Photosynthesis

1
Photosynthesis

• Learning objective
• To be able to explain that
• light energy is absorbed by electrons in
  chlorophyll
• Some excited electrons gain enough energy to
  leave chlorophyll
• The electrons are replaced by the photolysis of
  water
• the energy from these excited electrons is used
  to generate ATP and reduced NADP

2
Adaptations in a leaf

• Write down the adaptations in a leaf and how they
  facilitate photosynthesis
• Thin gases can diffuse in and out quickly
• Large surface area maximise light absorption
• Thin layer of wax to reduce evaporation from
  the upper surface
• Rich network of vascular tissue xylem to
  deliver water and minerals, phloem to remove
  products of photosynthesis
• Cells in upper epidermis have no chloroplasts
  more light gets through to pallisade layer
• Tightly packed pallisade cells larger surface
  area for photosynthesis
• Chloroplasts move within cytoplasm have the
  chance of obtaining as much light as possible
• Stomata surrounded by guard cells open in day
  to allow CO2 in, close at night to reduce water
  loss
• Spongy mesophyll - creates air spaces making gas
  exchange more efficient between the atmosphere
  and pallisade mesophyll.

3
Autotrophic nutrition

• Autotrophic nutrition is making your own food
• Autotrophs are able to use a source of energy to
  make complex organis molecules from simple
  inorganic materials
• Chemoautotrophs harness energy released in
  exergonic chemical reactions to synthesis their
  own food (mainly bacteria)
• Photoautotrophs use light energy to synthesise
  their own organic materials (green plants and
  algae)

4
Structure of cholroplasts
5
Phases of Photosynthesis
Light dependant reactions
Oxygen
Water
electrons
energy
Light Independent Reactions
Carbon Dioxide
Carbohydrate
6
Light dependant reaction

• Takes place in the thylakoids
• Thylakoid membrane is a phospholipid bilayer with
  chlorophyll molecules embedded in it
• Chlorophyll molecules absorb light energy and
  then transfer it to other protein molecules
• Light dependant reaction generates ATP

7
Chlorophyll

• Chlorophyll has evolved to make use of as much
  of the visible spectrum as possible
• Chlorophyll is a mixture of compounds that fall
  into two basic types
• Chlorophyll
• Carotenoids
• The two main chlorophylls are a and b

8
Absorption and Action Spectrums

• The action spectrum shows the rate of
  photosynthesis at different wavelengths.
• The absorption spectrum shows how strongly the
  pigments absorb at different wavelengths.

9

• Read the application box on page 6 and answer
  questions 1 - 3

10
Photosynthesis

• Learning objective
• To be able to explain that
• In the light-dependent reaction light energy
  excites electrons in chlorophyll
• the energy from these excited electrons is used
  to generate ATP and reduced NADP

11
Thylakoid membrane

• The first reactions of photosynthesis require
  light energy, and are called light dependant
  reactions which take place in the thylakoid
  membrane.
• The thylakoid membranes have the pigments
  arranged in funnel shaped photosystems

12
Photosystems absorb some wavelengths of light
and reflect others There are two photosystems I
and II that are in close proximity to one another
in the thylakoid membrane
13
Photosystem I

• Each photosystem holds the light absorbing
  pigments in the best position to maximize the
  absorbance of photons of light.
• Photosystem I contains chlorophyll a and absorbs
  best at 700 nm (P700)
• Photosystem II contains chlorophyll b and absorbs
  best at 680 nm (P680)

Chl. b 650
Chl. a 670
About 300 light trapping chlorphyll molecules
Chl. a 680
Chl. a 690
Reaction centre I
P700
Light reaction 1
Photosystem II
Chl. b 650
Chl. a 670
About 300 light trapping chlorphyll molecules
Chl. a 680
Reaction centre II
P690
Light reaction 2
14
Energy captured in the Light Dependant reaction

• Used for 2 things
• Making ATP from ADP and Pi is called
  phosphorylation. In the light dependant reaction
  it is called PHOTOphosphorylation because it
  involves light
• For splitting water into H and OH ions this is
  called photolysis
• For combining electrons, photons and NADP to
  produce reduced NADP (NADPH) powerful reducing
  agent used in the light independent reactions.

15
Electrons
Protons
Oxygen released
16
Photophosphorylation

• Excited electrons passed to electron acceptor
• Electrons pass along chain of electron acceptors
  down energy gradient
• Electrons lose energy at each stage in the chain
• Energy is used to add phosphate molecule to ADP -
  photophosphorylation

17
Photophosphorylation

• Uses PSI and PSII

2e-
NADPH H
NADP 2H
e-
ADP P
LIGHT
ATP
PSI P700
LIGHT
PSII P680
H2O
½ O2 2H
18

• Photosystem One - PS I Its primary pigment is a
  molecule of chlorophyll a. What is its peak
  absorbance?
• Photosystem Two - PS II Its primary pigment is a
  molecule of chlorophyll b. What is its peak
  absorbance?
• Where precisely in the plant does the light
  dependant stage occur?
• Which two compounds produced in the light
  dependant stage are used in the light independent
  stage?
• Which of the light dependant reactions of
  photosynthesis are involved in producing these
  compounds?

19

• 1. Photosystem One - PS IIts primary pigment
  is a molecule of chlorophyll a. What is its peak
  absorbance?
• PSI absorption peak at 700nm. It is called P700
• 2. Photosystem Two - PS II Its primary pigment
  is a molecule of chlorophyll b. What is its peak
  absorbance?
• PSII absorption peak at 680nm. It is called P680

20

• 3. Where precisely in the plant does the light
  dependant stage occur?
• In the thylakoid membranes of the chloroplasts
• 4. Which two compounds produced in the light
  dependant stage are used in the light independent
  stage?
• ATP and NADPH H
• 5. Which of the light dependant reactions of
  photosynthesis are involved in producing these
  compounds?
• ATP is produced by both cyclic and non cyclic
  photophosphorylation NADPH is produced by non
  cyclic photophosphorylation