Learn the Different Types of Plant Cells

Plant Tissue Stem
This is a typical dicotyledon stem (Buttercup). At center is an oval vascular bundle embedded in parenchyma cells (yellow) of the cortex of the stem. The vascular bundle contains large xylem vessels (center right) and the nutrient conducting phloem (orange). At the outer edge of the vascular bundle is sclerenchyma tissue. Power and Syred/Science Photo Library/Getty Images

Plant cells are eukaryotic cells that are the building blocks of plant tissue. They are similar to animal cells and have many of the same organelles. As a plant matures, its cells become specialized in order to perform vital functions such as nutrient transport and structural support. There are a number of important specialized types of plant cells. Some examples of specialized plant cells and tissues include: parenchyma cells, collenchyma cells, sclerenchyma cells, xylem, and phloem.

Parenchyma Cells

Starch Grains - Carbohydrates
This image shows starch grains (green) in the parenchyma of a Clematis sp. plant. Starch is synthesized from the carbohydrate sucrose, a sugar produced by the plant during photosynthesis, and used as a source of energy. It is stored as grains in structures called amyloplasts (yellow). STEVE GSCHMEISSNER/Science Photo Library/Getty Images

Parenchyma cells are usually depicted as the typical plant cell because they are not as specialized as other cells. Parenchyma cells have thin walls and are found in dermal, ground, and vascular tissue systems. These cells help to synthesize and store organic products in the plant. The middle tissue layer of leaves (mesophyll) is composed of parenchyma cells, and it is this layer that contains plant chloroplasts. Chloroplasts are plant organelles that are responsible for photosynthesis and most of the plant's metabolism takes place in parenchyma cells. Excess nutrients, often in the form of starch grains, are also stored in these cells. Parenchyma cells are not only found in plant leaves, but in the outer and inner layers of stems and roots as well. They are located between xylem and phloem and assist in the exchange of water, minerals, and nutrients. Parenchyma cells are the main components of plant ground tissue and the soft tissue of fruits.

Collenchyma Cells

Collenchyma Cells
These plant collenchyma cells form supporting tissue. Credit: Ed Reschke/Getty Images

Collenchyma cells have a support function in plants, particularly in young plants. These cells help to support plants, while not restraining growth. Collenchyma cells are elongated in shape and have thick primary cell walls composed of the carbohydrate polymers cellulose and pectin. Due to their lack of secondary cell walls and the absence of a hardening agent in their primary cell walls, collenchyma cells can provide structural support for tissues while maintaining felxibility. They are able to stretch along with a plant as it grows. Collenchyma cells are found in the cortex (layer between the epidermis and vascular tissue) of stems and along leaf veins.

Sclerenchyma Cells

Sclerenchyma - Plant Vascular Bundle
This images shows sclerenchyma at vascular bundles of a sunflower stem. Ed Reschke/Photolibrary/Getty Images

Sclerenchyma cells also have a support function in plants, but unlike collenchyma cells, they have a hardening agent in their cell walls and are much more rigid. These cells have thick secondary cell walls and are non-living once matured. There are two types of sclerenchyma cells: sclereids and fibers. Sclerids have varied sizes and shapes, and most of the volume of these cells is taken up by the cell wall. Sclerids are very hard and form the hard outer shell of nuts and seeds. Fibers are elongated, slender cells that are strand-like in appearance. Fibers are strong and flexible and are found in stems, roots, fruit walls, and leaf vascular bundles.

Conducting Cells

Xylem and Phloem in Dicotyledon plant
The center of this stem is filled with large xylem vessels for transporting water and mineral nutrients from the roots to the main body of the plant. Five bundles of phloem tissue (pale green) serve to distribute carbohydrate and plant hormones around the plant. Steve Gschmeissner/Science Photo Library/Getty Images

Water conducting cells of xylem have a support function in plants. Xylem has a hardening agent in the tissue that makes it rigid and capable of functioning in structural support and transportation. The main function of xylem is to transport water throughout the plant. Two types of narrow, elongated cells compose xylem: tracheids and vessel elements. Tracheids have hardened secondary cell walls and function in water conduction. Vessel elements resemble open-ended tubes that are arranged end to end allowing water to flow within the tubes. Gymnosperms and seedless vascular plants contain tracheids, while angiosperms contain both tracheids and vessel members.

Vascular plants also have another type of conducting tissue called phloem. Sieve tube elements are the conducting cells of phloem. They transport organic nutrients, such as glucose, throughout the plant. The cells of sieve tube elements have few organelles allowing for easier passage of nutrients. Since sieve tube elements lack organelles, such as ribosomes and vacuoles, specialized parenchyma cells, called companion cells, must carry out metabolic functions for sieve tube elements. Phloem also contains sclerenchyma cells that provide structural support by increasing rigidity and flexibility.

Sources:

  • Sengbusch, Peter v. “Supporting Tissues - Vascular Tissues.” Botany online: Supporting Tissues - Conducting Tissues, www1.biologie.uni-hamburg.de/b-online/e06/06.htm.
  • The Editors of Encyclopædia Britannica. “Parenchyma.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 23 Jan. 2018, www.britannica.com/science/parenchyma-plant-tissue.
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Bailey, Regina. "Learn the Different Types of Plant Cells." ThoughtCo, Feb. 9, 2018, thoughtco.com/types-of-plant-cells-373616. Bailey, Regina. (2018, February 9). Learn the Different Types of Plant Cells. Retrieved from https://www.thoughtco.com/types-of-plant-cells-373616 Bailey, Regina. "Learn the Different Types of Plant Cells." ThoughtCo. https://www.thoughtco.com/types-of-plant-cells-373616 (accessed April 23, 2018).