What are Cells Made Of? - Eukaryotic Plant Cells (Part 2 of 2)

Plant cells contain many organelles found in animal cells. It is a source of endless irony that student's first exposure to a self-prepared plant slide is an ATYPICAL cell - the onion cell. Onions are storage organs, much like potato or beetroot. They completely lack the quintessential plant organelle - the chloroplast. These sugar making factories are found only in plants and give plants their green colour; but are not the only difference between animal and plant cells, as we shall see on our tour.

These organelles are, perhaps, the most important organelle in the living world today. They use the energy from the sun to convert water and carbon dioxide into glucose (which is then respired by mitochondria - that's right, plants respire too!) Not only are these responsible for the vast majority of the oxygen in our atmosphere, form the basis of food chains (as they are autotrophs, rather than heterotrophs) they are also why plants (and by extension, much of the planet when seen from space) are green in colour.

Chloroplasts are easily identified with a light microscope. Under the electron microscope, chloroplasts are seen to possess a double membrane that surrounds a third membrane structure - the thylakoid stacks (they look like piles of coins) These stacks rest in the stroma matrix, which contains most of the enzymes (biological catalysts) required for the various reactions that we collectively call: Photosynthesis

It is thought that, like mitochondria, chloroplasts were originally a type of ancient prokaryote was engulfed by another, larger prokaryote. However, instead of being digested, the smaller prokaryote survived and struck up a symbiotic relationship with its would-be killer. The rest is history.

Thought to be an enlarged lysosome, these organelles play a number of vital roles in the plant cell.

Vacuoles are where the 'sap' of the cell is stored. Many important biomolecules that would otherwise affect the osmostic balance of the cell are stored in this organelle. It is surrounded by a membrane known as the Tonoplast.This has a structure similar to that of the plasma membrane, and controls which substances move into, and out of, the vacuole.

Vacuoles are also the cell's osmotic pressure control system. When plants wilt, they tend to lost water from the vacuole as much as possible. It is undesirable for the cytoplasm to lost water, as this would affect vital cellular processes. As such, the majority of cell size changes in plants are usually due to changes in the volume of the vacuole.

Without a skeleton (endo- or exo-) plants require a different strategy to allow themselves to reach for the sky; this role is fulfilled by the cell wall.

The cell wall is made of the most abundant natural polymer on the face of the planet - cellulose. This is not to say that cell wall structure is simple - far from it. There are many forms of cellulose, each with a different function. There are also other macromolecules such as peptidoglycans and pectins that contribute to the mechanical strength of cellulose

The cell wall is thought to be a remnant of the Prokaryotic Capsule. Its function is to provide mechanical strength to the plant, as well as allowing for turgor pressure to be built up. Turgor pressure is caused by the contents of the cell pressing firmly against the solid cell wall. Without this pressure, plants could not stand up. When plants lose water, there are less contents to push against the cell wall, turgor pressure drops, and the plant starts to wilt.

Whilst neighbouring animal cells can communicate quite easily, this is made much more difficult in plant cells due to the polysaccharide cell wall; think how difficult it is to text whilst wearing thick gloves. Plasmodesmata are like fingerless gloves - they are microscopic channels in the cell wall that allow for communication between cells via the Symplastic pathway. A variety of proteins, RNA molecules and other communication molecules can pass through these channels to allow for coordinated action by many plant cells.

That concludes our tour of the major unique sites offered by a typical plant cell. As I have mentioned, many atypical plant cells exist that do not contain one or many of the above organelles - the most common of these being onion cells. There are other, transient structures, that occur in eukaryotes, however this is not the place to discuss them (It would be better to discuss these in the context of the processes during which they appear.) I hope you have enjoyed this mini series, please comment below or contact me if you have any questions.

Until next time...