3.4 Cellular Energetics: Respiration and Photosynthesis

Cellular Respiration and Energy Production

Cellular energetics covers how cells obtain and use energy. Respiration breaks down nutrients to produce ATP, while photosynthesis captures light energy to build glucose. Together, these two processes form the foundation of energy flow through living systems and connect directly to cell structure, metabolism, and ecology.

more resources to help you study

practice questions

Overview of Cellular Respiration

Cellular respiration is the metabolic pathway that breaks down glucose and other organic molecules to release energy in the form of ATP (adenosine triphosphate). Every cell in your body depends on this process to power its work.

The overall equation:

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

Cellular respiration occurs in three main stages: glycolysis, the Krebs cycle (citric acid cycle), and the electron transport chain (ETC). Each stage happens in a different location within the cell and contributes differently to total ATP yield.

Stages of Cellular Respiration

1. Glycolysis (cytoplasm)

• Splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules
• Net yield: 2 ATP and 2 NADH per glucose
• Does not require oxygen, so it occurs in both aerobic and anaerobic conditions

2. Krebs Cycle (mitochondrial matrix)

• Pyruvate is first converted to acetyl-CoA (releasing one $CO_2$ per pyruvate), then acetyl-CoA enters the cycle
• Each turn of the cycle oxidizes the acetyl group completely to $CO_2$, generating NADH, $FADH_2$, and a small amount of ATP (as GTP)
• Two turns occur per glucose molecule (one per pyruvate)

3. Electron Transport Chain and Oxidative Phosphorylation (inner mitochondrial membrane)

• NADH and $FADH_2$ donate electrons to a series of protein complexes (Complex I → II → III → IV)
• As electrons pass through the chain, energy is released and used to pump $H^+$ ions across the inner membrane, creating a proton gradient
• $H^+$ ions flow back through ATP synthase, driving the synthesis of ATP via chemiosmosis
• Oxygen serves as the final electron acceptor, combining with electrons and $H^+$ to form water

This is where the bulk of ATP is made. The ETC alone accounts for roughly 26–28 of the 30–32 total ATP produced per glucose.

ATP powers cellular work such as:

• Biosynthesis (building proteins, DNA replication)
• Active transport (the sodium-potassium pump maintaining ion gradients)
• Muscle contraction (myosin pulling on actin filaments)

Mitochondria's Role in Respiration

Mitochondrial Structure and Function

Mitochondria are double-membrane organelles where most ATP production takes place. Their structure is directly tied to their function:

• The outer membrane is permeable to small molecules and ions
• The inner membrane is highly folded into cristae, which increase surface area for the electron transport chain complexes (I, II, III, IV) and ATP synthase
• The matrix (the space inside the inner membrane) contains the enzymes for the Krebs cycle, including citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase

The folding of the inner membrane is a key structural adaptation. More surface area means more room for ETC complexes, which means greater capacity for ATP production.

Mitochondrial Genetics and Endosymbiotic Theory

Mitochondria have their own circular DNA, their own ribosomes (similar in size to bacterial ribosomes), and the ability to replicate independently within the cell. These features are strong evidence for the endosymbiotic theory, which proposes that mitochondria originated from ancient aerobic bacteria that were engulfed by a primitive eukaryotic cell. Over time, the relationship became mutually beneficial and permanent.

The number of mitochondria per cell varies with energy demand. Skeletal muscle cells and neurons contain thousands of mitochondria because they consume large amounts of ATP. Less metabolically active cells, like skin cells, have far fewer.

Aerobic vs. Anaerobic Respiration

Aerobic Respiration

Aerobic respiration requires oxygen and includes all three stages: glycolysis, the Krebs cycle, and the electron transport chain. It produces approximately 30–32 ATP per glucose molecule, making it far more efficient than anaerobic pathways.

Most eukaryotes rely primarily on aerobic respiration, including animals, plants, fungi, and many bacteria. Even plants, which produce glucose through photosynthesis, use aerobic respiration to break that glucose down for energy.

Anaerobic Respiration and Fermentation

When oxygen is unavailable, cells can still produce ATP through glycolysis alone, yielding only 2 ATP per glucose. To keep glycolysis running, cells must regenerate $NAD^+$ from NADH. That's where fermentation comes in. Fermentation doesn't produce additional ATP; its purpose is to recycle $NAD^+$ so glycolysis can continue.

There are two main types:

• Lactic acid fermentation: Pyruvate is converted to lactate. This happens in your muscle cells during intense exercise when oxygen delivery can't keep up with demand. The buildup of lactate contributes to the burning sensation and fatigue you feel.
• Alcoholic fermentation: Pyruvate is converted to ethanol and $CO_2$. Yeast and some bacteria use this pathway. It's the basis of bread-making ($CO_2$ causes dough to rise) and alcohol production (beer, wine).

Some organisms, called facultative anaerobes (like E. coli), can switch between aerobic and anaerobic pathways depending on whether oxygen is available. This flexibility gives them a survival advantage in changing environments.

Photosynthesis and Energy Capture

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose. It's the entry point for almost all energy in biological systems.

The overall equation:

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

Notice this is essentially the reverse of cellular respiration. Photosynthesis occurs in two stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).

Light-Dependent and Light-Independent Reactions

Light-Dependent Reactions (thylakoid membranes of chloroplasts)

1. Photosynthetic pigments absorb light energy. Chlorophyll a is the primary pigment, while chlorophyll b and carotenoids (beta-carotene, xanthophylls) are accessory pigments that capture additional wavelengths and funnel energy to the reaction centers.
2. Light energy hits Photosystem II (PSII) first, exciting electrons to a higher energy level. Water molecules are split (photolysis) to replace those electrons, releasing $O_2$ as a byproduct.
3. Excited electrons pass through an electron transport chain between PSII and Photosystem I (PSI), and the energy released pumps $H^+$ into the thylakoid space, building a proton gradient.
4. $H^+$ flows back through ATP synthase, producing ATP (photophosphorylation).
5. At PSI, electrons are re-energized by light and ultimately used to reduce $NADP^+$ to NADPH.

The net products of the light reactions are ATP, NADPH, and $O_2$.

Light-Independent Reactions / Calvin Cycle (stroma of chloroplasts)

1. Carbon fixation: The enzyme RuBisCO attaches $CO_2$ to a 5-carbon molecule (RuBP), producing two 3-carbon molecules (G3P).
2. Reduction: ATP and NADPH from the light reactions are used to convert G3P into higher-energy forms.
3. Regeneration: Most G3P molecules are recycled to regenerate RuBP so the cycle can continue. For every 3 turns of the cycle (3 $CO_2$ fixed), one net G3P molecule exits the cycle.

It takes 6 turns of the Calvin cycle to produce enough G3P to assemble one glucose molecule.

The glucose produced is used by the plant for growth, cellular respiration, and storage as starch (amylose and amylopectin) or structural polysaccharides like cellulose.

Cellular Respiration and Photosynthesis in the Carbon Cycle

Complementary Nature of Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are complementary processes that drive the global carbon cycle:

• Photosynthesis removes $CO_2$ from the atmosphere and incorporates carbon into organic molecules
• Cellular respiration breaks down those organic molecules and releases $CO_2$ back into the atmosphere
• The $O_2$ released by photosynthesis is consumed by aerobic respiration; the $CO_2$ released by respiration is consumed by photosynthesis

This reciprocal relationship keeps atmospheric $CO_2$ and $O_2$ levels relatively stable under natural conditions.

Carbon Cycle Disruptions and Environmental Impact

Human activities have disrupted this balance. Burning fossil fuels (coal, oil, natural gas) releases carbon that was locked underground for millions of years, adding $CO_2$ to the atmosphere faster than photosynthesis can remove it. Deforestation, particularly in carbon-dense ecosystems like the Amazon rainforest, reduces the planet's photosynthetic capacity at the same time.

The result is rising atmospheric $CO_2$, which drives climate change through the greenhouse effect. Excess $CO_2$ also dissolves in ocean water, forming carbonic acid and lowering ocean pH, a process called ocean acidification that threatens marine ecosystems, particularly organisms that build calcium carbonate shells and skeletons (corals, mollusks).