The endomembrane system

Membranes that work as a team

A eukaryotic cell holds many organelles, each wrapped in a membrane. Some of them work so closely that they are treated as one system.

The endomembrane system is made of four parts:

1. the endoplasmic reticulum (ER),
2. the Golgi complex,
3. lysosomes,
4. vacuoles.

Each part has its own structure and job. But their functions are coordinated: materials pass from one to the next.

Mitochondria, chloroplasts and peroxisomes also have membranes. They are not counted in the endomembrane system, because their functions are not coordinated with these four.

The endoplasmic reticulum

Under the electron microscope, the cytoplasm is seen to hold a network of tiny tubes and sacs. This network is the endoplasmic reticulum.

The ER divides the space inside the cell into two parts. The space inside the ER is the luminal compartment. The cytoplasm outside it is the extra-luminal compartment.

There are two kinds of ER.

• Rough ER has ribosomes on its outer surface. It makes proteins and helps to secrete them. It is extensive in cells that send out proteins. It is continuous with the outer membrane of the nucleus.
• Smooth ER has no ribosomes. It is the main site where lipids are made. In animal cells, the steroid hormones, which are lipid-like, are made in the smooth ER.

The Golgi apparatus

In 1898, Camillo Golgi saw darkly stained, net-like structures near the nucleus. They are now called the Golgi apparatus, after him.

• It is made of many flat, disc-shaped sacs, the cisternae, about $0.5$ to $1.0$ µm across.
• The cisternae are stacked one above another, near the nucleus.
• The stack has two faces. The cis face is the forming face; it is convex. The trans face is the maturing face; it is concave.
• The two faces are quite different, but they are linked.

The main job of the Golgi apparatus is to package materials. Some go to targets inside the cell; others are secreted outside it.

Materials from the ER arrive in small vesicles, which fuse with the cis face. They move through the stack towards the trans face, and leave in new vesicles. This is why the Golgi works so closely with the ER.

Many proteins made on the rough ER are changed in the Golgi. It is an important site for forming glycoproteins and glycolipids.

Lysosomes

Lysosomes are small vesicles bound by a membrane. They are formed by packaging in the Golgi apparatus.

• They are rich in hydrolytic enzymes, or hydrolases: lipases, proteases, carbohydrases and others.
• These enzymes work best at an acidic pH.
• They can digest carbohydrates, proteins, lipids and nucleic acids.

Vacuoles

A vacuole is a space in the cytoplasm bound by a single membrane, the tonoplast.

• It holds water, sap, excretory products and other materials the cell does not need.
• In a plant cell, the vacuole can fill up to 90 per cent of the volume of the cell.
• The tonoplast moves several ions against their concentration gradient into the vacuole. So their concentration is higher in the vacuole than in the cytoplasm.
• In Amoeba, the contractile vacuole is important for excretion and for keeping the water balance.
• In many protists, food vacuoles form when the cell engulfs food particles.

You should find that with $20$, $30$ and $10$ minutes for the three stages, the protein reaches the surface after $60$ minutes. Whatever times you choose, the order stays the same: rough ER, then the cis face, then the trans face, then a vesicle.

Try it in Python

The program walks a protein along the same road and keeps a running clock.

You should see the protein leave the Golgi stack after $50$ minutes, and the vesicle after $60$ minutes. With $45$ minutes in the Golgi, it leaves the vesicle after $75$ minutes.

Practise with new numbers

Common mistakes

• Putting mitochondria in the endomembrane system. They have membranes, but their work is not coordinated with the ER and the Golgi.
• Swapping the two kinds of ER. Rough ER, with ribosomes, makes proteins; smooth ER makes lipids and steroids.
• Swapping the Golgi faces. Vesicles from the ER join the cis face; finished products leave from the trans face.
• Saying lysosomal enzymes work best in a neutral medium. They work best at an acidic pH.

Remember these

• Endomembrane system: ER, Golgi complex, lysosomes and vacuoles. Not mitochondria, chloroplasts or peroxisomes.
• Rough ER: proteins for secretion. Smooth ER: lipids, and steroid hormones in animal cells.
• Golgi: cisternae with a cis (forming) and a trans (maturing) face; packaging; glycoproteins and glycolipids.
• Lysosomes: hydrolases, best at acidic pH. Vacuoles: tonoplast; up to 90 per cent of a plant cell.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four parts of the endomembrane system.

2. Why does a cell that sends out a lot of protein have plenty of rough ER?

3. At which face of the Golgi stack do vesicles from the ER arrive?

4. How are lysosomes formed, and what do they contain?

5. How can ions be more concentrated in the vacuole than in the cytoplasm?