Membrane-Bound Organelles (College Board AP Biology)

• Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments
• Within cells, there are membrane-bound organelles that achieve compartmentalization on a smaller scale
• This allows separate metabolic reactions to take place in different regions of the cell
• Membranes contain membrane proteins, many of which are enzymes that perform their catalytic role at the membrane or just to one side of it
  • For example, many of the membrane proteins inside a mitochondrion catalyze the reactions of oxidative phosphorylation
• For details on the various membrane-bound organelles that enable compartmentalization within a cell, see Topic 2.1 Cell Structure: Subcellular Components

Membrane Proteins as Enzymes Diagram

Membrane proteins have many roles, including as enzymes on the inner surface of a cell (shown bottom left).

Membrane Protein Inside a Mitochondrion Diagram

The enzyme ATP synthase, an important enzyme in the pathways of respiration, and its location in the inner mitochondrial membrane.

Benefits of Membrane-Bound Organelles

• Membrane-bound organelles occur in eukaryotic cells. They 
  • Allow compartmentalization within a cell
    • For example, allowing the reaction of glycolysis to be kept separate (in the cytosol) from those of the Krebs cycle and oxidative phosphorylation (inside mitochondria)
    • This allows a cell to utilize anaerobic respiration even in the temporary absence of oxygen
  • To keep various reactions separate from each other
    • This minimizes the risk of conflicting side reactions occurring, that would have no metabolic benefit
    • This also minimizes the risk of enzyme reactions being accidentally inhibited by substrates and products from other biochemical pathways
  • Cells can increase the internal surface area for reactions by housing important membrane proteins within the membranes that surround organelles
    • Cells that have a high requirement for ATP, such as muscle cells, can produce more mitochondria to give a larger membranes with electron transport chains to produce ATP
    • Cells that have an active role in secretion of proteins, such as insulin producing cells in the pancreas, can increase the number of ribosomes / rough ER / Golgi to meet the demand for secreted insulin
    • Cells with a role in removal of waste, such as neutrophils and monocytes, can produce more lysosomes to fulfil their role more effectively