Cytoplasmic Organelles: MDCAT Biology notes

Membrane classification

• Single membrane: ER, Golgi complex, lysosomes, peroxisomes, glyoxysomes, vacuoles (bounded by the tonoplast).
• Double membrane: nucleus, mitochondria, chloroplasts.
• No membrane: ribosomes, centrioles, cytoskeleton, nucleolus.

Endoplasmic reticulum (ER)

The ER is a network of channels and cisternae running from the plasma membrane to the nuclear envelope.

• Rough ER is studded with ribosomes. It makes proteins for export or for membranes.
• Smooth ER handles lipid and steroid synthesis and detoxification of drugs and poisons, especially in liver cells. In muscle it helps transmit impulses (as sarcoplasmic reticulum), and it transports materials.
• Material moves from the ER to the Golgi, so the ER is the precursor of the Golgi.

Golgi complex

Discovered by Camillo Golgi, the Golgi complex is a stack of flattened sacs called cisternae. Vesicles arrive at the convex forming face and bud off from the concave maturing face. It is the cell's packaging and distribution centre. Its jobs are:

• Processing and modifying secretions. It adds carbohydrates to make glycoproteins and glycolipids.
• Budding off primary lysosomes.
• Forming plasma membrane, and in plants the new cell wall and middle lamella.
• Secretion, such as neurotransmitters. Secretory cells have many Golgi bodies.

The Golgi does not respire or digest.

Lysosomes

Lysosomes are single-membrane sacs of hydrolytic enzymes, discovered by de Duve. They are called "suicide bags". A primary lysosome fuses with a food vacuole or with a worn-out organelle to form a secondary lysosome (a digestive vacuole or autophagosome). Their functions are intracellular digestion, autophagy (digesting the cell's own old organelles), autolysis (self-destruction of the cell) and removing waste. They do not synthesize lipids.

Microbodies

• Peroxisomes contain oxidative enzymes that break down hydrogen peroxide.
• Glyoxysomes occur only in seedlings of oil-seed plants. They convert stored fat into carbohydrate.

Mitochondria and plastids

Mitochondria have a double membrane. The inner membrane folds into cristae, which carry the ETC proteins and ATP synthase. The matrix holds the Krebs cycle enzymes. Mitochondria contain circular DNA like bacteria and 70S ribosomes, so they make some of their own proteins and divide independently. Active muscle cells, as in athletes, have more mitochondria. Chloroplasts are also semi-autonomous: they contain pigment (chlorophyll) and their own DNA, and they reproduce independently.

Ribosomes and nucleolus

Ribosomes are made of rRNA and protein (they are ribonucleoprotein particles). Each has two subunits, and they are the site of protein synthesis and mRNA translation. The nucleolus makes rRNA and assembles ribosomes; it disappears during cell division. A string of ribosomes on one mRNA is a polysome, and $\mathrm{Mg^{2+}}$ controls subunit attachment. Nuclear pores allow exchange between the nucleus and the cytoplasm.

Cytoskeleton, centrioles and flagella

A centriole is made of nine triplets of microtubules. The centrosome, a pair of centrioles, lies in the cytoplasm near the nucleus. At division two pairs, four centrioles in total, carry $4 \times 9 = 36$ triplets. Cilia and flagella arise from basal bodies.

Common MDCAT traps

• Lysosomes come from the Golgi, not the ER.
• Secondary lysosomes include digestive vacuoles and autophagosomes.
• Detoxification happens in the SER; glycoprotein processing happens in the Golgi, not the SER.
• ATP synthase sits on the inner mitochondrial membrane.
• Cisternae belong to the Golgi and ER; cristae belong to mitochondria.

Quick revision

• Glyoxysomes are found in oil-seed seedlings.
• The tonoplast bounds the vacuole.
• The nucleolus makes ribosomes.
• Mitochondria and chloroplasts contain their own circular DNA.
• Microfilaments drive cyclosis.