Photosynthesis

Chemical Energy Where do you get your energy? You eat food! Carbohydrates and lipids have the most energy This energy is only useable after it is broken down Adenosine triphosphate (ATP) is a molecule that carries energy from the breakdown of food When ATP is broken down to ADP (adenosine diphosphate), energy is released

Energy Sources Heterotrophs Autotrophs (producers) Need to eat other organisms to gain energy Autotrophs (producers) Make their own food Chemoautotrophs- use chemical energy to make food Photoautotrophs- use energy from sunlight (through the process of photosynthesis)

Photosynthesis Overview Photosynthesis: the process through which energy from sunlight is converted to chemical energy Takes place in the chloroplasts Two basic processes: Light-dependent reactions capture energy from sunlight Light-independent reactions use the energy captured in light-dependent reactions to make sugars 6 CO2 + 6 H2O C6H12O6 + O2 Carbon dioxide + water light, enzymes sugar + oxygen

Chloroplast Grana: stacks of pancake-shaped thylakoid membranes Stroma: fluid surrounding the grana Chlorophyll: a molecule in the chloroplast that absorbs light energy Chlorophyll a and chlorophyll b Absorb mostly red and blue wavelengths of light; reflect green wavelengths This is how plants get their color!

Light-Dependent Reactions Energy is captured and transferred in the thylakoid membranes by two groups of molecules called photosystems Energy is transferred to electrons This is used to make energy-carrying molecules ATP and NADPH

Photosystem II and Electron Transport Energy absorbed from sunlight Chlorophyll and other molecules absorb energy Energy is transferred to electrons High energy electrons enter the electron transport chain (series of proteins in thylakoid membrane) Water molecules split Oxygen, hydrogen ions (H+), and electrons separated Oxygen is released as waste H+ ions transported Energy from electron transport chain is used to pump H+ ions from outside to inside the thylakoid membrane against a concentration gradient (active transport) H+ ions build up inside thylakoid membrane Electrons move on to photosystem I

Photosystem II

Photosystem I and Energy-Carrying Molecules Energy is absorbed from sunlight and energizes electrons NADPH (an energy molecule) is produced by electrons being added to NADP+ molecules H+ ions diffuse through a protein channel called ATP synthase in the thylakoid membrane Concentration is higher inside than outside (chemiosmotic gradient) As H+ ions flow through ATP synthase, ATP is formed by phosphorylation of ADP molecules

Photosystems I and II

Summary of Light-Dependent Reactions Energy is captured and transferred to electrons that enter the electron transport chain Water molecules are broken down into oxygen, H+ ions, and electrons Electrons provide energy for H+ ion transport and are added to NADP+ to form NADPH H+ ions flowing through ATP synthase make ATP

Products of Light-Dependent Reactions Oxygen is released as waste Small amounts of NADPH and ATP

Light-Independent Reactions Do not need sunlight Uses energy from light-dependent reactions Take place inside the stroma (fluid surrounding thylakoid membranes) Also called the Calvin Cycle Use carbon dioxide and ATP and NADPH molecules to build simple sugars

Calvin Cycle Carbon dioxide is added to 5-carbon molecules inside the Calvin cycle to form 6-carbon molecules Energy from light-dependent reactions is used to split 6-carbon molecules, forming two 3-carbon molecules Out of every 3 CO2 molecules added to the Calvin cycle, one high-energy 3-carbon molecule exits After two 3-carbon molecules have exited, they are bonded together to build a 6-carbon sugar 3-carbon molecules left in the cycle are changed back into 5-carbon molecules using ATP

Calvin Cycle Summary Calvin cycle turns 6 times Produces 6-carbon simple sugars Typically glucose molecules Most important enzyme: ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) Most abundant enzyme on earth

Photosynthesis

Photosynthesis Summary Light-dependent reactions Photosystem II Energy captured and transferred to electrons Water molecules split, releasing oxygen H+ ions build up inside thylakoid Photosystem I Energy absorbed, energizing electrons NADPH produced ATP produced by flow of H+ ions through ATP synthase Light-independent reactions Calvin cycle CO2 is broken down and ATP and NADPH is used to build 6-carbon sugar Uses the enzyme RuBisCO