Parenchyma Cells Definition
In plants, parenchyma is one of three types of ground tissue. Ground tissue is anything that is not vascular tissue or part of the dermis (skin) of the plant. In contrast to collenchyma and sclerenchyma cells, parenchyma cells primarily consists of all of the simple, thin walled, undifferentiated cells which form a large majority of many plant tissues.
Structure of Parenchyma Cells
Parenchyma cells are notable for their thin walls, and for being alive at maturity. Collenchyma cells tend to develop thicker secondary cell walls, to support structure. Sclerenchyma cells get both thicker walls and die off at maturity, producing tissues like bark and vascular tissue. The parenchyma cells have thinner walls and stay alive at maturity. While this makes them less useful in structural applications, the cells can move and store water and nutrients as well as divide quickly. This is important for the growth and repair functions of the parenchyma cells.
Each parenchyma cell may be a different shape, depending on its exact location and which tissue it is present in. However, it will always have a large central vacuole. This organelle is responsible for storing water and ions. This both creates a pressure between the parenchyma cells and their neighbors (called turgor pressure) and also allows the plant to store enormous amounts of water and nutrients. The thin walls of the parenchyma cells also allow the easy passage of sugars created in the leaves.
In fact, most photosynthesis takes place within specialized parenchyma cells found within leaves. These parenchyma cells, called chlorenchyma cells, contain chloroplasts. Chloroplasts are special organelles which carry out the process of photosynthesis, storing the energy of sunlight in the newly created bonds of sugar molecules. These sugars can then be converted into other sugars, fats, and oils, and stored in other parenchyma cells within the stems and roots. Potatoes, for example, are mostly parenchyma cells packed with stored starches. The plant would typically use the stores to survive the winter and get a boost the next spring.
Parenchyma Cells Functions
Healing and Repair
One of the most important functions of parenchyma cells is that of healing and repair. Parenchyma cells are unique in their meristematic nature. This means that the cells are pluripotent, having the ability to divide into a number of different cells. This plays an important role in how a plant can heal itself after a wound. While it may seem silly to think that a tree heals, the process is not much different to healing in a human body.
Parenchyma cells, once exposed to the outside when a wound occurs, are stimulated to start dividing. The cells divide towards the wound, differentiating into the different cell types which are needed, such as bark and epidermis. The parenchyma cells on the inside of the wound remain undifferentiated, and provide a source of meristematic cells in case the plant is attacked again. This process is responsible for healing in plants, from giant trees to a blade of grass.
Another important role parenchyma cells play is that of provider. While the other cell types provide much of the support and foundation on which the parenchyma cells operate, they produce a majority of the photosynthesis products. Simply through sheer numbers, parenchyma cells outnumber the other types. The chlorenchyma cells specifically do the majority of the photosynthesis.
However, photosynthesis would come to halt if the products had nowhere to go. Some parenchyma cells differentiate into part of the phloem, a special passageway for the sugars and products of photosynthesis to traverse the plant. These parenchyma cells allow the products to make it from the leaves, where they are created, all the way to the roots. The living cells have specialized proteins and channels which are used to help the sugars make their way efficiently to the roots and other tissues. These other parenchyma tissues need the sugars because they are internal and do not contain chloroplasts with which to create their own energy.
Nutrient and Food Storage
Humans rely on the storage ability of parenchyma cells as our main source of food. The entire food chain is based upon the storage of sugar within parenchyma cells. So, whether you eat meat or are a vegan, you need parenchyma cells. The large central vacuole within plant cells allows the storage of large amounts of soluble nutrients, which dissolve into the water. The plant can control the usage and distribution of the nutrients within cells via the activation of specific proteins and pathways. Parenchyma cells are a major storage place for ions, water, and all photosynthesis products. Many of the foods we know, like fruits and vegetables, are purposefully bred exaggerations of natural plant processes. Corn, potatoes, and wheat were all selected from less productive ancestors which stored higher amounts of nutrients in their parenchyma cells.