B2.2 Organelles and compartmentalizationIB Biology HL: Revision notes

An organelle is a discrete subunit of a cell adapted to a specific function. Nuclei, vesicles, ribosomes and the plasma membrane are organelles; the cell wall, cytoskeleton and cytoplasm are not.

NOS: organelle functions could only be studied after ultracentrifuges allowed cell fractionation: cells are homogenised in cold, isotonic buffer and centrifuged at increasing speeds, separating nuclei, then mitochondria, then vesicles and ribosomes. Progress often follows new techniques.

Separating the nucleus from the cytoplasm lets post-transcriptional modification (e.g. removal of introns) finish before mRNA meets ribosomes. In prokaryotes mRNA may meet ribosomes immediately.

In the cytoplasm, compartments concentrate enzymes and metabolites and separate incompatible processes. Lysosomes hold hydrolytic enzymes at low pH; they fuse with phagocytic vacuoles to digest engulfed material without harming the rest of the cell.

• Double membrane with a small intermembrane space: protons pumped into this small volume quickly build a steep concentration gradient for chemiosmosis.
• Cristae (folds of the inner membrane) give a large surface area for electron transport chains and ATP synthase. Cells with high ATP demand, such as heart muscle, have more cristae.
• The matrix compartmentalizes the enzymes and substrates of the Krebs cycle, keeping them concentrated.

• Thylakoid membranes, stacked in grana, give a large surface area holding photosystems, electron transport chains and ATP synthase.
• The small volume of fluid inside thylakoids lets a steep proton gradient form quickly.
• The stroma compartmentalizes the enzymes and substrates of the Calvin cycle; ATP and reduced NADP from the thylakoids pass straight into it.

The nucleus is surrounded by a double membrane (nuclear envelope) which separates transcription from translation. It needs nuclear pores so that mRNA can leave and proteins (e.g. DNA polymerase, histones) can enter.

During mitosis and meiosis the envelope breaks into vesicles, so spindle microtubules can reach the chromosomes; in telophase the vesicles re-fuse to form new nuclei.

Free ribosomes in the cytoplasm synthesize proteins retained in the cell (e.g. cytoplasmic enzymes). Ribosomes bound to the rough endoplasmic reticulum synthesize proteins that pass into the ER lumen for transport within the cell or secretion.

The Golgi apparatus is a stack of flattened membrane sacs (cisternae). It receives vesicles from the rough ER, processes proteins (e.g. adding carbohydrate) and packages them into secretory vesicles, which release them by exocytosis.

Vesicles are small membrane sacs that move materials. Clathrin assembles into a cage on the cytoplasmic side of the membrane, pulling it inwards so a coated vesicle buds off (e.g. in endocytosis).