Bioenergetics: Energy for Life

Bioenergetics Overview

• Bioenergetics is the study of how organisms acquire, convert, store, and use energy for life processes.
• Organisms primarily get energy from their surroundings.
• Plants use photosynthesis with sunlight, while animals consume food.
• This energy is converted into usable chemical energy and stored in ATP.
• ATP is a ready energy source used for growth, movement, repair, and reproduction.
• Oxidation-reduction (redox) reactions, where electrons transfer between molecules, are vital for metabolism.
• Oxidation involves a molecule losing electrons, while reduction involves gaining electrons.
• Electron flow drives ATP generation via cellular respiration and photosynthesis.
• Bioenergetics focuses on energy transformations in living organisms.

ATP: The Cell's Energy Currency

• Cells use adenosine triphosphate (ATP) as a nucleotide-based energy currency.
• Cells create ATP to store energy and break it down when energy is needed.
• The ATP molecule has three subunits: adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups.
• Covalent bonds between two phosphates in ATP are high-energy bonds.
• Breaking one phosphate bond releases about 7.3 kcal (7,300 calories) per mole of ATP.
• ATP + H2O → ADP + Pi + energy (7.3 kcal/mole) is the equation for ATP hydrolysis
• During common reactions, the outer phosphate bond breaks, forming adenosine diphosphate (ADP) and inorganic phosphate (Pi).
• In some cases, ADP breaks down further into adenosine monophosphate (AMP) and Pi.
• ADP + H2O → AMP + Pi + energy (7.3 kcal/mole) is the equation for ADP hydrolysis
• Cells derive energy by oxidizing food, storing it by combining ADP with Pi to form ATP.
• ATP forms during energy-releasing and breaks down during energy-consuming processes.
• ATP facilitates energy transfer between metabolic reactions.

Photosynthesis

• Autotrophs (plants, algae, some bacteria) use inorganic raw materials to produce organic food in carbohydrates form.
• Carbohydrates provide energy and convert into proteins, lipids, etc.
• Photosynthesis synthesizes glucose from carbon dioxide and water using sunlight (and chlorophyll), releasing oxygen as a byproduct.
• Photosynthesis is vital for life.
• The general equation for photosynthesis is 6 CO2 + 12 H2O + photons → C6H12O6 + 6 O2 + 6 H2O, or carbon dioxide + water + light energy → glucose + oxygen + water.
• Photosynthesis occurs in two phases: light and dark reactions.
• Light reactions occur on the thylakoid membranes of chloroplasts, while dark reactions occur in the stroma.

Light Reactions

• Light energy is used to form ATP and NADPH high-energy molecules.
• Chlorophyll absorbs light, causing two of its electrons to become high-energy electrons, which are then released.
• High-energy electrons travel an electron transport chain, releasing energy to produce ATP as they move to lower energy levels.
• Light breaks down water molecules, releasing oxygen and forming hydrogen atoms that give electrons to chlorophyll and become hydrogen ions.
• After the ATP production, chlorophyll electrons and water-derived hydrogen ions reduce NADP into NADPH (Nicotinamide Adenine Dinucleotide Phosphate - reduced).

Dark Reactions (Calvin Cycle)

• Carbon dioxide is reduced to create glucose.
• CO2 molecules bind with 5-carbon compounds to make unstable 6-carbon compounds that split into two 3-carbon compounds.
• The 3-carbon compounds convert into 3-carbon carbohydrates using ATP and hydrogen from NADPH (generated during light reactions).
• The 3-carbon carbohydrates are used to make glucose and regenerate the original 5-carbon compounds, which also uses ATP.

Role of Chlorophyll in Photosynthesis

• Thylakoid membranes in chloroplasts contain pigments, with chlorophyll-a as the main pigment, and chlorophyll-b and carotenoids as accessory pigments.
• Pigments transform sunlight into chemical energy for photosynthesis.
• Only about 1% of the light striking a leaf is absorbed; the remainder reflects or transmits.
• Blue and red lights drive more photosynthesis.
• Different pigments absorb different light wavelengths.
• Chlorophyll-a absorbs blue and red light, while accessory pigments absorb the wavelengths that chlorophyll-a does not.
• Reactions occur within a pigment upon light absorption, releasing electrons.
• High-energy electrons move through an electron transport chain, which creates ATP and reduces NADP to NADPH.

Cellular Respiration

• Organisms extract energy by breaking the C-H bonds in food through intracellular food oxidation known as cellular respiration.
• Glucose is the most common food source for cellular energy.

Aerobic Respiration

• Aerobic respiration occurs with oxygen.
• Aerobic respiration is a complete glucose oxidation that releases maximum energy.
• In the first phase, a glucose molecule (6-C) splits into two pyruvic acid molecules (3-C).
• Pyruvic acid molecules are completely oxidized (all C-H bonds are broken) with energy, releasing in the second phase.

Anaerobic Respiration (Fermentation)

• Anaerobic respiration occurs without oxygen.
• Anaerobic respiration incompletely oxidizes glucose, releasing less energy.
• The initial phase resembles aerobic respiration.
• Glucose breaks down into two molecules of pyruvic acid, which is then processed in two ways.

Alcoholic Fermentation

• During anaerobic respiration in bacteria and yeast, pyruvic acid breaks down into alcohol (C2H5OH) and CO2.
• 2(C3H4O3) → 2(C2H5OH) + 2CO2 is the equation for alcoholic fermentation.

Lactic Acid Fermentation

• Pyruvic acid is converted to lactic acid (C2H6O3) in skeletal muscles of humans and other animals during anaerobic respiration.
• 2(C3H4O3) + 4H → 2(C3H6O3) is the equation for lactic acid fermentation.

Importance of Fermentation

• The early Earth's atmosphere lacked free oxygen, so early organisms used anaerobic respiration for energy.
• Anaerobes, including bacteria and fungi, still use anaerobic respiration today.
• Muscles use anaerobic respiration when oxygen cannot meet the energy demand.
• Fermentation in fungi and bacteria produces useful products like cheese, yogurt, and alcohol.

Mechanism of Aerobic Respiration

• Glycolysis, the Krebs cycle, and the electron transport chain are the three stages of aerobic respiration.

Glycolysis

• Glucose (6C) breaks down into two pyruvic acid molecules (3C).
• It results in two ATPs and two NADH molecules.
• Glycolysis occurs in the cytoplasm.
• Oxygen is not required for glycolysis, so it also occurs in anaerobic respiration.

Krebs Cycle

• When Oxygen is available, pyruvic acid molecules migrate from the cytoplasm to the mitochondrial matrix.
• Pyruvic acid transforms into acetyl coenzyme-A, carbon dioxide, and NADH before the Krebs cycle.
• Acetyl coenzyme-A completely oxidizes to carbon dioxide, forming ATP and energy-rich NADH and FADH2 (Flavin Adenine Dinucleotide - reduced) compounds.

Electron Transport Chain

• This happens on the inner mitochondrial membranes where NADH and FADH2 revert to NAD and FAD by releasing electrons and hydrogen ions.
• The emitted electrons pass through an electron transport chain and produce energy to make ATP.
• At the end of the chain, electrons and hydrogen ions join with oxygen to form water.

Use of Respiratory Energy

• ATP produced during cellular respiration drives:
• Muscle contractions and movement
• Active transport of substances
• Synthesis of biomolecules
• Replication of DNA and mitosis
• Transmission of nerve impulses
• Maintenance of body temperature
• Breakdown and elimination of toxins

Key Points

• Electrons transfer between molecules during redox reactions.
• Oxidation involves a molecule losing electrons
• Reduction means it gains electrons.
• Electron flow (oxidation-reduction) fuels energy production.
• ATP serves as the cell's energy currency.
• Photosynthesis combines water and carbon dioxide using light and chlorophyll to produce carbohydrates and oxygen.
• Chlorophyll in light reactions of photosynthesis captures sunlight and makes ATP.
• Carbon dioxide reduces to produce glucose during dark reactions.
• Anaerobic respiration is incomplete glucose oxidation without oxygen, whereas aerobic respiration is complete oxidation with oxygen.
• Glucose splits into two pyruvic acid molecules during glycolysis.
• The pyruvic acid molecule in the Krebs cycle breaks down into hydrogen ions, high-energy electrons, and carbon dioxide.
• Electrons travel along an electron transport chain and emit energy, which converts ADP into ATP.