All plants are made of cells, and like animals, all of them start from a single cell. This cell is called an egg cell and once fertilized by pollen, it begins to divide countless times. These cells later differentiate and take different roles upon themselves, changing their size and shape as well. The structure of a plant can be divided into two systems: a shoot system, the part above the ground, and a root system, below the ground. While the roots function to both stabilize plant and to absorb water with nutrients, the shoot system is responsible for photosynthesis, production of flowers and the next generation of seeds. In order to understand how all these different types came up from a single cell, we need to look into plant cell biology first.
Observing a plant, we can look at it by different levels of an organization. At the top, there is a plant as an organism, which contains organs. The individual organs are made of tissues, and tissues are made of many cells. The cell is the basic, self-sustainable organic unit, however, it can still be broken up into smaller components such as its organelles. There is a very close relationship between the structure and function of the cell or the organ. For an example, the wide and spread out structure of a leaf is to absorb the light needed for photosynthesis. This can be seen even on the level of molecules, the photosynthesis would not be possible without the molecule of chlorophyll.
Most of the structures found in plant cells can also be found in cells of animals, and both can be divided into two main parts: the cytoplasm and the plasma membrane. The latter one is sort of a container that surrounds the soluble cytoplasm, which is the inside of the cell. The membrane allows for proteins to transverse through, connecting both the outer and inner side of the cell. There are also proteins that can be bound only to one side of the membrane, nonetheless, all of them have different functions depending on what the function of the whole cell is. Both animal and plant cells also contain a nucleus, covered by a nuclear membrane or also sometimes called a nuclear envelope, protecting the DNA. Both cells contain an organelle called mitochondria, known as the powerhouse of the cells because they generate most of the energy needed for life. They manage this through a complex set of chemical reactions, which in its simplest form take sugar and oxygen and convert it into the chemical form of energy used in cells, ATP, and releases carbon dioxide as its by-product. A mitochondria is comprised of different compartments to carry out its functions and is enclosed again by a membrane. What is interesting is that it carries a small amount of DNA, very similar to the DNA found in bacteria.
There are other organelles common for both animal and plant cell, including Golgi apparatus, a plasma porticulum and others. However, plants have three additional organelles specific only for them. The first of them is a cell wall, a very rigid part on the outside of the outer membrane. Everything on the inside, the plasma membrane and the cytoplasm, we call protoplast. Plants also contain an organelle called plastids, the most familiar of them is a chloroplast, which carries out the photosynthesis. In the center of a cell can be found a large sac, a central vacuole, responsible for the shape and other functions of the cell. The last organelle, plasmodesmata, is in the walls of a cell, connecting one cell to another. This enables communication and exchange of materials between individual cells, practically from the root up to the flower.
We are going to take a slightly deeper look at some of the most important components of a plant cell. Once again, starting from the outside, we are met with cell walls, which provide a protection for the cell. Apart from keeping the environment out and the cytoplasm in, it helps the plant to maintain its shape and prevents the excessive uptake of water. The cell wall is not only a passive barrier, but it is active as well in that it contains enzymes that affect the plant metabolism. It is variable in size, varying between 0.1 and 5 microns thick, and has a specific chemical composition. In general though, it is made from long molecules of alpha glucose, cellulose, that bind together to make a very strong hydrophilic fiber. In fact, they are so strong that they by themselves are one of the strongest molecules on earth per weight. These cellulose fibers are then cross-linked, bound together in a matrix and glued together by a polysaccharide called pectin. To have a better idea, you can compare it to a reinforced concrete.
The main fill of the cytoplasm is often a vacuole, which can provide up to 90% of the cell volume. It is enclosed by a membrane called tonoplast, which can be also connected to various other membranes within the cell. The main function of the vacuole is to serve as a storage for the cell, storing all kinds of compounds. From ions such as potassium, chloride or sodium, through dangerous metabolic byproducts, such as purple color anthocyanin (which is poisonous to the cell), to acids such as citric acid found in lemons or poisons such as nicotine in tobacco. Nonetheless, the vacuole has a major role in the growth and shape of the plant. The plant pumps water into the vacuole, putting pressure on the cytoplasm, resulting in an elongation of the cell without having to invest a lot of energy into making a new cell.
Let’s finally get to the green stuff though, the chloroplast. Like mitochondria, it converts energy into usable forms, but instead of taking sugar and oxygen it uses light and carbon dioxide to make sugar and oxygen. So the main function of the chloroplasts is to do photosynthesis, but they also make amino and fatty acids for the cell to use, making them essential for the metabolism of the whole plant. Chloroplasts contain chlorophyll, the almighty green. There are other types of plasts: a chromoplast giving colors to petals in flowers or to fruits, or amyloplasts in the roots of the plant, which accumulate starch as a source of energy. Chloroplasts can actually move within the cell accordingly to the light environment. In order to garner more or less energy for photosynthesis, they move so that they are positioned exactly to the energy needs of the plant.