Cell wall and the endomembrane system

Bacteria, algae, fungi and plants have a stiff cell wall outside the membrane; animal cells do not. Bacterial walls are made of peptidoglycan: thick in Gram-positive bacteria, and thin with an extra outer membrane in Gram-negative bacteria. Plant walls are made of cellulose, and fungal walls of chitin.

A young plant cell grows inside a thin primary wall. Later a thick secondary wall forms inside it, made rigid by lignin. Neighbouring cells are glued together by the middle lamella (made of calcium pectate), and their cytoplasm is linked through fine channels, the plasmodesmata.

The endomembrane system is a team of organelles that make, process, pack and deliver proteins and lipids: the ER, Golgi apparatus, lysosomes and vacuoles.

• Rough ER is studded with ribosomes and handles proteins that will be secreted. Smooth ER has no ribosomes and makes lipids, including phospholipids and cholesterol.
• George Palade traced the route a secreted protein takes: rough ER → Golgi apparatus → secretory vesicles → out of the cell.
• The Golgi apparatus (seen by Camillo Golgi in 1898) receives material at its cis face, modifies proteins (for example by adding sugars), and ships them out from its trans face. ER makes, Golgi packs, vesicles deliver.

Lysosomes are small sacs full of digesting enzymes (acid hydrolases). They digest food particles and worn-out parts of the cell itself (autophagy). If one enzyme is missing, material piles up: in Tay-Sachs disease, lipids build up in brain cells.

Vacuoles store water and other materials and help support the cell. In a mature plant cell, one large central vacuole fills almost 90% of the cell. Its membrane is the tonoplast.