Chapter 7: Capturing Solar Energy: Photosynthesis

7.1 What Is Photosynthesis?

• Living things need energy to survive, mostly from sunlight either directly or indirectly.

• Photosynthesis: The process by which light energy is captured and stored as chemical energy in organic molecules like sugar.

• Photosynthesis is crucial for life, providing fuel and oxygen.

• Performed by photosynthetic protists and bacteria in lakes and oceans, and mostly by plants on land.

• These organisms incorporate close to $100$ billion tons of carbon into their bodies annually.

Leaves and Chloroplasts Are Adaptations for Photosynthesis

• Leaves are adapted for photosynthesis with a flattened shape for large surface area exposure to the sun.

• Thinness ensures sunlight can reach the chloroplasts inside.

• Epidermis: Transparent cells forming the upper and lower surfaces, protecting the inner leaf and allowing light penetration.

• Cuticle: A transparent, waxy, waterproof covering on the epidermis that reduces water evaporation.

• Stomata: Adjustable pores in the epidermis through which the leaf obtains carbon dioxide ($CO_2$) from the air.

• Mesophyll: Layers of cells inside the leaf where most chloroplasts are located.

• Vascular bundles: Veins in the leaf that supply water and minerals and carry sugars to other parts of the plant.

• Bundle sheath cells: Surround the vascular bundles; lack chloroplasts in most plants.

• Photosynthesis occurs within chloroplasts, mostly in mesophyll cells. A single mesophyll cell often contains $40$ to $50$ chloroplasts.

• Chloroplasts are small (about $5$ micrometers in diameter).

• Chloroplast structure:- Double outer membrane enclosing the stroma.

  • Stroma: Semifluid substance within the chloroplast.

  • Thylakoids: Disk-shaped, interconnected membranous sacs embedded in the stroma.

  • Thylakoid space: Fluid-filled region enclosed by thylakoids.

  • Light reactions of photosynthesis occur in and adjacent to the thylakoids.

  • Calvin cycle reactions occur in the stroma.

Photosynthesis Consists of the Light Reactions and the Calvin Cycle

• Photosynthesis converts sunlight energy into chemical energy stored in glucose bonds, using carbon dioxide and water, and releases oxygen as a by-product.

• Overall chemical reaction for photosynthesis:$<br>6 CO<em>2 + 6 H</em>2O + \text{light energy} \rightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2$

• Photosynthesis involves dozens of reactions, each catalyzed by a separate enzyme.

• Occurs in two stages: light reactions and the Calvin cycle.

• Each stage takes place in a different region of the chloroplast but are connected by energy-carrier molecules.

Light Reactions

• Chlorophyll and other molecules in the thylakoid membranes capture sunlight energy converting some of it into chemical energy stored in ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

• Water is split, and oxygen gas is released.

Calvin Cycle

• Enzymes in the stroma use carbon dioxide from the atmosphere and chemical energy from ATP and NADPH to synthesize a three-carbon sugar that will be used to make glucose.

• The "photo" part refers to the capture of light energy, charging ADP and NADP+ to form ATP and NADPH.

• The "synthesis" part refers to the Calvin cycle, which captures carbon to synthesize sugar, powered by ATP and NADPH.

• The depleted carriers (ADP and NADP+) are then recharged by the light reactions into ATP and NADPH.

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?

• The light reactions capture sunlight energy, storing it as chemical energy in ATP and NADPH.

• Molecules that make these reactions possible, including light-capturing pigments and enzymes, are anchored in the thylakoid membranes.

Light Is Captured by Pigments in Chloroplasts

• The sun emits energy that spans a broad spectrum of electromagnetic radiation.

• The electromagnetic spectrum ranges from short-wavelength gamma rays, through ultraviolet, visible, and infrared light, to very long-wavelength radio waves.

• Light and other electromagnetic waves are composed of individual packets of energy called photons.

• The energy of a photon corresponds to its wavelength: short-wavelength photons, such as gamma and X-rays, are very energetic, whereas longer-wavelength photons, such as radio waves, carry lower energies.

• Visible light consists of wavelengths with energies that are high enough to alter biological pigment molecules (light-absorbing molecules) such as chlorophyll, but not high enough to break the bonds of crucial molecules such as DNA.

Absorption of Light

• When light strikes an object: it is either reflected, transmitted, or absorbed.

• Reflected or transmitted wavelengths determine the color of the object.

• Absorbed light can drive biological processes such as photosynthesis.

• Chloroplasts contain pigment molecules that absorb different wavelengths of light.

• Chlorophyll a: The key light-capturing pigment molecule in chloroplasts.- Strongly absorbs violet, blue, and red light, but reflects green, giving leaves their color.

• Accessory pigments: Absorb additional wavelengths of light energy and transfer their energy to chlorophyll a.- Chlorophyll b: A slightly different form of chlorophyll that absorbs some of the blue and red-orange wavelengths of light that are missed by chlorophyll a and reflects yellow-green light.

  • Carotenoids: Accessory pigments found in all chloroplasts that absorb blue and green light and appear mostly yellow or orange because they reflect these wavelengths.

  • Beta-carotene is a carotenoid that animals convert into vitamin A which is used to synthesize the light-capturing pigment in our eyes.

• In autumn, chlorophyll breaks down before carotenoids, revealing yellow and orange pigments as fall colors.

The Light Reactions Occur in Association with the Thylakoid Membranes

• Light energy is captured and converted into chemical energy by the light reactions that occur in and on the thylakoid membranes.

• These membranes contain many photosystems, each consisting of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins.

• Two photosystems—photosystem II and photosystem I—work together during the light reactions.

• Each photosystem has an electron transport chain located adjacent to it. Each electron transport chain consists of a series of electron-carrier molecules embedded in the thylakoid membrane.

• The overall path of electrons is photosystem II → electron transport chain II → photosystem I → electron transport chain I → NADP+.

Photosystem II Uses Light Energy to Create a Hydrogen Ion Gradient and to Split Water

1. Photons of light are absorbed by pigment molecules clustered in photosystem II.

2. Energy hops from one pigment molecule to the next until it reaches the photosystem II reaction center.

3. Energy boosts an electron from one of the reaction center chlorophylls to the primary electron acceptor, which captures the energized electron.

4. The reaction center of photosystem II must be supplied continuously with electrons to replace those that are boosted out of it when energized by light. Replacement electrons come from water.

5. Water molecules are split by an enzyme associated with photosystem II, liberating electrons that will replace those lost by the reaction center chlorophyll molecules. Splitting water also releases two hydrogen ions ($H^+$) that are used in forming the $H^+$ gradient that drives ATP synthesis. For every two water molecules split, one molecule of $O_2$ is produced.

6. Once the primary electron acceptor in photosystem II captures the electron, it passes the electron to the first molecule of electron transport chain II.

7. The electron travels from one electron carrier molecule to the next, losing energy as it goes. Some of this energy pumps $H^+$ across the thylakoid membrane into the thylakoid space, where it will be used to generate ATP.

8. The energy-depleted electron leaves electron transport chain II and enters the reaction center of photosystem I, where it replaces the electron ejected when light strikes photosystem I.

Photosystem I Generates NADPH

• Light strikes the pigment molecules of photosystem I.

• This light energy is passed to a chlorophyll a molecule in the reaction center.

• This energizes an electron that is absorbed by the primary electron acceptor of photosystem I, replacing the energy-depleted electron from electron transport chain II.

• The energized electron is passed along electron transport chain I until it reaches NADP+.

• When an NADP+ molecule (dissolved in the fluid stroma) picks up two energetic electrons along with one hydrogen ion, the energy-carrier molecule NADPH is formed.

The Hydrogen Ion Gradient Generates ATP by Chemiosmosis

• As an energized electron travels along electron transport chain II, some of the energy the electron liberates is used to pump $H^+$ into the thylakoid space, creating a high concentration of $H^+$ inside the space and a low concentration in the surrounding stroma.

• During chemiosmosis, $H^+$ flows back down its concentration gradient through a special type of channel that spans the thylakoid membrane.

• This channel, called ATP synthase, generates ATP from ADP and phosphate dissolved in the stroma as the $H^+$ flows through the channel.

• Analogy: $H^+$ gradient is like water stored behind a dam at a hydroelectric plant. The hydrogen ions can flow into the stroma only through ATP synthase channels as turbines generating electricity.

• Light Reactions Summary:

  • Chlorophyll and carotenoid pigments of photosystem II absorb light.

  • Energy is transferred to a reaction center chlorophyll a molecule, which ejects an electron.

  • The electron is captured by a primary electron acceptor molecule.

  • The electron is passed to electron transport chain II, releasing energy used to create a hydrogen ion gradient across the thylakoid membrane.

  • This gradient drives ATP synthesis by chemiosmosis.

  • Enzymes associated with photosystem II split water, releasing electrons, $H^+$, and oxygen.

  • Photosystem I absorbs light, ejecting an electron. The electron is replaced by an energy-depleted electron from electron transport chain II.

  • The energized electron passes through electron transport chain I.

  • NADPH is formed from NADP+ and $H^+$.

  • Overall products: NADPH, ATP, and oxygen.

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?

• Cells produce carbon dioxide as they burn sugar for energy but cannot fix carbon atoms in $CO_2$ to form organic molecules.

• Nearly all carbon fixation is performed by photosynthetic organisms.

• Carbon is captured from atmospheric $CO_2$ during the Calvin cycle using energy from sunlight harnessed during the light reactions.

• Discovered by Melvin Calvin, Andrew Benson, and James Bassham using radioactive isotopes of carbon.

The Calvin Cycle Captures Carbon Dioxide

• ATP and NADPH synthesized during the light reactions are dissolved in the fluid stroma surrounding the thylakoids.

• There, these energy carriers power the synthesis of the three-carbon sugar glyceraldehyde-3-phosphate (G3P) from $CO_2$ during the Calvin cycle.

• The Calvin cycle begins and ends with the same five-carbon molecule, ribulose bisphosphate (RuBP).

• Each "turn" captures three molecules of $CO_2$ and produces one molecule of G3P.

• The Calvin cycle has three parts: (1) carbon fixation, (2) the synthesis of G3P, and (3) the regeneration of RuBP that allows the cycle to continue.

Carbon Fixation

• Carbon from $CO_2$ is incorporated into larger organic molecules.

• The enzyme rubisco combines three $CO_2$ molecules with three RuBP molecules to produce three unstable six-carbon molecules that immediately split in half, forming six molecules of phosphoglyceric acid (PGA, a three-carbon molecule).

• Because carbon fixation generates this three-carbon PGA molecule, the Calvin cycle is often referred to as the C3 pathway.

Synthesis of G3P

• Occurs via a series of reactions using energy from ATP and NADPH.

• Six 3-carbon PGA molecules are rearranged to form six 3-carbon G3P molecules.

Regeneration of RuBP

• Three molecules of RuBP are regenerated from five of the six G3P molecules, powered by ATP.

• The remaining G3P molecule, the end product of photosynthesis, exits the Calvin cycle.

• Carbon fixation can be sabotaged because the enzyme rubisco is not very selective, and will cause either $CO<em>2$ or $O</em>2$ to combine with RuBP. If $O_2$ is fixed, the result is a wasteful process called photorespiration.

C4 Plants

• C4 plants capture carbon and synthesize sugar in different cells.

• The C4 pathway selectively captures carbon in their mesophyll chloroplasts.

• PEP carboxylase causes $CO_2$ to react with phosphoenolpyruvate (PEP).

• This produces oxaloacetate, from which the C4 pathway gets its name.

• Oxaloacetate is converted into malate, which diffuses from the mesophyll cells into bundle sheath cells.

• In C4 plants, Calvin cycle enzymes (including rubisco) are only present in the chloroplasts of the bundle sheath cells.

• In the bundle sheath cells, malate is broken down, forming pyruvate and releasing $CO<em>2$. This generates a high $CO</em>2$ concentration allowing rubisco to fix carbon with little competition from $O_2$, minimizing photorespiration.

• The pyruvate is transported back into the mesophyll cells. Here, ATP energy is used to convert pyruvate back into PEP, allowing the cycle to continue.

CAM Plants

• CAM plants capture carbon and synthesize sugar at different times.

• Unlike C4 plants, CAM plants do not use different cell types to capture carbon and to synthesize sugar. Instead, they perform both activities in the same mesophyll cells, but at different times; carbon fixation occurs at night, and sugar synthesis occurs during the day

• The stomata of CAM plants open at night

• The malate produced by the C4 pathway is then shuttled into the central vacuole where it is stored as malic acid until daytime.

• During the day, when stomata are closed to conserve water, the malic acid leaves the vacuole and re-enters the cytoplasm as malate.

• The malate is broken down, forming pyruvate and releasing carbon which enters the Calvin cycle (via rubisco) to produce sugar.

Carbon Fixed During the Calvin Cycle Is Used to Synthesize Glucose

• Two G3P molecules can be combined to form one six-carbon glucose molecule.

• Glucose can be used to synthesize sucrose (table sugar), starch, or cellulose.

• Some glucose molecules are also broken down during cellular respiration.

Calvin Cycle Summary

• Carbon fixation: Three RuBP capture three $CO_2$, forming six PGA.

• G3P synthesis: Energy from ATP and NADPH produces six G3P, one of which leaves the cycle.

• RuBP regeneration: ATP energy regenerates three RuBP molecules from the remaining five G3P molecules.

• Two G3P molecules combine to form glucose.