Photosynthesis Chapter 7

We depend on the solar energy of a star 93 million miles away—our Sun! We depend on the solar energy of a star 93 million miles away—our Sun! Almost all living things are somehow dependent, directly or indirectly, on the amount of solar energy that photosynthesizers can convert into chemical energy Almost all living things are somehow dependent, directly or indirectly, on the amount of solar energy that photosynthesizers can convert into chemical energy

PHOTOSYNTHESIS converts solar energy into the chemical energy of a carbohydrate PHOTOSYNTHESIS converts solar energy into the chemical energy of a carbohydrate

The organisms that undergo photosynthesis are called AUTOTROPHS The organisms that undergo photosynthesis are called AUTOTROPHS For most plants, photosynthesis occurs in the leaves For most plants, photosynthesis occurs in the leaves

Water is absorbed through the roots Water is absorbed through the roots Carbon dioxide enter the leaf through openings called stomata—the same carbon dioxide that we exhale (this is called transpiration) Carbon dioxide enter the leaf through openings called stomata—the same carbon dioxide that we exhale (this is called transpiration)

After entering the leaf, carbon dioxide and water diffuse into chloroplasts (the organelles that carry on photosynthesis) After entering the leaf, carbon dioxide and water diffuse into chloroplasts (the organelles that carry on photosynthesis)

The chloroplast The stroma, a double membrane, surrounds the chloroplast The stroma, a double membrane, surrounds the chloroplast Thylakoids, sometimes stacked to form grana, are also located inside the chloroplast Thylakoids, sometimes stacked to form grana, are also located inside the chloroplast

Chlorophyll and other pigments that are part of the thylakoid membrane are capable of absorbing solar energy—the energy that drives photosynthesis Chlorophyll and other pigments that are part of the thylakoid membrane are capable of absorbing solar energy—the energy that drives photosynthesis

Photosynthetic pigments Pigments are molecules that absorb or reflect wavelengths of light Pigments are molecules that absorb or reflect wavelengths of light There are pigments in chloroplasts that absorb light, called their absorption spectrum There are pigments in chloroplasts that absorb light, called their absorption spectrum

Chlorophyll a, chlorophyll b, and carotenoids are all pigments that participate in photosynthesis Chlorophyll a, chlorophyll b, and carotenoids are all pigments that participate in photosynthesis They absorb violets, blues, and reds and reflect green (which is why leaves appear green to us) They absorb violets, blues, and reds and reflect green (which is why leaves appear green to us)

Chlorophyll a and chlorophyll b are primary pigments and play an accessory role (accessory pigments) Chlorophyll a and chlorophyll b are primary pigments and play an accessory role (accessory pigments)

In autumn, chlorophyll disappears from leaves and carotenoids remain—which is why leaves appear yellow, orange, and red at this time In autumn, chlorophyll disappears from leaves and carotenoids remain—which is why leaves appear yellow, orange, and red at this time

Photosynthetic reaction Remember that photosynthesis is a redox reaction: carbon dioxide is reduced and water is oxidized Remember that photosynthesis is a redox reaction: carbon dioxide is reduced and water is oxidized

Revisit the equation for photosynthesis: you recall that glucose and oxygen are the end products Revisit the equation for photosynthesis: you recall that glucose and oxygen are the end products Two separate reactions occur during photosynthesis: one to produce the chemical energy of ATP and NADPH (“Light Reactions”) and one reaction to produce the glucose (“Calvin Cycle Reactions”) Two separate reactions occur during photosynthesis: one to produce the chemical energy of ATP and NADPH (“Light Reactions”) and one reaction to produce the glucose (“Calvin Cycle Reactions”)

Two types of reactions: light and Calvin Cycle reactions There are two sets of reactions that occur during photosynthesis: Light reactions (for ATP and NADPH, chemical energy, production) and Calvin Cycle reactions (for glucose production) There are two sets of reactions that occur during photosynthesis: Light reactions (for ATP and NADPH, chemical energy, production) and Calvin Cycle reactions (for glucose production) These two reactions are needed because enzymes are needed to produce carbohydrates These two reactions are needed because enzymes are needed to produce carbohydrates

Light Reactions and the Calvin Cycle: working together

Light reactions 1. Chlorophyll located within the thylakoid membranes absorbs solar energy and energizes electrons 1. Chlorophyll located within the thylakoid membranes absorbs solar energy and energizes electrons

Light reactions 2. these energized electrons move down an electron transport chain and energy is captured and later used for ATP production 2. these energized electrons move down an electron transport chain and energy is captured and later used for ATP production

Light reactions 3. Energized electrons are also taken up my NADP+, and electron carrier, which turns into NADPH 3. Energized electrons are also taken up my NADP+, and electron carrier, which turns into NADPH

Light reactions: summary Summary: Summary: Solar energy  chemical energy (ATP, NADPH) Solar energy  chemical energy (ATP, NADPH)

How light reactions generate ATP and NADPH: Photosystems This pathway uses two photosystems, photosystem I (PSI) and photosystem II (PSII) This pathway uses two photosystems, photosystem I (PSI) and photosystem II (PSII) A photosystem consists of a pigment complex and electron acceptor molecules within the thylakoid membranes A photosystem consists of a pigment complex and electron acceptor molecules within the thylakoid membranes

Photosystem I: Photosystem I is a water-splitter photosystem, which uses light energy to extract electrons from H 2 O Photosystem I is a water-splitter photosystem, which uses light energy to extract electrons from H 2 O This is the process that releases O 2, a waste product of photosynthesis This is the process that releases O 2, a waste product of photosynthesis

Photosystem II: Photosystem II is an NADPH-producing photosytem Photosystem II is an NADPH-producing photosytem It produces NADPH by transferring light- excited electrons from chlorophyll to NADP+ It produces NADPH by transferring light- excited electrons from chlorophyll to NADP+

An electron transport chain connecting the two photosystems releases energy that the chloroplast uses to make ATP An electron transport chain connecting the two photosystems releases energy that the chloroplast uses to make ATP However….we still need Glucose! So onward to the Calvin Cycle! However….we still need Glucose! So onward to the Calvin Cycle!

Calvin Cycle reactions: making sugar from Carbon Dioxide This takes place in the stroma This takes place in the stroma 1. Carbon dioxide is taken up and reduced to carbohydrate 1. Carbon dioxide is taken up and reduced to carbohydrate The ATP and NADPH that was formed during the Light Reaction carries out this reduction

Calvin Cycle 2. After production of a carbohydrate, ADP + P and NADP+ return to the Light Reaction where they become ATP and NADPH once more 2. After production of a carbohydrate, ADP + P and NADP+ return to the Light Reaction where they become ATP and NADPH once more

Calvin Cycle: summary Summary: Summary: Chemical energy  chemical energy Chemical energy  chemical energy (ATP, NADPH) (carbohydrate) (ATP, NADPH) (carbohydrate)

Calvin Cycle: the big picture The energy yield in Light Reactions is stored in ATP and NADPH The energy yield in Light Reactions is stored in ATP and NADPH These molecules are used by the Calvin Cycle reactions to reduce CO 2 to a carbohydrate, mostly to a carbohydrate called G3P (also called PGAL) These molecules are used by the Calvin Cycle reactions to reduce CO 2 to a carbohydrate, mostly to a carbohydrate called G3P (also called PGAL) G3P is then converted to all other organic molecules that the plant needs G3P is then converted to all other organic molecules that the plant needs

Stages of the Calvin Cycle 1) the enzyme “RuBP carboxylase” fixes CO2 to RuBP, producing a 6-carbon molecule that immediately breaks down to two C 3 molecules 1) the enzyme “RuBP carboxylase” fixes CO2 to RuBP, producing a 6-carbon molecule that immediately breaks down to two C 3 molecules

Stages of the Calvin Cycle 2) CO 2 (incorporated into an organic molecule) is reduced to a carbohydrate (CH 2 O) 2) CO 2 (incorporated into an organic molecule) is reduced to a carbohydrate (CH 2 O) This step requires NADPH and some of the ATP from the light reactions This step requires NADPH and some of the ATP from the light reactions

The Calvin Cycle For every three turns of the Calvin Cycle, the net gain is one G3P molecule For every three turns of the Calvin Cycle, the net gain is one G3P molecule The other 5 G3P molecules are used to reform three molecules of RuBP The other 5 G3P molecules are used to reform three molecules of RuBP This step also requires ATP for energy This step also requires ATP for energy It takes two G3P molecules to make one glucose molecule It takes two G3P molecules to make one glucose molecule