Adaptations of Mitochondria & Chloroplasts (HL) (HL IB 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 under the subtopic of oxidative phosphorylation

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

Structure of Mitochondria Diagram

The structure of a mitochondrion facilitates the process of aerobic cell respiration

Relationship between structure & function

• The structure of mitochondria makes them well adapted to their function
  • They have a double membrane and a small volume of intermembrane space; this means that this space can be used for the concentration build up of hydrogen ions required for respiration reactions
  • 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
  • Compartmentalisation of enzymes and substrates using the matrix ensures that respiration reactions, like the Krebs cycle, can happen more efficiently 

Structure

• Chloroplasts are the organelles in plant cells where photosynthesis occurs
• These organelles are roughly 2 - 10 μm in diameter (they are larger than mitochondria)
• Each chloroplast is surrounded by a double-membrane envelope
  • Each of the envelope membranes is a phospholipid bilayer
  • The outer membrane is permeable to a range of ions and small molecules
  • The inner membrane contains transport proteins that only allow certain molecules or ions to enter or leave the chloroplast
• Chloroplasts are filled with a cytosol-like fluid known as the stroma
  • CO₂, sugars, enzymes and other molecules are dissolved in the stroma
  • If the chloroplast has been photosynthesising there may be starch grains or lipid droplets in the stroma
• A separate system of membranes is found in the stroma
• This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids
  • The thylakoid membranes contain pigments, enzymes and electron carriers
  • These thylakoids stack up to form structures known as grana (singular – granum)
  • Grana are connected by membranous channels called stroma lamellae, which ensure the stacks of sacs are connected but distanced from each other

Chloroplast Structure Diagram

The structure of chloroplasts facilitates the process of photosynthesis

• The membrane system provides a large number of pigment molecules that ensure as much light as necessary is absorbed
• The pigment molecules are arranged in light-harvesting clusters known as photosystems
  • In a photosystem, the different pigment molecules are arranged in funnel-like structures in the thylakoid membrane
  • Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre

Adaptations of chloroplasts to photosynthesis

• Stroma:
  • The gel-like fluid contains enzymes that catalyse the reactions of the light-independent stage
  • Enzymes and their substrates are compartmentalised for reactions of the Calvin cycle
  • The stroma surrounds the grana and membranes, making the transport of products from the light-dependent stage into the stroma rapid
• Grana:
  • The granal stacks create a large surface area for the presence of many photosystems which allows for the maximum absorption of light
  • It also provides more membrane space for electron carriers and ATP synthase enzymes
• DNA:
  • The chloroplast DNA contains genes that code for some of the proteins and enzymes used in photosynthesis
• Ribosomes:
  • The presence of ribosomes allows for the translation of proteins coded by the chloroplast DNA
• Inner membrane of chloroplast envelope:
  • The selective transport proteins present in the inner membrane control the flow of molecules between the stroma and cytosol (the cytoplasm of the plant cell)
• Thylakoid space:
  • This is where a proton gradient develops (to generate ATP)
  • The space has a very small volume so a proton gradient can develop very quickly