Nucleus, Ribosomes, ER, Golgi and Lysosomes

The nucleus contains the cell's linear chromosomes. DNA replication and transcription (DNA to RNA) happen here; translation does not.

• The nuclear envelope is a double membrane. Its outer membrane is continuous with the rough ER.
• Nuclear pores let mRNA and ribosome subunits out and proteins (such as histones and enzymes) in.
• Chromatin is DNA wound round histone proteins. It condenses into visible chromosomes when the cell divides.
• The nucleolus has no membrane. It holds the genes for ribosomal RNA (rRNA), makes rRNA, and assembles ribosome subunits.
• In mitosis the envelope and nucleolus break down in prophase and reform in telophase, so a cell at metaphase shows no nucleus.
• Mature human red blood cells have no nucleus; skeletal muscle fibres have many.

Ribosomes carry out translation: they read mRNA and link amino acids, carried by tRNA, into a polypeptide.

• Made of ribosomal RNA and protein. They contain no DNA and have no membrane.
• Two subunits, large and small, which join on an mRNA.
• Sizes are given in S (Svedberg units, a measure of how fast a particle settles in a centrifuge). S values do not add: 50S + 30S makes a 70S ribosome.
• Free ribosomes make proteins that stay in the cytosol. Ribosomes bound to the rough ER make proteins for secretion, for membranes and for lysosomes. A single mRNA is often read by several ribosomes at once (a polysome).
• Some antibiotics (tetracycline, streptomycin) block 70S ribosomes, which is why they stop bacteria with little effect on the 80S ribosomes of human cytoplasm.

The endomembrane system is the set of membranes linked by vesicles: nuclear envelope, ER, Golgi, lysosomes, vesicles and the plasma membrane.

• Rough ER (RER): flattened sacs covered in ribosomes. Proteins made there enter its lumen, fold, and get their first sugar chains, becoming glycoproteins.
• Smooth ER (SER): tubes with no ribosomes. Makes lipids, phospholipids and steroid hormones; detoxifies drugs and alcohol in liver cells; stores $\mathrm{Ca^{2+}}$ in muscle (where it is called the sarcoplasmic reticulum).
• Golgi apparatus: a stack of flattened sacs. Vesicles from the ER arrive at one face, proteins are modified (sugars trimmed and added), sorted and packed, and vesicles leave from the other face. It also makes lysosomes and, in plants, cell wall polysaccharides.
• Every eukaryotic cell that makes proteins has a Golgi, including neurons; it is largest in secretory cells such as pancreatic and mucus cells.
• Vesicles bud from the RER, the Golgi and the plasma membrane.

• Lysosomes: single-membrane vesicles holding about fifty hydrolytic enzymes (proteases, lipases, nucleases). Their enzymes are made on the rough ER, and lysosomes form by budding from the Golgi apparatus. A proton pump keeps the inside at about pH 4.5 to 5, where the enzymes work best; the cytosol is about pH 7.2.
• Lysosomes digest material taken in by the cell and worn-out organelles (autophagy). A missing lysosomal enzyme causes a lysosomal storage disease (for example Tay-Sachs disease).
• Peroxisomes: single-membrane vesicles with oxidase enzymes and catalase, which breaks down toxic hydrogen peroxide into water and oxygen. They also break down fatty acids.
• Vacuoles: a plant cell has one large central vacuole bounded by the tonoplast, holding cell sap (water, ions, sugars, pigments). It keeps the cell turgid and stores substances. Freshwater protists use contractile vacuoles to pump out excess water.

Bulk transport moves large particles or large amounts in vesicles. It is active: it uses ATP.

• Endocytosis brings material in. The membrane folds in round it and pinches off as a vesicle, which often fuses with a lysosome.
• Exocytosis sends material out. A vesicle, usually from the Golgi, fuses with the plasma membrane and empties its contents outside. Its membrane becomes part of the plasma membrane.
• Exocytosis releases hormones (insulin), digestive enzymes, mucus, and neurotransmitters at every nerve ending.