Cellular Respiration

Overview and Importance

Cellular respiration is a fundamental catabolic process by which cells convert glucose into ATP, the primary energy currency used to power cellular activities. This process occurs in all prokaryotic and eukaryotic cells, though the mechanisms differ slightly between these groups. - ATP is used for: movement (flagella, cilia), organelle activity (lysosome, chloroplast, ER), muscle contraction, cell division (mitosis & meiosis), exocytosis & endocytosis, phagocytosis, and sperm motility. - Diffusion and osmosis are passive processes and do not require ATP. - Cellular respiration is a universal process, but eukaryotes utilize mitochondria, while prokaryotes do not.

Metabolic Pathways: Catabolic vs. Anabolic

Metabolic pathways are classified as either anabolic (building molecules, requiring energy) or catabolic (breaking down molecules, releasing energy). Cellular respiration is a catabolic pathway.

• Anabolic Pathways: Small molecules are assembled into larger ones; energy is required.

• Catabolic Pathways: Large molecules are broken down into smaller ones; energy is released.

Summary Reaction of Cellular Respiration

The overall reaction for cellular respiration is: - A single molecule of glucose yields approximately 30-32 ATP molecules. - Direct combustion of glucose releases too much energy; cellular respiration captures energy in manageable ATP packets.

Relationship to Photosynthesis

- Photosynthesis is an anabolic pathway (builds glucose), while cellular respiration is catabolic (breaks down glucose). - Chloroplasts (photosynthesis) and mitochondria (cellular respiration) both have complex internal membranes. - Glucose produced in photosynthesis is the substrate for cellular respiration.

Differences Between Plants and Animals

• Plants: Produce glucose in chloroplasts, then transfer it to mitochondria for cellular respiration.

• Animals: Obtain glucose by consuming plants or other animals that have consumed plants.

Three Sequential Steps of Cellular Respiration

Step 1: Glycolysis

Glycolysis is the first step in cellular respiration and occurs in the cytoplasm. It is a primitive process, present in both prokaryotic and eukaryotic cells, and does not require oxygen. - Process: Glucose (C6H12O6) is converted to 2 pyruvate molecules. - Energy Budget: 2 ATP are invested, 4 ATP are produced, yielding a net gain of 2 ATP. - Isomers: Glucose and fructose are both hexose sugars (C6H12O6) but differ in spatial arrangement.

Step 2: Citric Acid (Krebs) Cycle

The citric acid cycle occurs in the mitochondria and is limited to eukaryotic cells. Each glucose molecule produces two pyruvates, so the cycle runs twice per glucose. - Process: Pyruvate is converted to Acetyl CoA, which enters the cycle. - Products per cycle: 1 ATP, 3 NADH, 1 FADH2 (per pyruvate; cycle runs twice per glucose). - Total ATP: 2 ATP per glucose. - Key molecules: Citric acid (citrate, found in citrus fruits) and malic acid (malate, found in apples) are present in all mitochondria.

Step 3: Electron Transport Chain (ETC)

The ETC occurs on the inner mitochondrial membrane (cristae) and is exclusive to eukaryotes. It produces the largest amount of ATP. - NADH and FADH2 from previous steps donate electrons. - Oxygen is required (aerobic process); plants derive O2 from photolysis, animals inhale it. - ATP yield: 26-28 ATP per glucose. - ATP synthase: Enzyme recognized by the 1997 Nobel Prize (Paul Boyer & John Walker) for its role in ATP production.

Summary Table: Cellular Respiration Steps

Anaerobic Respiration and Fermentation

Overview

Anaerobic respiration and fermentation occur in organisms living in oxygen-poor or oxygen-lacking environments. These processes are less efficient, producing fewer ATP molecules. - Examples: Mud flats, hot sulfur springs, deep-sea vents, cake batter (yeasts). - Yeasts: Unicellular fungi that can be aerobic or anaerobic.

Fermentation By-Products

Different yeast varieties produce economically important by-products:

• Baking yeast: Glucose → ATP + CO2 (CO2 causes dough to rise)

• Alcoholic yeast: Glucose → ATP + Ethanol + CO2 (used in alcoholic beverage production)

Key Points and Applications

• ATP is the universal energy currency for cells.

• Cellular respiration is essential for life, powering nearly all cellular activities except passive transport.

• Fermentation is important in food and beverage industries.

• Nobel Prize awarded for discovery of ATP synthase, a key enzyme in ATP production.

Additional info:

• ATP synthase is a membrane-bound enzyme complex responsible for the synthesis of ATP from ADP and inorganic phosphate during oxidative phosphorylation.

• Fermentation pathways vary: lactic acid fermentation (in muscle cells) and alcoholic fermentation (in yeast).