1. Topic 3.8 - Photosynthesis
Topic 3 – Chemistry of Life

2. What is photosynthesis?
At IGCSE you learnt this equation for photosynthesis: …
……..
Write it down yourself!! You should know this!!!

3. Photosynthesis equation
Carbon + water Glucose + Oxygen
Dioxide
What are the reactants in this reaction?
What are the products?
Why are light and chlorophyll written on the arrow?
Light
Chlorophyll

4. Light Energy  Chemical Energy
Energy changes
In photosynthesis there is an energy change:
Light Energy  Chemical Energy

5. More about light
Light makes up part of the electromagnetic radiation spectrum:

6. More about light Light itself is split up into different colours.
Which colour has the shortest wavelength?
Which colour has the biggest frequency?
Which colour has the most energy?
All answers are “Violet”.

7. More about light
We can see the different colours in light when they are refracted (bent) by different amounts e.g. in a prism or rainbow.

8. Pigments White substances reflect all the colours of light.
Other substances (called pigments) absorb one or more of the wavelengths of light.
The wavelengths of light that are not absorbed are reflected
This is the colour it appears to be to our eyes.

9. Which colours are absorbed and reflected?
Absorbed: Orange, Yellow, Green, Blue, Indigo, Violet
Reflected: Red
Absorbed: Red, Orange, Yellow, Blue, Indigo, Violet
Reflected: Green
Absorbed: Red, Orange, Yellow, Green, Blue, Indigo, Violet
Reflected: None
Absorbed: Red, Orange, Green, Blue, Indigo, Violet
Reflected: Yellow

10. Photosynthesis uses pigments
Most plants are green because of a photosynthetic pigment called chlorophyll.
Where is chlorophyll found in the leaf?
There are several types of chlorophyll:
Chlorophyll a (main photosynthetic pigment)
Chlorophyll b (accessory photosynthetic pigment)
Carotenoids (accessory photosynthetic pigment)

11. Absorption of light by Chlorophyll
If white light is shone on to chlorophyll, what colour is reflected?
So what does this tell us about the absorption of green light?
Green light is not absorbed by chlorophyll

12. Absorption of light by Chlorophyll
What about the other colours of the spectrum?
Turn to page 48 in your text book and read section about how to find out the absorption spectrum of chlorophyll. Make notes on this for your homework.

13. Absorption spectra of photosynthetic pigments
You do not need to know the absorption spectrum of each individual photosynthetic pigment – just the overall shape on the next slide.

14. Also on P 49 in your text book

15. Absorption spectra of photosynthetic pigments
Chlorophyll absorbs
blue light and red light
…but reflects green light

16. The two stages of photosynthesis
The light dependent stage
This takes place in the chloroplast on the thylakoid membranes

17. Thylakoid membranes

18. The two stages of photosynthesis
The light dependent stage
This takes place in the chloroplast on the thylakoid membranes
Light energy splits water molecules into Hydrogen ions (H+), electrons and oxygen atoms (O2)
H2O  H+ + e- + O2

19. 2H2O O e- O e- e- e- e- e- O2 H H+ H e- H H

20. The two stages of photosynthesis
The light dependent stage
This takes place in the chloroplast on the thylakoid membranes
Light energy splits water molecules into Hydrogen ions (H+), electrons and oxygen atoms (O2)
ATP is also made

21. ATP
ATP (Adenosine Tri Phosphate) is often referred to as the “energy currency” of organisms.
It consists of adenosine linked to three phosphate ions:
Adenosine P P P

22. ADP ATP can also exist as ADP (Adenosine Di Phosphate):
It takes a lot of energy to join the extra phosphate ion to ADP to make ATP.
Energy
Adenosine P P P

23. ATP When ATP loses one of these phosphate ions, energy is released
So if we make ATP from ADP and P, we have made a moveable source of energy to be used around the cell.
Energy
Adenosine P P P

24. The two stages of photosynthesis
Remember, the light dependent stage has given us Oxygen, Hydrogen ions, electrons and ATP
The light independent stage
The useful product of photosynthesis is glucose (C6H12O6).
The light dependent stage has given us the Hydrogen but where does the Carbon and Oxygen come from?

25. The light independent stage
Hang on! Didn’t we make Oxygen from splitting water??
YES! But that is NOT where the oxygen comes from!!
The Carbon and Oxygen both come from the other reactant in photosynthesis which is….?
Carbon Dioxide

26. The light independent stage
In the light independent stage, the Hydrogen and ATP from the light dependent stage react with Carbon and Oxygen in carbon dioxide to make glucose (C6H12O6): H H C C O O
(from the splitting of water) 2 2
(from the air) 6 12 6
Glucose

27. Tracking Atoms in Photosynthesis…
Reactants
6 CO2
12 H2O
Products
C6H12O6 6O2

28. Tracking Atoms in Photosynthesis…
It’s actually a little more complicated than this when you look at the Chemistry:
Reactants
6 CO2
12 H2O
Products
C6H12O6 6H2O 6O2

29. Tracking Atoms in Photosynthesis…
12 molecules of water are needed as reactants to make the glucose.
6 molecules of water are made as products…
This is usually written overall as photosynthesis needs 6 molecules of water as reactants.
However, you may see it written differently (and more accurately!) in text books!

30. Measuring the rate of photosynthesis
Photosynthesis can be measured by seeing how quickly the reactants are used or how quickly the products are made.
Biologists do this in one of three ways:

31. Measuring the rate of photosynthesis
Measure the rate of reaction directly by:
Measure the use of CO2.
Measure the production of O2.
Look in your text book for diagrams of these methods (Page 49 – 50).
Measure the rate indirectly by:
Measure the increase in Biomass . (Organic molecules will build up during plant growth - the mass of living material is called the biomass).

32. Experiment
Shine a light on to some pond weed (Elodea) and count the number of bubbles of oxygen produced during 1 minute.
How could you use this set up to investigate the effect of a factor on photosynthesis?

33. Factors that limit photosynthesis
Look at the experimental setup in the classroom.
Write a research question for this experiment.
Gas pressure sensors with aquatic plant producing oxygen at different temperatures.

34. Compare these two experiments for good practice

35. Factors that limit photosynthesis
Light
a) As the intensity of light increases, the rate of reaction of photosynthesis increases.

36. Factors that limit photosynthesis
Light
(b) Light intensity has saturated the plant. Even if there is a further increase in light, there is no increase in the rate of photosynthesis.

37. Factors that limit photosynthesis
Light
(b) Light intensity has saturated the plant. Even if there is a further increase in light, there is no increase in the rate of photosynthesis.

38. Note that the light intensity to achieve maximum rate of photosynthesis is less than the intensity of light in the summer. Light is rarely a limiting factor in these conditions.
However light can limit photosynthesis in winter when there is not enough light.

39. Factors that limit photosynthesis
Temperature
Photosynthesis is controlled by enzymes. Therefore, you should be able to explain the shape of this graph:

40. Factors that limit photosynthesis
Temperature
Bear in mind that this is an amalgamation of several enzymes with many different optimum temperatures. However, many enzymes have a similar optimum temperature.

41. Factors that limit photosynthesis
Carbon Dioxide
a) As the concentration of carbon dioxide is increased the rate of photosynthesis gets faster.

42. Factors that limit photosynthesis
Carbon Dioxide
b) The concentration of carbon dioxide has saturated the plant. The maximum rate of reaction has been achieved. Further increases in carbon dioxide do not make the rate go faster.

43. Factors that limit photosynthesis
Carbon Dioxide
c) Note this is the normal concentration of carbon dioxide in the atmosphere! Carbon Dioxide can be a limiting factor.

44. Testing for photosynthesis
To test to see if photosynthesis has taken place, biologists conduct the “Starch Test”.
Where has the starch in a plant come from?
A plant is “destarched” by putting it in a dark place for 2-3 days.
What happens to any starch that has been made?

45. Testing for photosynthesis
A leaf from the plant can be experimented on and then the presence of starch ascertained.
What solution tests for starch? What happens to it if starch is present? What happens if starch is not present?
We can then use the presence or absence of starch to tell us about the presence or absence of photosynthesis.

46. Testing a leaf for starch
Look at this animation to remind you how to test a leaf for starch then try the experiment yourself.