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Plant cells

Let’s start with the most basic unit of life: cells. All living organisms are composed of cells, and are called unicellular when they are composed of a single cell or multicellular, when they have more than one cell. A plant is composed of millions of cells, organized into tissues (similar cells grouped together) and organs (different tissues grouped together). All living cells have common structures like the cell membrane, cytoplasm or ribosomes. Some groups of organisms have certain characteristics in their cells that are unique. Plant cells, for example, are different from animal, fungi, and bacteria cells. They have cell walls, a central vacuole, and chloroplasts. Cell walls in plants are made of cellulose, hemicellulose, and pectin, organic compounds called polysaccharides. The function of the cell wall is to protect the cell and provide mechanical support. A thicker cell wall provides more mechanical support and a thinner cell wall provides flexibility. The central vacuole is essential in the regulation of the turgor pressure, which is the pressure that liquid exerts against the cell wall (Figure ). This internal pressure is important for the physiology of plants because that’s how they are able to remain upright and not wilt. Plants also have unique organelles, called plastids, that have different functions: photosynthesis (chloroplasts), synthesis and storage of starches (amyloplasts), and synthesis of special pigments (chromoplasts).

Plant tissues

Plants are composed of different types of cells, which have different functions. The cells group together into tissues, which in plants can be simple (one type of cell) or complex (more than one type of cell). Simple plant tissues are parenchyma, collenchyma and sclerenchyma.

• Parenchyma cells are the least specialized type of plant cell and the most abundant in plants. They are usually rounded and can be found in all plant organs. In leaves they make up the mesophyll, where photosynthesis takes place (Chapter 4). In stems and roots they make up the cortex, which is responsible for storing carbohydrates and other substances needed for plants to function (Chapters 2 and 3). Parenchyma cells also participate in plant support and the transfer of nutrients in the phloem. Parenchyma cells are the cells that divide and produce new cells when plants are growing. Parenchyma cells make up the parenchyma tissue.
• Collenchyma cells are elongated and provide support to growing plant organs, like leaves. Their primary cell wall is thick, but lacks lignin, making them flexible. Collenchyma cells group into a tissue of the same name.
• Sclerenchyma cells are thick as their cell walls contain lignin, a rigid polysaccharide. They are mostly dead when mature, and their main function is to provide mechanical support, for example in the stem.

Complex plant tissues are xylem, phloem, epidermis and periderm:

• Xylem is the tissue that transports water and nutrients from the roots to the leaves. It is composed of specialized parenchyma cells, tube shaped cells called vessels and tracheids, form long tubes for longitudinal transportation and are usually dead at maturity, and ray cells for lateral conduction.
• Phloem is the tissue that transports the sugars produced in photosynthesis. Unlike xylem, it is composed of live cells called sieve tube members (tube shaped cells) and companion cells, which help control the flow of liquid on the sieve tube members.
• Epidermis is usually a one cell layer on the outside of the plant tissues for protection, and secretes a protective layer called cuticle. Different cells can be found here: parenchyma cells, guard cells in the stomata (openings for gas exchange on leaves), trichomes (hairs), root hairs in the roots, and glands (production of substances).
• Periderm is the outermost layer of woody plants, or outer bark. It is composed of cork cells and lenticels, which are clusters of parenchyma cells that help bring more oxygen into the stem.

In the upcoming four chapters we will cover the different plant organs (Chapters 2 – 5). All plants have four main organs with different functions: roots, stems, leaves, and reproductive organs (flowers, fruits, and seeds). Stems and roots transport water and nutrients, and support the plant. Leaves are where photosynthesis happens. Flowers, fruits, and seeds play a role in plant sexual reproduction and dispersal.