Photosynthesis: An OverviewSection 8.2

Inside Chloroplasts• Chloroplasts contain saclike photosynthetic

membranes called thylakoids where chlorophyll is located.

• The thylakoids are interconnected and arranged in stacks called grana.

• The fluid in chloroplasts is called the stroma.

Photosynthesis Overview

• Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high energy sugars and oxygen.

Light• Sunlight is white light which is a mixture of

colors in the visible spectrum of varying wavelengths.

• Plants gather the sun’s energy using light-absorbing molecules called pigments.

Chlorophyll• The principle pigment used by plants to collect light

energy is chlorophyll.• Two types of chlorophyll are found in plants:

chlorophyll a and chlorophyll b.• These pigments absorb light in the blue-violet and

red regions and reflect light in the green region. This is why plants are green!

Chlorophyll Molecular Structure

Accessory Pigments• Plants also contain red and orange pigments such as

carotene that absorb light in other regions.• The intense color of chlorophyll overwhelms these

accessory pigments.• Chlorophyll breaks down first in the fall, leaving

orange and red pigments.

Chromatography• A separation technique that utilizes the

differing polarities of the substances in a mixture.

• Chromatography can be used to separate the pigments in plant leaves.

High Energy Electrons• Once chlorophyll absorbs light energy, the

energy is transferred to electrons.• The plant uses electron carriers to transport

the high energy electrons.• One of the electron carriers is NADP+.

Nicotinamide Adenine Dinucleotide Phosphate

• NADP+ accepts two high energy electrons and a hydrogen ion to become NADPH.

• This enables some of the light energy to be trapped in chemical form.

• The electron carrier can move throughout the cell and use the energy they carry to help build molecules the cell needs.

Light Dependent Reactions Overview• Require the direct involvement of light and light-

absorbing pigments.• Uses energy from sunlight to produce ATP and other

energy-rich compounds (NADPH).• Water is required and oxygen is a byproduct.

Photosystems• Takes place in thylakoid membranes of the

chloroplast.• Thylakoids contain clusters of chlorophyll and

proteins known as photosystems.• The photosystems absorb sunlight and generate

high-energy electrons that are passed to a series of electron carriers in the thylakoid membrane.

Electron Transport Chain• A series of electron carrier proteins that

shuttle high-energy electrons during ATP-generating reactions.

Light Dependent Reactions Details

• Step 1: PS II absorbs light energy and uses it to energize electrons made by splitting water molecules.

• Step 2: High-energy electrons move down the transport chain and are used to pump H+ across the membrane against its concentration gradient.

• Step 3: PS I uses light energy to produce NADPH by reenergizing electrons.

• Step 4: The H+ ions move back across the membrane through ATP synthase, which generates ATP.

Helpful Videos

http://www.youtube.com/watch?v=joZ1EsA5_NY

http://www.youtube.com/watch?v=ww33L0lD37I

Light Independent Reactions Overview• No light is required.• ATP and other energy-rich compounds (NADPH) from

the light dependent reactions are used to produce sugars from carbon dioxide.

• Takes place in the stroma of the chloroplast.

Calvin Cycle Details

• Step 1: Carbon dioxide is added– Carbon dioxide is added to five-carbon molecules

already in the cycle to form three-carbon molecules.

– Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

• Step 2: High-energy three-carbon molecules formed– Energy from light dependent reactions is used by

enzymes to form high-energy three-carbon molecules.

• Step 3: Three-carbon molecules exit– Three-carbon molecules leave the cycle to form

sugars.– Plants form starch to store the sugars.

• Step 4: Three-carbon molecules recycled– Energy from light dependent reactions is used to

change the three-carbon molecules left in the cycle back to five-carbon molecules that are then used to restart the cycle.

Factors Affecting Photosynthesis

• Temperature• Light Intensity• Water

Temperature

• Enzymes needed to catalyze the reactions of photosynthesis work best between 0 and 35 degrees Celsius.

Light Intensity

• High intensity of light increases the rate of photosynthesis, but there is a maximum value for this rate.

Water• Shortages of water can slow or stop

photosynthesis and/or damage plant tissues.• Some plants that live in dry areas have waxy

coatings on their leave to prevent water loss.

C4 Photosynthesis• Have a specialized chemical pathway that

allows them to capture carbon dioxide in extreme conditions.

• Examples: corn, sugar cane, sorghum

CAM Plants• Like C4 plants, have a specialized chemical pathway

that allows them to capture carbon dioxide in extreme conditions.

• Stomata are only open at night and are sealed tightly during the day.

• Examples: pineapple trees, desert cacti