbon dioxide concentration affect photosynthesis?

Answer 38

Carbon dioxide is one of the raw materials required for photosynthesis. This means that the more carbon dioxide that is present, the faster the reaction can occur. This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply.

Question 39

What can the amount of chlorophyll be affected by?

Answer 39

The amount of chlorophyll present in a plant can be affected by diseases, lack of nutrients and loss of leaves.

Question 40

How does chlorophyll affect photosynthesis?

Answer 40

The number of chloroplasts (as they contain the pigment chlorophyll which absorbs light energy for photosynthesis) will affect the rate of photosynthesis. The more chloroplasts a plant has, the faster the rate of photosynthesis.

Question 41

How do you investigate the effect of light on gas exchange from a leaf using hydrogen carbonate indicator?

Answer 41

Place some hydrogen carbonate indicator in the bottom of four boiling tubes; three large leaves are detached from a plant and placed in the tops of three of the tubes. Each tube is sealed with a rubber bung. One tube is placed in bright light, one in dark and the third is covered with some translucent material. The fourth tube acts as a Control, to show that it is the presence of the leaf which affects the colour of the indicator.

Question 42

What are the results of the hydrogen carbonate indicator experiment?

Answer 42

The colour of the indicator left in the tube that was in the light was purple. The colour of the indicator left in the tube that was in the dark was yellow. The colour of the indicator left in the tube that was in dim light was orange. The control tube was also orange.

Question 43

How do you investigate photosynthesis using the evolution of oxygen from a water plant?

Answer 43

Take a bundle of shoots of a water plant and submerge them in a beaker of water underneath an upturned funnel. Fill a boiling tube with water and place it over the end of the funnel. As oxygen is produced, the bubbles of gas will collect in the boiling tube and displace the water. You can show that the gas collected is oxygen by relighting a glowing splint.

Question 44

How does a plant use glucose?

Answer 44

Glucose is used in chlorophyll, the form of lipids (such as oil in seeds), proteins and DNA, cellulose for cell walls, starch for storage and, also, sucrose for transport.

Question 45

What is the use of nitrate?

Answer 45

Nitrate is used by plants for making amino acids, proteins, chlorophyll, DNA and many other compounds.

Question 46

What are the deficiency symptoms of nitrate?

Answer 46

Deficiency symptoms include limited growth of the plant and older leaves turning yellow.

Question 47

What is the use of phosphate?

Answer 47

Phosphate is used by plants for making DNA and many other compounds; it is also a part of cell membranes.

Question 48

What are the deficiency symptoms of phosphate?

Answer 48

Deficiency symptoms include poor root growth and younger leaves turning purple.

Question 49

What is the use of potassium?

Answer 49

Potassium is used by plants for enzymes of respiration and photosynthesis to work.

Question 50

What are the deficiency symptoms of potassium?

Answer 50

Deficiency symptoms include leaves turning yellow with dead spots.

Question 51

What is the use of magnesium?

Answer 51

Magnesium is part of chlorophyll.

Question 52

What are the deficiency symptoms of magnesium?

Answer 52

Deficiency symptoms include leaves turning yellow.