1. PHOTOSYNTHESIS
BIO11

2. Photosynthesis
Photosynthesis is the process by which carbohydrates (an organic nutrient) are synthesized from inorganic sources using the energy of light.

3. PHOTOSYNTHESIS OVERVIEW
6CO2 + 6H2O + energy (sun)  C6H12O6 + 6O2

4. In order for photosynthesis to take place the following conditions must be met:
Light – plants use the visible light spectrum
Photosynthetic pigments – there are various photosynthetic pigments found in chloroplasts including: chlorophyll A, chlorophyll B, chlorophyll C, carotene, and xanthophyll (the most important being chlorophyll A)
Thylakoid membranes – specialized membranes of the chloroplasts that are embedded with chlorophyll. These are in stacks called grana, surrounded by a gel-like substance called the stroma.

5. Chloroplast

6. Photosynthesis overview
Photosynthesis occurs in two distinct phases:
The light reactions – known as the photophase because it requires light energy.
The dark reactions (Calvin cycle) – during this phase, light is not required; however, the ATP and NADPH produced by the light reactions are.

7. STRUCTURE OF A LEAF Leaves are a plant's main photosynthetic organs.
Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out.
The oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf.

8. STRUCTURE OF A LEAF
In which cells does photosynthesis take place?

9. STRUCTURE OF A LEAF
Mesophyll cells are specialized for photosynthesis.
These cells in the middle of the leaf contain many chloroplasts, the organelles that perform photosynthesis.

10. STOMA
The stoma allows for the plant to exchange gasses with its environment.
Stoma
Guard cells

11. PIGMENTS & CHLOROPHYLL
Pigment: any substance that absorbs light of a specific wavelength.
Photosynthetic Pigments absorb light (solar) energy and convert it to chemical energy.
Chlorophyll by itself can’t carry out photosynthesis - it will absorb light but radiate it back at a different wavelength.

12. PIGMENTS & CHLOROPHYLL
Electron Acceptors must also be present for photosynthesis.
Chlorophyll is found embedded in flattened membranes called Thylakoids in the Chloroplast.
Thylakoids are arranged I interconnected stacks called Granna.

13. Photosynthesis: An Overview of the Light and ‘Dark’ Reactions
Occurs in Photoautotrophs (organisms that can make their own using energy from the sun).
Photosynthesis takes place in the chloroplasts.
Photosynthesis includes two processes…
animation
LIGHT REACTIONS
Requires sunlight
Occurs in the granna of chloroplasts
Produces ATP and NADPH (used to power the Calvin cycle)
‘DARK’ REACTIONS
(a misnomer…aka Calvin cycle)
Doesn’t require sunlight (happens 24/7).
Occurs in the stroma of chloroplasts
Produces PGAL (which can later be used to make glucose)

14. Light reactions
Chlorophyll absorbs light energy into the thylakoid membrane and is said to be ‘energized’. The energized chlorophyll causes the following reactions to occur:
A water molecule is split into hydrogen ions and oxygen atoms. This split occurs due to chlorophyll’s strong attraction for electrons. The hydrogen ions are then held by the coenzyme NADP+. The oxygen atoms are released to the atmosphere as a by-product.
ADP adds a phosphate group to become ATP (phosphorylation). The energy stored in ATP is used during the Calvin cycle.

15. Overview of light dependent reactions

16. The Calvin Cycle
In Photosynthesis, ATP and NADPH are produced in photophosphorylation, aka the Light Reactions. This happens in the thylakoid but notice that the products are actually produced in the stroma. This sets up the next series of reactions, the Calvin cycle which happens completely in the stroma. This is where sugars are manufactured. Melvin Calvin discovered this cycle in 1940.

17. THE CALVIN CYCLE
The reactions that convert carbon dioxide (CO2) into carbohydrate (glucose) molecules and H2O is given off to the atmosphere.
Occurs in the stroma of chloroplasts.
The process occurs by a cyclic series of reactions.

18. … The Calvin Cycle
The end product of photosynthesis isn’t really glucose; it’s PGAL (phosphoglyceraldehyde). PGAL can be used to manufacture glucose, or other sugars, fatty acids or amino acids and as an energy source for cellular respiration.
During a series of reactions, ATP and NADPH produced by the light reactions help convert CO2 into PGAL.
The Calvin Cycle has three phases:
1st phase: Carbon Fixation
2nd phase: Reduction
3rd phase: Regeneration of the Carbon acceptor molecule

19. Phase 1: CARBON FIXATION
1st Phase: Carbon Fixation
1. Three five-carbon sugar molecules are the acceptors that bind 3 CO2 molecules (dissolved in the stroma). This reaction is catalyzed by the enzyme rubisco.
2. Three unstable 6-C molecules are produced (not shown) which quickly break down to give six 3-carbon molecules of the
3 x CO2 1 2
6 x
(3-C)
3 x
(5-C)
Rubisco
Phosphate
carbon
Animation: Calvin cycle

20. Phase 2: REDUCTION REACTIONS
2nd Phase: Reduction
3. The six 3-C molecules are phosphorylated and reduced to PGAL (phosphoglyceraldehyde), a three-carbon sugar. This phosphate bond is then broken and hydrogen is added from NADPH.
4. Six molecules of PGAL are produced. However, only one of the six molecules exits the cycle as an output (to make sugar, etc.) while...
3 x CO2 1 2
6 x 3-PGA
(3-C)
3 x RuBP
(5-C)
Rubisco
6 x ATP
6 x ADP 3
6 x 1,3 BPG
6 x NADPH
6 x NADP
6 x PGAL
(3-C)
6 x Pi 4
Animation: Calvin cycle
NOTE: PGAL is also referred to as G3P
1 x PGAL (3-C)

21. Phase 3: REGENERATION
3rd Phase: Regeneration of the Carbon acceptor molecule
5. ...the remaining five enter a complex process that regenerates more five-carbon sugar molecules to continue the cycle....
6. In this process, ATP is used to convert the five PGAL’s to three 5-C molecules.
7. Summary...
9 ATP used
6 NADPH used
1 PGAL produced
5-C regenerated
3 x CO2 1 2
6 x 3-PGA
(3-C)
3 x RuBP
(5-C)
Rubisco
6 x ATP
6 x ADP 3
3 x ADP
3 x ATP
6 x 1,3 BPG 6
6 x NADPH
6 x NADP
6 x Pi
5 x PGAL
(3 C)
6 x PGAL
(3-C) 5 4
Animation: Calvin cycle
1 x PGAL
(3-C)

22. Glucose formation
The sixth PGAL will be used to form a glucose molecule.
Since each glucose molecule (C6H12O6) is formed from two PGAL’s, two complete turns of the cycle are required.
NADP+, ADP and Pi are then released by the Calvin cycle and are recycled back to the thylakoid membrane to be transformed yet again by the light reactions.