1. Photosynthesis – Chapter 8
Read whole chapter.
Overall reaction: 6 H CO2  C6H12O6 + 6O2

2. Movement of water and gases
Water enters plants through the roots and is carried to leaves via tubular cells called xylem.
CO2 comes from the air and enters the leaf through tiny pores called stomata (singular = stoma).
O2 departs from the leaf through the same stomata.
A lot of water evaporates from the leaves through the stomata (transpiration). Only a little water is actually involved in the reactions of photosynthesis.
“Gas exchange” is a general term for all these processes happening at the stomata.

6. Visible light is important in biological processes
Why is the visible part of the spectrum used both by our eyes and by plant photosynthesis?
Longer wavelengths don’t have enough energy; they merely vibrate molecules (heat).
Shorter wavelengths have too much energy; they destroy molecules (cancer, mutations).
Visible light is “just right” – its energy excites electrons to higher energy states, and cells can harness that energy to do work.

7. However …
Human eyes and plant chloroplasts don’t use the all visible wavelengths equally.
Our eyes are most sensitive to greens.
Chlorophyll absorbs most strongly in the blue and red parts of the spectrum; green light is the part plants don’t use.

9. Photosynthesis has two parts
THE LIGHT REACTIONS
Light energy is captured.
High-energy nucleotides ATP and NADPH are formed.
Water is split.
Electrons go to NADPH
Oxygen is released as a by-product.
THE CALVIN BENSON CYCLE
ATP and NADPH give up their energy.
The energy is used to build glucose from CO2.

11. “Dark Reactions” is misleading.
The Calvin-Benson cycle does not require light. It could go on in the dark, BUT…
The Calvin-Benson cycle does require ATP and NADPH, and they are only produced when light is available!
Thus, no glucose is really produced in the dark.
A better term is “light-independent” reactions.

14. The Light Reactions
Chlorophyll and accessory pigment molecules are grouped into Photosystems on the thylakoid membranes
There are two kinds of photosystem: Photosystem II and Photosystem I
II comes before I in the metabolic processes.
The thylakoids are covered in thousands of photosystems.

15. Photosystems I and II
Each photosystem is a collection of pigment molecules (chl. a, chl. b, carotenes, etc.) clustered around one special molecule of chlorophyll a called the reaction center.
In Photosystem II, the reaction center is chlorophyll a P680, a chemical form that has maximum light absorbance at 680 nm.
In Photosystem I, the reaction center is chlorophyll a P700, a slightly different form that has maximum light absorbance at 700 nm.

16. The main metabolic pathway of the light reactions: Noncyclic Electron Flow
Light strikes pigment molecules of PsII.
Energy is passed from molecule to molecule, ending up at the reaction center, P680.
The energy excites the electrons of P680 and these high-energy electrons are donated to an electron transport chain.
P680 makes up for the two electrons it lost by taking them away from water (i.e. oxidizes the water)

17. Noncyclic Electron Flow (continued)
Electrons are passed through an electron transport chain. This generates ATP through chemiosmosis, just like in respiration.
At the same time as steps 1-5 are happening, P680 in Photosystem I is also being excited and giving up electrons. It replaces them by accepting the electrons from Photosystem II, after they exit the electron transport chain.
The high-energy electrons from Photosystem I are used to reduce NADP+ to NADPH.

20. A second pathway in the light reactions: Cyclic Electron Flow
Goes on at the same time as the non-cyclic pathway; involves only Photosystem I.
Excited P700 donates electrons to a different electron transport chain.
They pass through the chain and return to P700.
As the electrons pass through the chain, ATP is formed by chemiosmosis.
Only ATP is formed, no NADPH.

22. Synthesizing sugars: The Calvin-Benson Cycle
Reactants:
6 CO2
12 NADPH
18 ATP
6 RuBP (Ribulose bisphosphate, a 5C molecule)
Products:
1 Glucose
12 NADP+
18 ADP
6 RuBP

25. “rubisco” An essential enzyme, with a flaw
RuBP Carboxylase-Oxygenase
Carboxylation of RuBP is the essential first step of the Calvin cycle. Thus, it is not an exaggeration to say that all life depends on this enzyme!
However, in the presence of O2, rubisco can also catalyze the harmful oxidation of RuBP, a destructive process called photorespiration.

26. C4 photosynthesis C4 plants are found in sunny areas.
They first trap carbon in the form of a 4-carbon compound, which is transported within the leaf.
Later, this is broken down, releasing the CO2 again, so it can be used in the Calvin cycle.
BUT NOW - the Calvin cycle occurs in areas of the leaf where oxygen is less common, reducing photorespiration.

28. C4 Photosynthesis (cont.)
The enzyme that catalyzes the fixation of CO2 as a 4-C compound is PEP-carboxylase.
This enzyme is unaffected by oxygen, and cannot catalyze photorespiration.
BUT, PEP-carboxylase cannot function at cool temperatures, so C4 plants don’t grow well in the shade or in the winter (e.g. Bermuda-grass).

29. CAM photosynthesis Like C4, but carbon is trapped at night.
Stomata can remain closed during the day.
Highly inefficient way to trap energy, but prevents water loss and is thus a good survival adaptation for desert plants.