1Ribosomes: protein factories in every cell

A ribosome is made of ribosomal RNA (rRNA) and protein. It has no membrane, so it is a subcellular structure rather than a membrane-bound organelle. Its job is to build proteins: it moves along a molecule of messenger RNA (mRNA) and joins amino acids in the order the mRNA's sequence dictates (Unit 6 covers how).

Ribosomes are found in every known form of life: bacteria, archaea and eukaryotes. A structure this complex, built to one plan and doing one job everywhere, is best explained by common ancestry: all life inherited ribosomes from a shared ancestor.

Ribosomes sit in two places in a eukaryotic cell. Free ribosomes float in the cytosol and make proteins that stay in the cytosol. Bound ribosomes are attached to the outside of the endoplasmic reticulum and make proteins that enter the endomembrane system: proteins for export, for the plasma membrane, or for lysosomes.

2The endomembrane system

The endomembrane system is a group of membranes and membrane-bound organelles that work together to modify, package and transport proteins, lipids and polysaccharides around the cell. Its members are:

• the nuclear envelope, the double membrane around the nucleus;
• the endoplasmic reticulum (ER), rough and smooth;
• the Golgi complex;
• lysosomes;
• vacuoles and transport vesicles;
• the plasma membrane itself.

They count as one system because material moves between them in vesicles: small spheres of membrane that bud off one compartment and fuse with the next.

A secreted protein travels: ribosome on rough ER → inside the rough ER → vesicle → Golgi complex → vesicle → plasma membrane → out.

Figure 1 shows that route. Follow the numbers: the protein is made on the rough ER, carried in a transport vesicle to the Golgi, modified there (a carbohydrate is added), packed into a secretory vesicle, and released when that vesicle fuses with the plasma membrane.

3Endoplasmic reticulum, rough and smooth

The endoplasmic reticulum is a network of membrane tubes and flattened sacs that spreads through the cytoplasm from the nuclear envelope. It gives the cell mechanical support, helping it keep its shape, and it is a route for intracellular transport: materials travel through its inner space from one part of the cell to another.

Rough ER is studded with bound ribosomes, which is what makes it look rough. As the ribosome builds a protein, the growing chain is threaded into the ER's inner space. So rough ER does two things at once: it helps carry out protein synthesis, and it compartmentalises the new protein, keeping it apart from the cytosol so it can be folded and sent on into the endomembrane system rather than left loose in the cell.

Smooth ER has no ribosomes. Its jobs are lipid synthesis (for example, the phospholipids of new membranes, and steroids) and detoxification, where enzymes in its membranes change harmful substances into forms the cell can get rid of. Which special jobs smooth ER does in particular cell types is not examined.

4The Golgi complex

The Golgi complex (also called the Golgi apparatus) is a stack of flattened membrane sacs. Vesicles from the ER arrive at one face of the stack; finished products leave from the other face.

Inside the Golgi, newly made products are correctly folded and chemically modified. The best-known modification is glycosylation, the adding of carbohydrate chains to proteins to make glycoproteins. These changes decide what a protein can do, or where it will be sent: a carbohydrate tag works like an address. The Golgi then packages the products into vesicles for trafficking to the plasma membrane, to lysosomes, or out of the cell.

The exam stops there. Which specific phospholipids the Golgi makes, and how it sorts particular enzymes into lysosomes or secretory vesicles, is beyond the course.

5Mitochondria: a folded inner membrane

A mitochondrion has a double membrane. The outer membrane is smooth. The inner membrane is highly folded into cristae, and it encloses the fluid matrix. The two membranes create separate compartments, so different reactions of aerobic cellular respiration can happen in different places with different conditions (Unit 3 gives the detail).

The folds are the structure–function point. Many of the proteins that make ATP are embedded in the inner membrane. Folding packs far more membrane, and so far more of those proteins, into the same volume. More inner membrane means ATP is made more efficiently. Muscle cells, which use a lot of energy, have many mitochondria with dense cristae.

6Lysosomes: digestion and planned death

A lysosome is a membrane-enclosed sac full of hydrolytic enzymes: enzymes that break large molecules into smaller ones by hydrolysis (1.2). Lysosomes digest food particles taken in by the cell, worn-out organelles and invading bacteria, and the products (amino acids, sugars) are recycled.

The membrane is the key structure. It keeps powerful digestive enzymes away from the rest of the cell, so the cell can digest material without digesting itself.

Lysosomes also take part in apoptosis, programmed cell death: an orderly, controlled self-destruction of a cell that is no longer needed or is damaged. It removes cells that could harm the organism, and shapes the body as it develops.

7Vacuoles: storage and turgor

A vacuole is a membrane-bound sac, and different vacuoles do different jobs.

• Plant cells usually have one large central vacuole that can fill most of the cell. It stores water and nutrients. As it fills with water it presses the cytoplasm against the cell wall, producing turgor pressure, which keeps the cell firm and the plant upright. A wilting plant is a plant whose central vacuoles have lost water (2.7).
• Animal cells have vacuoles that are smaller and more numerous, storing materials the cell needs or has taken in.

8Chloroplasts

A chloroplast is found in plants and in photosynthetic algae. Like a mitochondrion it has a double membrane, and it is where photosynthesis happens: light energy is captured and used to make sugars from carbon dioxide and water (Unit 3). Animal cells and fungi have no chloroplasts. Why mitochondria and chloroplasts both have two membranes is the story of 2.10.

9Making claims from structures

AP asks you to make a scientific claim: a statement that answers the question, which you then support with evidence and reasoning. Cell structure is where you practise it.

Claim: cell X secretes large amounts of protein. Evidence: cell X has far more rough ER and Golgi membrane per cell than cell Y. Reasoning: proteins for export are made by ribosomes on the rough ER and are packaged for secretion by the Golgi, so a cell that exports a lot of protein needs more of both.

Write all three parts.