1. Unit 1 Communication, Homeostasis and Energy
Photosynthesis
Unit 1
Communication, Homeostasis and Energy

2. Which process evolved first on Earth – aerobic respiration or photosynthesis?
Give reasons for your answer
You are going to present your answer with your reasons to the rest of the class!!
Think about it!!! 10 minutes

3. Answer Plants and animals rely on aerobic respiration
Requires oxygen
Oxygen is a by-product of photosynthesis
Until photosynthesis evolved there was no free oxygen in the atmosphere
Photosynthesis evolved first!!
Answer

4. Test yourself Where in plants does photosynthesis take place?
What are the raw materials needed for photosynthesis?
What is the energy source for photosynthesis?
Draw a flow diagram showing how energy from sunlight is used to produce muscle contractions in your arm.

5. Learning outcomes Define the terms autotroph and heterotroph.
State that light energy is used during photosynthesis to produce complex organic molecules.
Explain how respiration in plants and animals depends upon the products of photosynthesis.
State that, in plants, photosynthesis is a two-stage process taking place in chloroplasts.

6. The importance of photosynthesis
Photosynthesis transfers light energy into the chemical potential energy of organic molecules.
Photosynthesis releases oxygen from water, so all aerobes depend on photosynthesis for their respiration.

7. Heterotroph and Autotroph
an organism that uses an external energy source and inorganic molecules to make complex organic molecules.
Chemoautotroph
Photoautotroph
Heterotroph
Organism that needs to take in complex organic molecules which act as a source of energy and as usable carbon compounds.

8. Energy in Living Organisms
In order to maintain life, organisms need a source of energy.
In most organisms this is provided by the oxidation of organic molecules.
Autotrophic nutrition
Synthesise organic materials from inorganic sources e.g. photosynthesis
Heterotrophic nutrition
Obtained in organic form 8

9. Photosynthesis: an outline
Photosynthesis (p/s) is the fixation of carbon dioxide and its reduction to carbohydrate, using hydrogen from water

10. Photosynthesis Equations
Word equation for photosynthesis
Light energy
Carbon dioxide + watercarbohydrate + oxygen
chlorophyll

11. Photosynthesis Equations
Overall Chemical Equation
Light energy
nCO nH20  (CH2O)n + nO2
chlorophyll
Balanced Equation for hexose sugars
Light energy
6CO H2O  C6H12O O2
chlorophyll

12. Photosynthesis experiments
Testing a leaf for starch
What are the requirements for photosynthesis
Light
Chlorophyll
carbon dioxide

13. Factors affecting the rate of photosynthesis
Factors limiting photosynthesis
chlorophyll (enzymes)
carbon dioxide
Light
Water
Temperature

14. Photosynthesis Photosynthesis is a 2 stage process
Light dependent reactions
thylakoid membranes
Light independent reactions
Stroma

15. Learning outcome
Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their functions.

16. Chloroplast

17. Chloroplast – electro micrographx y z

18. Chloroplast Structure
3 – 10μm diameter
Envelope of 2 phospholipid membranes
Stroma = fluid interior
Thylakoids are series of flattened sacs, which form stacks (grana) in places

19. Chloroplast function Grana
Provides a LSA to hold pigments, electron carriers, and enzymes for light dependent reactions.
Photosystems arranged in funnel like structure in thylakoid
Membrane of grana holds ATPsynthase (chemiosmosis)

20. Chloroplast Function Stroma
Site of light independent reactions (carbon fixation)
Contains sugars, organic acids and enzymes for Calvin cycle
Store starch grains
Loop DNA – codes for chloroplast proteins

21. Chloroplast Function Lamellae Do not contain chlorophyll
Form a network between the grana

22. Learning Outcomes Define the term photosynthetic pigment.
Explain the importance of photosynthetic pigments in photosynthesis.
State that the light-dependent stage takes place in thylakoid membranes and that the light-independent stage takes place in the stroma.

23. Trapping Light energy
The fate of light which strikes the leaf

24. Trapping the Light Energy
The fate of light which strikes the leaf 
Light shining on leaf (100%)
12% light reflected
83% light absorbed, but only 4% of this is used in photosynthesis
5% of light transmitted

25. These values will be affected by
the amount of chloroplasts in the leaf
how shiny the leaf is
how thick the leaf is
Features of light that make it important
spectral quality (colour)
intensity (brightness)
duration (time)
Visible light has a wavelength between 400nm and 700nm

26. Absorption of Light
Leaves contain a variety of photosynthetic pigments, of which chlorophyll is the most obvious.
It is these pigments which absorb light energy.
There are two different groups of pigments
chlorophylls – chlorophyll a, chlorophyll b
Carotenoids – xanthophyll, carotene
Different photosynthetic pigments absorb different wavelengths.

27. Absorption and Action Spectra
Absorption Spectrum
A graph of absorbance of different wavelengths of light by a pigment
Action Spectrum
A graph of the rate of photosynthesis at different wavelengths of light.
Chlorophylls absorb red and blue violet regions of light, and reflect green
Carotenoids absorb the blue-violet region of the spectrum.

28. Absorption Spectrum

29. Action Spectrum

30. Absorption and Action Spectra

31. The Chemistry of photosynthesis

32. Learning Outcomes
Outline how light energy is converted to chemical energy (ATP and reduced NADP) in the light-dependent stage.
Explain the role of water in the light-dependent stage.

33. Photosynthesis is a two-stage process
Evidence for this comes from experiments with isotopes of oxygen.
Plants provided with C18O2 combine the atoms into carbohydrates
Plants provided with H218O release the 18O atoms as oxygen gas
All the oxygen released by photosynthesis comes from water.

34. Harvesting Light
In p/s the light energy absorbed by the p/s pigments is converted to chemical energy.
The absorbed light energy excites electrons in pigment molecules.
In functioning photosystems this is the energy which drives the process of photosynthesis.

35. There are two categories of p/s pigment
Primary pigments
chlorophyll a
Accessory pigments
chlorophyll a, chlorophyll b and carotenoids

36. Light-dependent reactions
Water is split in a reaction called photolysis,
These reactions provide the energy to:
Synthesis ATP from ADP and Pi (photophosphorylation)
Transfer H+ and e- to NADP to form reduced NADP

37. Photophosphorylation
Photophosphorylation can be cyclic or non cyclic depending on the pattern of electron flow in one or both photo systems
Cyclic photophosphorylation
PSI
Non cyclic photophosphorylation
PSII & PSI

38. Harvesting Light Photosystems
Pigments are arranged into light harvesting clusters called photosystems
light energy absorbed by pigments is passed to the primary pigment, which acts as a reaction centre.

39. Photosystems Photosystem I Photosystem II
Arranged around chlorophyll a molecule with an absorption peak at 700 nm.
Reaction centre P700
Photosystem II
Chlorophyll a molecule with absorption peak at 680nm
Reaction centre P680

40. Light harvesting system

41. Cyclic Photophosphorylation
Electron
acceptor
Electron carriers
ADP + Pi
ATP
2e-
Light energy absorbed by Chlorophyll a
PSI

42. Cyclic Photophosphorylation
involves only photosystem I, which has a chlorophyll a with a reaction centre P 700.
An electron from the molecule is excited to a higher energy level.
It is captured by an electron acceptor, and then is passed back to one of the chlorophyll a P700 molecules.
This happens due to a chain of electron carriers .

43. Cyclic photophosphorylation
The whole process releases energy to make ATP from ADP and inorganic phosphate.
This ATP will then be used in the light – independent reaction.

44. Non cyclic photophosphorylation
Electron
Acceptor B
Electron
Acceptor A
ADP + Pi
ATP
Electron carrier
NADP
2e-
Electron carrier
2e-
NADPH+ H+
PSI
Light energy
Light energy
PSII
H2O ½O2 + 2e- + 2H+

45. Non-cyclic photophosphorylation
involves both photosystems.
Both absorb light and the electrons which are excited leave the reaction centres of P680 and P700 of the chlorophyll a molecules.
Electron acceptors pass the electrons along chains of electron carriers.
The P700 of the photosystem I absorbs electrons from photosystem II.
Replacement electrons from the photolysis of water go to photosystem II.

46. Non-cyclic photophosphorylation
The electrons lose energy passing along the electron chain and this goes towards synthesising ATP.
The photolysis of water releases two protons/H+s
H+ combine with electrons from photosystem I and NADP to give reduced NADP (NADPH + H+)

47. The Photolysis of water
H2O  2H e- + ½O2
Oxygen is released as a waste product
The H+ and e- are transferred to NADP to give reduced NADP
2H+ + 2e- + NADP  reduced NADP
The reduced NADP then passes onto the light independent reactions

48. Pupil Activity
Complete the diagram of Photophosphorylation

49. Prep Question – 10 marks
Describe the structure of a chloroplast and then give an account of the role played by chlorophyll in photosynthesis. Refer to action and absorption spectra in your answer.
Write in bullet points and include a diagram.

50. Jan 03 Question 1 (a) blue and red light used in photosynthesis;
(light of) wavelength 420 – 450 nm, gives high rate / AW;
(light of) wavelength 650 – 690 nm, gives high rate / AW;
(light of) wavelength of 500 – 650 nm / green light, less effective / reflected;
sharp / AW, drop after 680 – 690 nm;

51. (b) (i) (ii) (iii) chlorophyll a; chlorophyll b;
carotenoids / carotene;
xanthophylls;
phaeophytin;
(ii)
absorb/ trap/ capture / harvest, light / transfer energy / transfer electrons;
(iii)
granum/ thylakoid (membrane) / lamella / quantasome;

52. Light independent reactions
The light independent reaction involves the fixation of carbon dioxide, and it takes place in the stroma of the chloroplast.

53. Learning Outcomes
Outline how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP).
Explain the role of carbon dioxide in the light-independent stage (Calvin cycle).
State that TP can be used to make carbohydrates, lipids and amino acids.
State that most TP is recycled to RuBP.

54. a.k.a. Calvin cycle

55. Calvin Cycle Light independent reactions
The stages are:
Carbon dioxide is linked with a molecule of ribulose bisphosphate (RuBP), which is a 5 carbon sugar, using the enzyme ribulose bisphosphate carboxylase.
A highly unstable 6C structure is formed which immediately splits into 2 molecules of the 3 carbon compound glycerate-3-phosphate (GP).
GP is converted into triose phosphate (3C) with the addition of hydrogen from reduced NADP and energy from ATP

56. The fate of triose phosphate
Triose phosphate has two purposes within the cell
Synthesis of molecules
Synthesis of hexose sugars, starch and cellulose
Synthesis of amino acids
5/6 are used in the conversion to RuBP so that more CO2 can be taken up

57. The fate of the products of photosynthesis
Specialist carbohydrates
glucose
fructose
Sucrose
Cellulose
Lipids
Amino acids and proteins
Nucleic acids
Growth factors, vitamins, hormones, pigments

58. Factors limiting photosynthesis

59. Factors Limiting Photosynthesis
How can the rate of photosynthesis be measured?
Which Environmental factors could limit the rate of photosynthesis?

60. Learning outcomes
Discuss limiting factors in photosynthesis, with reference to carbon dioxide concentration, light intensity and temperature.
Describe how to investigate experimentally the factors that affect the rate of photosynthesis.

61. Factors affecting photosynthesis
Raw materials
Water
Carbon dioxide
Energy in the form of sunlight
Light independent stage requires a relatively high temperature
The light-dependent reactions are not directly affected by temperature, why is this?

62. Quick revision In the light-dependent stage what is water a source of?
Hydrogen ions used in chemiosmosis
Hydrogen ions accepted by NADP
Electrons to replace lost by oxidised chlorophyll

63. Limiting factors
If any of these factors are in short supply, it can limit the rate at which photosynthesis takes place
The factor in the shortest supply is known as the limiting factor.

64. Light Intensity

65. Light Intensity Light drives the light-dependent reactions
More light, more photosynthesis
At a point where increasing light intensity has no effect on the rate of photosynthesis, light is no longer the limiting factor

66. Carbon Dioxide Concentration
Carbon dioxide in air is about 0.04%
Carbon dioxide is needed for the Calvin cycle
If a plant is given extra CO2 they will photosynthesis faster

67. Carbon Dioxide Concentration
Over which part of this curve is carbon dioxide the limiting factor for photosynthesis?
Suggest why the curve flattens out at high levels of CO2.

68. Carbon dioxide concentration

69. temperature Temperature affects the kinetic energy of molecules
Higher the temperature, the faster the molecules move
More collisions
Rate of reaction increases
At temperatures that are too high, enzyme molecules denature and the rate of reaction slows down.

70. Learning outcomes
Describe the effect on the rate of photosynthesis, and on levels of GP, RuBP and TP, of changing carbon dioxide concentration, light intensity and temperature.

71. Effect of light on the Calvin Cycle
The Calvin cycle depends on the products from the light-dependent reactions.

72. Effect of light on the Calvin Cycle
Explain why the Calvin cycle stops running when there is no light and the TP is used up.

73. Effect of light on the Calvin Cycle
Make a copy of this diagram and add another line to show what you would expect to happen to the levels of RuBP during this 8 minute period.

74. Effect of temperature on the Calvin cycle
What effect would you expect a rise or a fall in temperature to have on the relative levels of GP, TP and RuBP?
When answering this assume that the temperature does not go high enough to denature the enzymes.
Explain your reasoning.

75. Effect of carbon dioxide concentration on the Calvin cycle
If CO2 is in short supply
Less for RuBP to react with
Less GP
Less TP
Initial accumulation of RuBP