Mitochondria & the Need for Energy (AQA A Level Biology)

• Mitochondria are rod-shaped organelles 0.5 - 1.0 µm in diameter
• They are the site of aerobic respiration in eukaryotic cells
• The function of mitochondria is to synthesize ATP
• Synthesis of ATP in the mitochondria occurs during the last stage of respiration called oxidative phosphorylation
  • This relies on membrane proteins that make up the ‘electron transport chain’ and the ATP synthase enzyme – the details of this are covered later in the notes

Structure

• Mitochondria have two phospholipid membranes
• The outer membrane is:
  • Smooth
  • Permeable to several small molecules

• The inner membrane is:
  • Folded (cristae)
  • Less permeable
  • The site of the electron transport chain (used in oxidative phosphorylation)
  • Location of ATP synthase (used in oxidative phosphorylation)

• The intermembrane space:
  • Has a low pH due to the high concentration of protons
  • The concentration gradient across the inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis

• The matrix:
  • Is an aqueous solution within the inner membranes of the mitochondrion
  • Contains ribosomes, enzymes and circular mitochondrial DNA necessary for mitochondria to function

The structure of the mitochondria

Relationship between structure & function

• The structure of mitochondria makes them well adapted to their function
• They have a large surface area due to the presence of cristae (inner folds) which enables the membrane to hold many electron transport chain proteins and ATP synthase enzymes
• More active cell types can have larger mitochondria with longer and more tightly packed cristae to enable the synthesis of more ATP because they have a larger surface area
• The number of mitochondria in each cell can vary depending on cell activity
  • Muscle cells are more active and have more mitochondria per cell than fat cells

• Living organisms are composed of cells, and within each cell, many activities and processes are constantly being carried out to maintain life
• Work in a living organism requires energy and usable carbon compounds

Essential work within organisms table

The source of energy & materials

• For nearly all organisms the sun is the primary source of energy
• The reactions of photosynthesis store energy in organic molecules
  • Light energy from the sun is transformed into chemical potential energy in the synthesis of carbohydrates
  • The carbohydrates formed are then used in the synthesis of ATP (from their breakdown) or are combined and modified to form all the usable organic molecules that are essential for all metabolic processes within the plant
  • Photosynthesis is carried out by the first organism in a food chain, such as plants and some other small organisms

• Respiration in all living cells releases energy from the breakdown of organic molecules
• Respiration involves the transfer of chemical potential energy from nutrient molecules (such as carbohydrates, fats and proteins) into a usable energy form (through the synthesis of ATP) that can be used for work within an organism

Glucose equations

glucose + oxygen →  carbon dioxide + water + energy

C₆H₁₂0₆ + 6 O₂ →  6 CO₂ + 6 H₂0 + 2870kJ

• Autotrophs are organisms that are able to synthesise their own usable carbon compounds from carbon dioxide in the atmosphere through photosynthesis
• Heterotrophs don’t have this ability. They require a supply of pre-made usable carbon compounds which they get from their food