Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1 · 1.1

The Cell Wall.

Put a plant cell beside an animal cell and one covering sets them apart. Where does it sit, how many layers does it have, and what is it made of? This note walks through it step by step.

1. What it is, and where it sits

The cell wall is the relatively rigid, non-living covering around a plant cell. Among plants and animals it belongs to plant cells alone. Fungi and bacteria have a wall too; animal cells do not. When Hooke looked in 1665, what he mostly saw was this wall.

In a plant cell the non-living wall lies between the middle lamella and the cell membrane. In unicellular plants and bacteria it lies outside the cell membrane.

A fully developed wall is divided into three layers: the middle lamella, the primary wall and the secondary wall. Take them one at a time.

2. Three layers, pits and plasmodesmata

The middle lamella starts forming in telophase of mitosis. The phragmoplast and the pectin-carrying vesicles of the Golgi body build it together. It is the layer two neighbouring cells share, so when it dissolves the two cells come apart.

On its inner side, cellulose, hemicellulose and glycoprotein pile up to make the primary wall. It is thin, 1-3 μm.

The secondary wall is the third layer. Once growth is complete, some cells, such as tracheids and fibres, lay it down on top of the primary wall. It is thick, 5-10 μm, made of cellulose and lignin, and itself has three layers. Meristematic cells never make one.

The wall is not equally thick everywhere. The pit field is its thinnest region, where no primary wall forms. Between two facing pits only the middle lamella remains, and that is the pit membrane. Two facing pits together are a pit pair.

Walls are also pierced. Fine pores in the walls of two neighbouring cells carry tubular strands of cytoplasm from one cell to the next. These are the plasmodesmata, singular plasmodesma.

The drawing below follows the book's Figure 1.3: a slice through the wall between two cells. Tap the five points, then match the names.

3. What each layer is made of

The middle lamella is rich in pectic acid and the insoluble salts calcium pectate and magnesium pectate, together called pectin. A little protopectin is there too.

The primary wall holds mainly cellulose, hemicellulose (xylans, arabans, galactans) and glycoprotein. The hemicellulose xyloglucan acts as a cross-link.

Many secondary walls contain lignin. Some walls contain suberin and wax.

Walls outside the plant kingdom differ in material: the fungal wall is made of chitin and the bacterial wall of lipid-protein polymers. Drop the five materials into the five wall boxes.

Taking the whole wall, it usually holds 40% cellulose, 20% hemicellulose, 30% pectin and 10% glycoprotein. Read the chart and work out three things: what cellulose and hemicellulose make together, how many grams of pectin sit in a given mass of wall, and which component is the largest.

4. Cellulose, from small to large

The main component of the wall is cellulose, a polysaccharide built from many β-D glucose molecules. 1000-3000 molecules together form one cellulose chain.

Start from one glucose molecule and drag. At each step many of the last unit bundle together into the next.

Climbing up from the chain: about 100 chains make one micelle, the smallest structural unit. About 20 micelles make one microfibril. 250 microfibrils make one macrofibril. Many macrofibrils make a fibre. The numbers 100, 20 and 250 are easy to mix up, so put the steps in order yourself.

5. What the wall does

The wall gives the cell a definite shape. It protects the living contents from injury. It gives strength and rigidity. It helps in the absorption and conduction of water and minerals. And it separates one cell from another. Five functions, and all five make a full exam answer.

6. Check yourself

Three questions in the exam's style. Start with the first.

The second is about the same wall.

In the last one, watch both the numbers and the names.

The whole note on a few cards.

Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.1.