Passive transport and osmosis

Mechanisms of passive transport

The main feature of passive transport is that the cell does not expend energy to move molecules. Substances always move from an area of high concentration to an area of low concentration (along the concentration gradient).

There are two main pathways of passive transport:

1. Simple diffusion. Substances pass directly through the lipid bilayer of the cell membrane. Gases — oxygen ($O_2$) and carbon dioxide ($CO_2$) — are transported in this way.
2. Transport through membrane channels. This is carried out using special channel-forming proteins. This pathway is necessary for the movement of ions, such as sodium ($Na^+$) and potassium ($K^+$).

Osmosis and osmoregulation

Osmosis is a specific type of diffusion in which exclusively water molecules move through a semipermeable membrane. The direction of movement is strictly determined: water tends to go where the concentration of dissolved substances (salts) is higher, leaving the area with their lower concentration.

As a result of water transfer, osmotic pressure begins to act on the membrane, which is created due to the presence of solutions inside the cell. Living organisms have the ability to osmoregulate — altering the concentration of substances to control pressure:

• In protozoa (e.g., amoebas), excess fluid is eliminated using contractile vacuoles.
• In the human body, osmoregulation is provided by a complex excretory system.

Cell turgor pressure

Turgor is the tense state of the cell wall that occurs under the influence of osmotic pressure.

The degree of membrane tension depends on three key factors:

• Internal osmotic pressure (it is what "inflates" the cell from the inside).
• External osmotic pressure of the environment.
• Physical elasticity of the cell wall itself.

Upon water loss, turgor decreases. This process invariably accompanies phenomena such as dehydration, wilting, aging, and autolysis (intracellular breakdown).

Types of solutions and their effect on the cell

Depending on the magnitude of osmotic pressure relative to the intracellular medium, solutions are divided into three groups:

• Isotonic solutions. Their osmotic pressure is equal to the pressure inside the cell. The volume of cells placed in them does not change. For humans, a 0.9% NaCl solution is physiological (isotonic). In medicine, it serves as the basis for intravenous drug administration, is used to replenish blood volume in blood loss, and to treat severe dehydration.
• Hypertonic solutions. The pressure in them is higher than in the cell. Water rushes into the external environment. In plants, this causes plasmolysis (the cytoplasm shrinks and pulls away from the wall), and human erythrocytes wrinkle. When a plant cell is placed back into water, the reverse process — deplasmolysis — is possible, but prolonged plasmolysis leads to death. In surgery, gauze bandages with a hypertonic solution are used to cleanse purulent wounds (“drawing out” pus and fluid).
• Hypotonic solutions. The pressure is lower than the intracellular pressure. Water penetrates inside the cell. Plant cells swell (turgor increases), and erythrocytes swell to such an extent that their membrane cannot withstand it and ruptures — hemolysis occurs. This is precisely why intravenous administration of such fluids is fatal.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Structure and functions of the membrane lipid bilayer
• Specificity of channel-forming proteins for ions
• The process of deplasmolysis and plant cell survival mechanisms
• The role of contractile vacuoles in protozoa
• Physiology of excretory organs in human osmoregulation

Other topics in the section "The Cell — The Elementary Unit of Life"

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