8.3 The Process of Photosynthesis

8.3 The Process of Photosynthesis

Review

Phototrophs: obtain energy from light

Chemotrophs: obtain energy from

chemicals

Autotrophs: obtain carbon from CO2

Heterotrophs: obtain carbon from organic

sources

Light-Dependent Reactions: ATP and NADPH

The light-dependent reactions occur in the thylakoids of

chloroplasts.

Photosystems - groups of chlorophyll and proteins in the

thylakoid. Absorb sunlight and generate high energy electrons that

are used to make sugars.

Photosystem II

Water molecules provide new

electrons to chlorophyll.

Enzymes break up the water

molecules into 2 electrons, 2

Hydrogen atoms, and 1

oxygen atom.

Oxygen is released into the air.

The H+ ions are released inside

the thylakoid.

Electron Transport Chain

Electron transport chain - a

system that carries high-

energy electrons during ATP-

generating reactions.

Electron transport chain

takes energy from the

electrons and uses it to pump

H+ ions from the stroma into

the thylakoid space.

Electron Transport Chain

At the end of the electron

transport chain, the

electrons pass to

photosystem I.

Photosystem I - pigments

use energy from light to

re-energize the electrons

At the end of a short second electron transport chain, NADP+

molecules in the stroma pick up the high-energy electrons and H+

ions at the outer surface of the thylakoid membrane to become

NADPH.

The difference in both charge and H+ ion concentration across

the membrane creates a gradient, or build up, of hydrogen

inside the thylakoid. The H+ now wants to diffuse out of the

thylakoid.

ATP synthase spans the thylakoid membrane and allows H+ ions

to pass through it, while creating ATP as result. This is called

chemiosmosis.

Light-Independent Reactions: Producing

Sugars

Light-independent reactions aka the Calvin cycle - plants use the energy

that ATP and NADPH contains to build stable high-energy carbohydrate

compounds that can be stored for a long time.

Summary of the Calvin Cycle

Calvin cycle - 6 CO2 → 1 6

carbon sugar.

The energy for the reactions

is supplied by compounds

produced in the light-

dependent reactions.

Temperature, Light, and WaterThe reactions of photosynthesis are made possible by enzymes that function best

between 0°C and 35°C.

High light intensity increases the rate of photosynthesis.

After the light intensity reaches a certain level, however, the plant reaches its

maximum rate of photosynthesis, as is seen in the graph.

Photosynthesis Under Extreme Conditions

In order to conserve water, most

plants under bright, hot

conditions close the small

openings in their leaves that

normally admit carbon dioxide.

This causes carbon dioxide within

the leaves to fall to very low

levels, slowing down or even

stopping photosynthesis.

C3 Plants, like soy, oats, wheat

and rice, when the climate is

hot and dry, these plants close

their stomata. This prevents

CO2 from entering the plant

cells.

Photorespiration is where no

ATP and no sugar is made in

the plant cell due to the fact

that carbon dioxide is depleted

and oxygen is building up.

C4 Plants, like corn and

sugarcane keep their

stomata closed most of the

time to conserve water.

They are able to prevent

photorespiration by using

special enzymes that

continue photosynthesis

even when carbon dioxide

levels are low.

CAM plants, like cacti and

pineapples, conserve water by

opening their stomata and

admitting CO2 only at night.

The plant is then able to bank a

large amount of CO2 to

continue photosynthesis during

the day.

Video ReviewCrash course: http://youtu.be/sQK3Yr4Sc_k

TED-Ed Calvin Cycle: http://youtu.be/0UzMaoaXKaM