PHOTOSYNTHESISPHOTOSYNTHESIS

Class Notes

PHOTOSYNTHESIS Photosynthesis is ability of a plant to turn

sunlight, air, and water into sugar (energy).

The overall reaction is: light

chlorophyll

CO2 + H2O -------> CH2O + O2

Photosynthesis is vital to life for two reasons

–1. The oxygen in the air comes from

photosynthesis. The plants continue to

replenish the oxygen in the air.–2. All of our food comes directly or

indirectly from photosynthesis.

Parts of a leafParts of a leaf

Stomatapalisade layerspongy layerveinschloroplast

thylakoidsgranastroma

The Chloroplast

-double membrane, stroma, thylakoid, grana

Light

Photosynthesis uses visible light.– Only visible light with intermediate

wavelengths has enough energy to cause chemical change without destroying biological molecules.

What are the colors of the visible spectrum?

Why does a leaf look green?

. Light is captured by

pigments. The main photosynthetic pigment is chlorophyll. There is chlorophyll a, b and c. Chlorophyll a is the major photosynthetic pigment and is found in all photosynthetic plants, protist, and cyanobacteria

CarotenoidsCarotenoids

CarotenoidsCarotenoids are accessory pigments found in all green plants. They absorb blue and green wavelengths and give a plant a yellow or orange color. In the fall when chlorophyll breaks down, it is the accessory pigments which give colors of fall. The red color of some autumn leaves is due to the anthocyanin pigments. These are not photosynthetic.

Stages of Photosynthesis

Chemiosmotic ATP synthesis

Oxygen production

Electron transport

Light absorption

LIGHT REACTIONS

Carbon Fixation

DARK REACTIONS

The Energy-Capturing Reactions

Light absorption – Chlorophyll a and b are antennae pigments

Electron transport– Electrons lost by chlorophyll a end up at

NADP– Produce oxygen

Chemiosmotic ATP synthesis– Powered by H ion reservoir

The Dark reactions use more ATP then NADPH.

Each turn of the cycle uses 9 ATP and only 6 NADPH.

How are more ATP acquired than NAPH since one ATP is produced in photosystem II and one NADPH is produced in photosystem I?

Cyclic and noncyclic electron flow

Noncyclic Electron Flow

Cyclic Electron Flow

The Dark ReactionsThe Dark Reactions The dark reactions take place outside

the thylakoid membrane in the stromastroma. The Calvin Cycle converts CO2 to sugar in a three phase process.

1.1. Carbon fixationCarbon fixation - The carbon dioxide is incorporated into an organic molecule (1C + 5C = 6C) called RuBP by the enzyme Rubisco. This 6 carbon structure is unstable and immediately splits into 2, 3 carbon molecules. This is the same molecule that glucose is split into.

2.2. Reduction Reduction – Each 3 carbon molecule is phosphorylated by ATP and NAPH to create G3P. The cycle must turn 3 times for a net gain of one molecule of G3P.

3.3. Regeneration of CORegeneration of CO2 2 acceptoracceptor – The rearrangement of 5 molecules of G3P into 3 molecules of RuBP required 3 more ATP.

( 9 ATP and 6 NADPH yields 1 G3P)

Intensity of lightIntensity of lightScarcity of water Scarcity of water

What controls photosynthesisWhat controls photosynthesis?

Ecological Aspects C4 PLANTS - Use more ATP but at high

temperatures they can photosynthesize faster -allowing plants to grow and reproduce faster. These include many weeds and important crops such as corn.

A special variation of C4 plants are the CAM plants. These are mostly desert plants. These plants keep the stomata closed during the day and open at night. They must conserve water!

Vocabulary

Glossary of terms – blade– stipules– petiole– vein– midrib

carotenoidschlorophyllelectromagnetic

spectrumgranum

NADP+

NADPHphotonphotosystemphotosystem Iphotosystem IIpigmentstromathylakoid

VocabularyVocabulary