IB Biology Topic B2.2.1: Organelles as Discrete Subunits of Cells That Are Adapted to Perform Specific Functions Notes

Organelles: Discrete Structures with Specialised Roles

1. A cell contains many structures, but not all of them are organelles.
2. An organelle is a discrete subunit within a cell that performs a specific function.
   1. "Discrete" means it exists as a separate, identifiable body, not just a region of cytoplasm.
3. Ribosomes, mitochondria, the nucleus, and the Golgi apparatus are all organelles.
   1. The cytoplasm, the cytoskeleton, and the cell wall are not.
4. Some organelles are surrounded by one or two phospholipid membranes.
   1. Others, like ribosomes, have no membrane at all.
5. Membranes matter because they create enclosed compartments where conditions can differ from the rest of the cell.

Membrane-Bound Organelles: Compartments with Controlled Environments

1. Membrane-bound organelles are enclosed by at least one phospholipid bilayer.
2. This creates an internal environment that can be chemically different from the cytoplasm.
   1. Reactions can then run under conditions that would be impossible or harmful elsewhere in the cell.

Double-Membrane Organelles

1. The nucleus is surrounded by a double membrane called the nuclear envelope.
   1. Pores in the envelope control what enters and exits, protecting the DNA while letting molecules like mRNA pass through.
   2. Inside, the DNA is organised as chromatin (DNA wrapped around histone proteins), which condenses into visible chromosomes during cell division.
2. Mitochondria also have a double membrane.
   1. The inner membrane folds inward to form cristae, which increase the surface area available for aerobic respiration.
   2. The fluid-filled space inside the inner membrane is the matrix.
3. Chloroplasts, found in plant cells and some protists, have a double membrane around an internal system of thylakoid membranes.