PhotosynthesisPhotosynthesis

AP Biology Unit 4

Vocabulary Review

• What is an autotroph?– 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

CO2

H2O

NADPH

RuBP (R

uDP)

C6H

12O

6

0% 0% 0%0%0%

1. CO2

2. H2O

3. NADPH

4. RuBP (RuDP)

5. C6H12O6

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Carbohydrate-synthesizing reactions of photosynthesis directly require

Lig

ht

Pro

ducts

of the

light

...

Dar

knes

s

O2

and H

2O

Chlo

rophy

ll an

d CO2

0% 0% 0%0%0%

1. Light

2. Products of the light reactions

3. Darkness

4. O2 and H2O

5. Chlorophyll and CO2

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The end products of the light-dependent reactions of photosynthesis are

ADP, H

2O, N

ADPH

ADP, P

GAL, RuBP (.

..

ATP, C

O2, H

2O

ATP, N

ADPH, O2

CO2,

H+,

PGAL

0% 0% 0%0%0%

1. ADP, H2O, NADPH

2. ADP, PGAL, RuBP (RuDP)

3. ATP, CO2, H2O

4. ATP, NADPH, O2

5. CO2, H+, PGAL

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