Overview of Cellular Respiration and Photosynthesis

Energy and Matter in Ecosystems

Energy and matter cycle through ecosystems, following the laws of conservation. Cellular respiration and photosynthesis are central to this cycle, enabling organisms to convert energy and matter into usable forms.

• Cellular respiration occurs in mitochondria of all eukaryotic cells (animals, plants, fungi, protists).

• Chloroplasts are found in plants and photosynthetic protists/algae; cyanobacteria (prokaryotes) contain chlorophyll but lack organelles.

• Prokaryotes perform cellular respiration using their cell membrane, not mitochondria or chloroplasts.

• The main role of mitochondria is to generate ATP using oxygen and glucose, producing CO2 and water as byproducts.

Cellular Respiration: Process and Stages

Requirements and Key Molecules

Cellular respiration is a continuous process in all cells, converting glucose and oxygen into ATP, the energy currency of the cell.

• Glucose is the fuel; plants synthesize it via photosynthesis, animals obtain it from food.

• Oxygen is the final electron acceptor in the Electron Transport Chain (ETC).

• Gas exchange in animals occurs in the lungs via diffusion along concentration gradients.

• ATP is essential for cellular processes; its absence leads to cell death (e.g., cyanide poisoning blocks ATP production).

• NADH and FADH2 are electron carriers involved in redox reactions, transporting electrons to the ETC.

Stages of Cellular Respiration

Cellular respiration consists of three main stages, each occurring in specific cellular locations.

• Glycolysis (cytoplasm): Splits glucose into pyruvate, generating ATP and NADH.

• Citric Acid Cycle (mitochondrial matrix): Further breaks down pyruvate, releasing CO2, producing NADH, FADH2, and ATP.

• Oxidative Phosphorylation (inner mitochondrial membrane): Uses NADH/FADH2 in the ETC to create a proton gradient, powering ATP synthase.

Detailed Breakdown of Cellular Respiration Stages

• Glycolysis: One 6-carbon glucose is split into two 3-carbon pyruvate molecules, generating 2 ATP and NADH.

• Citric Acid Cycle: Pyruvate is further broken down, releasing CO2, producing NADH, FADH2, and 2 ATP.

• Electron Transport Chain (ETC): NADH and FADH2 transfer electrons, pumping protons to create a gradient.

• ATP Synthase: Uses the proton gradient to synthesize ATP from ADP and phosphate; oxygen is the final electron acceptor, forming water.

• Most ATP (about 28-30 per glucose) is produced during oxidative phosphorylation; glycolysis and the citric acid cycle each contribute 2 ATP.

• Poisons like cyanide disrupt the ETC, halting ATP production and causing cell death.

Equation for Cellular Respiration:

Anaerobic Respiration and Alternative Substrates

Fermentation Pathways

When oxygen is unavailable, cells perform fermentation, a less efficient process for ATP production.

• Lactic acid fermentation (animals): Produces lactate.

• Alcohol fermentation (yeast): Produces ethanol and CO2.

• Fermentation yields only 2 ATP per glucose, compared to aerobic respiration.

Alternative Substrates

• Cells can use fats and proteins for respiration; fats yield more ATP but require greater input, proteins are used as a last resort (can result in muscle loss).

• Feedback inhibition regulates metabolic cycles, slowing ATP production when sufficient ATP accumulates.

Photosynthesis: Process and Comparison to Respiration

Photosynthesis in Chloroplasts

Photosynthesis is the process by which plants and some protists synthesize glucose from light, CO2, and water.

• Occurs in chloroplasts, primarily in plant leaves.

• Plants are photoautotrophs, making their own food.

• Chloroplasts have a double membrane and internal thylakoid discs containing chlorophyll.

Stages of Photosynthesis

• Light-dependent reactions: Use sunlight and water to produce ATP, NADPH, and oxygen (byproduct).

• Calvin cycle: Uses ATP, NADPH, and CO2 to synthesize glucose; ATP produced here is used for glucose synthesis, not general cellular energy.

Equation for Photosynthesis:

Comparison of Photosynthesis and Cellular Respiration

• Photosynthesis builds glucose from CO2; respiration breaks glucose down to CO2.

• Electron transport in chloroplasts is similar to mitochondria but less efficient and oriented "inside out" (proton gradient inside thylakoid disks).

• ATP synthase in chloroplasts produces less ATP than in mitochondria.

Key Differences and Evolutionary Insights

Structural and Functional Comparisons

The ATP synthase and electron transport chains in mitochondria and chloroplasts are structurally similar, suggesting a common evolutionary origin.

• Mitochondria generate ATP for cellular functions; chloroplasts focus on glucose synthesis for ATP generation in mitochondria.

• Photosynthetic electron transport uses light energy, resulting in lower ATP yield compared to respiration.

• Plants rely on mitochondria for most of their ATP needs.

• Understanding efficiency and orientation differences is crucial for mastering these processes.

Summary Table: Comparison of Cellular Respiration and Photosynthesis

Example

• During exercise, muscle cells increase ATP demand, relying on cellular respiration; if oxygen is limited, lactic acid fermentation occurs, causing muscle fatigue.

• Plants use photosynthesis to produce glucose, which is then used in mitochondria for ATP production.