What is a Cell Wall? All living things are composed of cells. They are the building blocks of all life. Cells come in many different shapes and have different functions. Plant and animal cells are different, too. The main difference between plant and animal cells is that plant cells have a cell wall on the outer layer, whereas animal cells only have a cell membrane. The
cell wall is a protective layer outside the cell membrane that also provides support for the cell\’s structure. The cell wall gives the plant its actual shape. It acts as a gatekeeper, because it determines what can come in and out of the cell in order to keep the cell protected. It is kind of like the outside bricks of a castle, only, as you will learn as you read on, there are holes throughout this castle. Those holes do make the cell vulnerable, but they are important to the function of the cell. A redwood tree and a dandelion both have cell walls on the outside of all of their cells.
The cell walls are there to give the plants their shape and support; however, the cell walls act and are constructed a little different to meet the needs of the particular plant. For instance, a 100-foot redwood tree needs a very strong and rigid plant cell wall so that it can grow to its great height and not fall over in the wind. On the other hand, a little yellow dandelion out in the field needs to have more plasticity so that it can bend, not break, as the wind blows through the field. Have you ever forgotten to water the flowers? They may not be able to talk, but they will let you know when they\’re thirsty, as they begin to droop over. Their shape is still being maintained by the cell wall so that, as soon as you water the plant, it can pick itself back up again. On the other hand, if you water too much, the cell wall also makes sure that the cell does not burst.
It protects the cell from over-expansion. The cell wall protects the plant and cells from the many insects and pathogens that could harm the plant, but the cell wall still has its vulnerable areas. There are holes all over the cell wall called plasmodesmata. These are holes that allow for nutrients to enter the cell as well as waste to exit the cell. These small holes can cause the cell to lose water, and this is when the plant will start to droop. But as soon as the plant can get a drink, it will bounce right back up to its proper shape. Like mitochondria, chloroplasts likely originated from an ancient symbiosis, in this case when a nucleated cell engulfed a photosynthetic prokaryote. Indeed, chloroplasts resemble modern cyanobacteria, which remain similar to the cyanobacteria of 3 million years ago.
However, the evolution of photosynthesis goes back even further, to the earliest cells that evolved the ability to capture light energy and use it to produce energy-rich molecules. When these organisms developed the ability to split water molecules and use the electrons from these molecules, photosynthetic cells started generating oxygen an event that had dramatic consequences for the evolution of all living things on Earth (Figure 1). Today, chloroplasts retain small, circular genomes that resemble those of cyanobacteria, although they are much smaller. (Mitochondrial genomes are even smaller than the genomes of chloroplasts. ) Coding sequences for the majority of chloroplast proteins have been lost, so these proteins are now encoded by the nuclear genome, synthesized in the cytoplasm, and transported from the cytoplasm into the chloroplast.