1. Photosynthesis, the light reaction
Plant Physiology
Photosynthesis, the light reaction

2. Photosynthesis
Photosynthesis is the process that converts solar energy into chemical energy
Nourishes almost the entire living world
Plants, algae
Autotrophs sustain themselves without eating anything derived from other organisms
Producers of the biosphere, make organic molecules from CO2 and other inorganic molecules
Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2
Heterotrophs obtain their organic material from other organisms
Consumers of biosphere
Almost all Heterotrophs depend on photoautotrophs for food and O2

3. Photosynthesis converts light energy into chemical energy of food
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
The structural organization of these cells allows for the chemical reactions of photosynthesis

4. Chloroplasts: The Sites of Photosynthesis in Plants
Leaves are the major locations of Photosynthesis
Their green color is from Chlorophyll, the green pigment within chloroplasts
Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
CO2 enters and O2 exits the leaf through microscopic pores called Stomata

5. Chlorophylls are Leaf Pigments
Chlorophylls collect light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid
Chlorophyll a initiates the light-dependent reactions, primary pigment in plants and cyanobacteria
-absorbs violet-blue and red light
Chlorophyll b secondary pigment absorbing light wavelengths that chlorophyll a does not absorb
Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll

6. The Structure of Chlorophyll
The positive charges on the Mg2+ ion attract electrons
The electrons bounce around the porphyrin ring

7. Pigments (molecules that absorb visible light)
Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing.
photon: a particle of light
-acts as a discrete bundle of energy
-energy content of a photon is inversely proportional to the wavelength of the light
photoelectric effect: removal of an electron from a molecule by light
-occurs when photons transfer energy to electrons

8. Absorption of Light Energy
Light energy is absorbed by electrons
The energy causes electrons to jump shells; the more energy absorbed, the further away electrons move from the nucleus
The energy may be shed as fluorescence
Or transferred in the form of an electron to another molecule

9. How Electrons Capture Energy
Electrons can absorb radiant energy.
Radiant energy comes in parcels called photons
When electrons absorb energy, they hop to a higher shell.
When electrons release energy, drop back to the lower shell.
The energy released is a kind of light energy called fluorescence.

10. The Visible Spectrum of Light
Photosynthesis uses only small visible portion of the electromagnetic spectrum
Wavelengths of visible light most important for photosynthesis.
The symbol for wavelength is λ

12. Energy for all life on Earth ultimately comes from photosynthesis.
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Oxygenic photosynthesis is carried out by:
cyanobacteria, 7 groups of algae, all land plants
sunlight

13. Internal Structure of a Leaf
Main site of Photosynthesis

14. The Chloroplast The site of light harvesting or energy capture
The site of the start of carbohydrate synthesis
(Inside of thylakoid)
The Chloroplast

15. Photosynthesis Overview
Photosynthesis takes place in chloroplasts.
thylakoid membrane – internal membrane arranged in flattened sacs
contain chlorophyll and other pigments
Organized into photosystems
Capture light and transfer energy (to pigment molecules)
grana – stacks of thylakoid membranes
stroma – semiliquid substance surrounding thylakoid membranes (houses the enzymes to make organic molecules)

16. Photosynthesis in the Chloroplast
The light-dependent reactions (the harvesting of light) occur on thylakoid membranes
The carbon fixation reactions (formation of carbohydrate) occur in the stroma

17. Photosynthesis Overview
Photosynthesis takes place in 3 stages:
Capturing energy from sunlight
Using the energy to make ATP and reduce NADP+ to NADPH
(nicotinamide adenine dinucleotide phosphate)
Using the ATP and NADPH to synthesize organic molecules (glucose) from CO2

18. Photosynthesis Overview
Photosynthesis is divided into:
light-dependent reactions
-capture energy from sunlight
-make ATP and reduce NADP+ to NADPH
carbon fixation reactions (light-independent reactions)
-use ATP and NADPH to synthesize organic molecules from CO2

19. Photosynthesis Overview
Photosynthesis takes place in the green portions of plants
Leaf of flowering plant contains mesophyll tissue
Cells containing chloroplasts
Specialized to carry on photosynthesis
CO2 enters leaf through stomata
Diffuses into chloroplasts in mesophyll cells
In stroma, CO2 fixed to C6H12O6 (sugar)
Energy supplied by light

20. Photosystem Organization
A photosystem consists of
1. an antenna complex (light harvesting complex) of hundreds of accessory pigment molecules that gather photons and feeds energy to reaction center
2. a reaction center of one or more chlorophyll a molecules pass electrons out of photosystem (photochemical reactions)
In summary, energy of electrons is transferred through the antenna complex to the reaction center.

21. Photosystem Organization
At the reaction center (transmembrane protein complex), the energy from the antenna complex is transferred to chlorophyll a.
This energy causes an electron from chlorophyll to become excited.
The excited electron is transferred from chlorophyll a to an electron acceptor.
Water donates an electron to chlorophyll a to replace the excited electron.

22. pheophytin
Converting light to chemical energy

23. Photosynthesis Overview

24. Light Reactions
Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway.
Both pathways produce ATP; only the noncyclic pathway also produces NADPH.
ATP production during photosynthesis is called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.

25. Light Reactions: The Noncyclic Electron Pathway
Takes place in thylakoid membrane
Uses two photosystems, PS-I and PS-II (consists of pigment complexes)
PS II captures light energy
Causes an electron to be ejected from the reaction center (chlorophyll a)
Electron travels down electron transport chain to PS I
Replaced with an electron from water
causes H+ to concentrate in thylakoid chambers
causes ATP production
PS I captures light energy (electrons and H)
Transferred permanently to a molecule of NADP+
Causes NADPH production

26. Light Reactions: Noncyclic Electron Pathway

27. Light Reactions: The Cyclic Electron Pathway
Uses only photosystem I (PS-I)
Begins when PS I complex absorbs solar energy
Electron ejected from reaction center
Travels down electron transport chain
Causes H+ to concentrate in thylakoid chambers
Which causes ATP production
Electron returns to PS-I (cyclic)
Pathway only results in ATP production

28. Light Reactions: Cyclic Electron Pathway

29. The Organization of the Thylakoid Membrane
PS II consists of a pigment complex and electron-acceptor molecules; it oxidizes H2O and produces O2.
The electron transport system consists of cytochrome complexes and transports electrons and pumps H+ ions into the thylakoid space.
PS I has a pigment complex and electron-acceptor molecules; it is associated with an enzyme (oxidoreductases) that reduces NADP+ to NADPH.
ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.

30. ATP Production
Thylakoid space acts as a reservoir for hydrogen ions (H+)
Each time water is oxidized, two H+ remain in the thylakoid space
Electrons yield energy
Used to pump H+ across thylakoid membrane
Move H+ from stroma into the thylakoid space
Flow of H+ back across thylakoid membrane
Energizes ATP synthase
Enzymatically produces ATP from ADP + P
This method of producing ATP is called chemiosmosis