10 Photosynthesis: Energy from Sunlight. 10 Photosynthesis: Energy from Sunlight 10.1 What Is Photosynthesis? 10.2 How Does Photosynthesis Convert Light.

10Photosynthesis: Energy

from Sunlight

10 Photosynthesis: Energy from Sunlight

10.1 What Is Photosynthesis?

10.2 How Does Photosynthesis Convert Light Energy into Chemical Energy?

10.3 How Is Chemical Energy Used to Synthesize Carbohydrates?

10.4 How Have Plants Adapted Photosynthesis to Environmental Conditions?

10.5 How Does Photosynthesis Interact with Other Pathways?

10 Photosynthesis: Energy from Sunlight

Opening Question:

What possible effects will increased atmospheric CO2 have on global food production?

To predict how plants will respond to rising CO2 levels, biologists have performed large-scale experiments. Photosynthesis rates increase as atmospheric CO2 concentration increases.


Photosynthesis: “synthesis from light”

Energy from sunlight is captured and used to convert CO2 to more complex carbon compounds.

Figure 10.1 The Ingredients for Photosynthesis


Using stable 18O isotopes, Ruben and Kamen determined that water is the source of O2 released during photosynthesis:

Figure 10.2 The Source of the Oxygen Produced by Photosynthesis

Working with Data 10.1: Water Is the Source of the Oxygen Produced by Photosynthesis

The stable isotope 18O was used to confirm the hypothesis that O2 generated during photosynthesis came from water.

Algal cells were exposed to water, and CO2 generated from K2CO3 and KHCO3

–.


Experiment 1: water contained more 18O than 16O.

Experiment 2: the CO2 contained more 18O than 16O.

A mass spectrometer measured the isotopic content of reactants and the O2 produced.


Question 1:

In Experiment 1, was the isotopic ratio of O2 similar to that of H2O or to that of CO2?

What about in Experiment 2?

Working with Data 10.1, Table 1


Question 2:

What can you conclude from these data?


Photosynthesis is an oxidation–reduction process.

Oxygen atoms in H2O are in a reduced state; they are oxidized to O2.

Carbon atoms are in the oxidized state in CO2; they are reduced to a carbohydrate.


Water is the donor of protons and electrons in oxygenic photosynthesis.

In anoxygenic photosynthesis, other molecules donate the protons and electrons.

Example: purple sulfur bacteria use H2S.


Two pathways occur in different parts of the chloroplast:

• Light reactions: Convert light energy to chemical energy as ATP and NADPH.

• Light-independent reactions: Use ATP and NADPH (from the light reactions) plus CO2 to produce carbohydrates.

Figure 10.3 An Overview of Photosynthesis


Light is a form of energy—electromagnetic radiation.

It is propagated as waves—the amount of energy is inversely proportional to its wavelength.

Light also behaves as particles, called photons.

Figure 10.4 The Electromagnetic Spectrum


Certain molecules absorb photons of specific wavelengths.

When a photon hits a molecule, it can:

• Bounce off—scattered or reflected

• Pass through—transmitted

• Be absorbed, adding energy to the molecule (excited state)

In-Text Art, Ch. 10, p. 189


The absorbed energy boosts an electron in the molecule into a shell farther from the nucleus.

This electron is held less firmly— making the molecule more unstable and reactive.


Molecules that absorb specific wavelengths in the visible range are called pigments.

Other wavelengths are scattered or transmitted, which imparts the colors that we see.

Chlorophyll absorbs blue and red light and scatters green.


Absorption spectrum: plot of wavelengths absorbed by a pigment.

Action spectrum: plot of photosynthesis against wavelengths of light to which it is exposed.

The rate of photosynthesis can be measured by the amount of O2 released.

Figure 10.5 Absorption and Action Spectra


The major pigment in photosynthesis is chlorophyll a.

It has a hydrocarbon “tail” that anchors it in a protein complex in the thylakoid membrane called a photosystem.

Figure 10.6 The Molecular Structure of Chlorophyll a


Chlorophyll a and accessory pigments (chlorophylls b and c, carotenoids, phycobilins) are arranged in light-harvesting complexes, or antenna systems.

Several complexes surround a reaction center in the photosystem.

Figure 10.7 Energy Transfer and Electron Transport (Part 2)


Light energy is captured in the light harvesting complexes and transferred to the reaction centers.

Accessory pigments absorb light in other wavelengths, increasing the range of light that can be used.

Types of accessory pigments characterize different groups.


When a pigment molecule absorbs a photon, the excited state is unstable and the energy is quickly released.

The energy is absorbed by other pigment molecules and passed to chlorophyll a in a reaction center.

Figure 10.8 Noncyclic Electron Transport Uses Two Photosystems


The excited chlorophyll a molecule (Chl*) gives up an electron to an acceptor.

Chl* + acceptor → Chl+ + acceptor –

A redox reaction: The chlorophyll gets oxidized; the acceptor molecule is reduced.


The electron acceptor is the first in a chain of carriers in the thylakoid membrane.

The final electron acceptor is NADP+, which gets reduced:

NADP+ + H+ + 2e– → NADPH


Noncyclic electron transport uses two photosystems:

• Photosystem I has P700 chlorophyll —absorbs best at 700 nm.

• Photosystem II has P680 chlorophyll —absorbs best at 680 nm.


Photosystem II:

When excited chlorophyll (Chl*) gives up its electron, it is unstable, and grabs another electron (it is a strong oxidizer).

The electron comes from water:

2Chl+ + H2O → 2Chl + 2H+ + ½O2


The energetic electrons are passed through a series of membrane-bound carriers to a final acceptor at a lower energy level.

A proton gradient is generated and is used by ATP synthase to make ATP.


Photosystem I:

An excited electron from the Chl* reduces an acceptor.

The oxidized Chl+ takes an electron from the last carrier in photosystem II.

The energetic electron is passed through several carriers and reduces NADP+ to NADPH.

Figure 10.8 Noncyclic Electron Transport Uses Two Photosystems


Cyclic electron transport:

Uses photosystem I and electron transport to produce ATP instead of NADPH.

Cyclic: the electron from the excited chlorophyll passes back to the same chlorophyll.

Figure 10.9 Cyclic Electron Transport Traps Light Energy as ATP


ATP is formed by photophosphorylation, a chemiosmotic mechanism.

H+ is transported across the thylakoid membrane into the lumen, creating an electrochemical gradient.

Figure 10.10 Photophosphorylation


Water oxidation creates more H+ in the thylakoid lumen and NADP+ reduction removes H+ in the stroma.

Both reactions contribute to the H+ gradient.


High concentration of H+ in the lumen drives movement of H+ back into the stroma through protein channels.

The channels are ATP synthases that couple movement of protons with formation of ATP.


CO2 fixation: CO2 is reduced to carbohydrates.

Occurs in the stroma.

Energy in ATP and NADPH is used to reduce CO2.


Calvin and Benson used the 14C radioisotope to determine the sequence of reactions in CO2 fixation.

They exposed Chlorella to 14CO2, then extracted the organic compounds and separated them by paper chromatography.

Figure 10.11 Tracing the Pathway of CO2 (Part 1)

Figure 10.11 Tracing the Pathway of CO2 (Part 2)


The first compound to be formed is 3-phosphoglycerate, 3PG, a 3-carbon sugar phosphate.


The pathway of CO2 fixation is cyclic: the Calvin cycle.

CO2 first binds to 5-C RuBP; the 6-C compound immediately breaks down into two molecules of 3PG.

The enzyme rubisco (ribulose bisphoshate carboxylase/oxygenase) is the most abundant protein in the world.

Figure 10.12 RuBP Is the Carbon Dioxide Acceptor

Working with Data 10.2: Tracing the Pathway of CO2

• In experiments to determine the reactions of photosynthesis, the green alga Chlorella was exposed to CO2 made with the radioactive 14C isotope.

• Cells were exposed for various periods of time.


• The first reaction in CO2 fixation can occur in the dark.

• Cells were exposed to 20 minutes of light followed by various lengths of darkness.

Working with Data 10.2, Table 1


• Question 1:

• Using the data in the table, plot radioactivity in 3PG versus time.

• What do the data show?


• Question 2:

• Why did the amount of radioactively labeled RuBP go down after 30 seconds in the dark?


The Calvin cycle:

• Fixation of CO2 to 3PG

• Reduction of 3PG to G3P

• Regeneration of RuBP, the CO2 acceptor.

For every turn of the cycle, one CO2 is fixed and one RuBP is regenerated.

Figure 10.13 The Calvin Cycle


Glyceraldehyde 3-phosphate (G3P) is the product of the Calvin cycle.


Some G3P is exported to the cytosol and converted to hexoses (glucose and fructose) used in respiration.

Hexoses may be converted to sucrose and transported to other parts of the plant and used for energy or to build other molecules.


Some G3P is used to synthesize glucose and starch within the chloroplast.

The stored starch is used at night so that photosynthetic tissues can continue to export sucrose to the rest of the plant.


Products of the Calvin cycle are crucial to the entire biosphere.

The covalent bonds generated by the cycle provide almost all of the energy for life.


Photosynthetic organisms (autotrophs) use this energy for growth, reproduction, and development.

Heterotrophs cannot photosynthesize and depend on autotrophs for both energy and raw materials.


The Calvin cycle is stimulated by light:

• Protons pumped from stroma into thylakoids increase the pH, which favors activation of rubisco.

• Electron transport reduces disulfide bonds in Calvin cycle enzymes to activate them.

Figure 10.14 The Photochemical Reactions Stimulate the Calvin Cycle


Rubisco is an oxygenase as well as a carboxylase.

It can add O2 to RuBP instead of CO2, reducing the amount of CO2 fixed.

Its affinity for CO2 is about 10 times higher, thus carboxylation is usually favored.


But if O2 concentration in the leaf is high, O2 combines with RuBP, resulting in photorespiration:

RuBP + O2 → phosphoglycolate + 3PG

Phosphoglycolate (2 carbons) does not enter the Calvin cycle, but another metabolic pathway converts it to 3PG.


2 phosphoglycolate + O2 → 3PG + CO2

75% of the carbon from phosphoglycolate are recovered for the Calvin cycle; photorespiration reduces CO2 fixation by 25%.

Photorespiration consumes O2 and releases CO2. Occurs only in the light.

Figure 10.15 Organelles of Photorespiration (Part 1)

Figure 10.15 Organelles of Photorespiration (Part 2)


Photorespiration is more likely at high temperatures.

On hot, dry days stomata (leaf pores) are closed to prevent water loss. CO2 concentration falls as it is used in photosynthesis, and thus O2 concentration increases— photorespiration occurs.


Plants differ in how they fix CO2.

C3 plants: first product of CO2 fixation is 3PG. Cells in the leaf mesophyll have abundant rubisco.

On hot days, plants close stomata to conserve water, which limits entry of CO2 and photorespiration occurs.


In C4 plants, oxaloacetate (4 carbons) is the first product of CO2 fixation.

They have a mechanism to increase CO2 near rubisco and isolate it from O2.

CO2 is fixed in mesophyll cells by PEP carboxylase into a 3-carbon compound, phosphoenolpyruvate (PEP), then to oxaloacetate.

PEP carboxylase has no oxygenase activity and fixes CO2 even when levels are low.

Figure 10.16 Leaf Anatomy of C3 and C4 Plants


Oxaloacetate is converted to malate; it diffuses to bundle sheath cells, which have modified chloroplasts that concentrate CO2 around rubisco.

Malate is decarboxylated to pyruvate and CO2. Pyruvate moves back to mesophyll cells to regenerate PEP, which requires ATP.

The CO2 enters the Calvin cycle.

Figure 10.17 The Anatomy and Biochemistry of C4 Carbon Fixation (Part 1)

Figure 10.17 The Anatomy and Biochemistry of C4 Carbon Fixation (Part 2)


C4 plants must use some energy to “pump up” CO2 concentration in bundle sheath cells.

In cool, cloudy conditions, C3 plants have an advantage, but in warmer, dryer climates, C4 plants have the advantage, since photorespiration does not occur.


C3 plants are more efficient; but C4 plants may have evolved in response to declining CO2 levels 12 mya.

Atmospheric CO2 levels have been increasing over the last 200 years. Further increases may give C3 plants an advantage if CO2 becomes high enough to prevent photorespiration.


Some plants have crassulacean acid metabolism (CAM).

CO2 is initially fixed into a 4-C molecule by PEP carboxylase, but fixation and the Calvin cycle are separated in time, not space.

Night: CO2 fixed by PEP carboxylase; stomata are open, but less water loss occurs. Malate is stored.


Day: Stomata close to conserve water; malate moves to chloroplasts and is decarboxylated.

This supplies CO2 for the Calvin cycle, and light reactions provide ATP and NADPH.

CAM plants include water-storing plants (succulents) of the family Crassulaceae, many cacti, pineapples, and others.

Table 10.1


Green plants can synthesize all the molecules they need from simple starting materials: CO2, H2O, phosphate, sulfate, ammonium ions, and other minerals.

They use the carbohydrates produced in photosynthesis to produce energy by respiration and (rarely) fermentation. Respiration occurs in both the light and dark.


Photosynthesis and respiration are closely linked through the Calvin cycle.

Partitioning of G3P is important:

• Some goes to the cytosol and enters glycolysis and cellular respiration or is used to make other compounds.

• Some enters gluconeogenesis— sugars are formed and transported to other parts of the plant.

Figure 10.18 Metabolic Interactions in a Plant Cell (Part 1)

Figure 10.18 Metabolic Interactions in a Plant Cell (Part 2)


Only 5% of total sunlight energy is transformed to the energy of chemical bonds.

Understanding the inefficiencies of photosynthesis may be important as climate change drives changes in photosynthetic activity of plants.

Figure 10.19 Energy Losses in Photosynthesis

10 Answer to Opening Question

Higher CO2 concentration generally leads to increased photosynthesis, especially in C3 plants.

C3 crops such as wheat and rice may grow more, but the parts that we eat (seeds) may not grow more.

10 Answer to Opening Question

Increased plant growth may be countered by the effects of CO2 on climate—increased temperatures and changing rainfall patterns.

Biologists estimate that increased CO2 will result in moderately increased food production.

10 Photosynthesis: Energy from Sunlight. 10 Photosynthesis: Energy from Sunlight 10.1 What Is Photosynthesis? 10.2 How Does Photosynthesis Convert Light.

Documents

light energy

photosynthesis rates

anoxygenic photosynthesis

chemical energy

light reactions

source of o2

co2 levels

lightindependent reactions