Ch. 10 Notes Continued

Today Mrs. Lyon had to leave early, so we just took notes in class.

• The vocab quiz has been moved to tomorrow.
• Mrs. Lyon will go over the notes we took today in class tomorrow.

Here's how far B block got today:

The Light Reactions

Light is electromagnetic energy and it behaves as though it is made up discrete particles called photons--each of which has a fixed quantity of energy.

Substances that absorb light are called pigments, and different pigments absorb light of different wavelengths. Chlorophyll is a pigment that absorbs not only red and blue but also green. This is why we see summer leaves as green.



Sunlight encompasses a broad spectrum of light, most of which is not absorbed by chlorophyll. Only certain wavelengths of light spectrum are utilized in providing plants with energy. This is the spectrum of light absorption for chlorophyll, as you can see the rate of absorption is highest in the red and blue area, whereas it is lowest in the green. The green light is reflected back, giving us the leaf color we see.






When chlorophyll absorbs light energy in the form of photons, one of the molecule’s electrons is raised to an orbital of higher potential energy. The chlorophyll is then said to be in an “excited” state.

Photons of light are absorbed be certain groups of pigment molecules in the thylakoid membrane of the chloroplasts. These groups are called photosystems. Photosystems have an antenna complex made up of chlorophyll molecules and caretenoid molecules (accessory pigments in the thylakoid membrane); this allows them to gather light effectively.


There are two photosystems in the thylakoid membrane that are important to photosynthesis-- photosystem I (PSI) and photosystem II (PSII). Each of these photosystems has a reaction center (the site of the first light-driven chemical reaction of photosynthesis)



Here are the major steps of the light reactions of photosynthesis:

1. Photosystem II absorbs light in the 680 nm wavelength range. An electron in the reaction center chlorophyll (called P680) becomes excited and then is captured by a primary electron acceptor. The reaction center chlorophyll is oxidized and needs an electron.
2. An enzyme supplies the missing electron taken from photolysis water (the splitting of water) to P680; what is split in the process, and a free oxygen is created-- this oxygen combines with another oxygen to form O2
3. The original excited electron passes from the primary electron acceptor of photosystem II to photosystem I through an electron transport chain.
4. The energy from the transfer of electrons down the electron transport chain is used to phosphorylate ADP to ATP in the thylakoid membrane, in a process called noncyclic photophosphorylation. Later this ATP will be used as energy in the formation of carbohydrates, in the dark reactions, or the Calvin cycle.
5. The electrons that get to the end of the electron transport chain donated to the chlorophyll in the P700 in photosystem I. (this meeds for an electron be PSI is created when light energy excites an electron in P700, and that electron is taken up by the primary acceptor of photosystem I).
6. The primary electron acceptor of photosystem I passes along the excited electrons to another electron transport chain, which transmits them to ferredoxin, and then finally to NADP+, which is reduced to NADPH, the second of the two important light-reaction products.

An alternative to noncyclic electron flow in cyclic electron flow. While non cyclic electron flow produces nearly equal quantities of ATP and NADHP, the Calvin cycle reactions use more ATP than NADPH. In the cyclic electron flow, photosystem II is bypassed, and the electrons from ferredoxin cycle back to a portion of the electron transport chain of PSII and its cytochromes and the to P700. Neither NADPH nor oxygen is produced, but ATP is still a product. This process is called cyclic photophosphorylation. Cyclic photophosphorolation can occur in some photosynthetic bacteria.

The Calvin Cycle

In the course of the Calvin cycle, CO2 is converted to a carbohydrate called glyceraldehyde-3-phosphate (G3P), and ATP and NADPH are both consumed. But in order to make one molecule of G3P, the cycle must go go through three rotations and fix three molecules of CO2.

These are the steps of the Calvin cycle:

1. The three CO2 molecules are attached to three ribose biphosphate molecules (RuBPs); these reactions are catalyzed by rubisco to produce an unstable product that immediately splits into two three-carbon compounds called 3-phosphoglycerate
2. 2.) The 3-phosphoglycerate molecules are phosphorylated to become 1, 3-diphosphoglycerate
3. Next NADPHs reduce the 1, 3-diphosphoglycerates to create glyceraldehyde-3-phosphates (G3P)
4. Finally the RuBP is regenerated as the 5 G3Ps are reworked into 3 of the starting molecules, with the expenditure of 3 ATP molecules
   

The results of the Calvin cycle (which produces one G3P molecules for each trip through the cycle) are that:

• 9 molecules of ATP are consumed (to be replenished by the light reactions)

• 6 molecules oh NADPH are consumed (also to be replenished by the light reactions)

• The G3P that was created is later metabolized into larger carbohydrates

Review Questions:

1. The final product of the Calvin cycle is

A) RuPB
B) PGA
C) ATP
D) G3P

2.) Colors of light most useful in photosynthesis are

A) green, yellow, and orange
B) red, violet, and blue
C) infrared, red, and yellow
D) red, white, and blue

3.) The pigment molecules responsible for photosynthesis are located in the

A) mitochondria
B) cytoplasm of the cell
C) stroma of the chloroplast
D) thylakoid membrane of the chloroplast
E) all of the above

4.) Which of the following occurs during the light-dependent reactions of plants?

A) electron transport
B) chemiosmosis
C) splitting of water
D) all of the above
E) none of the above

5.) Production of one molecule of 3-phosphoglyceraldehyde requires how many turns of the Calvin cycle?

A) 1
B) 2
C) 3
D) 6
E) 12

The answers are D, B, D, D, C.