All cells have membranes. The membrane is a flexible sort of bag that keeps the cell together. Unlike a regular bag, the cell membrane controls what enters and leaves the cell. Plant cells also have a cell wall right outside the membrane, sort of like a trash bag inside a trash can. Animal cells do not have cell walls.

The cell wall is made of tough carbohydrates. If you have ever chewed on celery, and just kept chewing, you know how tough those walls can be. The walls give the plant structure so they can stand up tall. Even plants that are very flexible, like grass or dandelions have tough cell walls. The cell walls are made of cellulose (long chains of sugars) and pectin (a protein). Shrubs and trees add even tougher walls made of lignin (which contains sugars and other things). We just call the tough material wood.

The cell walls keep plant cells from bursting. Imagine holding the top of a trash bag around a hose and filling the bag with water. Eventually, the bag might stretch until it burst. But if you put the same bag in a trash can before you filled it, it wouldn’t stretch much and wouldn’t burst. This is sort of how plants avoid taking in too much water.

The cellulose, pectin, and wood that make up cell walls do not dissolve in water or break down easily. Cell membranes start to break down almost as soon as something dies because they do not have fibers to make them tough.

Animal cells need some kind of support. Small animals can just use water inside or outside their bodies to support them. We need bones and muscles for our support. Insects and spiders use a hard outer covering.

How do you think having a cell wall might make it harder for a cell to divide?

What an interesting question! Plant cell walls comprise of mostly polysaccharides, which are chains of sugar, while the cell membranes are made of mostly lipids. Maybe an analogy for the cell wall would be a piece of wood, and the cell membrane would be a balloon with water in it. These sugars of the cell walls are much more rigid than the lipids of the cell membrane, so cell walls can retain its structure, despite the cell being dead.

This does not apply to animal cells because after the cells die, there is no control over the amount of liquid in the cell so the cells will likely either lose a lot of water and wither or gain a lot of water and explode.

It is also worth noting that plant cells also have cell membranes, in addition to the cell walls, but since cell walls are much more durable, we generally only think about cell walls when we talk about plants.

In all cellular lives, phospholipid is the building block of the membrane. Phospholipid has a polar head and a non-polar tail. The polar head is not repelled by water, another polar molecule, and hence called hydrophilic. The non-polar tail is repelled by water and hence hydrophobic.

The cell membrane is made of a lipid bilayer. In a lipid bilayer, the non-polar tails hide in the middle while the polar heads expose to each side of the membrane. Those two interactions and the interaction between the tails of adjacent molecules held the membrane together. Since those interactions do not share electrons between each of their parts, in chemistry those interactions are called non-covalent interaction.

Non-covalent interaction is weak comparing to covalent interaction. A detergent like soap can easily disrupt a cell membrane. On the other hand, in the cell wall, there are covalent cross-links to strongly bond each building block together. As a consequence, cell wall is much stronger than any cell membranes and can easily outlast the cell dwell within.

Cell walls are tough and stiff. Plant cells have cell walls. Cell walls are outside of cell membranes on the cells of plants, fungi, bacteria, and archaea.

Cell membranes are fragile and flexible. All cells have cell membranes. Yes, the stiff cell walls survive cell death, but the fragile membranes fall apart when cells die.

Cell walls are made up of resilient chemicals - cellulose for plants and most algae; chitin for fungi; peptidoglycan for bacteria. The cell can die and the cell's membrane and most of the cell's internals can rot away before the cell wall will rot. Cell walls do eventually degrade, though, or else the world would be completely buried in cellulose from all of the world's plant matter through the ages.

Animal cells do not have cell walls. They do have cell membranes, but so does every other cell, including those cells that also have cell walls. Cell membranes are made of soapy phospholipids and some amount of cholesterol, both of which are much more degradable than the tougher materials that cell walls are made out of.