Learning Intentions To know what is meant by

Learning Intentions • To know what is meant by the concept of limiting factors • To know how photosynthesis can be measured • To know how temperature, carbon dioxide concentration and light intensity affect the rate of photosynthesis

The law of limiting factors • At any given moment , the rate of a physiological process is limited by the factor that is at its least favourable value • This factor is called a limiting factor • Changing the levels of the other factors will not alter the rate of the process

Three limiting factors for photosynthesis • Carbon dioxide • Light • Temperature

The graph shows the effect of different conditions on the rate of photosynthesis.

Name the factor which is limiting photosynthesis at A. Give a reason for your answer.

Carbon dioxide is limiting the rate of photosynthesis at B. How does the graph show this?

What would the grower need to consider before increasing the temperature during winter to 25 °C ?

Answers

Carbon Dioxide • In a greenhouse in full crop, the level of CO 2 inside on a sunny day will be well _______ that outside and so photosynthesis is _______. Growers can add extra CO 2 by one of two main methods: • Burning ______ and releasing the flue gases into the greenhouse. • Adding pure ______ from a tank of liquid outside.

Light • Artificial lighting can be used in winter to boost natural daylight levels – but it _____ a lot! Surprisingly, plants generally do not like ________ light. The length of day has a dramatic effect on the _________ of most plants.

Temperature • Photosynthesis is a series of chemical reactions and so is dependent on _________. As always, more kinetic energy means more __________ and so more _______________ are formed and the action goes faster – up to the point (35 – 40 o C) when the enzymes are __________. BUT that assumes that all other factors are optimal. In the real world, this is not the case – CO 2 is ___________ limiting in summer and _____________ and __________ are limiting in winter. In summer, the main problem is keeping the temperature down.

• This is where shading is supposed to help (less radiation = less heat absorbed); a better solution is more _________ (use a fan) and pouring water on the floor, when its evaporation will cool the atmosphere. • The light reactions of photosynthesis are _____ temperature dependent. • The dark reactions of photosynthesis are ___________ dependent enzymatic processes.

• These reactions do have an _________ temperature. Photosynthesis by most plants increases only up to about 25 o C. The rate levels out and then actually __________ as the temperature approaches or exceeds human body temperature. •

Compensation Point • Every plant has a light level where rate of photosynthesis = rate of respiration and no net gas exchange takes place. • This is known as the Compensation Point and is passed every day at dawn and dusk.

The rate of photosynthesis is usually measured by measuring either • The volume of oxygen released by a plant or • The volume of carbon dioxide taken up by a plant

Photosynthometer/Audus

• www. biology 4 all. com/resources_library/sour ce/61 c. swf

1 • Why does the apparatus • Because any air need to be airtight? escaping from or entering the apparatus will respectively decrease or increase the volume of gas measured, which will give an unreliable result.

2 • Suggest an advantage to • To ensure there is providing an additional sufficient CO 2 and this source of carbon does not limit the rate dioxide by adding of photosynthesis potassium hydrogen carbonate to the pond water?

3 • Suggest why the plant is • To prevent kept in the dark before photosynthesis and to the experiment begins. allow any oxygen produced before the experiment to disperse.

4 • Why is the lab darkened • The lab was darkened to except for the avoid extra light which experimental light could have stimulated source? extra photosynthesis. • Why is the temperature • Temperature was kept constant? constant because this also affects the rate of photosynthesis.

5 • What are the main sources of inaccuracy in this experiment? • Temperature may vary as the lamp heats the water. (This should be avoided by the water bath) • The carbon dioxide concentration of the water may vary during the experiment, especially if potassium hydrogencarbonate was added earlier. • Any stray light which is admitted to the laboratory will affect photosynthesis.

6 • If the gas is collected analysed it is found not to be pure oxygen. Can you account for this? • As the bubble of oxygen rises through the water some of the dissolved nitrogen will come out of solution and enter the bubble, and some of the oxygen will dissolve. This exchange is due to the different partial pressures (concentrations) of oxygen and nitrogen in the bubble and the water, there being a tendency for them to come to equilibrium with time. Traces of water vapour and carbon dioxide will also be present in the collected gas.

7 • Suggest reasons why • For the reasons measuring the volume discussed in answer to of gas produced by the previous question plant in this experiment and also because the may not be an accurate volume of oxygen measure of produced will be less photosynthesis. than that produced by photosynthesis as some of the oxygen will be used up in cellular respiration.

8 • Why is it advisable to • The amount of oxygen aerate the water before produced by beginning the photosynthesis must all experiment? be collected. If the water is not saturated with air, some of the oxygen released in photosynthesis will dissolve in the water and reduce the amount recorded.

Learning Intentions • To know what is meant by the concept of limiting factors • To know how photosynthesis can be measured • To know how temperature, carbon dioxide concentration and light intensity affect the rate of photosynthesis