Photosynthesis

• Review the following terms:– Autotrophs and heterotrophs– The structure of chloroplasts and cell

membrane– Electron transport chain– The functions of ATP and NADH– Potential and kinetic energy – Organic and inorganic molecules

Photosynthesis in Overview• Process by which plants and other autotrophs

store the energy of sunlight and use this energy to make sugars and other organic molecules.

• Requires sunlight, water, and carbon dioxide.• The main purpose of photosynthesis is to make

organic molecules (carbohydrates).• Overall equation:

6 CO2 + 6 H20 C6H12O6 + 6 O2

• Occurs in the leaves of plants in the chloroplasts.• Oxygen is also produced in this process. This is

the oxygen that we use to breath in for cellular respiration.

Leaf Structure• Most photosynthesis occurs in the mesophyll layer of the

leaf.• Gas exchange of CO2 and O2 occurs at openings called

stomata surrounded by guard cells on the lower leaf surface.

• Stomata are able to open and close because water is also evaporated through them into the atmosphere from the plant.

• When stomata are open, CO2 can freely enter the leaf but water also leaves at the same time.

http://croptechnology.unl.edu/animation/transpiration.swf

Chloroplast Structure• Inner membrane

called the thylakoid membrane.

• Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana)

• Stroma is a liquid surrounding the thylakoids.

Pigments• Pigment is any molecule that is able to absorb

light . Only light that is absorbed by pigments is useful for photosynthesis.

• Chlorophyll a is the most important photosynthetic pigment.

• Other pigments called antenna or accessory pigments are also present in the leaf.– Chlorophyll b– Carotenoids (orange / red)– Xanthophylls (yellow / brown)

• These pigments are embedded in the membranes of the chloroplast in groups called photosystems.

Photosynthesis: The Chemical Process• Occurs in two main phases.

– Light reactions– The Calvin Cycle

• Light reactions are the “photo” part of photosynthesis. Light is absorbed by pigments and the energy of light is used to form ATP and NADPH molecules. This takes place in the grana of the chloroplast.

• The Calvin cycle is the “synthesis” part of photosynthesis. Trapped energy from the sun is converted to the chemical energy of sugars by using the ATP and NADPH that was produced in the first process. This process takes place in the stroma of the chloroplast.

Light Reactions (aka light-dependent process)

• We are going to go over the steps of this process together in class.

• The products of the light reaction are ATP, NADPH and oxygen (O2).

• ATP and NADPH move to the Calvin cycle to help to synthesize carbohydrates.

• Oxygen is released as a waste product or used up in cellular respiration

• The amount of ATP and NADPH molecules depends on the concentration of water in the plant and on the intensity of sunlight. The rate of the reaction is determined by these environmental factors

• http://vcell.ndsu.nodak.edu/animations/photosynthesis/movie.htm and • http://vcell.ndsu.nodak.edu/animations/photosystemII/movie.htm

The Calvin Cycle• During this process, carbohydrates are formed.

This is the only process on the earth that can form organic molecules from inorganic ones. All other organic molecules (big 4) forms from carbohydrates.

• This cycle requires ATP, NADPH and CO2 to take place in the stroma of the chloroplast.

• ATP, NADPH are from the light reaction, while CO2 has to be taken in from the atmosphere through the stomata of the leaves.

• We will discuss this process in steps in class• http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Relationship Between the Two Stages of Photosynthesis

Factors Affecting Photosynthesis

• Shortage of water can slow down or stop photosynthesis.

• Enzymes that are necessary for photosynthesis function only within a certain temperature range.

• Light intensity is also a factor that can speed up or slow down photosynthesis.

• Carbon dioxide concentration is crutial

Alternative Pathways of Photosynthesis

• These pathways adapt to perform photosynthesis in dry and hot environment

• They are more efficient than the traditional C3 pathway (what we learned earlier)

• Plants with alternative pathways have a slightly different Calvin cycle.

• In C4 plants the location of the cycle is different, in CAM plants the timing is different.

C4 Pathway

• CO2 trapping (fixation) and the Calvin cycle take place in two separate location.

• CO2 fixation is in the mesophyll cells of the leaf, the Calvin cycle takes place in the bundle sheath cells (around the veins of the leaf) that are low in oxygen.

• Examples of C4 plants: corn, sugar cane

CAM pathway

• Occurs in succulent plants (cacti)• Carbon fixation (trapping CO2) takes place

at night when the stomata are open• Calvin cycle takes place during the day,

when stomata are closed• This way plants do not lose much water

during hot and dry days.