Photosynthesis Trapping Light Energy to Build Carbohydrates

Which organisms are photosynthetic? Photoautotrophs Plants Algae Some bacteria

What is photosynthesis?

Process by which plants use light energy to build carbohydrates (food). Chemical equation: light + chlorophyll 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 carbon dioxide + water yields carbohydrate + oxygen gas in the presence of light & chlorophyll

Why are plants mostly green? Chlorophyll Green pigment Located in chloroplasts Are they always green? No! Think about the leaves in the fall – what happens to them? Chlorophyll is the most dominant pigment so we see it most, but when it dies out the other pigments show through

Chloroplast Structure Chloroplast Membranes Two of them Inner and outer Both composed of a lipid bilayer Thylakoids Disks of membranes Contain chlorophyll Grana Stacks of thylakoids Stroma Fluid inside the chloroplast

A closer look at where it all takes place…

Photosynthesis Reactions Light Reactions (aka light dependent reactions) Needs light Takes place in the thylakoids Calvin Cycle (aka light independent reactions) Does not need light Takes place in the stroma

Light Reactions Trap Light Energy GOAL: To make NADPH & ATP for the Calvin Cycle BY - PRODUCT: Oxygen gas

1: Energy is Captured from Sunlight Chlorophyll, located in the thylakoid membranes, absorbs light Light energy excites chlorophyll electrons Excited electrons enter the electron transport chain (ETC)

2: Photolysis Photolysis: An enzyme splits water molecules 2H 2 O  O 2 + 4H + + 4e - Oxygen is released by chloroplasts as a waste product! Oxygen gas supplies nearly all the oxygen in the atmosphere

Stage 2B: Electron Transport Chain Excited electrons from chlorophyll and H + from split water travel through the thylakoids and move to NADP + Nicotinamide dinucleotide phosphate (you do not need to know this name) Coenzyme: act as carriers in many biological processes (“the shuttle bus”) NADPH is formed An electron carrier that moves electrons between the rxns of photo. e - + H + + NADP +  NADPH Travels to the Calvin Cycle (Stage 3) ATP is formed and travels to the Calvin Cycle (stage 3) to provide energy

Light reaction re Takes place in the thylakoid membrane Chlorophyll absorbs light Water is split Gives off oxygen as by product NADPH and ATP are produced and sent to the Calvin Cycle

Stage 3: Calvin Cycle The light independent Reactions – do not have to take place in light GOAL: To make simple sugars

Carbon Fixation Calvin Cycle consists of carbon fixation CO 2 from the air is converted into a form that is usable by living things “fixed” into 6 carbon sugars Adding hydrogens (that’s why it needs NADPH) Eventually become glucose and other sugars

Calvin Cycle Reaction Basics Reactants 5-carbon sugar molecule CO 2 H from NADPH Products Simple sugars Ex: glucose Used to make other carbohydrates ATP from Light Reactions powers the Calvin Cycle

The Overall Net Result of Photosynthesis The overall result of photosynthesis is that for every 6 carbon dioxide molecules (plus 6 water) that a plant uses, 1 glucose molecule (plus 6 oxygen) is created.

ELECTRON CARRIERS

Discussion Questions Where do the light reactions take place? What are the products of the light reactions? Where do the chlorophyll replacement electrons come from? Where do the dark reactions take place? Where does the NADPH in the Calvin Cycle come from? Where does the ATP in the Calvin Cycle come from? What are the products of the Calvin Cycle? What are chemoautotrophs?