chapter 17 pt 4

Question 1

Limiting factors:

Answer 1

When one of the factors needed for a plant to photosynthesise is in short supply, it reduces the rate of photosynthesis, and is therefore a limiting factor.

Question 2

The factors that affect the rate of photosynthesis are:

Answer 2

Light intensity

Carbon dioxide concentration

Temperature

Question 3

Light intensity

Answer 3

light is needed as an energy source.
As light intensity increases, ATP and reduced NADP are produced at a higher rate.

Question 4

Carbon dioxide concentration

Answer 4

carbon dioxide is needed as a source of carbon, so if all other conditions are met, increasing the carbon dioxide concentration increases the rate of carbon fixation in the Calvin cycle and, therefore, the rate of TP production.

Question 5

Temperature

Answer 5

affects the rate of enzyme-controlled reactions.
As temperature increases, the rate of enzyme activity increases until the point at which the proteins denature.
An increase in temperature increases the rates of the enzyme-controlled reactions in photosynthesis, such as carbon fixation.
The rate of photorespiration, however, also increases above 25°C meaning higher photosynthetic rates may not be seen at higher temperatures even if enzymes are not actually denatured.

Question 6

Stomata on plant leaves and other surfaces will…

Answer 6

close to avoid water loss by transpiration during dry spells when plants undergo water stress.
The closure of stomata stops the diffusion of carbon dioxide into the plant, reducing the rate of the light-independent reaction, and eventually stopping photosynthesis.
Although water is required for photosynthesis it is never considered a limiting factor because for water potential to have become low enough to limit the rate of photosynthesis the plant will already have closed its stomata and ceased photosynthesis.
Plants, except those with adaptations to tolerate drought conditions, are unlikely to survive these conditions.

Question 7

The law of limiting factors states..

Answer 7

that the rate of a physiological process will be limited by the factor which is in shortest supply.

Question 8

Investigating the factors that affect the rate of photosynthesis: data loggers

Answer 8

Data loggers are electronic devices that record data over time using sensors.

Physical properties are recorded such as light intensity, temperature, pressure, pH (which can be used as a measure of carbon dioxide concentration), and humidity.

Readings can be displayed in graphical form or on a spreadsheet.

They are usually equipped with a microprocessor (which inputs digital data) and internal memory for data storage.

Data loggers can usually interface with a computer

Readings are taken with high degrees of accuracy and can be taken over long periods of time.

They can be set to take many readings in a short period of time or used when there is a risk involved, for example, extreme cold or heat.

Question 9

The factors affecting rate of photosynthesis can be investigated using a live pond weed, such as Elodea.

Answer 9

The rate of photosynthesis can be estimated by calculating the rate of oxygen produced, carbon dioxide used, or increase in dry mass of a plant.
Apparatus could be set up as shown in Figure 2.
Sodium hydrogen carbonate would be used to provide carbon dioxide.
The pond weed should be kept illuminated before use.
The apparatus should be left to equilibrate for 10 minutes or so before readings are taken.
The oxygen sensor may also need to be calibrated using the oxygen concentration of air (21%).
The software can be set to take readings at desired intervals for the required length of time.

Question 10

diagram of The factors affecting rate of photosynthesis can be investigated using a live pond weed, such as Elodea.

Answer 10

Question 11

graph of The factors affecting rate of photosynthesis can be investigated using a live pond weed, such as Elodea.

Answer 11

Question 12

The effect of reducing light intensity on the Calvin cycle:

Answer 12

Reducing light intensity will reduce the rate of the light-dependent stage of photosynthesis.
This will reduce the quantity of ATP and reduced NADP produced.
ATP and reduced NADP are needed to convert GP to TP.
The concentration of GP will therefore increase and the concentration of TP will decrease.
As there will be less TP to regenerate RuBP, the concentration of RuBP will also decrease.
The reverse will happen when the light intensity is increased

Question 13

The effect of reducing light intensity on the Calvin cycle diagram

Answer 13

Question 14

The effect of decreasing temperature on the Calvin cycle

Answer 14

All the reactions making up the Calvin cycle are catalysed by enzymes, for example, RuBisCO in carbon fixation.
At lower temperatures enzyme and substrate molecules have less kinetic energy resulting in fewer successful collisions and a reduced rate of reaction.
This means decreasing temperature results in lower concentrations of GP, TP, and RuBP.
The same effect will be seen at high temperatures as enzymes will be denatured - this is irreversible.

Question 15

The effect of reduced carbon dioxide concentration on the Calvin cycle

Answer 15

As carbon dioxide is an essential substrate of the Calvin cycle, low concentrations will lead to reduced concentrations of GP (as there is less carbon dioxide to be fixed) and TP.
The concentration of RuBP will increase as it is still being formed from TP but not being used to fix carbon dioxide.

Question 16

Artificial photosynthesis, a win-win solution:
part 1

Answer 16

The burning of fossil fuels, and respiration, is continually releasing huge quantities of carbon dioxide into the atmosphere.
The overall concentration of carbon dioxide, a greenhouse gas, is increasingly leading to more heat from the Sun being trapped in the atmosphere.
This is enhanced global warming and is causing the polar ice caps to melt, increasing sea levels, and changing the climate around the world.
Fossil fuels have a limited supply and will eventually run out, leading to fuel shortages, and many parts of the world already suffer from food shortages.
Therefore, we have a surplus of carbon dioxide, which needs removing, and a shortage of fuel and food, both of which are forms of biomass produced by plants using carbon dioxide.

Question 17

Artificial photosynthesis, a win-win solution:
part 2

Answer 17

Photosynthesis would appear to offer a solution.
It uses carbon dioxide and energy from the Sun to produce carbohydrates.
Carbohydrates can be used as both food and fuel.
It is said that ‘more energy hits Earth from the Sun in one hour than mankind uses in an entire year’, so there is no shortage of energy.
We already collect and use energy from the Sun in the form of solar power but the Sun doesn’t always shine and at the moment there are no practical ways to store energy for a ‘rainy day’.
However, the carbohydrate fuel produced by plants can be stored for long periods.
The problem is that the process of photosynthesis, which has taken millions of years to evolve, is still not particularly efficient, so relying on plants is not the answer.
Artificial photosynthesis is seen as a possible solution.
By improving on the natural process of photosynthesis carried out by plants, more carbon dioxide could be removed from the atmosphere and more carbohydrate products could be produced which could help with fuel and food shortages.

Question 18

Different types of photosynthesis:

Answer 18

Most plants use the form of photosynthesis that you have learnt about in this chapter.
This is referred to as C3 photosynthesis, and it is most efficient in cool, wet climates with average sunshine values.
Plants that live in hot, arid climates like the desert which are exposed to intense sunlight use different types of photosynthesis.
Plants which use C4 and crassulacean acid metabolism (CAM) types of photosynthesis use water more efficiently and can photosynthesise at faster rates at higher temperatures and light intensities.
Plants are adapted to their different environments to photosynthesise as efficiently as possible.

Question 19

C4 photosynthesis:

Answer 19

Plants adapted to high temperatures and limited water supply.

They are able to fix carbon dioxide more efficiently and so do not need to have their stomata open for as long as C3 plants meaning there is less water lost by transpiration.

PEP carboxylase present in mesophyll cells, which first fixes carbon dioxide, is not inhibited by oxygen (like RuBisCO), increasing the efficiency of fixation.

The four-carbon molecules produced are transported to bundle sheaths, formed from tightly packed cells, deeper inside the plant.

These molecules are then decarboxylated, and the carbon dioxide is then fixed by RuBisCO and enters the Calvin cycle.

As RuBisCO is shielded from atmospheric oxygen, the waste of resources by photorespiration is reduced.

Corn is an example of a C4 plant.

Question 20

CAM photosynthesis:

Answer 20

plants open their stomata at night, usually closing them during the day, again reducing water loss by transpiration.

Carbon dioxide is converted to an acid and stored during the night.

During the day the acid is broken back down releasing carbon dioxide to RuBisCO.

During very dry spells stomata can remain closed night and day.

The carbon dioxide released from respiration is used in photosynthesis and the oxygen released by photosynthesis is used for respiration.
Cacti are CAM plants.