In fact, the virus is a large area, and it also includes a branch-sub-virus (for example, prion is a kind of sub-virus). Sub-virus is a kind of organism with simpler structure than virus. But from a macro point of view, sub-viruses also belong to the category of virology. Therefore, for high school biology knowledge, except viruses, other organisms are composed of cells (including eukaryotes and prokaryotes).
Observing plant cells under an optical microscope, we can see that their structures are divided into the following four parts (Figure 3- 1- 1).
Cell wall is located in the outermost layer of plant cells and is a transparent thin wall. Mainly composed of cellulose, with large pores and free penetration of substance molecules. Cell walls support and protect cells.
Cell membrane The inner side of the cell wall is closely attached to an extremely thin membrane, which is called cell membrane. This membrane composed of protein molecules and lipid molecules allows small molecules such as water and oxygen to pass freely, while some ions and macromolecules cannot pass freely. So it not only protects the inside of the cell, but also controls the entry and exit of substances: it prevents useful substances from seeping out of the cell at will, and it also prevents harmful substances from entering the cell easily.
Cell membrane is difficult to distinguish under optical microscope. Observing with electron microscope, we can know that the cell membrane is mainly composed of protein molecules and lipid molecules. In the middle of the cell membrane is a phospholipid bilayer, which is the basic skeleton of the cell membrane. On the outside and inside of phospholipid bilayer, there are many spherical protein molecules embedded in different depths of phospholipid bilayer (Figure 3- 1-2) or covered on the surface of phospholipid bilayer. Most of these phospholipid molecules and protein molecules have fluidity, so it can be said that the cell membrane has certain fluidity. This structural feature of cell membrane is very important for it to complete various physiological functions.
The thick and transparent substance wrapped on the cell membrane in cytoplasm is called cytoplasm. Some refractive particles can also be seen in the cytoplasm. Most of these particles have certain structure and function, similar to various organs of living things, so they are called organelles. For example, in the mesophyll cells of green plants, you can see many green particles, which are organelles called chloroplasts. Photosynthesis of green plants is carried out in chloroplasts. In the cytoplasm, one or several vacuoles can often be seen, and the vacuoles are filled with liquid, which is called cell fluid. In mature plant cells, vacuoles merge into a central vacuole, accounting for more than half of the whole cell.
The cytoplasm is not frozen and static, but moves slowly. In cells with only one central vacuole, the cytoplasm often circulates around the vacuole, which promotes the transport of intracellular substances and strengthens the interconnection between organelles. Cytoplasmic movement is a life phenomenon that consumes energy. The more vigorous the life activity of cells, the faster the cytoplasmic flow, and vice versa. When cells die, the flow of cytoplasm stops.
In addition to chloroplasts, there are some organelles in plant cells, which have different structures and functions and * * * complete the life activities of cells. The structure of these organelles needs to be observed by electron microscope. The cell structure observed under the electron microscope is called submicroscopic structure (Figure 3- 1-3).
Mitochondria are linear and granular, hence the name. On mitochondria, there are many kinds of particles related to respiration, that is, many kinds of respiratory enzymes. It is the place where cells breathe, through which they oxidize and decompose organic matter and release energy for the life activities of cells, so some people call mitochondria the "power station" or "power factory" of cells.
Endoplasmic reticulum (ER) is a reticular pipeline system composed of membranes in cytoplasm. It is connected with cell membrane and plays an important role in the synthesis and transportation of protein and other substances in cells.
Ribosome Ribosome is a kind of granular body, which mostly exists on the outer surface of endoplasmic reticulum membrane and is an important basis for the synthesis of protein.
Centrosome centrosome exists in animal cells and some lower plant cells, and is called centrosome because it is located near the nucleus. Centrosome is closely related to cell mitosis.
The cytoplasm of the nucleus contains a nearly spherical nucleus, which is composed of more viscous substances. The nucleus is usually located in the center of the cell. The nuclei of mature plant cells are often pushed to the cell edge by the central vacuole. There is a substance in the nucleus that is easily dyed dark by basic dyes such as magenta and hematoxylin. This substance is called chromatin. The substance used by organisms to transmit seeds and generations, that is, genetic material, is on chromatin. When cells undergo mitosis, chromatin becomes chromosomes.
Most cells have only one nucleus, and some cells contain two or more nuclei, such as muscle cells and liver cells. The nucleus can be divided into four parts: nuclear membrane, chromatin, nuclear fluid and nucleoli. The nuclear membrane is connected with endoplasmic reticulum, and chromatin is located between the nuclear membrane and nucleolus. Chromatin is mainly composed of protein and DNA. DNA is an organic macromolecule, also called deoxyribonucleic acid, which is the genetic material of living things. During mitosis, chromosomes are copied, and DNA is also copied into two parts, which are evenly distributed to two daughter cells, so that the number of chromosomes in offspring cells is constant, thus ensuring the stability of genetic characteristics of offspring.
Compared with plant cells, animal cells have many similarities, such as cell membrane, cytoplasm, nucleus and other structures. However, there are some important differences between animal cells and plant cells. For example, the outermost animal cell is a cell membrane, but there is no cell wall. There is no chloroplast in the cytoplasm of animal cells and no central vacuole is formed (Figure 3- 1-4).
In short, plants and animals are made up of cells. Cell is the basic unit of organism structure and function.
Find some yourself.