English name: CELL is abbreviated as C in the article.

Cell is composed of protoplasm and nucleus (or pseudonucleus) surrounded by membrane, which is the basic unit of organism structure and function, and also the basic unit of life activities. Cells can proliferate through division, which is the basis of individual development and systematic development of organisms. Cells are either independent living units, or multiple cells form cell groups or tissues, or organs and organisms; Cells can also divide and reproduce; Cell is the basic unit of heredity and has genetic totipotency.

All living things are made up of cells except viruses. There are both unicellular organisms and multicellular organisms in nature. Cell is the basic structure and functional unit of organism. Cells are an indispensable part of the biological world.

Cellular structure

Observing a plant cell under an optical microscope, we can see that its structure is divided into the following four parts.

Cell epidermis

Located in the outermost layer of plant cells, it 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 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.

cytoplasm

The thick and transparent substance wrapped on the cell membrane 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 electron microscope is called submicroscopic structure.

mitochondria

It is 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.

chloroplast

Chloroplast is an important organelle in green plant cells, and its main function is photosynthesis. Chloroplast consists of bilayer membrane, thylakoid and matrix. The thylakoid is a small flat thylakoid structure. On thylakoid membrane, there are pigments and enzymes necessary for photosynthesis. Many thylakoids are superimposed to form particles. Matrix is filled between particles, which contains enzymes related to photosynthesis. The matrix also contains DNA.

endoplasmic reticulum

Endoplasmic reticulum (ER) is a reticular pipeline system composed of membranes in cytoplasm, which is widely distributed in cytoplasm matrix. It is connected with cell membrane and plays an important role in the synthesis and transportation of protein and other substances in cells.

There are two kinds of endoplasmic reticulum: one is smooth surface; The other is that many small particles are attached to it. Endoplasmic reticulum increases the membrane area in cells, and many enzymes attach to the membrane, which provides favorable conditions for normal chemical reactions in cells.

Golgi apparatus

Golgi apparatus is ubiquitous in plant cells and animal cells. It is generally believed that Golgi apparatus in cells is related to the formation of cell secretions. Golgi body itself has no function of synthesizing protein, but it can process and transport protein. Golgi apparatus is related to the formation of cell wall during plant cell division.

ribosome

Ribosomes are oval granular bodies, some of which are attached to the outer surface of endoplasmic reticulum membrane, while others are free in the cytoplasmic matrix, which is an important basis for the synthesis of protein.

centrosome

Centrosomes exist in animal cells and some lower plant cells, and are called centrosomes because they are located near the nucleus. Each centrosome consists of two centrosomes arranged vertically to each other and perpendicular to the surrounding substances. The centrosome of animal cells is closely related to mitosis.

vacuole

Vacuoles are vesicular structures in plant cells. Vacuoles in mature plant cells are very large, accounting for 90% of the whole cell volume.

There is a vacuole membrane on the surface of vacuole. There is cell fluid in vacuoles, which contains substances such as sugar, inorganic salts, pigments, protein, etc., and the concentration can reach a high level. Therefore, it plays a regulatory role in the environment where cells are located, which can keep cells in a certain osmotic pressure and keep them in an expanded state.

Lysosomes lysosomes are organelles with a single membrane sac structure in cells. It contains a variety of hydrolases and can decompose a variety of substances.

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.

There is RNA, which is a single-stranded DNA in replication. It transmits protein and is called the messenger of DNA.

It is composed of protoplasm and nucleus (or pseudonucleus) surrounded by membrane, which is the basic unit of organism structure and function, and also the basic unit of life activities. Cells can proliferate through division, which is the basis of individual development and systematic development of organisms. Cells are either independent living units, or multiple cells form cell groups or tissues, or organs and organisms; Cells can also divide and reproduce; Cell is the basic unit of heredity and has genetic totipotency (plant)

Animal nuclei are omnipotent.

Cytology is a branch of biology that studies the structure and function of cells.

Cell is the basic unit that constitutes the form and function of an organism, and it is composed of many parts. Therefore, the study of cell structure should not only know which parts it is composed of, but also further understand the composition of each part. Accordingly, we should not only know the function of the whole cell, but also understand the functional relationship of each part.

Biological physiological functions and all life phenomena are expressed on the basis of cells. Therefore, cytology is very important for understanding the inheritance, development and physiological functions of organisms, as well as pathology, pharmacology and agricultural breeding as the basis of medical treatment.

The vast majority of cells are very small, beyond the limits of human vision, and observation of cells must use a microscope. So in 1677, when Levin Hook observed the animal's "sperm" with his own simple microscope, he didn't know it was a cell. The word "cell" was named after robert hooke observed cork slices in 1665 and saw cells in cork. In fact, these cells are not living structures, but gaps formed by cell walls, and the term cell has always been used.

In the early stage of cytology, although many tiny objects, such as bacteria and ciliates, were observed with a simple microscope, the main purpose was to observe some developmental phenomena, such as the metamorphosis of butterflies and the structure of sperm and eggs. It was not until 1827 that Bell discovered the eggs of mammals that he began to observe the cells themselves carefully. The achromatic objective lens developed before and after this, the introduction of fuchsin and hematoxylin as dyes to dye the nucleus, and the start of slicer and slicing technology have created favorable conditions for more detailed observation of cells.

German biologists Schleiden and Wang Shi have played a great role in promoting cell research. In 1838, the former describes that cells are produced in a clayey parent material through a process similar to crystallization, and plants are regarded as the * * * isomorphism of cells. Inspired by him, Shi Wan firmly believed that animals and plants are composed of cells, and pointed out the consistency of their structure and growth, and put forward the cell theory in 1839.

At the same time, Czech animal physiologist Pukenye put forward the concept of protoplasm; German zoologist Sebold concluded that protozoa are single-celled. On the basis of studying connective tissue, German pathologist Fairshaw put forward the famous saying that "all cells come from cells" and founded cytopathology.

/kloc-From the mid-9th century to the early 20th century, great progress has been made in the study of cell structure, especially the nucleus. German botanist Strasbourg first described the colored objects in plant cells in 1875, and came to the conclusion that the same plant has a certain number of colored objects; Baranetski described the spiral structure of colored objects in 1880, and the colored particles were found in Pfitzner the next year. It was not until 1888 that Valdeir officially named the colored objects in the nucleus as chromosomes. German scholar Henkin observed the X chromosome in insect sperm cells in 189 1, and Stevens and Wilson observed the Y chromosome in 1902.

German botanist Hoffmeister described the indirect division of plants and animals in 1867 and 1873 respectively. Froemming, a German cytologist, put forward the name of mitosis rather than indirect division after discovering the longitudinal division of chromosomes in 1882, and H described the chromosome distribution during indirect division. After him, Strasbourg divided mitosis into prophase, metaphase, anaphase and anaphase, which is still common until now. He and other scholars also observed the meiosis of plants, and finally distinguished the number of haploid and diploid chromosomes through further research.

The understanding of cytoplasmic structure lags behind that of nucleus or chromosome, and this situation will be improved in the long run. Especially after the beginning of the 20th century, with the cytogenetics research on the chromosome basis of genetic phenomena such as separation, recombination, linkage and exchange, the understanding of chromosomes has been deepened. But at the same time, except for some physiological functions of cell physiology, the understanding of cytoplasmic structure has not made much progress. It was not until the 1940s that the electron microscope was widely used, and a set of techniques for embedding and slicing specimens was gradually improved and great changes took place.

1900 After Mendel's research results were rediscovered, the study of genetics strongly promoted the progress of cytology. Morgan, an American geneticist and embryologist, studied the inheritance of Drosophila and found that individuals who occasionally have white eyes are always male; Combined with the existing knowledge about sex chromosomes, the appearance of white-eyed men is explained, and the genetic phenomenon is explained from the cell. Genetic factors may be located on chromosomes. Cytology and genetics are linked, quantitative and physiological concepts are obtained from genetics, qualitative, material and narrative concepts are obtained from cytology, and cytogenetics is gradually produced.

In 1920, American cytologist Sutton further pointed out that the parallel phenomenon between genetic factors and chromosome behavior inevitably means that genetic factors are located on chromosomes, and mentioned that if two pairs of factors are located on the same chromosome, they may or may not be inherited according to Mendel's law, which shows the concept of linkage and deepens the research on mature division, especially chromosome pairing and chromosome exchange.

In addition, it is also found that radiation and temperature can cause mutation in Drosophila, which is more conducive to the experimental study of chromosomes because of its high mutation frequency. All kinds of mutations caused by radiation, including gene translocation, inversion and deletion, exist in chromosomes. The gene arrangement map of chromosomes can be calculated by crossing mutants with wild types and statistically processing their offspring. The extensive study on the morphology of sex chromosomes has also found a cytological basis for the determination of male and female sex.

In the early 1940s, the technical methods of other disciplines were used in cytological research one after another, which opened up a new situation and formed some new fields. First of all, the application of electron microscope produces ultramicroscopic morphology.

Belgian zoologist Blachais studied the significance of nucleic acid in development by using special staining methods in embryology. Almost at the same time, the Swedish biochemist kasper Song created an ultraviolet cell spectrophotometer to detect the existence of protein, DNA and RNA in cells according to the absorption of certain wavelengths by various substances. Their work has attracted people's attention to the role of nucleic acids in cell growth and differentiation. On the basis of their work, cytochemistry has been developed to study the chemical composition of cells, which can supplement the study of morphology and increase some understanding of cell structure.

In the 1940s, biochemical studies on the functions of various parts of cells were gradually carried out, resulting in biochemical cytology. First, homogenate-mechanically grinding cells in a suitable solution-and differential centrifugation are used to obtain mitochondria, microsomes, hyaluronic acid and nuclei. By studying them separately, we can know the existence and distribution of some substances and enzymes, and where some metabolic processes are carried out. Some studies on mitochondria and microsomes show that many basic biochemical processes are carried out in cytoplasm rather than in nucleus. This method, combined with in-depth morphological research, leads to a deeper understanding of intracellular processes.

Although tissue culture developed greatly in 1930s, it can only cultivate tissue blocks, but not single cells of normal tissues, and its importance has not been fully demonstrated. Using cultured cells, we can study many problems that cannot be studied as a whole, such as cell nutrition, exercise, behavior and the relationship between cells. Almost all kinds of tissues, including some invertebrates, have been cultivated.

Under good culture conditions, the growth of various cells grown from tissue blocks is different. Morphology can be basically divided into three types: epithelium, connective tissue and wandering cells. Sometimes, cultured cells show characteristics that normal tissues do not have in organisms. For example, if the medium contains substances that enhance surface activity, cells of various tissues can acquire phagocytosis. However, they still maintain their unique properties and potential, because if they change the culture environment or move back to the original parts of animals, they can still grow as they are.

It is worth mentioning that the growth of fibroblasts in culture is also affected by substrates. Generally speaking, they grow radially and aimlessly from tissue blocks. However, if the culture medium is artificially placed under tension in a certain direction, or traces are artificially made on the substrate, cells will grow along the direction of tension or along the traces. This phenomenon may explain the functional adaptation of connective tissue and tendon as a whole-they always grow and differentiate in the direction of tension.

It can be seen that the study of cells, the deepening of submicroscopic structure after the use of electron microscope and the deepening of functions after the application of biochemical technology have created conditions for the formation of cell biology-the study of cell life phenomena at the molecular level. Therefore, under the influence of outstanding achievements in molecular genetics and molecular biology, a new discipline of cell biology was quickly formed.

Generally, cells are so small that their faces can only be seen clearly under a microscope. Generally, skeletal muscle cells are as long as1-40 mm. However, there are also communities as long as1m.

Neuroanatomists have found that in the nervous system of mammals, some neurons (that is, nerve cells) specialize in motor functions, and their prominent parts can be as long as 1 meter. Their cell bodies are located in the cerebral cortex or spinal cord gray matter, but their protruding ends can reach far away. Located in the cerebral cortex are called pyramidal cells. They have a long process called axon. Axon is a channel used to transmit information, and the motor instructions sent by the brain reach the spinal cord through the brain stem along this line. The cells in the spinal cord that receive instructions from the cerebral cortex are called motor neurons in the anterior horn of the spinal cord. It also has a very long axon, which passes through the cone-shaped tube and reaches the dominated muscle along the spinal nerve, and converts the motor instructions of the brain into the signal of muscle movement, so that the muscle moves with the intention of the brain.

The structure and function of cells are consistent. The distance from cerebral cortex to spinal cord and spinal cord to muscle is very long, so the nerve cells that establish such a long-distance connection between these two parts must have a specific structure, which is why there are such long protrusions. In addition, the bigger an animal is, the longer its motor neurons are.

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.

Human cells

1. The largest cell in human body is a mature egg cell (diameter 0. 1mm).

2. The smallest cell in human body is lymphocyte (6 microns in diameter).

3. The longest-lived cells are nerve cells.

4. The cell with the shortest life span is white blood cell.

Chemical composition of cells

The basic elements of a cell are O, C, H, N, Si, K, Ca, P and Mg, of which O, C, H and N account for more than 90%. Cytochemicals can be divided into two categories: inorganic substances and organic substances. Water is the most important component in inorganic substances, accounting for about 75%-80% of the total content of cellular substances.

I. Water and inorganic salts

(1) Water is the most basic substance of protoplasm.

Water is not only the most abundant in cells, but also plays a key role in the origin of life and the formation of ordered structure of cells because of its unique physical and chemical properties. It can be said that there is no life without water. Water exists in cells in two forms: one is free water, accounting for about 95%; The other is bound water, which is bound to protein by hydrogen bonds or other bonds, accounting for about 4% ~ 5%. With the growth and aging of cells, the water content of cells gradually decreases, but the water content of living cells will not be lower than 75%.

The main function of water in cells is to dissolve inorganic substances, regulate temperature, participate in enzyme reactions, participate in substance metabolism, and form an orderly structure of cells. The reason why water has so many important functions is inseparable from its unique properties.

1. Water molecules are dipoles.

From the chemical structure, water molecules seem to be very simple, consisting of only two hydrogen atoms and 1 oxygen atom (H2O). However, the charge distribution in water molecules is asymmetric, one side is positively charged and the other side is negatively charged, thus showing polarity, which is a typical dipole (Figure 3-3 1). Because of this characteristic, water molecules can combine with positive and negative charges in protein. Each amino acid in protein can bind 2.6 water molecules on average.

Water molecules have electrostatic effect because of their polarity, so they are good solvents for some ionic substances (such as inorganic salts).

2. Hydrogen bonds can be formed between water molecules.

Because water molecules are dipoles, weak hydrogen bonds can be established between water molecules and between water molecules and other polar molecules. In water, each oxygen atom can form two hydrogen bonds with the hydrogen atoms of two other water molecules. The hydrogen bond force is very weak, so the hydrogen bond between molecules is often in the process of disconnection and reconstruction.

3. Water molecules can be decomposed into ions.

Water molecules can be decomposed into hydroxyl ions (OH-) and hydrogen ions (H+). Under standard conditions, a small amount of water molecules are always dissociated into ions, and about 107mol/L water molecules are dissociated, which is equivalent to 2 out of every 109 water molecules. However, the electrolysis of water molecules is not stable, and it is always in the dynamic balance of mutual transformation between molecules and ions.

(2) Inorganic salts

The content of inorganic salts in cells is very small, accounting for about 65438 0% of the total cell weight. Salt dissociates into ions in cells, and the concentration of ions plays many important roles besides regulating osmotic pressure and maintaining acid-base balance.

The main anions are Cl-, PO4 and HCO-3, among which phosphate ion is the most important in cell metabolism: ① it plays a key role in energy metabolism of various cells; ② It consists of nucleotide, phospholipid, phosphoprotein and phosphorylated sugar; ③ Adjust the acid-base balance and buffer the pH of blood and tissue fluid.

The main cations are Na+, K+, Ca2+, Mg2+, Fe2+, Fe3+, Mn2+, Cu2+, Co2+ and Mo2+.

Second, the organic molecules of cells

There are thousands of organic substances in cells, accounting for more than 90% of dry cell weight. They are mainly composed of carbon, hydrogen, oxygen, nitrogen and other elements. Organic compounds are mainly composed of protein, nucleic acids, lipids and sugars, accounting for more than 90% of dry cell weight.

(1) protein

In life activities, protein is an extremely important macromolecule, and almost all life activities are related to the existence of protein. Protein is not only the main structural component of cells, but more importantly, the bio-specific catalyst-enzyme is protein, so the metabolic activity of cells cannot be separated from protein. A cell contains about 104 kinds of protein, and the number of molecules reaches 10 1 1.

(2) nucleic acid

Nucleic acid is the carrier molecule of biological genetic information, and all living things contain nucleic acid. Nucleic acid is a macromolecule formed by the polymerization of nucleotide monomers. Nucleic acid can be divided into RNA and deoxyribonucleic acid When the temperature rises to a certain height, double-stranded DNA dissociates into single-stranded DNA, which is called denaturation or melting. This temperature is called the melting temperature (Tm). The melting temperature of DNA with different base composition is different, and the Tm of G-C is higher for DNA with more (three hydrogen bonds). Those molecules with more A-T pairs (2 hydrogen bonds) have lower Tm. When the temperature drops below a certain temperature, the complementary single strand of denatured DNA can restore the double helix structure of DNA by forming hydrogen bonds between paired bases. This process is called renaturation or annealing.

There are three main conformations of DNA.

B-DNA: Watson &; In the right-handed helix model proposed by Click, there are 10 bases in each turn, the helix twist angle is 36 degrees, the pitch is 34A, the helix rise value of each base pair is 3.4A, and the base inclination angle is -2 degrees.

A-DNA: right-handed helix, with 10.9 bases per turn, helix twist angle of 33 degrees, pitch of 32A, helix rising value of each base pair of 2.9A, and base inclination angle of 13 degrees.

Z-DNA: left-handed helix, with 12 bases per revolution, helix twist angles of -5 1 degree (G-C) and -9 degrees (C-G), and pitch of 46A. The helix rising values of each base pair are 3.5a (G-C) and 4.1respectively.

(3) sugar

There are both monosaccharides and polysaccharides in cells. Monosaccharides in cells exist as energy sources and raw materials of sugar-related compounds. The important monosaccharides are pentose and hexose, in which ribose is the most important pentose and glucose is the most important hexose. Glucose is not only the key monosaccharide of energy metabolism, but also the main monomer of polysaccharide.

Polysaccharide plays an important role in cell structure. Polysaccharides in cells can be basically divided into two categories: one is nutritional reserve polysaccharide; The other is structural polysaccharide. There are two kinds of polysaccharides as food reserves, starch in plant cells and glycogen in animal cells. Structural polysaccharides in eukaryotic cells mainly include cellulose and chitin.

(4) Lipids

Lipids include fatty acids, neutral fats, steroids, waxes, glycerophosphates, sphingolipids, glycolipids, carotenoids, etc. Lipid compounds are insoluble in water but soluble in nonpolar organic solvents.

1, neutral fat

① Glyceryl ester: It is a triglyceride formed by the combination of carboxyl group of fatty acid and hydroxyl group of glycerol. Glyceryl ester is the main storage form of fat in animals and plants. When there are excess carbohydrates, protein or lipids in the body, they can be converted into glycerides and stored. Glyceryl ester is an energy substance, which releases twice as much energy as sugar or protein when it is oxidized. When nutrition is deficient, glycerides should be used to provide energy.

(2) Wax: fatty acids are esterified with ethanol to form wax (such as beeswax). Wax has a long hydrocarbon chain and a higher melting point than glyceride. Cells do not contain wax, but some cells can secrete wax. Such as: wax film secreted by plant epidermal cells; The wax glands of Homoptera insects, such as the cerumen glands of the external auditory canal of higher animals.

2. Phospholipids

Phospholipids are very important to the structure and metabolism of cells. They are the basic components of biofilm and participants in many metabolic pathways. It can be divided into glycerophosphate and sphingomyelin.

3. Sugar and fat

Glycolipid is also a component of cell membrane, which is related to cell recognition and surface antigenicity.

4. Terpenoids and steroids

These two compounds are derivatives of isopentene and contain no fatty acids.

The main terpenoids in biology are carotene and vitamins A, E, K, etc. There is also a polyterpene alcohol phosphate, which is the carrier of glycosyltransferase in cytoplasm.

Steroids are also called steroids, in which cholesterol is a component of the membrane. Other steroids are hormones, such as estrogen, androgen and adrenal hormone.

Three. Enzymes and biocatalysts

(1) enzyme

Enzymes are protein catalysts, whose main function is to reduce the activation energy of chemical reactions and increase the probability of reactant molecules crossing the activation energy barrier to complete the reaction. The action mechanism of enzyme is that enzyme and substrate combine temporarily in the reaction to form enzyme-substrate activation complex. This kind of complex has a low demand for activation energy, so the number of complex molecules crossing the activation energy barrier per unit time is more than that of simple molecules. After the reaction is completed, the enzyme molecules are released from the enzyme-substrate complex immediately.

The main characteristics of enzyme are: high catalytic ability, high specificity and adjustability; Appropriate pH value and temperature are needed; Only the reaction allowed by thermodynamics can be catalyzed, and both positive and negative reactions can be catalyzed, which can accelerate the reaction to reach equilibrium in essence.

Some enzymes need non-protein cofactors to be active. Cofactors can be complex organic molecules, metal ions, or both. The complete protein-cofactor complex is called holoenzyme. The cofactor is removed from the whole enzyme, and the remaining protein part is called apoenzyme protein.

(2) RNA catalyst

T cech 1982 found that the precursor of tetrahymena can self-process to produce mature rRNA products without any protein. This processing method is called self-splicing. Later, it was found that this cleaved RNA intron sequence had catalytic activity similar to that of an enzyme. This RNA sequence is about 400 nucleotides long and can be folded into a complex surface structure. It can also be combined with another RNA molecule and cut at a certain site, so this catalytic RNA sequence is called ribozyme. Later, it was found that RNA with catalytic activity existed not only in Tetrahymena, but also in prokaryotes and eukaryotes. A typical example is ribosomal peptidyl transferase. In the past, it was always thought that protein in ribosomes was the catalyst for peptide chain synthesis, but in fact, RNA, not protein, was the component with peptidyl transferase activity and catalytic peptide bond formation, and protein in ribosomes only played the role of scaffold.