PHOTOSYNTHESIS

BIO11

Photosynthesis

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

PHOTOSYNTHESIS OVERVIEW

6CO2 + 6H2O + energy (sun) C6H12O6 + 6O2

• In order for photosynthesis to take place the following conditions must be met:1. Light – plants use the visible light spectrum2. 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)

3. 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.

Chloroplast

Photosynthesis overview

• Photosynthesis occurs in two distinct phases:1. The light reactions – known as the photophase

because it requires light energy.2. The dark reactions (Calvin cycle) – during this

phase, light is not required; however, the ATP and NADPH produced by the light reactions are.

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.

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

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.

STOMA• The stoma allows for the plant to

exchange gasses with its environment.

Stoma

Guard cells

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.

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.

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…

• 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)

http://simple animation

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.

Overview of light dependent reactions

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.

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.

… 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

3 x CO2

6 x(3-C)3 x

(5-C)

1

Rubisco

2

11stst Phase: Carbon Phase: Carbon FixationFixation

1.1. Three five-carbon sugar Three five-carbon sugar molecules are the molecules are the acceptors that bind 3 COacceptors that bind 3 CO22 molecules (dissolved in molecules (dissolved in the stroma). the stroma). This reaction This reaction is catalyzed by the is catalyzed by the enzyme enzyme rubiscorubisco..

2.2. Three Three unstable 6-C unstable 6-C molecules are produced molecules are produced (not shown) which (not shown) which quickly break down to quickly break down to give give sixsix 33-carbon -carbon molecules of themolecules of the

Phase 1: CARBON FIXATION

Animation: Calvin cycle Phosphate

carbon

3 x CO2

3 x RuBP (5-C)

6 x ATP

6 x ADP

6 x NADPH

6 x NADP6 x Pi6 x PGAL

(3-C)

1 x PGAL (3-C)

1

Rubisco

2

4

3

22ndnd Phase: Reduction Phase: Reduction3.3. The six 3-C molecules are The six 3-C molecules are

phosphorylated and phosphorylated and reduced to reduced to PGALPGAL ((phosphoglyceraldehyde)phosphoglyceraldehyde), , a three-carbon sugar. This a three-carbon sugar. This phosphate bond is then phosphate bond is then broken and hydrogen is broken and hydrogen is added from NADPH.added from NADPH.

4.4. SSixix molecules of PGAL are molecules of PGAL are produced. However, only produced. However, only one of the six molecules one of the six molecules exits the cycle as an output exits the cycle as an output (to make sugar, etc.) (to make sugar, etc.) whilewhile......

Phase 2: REDUCTION REACTIONS

Animation: Calvin cycle

6 x 1,3 BPG

NOTE: PGAL is also NOTE: PGAL is also referred to as G3Preferred to as G3P

6 x 3-PGA(3-C)

3 x CO2

3 x RuBP (5-C)

6 x ATP

6 x ADP

6 x NADPH

6 x NADP6 x Pi

6 x PGAL(3-C)

1 x PGAL(3-C)

5 x PGAL(3 C)

3 x ADP

3 x ATP

1

Rubisco

2

4

5

3

6

33rdrd Phase: Regeneration Phase: Regeneration of the Carbon of the Carbon acceptor molecule acceptor molecule

5.5. ... ...the remaining five the remaining five enter a complex process enter a complex process that regenerates more that regenerates more five-carbon sugar five-carbon sugar molecules to continue molecules to continue the cycle....the cycle....

6.6. In this process, ATP is In this process, ATP is used to convert the used to convert the five PGAL’s to three five PGAL’s to three 5-C molecules.5-C molecules.

7.7. Summary...Summary...9 ATP 9 ATP usedused6 NADPH 6 NADPH usedused1 PGAL 1 PGAL producedproduced 5-C regenerated5-C regenerated

Phase 3: REGENERATION

Animation: Calvin cycle

6 x 1,3 BPG

6 x 3-PGA(3-C)

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.