School of Sciences, Lautoka Campus BIO509 Botany Lecture 26: Photosynthesis

Autotrophs and Heterotrophs Autotrophs – organisms that use light energy from the sun to produce food; also called producers Examples: plants, blue-green algae, some prokaryotes. Heterotrophs – organisms that obtain energy from the foods they consume; also called consumers Examples: animals, fungus, some prokaryotes and protists.

Photosynthesis is a process by which plants and some other organisms use light energy to convert water and carbon dioxide (CO2) into oxygen and carbohydrates such glucose. Takes place inside chloroplasts. Photosynthesis is essentially the only mechanism of energy input into the living world.

Inputs and outputs Needed (Reactants): Light Water Carbon dioxide Given Off (Products): Sugars Oxygen

FORMULA OF PHOTOSYNTHESIS 6 CO2 + 6 H2O  C6H12O6 + 6 O2 Carbon dioxide + water  sugar + oxygen

Major steps in photosynthesis Light – dependent reactions (light reactions) Light –independent reactions (Dark reactions) Dark reactions use products from light reactions. Both reactions occur simultaneously.

Light reactions Involves the excitation of chlorophylls. Takes place inside the chloroplast.

Thylakoids – saclike photosynthetic membranes Grana – stacks of thylakoids Stroma – region outside thylakoid membranes

Light Absorption in the Chloroplasts Chloroplasts in plants cells absorb light energy from the sun during the light dependent reactions. Photosynthetic cells may have thousands of chloroplasts. Chlorophylls and accessory pigments such as caretonoids function to absorb light. Light-capturing pigments in the grana are organized into photosystems

Chlorophyll Absorbs red & blue light Reflects green light

Light dependent reactions The main function of the light reaction is to generate the NADPH and ATP required for carbon reduction. NADPH and ATP is generated through a series of reactions that constitute the photosynthetic electron transport chain.

Water is split, giving off oxygen and electrons. Light is absorbed by chlorophyll a which "excites" the electrons in the chlorophyll molecule. Electrons from split water are passed through a series of carriers along the electron transport chain. ATP is released. Some hydrogen from the split water molecules are used to reduce NADP (nicotin amide adenine dinucleotide phosphate.) to NADPH (reduced nicotin amide adenine dinucleotide phosphate)

ATP & NAPDH ATP and NAPH store energy in their bonds. ATP loses a Phosphate to become ADP, releasing energy. NAPH loses a hydrogen to become NADP+, releasing energy.

Photosynthetic electron transport chain (ETC) Key components Two large multimolecular pigment protein complexes known as Photosystem I and photosystem II. Two photosystems operate in series linked by a protein aggregate called the cytochrome complex.

The overall effect of ETC is to extract low energy electrons from water and using light energy trapped by chlorophyll to increase their energy level to finally produce strong reductant NADPH.

Electrons are picked up by Plastoquinone (PQ) (which also picks up H+ Electrons are picked up by Plastoquinone (PQ) (which also picks up H+ from stroma) and moves the electron to cytochrome b6f (while moving the H+ into the lumen). Plastocyanin (PC) moves the electron to PSI. Using energy from light this electron is moved to the ferredoxin complex where it combines with NADP and H+ to form NADPH.

Photophosporylation : light-driven phosphorylation or ATP synthesis ATP is synthesised in the CF0-CF1 complex (which contains ATP synthase) located at the end of each ETC complex. The proton gradient: higher concentration in the lumen compared to the lower concentration in the stroma:

At the end of the light-dependent reactions (ETC) we have: ATP and NADPH Oxygen is given off

Light- Independent Reactions This is also known as Calvin Cycle. C3 Cycle (the first stable product of carbon assimilation is a 3 carbon compound) Carbon Reduction Cycle (PCR)

Fixation of CO2 by Calvin Cycle using ATP and NADPH from light reaction occurs in three steps Carboxylation of CO2 acceptor Ribulose biphosphate (RuBP ) forming two molecules of 3- phosphoglycerate (3PGA). Reduction of 3PGA forming glyceraldehyde- 3-phosphate (G3P) which is carbohydrate. Regeneration of CO2 acceptor RuBP from G3P.

RuBP – ribulose 1,5 biphosphate PGA – 3-phosphoglyceric acid G3P – glyceraldehyde 3- phosphate

There are several steps in the Calvin Cycle that require enzymes. The most important enzyme in the Calvin Cycle is ribulose, 1,5 bisphosphate caroxylate-oxygenase (rubisco). Rubisco catalyses the first reaction in the Calvin Cycle

Importance of Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules The hydrocarbon skeleton of G3P can form Fatty acids and glycerol to make plant oils Glucose phosphate (simple sugar) Fructose (which with glucose = sucrose) Starch and cellulose Amino acids

What factors affect the process of photosynthesis?

Questions??