CHAPTER 6 - PHOTOSYNTHESIS

ENERGYEnergy is the ability to do work. All living things depend on energy. Energy comes in the form of light, heat,

electricity, or sound. Energy can be stored in chemical

compounds.

Autotrophs and Heterotrophs Autotrophs are organisms that make their own

food. Use light energy from the sun to produce food Plants are examples of autotrophs.

Heterotrophs obtain energy from the food they consume. Unable to directly use the sun’s energy.

Must consume other organisms either by ingesting (eating) them or decomposing them.

Animals are examples of heterotrophs.

The Photosynthesis Equation

Photosynthesis is the process whereby plants use the energy of sunlight to convert water and carbon dioxide into oxygen and high-energy carbohydrates.

Carbon dioxide +water sugar and oxygen In addition to water and carbon dioxide,

photosynthesis requires light and chlorophyll, a molecule in chloroplasts.

The Photosynthesis Equation

3 Requirements for Photosynthesis:

1.Sunlight2.Pigments3.Energy storing compounds

1. LIGHT

Light travels to the Earth in the form of sunlight We perceive sunlight as white light, but is really a

mixture of many different wavelengths of light Wavelengths of light that are visible to us are

known as the visible light spectrum

2. PIGMENTS

Pigments are light absorbing molecules that help plants gather the sun’s energy The main pigment found in plants is chlorophyll

Chlorophyll absorbs red and blue wavelengths of light, but it reflects green making the plant appear green

When the pigments absorb light they are also absorbing the energy in that light, producing more energy for the cell

INSIDE A CHLOROPLAST Photosynthesis takes place in chloroplasts. Chloroplasts contain saclike photosynthetic

membranes called thylakoids. Thylakoids contain clusters of chlorophyll and other

pigments and proteins known as photosystems that are able to capture sunlight.

Light dependent reactions take place here. Thylakoids are arranged in stacks known as grana. The region outside the thylakoid membrane is

called the stroma. Light independent (dark) reactions take place here.

3. Energy Storing Compounds Used to trap high energy electrons into

chemical bonds. Occurs in 2 ways:

1. Electron carrier NADP+ accepts a pair of high energy electrons and gets converted to NADPH.

2. AMP is converted to ADP which is then converted to ATP.

NOTE: The energy stored in these molecules is released by breaking chemical bonds to generate things the cell needs, like glucose!

Light and Dark Rxns Light-dependent reactions produce oxygen and

make energy storing compounds (ATP and NADPH). Occurs by converting ADP and NADP+ into ATP and

NADPH. These reactions REQUIRE light.

Light-independent (dark) reactions use the energy stored in NADPH and ATP to make glucose. Glucose is more stable and can store up to 100 times

more energy than NADPH and ATP. These reactions do not require light.

Light Dependent Rxns

Photosynthetic membranes of chloroplast (thylakoids) contain chlorophyll. This is where the light reactions occur.

The light reactions are divided into 4 processes: 1. Light absorption 2. Electron transport 3. Oxygen production 4. ATP production

1. Light Rxns - Light absorption

Green plants contain photosystems. Clusters of pigment molecules that absorb

energy from sunlight. High energy electrons move through the

photosystems and are then released to electron carriers.

2. Light Rxns - electron transport

During electron transport, high energy electrons are passed along electron carriers in the photosynthetic membrane. These carriers are called the electron transport

chain. At the end of the chain, high energy electrons are

passed to NADP+ converting it to NADPH.

3. Light Rxns - Oxygen Production

Electrons are getting used up by chlorophyll and must be replaced! This occurs by taking electrons from water to

replace those used by chlorophyll. Electrons are removed from water molecules

leaving H+ ions and oxygen. Oxygen is a by-product of the splitting of water by

is NEEDED by us!

4. Light Rxns - ATP production

Because hydrogen (H+) ions were released inside the thylakoid membrane as a product of the splitting of water molecules, the inside of the membrane becomes positively charged, while the outside is negatively charged. This difference in charges creates a gradient that

provides the energy to make ATP from ADP.

Light Rxns - SUMMARY

The light reactions USE: water, light energy, chlorophyll pigments

The light reactions PRODUCE: oxygen, NAPDH, ATP

Dark Rxns -The Calvin Cycle

ATP and NADPH can hold large amounts of chemical energy, but only for a few minutes.

The Calvin Cycle uses CO2 as well as ATP and NADPH from the light-dependent reactions to produce glucose that can be stored in the plant for long periods of time.

This process does not require light, but often takes place while the sun is shining.

Calvin Cycle

Carbon dioxide molecules enter the cycle from the atmosphere.

The carbon dioxide molecules combine with 5-carbon molecules. This reaction is catalyzed by the enzyme rubisco.

The result is 3-carbon molecules.

Calvin Cycle

The energy from breaking ATP into ADP and NADPH into NADP+ is used to convert the 3-carbon molecules into PGAL.

Most PGAL is recycled. 1 of 6 PGAL molecules formed is used to

make glucose. Plants use glucose for energy. Organisms that eat plants indirectly also use this

energy from glucose.

Calvin Cycle - SUMMARY

Calvin Cycle USES: NADPH, ATP ,CO2

Calvin cycle PRODUCES: glucose (C6H12O6)