Photosynthesis: How Do Organisms Get Energy From

the Sun?

Chapter 7

Johann Baptista van Helmont• Christian, Chemist, Physician, Philosopher• Recognized gas as a form of matter• Aristotle claimed plants extract materials

from the soil• Planted a 5 lb. willow tree in 200 lbs. of soil• Gave only water for 5 years• Tree weighed 169 lbs; soil lost 2 oz.• But, he thought the weight came from the

water.

• Hydroponics

• Microscopists discover stomata in plant leaves

Joseph Priestly

Put a candle in a bell jar →

Candle goes out

Put a mouse in a bell jar →

Mouse dies

Put a plant and a mouse in a bell jar →

Mouse lives

Photosynthesis 6 CO2 + 6 H2O → C6H12O6 + 6 O2

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

Cellular respiration

Plants, algae and many photosynthetic bacteria can harness the sun’s energy and turn it into sugar.

Autotrophs

What about proteins?

Proteins contain C,H,O, and N.

Nitrogen and other trace elements are taken in from the soil.

Fertilizer - 10:5:5

Nitrogen : Phosphorus : Potassium

Do plants always “breathe out “oxygen?

• When plants break down glucose for energy they do it in the same manner as other organisms, and then they give off CO2.

• They cannot give off oxygen if there is no light.

Photosynthesis is a two “step” process

one set of reactions requires light

the second set of reactions does not – light independent (“dark”) reactions

The light reactions are called photophosphorylation because they use light to add a phosphate group to ADP to make ATP.

Light

It’s a particle

discrete units – photons

travels in a straight line

No, it’s a wave

wavelength – can we see it?

What color is it?

ROY G BIV

Why do plants appear green?

Pigments – have electrons that can be more easily excited.

Chlorophylls – alpha and beta absorb most of the photons in plants.

Carotenoids – yellow and orange pigments that transfer energy to chlorophyll

Phycobilins – red and blue pigments in red algae and cyanobacteria

Very few chlorophyll molecules actually photosynthesize.

Most chlorophyll molecules along with the carotenoids form an energy gathering system called an antenna complex.

Energy is transferred to a photochemical reaction center.

So, in a nutshell:

1. Chlorophyll and carotenoids absorb light.

2. The energy is transferred to the reaction center.

3. The energy splits oxygen from water and forms chemical bonds.

Plants have two distinct sets of reactions:

Photosystem I

Photosystem II

Photosystem I absorbs light of 700nm best

(P700)

Photosystem II absorbs light of 680 nm (also into the blue and violet range)

(P680)

What does this have to do with me?

Plants grown indoors need a range of light colors to grow well.

Fluorescent bulbs have very little 700 nm light.

Incandescent bulbs have very little 680 nm light.

Use both for maximum growth.

The action takes place across the thylakoid membrane.

Chlorophyll is found in the thylakoid membrane in association with proteins.

When P700 absorbs light, electrons are excited, move to outer orbitals, and P700 becomes a good electron donor.

When it gives up its electron, it becomes oxidized.

The electron can then travel one of two paths:

Cyclic photophosphorylation – electron is given to an electron transport chain, ATP is formed and the electron is given back to P700

Noncyclic Photophosphorylation – electron is transferred to NADP+ → NADPH.

This provides the reducing power for the formation of glucose.

But now P700 is left oxidized. Where do we get the electrons to reduce it again?

From Photosystem II !

Photosystem II uses P680

The electrons from P 680 go into their own electron transport chain where ATP is produced by noncyclic photophosphorylation. The final electron acceptor is P700.

But now P680 is oxidized. Where can it get electrons?

It gets them by splitting water and releasing oxygen.

Water H+, electrons and O2

Until cyanobacteria figured out how to do this, there was very little oxygen in the Earth’s atmosphere.

Now we have ATP and NADPH formed →

The light-independent reactions which make glucose.

Glucose is a more stable energy storage molecule than ATP.

In the stroma, the Calvin cycle takes the hydrogens from water, the carbon and oxygen from carbon dioxide and makes glucose.

Calvin Cycle

1. Capturing carbon

CO2 + ribulose biphosphate → 6 carbon compound → 2 3-carbon molecules

This is catalyzed by an enzyme called ribulose biphosphate carboxylase or Rubisco

This is a very slow reaction, so plants produce a lot of it.

It is probably the most abundant protein on the planet!!

2. Making sugar

The end product of the Calvin Cycle is a three carbon compound called glyceraldehyde phosphate.

Some enters the cytosol of the cell, where it can be turned into glucose, fats or amino acids.

3. Regenerating ribulose biphosphate

Some remains in the Calvin cycle and is used to make more ribulose biphosphate.

It takes 18 molecules of ATP and 12 molecules of NADPH to make one molecule of glucose.

What factors affect photosynthesis?

• Wavelength of light

• Intensity of the light

• Length of the day

• Length of the growing season

• Pollution

• Other taller vegetation

• Availability of carbon dioxide and water

Balance between CO2 and H2O

• If the stomata are open all the time, the plant could dehydrate.

• If the stomata are closed all the time, CO2 can’t get in and O2 can’t leave.

• Hot, dry conditions → photorespiration, wastes about half of carbohydrate produced.

Some plants make a 4 carbon molecule instead of the 3 carbon phosphoglycerate.

This is oxaloacetic acid → 3-carbon molecule → glucose production and CO2 which goes back to the Calvin Cycle.

These plants are called C4 plants.

Corn and sugar cane survive well in hot, dry climates.

The other plants are called C3 plants.

Desert plantsCacti use a modified C4 pathway callled

crassulacean acid metabolism or CAM. These plants open the stomata only at night and store CO2 in a four carbon molecule for use the next day.

Requires a lot of energy, so growth rates are very slow.