1. Photosynthesis
Introduction

2. What is photosynthesis?
Photosynthesis: to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules
Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes
Nourishes almost all of the living world directly or indirectly!

3. Autotrophs
Autotroph: an organism that obtains organic food molecules without eating other organisms or substances derived from other organisms
They make their own food!
Autotrophs can be separated by the source of energy that drives their metabolism.
Photoautotrophs use light as the energy source.
Chemoautotrophs harvest energy from oxidizing inorganic substances, including sulfur and ammonia.
Chemoautotrophy is unique to bacteria.

4. Heterotrophs
Heterotroph: an organism that obtains organic food molecules by eating other organisms or their byproducts
Consumers of the biosphere
Some feed on plants and other
animals.
Others decompose and feed on
dead organisms and on organic
litter, like feces and fallen leaves.
Almost all depend on photoautotrophs
for food and for oxygen, a byproduct
of photosynthesis.

5. Where does photosynthesis take place?
Chloroplasts are the site of photosynthesis in plants
half a million chloroplasts per square millimeter of leaf surface!
Chlorophyll: the green pigment in the chloroplasts.
important for the absorption
of light energy during
photosynthesis

6. Leaf Anatomy Mesophyll: the tissue in the interior of the leaf
Where chloroplasts are found
Stomata: microscopic pores that allow CO2 to enter and O2 to exit the leaf
Veins: deliver water from the roots and carry off sugar from mesophyll cells to other plant areas.

7. Chloroplast Parts
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
Inside area is called
the thylakoid space
Stacked into grana

8. The Equation
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
hydrogen extracted from water is incorporated into sugar along with the carbon dioxide
the oxygen is released to the atmosphere
Photosynthesis is a redox reaction.
reverses the direction of electron
flow in respiration
Water is split and electrons
transferred with H+ from water
to CO2, reducing it to sugar.

9. 1. Light Reactions: Overview
light reactions: convert solar energy to chemical energy
light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP+,forming NADPH.
NADPH, an electron acceptor, provides energized electrons, reducing power, to the Calvin cycle.
also generates ATP by photophosphorylation for the Calvin cycle
occurs in the thylakoids

10. 2. Calvin Cycle: Overview
Calvin cycle: incorporates CO2 from the atmosphere into an organic molecule and uses energy from the light reaction to reduce the new carbon piece to sugar.
begins with the incorporation of CO2 into an organic molecule via carbon fixation
carbon backbone is reduced with electrons provided by NADPH
ATP from the light reaction
also powers parts of the
Calvin cycle
occurs in the stroma

11. Review Draw a diagram of the chloroplast. Label all the parts.
Label where the light reaction and the Calvin cycle (dark reaction) occur.
Write a detailed three-sentence summary of photosynthesis.

12. Photosynthesis Details!
The Light Reactions and the Calvin Cycle

13. How it all fits together…

14. Why do we see green?
Different pigments absorb photons of different wavelengths.
A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

15. Absorption Spectra
light reactions: perform work with wavelengths of light that are absorbed.
In the thylakoid are several pigments that differ in their absorption spectrum.
Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.

16. Photon Absorption
When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy.
Photons are absorbed by clusters of pigment molecules in the thylakoid membranes.

17. Excited Electrons!
Excited electrons are unstable… they drop to their ground state in a billionth of a second, releasing heat energy.
Car in the sun gets hot!
Some photons release
light too…

18. Photosystems
Photosystems: the “light-harvesting units”, made of chlorophyll, proteins and other organic molecules
consists of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules
Energy is transmitted from pigment molecule to pigment molecule until it reaches a particular chlorophyll a: the reaction center
Like a satellite dish!
Primary electron acceptor: captures the excited electron

19. Two Types of Photosystems
Photosystem I: has a reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm
Photosystem II: has a reaction center with a peak at 680nm
differences due to the proteins associated with each reaction center
These two photosystems work together to use light energy to generate ATP and NADPH.

20. Noncyclic Electron Flow… the predominant route: produces both ATP and NADPH

21. The steps…
Photosystem II absorbs light, 2 excited electrons are passed to P680 (chlorophyll a). Then the electrons are captured by the primary electron acceptor.
Water is split to replace the lost electrons
splits into 2 H+ and an oxygen atom which combines with another to form O2
3. Excited electrons “fall” down the electron transport chain to Photosystem I

22. Energy of “falling” electrons is used to make ATP using chemiosmosis across the thylakoid membrane: noncyclic photophosphorylation
ATP is used by the Calvin Cycle
5. The falling electrons fill a “hole” in P700 (chlorophyll a) in Photosystem I. This hole is created when photons excite electrons on the photosystem I complex. The light energy sends 2 electrons to another primary electron acceptor
Electrons “fall” down a second electron transport chain. Electrons are picked up by NADP+ to form NADPH
NADPH will go to the Calvin Cycle

23. Summary
The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH

24. Calvin says, “I need more ATP!”
Problem: Noncyclic electron flow produces about the same amount of ATP and NADPH, but the Calvin Cycle uses MORE ATP
Cyclic Electron Flow makes up the difference.
Cyclic Electron Flow: uses only photosystem I, but electrons are sent down the first electron transport chain to make ATP
generate ATP by cyclic photophosphorylation

25. Cyclic Electron Flow

26. Chemiosmosis (again?!)
Yup! …electron transport chain pumps protons across a membrane as electrons are passed along a series of more electronegative carriers.
This builds the H+ gradient across the membrane.
ATP synthase
molecules
generate ATP
as H+ diffuses
back across the
membrane.

29. The Calvin Cycle
Carbon enters the cycle in the form of CO2 and leaves as sugar
The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P)
G3P is the starting material for making other organic compounds, including glucose and other carbohydrates.
spends the energy of ATP and the reducing power of electrons carried by NADPH to make the sugar
1 G3P = 9 ATP and 6 NAPDH
The cycle must take place 3 times,
fixing 3 molecules of carbon dioxide
3 Phases…

30. Phase 1: Carbon fixation
each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).
This enzyme rubisco catalyzes the first step
The 6-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate per CO2.

31. Phase 2: Reduction
each 3-phospho-glycerate receives another phosphate group from ATP
NADPH donates a pair of electrons
Six molecules of G3P are produced, but only 1 exits the cycle (others are reused)
G3P can be used to make glucose and other organic compounds

32. Phase 3: Regeneration of RuBP
five G3P molecules are rearranged to form 3 RuBP molecules.
Requires 3 ATP (one per RuBP) to complete the cycle and prepare for the next.

33. Now I need a nap! 