BSCI 170: Photosynthesis

Exam 2 material

What did Jan Baptist Van Helmont discover

the mass of a tree comes from water and carbon in the air

What’s the role of chloroplasts?

to capture light energy from the sun and covert it to chemical energy

Autotrophs & photautotrophs

self feeders

• can produce organic molecules from inorganic forms of C (like CO_² in air)

photoautortrophs: use light energy to drive this process ^

• they are also produces of biosphere

Heterotrophs

• feed on others; cannot make their own food (C-C bonds)

Where does photosynthesis occur?

thylakoids of the chloroplast

Photosynthesis equation

• Anabolic

• light + 6CO_² + 6H₂O —> C₆H₁₂O₂ + 6O₂

electrons (& H_⁺⁾ from ___ are added to ___

• H₂O, CO₂

H₂O is ____ to O_2.

oxidized

CO_{2 is} ____ to a carbohydrate (CH₂O)_n

reduced

Is photosynthesis exergonic or endergonic?

endergonic; electrons gain energy in the process

2 stages of photosynthesis

• photo: light dependent reactions (1st)

• synthesis: light independent reactions (2nd)

What is the purpose of the light independent reactions (synthesis)?

to reduce inorganic atmospheric carbon (CO₂) into carbohydrate (CH₂O)_n

What does the Calvin cycle use to convert inorganic C into sugar?

ATP and NADPH (same as NADH, just with phosphate)

What are the 3 steps of the Calvin Cycle?

1. Carbon fixation

2. Reduction

3. Recognition of CO₂ acceptor

Carbon Fixation (Calvin cycle)

• 1st step

• 3 RuBP+ 3 CO₂ ——> 6 3-PG

• ribulose biphoshate (rubisco) + CO₂ come in to create 3 carbon phosphoglycerate

What is Rubisco

• enzyme used during crabon fixation

• most abundant protein on earth (> 40 lbs per person)

Reduction of carbon (Calvin cycle)

• 2nd step

• products of carbon fixation are phosphorylated & reduced

Input:

• ATP used to phosphorylates

• NADPH used to add H & high energy electron

Output:

• 1 G3P is removed

Recognition of CO₂ Acceptor (Calvin Cycle)

• 3rd step

• ATP atp is added to rearrange 3C G3P molecule back into 5C RuBP so that it is ready to accept more C

How many CO₂ are fixed in the Calvin cycle to make one 3-Carbon molecule (G3P)?

3

How many ATPs are consumed in the Calvin cycle to make one 3-Carbon molecule (G3P)?

9 ATPs

How many NADPH’s are consumed in the Calvin cycle to make one 3-Carbon molecule (G3P)?

6 NADPHs

How many ATPs are consumed for each carbon fixed into organic form in the Clavin cycle?

3 ATPs

How many NADPH’s are consumed for each carbon fixed into organic form in the Clavin cycle?

2 NADPH

What happens to the G3P?

• In starch synthesis: Energy storage

• In sucrose synthesis: energy transprot

• oxidation to make ATP in cytosol & mitochondria

Visible light spectrum

400-700 nm

List the 3 photosynthetic pigments

• chlorophylls: green

• carotenoids: yellow, orange

• phycobilin & phycoerythrin: red

What happens during the light dependent reactions?

Absorbed light energy is converted and stored transiently in the form of ATP & NADPH

What are the roles of NADPH & ATP

• ATP provides energy to drive anabolic reactions

• NADPH: electron carrier like NADH, provides electrons needed to reduce CO₂

Photosystem II

• functions first

• P680 pigment

• captures photon → excited proton goes through ETC —→ Phosphorylation

• replaces lost electron by oxidizing water (water splitting complex)

photophosphorylation

Excited electrons (from light) are used to add phosphate group to ADP to form ATP:

• exergonic electron transport drives H⁺ from stroma to thylakoid space

• —> H⁺ gradient stores energy

• —> H⁺ returning back to stroma through ATP synthase provides the free energy needed to drive phosphorylation of ADP (chemioosmosis)

After making ATP, how to electrons get enough energy to reduce NADPH⁺

• PS I energizes the electrons again by accepting an electron from the ETC, a photon excites it, and then gets captured away by NADP⁺

• NADP⁺ becomes NADPH (in stroma)

What happens to the ATP and NADPH produced by light-dependent reactions?

they’re used in the calvin cycle

Whats the problem with Rubisco?

• it can either add O₂ or CO₂ to RuBp

• reacting with oxygen defeats the purpose of photosynthesis

  • produces 1 3-C and 1 2-C compound

  • 2-C is toxic

Carbon fixation by C₄ Plants

• separate carbon fixation from where rubisco is

• C fixation happened 2x

• Rubisco avoids O₂

Carbon fixation by CAM plants

• fix carbon at night

• don’t have to open stromata & dry out during day

• can clear out O₂ so that it does not come in contact with rubisco

Alternate Carbon fixation pathways (to limit O₂ in presence of rubisco)

• fix carbon 2x

• 1st enzyme uses only CO₂ as substate

• CO₂ subsequently released in presence of rubisco and fixed again

  • different place or time