Energy and Respiration

• Cells need energy for active transport, movement and anabolic reactions such as DNA replication and protein synthesis.
• ATP is the universal energy currency; it is made by substrate-linked phosphorylation and by chemiosmosis.
• Lipids release about twice as much energy per gram as carbohydrates or proteins. RQ = CO₂ produced ÷ O₂ taken in.
• Aerobic respiration: glycolysis (cytoplasm), link reaction and Krebs cycle (matrix), oxidative phosphorylation (inner membrane).
• Anaerobic respiration: lactate fermentation in mammals, ethanol fermentation in yeast; far lower ATP yield.
• Rice is adapted to waterlogged soil. Respirometers and redox indicators (DCPIP, methylene blue) measure respiration.

Why organisms need energy

• Active transport: moving ions and molecules against a concentration gradient using carrier proteins, e.g. the Na⁺/K⁺ pump.
• Movement: muscle contraction, movement of cilia and flagella, movement of chromosomes on the spindle, vesicles moving along the cytoskeleton.
• Anabolic reactions (building large molecules from small ones): DNA replication (activated nucleotides) and protein synthesis (activating amino acids and joining them on ribosomes), also polysaccharide synthesis.
• Also maintaining body temperature in mammals and birds, and transmitting nerve impulses.

ATP: the universal energy currency

ATP (adenosine triphosphate) is a phosphorylated nucleotide: adenine, ribose and three phosphate groups. Hydrolysis by ATPase gives ADP + Pᵢ and releases about 30.5 kJ mol⁻¹.

Features that suit it to its role

• Hydrolysis releases a small, manageable amount of energy, enough for most reactions with little wasted as heat.
• Hydrolysis is a single, rapid step, catalysed by one enzyme, so energy is available quickly.
• It is water-soluble and easily transported within the cell.
• It cannot pass out through the cell surface membrane, so it stays in the cell.
• It is quickly regenerated from ADP and Pᵢ, so a small pool turns over very rapidly.
• It is used by all cells for all energy-requiring processes, which is why it is called universal.
• The phosphate group can be transferred to another molecule (phosphorylation), making that molecule more reactive.

How ATP is made

• Substrate-linked phosphorylation: a phosphate is transferred directly from a substrate molecule to ADP (in glycolysis and the Krebs cycle).
• Chemiosmosis: protons diffuse down a gradient through ATP synthase in the inner membrane of mitochondria and in the thylakoid membranes of chloroplasts.

Respiratory substrates and their energy values

Lipids

Respiratory quotient

respiratory quotient (RQ)