Photosynthesis AP Biology Unit 4

Vocabulary Review What is an autotroph? What is a heterotroph? Make their own energy What is a heterotroph? Aren’t able to make own energy– have to get it from another source

Photosynthetic Organisms Photosynthesis is one way to make your own energy Who does it? Plants, algae, some other protists, some prokaryotes (ex. cyanobacteria)

Location of Photosynthesis Photosynthesis occurs in the chloroplasts Organelles that contain the pigment chlorophyll and other pigments Openings in the leaf (stomata) allow CO2 (reactant) to enter and O2 (product) to leave

Equation for Photosynthesis 6 CO2 + 6 H2O + ENERGY  C6H12O6 + 6 O2 In terms of reactants and products, it is the reverse of cellular respiration, but process is different.

Stages of Photosynthesis Light Reactions Captures light energy to produce ATP and NADPH Calvin Cycle (“Dark” Reactions) Uses ATP and NADPH to “fix” carbon into a carbohydrate “fix” means to incorporate the carbon into a compound

Light Reactions Main point = convert sunlight energy into chemical energy (ATP and NADPH) Sunlight can be divided into different wavelengths of light– only some are useful in photosynthesis

Light Reactions Photosynthetic pigments in the chloroplast absorb different wavelengths Chlorophyll a Chlorophyll b Carotenoids

Question… Why are most plants green? The chlorophyll and carotenoid molecules absorb the blue-violet and red light and use it to perform photosynthesis They reflect green light – colors we see are the reflected wavelengths.

Light Reactions: Photosystems Photosystems are the complex of pigments that receives the sunlight to begin the process Photosystems absorb energy to excite an electron in chlorophyll There are 2 different photosystems in chlorophyll – I and II.

Light Reactions: Noncyclic Flow Step 1: Light excites electrons in photosystems I and II Step 2: Excited electrons are passed down an electron transport chain – generates ATP and NADPH

Noncyclic Flow Excited electron from Photosystem II fills the space left in Photosystem I Water is split (photolysis) to provide an electron to fill the space left in Photosystem II

Cyclic Electron Flow Under certain circumstances, the excited electrons will only flow through Photosystem I (not II)

Cyclic Electron Flow Generates ATP Doesn’t generate NADPH Runs because the Calvin Cycle needs more ATP than NADPH May shift from noncyclic to cyclic flow when NADPH levels are high

Electron Transport Chain very similar to cellular respiration An H+ concentration gradient across the thylakoid membrane drives ATP synthesis

Calvin Cycle Also called the Dark Reactions, Non-light Dependent Reactions, or the Calvin Benson Cycle Uses the ATP and NADPH produced in the light reactions to produce glucose using CO2

Calvin Cycle: Rubisco Rubisco is the enzyme that first “traps” CO2 into the cycle Through a series of reactions, CO2 is used to eventually make glucose

Calvin Cycle CO2 only contributes 1 carbon  6 “turns” of the calvin cycle are needed to produce one glucose. RuBP also has to be regenerated so that cycle can continue

The O2 released during photosynthesis comes from NADPH RuBP (RuDP) C6H12O6 15

Carbohydrate-synthesizing reactions of photosynthesis directly require Light Products of the light reactions Darkness O2 and H2O Chlorophyll and CO2 15

The end products of the light-dependent reactions of photosynthesis are 15 ADP, H2O, NADPH ADP, PGAL, RuBP (RuDP) ATP, CO2, H2O ATP, NADPH, O2 CO2, H+, PGAL