PHOTOSYNTHESISPHOTOSYNTHESISThe BasicsThe Basics

BIO11BIO11

McIntyreMcIntyre

PhotosynthesisPhotosynthesis

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

PHOTOSYNTHESIS OVERVIEWPHOTOSYNTHESIS OVERVIEW

6CO6CO22 + 6H + 6H22O + energy (sun) O + energy (sun) C C66HH1212OO66 + 6O + 6O22

In order for photosynthesis to take place the In order for photosynthesis to take place the following conditions must be met:following conditions must be met:

1.1. Light – plants use the visible light spectrumLight – plants use the visible light spectrum

2.2. Photosynthetic pigments – there are various Photosynthetic pigments – there are various photosynthetic pigments found in chloroplasts photosynthetic pigments found in chloroplasts including: chlorophyll A, chlorophyll B, chlorophyll including: chlorophyll A, chlorophyll B, chlorophyll C, carotene, and xanthophyll (the most important C, carotene, and xanthophyll (the most important being chlorophyll A)being chlorophyll A)

3.3. Thylakoid membranes – specialized membranes of Thylakoid membranes – specialized membranes of the chloroplasts that are embedded with chlorophyll. the chloroplasts that are embedded with chlorophyll. These are in stacks called grana, surrounded by a These are in stacks called grana, surrounded by a gel-like substance called the stroma.gel-like substance called the stroma.

ChloroplastChloroplast

Photosynthesis overviewPhotosynthesis overview

Photosynthesis occurs in two distinct phases:Photosynthesis occurs in two distinct phases:1.1. The light reactions – known as the photophase The light reactions – known as the photophase

because it requires light energy.because it requires light energy.

2.2. The dark reactions (Calvin cycle) – during this The dark reactions (Calvin cycle) – during this phase, light is not required; however, the ATP and phase, light is not required; however, the ATP and NADPH produced by the light reactions are.NADPH produced by the light reactions are.

STRUCTURE OF A LEAFSTRUCTURE OF A LEAF Leaves are a plant's main Leaves are a plant's main photosynthetic organs.photosynthetic organs.

Leaves must permit carbon Leaves must permit carbon dioxide access to the dioxide access to the photosynthetic cells but impede photosynthetic cells but impede water from diffusing out.water from diffusing out.

The oxygen that is a waste The oxygen that is a waste product of photosynthesis must product of photosynthesis must be allowed to escape from the be allowed to escape from the leaf.leaf.

STRUCTURE OF A LEAFSTRUCTURE OF A LEAF

• In which cells does photosynthesis take place?In which cells does photosynthesis take place?

STRUCTURE OF A LEAFSTRUCTURE OF A LEAF

Mesophyll cells are specialized Mesophyll cells are specialized for photosynthesis.for photosynthesis.

These cells in the middle of These cells in the middle of the leaf contain many the leaf contain many chloroplasts, the organelles chloroplasts, the organelles that perform photosynthesis.that perform photosynthesis.

STOMASTOMA The stoma allows for the plant to The stoma allows for the plant to exchange gasses with its environment.exchange gasses with its environment.

Stoma

Guard cells

PIGMENTS & CHLOROPHYLLPIGMENTS & CHLOROPHYLL Pigment: any substance that Pigment: any substance that absorbs light of a specific absorbs light of a specific wavelength.wavelength.

Photosynthetic Pigments absorb Photosynthetic Pigments absorb light (solar) energy and light (solar) energy and convert it to chemical energy.convert it to chemical energy.

Chlorophyll by itself can’t Chlorophyll by itself can’t carry out photosynthesis - it carry out photosynthesis - it will absorb light but radiate will absorb light but radiate it back at a different it back at a different wavelength.wavelength.

PIGMENTS & CHLOROPHYLLPIGMENTS & CHLOROPHYLL Electron Acceptors must also be Electron Acceptors must also be present for photosynthesis.present for photosynthesis.

Chlorophyll is found embedded Chlorophyll is found embedded in flattened membranes called in flattened membranes called Thylakoids in the Chloroplast.Thylakoids in the Chloroplast.

Thylakoids are arranged I Thylakoids are arranged I interconnected stacks called interconnected stacks called Granna.Granna.

Photosynthesis: An Overview Photosynthesis: An Overview of the Light and ‘Dark’ of the Light and ‘Dark’

ReactionsReactions Occurs in Photoautotrophs (organisms Occurs in Photoautotrophs (organisms that can make their own using energy that can make their own using energy from the sun).from the sun).

Photosynthesis takes place in the Photosynthesis takes place in the chloroplasts.chloroplasts.

Photosynthesis includes two processes…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 reactionsLight reactions Chlorophyll absorbs light energy into the thylakoid Chlorophyll absorbs light energy into the thylakoid

membrane and is said to be ‘energized’. The energized membrane and is said to be ‘energized’. The energized chlorophyll causes the following reactions to occur:chlorophyll causes the following reactions to occur: A water molecule is split into hydrogen ions and oxygen A water molecule is split into hydrogen ions and oxygen

atoms. This split occurs due to chlorophyll’s strong atoms. This split occurs due to chlorophyll’s strong attraction for electrons. The hydrogen ions are then held attraction for electrons. The hydrogen ions are then held by the coenzyme NADPby the coenzyme NADP+.+. The oxygen atoms are released The oxygen atoms are released to the atmosphere as a byproduct. to the atmosphere as a byproduct.

ADP adds a phosphate group to become ATP ADP adds a phosphate group to become ATP (phosphorylation). The energy stored in ATP is used (phosphorylation). The energy stored in ATP is used during the Calvin cycle.during the Calvin cycle.

Overview of light dependent Overview of light dependent reactionsreactions

The Calvin CycleThe Calvin Cycle In Photosynthesis, ATP and NADPH are produced in In Photosynthesis, ATP and NADPH are produced in photophosphorylationphotophosphorylation, aka the Light , aka the Light

Reactions. This happens in the Reactions. This happens in the thylakoid thylakoid but notice that the products are actually produced in 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 the stroma. This sets up the next series of reactions, the Calvin cycle which happens completely in the in the stromastroma. This is where sugars are manufactured. Melvin Calvin discovered this cycle in . This is where sugars are manufactured. Melvin Calvin discovered this cycle in 1940.1940.

THE CALVIN CYCLETHE CALVIN CYCLE The reactions that convert carbon The reactions that convert carbon

dioxide into carbohydrate molecules dioxide into carbohydrate molecules occur in theoccur in the stroma of chloroplastsstroma of chloroplasts..

CO2 is converted to glucose (a CO2 is converted to glucose (a carbohydrate) and H2O is giving off to carbohydrate) and H2O is giving off to the atmosphere. the atmosphere.

The process occurs by a cyclic series The process occurs by a cyclic series of reactions.of reactions.

… … The Calvin CycleThe Calvin Cycle The end product of photosysnthesis The end product of photosysnthesis isn’t really glucoseisn’t really glucose; it’s ; it’s

PGALPGAL ( (pphosphohosphogglycerlyceralaldehydedehyde). ). PGALPGAL can be used to can be used to manufacture glucose, or other sugars, fatty acids or amino manufacture glucose, or other sugars, fatty acids or amino acids and as an energy source for cellular respiration.acids and as an energy source for cellular respiration.

During a series of reactions, ATP and NADPH During a series of reactions, ATP and NADPH produced by the light reactions help convert produced by the light reactions help convert COCO22 into PGAL. into PGAL.

The Calvin Cycle has three phases:The Calvin Cycle has three phases: 1st phase1st phase: Carbon Fixation: Carbon Fixation 2nd phase2nd phase: Reduction: Reduction 3rd phase3rd phase: Regeneration of the Carbon acceptor molecule: 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 formationGlucose formation The sixth PGAL will be used to form a The sixth PGAL will be used to form a

glucose molecule.glucose molecule. Since each glucose molecule Since each glucose molecule

(C(C66HH1212OO66) is formed from two PGAL’s, ) is formed from two PGAL’s, two complete turns of the cycle are two complete turns of the cycle are required.required.

NADP+, ADP and Pi are then released NADP+, ADP and Pi are then released by the Calvin cycle and are recycled by the Calvin cycle and are recycled back to the thylakoid membrane to back to the thylakoid membrane to be transformed yet again by the light be transformed yet again by the light reactions.reactions.