Photosynthesis

Ecological importance

• Life made of carbon based molecules 

• Source of C is CO2

• CO2→ C-based macromolecules= carbon fixed

• Only autotrophic organisms do carbon fixation 

Photoautotrophs:

• Light self feeders 

• Use sunlight E to make organic molecules from CO2

• Ex: plants, cyanobacteria, etc… 

Photons

• particles of light E 

How does light behave?

• as particle and wave 

what is the relationship between a wave e- and wavelength

inversely proportional

why type of light to bio systems use and why?

• visible light because it contains enough energy to be useful but not too much energy that it breaks molecules 

Effects of photons on electrons 

• Photons are E that can be absorbed by e—> energized e-

• Energized e- shift from lower E orbital to higher E orbital 

Trick plants can do

• capture photons and conv. To chemical energy by absorbing energy (energizing e-) 

When have an energized e- 2 outcomes:

1. Return to lower orbital (ground state), emit heat, emit light (fluorescence) 

2. Leave atom, captured by acceptor, occurs in photo synthesis (redox reaction) 

Plant organization: 

• Leaves→ main site of photosynthesis

• Chloroplasts: primary eukaryotic photosynthetic organelle 

Chloroplast structure:

• outer membrane contains the whole structure  

• Intermembrane space: space between inner and outer membrane 

• Innermemberane 

• Stroma: carbon fixation reactions happen here, calvin cycle occurs here, inside the inner membrane

• Thylakoid: stacked, coiled, stretched back and forth to max surface area, have enzymes involved in light dependent reactions

• Thylakoid space

Photosynthetic pigment 

• Captures light energy for photosynthesis

• Embedded in thylakoid membrane

• Ex: chlorophyll a (most common one)

pigment

• substance that absorbs visible light

Overview of photosynthesis: 

6CO2 + 6H2O + E—> C6H12O6 + 6O2 

• Anabolic

• endergonic , E input from sunlight  

Photosynthetic redox reactions: 

• H2O oxidized, CO2 reduced 

• C is reduced 

• Oxygen is oxidized 

• E- transferred to Hydrogen 

2 stages to photosynthesis 

1. “Photo”: light dependent reactions 

2. “Synthesis”: calvin cycle 

light dependent reactions 

• Take light energy from sun and convert it to 2 diff things 

• Light E→ chemical E (ATP and NADPH) (both potential E)

• (NADPH is e- carrier) 

where do light dependent reactions occur?

• Occur in photosystems: capture photons and initiate a series of reactions 

Photosystems 1 and 2: 

• Protein complexes containing specific pigments 

• Located in thylakoid membrane 

Job of photosystems:

• capture light E, transfer excited e- 

• Linear e- flow: 

Both photosystems involved, same 3 things in each: 

• Boost e- 

• Use E in e-

• Replace e- 

• Many steps involve redox reactions 

where does H2O enter the plant?

through the roots

where does CO2 enter the plant?

through small openings called the stomata

where does O2 leave the plant?

through small openings also (stromata)

chloroplast

organelle that carries out photosynthesis

chlorophyll

pigment responsible for absorbing light E, it absorbs red and blue light and reflects green light, it occurs inside thylakoid

what does the light dependent reaction oxidize?

it oxidizes water into O2 gas

what does the light dependent reaction reduce?

NADP+ is reduced to NADPH

in light dependent reactions what does some of the energy transferred by light do?

it makes ATP from ADP and P (phosphate)

light dependent products

oxygen gas, ATP, and NADPH

light dependent reactants

H2O, NADP+, ADP + P, and light

what does the calvin cycle take in?

CO2

what does the calvin cycle reduce

CO2 to sugar (ex: glucose)

in the calvin cycle what is oxidized?

NADPH is oxidized back to NADP+

what energizes the calvin cycle?

ATP transitioning back to ADP + P

what are the reactants of the calvin cycle?

CO2, ATP, and NADPH.

what are the products of the calvin cycle?

sugar (ex: glucose), NADP+, ADP + P

what is the first step of photosynthesis

• Photon hits pigment in PSII 

• e- absorbs E 

• Excited e- transferred to ETC (redox reactions) 

• Basically captured energy and moved it to something else

step 2 of photosynthesis

• e- from PSII moves through electron transport chain (ETC) 

• Generates H+ gradient inside thylakoid space (active transport) 

• H_ diffuses thought ATP synthase (facilitated diffusion) 

• End result of ETC: transport energy from electron to ATP

step 3 of photosynthesis

• PSII in oxidized form is extremely strong oxidizing agent

• Oxidizes (takes e- from) H2O 

• E- transferred to PSII 

• PSII returns to reduced form 

• O2 released as by product 

• This is where atmospheric O2 comes from 

step 4 of photosynthesis

• Boost PSI e- 

• e- in PSI (photosystem 1) pigments absorbs photon, becomes energized 

• PSI oxidized, ETC reduced 

step 5 of photosynthesis

• Use E in e- (to make NADPH)

• e- transferred to NADPH+--> NADPH synthesized (catalyzed by NADPH+ reductase) 

step 6 of photosynthesis

• Replace PSI e- (with PSII e-) 

• After PSII e- travels down ETC, E has been used 

• Now low-E-e- donated to PSI→ replaces lost e- 

carbon fixation/ calvin cycle:

• Formation of carbohydrates from CO2

Calvin Cycle (C3 Cycle):

• Carbon fixation method used by most plants 

• Occurs in stroma 

• Does not directly require light 

• does require ATP and NADPH from light dependent reactions 

• 3 phases

1st phase of calvin cycle

• Carbon fixation: 

• CO2 (1C) bound to ribulose biphosphate (RuBP, 5C) 

• Catalyzed by rubisco (enzyme)

• Results in unstable 6C compound 

2nd phase of calvin cycle

• reduction: 

• ATP and NADPH from light- dependent reactions used 

• Results in 2 molecules of glyceraldehyde-3-phosphate (G3P, 3C each) 

3rd phase of calvin cycle

• RuBP Regeneration

• Two options G3P 

• Can removes 2 G3P, make glucose (6C) 

• Can use ATP to recycle back to RuBP