The Cell Membrane, Wall and Endomembrane System: Quick Revision

A brief note on The Cell Membrane, Wall and Endomembrane System. Read it before a test. Try each question at the end before you open its answer.

The cell membrane

• A lipid bilayer: polar heads outwards, hydrophobic tails inwards; mostly phosphoglycerides, with cholesterol.
• Proteins: integral (buried) and peripheral (on the surface); carbohydrates on the outer side.
• Human red blood cell membrane: about 52 per cent protein and 40 per cent lipid.
• Fluid mosaic model (Singer and Nicolson, 1972): a quasi-fluid lipid with proteins that move sideways.
• Passive transport: diffusion and osmosis, no energy. Active transport: against the gradient, using ATP, as in the Na+/K+ pump.

The cell wall

• Non-living and rigid, outside the membrane of plants and fungi.
• Algae: cellulose, galactans, mannans, calcium carbonate. Land plants: cellulose, hemicellulose, pectins, proteins.
• Primary wall: young, can grow. Secondary wall: on the inner side.
• Middle lamella: calcium pectate, glues cells. Plasmodesmata: join the cytoplasm of neighbours.

The endomembrane system

• ER, Golgi complex, lysosomes, vacuoles. Not mitochondria, chloroplasts or peroxisomes.
• Rough ER: ribosomes, proteins for secretion. Smooth ER: lipids, steroid hormones.
• Golgi (Camillo Golgi, 1898): stacked cisternae; cis (forming) and trans (maturing) faces; packaging; glycoproteins and glycolipids.
• Lysosomes: from the Golgi; hydrolases active at acidic pH.
• Vacuoles: bound by the tonoplast; up to 90 per cent of a plant cell; contractile vacuole in Amoeba.

A worked example

A membrane sample from red blood cells weighs $300$ µg. How much protein and how much lipid does it hold?

1. Protein. $\dfrac{52}{100} \times 300 = 156$ µg.
2. Lipid. $\dfrac{40}{100} \times 300 = 120$ µg.
3. The rest. $300 - 156 - 120 = 24$ µg, mostly carbohydrate.

Practice with new numbers

A program to try

The program checks which organelles belong to the endomembrane system.

Mistakes to avoid

• The tails, not the heads, face the middle of the bilayer.
• Osmosis is passive; the Na+/K+ pump is active and uses ATP.
• The secondary wall forms inside the primary wall, towards the membrane.
• Mitochondria are not in the endomembrane system.
• ER vesicles join the cis face; products leave from the trans face.

Check yourself

1. Who proposed the fluid mosaic model?

2. Is the movement of water by osmosis active or passive?

3. What is the middle lamella mainly made of?

4. Which kind of ER makes steroid hormones?

5. Name the membrane round a plant vacuole.