PhotosynthesisPhotosynthesis

I. How do living things get I. How do living things get the energy they need to the energy they need to live? live?

Photosynthesis:Photosynthesis: The process by which The process by which plants (plants (autotrophsautotrophs) and some other ) and some other organisms use sunlight to synthesize organisms use sunlight to synthesize foods from carbon dioxide and water. It foods from carbon dioxide and water. It involves the green pigment chlorophyll involves the green pigment chlorophyll and generates oxygen as a byproduct.and generates oxygen as a byproduct.

What does autotroph mean?What does autotroph mean?

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I. How do living things get I. How do living things get the energy they need to the energy they need to live? live?

Reactants Reactants Products Products

COCO22 + H + H22O + light energy O + light energy C C66HH1212OO66 + O + O22

_______ _____ _________ ________ ____________ _____ _________ ________ _____

II. Where does all of this happen?II. Where does all of this happen?

II. Where does all this II. Where does all this happen?happen? Inside plant cells there Inside plant cells there

are organelles called are organelles called chloroplastschloroplasts. Inside the . Inside the chloroplast there are chloroplast there are stacks of membranes stacks of membranes called grana. called grana.

Each granum contains Each granum contains several layers of thylakoid several layers of thylakoid membranes. membranes.

II. Where does all this II. Where does all this happen?happen?

Each thylakoid Each thylakoid membrane is packed membrane is packed with chlorophyll (a green with chlorophyll (a green pigment) and proteins pigment) and proteins that carry out that carry out photosynthesis.photosynthesis.

The space between The space between grana is called the grana is called the stroma.stroma.

III. So how does it III. So how does it actually happen?actually happen?

There are two stages of photosynthesis:There are two stages of photosynthesis:I.I. Light-dependent reactions (In thylakoid Light-dependent reactions (In thylakoid

membrane)membrane)

1.1. Light strikes the first Light strikes the first photosystem photosystem (a group of (a group of proteins), which causes some electrons to proteins), which causes some electrons to get “excited” (jump to a higher energy level). get “excited” (jump to a higher energy level).

2. Since excited electrons are unstable, they 2. Since excited electrons are unstable, they almost immediately transfer the energy to a almost immediately transfer the energy to a nearby molecule.nearby molecule.

A photosystem:A photosystem:

III. So how does it III. So how does it actually happen?actually happen?

3. The excited electrons are replaced by 3. The excited electrons are replaced by splitting a water molecule into hydrogen splitting a water molecule into hydrogen ions (Hions (H++) and oxygen (O) and oxygen (O22).).

4. Oxygen diffuses out of the chloroplast as 4. Oxygen diffuses out of the chloroplast as waste.waste.

5. The excited electrons travel down a 5. The excited electrons travel down a chain of membrane proteins called the chain of membrane proteins called the electron transport chainelectron transport chain (ETC). (ETC).

III. So how does it III. So how does it actually happen?actually happen?

6. The energy given off by this process is used to 6. The energy given off by this process is used to pump hydrogen (Hpump hydrogen (H++) ions across the ) ions across the membrane.membrane.

7. More excited electrons from the second 7. More excited electrons from the second photosystem are added to NADPphotosystem are added to NADP++ to make to make NADPH.NADPH.

8. The H8. The H++ concentration gradient is used to power concentration gradient is used to power a membrane protein called a membrane protein called ATP synthaseATP synthase, , which produces ATP.which produces ATP.

Let’s watch a movie of it.Let’s watch a movie of it.

Light reactions

Cool – ATP synthase

In summary:In summary:

Light + HLight + H22O O

ATP + NADPH + OATP + NADPH + O22

III. So how does it III. So how does it actually happen?actually happen?

II. Light-independent reactions, aka the Calvin II. Light-independent reactions, aka the Calvin Cycle (in stroma)Cycle (in stroma)

1.1. COCO22 is added to a 5-carbon sugar called is added to a 5-carbon sugar called

ribulose bisphosphateribulose bisphosphate (RuBP). The enzyme (RuBP). The enzyme that catalyzes this reaction is called rubisco. that catalyzes this reaction is called rubisco.

2. The 6-carbon sugar splits in half to form two 2. The 6-carbon sugar splits in half to form two molecules of 3-phosphoglycerate (3-PGA).molecules of 3-phosphoglycerate (3-PGA).

III. So how does it III. So how does it actually happen?actually happen?

3. ATP and NADPH from the light reactions 3. ATP and NADPH from the light reactions provide energy and electrons to convert provide energy and electrons to convert 3-PGA into glyceraldehyde 3-phosphate 3-PGA into glyceraldehyde 3-phosphate (G3P).(G3P).

4. After three turns of the cycle, six 4. After three turns of the cycle, six molecules of G3P are made. molecules of G3P are made.

III. So how does it III. So how does it actually happen?actually happen?

5. Out of these six G3P molecules, five of 5. Out of these six G3P molecules, five of them enter a process that makes more them enter a process that makes more RuBP to continue the cycle.RuBP to continue the cycle.

6. Only one of the six G3P molecule exits 6. Only one of the six G3P molecule exits the cycle. This G3P can be converted the cycle. This G3P can be converted into into glucoseglucose or other sugars that the plant or other sugars that the plant needs.needs.