Lesson 2-4. Biology Grade 10-11.
Lecture 1. Cell Theory
A cell is an elementary membrane system capable of self-regulation, self-preservation, and self-reproduction.
The structure and functions of the cell, as well as the universality of all cellular forms, are reflected in the cell theory.
First principle of cell theory:
All organisms, except viruses, consist of cells and products of their vital activity. The products of vital activity include tissue fluid, lymph, blood, i.e., the internal environment of the organism.

Second principle of cell theory:
All cells have fundamental similarities in their structure and functions. For example, all cells have the plasma membrane apparatus (PMA), all eukaryotic cells have internal membrane organelles such as the Golgi complex, endoplasmic reticulum (ER), etc., all cells exhibit matrix processes, and all cells have an anaerobic stage of energy metabolism expressed by glucose breakdown or glycolysis. The commonality of structures and functions characteristic of cells indicates their homology, i.e., common origin.

Third principle of cell theory:
All existing cells originated from the division of pre-existing cells (this hypothesis was proposed by Rudolf Virchow).

Fourth principle of cell theory:
The activity of a multicellular organism is the sum of the activities of its constituent units, i.e., cells, taking into account the connection between cells.

Assignments:
Memorize the principles of the cell theory.

Lecture 2. Structure of Biomembranes
Biological membranes consist of protein, lipid, and carbohydrate components.

Protein and lipid components are abundant in the membrane and are called major components. The carbohydrate component is usually minor, except in plant cells. Membrane lipids are mainly phospholipids, to a lesser extent glycolipids and lipoproteins. Cholesterol and some fat-soluble vitamins are typically present. Lipids play an important role as they form the bilipid layer, which is the structural basis of all biological membranes. Membranes are characterized by lipid asymmetry, providing fundamental properties to all membranes. This asymmetry is achieved because lipids can move laterally within a monolayer and perform "flip-flop" transitions, i.e., move from one monolayer to another. The fatty acid radical length in membrane lipids is almost constant; membrane fluidity mainly depends on fatty acid saturation. Thanks to the "flip-flop" transition, the outer monolayer concentrates lipids with saturated fatty acid radicals, giving it particular rigidity, while the inner monolayer contains lipids with unsaturated radicals. The "flip-flop" transition requires high energy and is catalyzed by the enzyme flippase.

The bilipid layer performs the following functions:
Structural – this function is proven by the action of phospholipases. For example, phospholipase A cleaves lipid tails causing cell lysis; it is found in the venom of many snakes. Phospholipase C can cleave lipid heads, destroying the bilipid layer; it is present in cholera toxin.
Insulating – the bilipid layer only allows small uncharged molecules (alcohols, H2O, O2, CO2) to pass, as charged molecules get stuck at the head groups and hydrophobic molecules in the tail region. Therefore, the bilipid layer creates an electrochemical gradient of ions across the membrane. The difference in gradients is called the membrane potential; thus, all cells have a resting potential where the outer membrane surface is partially positive and the inner partially negative. The membrane is polarized, and the membrane potential ensures normal pressure in the cell. Disruption leads to cell death due to unequal water flow.

Membrane lipids regulate membrane fluidity. In non-mammals, fluidity depends on fatty acid saturation; in mammals, on cholesterol. Many membrane proteins function only within a specific protein environment. Fluidity depends on external factors such as temperature and pressure: increased pressure and decreased temperature make membranes more rigid, while decreased pressure and increased temperature increase fluidity. Fluidity affects almost all cell processes, including substance transport, metabolite transport, membrane potential maintenance, and osmotic pressure. Disruption in nerve cells can cause irreversible damage by preventing impulse transmission.

Changing membrane fluidity can be used medically. Local anesthetics like novocaine, lidocaine, and anesthesin are large hydrophobic molecules that integrate into the bilipid layer like cholesterol, reducing fluidity and nerve impulse conduction, producing anesthesia. General anesthetics (chloroethane) increase membrane fluidity, also blocking nerve impulses. Their effect is reversed faster under high pressure (used in hyperbaric chambers to wake patients from anesthesia). Inert gases such as helium integrate easily into membranes. Divers at great depth experience high pressure, reducing membrane fluidity; inert gases can be added to breathing mixtures to prevent this. Rapid ascent causes inert gases to enter blood and tissues intensely, causing decompression sickness with muscle and joint pain and nervous system disorders

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