Cellular Respiration and Photosynthesis: An Overview

$$\begin{aligned} ATP + H_2O \rightarrow ADP + P_i \end{aligned}$$

Chemical reactions are classified by their energy flow: exergonic reactions release free energy, while endergonic reactions require an input of energy to proceed.

Redox Reactions and Electron Carriers

Energy transfer in biological systems is almost always achieved through redox reactions. These involve the movement of electrons between molecules. To remember the difference, use the mnemonic OIL RIG: Oxidation Is Loss (of electrons), and Reduction Is Gain (of electrons).

Because electrons are highly energetic, cells use specialized "taxis" to transport them. These electron carriers, such as NADH and FADH$_2$, pick up high-energy electrons from metabolic pathways and deliver them to the electron transport chain.

Cellular Respiration: Harvesting Energy

Cellular respiration is the process of breaking down glucose to harvest energy in the form of ATP. The overall aerobic equation is:

$$\begin{aligned} C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{energy (ATP and heat)} \end{aligned}$$

. In this process, glucose is oxidized and oxygen is reduced.

The Four Stages of Respiration

1. Glycolysis: Occurring in the cytosol, this stage splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules. This process yields a net of 2 ATP and 2 NADH. It is a form of substrate-level phosphorylation, where ATP is produced directly during the reaction.
2. Pyruvate Oxidation: The pyruvates enter the mitochondrial matrix, where they are converted into acetyl-CoA. This step releases $CO_2$ and produces NADH.
3. Citric Acid (Krebs) Cycle: Also in the mitochondrial matrix, acetyl-CoA is fully oxidized. This cycle produces $CO_2$, a small amount of ATP (or GTP), and a large quantity of the electron carriers NADH and FADH$_2$.
4. Oxidative Phosphorylation: This final stage occurs in the inner mitochondrial membrane and produces the bulk of the cell's ATP. It consists of two parts:
   • The Electron Transport Chain (ETC): NADH and FADH$_2$ donate electrons to a series of proteins. As electrons move down the chain, their energy is used to pump $H^+$ ions from the matrix into the intermembrane space, creating an electrochemical gradient.
   • Chemiosmosis: The $H^+$ ions flow back into the matrix through a specialized protein called ATP synthase. This flow drives the phosphorylation of ADP into ATP.

Crucial Concept: Oxygen serves as the final electron acceptor at the end of the ETC. It combines with electrons and $H^+$ to form water ($H_2O$). Without oxygen, the chain backs up, and ATP production via oxidative phosphorylation halts.

Fermentation: Life Without Oxygen

When oxygen is unavailable, cells cannot perform oxidative phosphorylation. To prevent glycolysis from stopping, cells undergo fermentation. The primary goal of fermentation is not to make more ATP, but to regenerate NAD$^+$ from NADH so that glycolysis can continue to produce its net 2 ATP per glucose.

• Lactic acid fermentation produces lactate.
• Alcoholic fermentation produces ethanol and $CO_2$.

Photosynthesis: Building Energy Stores

While respiration breaks down sugars, photosynthesis builds them. The simplified equation for photosynthesis is:

$$\begin{aligned} 6 CO_2 + 6 H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6 O_2 \end{aligned}$$

. This occurs in the chloroplasts, specifically involving the thylakoids (where light is absorbed) and the stroma (the fluid surrounding the thylakoids).

Light-Dependent Reactions and the Calvin Cycle

The process is divided into two main stages:

1. Light-Dependent Reactions (in the thylakoid membrane): Chlorophyll absorbs light energy, which is used to split water molecules, releasing $O_2$ as a byproduct. The energy from the light is used to create two "batteries": ATP and NADPH.

2. The Calvin Cycle (in the stroma): Also known as light-independent reactions, the Calvin cycle uses the ATP and NADPH from the light reactions to "fix" carbon from $CO_2$ into organic molecules. The cycle follows three phases:

• Carbon Fixation: Rubisco attaches $CO_2$ to RuBP.
• Reduction: ATP and NADPH are used to convert the molecules into G3P (a three-carbon sugar).
• Regeneration: ATP is used to rebuild RuBP so the cycle can continue.

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