Photosynthesis!

Plant questions?

• How do plants obtain energy?

• Why do plants need light?

• Why do plants need water?

• Why are plants green?

http://fig.cox.miami.edu/~cmallery/150/phts/phts.htm

• The best known form of photosynthesis is the one carried out by higher plants and algae, as well as by cyanobacteria and their relatives, which are responsible for a major part of photosynthesis in oceans.

The very basics:

• Energy from light is used to convert CO2, H2O into sugar

• O2 is a byproduct or “leftover” and is released

Photosynthesis:

• In: 6CO2+6H2O+Energy

• Out: C6H12O6+6O2

Anatomy of a leaf (photosynthetic organ)

Leaf functions:• “Trap” sunlight• Perform photosynthesis• Move food to storage

areas in the plant

• Absorb CO2 from the air

• Minimize water loss from evaporation

Leaf structure is related to function…

Why are leaves green?

Because….

• Their cells are full of chloroplasts…which contain chlorophyll…a pigment that reflects green light…which we see….so leaves are green

Summary of chloroplast structure

• Enclosed in a double membrane• Contain Thylakoid disks called grana

(pigments embedded in Thylakoid membrane)

• The liquid part of chloroplasts is the Stroma – Stroma contains

• Electron Transport System• “Loose” enzymes

Photosynthesis takes place in 2 stages – each made up of a set of reactions:

• Light Dependent Reactions– Convert light energy to chemical energy in the

form of ATP & NADPH

• Light Independent Reactions (Calvin-Benson Cycle)– Convert chemical energy from ATP & NADPH

to long term storage form: glucose/starch

Where does the light come from?

• What are photons?– Light particles– No mass– Travel as a wave

• Photons & Energy– The smaller the

wavelength, the higher the energy per photon

Sunlight is a mixture of the colors of the rainbow

Photosynthetic Pigments: Light energy captors

• Photosynthetic plant pigments

• Embedded in membranes of thylakoid disks

• Consist of different varieties of chlorophylls and other accessory pigments such as beta-carotene

Pigments

• Pigments absorb light energy and convert it to chemical energy.

• Chlorophyll: GREEN (most important in photosynthesis)

• Carotenoids: YELLOW, ORANGE & RED.

• Photosynthesis only takes place when chlorophyll is present.

Photosynthetic pigments absorb the energy in specific colors of light

What colors are absorbed?What colors reflected?

Different pigments absorb different colors of light

This allows plants to use most of the available light

1. Light Reaction

• needs light energy

• takes place in the thylakoids

• needs water (H2O), which is divided into electrons and oxygen (which is liberated)

• uses 2 photosystems to capture the sun’s energy

• produces ATP which is used in the dark rx (Calvin-Benson cycle)

The Light ReactionTwo Steps:

1. Trapping Energy from the Sun.2. Splitting of Water

Animation of the Light Reaction:Animation of the Light Reaction:http://www.biology4all.com/resources_library/source/61a.swf

LIGHT REACTION PRODUCTS

• Energy– ATP– NADPH

• Oxygen (which is released into the atmosphere)

2.The Dark Reaction (Calvin cycle)

- uses CO2 to make glucose (“fixation” of carbon dioxide)

- takes place in the stroma of the chloroplast

- also known as C3 bc of the 3 carbon molecule (PGA) that is formed

- there are 2 other methods that plants may use to make glucose… C4 and CAM

Calvin Cycle : the synthesis

The rate of photosynthesis

- which factors affect the rate of “fixed” glucose?

1) The rate of photosynthesis can be limited by physical factors such as temperature. The lower the temperature, the slower the photosynthesis.

2) The availability of light is a factor that limits the rate of photosynthesis. When the amount of light decreases, the rate of photosynthesis also decreases

The C4 & CAM plants

The Calvin cycle and the “C4 Plants”

- in certain climates, the sun is too abundant, and it never limits photosynthesis- however, these climates are also very try and hot- here, it’s the CO2 that limits photosynthesis

- we can think of it as the availability and the loss of water (H2O)- when the plant photosynthesises in the sun, the CO2 must enter the leaves through the stomatas (little holes under the leaves)- but, when these holes are open, H2O is lost and the plant dehydrates

- if you close the stomatas, CO2 cannot enter so this limits photosynthesis- in C4 plants, the stomatas are only partially opened during the day- these plants have an enzyme that can transform the CO2 into a 4-carbon molecule- these molecules are stored in the different cells and the CO2 can be released when the plant lacks any

Examples of C4 plants are corn, sugar canes

Crassulacean Acid Metabolism (CAM)

- the ultimate prevention of CO2 loss is found in desert plants like the cactus- in these plants, the stomatas are only opened at night

- the plants fixes the CO2 into 4-carbon molecules during the night and transfer the carbon to the Calvin cycle during the day

- in the day, the stomatas are completely closed and there is no water loss

Comparison between photosynthesis and aerobic resp.

Global rx of photosynthesis:6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Global rx of aerobic cellular resp:C6H12O6 + 6O2 + 38 ADP + 38 P → 6CO2 + 6H2O + 38 ATP

- in a way, photosynthesis is the opposite of aerobic resp.

- the products of photosynthesis are the raw materials (reactants) of resp.

PHOTOSYNTHESIS RESPIRATIONWhere? In cholorophyll-bearing cells In all cells

When? In the presence of light All the time

Input? Carbon dioxide and water Reduced carbon compounds and

oxygen

Output? Reduced carbon compounds, Carbon dioxide and water

oxygen, and waterEnergy sources? Light Chemical bonds

Energy result? Energy stored Energy released

Reaction ? Reduction of carbon Oxidation of carbon compounds compounds

Energy carrier(s) NADP NAD and FAD