PHOTOSYNTHESIS

IB Biology HLE. McIntyre

Simple Photosynthesis Overview

Simplified Chemical summary:

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

Properties of Light

• Electromagnetic Radiation and the Visible Light Spectrum

• Engleman’s experiment showing which wavelength of visible light is best for photosynthesis

Structure of a Leaf• Look at the various cells

in the cross section of the leaf. In which cells does photosynthesis take place?

• Take this test...

‘Palisade’ means to surround

with a wall in order to fortify

Stoma

Stoma

Guard cells

• This structure allows for the plant to exchange gasses with its environment. What gasses??

Chloroplast structure

• http://Animation: Show first 20 sec for chloroplast anatomy (link #2)

Micrograph of Chloroplast

http://indycc1.agri.huji.ac.il/~zacha/chloroplast.jpg

take a quiz!

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Label your diagram!

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

Photosystems• Photosystems are arrangements

pigment-protein complexes. They contain chlorophyll and other accessory pigments packed into thylakoids.

• Many prokaryotes have only one photosystem, Photosystem I. Eukaryotes have Photosystem I plus Photosystem II.

• Photosystem I was the first to evolve.

Graphic: http://kvhs.nbed.nb.ca/gallant/biology/photosystem.jpg

Photosystem I uses Photosystem I uses chlorophyll achlorophyll a, in the form referred to as , in the form referred to as P700P700. . It absorbs light up to 700 nm. Photosystem II uses a form of It absorbs light up to 700 nm. Photosystem II uses a form of chlorophyll a known as chlorophyll a known as P680P680. It absorbs light up to 680 nm.. It absorbs light up to 680 nm.

…Photosystems• The accessory pigments (chlorophyll b, carotenoids , and

xanthophylls) play an indirect role in the formation of glucose through photosynthesis. These pigments provide chlorophyll a with the energy that they have captured from the sun. These pigments capture varying wavelengths of light and thus allow the plant to receive sun energy across a greater spectrum. Accessory pigments absorb energy that chlorophyll a does not absorb.

• Some carotenoids play a role in energy absorption rather than in photosynthesis. They absorb light to prevent damage to chlorophyll. The energy is lost as heat.

• Why do leaves of deciduous trees turn pretty colors in autumn?

Image: http://www.thrivingnow.com/for/photos/image_med/82/

A Closer Look a Photosystems…

The Chlorophyll MoleculeHow does the chlorophyll molecule stay in the correct orientation when embedded in the thylakoid membrane?

Light Absorption by Various Pigments

• http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect10.htm

• Why do most photosynthetic organisms look green?

……more detail

Phosphorylation

• Phosphorylation: The chemical addition of a phosphate group (phosphorous and oxygen) to a compound. i.e. adding Pi to ADP to get ATP

• Photophosphorylation is addition of a phosphate using the sun’s energy!

• There are two types of photophosphorylation; cyclic and non-cyclic.

Cyclic Photophosphorylation

• Cyclic photophosphorylation probably occurs in plants when there is too little NADP+ available (more on this later).

• Cyclic photophosphorylation is also seen in certain photosynthetic bacteria. Note that the bacteria have no chloroplasts. All structures are embedded in the membrane. The proton gradient is created between the cell membrane and the capsule.

Cyclic Photophosphorylation• A single photosystem is involved.• A photon of light strikes a pigment

molecule in the P700 antenna system.• The energy eventually reaches a

molecule of P700 (specialized chlorophyll a - the ‘reaction centre’). This electron is ejected from the photosystem.

• The energized electron leaves P700 and is passed to an acceptor molecule; Ferrodoxin (fd).

• The electron is then passed through the cytochrome b6f complex. This complex pumps protons (H+) into the space between bacterium’s cell membrane and capsule (or in the case of plants, inside the thylakoid). This creates a proton gradient.

• Protons can only cross back through the membrane via ATP synthase. ATP synthase uses the energy flow of protons (proton motive force) to make ATP (Phosphorylaion).

Animation 1: Development of Proton Motive Force (proton gradient) via chemiosmosis

Animation 2: Formation of ATP from Proton motive force

Animation 3: ATP synthase

…Cyclic Photophosphorylation• The electron is then passed through

the plastocyanin (pC).• The electron is passed back to the

reaction centre.• The electron’s energy is gradually

lost during this process. • The de-energized electron returns to

the chlorophyll a molecule to be energized again.

• We call this process cyclic photophosphorylation because electrons return to the photosystem and are then again energized. The process is a cycle!

• The energy released during this electron transport generates a proton gradient which is used to produce ATP.

• Animation: (non) cyclic photophosphorylation animation

Light Reactions and Non-Cyclic Photophosphorylation

Hmmmm…

Try to interpret this diagram in laymen’s terms.

Non-cyclic

photophosphorylation

…Light Reactions and Non-Cyclic Photophosphorylation• Happens in PLANTS. Two photosystems are involved.

• A photon hits Photosystem II (PS II or P680). This energy is relayed to the reaction centre via accessory pigments. A high energy electron is emitted.

• …meanwhile, an enzyme in PS II (enzyme Z) splits water. The oxygen is released as a byproduct. Electrons from water are used to replace those lost by PS II.

• The electron excited in PS II then travels to plastoquinone (Q), then to the b6f complex (proton pump).

Proton pump

NADP

Reductase

FdQ PC

Animation: (non) cyclic photophosphorylation animation

Proton pump

NADP

Reductase

FdQ PC

Animation: (non) cyclic photophosphorylation animation

• The proton pump uses this energy to pump protons across the thylakoid membrane, from the stroma into the thylakoid space. These protons can only exit the thylakoid via ATP synthase. The flow of protons (proton motive force) through ATP synthase is used to make ATP. ATP production in this manner is called Chemiosmosis.

…Light Reactions and Non-Cyclic Photophosphorylation

..Non-Cyclic Photophosphorylation• The electron then goes to plastocyanin (PC) and then to PS I.

• Remember, the electron has lost energy because…the proton pump used it up! It’s now de-energized!

• …A photon hits PS I (P 700). Energy is passed from accessory pigments to reaction centre which ejects a high energy electron.

Animation: (non) cyclic photophosphorylation animation

Proton pump

NADP

Reductase

FdQ

PC

The de-energized The de-energized electron replaces electron replaces the electron lost the electron lost from PS I.from PS I.

…Non-Cyclic Photophosphorylation

Animation: (non) cyclic photophosphorylation animation

Proton pump

NADP

Reductase

FdQ

PC

The electron is then passed to ferrodoxin (Fd) and then to NADP reductase, which The electron is then passed to ferrodoxin (Fd) and then to NADP reductase, which uses the newly energized electron to reduce NADP to NADPH.uses the newly energized electron to reduce NADP to NADPH.

The ATP and NADPH produced during non-cyclic photophosphorylation go to the The ATP and NADPH produced during non-cyclic photophosphorylation go to the Calvin cycle to provide energy and raw materials to make SUGAR! Calvin cycle to provide energy and raw materials to make SUGAR!

NON-cyclic photo-phosphorylation…

Non-cyclic

photophosphorylation

Does this make sense now?

Watch the animation, then answer this question:

Where do the protons come from that go through ATP synthase?

Cyclic vs. non-cyclic photophosphorylation in plants.

• Cyclic photophosphorylation occurs less commonly in plants than noncyclic photophosphorylation does. Examine the two diagrams below. What are the similarities and differences?

Examine the formula that summarizes photosynthesis…

CO2 + H2O C6H12O6 + O2

sunlight

You should know…

• Where the O2 byproduct comes from…

Infer…

• Where the carbon in glucose comes from…

• Where the hydrogen in glucose comes from…

• Where the oxygen in glucose comes from…

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 end product of photosysnthesis isn’t really

glucose; it’s PGAL (phosphoglyceraldehyde). PGAL (AKA G3P) can be used to manufacture glucose, or other sugars, fatty acids or amino acids.

• The Calvin Cycle has three phases:

• 1st phase: Carbon Fixation

• 2nd phase: Reduction

• 3rd phase: Regeneration of the Carbon acceptor molecule (RuBP)

3 x CO2

6 x PGA(3-C)3 x RuBP

(5-C)

1

Rubisco

2

11stst Phase: Carbon Phase: Carbon FixationFixation

1.1. Three five-carbon sugar Three five-carbon sugar molecules called ribulose molecules called ribulose bisphosphate, or bisphosphate, or RuBPRuBP, , are the acceptors that are the acceptors that bind 3 CObind 3 CO22 molecules molecules

(dissolved in the stroma). (dissolved in the stroma). This reaction is catalyzed This reaction is catalyzed by the enzyme by the 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 molecules of the molecules of the threethree-carbon -carbon phosphoglyceric acid phosphoglyceric acid ((PGAPGA))..

The Calvin CycleThe Calvin Cycle

Animation: Calvin cycle Phosphate

carbon

3 x CO2

6 x PGA(3-C)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 PGA molecules are The six PGA molecules are

phosphorylated to sixphosphorylated to six 1,3 1,3 BPGBPG (1,3 (1,3 bbisphosisphospphohogglycerate) as lycerate) as each PGA accepts a high each PGA accepts a high energy P from ATP. 1,3 energy P from ATP. 1,3 BPG is reduced to BPG is 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......

The Calvin CycleThe Calvin Cycle

Animation: Calvin cycle

6 x 1,3 BPG

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

3 x CO2

6 x PGA(3-C)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 (RuBP)(RuBP)

5.5. ...the remaining five ...the remaining five enter a complex enter a complex process that process that regenerates more regenerates more RuBP to continue the RuBP to continue the cycle....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 RuBP’s.RuBP’s.

7.7. Summary...Summary...9 ATP 9 ATP usedused6 NADPH 6 NADPH usedused1 PGAL 1 PGAL producedproduced RuBP regeneratedRuBP regenerated

The Calvin CycleThe Calvin Cycle

Animation: Calvin cycle

6 x 1,3 BPG

Photosynthetic Rate

• Photosynthetic rate is often measured as the rate of CO2 absorption per unit area of the leaf. (mmolCO2/m2/s)

How does Irradiance Affect Rate of Photosynthesis?

• Light-compensation point: the point on a light-response curve at which

photosynthetic CO2 uptake = respiratory CO2 evolution

• Light saturation point: the irradiance level at which the carbon fixation levels reach a maximum rate.

• http://www.marietta.edu/~spilatrs/biol103/photolab/compexpl.html

How does Irradiance affects Rate of Photosynthesis?

• How does irradiance initially affect rate of CO2 uptake?

As irradiance increases, CO2 uptake increases in a linear fashion.

• Describe CO2 absorption in absence of light. Explain.

It is negative. Plant PRODUCES CO2 due to cell respiration.

• What is the significance of the light saturation point?

• What is the significance of the light saturation point?

the maximum irradiance that can be used by the plant. Not enough enzymes to take advantage of increased light intensities.

• Explain the significance of the flat portion of the curve.

How Temperature affects Rate of Photosynthesis

• Temperature affects enzyme efficacy. Enzymes will work within an optimal temperature range. They can become denatured if the temperature is outside this range.

• How does temperature affect photosynthetic rate? Explain.

Interpret the graph!

Overview of light dependent reactions