SB6 - Plant Structures and their Functions

Question 1

What is photosynthesis?

Answer 1

Photosynthesis is the process by which plants create glucose and oxygen from carbon dioxide and water. It occurs in chloroplasts and is an endothermic reaction.
energy
by light
Carbon dioxide + water —> glucose + oxygen

Question 2

Describe the structure of a leaf

Answer 2

Leaves have a waxy cuticle on the outside followed by the upper epidermis. There is a layer of palisade mesophyll cells (which are packed with chloroplasts) then spongy mesophyll cells (which have air spaces to allow for gas exchange). Finally there is the lower epidermis with stomata.

Question 3

How are guard cells adapted?

Answer 3

Stomata are microscopic pores which allow carbon dioxide to diffuse into the leaf, but which also allow water out. The stomata are opened and closed by guard cells. When it is light, water flows into the guard cells, making them rigid, which opens the stoma. At night, water flows out of the guard cells, so they lose their rigidity and the stoma shuts.

Question 4

What is a limiting factor?

Answer 4

A limiting factor is a factor which prevents the rate of reaction increasing beyond a certain point.

Question 5

What are the limiting factors for photosynthesis?

Answer 5

• light intensity
• temperature
• carbon dioxide concentration

Question 6

How does light intensity affect the rate of photosynthesis?

Answer 6

The higher the light intensity, the faster the rate of photosynthesis until a certain point when the rate plateaus and won’t increase any more even if the light intensity is increased. Something else is now the limiting factor.

Question 7

How does carbon dioxide concentration affect the rate of photosynthesis?

Answer 7

The higher the carbon dioxide concentration the faster the rate of photosynthesis until a certain point when the rate plateaus. A different factor is now the limiting factor.

Question 8

How does temperature affect rate of photosynthesis?

Answer 8

The higher the temperature, the faster the rate of photosynthesis, until a certain point when the rate decreases due to the enzymes used in photosynthesis becoming denatured.

Question 9

What is the inverse square law?

Answer 9

To calculate the light intensity when the distance of a light source changes, we use:

                                original light intensity x original distance^2 New light intensity = ————————————————————
                                                     new distance^2

Question 10

Core practical - light intensity and photosynthesis

Answer 10

• decide on the different substances between the algae and the lamp you are going to use
• for each distance you will need one clear glass bottle plus one extra
• add 20 algal balls to each bottle
• add the same amount of indicator solution to each bottle and put on the bottle caps
• your teacher will have a range of bottles showing the colours of the indicator at different pHs. Compare the colour in your tubes with this pH range to work out the pH at the start.
• set up a tank of water between the lamp and the area you will place your tubes
• cover one bottle in kitchen foil so that it is in the dark
• measure the distances from the lamp and place a bottle at each one. Put the bottle in kitchen foil next to the closest one
• turn on the lamp and wait until you see obvious changes in the bottle colours
• compare the colours of all your bottles with the pH range bottles
• for each bottle, calculate the change in pH/hour
• plot a graph or chart of your results

Question 11

How are root hair cells adapted?

Answer 11

They have a large surface area, increasing the rate of water absorption. They also have thin cell walls, so the water is not slowed down.

Question 12

How can water get from the root hair cells to the xylem vessels?

Answer 12

Water can diffuse through the cell walls of the root cells, due to their open structure, towards the middle of the root (where the xylem vessel is). It can also be absorbed by the cytoplasm and move through root cells by osmosis.

Question 13

What is transpiration?

Answer 13

The flow of water and dissolved mineral ions into a root, up the stem and out of the leaves. A potometer can be used to measure the rate of transpiration.

Question 14

Describe the transpiration stream

Answer 14

Transpiration transports water and dissolved mineral ions from the roots to the leaves. Water moves from the soil to the root hair cells by osmosis. The water can flow through the cell walls by diffusion or the cytoplasm by osmosis until it reaches the xylem vessel. Water can move continuously up the xylem because each water molecule is bonded to the next by weak electrostatic forces. This means that as one molecule of water is pulled up the xylem, the next will follow. The water molecules are also bonded to the cell wall, meaning they can only move up the xylem not down. When the water reaches the leaf, it evaporates into the air spaces in the spongy mesophyll cells. The water concentration is higher in the air pocket than inside, so the water molecules diffuse out of the leaf through the stomata, causing the next water molecules to evaporate into the air space.

Question 15

What factors affect the rate of transpiration?

Answer 15

• wind (high wind =moves water molecules away from the stomata = maintains steep concentration gradient)
• humidity (high humidity = lots of water vapour in the air = shallow concentration gradient)
• temperature (higher temperature = particles move faster = faster diffusion)
• light intensity ( greater light intensity = wider stomata = faster transpiration)

Question 16

Describe how the xylem vessels are adapted

Answer 16

• xylem cells are dead so they form an empty tube for water to flow through
• the lack of cell walls between the cells means that water flow is not slowed down
• tiny pores allow water and mineral ions to enter and leave the xylem vessels
• thick side walls and rings of lignin form rigid tubes that will not burst due to the water pressure or collapse and provide support

Question 17

Describe how phloem is adapted

Answer 17

• holes in the end of the cell walls allow liquids to flow from one sieve cell to the next
• very small amount of cytoplasm and no nucleus so more room for sucrose solution
• pores so that sucrose solution can be pumped
• companion cells actively pump sucrose into or out of sieve cells, the increased pressure causes the sucrose solution to flow up to growing shoots or down to storage organs

Question 18

What is translocation?

Answer 18

The movement of dissolved sucrose from the leaves to other parts of the plant. Translocation occurs in the sieve cells of phloem tissue and requires active transport. As sucrose is pumped into the sieve cells the increased pressure causes the sucrose solution to flow.

Question 19

What are some adaptations of a leaf?

Answer 19

• large surface area to ensure enough light is collected
• chloroplasts can move towards light or away from it
• leaves are thin so carbon dioxide doesn’t have far to diffuse before reaching photosynthesising cells
• irregularly shaped spongy mesophyll cells don’t fit together well, creating air spaces, allowing gas exchange
• xylem vessels supply water for photosynthesis
• epidermis cells protect leaf and are transparent, allowing light through
• waterproof waxy cuticle, causing less water loss and helps stop microorganisms getting into the leaf

Question 20

How are some plants adapted for extreme conditions?

Answer 20

In winter, many deciduous plants lose their leaves, preventing water loss when the soil may be frozen. Reduce water loss by having stomata in pits or having tiny hairs to trap water vapour. Spines instead of leaves to minimise surface area and reduce water loss. Stomata only open at night. Stem stores water. Thick cuticle.

Question 21

What is a tropism?

Answer 21

Responding to a stimulus by growing towards or away from it.

Question 22

How does phototropism work?

Answer 22

Auxins are produced in the shoot, where they cause cell elongation. They are photophobic, so move to the shady side of a shoot when light hits it. This causes the cells on the shady side to elongate at a faster rate than those on the sunny side, causing the plant to bend towards the sun.

Question 23

How does gravitropism work?

Answer 23

Auxins are found in root tips, where they inhibit cell elongation. They naturally move under gravity to the lower side of the root, where the inhibit the elongation of the cells on the lower side. The cells on the upper side elongate at a faster rate, causing the root to bend downwards.

Question 24

What are some uses of auxins?

Answer 24

Artificial auxins can be used to make selective weed killers, which cause weeds to uncontrollably grow and then die. They are better than normal herbicides because they won’t kill the crop. Auxins are also used in hormone rooting powders, which can allow large numbers of identical plants to be grown, which is quicker than growing from seed and will ensure the genetic characteristics you want.

Question 25

What are some uses of gibberellins?

Answer 25

Gibberellins trigger germination in seeds, increasing yields. They can also be used to produce seedless fruits (e.g. grapes) and bigger fruits. They can also be used to trigger flowering.

Question 26

What is photoperiodism?

Answer 26

The response of organisms to the number of hours of daylight in 24 hours.

Question 27

What are some uses of ethene?

Answer 27

Ethene can be used to increase the rate of fruit ripening, so that it reaches supermarkets in the perfect condition.