Chapter 6

Pathways of Photosynthesis

Photosynthesis produces food and released oxygen

Photosynthesizers are autotrophs that produce their own food

• Photosynthesis– Converts solar energy to chemical energy (glucose)

• Autotrophs– Produce their own food

• Heterotrophs– Consumers

• Nearly all food chains start with plants or algae

In eukaryotes, chloroplasts carry out photosynthesis

• Materials for photosynthesis– CO2

• Comes through stomata• Comes from organisms breathing out

– H2O• Comes in through roots

• The materials diffuse into chloroplasts, where photosynthesis occurs

• Thylakoids are within chloroplasts and chlorophyll is found inside of them

• Chlorophyll is within thylakoids and absorbs solar energy

Photosynthesis involves two sets of reactions: the light reactions and the Calvin

cycle reactions• Oxidation

– Loss of hydrogen atoms– Loses electrons

• Reduction– Gain of hydrogen atoms– Gains electrons

• This type of reactions is called a redox reaction

• Light reactions– Only occur in the presence of light

– Energized electrons from the breakdown of H2O are taken up by a coenzyme (NADP+)

– NADP+ also combines with H+ (a reduction reaction) to form NADPH

– ATP is also produced

• Calvin Cycle– Light independent (can occur with or without light)

– CO2 reduces to CH2O, which can be used to form glucose

– H+ comes from NADPH– Energy comes from ATP

The light reactions capture solar energy

Solar energy is absorbed by pigments

• Solar energy– Radiant energy from the sun

• Visible light– Type of radiation used for

photosynthesis and vision– Shorter wavelength means more

energy• Violet-shortest wavelength, most

energy

• Red-longest wavelength, least energy

• Chlorophyll a, chlorophyll b, and carotenoids are found in photosynthesizing cells and are capable of absorbing various portions of visible light

• Green is reflected instead of absorbed, this is why plants appear green

During fall, enzymes that rebuild chorophyll work at a slower speed due to lower temperatures. Chlorophyll eventually disintegrates, leaving only carotenoids to absorb light. Carotenoids reflect yellow, orange, and red, which is why leaves appear to change colors.

Solar energy boosts electrons to a higher energy level

Antenna molecule– Takes in light energy– Pass energy to the

reaction center

Electrons release their energy as ATP forms

• Chloroplasts use electrons energized by solar energy to generate ATP by way of electron transport chain– High energy electrons

enter– Low energy electrons leave– Energy is released each

time the electron is transferred

– Energy is ultimately used to make ATP

The Calvin cycle reactions synthesize carbohydrates

ATP and NADPH from the light dependent reactions are needed to produce a carbohydrate

• CO2 comes in from the atmosphere and combines with RuBP (an enzyme that speeds the Calvin cycle)

• CO2 is reduced to form CH2O– ATP and NADPH are needed for this to occur– ATP breaks down to ADP + P– NADPH breaks down to NADP+

• G3P (glyceraldehyde-3-phosphate) (CH2O)is then produced, which is the building block for many carbohydrates (glucose, starch, and cellulose) and RuBP

• The ADP + P and NADP+ can go back to the light reactions to form ATP and NADPH again

http://media.pearsoncmg.com/bc/bc_0media_bio/bioflix/bioflix.htm?cc5photosynthesis

Tropical rain forests and global climates change

• Rain forest vegetation absorbs large amounts of CO2

• CO2 traps radiant heat from the sun and warms the world, similar to a greenhouse, which is why it is called a greenhouse gas

• Without greenhouse gasses, Earth would be about 33˚C cooler

• With excess greenhouse gasses, climate patterns will be disrupted (more heatwaves, droughts, and storms) and more hurricanes and tornadoes will occur

• Burning fossil fuels releases CO2, clearing rainforests usually involves burning them (slash and burn), releasing the trapped CO2, and reducing the amount of vegetation that can absorb CO2

• At warmer temperatures, photosynthesis rates have been seen to decrease

• Cut down forests => more CO2 in the air => warmer temperatures => lower photosynthesis rates => less CO2 is absorbed

• Protecting remaining forests and reestablishing deforested areas may help to slow climate change