A plant cell refers to any cell of a plant. Plant cells are presumed to have evolved from the early green algae and probably first occurred in the early Paleozoic era, more than 500 million years ago. Plant cells were first observed by an English natural philosopher, Robert Hooke. With the availability of a microscope, he was able to see plant cells from a bottle cork sample. He noticed microscopic pores that resembled a honeycomb. He, then, coined the term cells to refer to these tiny pores. Initially, he thought that they were empty cells. With the availability of more advanced microscopes, scientists were able to observe the living components of plant cells. Typically, a simple plant cell has membrane-bound cell structures (called organelles) that are suspended in the cytoplasm.
Plant cell vs. animal cell
A plant cell, similar to an animal cell, is eukaryotic. Eukaryotic cells are characterized by the presence of organelles, particularly nucleus, as opposed to prokaryotic cells that lack them. Although plant cell and animal cell are similar in this regard they have recognizable structural differences. A plant cell has a rigid cell wall that is absent in an animal cell. There is also a central vacuole that occupies much space in a plant cell. In an animal cell, the vacuoles are numerous but small. Plant cells have many chloroplasts whereas animal cells lack them. Chloroplasts are key organelles in photosynthesis. Thus, plants are capable of making their own food directly from inorganic sources whereas animals rely on available food from organic sources. They also differ in how a cell divides. A plant cell divides by forming a cell plate between two daughter cells during the latter phase of cell division. In contrast, an animal cell forms a cleavage furrow.
Plant cell structure
A typical plant cell structure includes organelles, cytoplasmic structures, cytosol, cell membrane (also called plasma membrane), and cell wall. Plant cell organelles include plastids, nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. The nucleus is the organelle that modulates the metabolic activities of the cell. It contains most of the cell’s genetic material. Other genetic materials are located in the semi-autonomous organelles, the mitochondria and the chloroplasts. The mitochondria are the organelles that provide the cells chemical energy, e. g. ATP, by cellular respiration. The chloroplasts are chlorophyll-containing organelles that play an important role in the photosynthetic process. They are the light energy (photon)-harvesting organelles. They convert inorganic salts, water, and carbon dioxide into complex organic material (e.g. glucose molecules) as driven by the light energy from a light source (e.g. sunlight). Apart from chloroplasts, there are also other types of plastids found in a plant cell. They are the chromoplasts and the leucoplasts. Chromoplasts contain accessory pigments whereas leucoplasts contain stored food. The endoplasmic reticulum is the organelle involved in protein synthesis. Together with the ribosomes, they create proteins that will be transported intracellularly or extracellularly. Proteins that are for transport are passed on to the Golgi apparatus where they are packaged and sorted. Lysosomes, peroxisomes, and cytoskeletons (microtubules, actin filaments, and intermediate filaments) are also present in plant cells. A central vacuole is present and often the largest cytoplasmic structure in a plant cell. It is essential to plant due to its role in osmoregulation. All these cytoplasmic structures are suspended in the fluid component of the cytoplasm, the cytosol. The cell membrane contains the organelles, other cytoplasmic structures, and the cytosol. It is a semi-permeable, double-membraned covering that is made up of lipids, carbohydrates, and proteins. On the cell’s exterior is a cell wall that is responsible for the cells turgidity, rigidity, strength, and resiliency against mechanical stress. A plant cell wall may be made up of two layers of cell walls, a primary and a secondary wall. A primary cell wall consists of cellulose, pectin, and hemicellulose. Over time, it could deposit another layer called the secondary cell wall. It is characteristically thick due to lignin deposition. For the generalized plant cell model, refer to the illustration of a plant cell on the right.
Fun activity: Plant Cell Coloring worksheet
Types of plant cells
Plants are comprised of several cells that may be organized to plant tissues and organs that perform a particular function. Thus, plant cells may be classified into types based on the tissues they are associated with. The common plant cell types are meristematic cells, parenchyma cells, collenchyma cells, sclerenchyma cells, and reproductive cells. Plant cells in a tissue communicate by plasmodesmata. They are microscopic channels that link plant cells by thin strands. They facilitate transport between individual plant cells.
Meristematic cells are the cells of the meristem. These cells are not yet differentiated and divide actively by mitosis. They are the stem cells of plants, capable of giving rise to any plant cell type.
Parenchyma cells are cells of the parenchyma tissues. They have distinctively thin walls (due to the absence of a secondary wall deposition) and remain alive at maturity. They are involved chiefly in photosynthesis, food storage, secretion, and phloem loading. Parenchyma cells are the most common plant cell type. They occur in vascular bundles, leaves, and epidermis. Parenchyma cells that are involved in photosynthesis are called chlorenchyma cells. Guard cells are another specialized type of parenchyma cells. They regulate the opening and closure of stomata (plant pores for gas exchange).
Collenchyma cells are cells of the collenchyma tissues. Similar to parenchyma cells, they also lack secondary cell wall. They are also often alive at maturity. They differ, however, from parenchyma cells in having thicker primary cell wall albeit the wall thickenings are irregular. Collenchyma cells are typically found in the growing shoots and leaves where they provide structural support.
Sclerenchyma cells are cells of the sclerenchyma tissues. They have thick walls due to secondary wall and lignin deposition. Unlike the parenchyma and collenchyma cells, the sclerenchyma cells are dead at maturity. They lose their protoplast. Examples of sclerenchyma cells are sclerenchyma fibers, sclereids and bast fibers.
Unlike animals, plants have an alternation of generations, i.e. the alternating phases of sporophyte and gametophyte. The sporophyte is a plant form in diploid condition. It eventually bears sporangia that produce spores. A spore is a haploid reproductive cell involved in asexual reproduction and gives rise to a gametophyte.
Each haploid spore divides mitotically to become the gametophyte. Thus, a gametophyte is a plant form in haploid condition. It bears gametangia that produce sex cells (gametes). The sex cells are reproductive cells involved in the sexual reproduction. The female sex cell is an egg cell whereas the male sex cell is a sperm cell. These cells are haploid.
Thus, when they unite they form a diploid zygote. The zygote, then, develops into a new sporophyte.
Plant cells are important as part of the biotic component of an ecosystem. They are the primary producers in which non-autotrophic organisms rely upon as a source of nutrient and organic matter. Plant cells are also important commercially. Fibers (e.g. phloem fibers) are collected for their commercial value. They are marketed as jute, hemp, flax, ramie, rattan, and kenaf. They also produce compounds that have medicinal and industrial value.
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