Microorganisms have no special organs to absorb nutrients, and their absorption of nutrients depends on the whole cell surface. At present, it is believed that the absorption of various nutrients depends on the action of plasma membrane, which has many pores, and various nutrients penetrate the cell membrane through different absorption methods. Whether nutrients can enter cells depends on three factors:

① Properties of nutrients (relative molecular weight, mass, solubility, electronegativity, etc.). ); ② Environment (temperature, PH value)

Wait a minute. ); ③ Permeability barrier of microbial cells (plasma membrane, cell wall, envelope, etc.). ). According to the characteristics of material transport process, the transport modes of materials can be divided into free diffusion, facilitated diffusion, active transport and group transfer.

First, free diffusion Free diffusion is also called simple diffusion. Protoplasma membrane is a semi-permeable membrane, and nutrients diffuse from high concentration extracellular environment to low concentration intracellular through small holes in plasma membrane. Free diffusion is non-specific, but the size and shape of water-containing pores in plasma membrane are selective to the nutrient molecules involved in diffusion. It has the following characteristics: ① there is no reaction during diffusion; ② No energy consumption; Can't transport against concentration; ③

The transmission rate is directly proportional to the concentration difference of substances inside and outside the membrane. Free diffusion is not the main way for microbial cells to absorb nutrients. Water is the only molecule that can freely cross the plasma membrane by diffusion. Fatty acids, ethanol, glycerol, some gases (

O2, CO2) and some amino acids can also freely diffuse into and out of cells to some extent. 2. Like free diffusion, facilitated diffusion is also a passive transport mode of substances across the membrane, in which ① no energy is consumed, ②.

The molecular structure of substances involved in transport does not change, ③ it cannot be transported against the concentration, and ④ the transport rate is directly proportional to the concentration difference of substances inside and outside the membrane. ⑤

Operators need to participate. Nutrients entering cells through facilitation diffusion mainly include amino acids, monosaccharides, vitamins and inorganic salts. Generally speaking, microorganisms transport corresponding substances through specific carrier proteins, but some microorganisms transport the same substance through more than one carrier protein. Third, active transportation Active transportation is a major mode of material transportation widely existing in microorganisms. Compared with the above two transportation modes, one of its important features is that materials need to consume energy in the transportation process and can be transported reversely and centrally. In the process of active transportation, different microorganisms need different energy to transport substances. Aerobic microorganisms and facultative anaerobic microorganisms directly use respiratory energy, anaerobic microorganisms use chemical energy, and photosynthetic microorganisms use light energy. The similarity between active transport and facilitated diffusion is that carrier protein is also needed in the process of material transport. The carrier protein develops its affinity with the transported substance through conformational changes, so that the two can combine and separate reversibly, thus completing the transmembrane transport of the corresponding substance. The difference is that the conformational change of carrier protein during active transport requires energy. 4. Group displacement is another active transportation mode. Different from the active transportation mode, it has a complex transportation system to complete the transportation of substances, and substances change chemically during transportation. Group transfer mainly exists in anaerobic and facultative anaerobic cells, and is mainly used to transport sugar. Fatty acids, nucleosides and bases can also be transported in this way. In the process of studying the absorption of glucose and lactose by Escherichia coli, it was found that these sugars existed in the cytoplasm in the form of phosphate sugars after entering cells, indicating that these sugars were phosphorylated during transportation, and the phosphate groups came from phosphoenolpyruvate (

PEP), so this group of translocation is also called phosphoenolpyruvate-phosphoglycosyltransferase transport system (PTS). PTS usually consists of five kinds of protein, including enzyme I and enzyme II.

And thermally stable protein (HPr) with low relative molecular weight. In the process of sugar transportation, the phosphate group on PEP is gradually phosphorylated and dephosphorylated by enzyme I and HPr, and finally in enzyme II.

Under the action of sugar, phosphate sugar is produced in cytoplasm. PEP-P+HPr→HPr-P+ Enzyme I → Enzyme I+ Pyruvate I-P+HPr → Enzyme III+ Enzyme IHPr-P+ Enzyme III → Enzyme III-P+HPr Sugar+Enzyme III-P → Sugar -P+ Enzyme III.