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Plant cells are eukaryotic cells or cells with a membrane-bound nucleus. Unlike prokaryotic cells, the DNA in a plant cell is housed within a nucleus that is enveloped by a membrane. In addition to having a nucleus, plant cells also contain other membrane-bound organelles (tiny cellular structures) that carry out specific functions necessary for normal cellular operation. Organelles have a wide range of responsibilities that include everything from producing hormones and enzymes to providing energy for a plant cell.

Plant cells are similar to animal cells in that they are both eukaryotic cells and have similar organelles. However, there are a number of differences between plant and animal cells. Plant cells are generally larger than animal cells. While animal cells come in various sizes and tend to have irregular shapes, plant cells are more similar in size and are typically rectangular or cube shaped. A plant cell also contains structures not found in an animal cell. Some of these include a cell wall, a large vacuole, and plastids. Plastids, such as chloroplasts, assist in storing and harvesting needed substances for the plant. Animal cells also contain structures such as centrioles, lysosomes, and cilia and flagella that are not typically found in plant cells.

Plant Cell Organelles

The following are examples of structures and organelles that can be found in typical plant cells:

Cell (Plasma) Membrane: This thin, semi-permeable membrane surrounds the cytoplasm of a cell, enclosing its contents.

Cell Wall: This rigid outer covering of the cell protects the plant cell and gives it shape.

Chloroplast: Chloroplasts are the sites of photosynthesis in a plant cell. They contain chlorophyll, a green pigment that absorbs energy from sunlight.

Cytoplasm: The gel-like substance within the cell membrane is known as cytoplasm. It contains water, enzymes, salts, organelles, and various organic molecules.

Cytoskeleton: This network of fibers throughout the cytoplasm helps the cell maintain its shape and gives support to the cell.

Endoplasmic Reticulum (ER): The ER is an extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). The ER synthesizes proteins and lipids.

Golgi Complex: This organelle is responsible for manufacturing, storing and shipping certain cellular products including proteins.

Microtubules: These hollow rods function primarily to help support and shape the cell. They are important for chromosome movement in mitosis and meiosis, as well as cytosol movement within a cell.

Mitochondria: Mitochondria generate energy for the cell by converting glucose (produced by photosynthesis) and oxygen to ATP. This process is known as respiration.

Nucleus: The nucleus is a membrane-bound structure that contains the cell's hereditary information (DNA).

Nucleolus: This structure within the nucleus helps in the synthesis of ribosomes.

Nucleopore: These tiny holes within the nuclear membrane allow nucleic acids and proteins to move into and out of the nucleus.

Peroxisomes: Peroxisomes are tiny, single membrane bound structures that contain enzymes which produce hydrogen peroxide as a by-product. These structures are involved in plant processes such as photorespiration.

Plasmodesmata: These pores or channels are found between plant cell walls and allow molecules and communication signals to pass between individual plant cells.

Ribosomes: Consisting of RNA and proteins, ribosomes are responsible for protein assembly. They can be found either attached to the rough ER or free in the cytoplasm.

Vacuole: This plant cell organelle provides support for and participates in a variety of cellular functions including storage, detoxification, protection, and growth. When a plant cell matures, it typically contains one large liquid-filled vacuole.

Plant Cell Types

As a plant matures, its cells become specialized in order to perform certain functions necessary for survival. Some plant cells synthesize and store organic products, while others help to transport nutrients throughout the plant. Some examples of specialized plant cell types and tissues include: parenchyma cells, collenchyma cells, sclerenchyma cells, xylem, and phloem.

Parenchyma Cells

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 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 flexibility. 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 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

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.

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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Learn About Plant Cell Types and How They're Like Animal Cells

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Plant cells are eukaryotic cells or cells with a membrane-bound nucleus. Unlike prokaryotic cells, the DNA in a plant cell is housed within a nucleus that is enveloped by a membrane. In addition to having a nucleus, plant cells also contain other membrane-bound organelles (tiny cellular structures) that carry out specific functions necessary for normal cellular operation.

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Plant Cell Structure

Plants are unique among the eukaryotes, organisms whose cells have membrane-enclosed nuclei and organelles, because they can manufacture their own food. Chlorophyll, which gives plants their green color, enables them to use sunlight to convert water and carbon dioxide into sugars and carbohydrates, chemicals the cell uses for fuel.

Like the fungi, another kingdom of eukaryotes, plant cells have retained the protective cell wall structure of their prokaryotic ancestors. The basic plant cell shares a similar construction motif with the typical eukaryote cell, but does not have centrioles, lysosomes, intermediate filaments, cilia, or flagella, as does the animal cell. Plant cells do, however, have a number of other specialized structures, including a rigid cell wall, central vacuole, plasmodesmata, and chloroplasts. Although plants (and their typical cells) are non-motile, some species produce gametes that do exhibit flagella and are, therefore, able to move about.

Plants can be broadly categorized into two basic types: vascular and nonvascular. Vascular plants are considered to be more advanced than nonvascular plants because they have evolved specialized tissues, namely xylem, which is involved in structural support and water conduction, and phloem, which functions in food conduction. Consequently, they also possess roots, stems, and leaves, representing a higher form of organization that is characteristically absent in plants lacking vascular tissues. The nonvascular plants, members of the division Bryophyta, are usually no more than an inch or two in height because they do not have adequate support, which is provided by vascular tissues to other plants, to grow bigger. They also are more dependent on the environment that surrounds them to maintain appropriate amounts of moisture and, therefore, tend to inhabit damp, shady areas.

It is estimated that there are at least 260,000 species of plants in the world today. They range in size and complexity from small, nonvascular mosses to giant sequoia trees, the largest living organisms, growing as tall as 330 feet (100 meters). Only a tiny percentage of those species are directly used by people for food, shelter, fiber, and medicine. Nonetheless, plants are the basis for the Earth's ecosystem and food web, and without them complex animal life forms (such as humans) could never have evolved. Indeed, all living organisms are dependent either directly or indirectly on the energy produced by photosynthesis, and the byproduct of this process, oxygen, is essential to animals. Plants also reduce the amount of carbon dioxide present in the atmosphere, hinder soil erosion, and influence water levels and quality.

Plants exhibit life cycles that involve alternating generations of diploid forms, which contain paired chromosome sets in their cell nuclei, and haploid forms, which only possess a single set. Generally these two forms of a plant are very dissimilar in appearance. In higher plants, the diploid generation, the members of which are known as sporophytes due to their ability to produce spores, is usually dominant and more recognizable than the haploid gametophyte generation. In Bryophytes, however, the gametophyte form is dominant and physiologically necessary to the sporophyte form.

Animals are required to consume protein in order to obtain nitrogen, but plants are able to utilize inorganic forms of the element and, therefore, do not need an outside source of protein. Plants do, however, usually require significant amounts of water, which is needed for the photosynthetic process, to maintain cell structure and facilitate growth, and as a means of bringing nutrients to plant cells. The amount of nutrients needed by plant species varies significantly, but nine elements are generally considered to be necessary in relatively large amounts. Termed macroelements, these nutrients include calcium, carbon, hydrogen, magnesium, nitrogen, oxygen, phosphorus, potassium, and sulfur. Seven microelements, which are required by plants in smaller quantities, have also been identified: boron, chlorine, copper, iron, manganese, molybdenum, and zinc.

Thought to have evolved from the green algae, plants have been around since the early Paleozoic era, more than 500 million years ago. The earliest fossil evidence of land plants dates to the Ordovician Period (505 to 438 million years ago). By the Carboniferous Period, about 355 million years ago, most of the Earth was covered by forests of primitive vascular plants, such as lycopods (scale trees) and gymnosperms (pine trees, ginkgos). Angiosperms, the flowering plants, didn't develop until the end of the Cretaceous Period, about 65 million years agojust as the dinosaurs became extinct.

Leaf Tissue Organization - The plant body is divided into several organs: roots, stems, and leaves. The leaves are the primary photosynthetic organs of plants, serving as key sites where energy from light is converted into chemical energy. Similar to the other organs of a plant, a leaf is comprised of three basic tissue systems, including the dermal, vascular, and ground tissue systems. These three motifs are continuous throughout an entire plant, but their properties vary significantly based upon the organ type in which they are located. All three tissue systems are discussed in this section.

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Molecular Expressions Cell Biology: Plant Cell Structure

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The basic plant cell has a similar construction to the animal cell, but does not have centrioles, lysosomes, cilia, or flagella. It does have additional structures, a rigid cell wall, central vacuole, plasmodesmata, and chloroplasts. Explore the structure of a plant cell with our three-dimensional graphics.