PLANT NUTRITION

Question 1

Photosynthesis

Answer 1

The process by which plants manufacture carbohydrates from raw materials using energy from light

Question 2

Limiting factor

Answer 2

Something present in the environment in such short supply that it restricts life processes

Question 3

PHOTOSYNTHESIS WORD EQUATION

Answer 3

the glucose produced is converted to starch for storage in the leaf..

raw materials for photosynthesis are carbon dioxide, water and light energy. The products are glucose (starch) and oxygen.

Question 4

The process of photosynthesis

Answer 4

1 Green plants take in carbon dioxide through their leaves. This happens by diffusion.

2 Water is absorbed through plants’ roots by osmosis and transported to the leaf through xylem vessels.

3 Chloroplasts, containing chlorophyll, are responsible for trapping light energy. This energy is used to break up water molecules and then to bond hydrogen and carbon dioxide to form glucose.

4 Glucose is usually changed to sucrose for transport around the plant, or to starch for storage.

5 Oxygen is released as a waste product, or used by the plant for respiration..

Question 5

Factors needed for photosynthesis

Answer 5

First, the plant is destarched. This involves leaving the plant in the dark for 48 hours. The plant uses up all the stores of starch in its leaves. One plant (or leaf) is exposed to all the conditions needed – this is the control. Another plant (or leaf) is deprived of one condition (this may be light or carbon dioxide). After a few hours, the starch test is carried out on the control and test plant/ leaf. The equation for photosynthesis shows the raw materials that a plant needs to make its food. Some plants have variegated leaves – only some parts of each leaf contain chlorophyll. When tested for starch, only the parts of the leaf with chlorophyll will contain starch. The carbon dioxide around a plant can be controlled by keeping the plant in a sealed container with a carbon dioxide absorber such as sodium hydroxide pellets. The control plant would be in an identical container, without the carbon dioxide absorber. When light intensity, carbon dioxide or temperature is increased, the rate of photosynthesis increases. However, there comes a point when further increases do not increase the rate.

Question 6

chrolopphyll number equation

Answer 6

Chlorophyll traps light energy and transfers it into chemical energy in molecules for the synthesis of carbohydrates. First, glucose is formed (C6H12O6 in the equation). However, this is converted into sucrose for translocation around the plant. The sucrose is changed to starch for storage. This is insoluble and causes no osmotic problems. Other carbohydrates, e.g. cellulose for making cell walls, can also be synthesised from sucrose. Sucrose is also an energy source for the plant.

Question 7

Factors affecting the rate of photosynthesis

Answer 7

As light intensity increases, so does the rate of photosynthesis. This can be demonstrated as shown in Figure 6.1 using an aquatic plant such as Elodea. The light intensity (I) is related to the distance (d) between the lamp and the plant (I = 1 / d2). As the lamp is moved closer, the light intensity increases. The rate of photosynthesis is directly proportional to the light intensity, as shown by the graph in Figure 6.1. However, the photosynthetic rate cannot be increased indefinitely: a point is reached where all the chloroplasts cannot trap any more light.

Question 8

glasshouse systems

Answer 8

Glasshouses are used in some countries to control conditions for plant growth, especially when growing conditions outside are not ideal. The glass helps trap heat inside and atmospheric conditions can be controlled. Carbon dioxide enrichment Atmospheric air contains only 0.04% carbon dioxide, so it can easily become a factor that limits the rate of photosynthesis. A glasshouse is a closed system, so the content of the air in it can be controlled. For example, the amount of carbon dioxide can be increased by burning fossil fuels in the greenhouse, or releasing pure carbon dioxide from a gas cylinder. Optimum light If light conditions in a glasshouse are not optimum (e.g. in winter), they can be improved by using artificial lights. Optimum temperature If the temperature is a limiting factor, e.g. in winter, it can be raised by using a heating system. If fossil fuels are burned, there is also a benefit from the carbon dioxide produced. Hydrogencarbonate indicator can be used to investigate the effect of gas exchange of an aquatic plant kept in the light and in the dark. Fresh indicator is a pink/ red colour. If there is a build-up of carbon dioxide (no photosynthesis, but respiration is producing carbon dioxide), the decrease in pH turns the indicator yellow. If the carbon dioxide level drops (e.g. during photosynthesis), the indicator turns purple.

Question 9

parts of leaf

Answer 9

Question 10

Adaptations of a leaf for photosynthesis

Answer 10

• Their broad, flat shape offers a large surface area for absorption of sunlight and carbon dioxide.
• Most leaves are thin and the carbon dioxide has to diffuse across only short distances to reach the inner cells.
• The large spaces between cells inside the leaf provide an easy passage through which carbon dioxide can diffuse.
• There are many stomata (pores) in the lower surface of the leaf. These allow the exchange of carbon dioxide and oxygen with the air outside.
• There are more chloroplasts in the upper (palisade) cells than in the lower (spongy mesophyll) cells. The palisade cells, being on the upper surface, will receive most sunlight and this will reach the chloroplasts without being absorbed by too many cell walls.
• The branching network of veins provides a good water supply to the photosynthesising cells.

Question 11

nitrate ions

Answer 11

Nitrate ions are needed for making amino acids. These are the building blocks of proteins. Remember that all proteins contain the element nitrogen (see Chapter 4). Each amino acid is formed by combining sugars, made during photosynthesis, with nitrate. The amino acids are made into long chains by bonding them together. The proteins are used to make cytoplasm and enzymes.

Question 12

magnesium ions

Answer 12

Magnesium ions are needed to make chlorophyll. Each chlorophyll molecule contains one magnesium atom. Plants need chlorophyll to trap light to provide energy during photosynthesis.

Question 13

Nitrate ion and magnesium ion deficiency

Answer 13

You already know the importance of nitrate ions for protein synthesis. If the plant has a nitrate ion deficiency, it will not be able to make proteins, so growth will slow down. The stem becomes weak and lower leaves become yellow and die, while upper leaves turn pale green. You already know the importance of magnesium ions for synthesis of chlorophyll. If the plant has a magnesium ion deficiency, it will not be able to make chlorophyll. Leaves turn yellow from the bottom of the stem upwards. Plant growth will suffer because it will have reduced photosynthesis. Yellowing of leaves due to lack of magnesium ions is called chlorosis

Question 14

Transpiration

Answer 14

The loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by the diffusion of water vapour through the stomata

Question 15

Translocation

Answer 15

Movement of sucrose and amino acids in the phloem from regions of production (source) to regions of storage or to regions where they are used for respiration or growth (sink)

Question 16

Transport in plants

Answer 16

Two types of tissues are present in plants to transport materials. The xylem carries water and salts, as well as providing support for the plant. The phloem carries food substances – sugars and amino acids. Xylem and phloem are found in vascular bundles in roots (Figure 8.1), stems (Figure 8.2) and leaves (Figure 6.8).

Question 17

water uptake

Answer 17

Root hair cells form on young roots to increase the surface area of the root for absorption of water and mineral ions, as well as providing anchorage for the plant. Figure 8.3 shows a root hair cell. The cell extension (the hair) increases the surface area of the cell to make it more efficient in absorbing materials.

Question 18

Passage of water through root, stem and leaf

Answer 18

Water passes through the root hair cells to the root cortex cells by osmosis, reaching the xylem vessels in the centre (see Figure 8.1). When water reaches the xylem, it travels up these vessels through the stem to the leaves. Mature xylem cells have no cell contents, so they act like open-ended tubes allowing free movement of water through them. In the leaves, water passes out of the xylem vessels into the surrounding cells (mesophyll cells). Mineral ions are also transported through the xylem. Root hair cells have a large surface area and they are very numerous on young roots. This increases the rate of the absorption of water by osmosis and ions by active transport.

Question 19

Transpiration

Answer 19

Transpiration is the loss of water vapour from a leaf. Water in the leaf mesophyll cells forms a thin layer on their surfaces. The water evaporates into the air spaces in the spongy mesophyll. This creates a high concentration of water molecules. They diffuse out of the leaf

into the surrounding air, through the stomata, by diffusion.

Question 20

Factors affecting transpiration rate

Answer 20

Question 21

The loss of

Answer 21

water vapour in the process of transpiration is related to the large surface area represented by the cell surfaces in the mesophyll of the leaf, the interconnecting air spaces in the spongy mesophyll and the stomata.

Question 22

Mechanism of water uptake

Answer 22

Water enters root hair cells by osmosis. This happens when the water potential in the soil surrounding the root is higher than in the cell. As the water enters the cell, its water potential becomes higher than in the cell next to it, e.g. in the cortex. Therefore, the water moves, by osmosis, into the next cell. The process is repeated until water reaches the xylem. Water also passes from cell to cell along the cell walls.

Question 23

Mechanism of water movement through a plant

Answer 23

Water vapour evaporating from a leaf creates a kind of suction (a transpiration pull) as water molecules are held together by cohesion. Therefore, the water forms a column and is drawn into the leaf from the xylem. This creates a transpiration stream, pulling water up from the root. Xylem vessels act like tiny tubes, drawing water up the stem by capillary action. Roots also produce a root pressure, forcing water up xylem vessels. Refer back to Chapter 2 to remind yourself of the structure of xylem tissue.

Question 24

WILTING

Answer 24

Wilting Young plant stems and leaves rely on their cells being turgid to keep them rigid. If the amount of water lost from the leaves of a plant is greater than the amount taken into the roots, the plant will have a water shortage. Cells become flaccid if they lack water, and they will no longer press against each other. Stems and leaves then lose their rigidity and wilt.

Question 25

Transpiration rate

Answer 25

An increase in temperature increases the kinetic (movement) energy of the water molecules, so they diffuse faster. Transpiration is likely to be faster on a hot day than on a cold day. An increase in humidity lowers the transpiration rate. This is because it increases the concentration of water molecules outside the leaf, reducing the concentration gradient for diffusion.

Question 26

Translocation

Answer 26

Translocation is the movement of sugars and amino acids through the phloem sieve tubes of the plant. The source is where the materials are produced (usually the leaves) and the sink is the region they are transported to. This may be for storage (e.g. in the roots or developing parts of the plants – new leaves, fruits, seeds, etc.), respiration or growth. During the life of a plant, a region that originally acted as a sink may become a source.

For example, sugars stored in the leaves of a bulb in the summer (acting as sink) may be translocated to a growing flower bud or stem the following spring. The bulb is now acting as a source.