15.1 Photosynthesis Part 1

Cellular Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Photosynthesis Equation

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Photosynthesis

A process that transforms light energy into chemical energy (the potential energy found in electrons that participate in covalent bonds, usually in sugars or other carbohydrates).

Light Reaction

A light-dependent reaction that makes water provides essential electrons to create the 12 Hs of glucose, 12 Hs of water, and 12 oxygen of O₂. The process takes light energy to convert it into chemical energy via electron transport in the thylakoid membrane.

Light Reactions Sum

For every water split, 2 electrons, 2 H+, and an O atom are released.

As the 2 electrons move from Photosystem II to Photosystem I, their physical movement provides the energy to pump protons into the thylakoid lumen. Reducing one molecule of NADP+ requires 2 electrons and 1 H+.

1.5 ATP are generated and 1 NADP+ is reduced into NADPH.

Light reactions photosynthesis: 12 waters = 18 ATP + 12 NADPH

Dark Reaction/Calvin Cycle

Light-independent reaction that fixes (reduces) CO₂ into 6 Cs of glucose, 6 Os of glucose, and 6 Os for water. It takes chemical energy (ATP, NADPH) and puts it into the chemical bonds between carbons to make sugar in the stroma.

Chloroplasts

A specialized double-membrane organelle found in plant cells and green algae that conducts photosynthesis.

Thylakoid

Membrane-bound compartment within plant and algae chloroplasts. The extensive membranes provide a large surface area for many copies of the photosynthetic machinery (PSII, PSI, the ETC, and ATP synthase) to reside, meaning that a lot of ATP and NADPH production can occur in a small space.

Pigment

A molecule that absorbs specific wavelengths of light.

Chlorophyll

The primary photosynthetic pigment in the thylakoid that absorbs light in the 400 to 700 nm photosynthetically active range (PAR). Has a Mg porphyrin ring that acts as the light-absorbing head in of the molecule. Only handles one electron at a time during the excitation process.

The absorption of light causes electrons to move to excited states, and as the electron drops back down to its normal level, energy is released as heat and fluorescence viewable with a UV light. The energy released excites an electron in a neighboring chlorophyll molecule.

Special Pair

A tightly bound dimer of chlorophyll a located at the reaction center of a photosystem. It uses the resonance energy collected from other chlorophylls to drive the transfer of electrons to electron carriers.

The Mg-porphyrin ring is the part of this chlorophyll pair that loses and passes the electron to the primary electron acceptor.

Thylakoid Membrane

A specialized, internal membrane system that houses chlorophyll pigments. Where light-dependent reactions of photosynthesis occur.

Thylakoid Lumen

The water-filled compartment enclosed by the thylakoid membrane inside chloroplasts and cyanobacteria.

Grana

Stacks of thylakoids, interconnected by tubular extensions called stromal lamellae.

Stroma

The space between the grana and the inner membrane of the chloroplast.

Inter-Membrane Space

The fluid-filled area situated between the outer and inner membranes of organelles like mitochondria and chloroplasts.

Photosystems

Protein light-harvesting complexes found in the thylakoid membrane that contain pigment molecules (chlorophyll and others). When a photon strikes a pigment molecule in a light-harvesting complex (outer areas), the energy is passed from molecule to molecule until it reaches the reaction center complex (inner area). Here, an excited electron from the special pair of chlorophyll a molecules is transferred t the primary electron acceptor.

Photosystem II (PSII)/P680

A photosystem that absorbs light first, working best within 680nm wavelength. Water is oxidized at the reaction-center complex for this photosystem into ½ O₂ and 2H+.

These products are dropped off in the lumen side, oxidizing O₂ and reducing the reaction-center complex with 2 electrons. O₂ is released as a byproduct into the atmosphere or used for cellular respiration, while the H ions are used to create an electrochemical gradient that drives ATP synthesis.

Leftover electrons from photolysis replenish those passed onto the electron carrier.

PSII Electron Replenishment

PSII already has electrons sitting in its chlorophyll (P680) from a previous round of water splitting. Sunlight hits PSII. It ejects one of its active electrons into the ETC. This electron is now on its way to PSI. PSII is now missing an electron. It has a positive charge (P680+) and is desperate for a replacement.

Water is split (photolysis) and electrons are yielded. One of the "leftover" electrons from water is handed to PSII to fill that exact vacancy. The new electron from water is sitting inside PSII. When the next photon of light hits PSII, this exact water electron gets energized and ejected into the ETC toward PSI.

Photosystem I (PS1)/P700

A complex of molecular machines that receive low-energy electrons at the end of the electron transport chain, use the energy in sunlight to re-excite those electrons to a high-energy state, and pass them on to electron carriers that either feed the ETC or an enzyme that catalyzes the reduction of NADP+ to NADPH. It is more electronegative than PSII, working best within 700 nm.

Energy from the passage of electrons to more electronegative complexes is used to pump protons into the thylakoid lumen.

Electron Transfer

Electrons enter the complex at the bottom middle of PSI at a site where the electron carrier from the ETC binds. Once the electrons reach an excited state, they are passed up and out of the complex to another electron carrier, which binds to the complex at the top middle.

In most cases, the electron carrier passes these excited electrons to an enzyme that reduces NADP+ to NADPH. NADPH is a molecule that is essential for reducing the carbon atoms in CO₂ and producing sugars (photosynthesis equation).

Chemiosmotic Phosphorylation

H+ travels out of the thylakoid lumen and into the stroma through ATP synthase, creating ATP by going down the concentration, pH, and charge gradients.

The intermembrane space of mitochondria is equivalent to the thylakoid lumen. Both the intermembrane space and the thylakoid lumen serve as the "high-proton reservoirs" (high concentration, low pH) that power ATP synthesis.

There are some slight differences in ATP synthases, but the catalytic core (the part that makes ATP) has about 70% sequence similarity between mitochondria and chloroplasts.

In regards to ETCs, which part of the mitochondria is analogous to the chloroplast?

High energy electrons in photosynthesis are excited by light energy, whose absorption of a photon changes the energy state and used in the electron chain to high energy molecules (ATP).

High energy electrons in respiration are used to transfer energy into other high energy molecules (ATP) via bonds, but are not actually in an excited state.

What is different between how we describe “high energy electrons” in photosynthesis vs. respiration?

Both electron transport chains use electrons being passed along down the chain to pump protons to make a proton gradient and generate ATP by using a proton gradient to power ATP synthase.

The electron chains are different in several ways (different # of proton pumps, different membranes, different spaces, etc.)

Consider the following statement. “The electron transport chain in mitochondria and chloroplasts work in the same way to generate ATP.”

What is accurate and inaccurate about the above statement?