Topic 6 - Plant Structures and their functions

Question 1

Describe photosynthetic organsisms

Answer 1

During photosynthesis, photosynthetic organisms, such as green plants and algae, use energy from the Sun to make glucose. Some of the glucose is used to make larger, complex molecules that the plants or algae need to grow. These make up the organism’s biomass- the mass of living material. The energy stored in the organisms’ biomass then works its way through the food chain as animals eat them and each other. So photosynthetic organisms are the main producers of food for nearly all life on Earth.

Question 2

What is type of reaction is photosynthesis in plants and algae?

Answer 2

An endothermic reaction that uses light energy to react carbon dioxide and water to produce glucose and oxygen

Question 3

What are the limiting factors of photosynthesis?

Answer 3

Light intensity, Concentration of CO2 and the temperature affects the rate of photosynthesis.

Question 4

How does not enough light slow down the rate of photosynthesis?

Answer 4

Light transfers the energy needed for photosynthesis. As the light level is raised, the rate of photosynthesis increases steadily(the rate is directly proportional to the light intensity). But this is only until a certain point where afterwards it doesn’t affect it and it will either be temperature or CO2 level that affects it. The distance from the lamp and light intensity are inversely proportional to each other. However, light intensity decreases in proportion to the square of the distance. This is called the inverse square law

Question 5

How does too little carbon dioxide slow down photosynthesis?

Answer 5

CO2 is one of the raw materials needed for photosynthesis. As with light intensity, increasing the CO, the increases of the rate of photosynthesis up to a point. After this the graph flies out, showing that CO2 is no longer the limiting factor. As long as light and CO, are in must supply temperature then the factor limiting photosynthesis must be temperature.

Question 6

How does temperature affect the rate of photosynthesis?

Answer 6

Usually, if the temperature is too low it’s because it’s too low-the enzymes needed for photosynthesis. This happens at about 45 ° C (pretty hot for outdoors, but greenhouses can not get that hot if you’re not careful).

Question 7

What can be use to investigate the effect of light intensity on the rate of photosynthesis?

Answer 7

Canadian pondweed (an aquatic plant) can be used to measure the effeot of light on the rate of photosunthesis. The rate at which the pondweed produces oxygen at the rate at which it’s photosynthesising - the faster the rate of oxygen production, the faster the rate of photosynthesis.

Question 8

Describe the method of the investigation of the effect of light intensity on the rate of photosynthesis. (6)

Answer 8

1) The gas syringe should be empty to start with. Sodium hydrogencarbonate may be added to the water to make sure it has sufficient carbon dioxide (sodium hydrogen carbonate releases CO2 in solution).
2) A source of white light is placed at a specific distance from the pondweed.
3) The pond is left to photosynthesise for a set amount of time.
4) As it photosynthesises, the oxygen released wants to collect in the gas syringe. This allows you to accurately measure the volume of oxygen produced.
5) The whole experiment is repeated with the light source at different distances from the pondweed. The rate of oxygen produced at each distance can be calculated .
6) For this experiment, any variable that could affect the results should be controlled, e.g. the temperature (which can be controlled by putting the conical flask in a water bath) and the carbon dioxide concentration (which can be controlled by adding a set amount of sodium bicarbonate to a set volume of water).

Question 9

Explain how the structure of the root hair cells is adapted

Answer 9

The cells on the surface grow in hairs which cover each branch of a root. This gives a large surface area for absorbing water and mineral ions from the soil. The concentration of mineral ions is usually higher in the root hair cells than in the soil around them, so mineral ions are absorbed by active transport. Water is absorbed by osmosis

Question 10

Describe what Phloem tubes do and how they are adapted to their function.

Answer 10

Phloem tubes are made of columns of living cells. They transport food substances (mainly sucrose) made in the leaves to the rest of the plant for immediate use or storage. This process is called translocation and requires energy from respiration. The transport goes in both directions.

Question 11

Describe what Xylem tubes do and how they are adapted.

Answer 11

Xylem tubes are made of dead cells joint end to end with no end walls between them and a hole down the middle. They are strengthened with a material called lignin. They carry water and mineral ions from the roots to the stem and leaves. The movement of water from the roots through the xylem and out of the leaves is called the transpiration stream

Question 12

Explain how water and mineral ions are transported through the plant by transpiration

Answer 12

Transpiration is caused by the evaporation and diffusion of water from a plant’s surface. Most transpiration happens at the leaves. The loss of water oreates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it. This in turn means more water is drawn up from the roots, and so there’s a constant transpiration stream of water through the plant. The transpiration stream carries mineral ions that are dissolved in the water along with it.

Question 13

How is sucrose transported around the body?

Answer 13

Phloem transports sucrose and amino acids up and down the plant. This is called translocation. In general, this happens between where these substances are made (the sources) and where they are used or stored (the sinks).

This means, for example, that sucrose is transported:

• from sources in the root to sinks in the leaves in spring time
• from sources in the leaves to sinks in the root in the summer

Question 14

How is the structure of a leaf adapted to photosynthesis and gas exchange?

Answer 14

• Broad so there is a large surface area exposed to light which is needed for photosynthesis
• The Palisade layer has lots of chloroplast, near the top of the leaf so they can get the most light.
• The upper epidermis is transparent so light can get to the palisade layer
• The xylem and phloem provide provide the leaf with water and take away the glucose produced; and they support the structure.
• The epidermal tissues are covered with a waxy cuticle to reduce water loss
• The lower epidermis as lots of stomata which lets Carbon Dioxide diffuse directly into the leaf. The spongy mesophyll has air spaces which increase the rate of diffusion of gases in and out of leaf cells.

Question 15

How does light intensity affect the rate of transpiration?

Answer 15

The brighter the light, the greater the transpiration rate. Stomata closes when it gets dark as photosynthesis can’t happen in the dark so they don’t need to let CO2 in. When the stomata is closed, very little water can escape so there is no need for transpiration.

Question 16

How does temperature affect the rate of transpiration?

Answer 16

The warmer it is, the greater the rate of transpiration. When it’s warm, the water particles have more energy to evaporate and diffuse out of the stomata.

Question 17

How does air flow affect the rate of transpiration?

Answer 17

The better the air flow around the leaf, the greater the rate of transpiration. If air flow is poor, water vapour just surrounds the leaf and doesn’t move away. This means that there is high concentration outside the leaf as well as in so diffusion doesn’t happen as quickly. If there is good air flow, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly.

Question 18

How can you calculate the transpiration rate?

Answer 18

Distance moved ÷ time taken

Question 19

How do cacti have adaptations to help them conserve water? (6)

Answer 19

• Small leaves or spines instead of leaves to reduce the surface area for water loss and to stop animals eating them.
• Curled leaves or hairs reduce the air flow close to the leaf by trapping water vapour near the surface and reducing diffusion of water.
• Thick waxy cuticle to reduce evaporation
• Thick, fleshy stem which stores water
• Fewer stomata or stomata that only open at night to reduce water loss by evaporation
• Stomata sunken in pits so it is lower than the surface which reduces the air flow close to the stomata.

Question 20

How do plants regulate its growth and development?

Answer 20

They produce chemicals called hormones.

Question 21

What are auxins?

Answer 21

Plant hormones which control growth at the tips of shoots and roots. Auxins are produced in the tips and they diffuse backwards to stimulate the cell elongation process which occurs in cells just behind the tips. Auxins promote growth in shoots but inhibits growth in the root.

Question 22

How do shoots grow in response to light and gravity?

Answer 22

SHOOTS ARE POSITIVELY PHOTOTROPIC (grow towards light)
1) When a shoot tip is exposed to light, it accumulates more auxin on the side that’s in the shade than the side that’s in the light.
2) This makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light.
By bending towards the light, the shoot will be able to absorb more light for photosynthesis, which enables the plant to grow. Shoots growing completely in the dark will be tall and spindly the auxin in the tips makes them elongate quickly on all sides. A taller shoot has a better chance of finding light.
SHOOTS ARE NEGATIVELY GRAVITROPIC (grow away from gravity)
1) When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
2) This causes the lower side to faster, bending the shoot upwards.

Question 23

How do roots grow in response to light and gravity?

Answer 23

ROOTS ARE POSITIVELY GRAVITROPIC (grow towards gravity)
1) A root growing sideways will also have more auxin on its lower side.
2) But in a root the extra auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards.
ROOTS ARE NEGATIVELY PHOTOTROPIC (grow away from light)
1) If a root starts being exposed to some light, more auxin accumulates on the more shaded side.
2) The auxin inhibits cell elongation on the shaded side, so the root bends downwards, back into the ground. Roots that are underground aren’t exposed to light. They grow downwards due to positive gravitropism.

Question 24

How are auxins used commercially?

Answer 24

Selective weed killers kill some plants, but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass. The selective weed killer contains auxins which causes the weeds to grow too quickly and die. Because the weeds have broader leaves, the weed killer is absorbed in larger quantities by the weeds than it is by the grass. Selective weed killers can reduce biodiversity within treated areas due to specific plants being killed.

Rooting powder also contains auxins to promote growth. A small potted plant bring dipped in rooting powder Plant cuttings can be dipped Selective weed killers kill some plants, but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass. The selective weed killer contains a growth hormone that causes the weeds to grow too quickly and die. Because the weeds have broader leaves, the weed killer is absorbed in larger quantities by the weeds than it is by the grass. Selective weed killers can reduce biodiversity within treated areas due to specific plants being killed.

Rooting powder contains plant hormones to promote growth.

A small potted plant bring dipped in rooting powder
Plant cuttings can be dipped Selective weed killers kill some plants, but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass. The selective weed killer contains a growth hormone that causes the weeds to grow too quickly and die. Because the weeds have broader leaves, the weed killer is absorbed in larger quantities by the weeds than it is by the grass. Selective weed killers can reduce biodiversity within treated areas due to specific plants being killed.

Rooting powder contains plant hormones to promote growth. A small potted plant bring dipped in rooting powder. Plant cuttings can be dipped in hormone rooting powder before planting.

Question 25

How are gibberellins used commercially?

Answer 25

Gibberellins can be used to:

• end seed dormancy
• promote flowering
• increase fruit size

Seed dormancy must be broken for seeds to germinate, and this can be done by using gibberellins.

Gibberellins can promote flowering, which can result in more financially profitable flowers to sell due to the increased speed of flower growth. More attractive flowers and larger specimens are also produced.

Flowering also has an impact on the rate of fruit growth. Gibberellins are used in the production of seedless fruit such as seedless grapes, which are normally sprayed with gibberellin to increase the size of each grape.

Question 26

How is ethene used commercially?

Answer 26

This ripening hormone is added to the fruit and it will ripen on the way to the supermarket and it will be perfect just as it reaches the shelves.