Cell membrane wraps cells, which keeps them relatively stable and normal life activities. In addition, the absorption of nutrients and the excretion of metabolites necessary for cells must pass through the cell membrane. Therefore, the characteristic that the cell membrane selectively allows some molecules to enter or leave the cell is called selective permeability, which is the most basic function of the cell membrane. If cells lose this function, they will die.
Cell membrane not only regulates and controls the exchange of substances inside and outside the cell through selective osmosis, but also helps the cell to ingest droplets and capture food particles from the external environment through "drinking water" and "exocytosis" to supply the demand for nutrients for cell regeneration activities. Cell membrane can also be stimulated by external signals, making cells respond, thus regulating cell life activities. Cell membrane is not only the physical barrier of cells, but also an important structure with complex functions in cell life activities.
An important morphological feature obtained from the evolution of primitive life to cells is that a membrane structure, that is, cell membrane, appears outside biological substances. Cell membrane is located on the cell surface, usually 7 ~ 8 nm thick, and consists of lipids and protein. Its most important feature is semipermeability, or selective permeability, which has a strong selective permeability to substances entering and leaving cells. Cell membrane and intima system are called biomembrane, which have the same basic structural characteristics.
According to the physiological and biochemical characteristics of cells, it is speculated that the plasma membrane is a lipid barrier, a lipid bilayer and a three-splint structure composed of protein, phospholipid and protein. At the same time, electron microscope observation also proved that the parenchyma membrane is indeed a dark-bright-dark three-layer structure. Subsequently, the freeze etching technique showed that there were protein particles in the bilayer film. Immunofluorescence technique proved that protein in plasma membrane was mobile. Accordingly, S.J.Singer et al. proposed a fluid mosaic model of biofilm in 1972, as shown in Figures 7-4-3 and 7-4-4. Its structural characteristics are as follows: the skeleton of biofilm is a phospholipid bilayer, in which protein molecules are embedded in different ways, and sugar molecules form glycolipids and glycoproteins on the surface of cell membrane; The uneven distribution of lipid and protein on the inner and outer surfaces of the biofilm reflects the different functions on both sides of the membrane. The lipid bilayer has fluidity, and its lipid molecules can move freely, and protein molecules can also move laterally in the lipid bilayer.
The basic structure of cell membrane: (1) lipid bilayer: phospholipid, cholesterol and glycolipid. There are about 109 lipid molecules on each animal membrane, that is, there are about 5x 106 lipid molecules on each square micron membrane. (2) Membrane proteins are divided into endogenous proteins and exogenous proteins. Intrinsic protein directly binds to the hydrophobic part of phospholipid through hydrophobic part, with polarity at both ends, which runs through the inside and outside of the membrane; The external protein is bound to the external end of the internal protein through a non-valence bond, or to the hydrophilic head of the phospholipid molecule. Such as vectors, specific receptors, enzymes and surface antigens. (3) Membrane sugar and sugar coating: glycoprotein and glycolipid.
Organelle is divided into: mitochondria; Chloroplast; Endoplasmic reticulum; Golgi apparatus; Ribosome; Lysosome; Vacuole; Centrosome.
Mitochondria are the main places for cells to carry out aerobic respiration. Also known as "power workshop".
Chloroplast is the place where green plants carry out photosynthesis.
Endoplasmic reticulum is the place where protein is synthesized and processed.
Golgi apparatus is the place where protein from endoplasmic reticulum is processed, classified and packaged.
Ribosomes are places where protein is produced.
Lysosomes decompose aging and damaged organelles, engulf and kill invading viruses or bacteria.
Vacuoles regulate the intracellular environment and keep plant cells strong. Contains pigment.
Centrosomes are related to mitosis of lower plant cells and animal cells. It consists of two centrioles perpendicular to each other.
Endoplasmic reticulum (ER)
Generally, eukaryotic cells have endoplasmic reticulum, and only a few highly differentiated eukaryotic cells, such as human red blood cells and prokaryotic cells, have no endoplasmic reticulum. Under the electron microscope, we can see that endoplasmic reticulum is a complex endometrial structure, which is a flat sac or tube surrounded by a single membrane. These cavities are connected to each other and to the nuclear envelope. According to its function, endoplasmic reticulum can be divided into smooth endoplasmic reticulum and rough endoplasmic reticulum. The particles attached to rough endoplasmic reticulum are ribosomes, which are the sites for protein synthesis. Therefore, the main function of rough endoplasmic reticulum is to synthesize secreted proteins, membrane proteins and proteins in endoplasmic reticulum and lysosomes. Glycosylation modification, folding and assembly of synthetic protein also occur in endoplasmic reticulum. The second is to participate in making more movies. There are no ribosomes on the smooth endoplasmic reticulum, but many active enzymes are embedded in the membrane. The main function of smooth endoplasmic reticulum is to synthesize lipids, including fats, phospholipids and sterols.
Ribosome (ribosome)
Ribosome is the site of protein synthesis, which is composed of rRNA and protein. Protein is on the surface and rRNA is inside. They are bound by valence bond. Ribosome is a collection of many enzymes, and it has several active centers that undertake the function of protein synthesis. Each active center consists of a special group of protein, and each enzyme or protein only has catalytic activity in the whole structure.
The number of ribosomes per cell can reach millions. Free ribosomes synthesizes protein retained in cytoplasm, such as structural proteins in membrane; Ribosomes attached to the endoplasmic reticulum synthesize and secrete extracellular protein, then transport to S-ER to form secretory vesicles, and then transport to Golgi apparatus, where they are processed and excreted.
Golgi apparatus
It consists of a series of flat sacs and vesicles, which secrete exuberant cells and are relatively developed. Golgi apparatus confirmed by electron microscope is a flat vesicle surrounded by a single membrane, and piles of vesicles are not interconnected like endoplasmic reticulum. A cell has only a few piles of Golgi apparatus, at most hundreds.
(1) is the final processing and packaging place of cell secretions, and secretory vesicles are discharged from cells through efflux.
(2) It can synthesize polysaccharides, such as mucus and various extracellular polysaccharides of plant cells.
lysosome
Lysosomes are vesicles produced by the rupture of Golgi apparatus and wrapped by a single membrane. The quantity can be more or less, with different sizes. They contain more than 60 kinds of acid enzymes, which can hydrolyze polysaccharides, phospholipids, nucleic acids and protein. Some of these enzymes are water-soluble, while others are bound to cell membranes. The pH value of lysosomes is about 5, which is the optimum pH value for enzymatic reaction. According to the different stages of lysosomes completing physiological functions, they can be roughly divided into primary lysosomes, secondary lysosomes and residues. Lysosomes have two functions: first, they fuse with food vacuoles to digest large particles such as food or pathogenic bacteria swallowed by cells into biological macromolecules, and the residues are excreted outside the cells; Second, in the process of cell differentiation, some aging organelles and biological macromolecules are trapped in lysosomes and digested, which is the need of self-reorganization.
Mitochondria (mitochondria)
Mitochondria have a double membrane structure, and the outer membrane is a smooth and continuous boundary membrane. The intima repeatedly extends and folds into the internal space, forming a ridge. The intima and adventitia are not connected, forming a membrane cavity. Under the light microscope, mitochondria are granular or short rod-shaped, with a transverse diameter of 0.2um~8um ~ 8 um, which is the size of bacteria. Mitochondria is an important part of ATP production in cells, and it is a power plant or energy converter in cells. Mitochondria are semi-autonomous, and there are circular DNA molecules and 70S ribosomes in the cavity, which can differentiate themselves, but some protein needs to be synthesized in the cytoplasm.
Chloroplast (chloroplast)
The chloroplast of higher plants is like a convex lens, with a double membrane structure, and there is no connection between the two membranes. Chloroplasts have complex membrane structures and are suspended in the matrix. These membranes, also called thylakoids, may have nothing to do with chloroplast intima. The thylakoid is also a double-layer membrane structure with a flat disc shape. Thylakoids are usually dozens of basal particles stacked together, and there are photosynthetic pigments and electron transfer systems on the thylakoid membrane.
Chloroplast is ubiquitous in green plants and algae, and it is the place of photosynthesis. At the same time, chloroplasts also have their own unique double-stranded circular DNA, ribosome and protein biosynthetic enzymes, which can synthesize their own necessary part of protein, so the hypothesis of the origin of * * * in chloroplasts has been recognized by many people.
microbody
Monolayer membrane vesicles containing enzymes are similar in function to lysosomes, but the enzymes contained are different from lysosomes. Microbodies help many substances to be transformed into other substances in a short time. Peroxisome is a kind of microorganism existing in animal and plant cells, and some enzymes contained in it can oxidize and decompose fatty acids to produce hydrogen peroxide.
Glyoxal cycle exists in lipid-rich plant cells, and some enzymes can convert fatty acid core oil into enzymes needed for early plant growth.
vacuole
In mature living plant cells, there is often a large central vacuole filled with liquid, which is formed by the fusion of small vacuoles during cell growth and development, and is a bubble filled with water and liquid surrounded by a single membrane. Vacuoles contain inorganic salts, amino acids, sugars, pigments and other metabolites, and even toxic compounds, which are in a hypertonic state, making cells in a state of imbibition and filling.
Cytoskeleton
In the cytoplasm of eukaryotic cells, there is a three-dimensional grid structure consisting of protein fibers-cytoskeleton. Protein fibers include microtubules, microfilaments and intermediate fibers, which have self-assembly and disassembly functions through phosphorylation and dephosphorylation, which is also a process of information transmission. Cytoplasmic organelles, enzymes and many protein are fixed on the cytoskeleton, which enables them to perform their respective functions in an orderly manner.
Cytoskeleton network system plays an important role in cell morphology construction, cell movement, material transport, energy conversion, information transmission, cell differentiation and cell transformation.
Microfilament (microfilament)
Microfilaments (actin fibers) refer to skeletal fibers composed of actin in eukaryotic cells. Functions of microfilament: muscle contraction, microvilli, strain fiber, cytoplasmic circulation and amoeba movement, cytoplasmic division ring.
microtubule
Microtubules are composed of two types of tubulin subunits α and β. These two proteins form tubulin dimer, which is the basic unit of microtubule assembly. Microtubule is a long tubular organelle structure composed of tubulin dimer, and its wall is composed of 13 fibrils. Microtubules can be assembled into single tube, double tube (in cilia and flagella) and triple tube (in centriole and matrix). Functions of microtubules: maintaining cell morphology, intracellular transport, flagella movement and ciliary movement, spindle and chromosome movement, grana and centriole.
Intermediate fiber (intermediate fiber)
After the synthesis of intermediate fibrin, it is basically assembled into intermediate fiber, and there are few free monomers. Under certain physiological conditions, there are similar intermediate fiber structures in plant cells. According to their tissue origin and immunogenicity, intermediate fibers can be divided into six categories: keratin fibers, wavy fibers, desmin fibers, nerve fibers, glial fibers and nuclear laminin. Intermediate fibers are closely related to microtubules, which may affect the assembly and stability of microtubules. In addition, the intermediate fiber extends from the nuclear fiber layer through the cytoplasm, which not only supports cell rigidity and coordinates the structure that produces movement, but more importantly, the intermediate fiber is related to important life activities such as cell differentiation, intracellular information transmission, nuclear gene transmission and nuclear gene expression.
Flagella, cilia and centriole (flagella, cilia, centriole)
An appendage on the surface of a cell whose function is movement. Flagella and cilia have the same basic structure, and the main difference lies in length and quantity. Flagella is long and few, cilia are short, and it often covers the whole cell surface. The basic structure of both is microtubules. The basal part is connected with the grana (9(3)+0) buried in cytoplasm. Centroid particles are similar to basal particles in structure, buried in the centrosome, and many microtubules are produced from the centrosome.
cytosol
The liquid part of cytoplasm except organelles. Rich in protein, accounting for 25 ~ 50% of cells; It contains a variety of enzymes and is a place for cell metabolic activities; There are also various cellular inclusions, such as glycogen, fat droplets of fat cells, pigment particles and so on.