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.
   1. Thylakoids stack into grana and contain chlorophyll, which absorbs light for photosynthesis.
   2. The fluid around the thylakoids is the stroma.

Single-Membrane Organelles

1. The rough endoplasmic reticulum (rough ER) is a network of flattened membrane sacs studded with ribosomes.
   1. It makes proteins destined for secretion, for insertion into membranes, or for use in lysosomes.
2. The smooth endoplasmic reticulum (smooth ER) has no ribosomes.
   1. It handles lipid synthesis, steroid hormone production, and detoxification.
3. The Golgi apparatus is a stack of membrane sacs called cisternae.
   1. It receives proteins from the rough ER, modifies and sorts them, and packages them into vesicles for transport or secretion.
4. Lysosomes are membrane-bound sacs filled with hydrolytic (digestive) enzymes.
   1. Their acidic interior (about pH 4.5 to 5) is ideal for enzymes that break down worn-out organelles and cellular debris.
   2. The membrane keeps these digestive enzymes safely contained.
5. Vesicles are small, single-membrane sacs that shuttle materials around the cell.
   1. They carry substances to the plasma membrane for secretion (exocytosis) and bring materials into the cell (endocytosis).

Non-Membrane-Bound Organelles: No Enclosure Needed

1. Ribosomes are made of ribosomal RNA and protein, with no surrounding membrane.
   1. They are the site of protein synthesis (translation).
   2. Ribosomes are found in every living cell, including prokaryotes.
2. Centrioles are cylindrical structures made of microtubules, found in most animal cells but not in plant cells.
   1. During cell division they help organise the spindle fibres that pull chromosomes apart.
3. Proteasomes are barrel-shaped protein complexes that break down damaged or unneeded proteins.
   1. Proteins tagged with ubiquitin are directed to proteasomes for degradation.

Structure Reflects Function in Every Organelle

1. Every organelle's structure is adapted to what it does.
   1. Mitochondria have folded inner membranes because aerobic respiration needs a large surface area.
   2. Lysosomes stay acidic because their hydrolytic enzymes only work at low pH.
   3. The rough ER is flattened and extensive to give maximum surface area for ribosomes and protein processing.
2. Ribosomes are small, compact machines present in huge numbers because protein synthesis must happen fast and continuously.
   1. They lack membranes because they need to work everywhere: free in the cytoplasm, on the ER, and inside mitochondria and chloroplasts.

Studying Organelles: Electron Microscopy and Cell Fractionation

1. Most organelles are too small to see with a light microscope.
2. Electron microscopes use beams of electrons instead of light, reaching magnifications up to 500,000x and resolutions below 1 nm.
   1. Transmission electron microscopes (TEMs) pass electrons through thin sections to reveal internal detail like cristae, thylakoids, and nuclear pores.
   2. Scanning electron microscopes (SEMs) bounce electrons off surfaces to produce detailed 3D images.
3. Cell fractionation isolates organelles by breaking cells open and spinning the mixture in an ultracentrifuge.
   1. Cells are first homogenised in a cold, buffered, isotonic solution.
   2. At increasing speeds, heavier organelles pellet first (nuclei), followed by mitochondria, then ER fragments, then ribosomes.
   3. This is called differential centrifugation.