The bodies of living organisms can be a single cell, their group or a huge cluster, numbering billions of such elementary structures. The latter include most higher plants. The study of cells - the main element of the structure and functions of living organisms - is engaged in cytology. This branch of biology began to develop rapidly after the discovery of the electron microscope, the improvement of chromatography and other methods of biochemistry. Consider the main features, as well as features by which the plant cell differs from the smallest structural units of the structure of bacteria, fungi and animals.
Discovery of a cell by R. Hooke
The theory of the tiny structural elements of all living things has passed the path of development, measured in hundreds of years. The shell structure of plant cells was first seen in his microscope by the British scientist R. Hooke. The general principles of the cellular hypothesis were formulated by Schleiden and Schwann; before that, other researchers made similar conclusions.
The Englishman R. Hook examined under a microscope a section of an oak cork and presented the results at a meeting of the Royal Society in London on April 13, 1663 (according to other sources, the event occurred in 1665). It turned out that the tree bark consists of tiny cells, called by Hooke "cells". The scientist considered the walls of these chambers, forming a pattern in the form of bee honeycombs, as living matter, and recognized the cavity as a lifeless, auxiliary structure. It was further proved that inside the cells of plants and animals they contain a substance, without which their existence is impossible, and the activity of the whole organism.
Cell theory
An important discovery of R. Hooke was developed in the works of other scientists who studied the structure of animal and plant cells . Similar structural elements were observed by scientists on microscopic sections of multicellular fungi. It was found that the structural units of living organisms have the ability to divide. Based on the research, representatives of German biological science M. Schleiden and T. Schwann formulated a hypothesis, which later became a cellular theory.
A comparison of plant and animal cells with bacteria, algae and fungi allowed German researchers to come to the following conclusion: the “chambers” discovered by R. Huck are elementary structural units, and the processes that go on in them are the basis of the vital activity of most organisms on Earth. An important addition was made by R. Virchow in 1855, noting that cell division is the only way of their reproduction. The Schleiden-Schwann theory with refinements has become universally recognized in biology.
The cell is the smallest element in the structure and activity of plants
According to the theoretical principles of Schleiden and Schwann, the organic world is one, which proves a similar microscopic structure of animals and plants. In addition to these two kingdoms, cellular existence is characteristic of fungi, bacteria, and is absent in viruses. The growth and development of living organisms is ensured by the emergence of new cells in the process of division of existing ones.
A multicellular organism is not just an accumulation of structural elements. Small units of the structure interact with each other, forming tissues and organs. Unicellular organisms live in isolation, which does not prevent them from creating colonies. The main signs of the cell:
- ability to exist independently;
- own metabolism;
- self-reproduction;
- development.
In the evolution of life, one of the most important stages was the separation of the nucleus from the cytoplasm with the help of a protective membrane. The connection has been preserved, because separately these structures cannot exist. Currently, there are two sub kingdoms - nuclear-free and nuclear organisms. The second group consists of plants, fungi and animals, the study of which is carried out by the relevant sections of science and biology as a whole. A plant cell has a nucleus, cytoplasm and organoids, which will be discussed below.
Plant cell diversity
At the fracture of a ripe watermelon, apple or potato, one can notice with the naked eye structural "cells" filled with liquid. These are fruit parenchyma cells with a diameter of up to 1 mm. Bast fibers are elongated structures whose length is significantly greater than the width. For example, a plant cell called cotton reaches a length of 65 mm. The fibers of flax bast and hemp have linear dimensions of 40-60 mm. Typical cells are much smaller — 20-50 microns. Such tiny structural elements can only be examined under a microscope. Features of the smallest structural units of the plant organism are manifested not only in differences in shape and size, but also in the functions performed in the composition of the tissues.
Plant cell: the main features of the structure
The nucleus and cytoplasm are closely interconnected and interact with each other, which is confirmed by research by scientists. These are the main parts of a eukaryotic cell, all other structural elements depend on them. The core serves to accumulate and transmit the genetic information necessary for protein synthesis.
The British scientist R. Brown in 1831 for the first time noticed a special body (nucleus) in the cell of an orchid family plant. It was a nucleus surrounded by a semi-liquid cytoplasm. The name of this substance means in literal translation from Greek "primary mass of the cell." It may be more fluid or viscous, but must be covered with a membrane. The outer shell of the cell consists mainly of cellulose, lignin, wax. One of the hallmarks of plant and animal cells is the presence of this strong cellulose wall.
The structure of the cytoplasm
The inner part of the plant cell is filled with hyaloplasm with tiny granules suspended in it. Closer to the membrane, the so-called endoplasm passes into a more viscous exoplasm. It is these substances that the plant cell is filled with that serve as the site of biochemical reactions and transport of compounds, the placement of organoids and inclusions.
About 70–85% of the cytoplasm is water, 10–20% is proteins, other chemical components are carbohydrates, lipids, and mineral compounds. Plant cells have a cytoplasm, in which, among the final products of synthesis, bioregulators of functions and reserve substances (vitamins, enzymes, oils, starch) are present.
Nucleus
A comparison of plant and animal cells shows that they have a similar structure of the nucleus located in the cytoplasm and occupying up to 20% of its volume. The Englishman R. Brown, who first examined under the microscope this most important and constant component of all eukaryotes, gave it a name from the Latin word nucleus. The appearance of nuclei usually correlates with the shape and size of cells, but sometimes differs from them. Mandatory structural elements are membrane, karyolymph, nucleolus, and chromatin.
There are pores in the membrane that separates the nucleus from the cytoplasm. Through them, substances enter the nucleus into the cytoplasm and vice versa. Karyolymph is a liquid or viscous nuclear content with sections of chromatin. The nucleolus contains ribonucleic acid (RNA), which penetrates the cytoplasmic ribosomes to participate in protein synthesis. Another nucleic acid, deoxyribonucleic acid (DNA), is also present in large quantities. DNA and RNA were first found in animal cells in 1869, subsequently found in plants. The nucleus is the "control center" of intracellular processes, a place to store information about the hereditary traits of the whole organism.
Endoplasmic reticulum (EPS)
The structure of animal and plant cells has significant similarities. The inner tubules filled with substances of different origin and composition are necessarily present in the cytoplasm. The granular variety of EPS differs from the agranular type by the presence of ribosomes on the surface of the membranes. The first is involved in the synthesis of proteins, the second plays a role in the formation of carbohydrates and lipids. As the researchers established, the channels not only penetrate the cytoplasm, they are connected with each organoid of a living cell. Therefore, the value of EPS is rated very highly as a participant in metabolism, a system of communication with the environment.
Ribosomes
The cell structure of plants or animals is difficult to imagine without these small particles. Ribosomes are very small; they can only be seen with an electron microscope. Proteins and molecules of ribonucleic acids predominate in the composition of the bodies; there is a small amount of calcium and magnesium ions. Almost the entire amount of RNA cells is concentrated in the ribosomes, they provide protein synthesis, "collecting" proteins from amino acids. Then the proteins enter the EPS channels and are carried by the network throughout the cell, penetrate into the nucleus.
Mitochondria
These organelles of the cell are considered its energy stations, they are visible when magnified in a conventional light microscope. The number of mitochondria varies in a very wide range, there may be units or thousands. The structure of the organoid is not very complex, there are two membranes and a matrix inside. Mitochondria are composed of lipid protein, DNA and RNA, and are responsible for the biosynthesis of ATP - adenosine triphosphoric acid. For this substance, the cells of plants or animals are characterized by the presence of three phosphates. The splitting off of each of them gives the energy necessary for all vital processes in the cell itself and throughout the body. On the contrary, the addition of phosphoric acid residues makes it possible to store energy and transfer in this form throughout the cell.
Look at the organelles of the cell in the image below and name the ones you already know. Pay attention to the large bubble (vacuole) and green plastids (chloroplasts). It’s about them.
Golgi complex
A complex cellular organoid consists of granules, membranes and vacuoles. The complex was opened in 1898 and was named in honor of the Italian biologist. Features of plant cells are the uniform distribution of Golgi particles throughout the cytoplasm. Scientists believe that the complex is necessary to regulate the content of water and waste products, remove excess substances.
Plastids
Only plant tissue cells contain green organelles. In addition, there are colorless, yellow and orange plastids. The type of plant nutrition is reflected in their structure and functions, and they are able to change color due to chemical reactions. The main types of plastids:
- orange and yellow chromoplasts formed by carotene and xanthophyll;
- chloroplasts containing chlorophyll grains - a green pigment;
- leukoplasts are colorless plastids.
The structure of plant cells is associated with the chemical reactions of organic matter synthesis from carbon dioxide and water using light energy. The name of this amazing and very complex process is photosynthesis. The reactions are carried out thanks to chlorophyll, it is this substance that is capable of capturing the energy of a ray of light. The presence of green pigment explains the characteristic color of leaves, herbaceous stems, unripe fruits. Chlorophyll in structure is similar to the hemoglobin of the blood of animals and humans.
The red, yellow and orange color of various plant organs is due to the presence of chromoplasts in the cells. Their basis is a large group of carotenoids that play an important role in metabolism. Leukoplasts are responsible for the synthesis and accumulation of starch. Plastids grow and multiply in the cytoplasm, along with it move along the inner membrane of the plant cell. They are rich in enzymes, ions, and other biologically active compounds.
Differences in the microscopic structure of the main groups of living organisms
Most cells resemble a tiny sac filled with mucus, bodies, granules and vesicles. Often there are various inclusions in the form of solid crystals of minerals, drops of oils, starch grains. Cells are in close contact in the composition of plant tissues, life as a whole depends on the activity of these smallest structural units that make up the whole.
With a multicellular structure, there is a specialization that is expressed in various physiological problems and functions of microscopic structural elements. They are determined mainly by the location of tissues in the leaves, root, stem or generative organs of the plant.
We highlight the main elements of the comparison of plant cells with elementary units of the structure of other living organisms:
- The dense shell, characteristic only of plants, is formed by fiber (cellulose). In fungi, the membrane consists of strong chitin (a special protein).
- Cells of plants and fungi differ in color due to the presence or absence of plastids. Taurus, such as chloroplasts, chromoplasts and leukoplasts, are present only in plant cytoplasm.
- There is an organoid that distinguishes animals - this is centriole (cell center).
- Only in the plant cell is there a large central vacuole filled with liquid contents. Usually this cell juice is colored with pigments in different colors.
- The main reserve compound of the plant organism is starch. Mushrooms and animals accumulate glycogen in their cells.
Among algae, many single, free-living cells are known. For example, such an independent organism is chlamydomonas. Although plants differ from animals in the presence of a cellulose cell wall, germ cells lack such a dense membrane - this is another proof of the unity of the organic world.