PhotosynthesisChapter 8

Where does all of our energy come from?

How do plants get food? Photosynthesis- Process by which green

plants or organism with chlorophyll convert light energy into chemical energy in the bonds of carbohydrates

Autotrophic – Can transfer energy to produce food – can synthesize food

Heterotrophic – Must obtain energy from preformed food – gotta eat food!

What is needed for autotrophs to photosynthesize? Carbon dioxide (atmospheric) Water (in the soil or air) Sunlight (or artificial light source) Chlorophyll (present in leaves and

sometimes stems) Enzymes (to regulate the rate of the

reaction)

Who helped us figure this out? Van Helmont (1600s)—experiment with

potted plant to determine what a plant uses to grow.

Conclusion: mass of tree comes from water

He got it only half right: there is also CO2!

Who helped us figure this out? Joseph Priestly (1770s) experiment with

candle, sprig of mint and a jar

Who helped us figure this out? Ingen-Housz—repeated Priestley’s

experiment but put the sprig of mint in the dark

Candle would not burn for as long in the dark, so LIGHT is important for plants

Putting it all together The experiments performed by van

Helmont, Priestley, and Ingenhousz led to work by other scientists who finally discovered that in the presence of light, plants transform carbon dioxide and water into carbohydrates, and they also release oxygen

THE FORMULA CO2 + 6H20 + light C6H12O6

chlorophyll

enzymes

Seems simple, huh? Takes EIGHTY different chemical reactions

from start to finish

Where does Photosynthesis happen? In the chloroplasts!

Chloroplasts are oval structure consisting of stacks called grana (photosynthetic membranes) and a liquid called stroma.

Chlorophyll is found in the stacked grana

What are the pigments Two main ones

Chlorophyll a and Chlorophyll b Other accessory pigments

Absorb other wavelengths of light Carotenoids, xylophylls, anthocyanin

Light White light is a mixture of all the

wavelengths of visible light Color is all about the reflection of light

Absorption and Reflection Chlorophyll absorbs red/blue light the

best It mostly reflects green light THIS IS WHY PLANTS ARE GREEN!

Why do the leaves change? When [water] is low, plant pigments

break down Mostly green pigment, that goes away

first

ATP When chlorophyll absorbs light, it is

absorbing energy It stores it in the bonds of Adenosine

Triphosphate (ATP)

ATP Made of

Adenine (nitrogenous base)

Ribose (5 carbon sugar)

3 phosphate groups

This is temporary storage

ATP

ADP

AMP

ATP Is constantly made and broken down

Think of ATP like cash in your pocket Glucose like an ATM card Glycogen like your savings account Lipids like a Savings Bond

ATP v ADP

2 Main steps of PS1. Light reactions—occurs only in the

presence of light Occurs in the grana (thylakoids) of the

chloroplasts Also known as Photolysis because light is

used to split water molecules into hydrogen and oxygen

2 Main steps of PS1. Dark Reactions—can occur in light or

darkness. Follows light reactions Occurs in the stroma of the chloroplasts Also known as Carbon fixation because

CO2 will get “fixed up” with the hydrogens and energy from the light reaction

Photosynthesis Overview

Light Reactions1. The light energy strikes the leaf, passes

into the leaf and hits a chloroplast inside an individual cell.

2. The light energy, upon entering the chloroplasts, is captured by the chlorophyll inside a granum.

3. Inside the grana some of the energy is used to split water into hydrogen and oxygen.

4. The oxygen is released into the air.

Light-dependent reactions

1. Light energy excites chlorophyll’s electrons in Photosystem II.

2. The high-energy electrons are passed along to the Electron Transport Chain, then Photosystem I.

3. Water is broken into 2 electrons, 2 H+ ions, and 1 oxygen atom.

This is the part of the equation where water is used

and oxygen is created!

6CO + 6H2O ------ C6H12O6 + O2

The electrons replace those lost by chlorophyll, the oxygen is stored until it is released, and the H+ ions are pumped from the stroma into the inner thylakoid space.

IN THE THYLAKOID MEMBRANE

4. In Photosystem I, the high energy electrons are used to add H+ ions to the molecule NADP+, creating NADPH (which is now storing the energy of those electrons).

5. Finally, all of the H+ ions in the inner thylakoid space start to diffuse out through a carrier protein known as ATP synthase. As they pass through, their energy is used to add a phosphate to ADP, creating ATP.

Review of Light-Dependent reactions in the photosystems of the thylakoid membrane:

Reactants…light energy, water

Products…NADPH, ATP (both energy carriers), Oxygen (given off)

Light energy has now been captured in chemical bonds.

The Dark Reactions5. The hydrogen is taken to the stroma along with

the grana's remaining light energy.6. Carbon dioxide enters the leaf and passes into the

chloroplast.7. In the stroma the remaining light energy is used to

combine hydrogen and carbon dioxide to make carbohydrates. This occurs during the Calvin Cycle

8. The energy -rich carbohydrates are carried to the plant's cells.

9. The energy- rich carbohydrates are used by the cells to drive the plant's life processes.

Light-dependent reactions

1. Light energy excites chlorophyll’s electrons in Photosystem II.

2. The high-energy electrons are passed along to the Electron Transport Chain, then Photosystem I.

3. Water is broken into 2 electrons, 2 H+ ions, and 1 oxygen atom.

This is the part of the equation where water is used

and oxygen is created!

6CO + 6H2O ------ C6H12O6 + O2

The electrons replace those lost by chlorophyll, the oxygen is stored until it is released, and the H+ ions are pumped from the stroma into the inner thylakoid space.

IN THE THYLAKOID MEMBRANE

4. In Photosystem I, the high energy electrons are used to add H+ ions to the molecule NADP+, creating NADPH (which is now storing the energy of those electrons).

5. Finally, all of the H+ ions in the inner thylakoid space start to diffuse out through a carrier protein known as ATP synthase. As they pass through, their energy is used to add a phosphate to ADP, creating ATP.

Review of Light-Dependent reactions in the photosystems of the thylakoid membrane:

Reactants…light energy, water

Products…NADPH, ATP (both energy carriers), Oxygen (given off)

Light energy has now been captured in chemical bonds.