Topic 6 - Plant structures and their functions

Question 1

Describe photosynthetic organisms in relation to their role in the lifecycle

Answer 1

The main producers of food, and therefore of biomass

Question 2

Describe 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

Explain the effect of temperature as a limiting factor on the rate of photosynthesis

Answer 3

With an increase in temperature, the rate of photosynthesis increases. As this reaction is controlled by enzymes, this trend continues up to a certain temperature until the enzymes begin to denature and the rate of reaction decreases

Question 3

Explain the effect of light intensity as a limiting factor on the rate of photosynthesis

Answer 3

The higher the light intensity, the faster the rate of reaction until another factor becomes limiting. At very high light intensities photosynthesis is slowed but these are not naturally occurring.

Question 4

What is the rate of photosynthesis directly proportional/ inversely proportional to?

Answer 4

The rate of photosynthesis is directly proportional to light intensity and inversely proportional to the distance from a light source

Question 5

Describe a limiting factor

Answer 5

An environmental condition which, when in shortage, slows down the rate of reaction as, despite increases in other factors the rate of photosynthesis will not increase any more. This can be soon on a graph as the curve levelling off.

Question 5

Explain the effect of carbon dioxide concentration as a limiting factor on the rate of photosynthesis

Answer 5

As the concentration of carbon dioxide increases, the rate of reaction increases until another factor becomes limiting

Question 6

Describe the Core Practical which investigates the effects of light intensity on the rate of photosynthesis

Answer 6

-Pondweed placed in test tube full of water.
-Lamp placed at measure distance
-The bubbles produced over one minute periods are recorded, the distance of the lamp from the tube changing for each period.
-The rate of photosynthesis is the number of bubbles produced per minute

Question 6

How could the photosynthesis core practical be improved?

Answer 6

-By using a gas syringe to more actually measure the volume of oxygen produced.

-An LED bulb used as it won’t change the water’s temperature

Question 7

What are the variables in the photosynthesis core practical?

Answer 7

Independent variable = Distance from the light source/light intensity
Dependent variable = The number of bubbles produced per minute
Control variables = Temperature, using the same piece of pondweed each time

Question 8

Explain how the structure of root hair cells is adapted to absorb water and mineral ions

Answer 8

-Have a large surface area, meaning more water can move in via osmosis
-The large vacuole affects the speed of movement of water from the soil to the cell to make it more efficient
-Mitochondria provide energy from respiration for the active transport of mineral ions to the root hair cell

The root hair cells takes water and mineral ions from the soil as they are found in the tips of roots

Question 8

What is the inverse square law?

Answer 8

Light intensity is directly proportional to 1/distance²

Question 9

Explain how the structure of the xylem is adapted to its function

Answer 9

-Upon formation, a chemical called lignin is deposited which causes the cells to die (become lignified). They become hollow and are joined end-to-end to form a continuous tube so water and mineral ions can move through.
-Lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water.

Xylem cells are specialised to transport water and mineral ions from the plant roots to the shoots

Question 10

Explain how the structure of the phloem is adapted to its function

Answer 10

-Cell walls of each cell form structures called sieve plates when they break down, allowing the movement of substances from cell to cell. Sieve plates are specialised for transport and have no nuclei but a cytoplasm that connects one cell to the next.
-Unlike xylem, these cells are alive
-Despite losing many sub-cellular structures, the energy these cells need to be alive is supplied by the mitochondria of the companion cells (which are attached to sieve plates)
-These cells use this energy to transport sucrose around the plant.

Phloem cells are specialised to carry the products of photosynthesis to all parts of the plant

Question 11

Explain the structure and function of the stomata

Answer 11

The stomata allows gas exchange and for water vapour to exit the cell. It has guard cells either side which are swollen with the stomata open (in the day) when they contain water and rigid and closed over the stomata when they contain less water (at night). The stomata is within the lower epidermis.

Question 12

Why are their large air spaces in leaves?

Answer 12

The spongy mesophyll layer contains large spaces which provide a large surface area for carbon dioxide to enter the cells and oxygen to leave by diffusion. Water vapour may also enter the cells here.

Question 13

Describe palisade cells

Answer 13

Palisade cells are packed with chloroplasts to absorb lots of sunlight for photosynthesis

Question 14

Describe the upper epidermis

Answer 14

Tightly packed cells which are transparent to let light through.

Question 15

Describe the leaf’s cuticle

Answer 15

It is a waxy coating which reduces water loss by transpiration.

Question 16

Describe three adaptations a leaf has for photosynthesis and gas exchange.

Answer 16

-Thin: So diffusion distances for the gases are short
-Shiny: So light is reflected off to prevent it from overheating and denaturing the enzymes
-Flat: Large surface area for light to fall on

Question 17

Define transpiration

Answer 17

The loss of water vapour from a plant through the stomata

Question 18

Define translocation

Answer 18

The transport of food substances made in the leaves up or down the phloem, for use immediately or storage.
Translocation is from the sources (where it is made) to the sinks (where it is used or stored).

Question 19

How does light intensity effect water uptake by a plant?

Answer 19

Increases in light intensity lead to an increased rate of photosynthesis, so more stomata open to allow gaseous exchange to occur. This means more water can evaporate, leading to an increased rate of transpiration.

Question 20

How does air movement effect water uptake by a plant?

Answer 20

Increased air movement increases water uptake as if more air is moving away from the leaf due to it being blown away, then the concentration of water vapour surrounding the leaf will be lower. This will mean there will be a steeper concentration gradient resulting in diffusion happening faster. This will increase the rate of transpiration.

Question 21

How does temperature effect water uptake by a plant?

Answer 21

Increases in temperature cause the molecules to move faster, resulting in evaporation happening at a faster rate and therefore the rate of transpiration increase.
The rate of photosynthesis increases, meaning more stomata are open for gaseous exchange, so more water evaporates and the rate of transpiration increases.

Question 22

How do you calculate rate calculations for transpiration?

Answer 22

-Fill a potometer with water.
-Then, cut the end of a shoot under water to ensure the xylem remains water-filled and prevents air locks.
-Insert the shoot into the rubber tubing at the end of the potometer and raise it so that a bubble of air is taken up.
-Lower the photometer into the water and record the distance travelled by the air bubble over time.

Question 23

Why do you measure uptake of water for rate calculations for transpiration?

Answer 23

Measuring the uptake of water by the plant gives an indication to the rate of transpiration because water is only taken up if water leaves the plant.

Question 24

Explain how plants are adapted to survive in extreme environments

Answer 24

-Many desert plants do not have leaves, or have small leaves, to reduce the amount of water lost via transpiration.
-Many leaves have a waxy cuticle to prevent evaporation of water in environments where water is scarce
-Stomata can be closed to prevent evaporation of water in extreme environments when water is scarce

Question 25

Explain phototropism

Answer 25

Plant shoots contain auxin which promotes the growth of shoots.
When the plant is exposed to light on one side, the sunlight breaks down the auxin on that side so that there is a higher cone of auxin on the side facing away from the sun.
The auxin stimulates cells to grow more, encouraging cell elongation so that the shoot bends towards the light.
The plant receives more light, so photosynthesis can occur at a faster rate.

Question 26

Explain negative gravitropism

Answer 26

In shoots, auxin gathers in the lower side. The cells of the shoot grow more on the side with the most auxin so cell growth is stimulated here. This makes the shoot bend and grow away from the ground. This is beneficial as light levels are likely to be higher further away from the ground.

Question 27

Explain positive gravitropism

Answer 27

In roots, auxin moves to the lower side. The cells of the root grow more on the side with less auxin, so it stimulates cells to grow and curve upwards. Auxin slows growth in the root and curves downwards. This is beneficial as there are likely to be increased levels of water and nutrients lower down.

Question 28

Describe the commercial uses of auxin

Answer 28

-In selective weed killers, auxin can use uncontrollable growth that kills plants with broad leaves (as many weeds have), such as dandelions and chickweeds.

-In rooting powders, auxin can promote the growth of the plant shoots of cut roots so that they develop quickly. This means identical plants can be quickly produced in large numbers from cuttings much faster than growing plants from seeds.

Question 29

Describe the commercial use of gibberellin

Answer 29

-When sprayed, it can override photoperiodism, allowing plants to flower outside of certain seasonal times.
-When sprayed it can cause flowers to develop seedless fruit without fertilisation
-When sprayed on carnation plants, they can increase the length of stems, number or size of flowers or cause flowering when the plant wouldn’t normally.

Question 30

Describe the commercial use of ethene/ethylene

Answer 30

It is used in bananas when they reach the country they will be sold in (e.g. UK) to treat them and make them ripe as they were shipped unripe.

In nature it is a gas given off by a plant which causes nearby fruit to ripen.