BISC 1303 - Chapter 8: Photosynthesis

Organisms can be

Autotrophs and heterotrophs

-how an organism gets its carbon

Autotrophs

an organism that gets carbon from inorganic sources

Heterotrophs

an organism that consumes organic substances or other organisms for carbon

E.G. - animals, fungi, most protists, most bacteria

Photoautotroph

an organism capable of producing its own organic compounds from inorganic carbon and energy from sunlight

E.G. - plants, some bacteria, some protists

Photosynthesis

CO₂ + H₂O + light (energy) → C₆H₁₂O₆ (glucose) + O₂

2 parts:

light-dependent reactions (LDR): H₂O + photons → O₂ + ATP + NADPH

light-independent reactions (LIR): CO₂ + ATP + NADPH → C6H12O6 (glucose)

Electromagnetic Radiation

visible light is a type of electromagnetic radiation, has specific wavelengths, different colors of visible light have different wavelengths

light has a wave nature and a particle nature: particles called photons (carry energy)

Pigment

a molecule that is capable of absorbing certain wavelengths of light and reflecting others

E.G. - chlorophyll, beta-carotene

Chlorophyll

harness energy from photons for photosynthesis

Beta-carotene (a carotenoid)

dispose of excess energy

A Photosystem

hundreds of chlorophylls and carotenoids are arranged into “light-harvesting complexes”

Step 1 of LDR

photons strike pigments in photosystem 2, excite e- of chlorophylls to a higher energy state

Step 2 of LDR

energy of excited e- is passed from one chlorophyll to another until it reaches reaction center (RC) chlorophyll

Step 3 of LDR

RC chlorophyll delivers high energy e- through an ETC

RC chlorophyll replaces lost e- through water ionization

Step 4 of LDR

high energy e- passed through series of carriers, lowered energy with each passing

energy used to pump H+ across membrane

creates proton gradient → used by ATP synthase to generate ATP

Step 5 of LDR

low energy e- arrive at photosystem 1’s RC chlorophyll

Step 6 of LDR

photons excite of e- in photosystem 1 chlorophyll, energy moves to RC chlorophyll

Step 7 of LDR

higher energy e- are used to reduce NADP+ to NADPH (electron carriers)

Light-Independent Reactions

take place in the “stroma” - cytoplasm of chloroplasts

steps 1-3 are the Calvin Cycle

Step 1 of LIR

an enzyme called RuBisCO performs carbon fixation by attached CO₂ to Ribulose Biphosphate (5-carbon), creating 3-phosphoglyceric acid aka 3-PGA (3-C x2)

Carbon fixation

the process of converting inorganic CO₂ gas into organic compounds

Step 2 of LIR

ATP and NADPH+ used to reduce 3-PGA to GLyceraldehyde-3-phosphate aka G3P (3-C)

Step 3 of LIR

ATP is used to regenerate Ribulose Bisphosphate from some G3P

Step 4 of LIR

*in cytoplasm G3P is used to build amino acids, lipids and sugars

converted to glucose through glycolysis

same enzymes from cellular respiration, working in reverse