HL Biology Photosynthesis

Starch Granule

Stores extra glucose in the chloroplast

Double Membrane

Evidence of endosymbiosis, since the inner membrane is similar to a prokaryotic membrane and the outer membrane seems to have developed based on the host cell’s membrane

Stroma

The fluid-filled space in the chloroplast which has a suitable pH and enzymes available for the Calvin Cycle.

Thylakoid

Hosts ETC, ATP Synthase, and Chlorophyll Synthases for photophosphorylation. Its small volume allows for rapid formation of high proton concentrations in its lumen.

Thylakoid Lumen

Where a high volume of H+ is stored to create the concentration gradient required for the light-dependent reactions/chemiosmosis

Thylakoid Membrane

Where the light-dependent reactions occur, containing pigments like chlorophyll that capture light energy to begin the ETC, therefore powering the rest of the reactions to make ATP

Photosystems

Protein complexes in the thylakoid that store pigments to capture light energy

Photosystem II

The first photosystem that begins the ETC and where photolysis of water happens, contributing to the high concentration of of H+ in the lumen and replenishing excited e^-

Photosystem I

The second photosystem in the ETC that reduces NADP+ → NADPH after being photoactivated, but no H₂O photolysis because its e^- is replenished by the rest of the ETC

Lamella

Connects/separates grans (thylakoid stacks)

Granum

Stacks of thylakoids, whose shape increases the SA available for the light-dependent reactions and whose small volumes quickly accumulate ions to create concentration gradients

What is the visible light spectrum? Which wavelengths are red, green, and violet?

400-700 nanometers in wavelength, 625-750nm, 500-565nm, 380-450nm

How do chloroplasts relate to cyanobacteria?

One theory describes that chloroplasts evolved after cyanobacteria were swallowed by other cells because they have similar features: double membranes with photosystems, their own DNA and ribosomes, and similar functions

Describe the 1st step of the light-dependent reactions

Photoactivation of Photosystem II

• Photo strikes a pigment molecule, making excited e^- that is passed to an (intermediate) e^- carrier to start the ETC

Describe the 2nd step of the light-dependent reactions

Photolysis of Water

• To replace from step 1, H₂O is split into H+ and O₂. 

• The H+ contributes to the proton gradient in the lumen

• This process is the main source of O₂ in photosynthesis, which is a waste product

Describe the 3rd step of the light-dependent reactions

Photophosphorylation of ATP

• After being photoactivated in Photosystem I, e^- are passed along ETC and H+ is pumped across the thylakoid membrane to create a high concentration in the thylakoid lumen

• This concentration allows for chemiosmosis, phosphorylating ADP into ATP

Describe the 4th step of the light-dependent reactions

NADP+ Reduction

• When Photosystem I is photoactivated, the excited e^- produced combine with H+ and NADP+ to reduce it into NADPH

• e^- is replenished by electrons who have completed the ETC/traveled through Photosystem II, so no H₂O photolysis happens here

What are the final products of the light dependent reaction? What is the overall equation?

2 NADPH, 3 ATP, O₂
2H₂O+ 2NADP+ +3ADP+3Pi​ + light→O₂ + 2NADPH + 3ATP

What other names do the Light-Independent Reactions go by?

The Calvin Cycle, the Dark reactions

Describe the 1st step of the light-independent reactions

Carbon Fixation to form Glycerate-3-Phosphate (GP)

• Ribulose Bisphosphate (RuBP) + CO₂ → GP

• This occurs by inserting CO₂ into RuBP to make a 6C compound, which splits into two strands that each have a P atom on the end

Describe the 2nd step of the light-independent reactions

Reduction of GP → Triose Phosphate (TP)

• Energy released from ATP → ADP + Pi and from NAD+ → NADH from the Calvin cycle powers the reduction of GP → TP

Describe the 3rd step of the light-independent reactions

1 TP Leaves the cycle, 5 TP continue

• 1 TP leaves to form glucose or other organic molecules (ex. amino acids)

Describe the 4th step of the light-independent reactions

5TP converted to RuBP

• The Pi from the ATP → ADP + Pi reaction used to reduce GP is added to to the 5TP to reform RuBP

How many cycles are required to make 1 TP?

3 Cycles (1 CO₂ required each cycle)

How many TP are required to make 1 glucose?

2 TP (1 TP leaves per cycle, so 2 cycles required)

What are 3 limiting factors on the rate of photosynthesis?

CO₂, light intensity, temperature

Why are Greenhouses used to test the rate of photosynthesis?

You can change CO₂ because the space is enclosed, but you can still control for heat and light. 

An O₂ sensor can be used to measure the rate of photosynthesis, or the amount of glucose content in plants can be used

What is FACE?

Free Air CO₂ Experimentation (FACE) involves enriching air surrounding plants with CO₂ to observe a more realistic environment, which is useful for future predictions involving the increase in greenhouse gases

They found that plants did better with more CO_²

What are the action spectra and absorption spectra

• Action spectra shows the amount of O₂ production while plants are exposed to different wavelengths of light

• Absorption spectra show the absorption of light by pigment molecules (ex. chlorophyll A, chlorophyll B

Describe features of the action and absorption spectras

Absorption: Y axis is labeled Absorbance, graph starts at 400nm, trough in the green range (500-565nm), peak in red-orange (625-750nm) and violet (380-450nm)

Action: Y axis is labeled Rate of Photosynthesis/O₂ production, then the same as absorption

What does the curve of O₂ production as temperatures increase look like?

A bell curve, where O₂ production increases until a peak optimum temperature, then begins decreasing as enzymes denature

Compare and contrast the structure of mitochondria and chloroplasts

Mitochondria

• Matrix 

• Protein pumps along inner membrane 

• Granum for extra SA

Chloroplast

• Stroma

• Photosystems along thylakoid membrane

• Cristae for extra SA

Both

• Double Membrane

• 70s Ribosomes and circular double-stranded DNA

• Method of extra SA

• Produce ATP

Compare and contrast the substrates used and produced made in mitochondria and chloroplasts

Mitochondria

• Substrates: Glucose, O₂, ADP

• Product: CO₂ + H₂, ATP

Chloroplast

• Substrates: CO₂, H₂, ADP

• Product: Glucose, ATP, O₂

Why does O₂ production plateau as light intensity increases?

Although there’s more light available for more electrons to be excited, driving more reactions, other limiting factors cause photosynthetic rate to plateau: CO₂ will likely run out, or RuBisCO enzymes become oversaturated

Where is the location of the light-dependent and independent reactions?

Light dependent reactions are in the thylakoid membrane, photosystems are embedded along it. The concentration gradient created in the reactions are located in the thylakoid lumen.

Light independent reactions (the calvin cycle) are in the stroma.

Compare and Contrast Oxidative Phosphorylation and Photophosphorylation

Oxidative:

• Inner mitochondrial membrane (and along cristae)

• Energy from NADH and FADH₂ (via Krebs cycle)

• O₂ is final electron acceptor

• H+ concentration high in matrix

• H₂O is product

Photophosphorylation:

• Thylakoid Membrane

• Energy from photons

• NADP+ is final electron acceptor, reduces to NADPH

• H+ concentration high in stroma

• O₂ is product

Both:

• Involve ATP synthase and chemiosmosis

• Final product is ATP

Compare and Contrast the Krebs Cycle and the Calvin Cycle

Krebs:

• Begins with pyruvate

• Performed in the mitochondrial matrix

• Produces NADH, FADH₂ and ATP

• CO₂ waste product

Calvin:

• Begins with CO₂ as a reactant

• Performed in the chloroplast stroma

• Produces NADP+, ADP, glucose 

Both:

• Cyclical metabolic pathways

• Both are catalyzed by enzymes

• Is related to the Electron Transport Chain (Krebs feeds it, Calvin gets replenished by it)