Passive Transport

Passive Transport Section 1

Cells live in a liquid environment where the exchange of substances between the cell and its surroundings must pass through the cell membrane. We know that there is a significant difference in the substance content inside and outside the cell, which relates to the function of the cell membrane. What are the different characteristics of transmembrane transport for different substances?

Water Movement Principle

When a drop of red ink is dropped into a glass of water, the water quickly turns red, a result of solute molecules diffusing in water. The rise of the liquid level in the funnel shown in the "Problem Exploration" is the result of solvent—water molecules from the beaker—diffusing into the funnel through a semi-permeable membrane. The diffusion of water molecules (or other solvent molecules) through a semi-permeable membrane is called osmosis. If there is a concentration difference on both sides of the semi-permeable membrane, water molecules will osmotically move from the side with higher relative water content to the side with lower relative water content.

Is the movement of water molecules through the cell membrane in and out of cells based on the same principle? Does the cell membrane act as a semi-permeable membrane?

Effects of Placing Mammalian Red Blood Cells in Different Concentrations of Sodium Chloride Solutions

After a period of time, red blood cells will undergo the following changes. The principles governing water movement in and out of other animal cells are the same as those governing red blood cells, both relying on osmosis.

Water Movement in Plant Cells

We know that plant cells have a distinct structure compared to animal cells. Plant cells have a cell wall outside the cell membrane. Research shows that for water molecules, the cell wall is fully permeable, allowing water molecules to freely pass through. The function of the cell wall is mainly to protect and support the cell, with less elasticity. In mature plant cells, the central vacuole occupies most of the cell space (Figure 4-2), squeezing the cytoplasm into a thin layer. Thus, the liquid environment within the cell mainly refers to the cell sap inside the vacuole. The cell membrane, vacuole membrane, and the cytoplasm between these two layers are collectively called the protoplast. When discussing water movement into and out of cells later on, it mainly refers to water passing through the protoplast into and out of the vacuole.

From the above exploration activities, it can be seen that the protoplast of plant cells acts like a semi-permeable membrane, and plant cells also undergo absorption and loss of water through osmosis. When the concentration of cell sap is lower than that of the external solution, water in the cell sap passes through the protoplast into the external solution, causing both the cell wall and protoplast to shrink to some extent. As the cell continues to lose water, due to the greater elasticity of the protoplast compared to the cell wall, the protoplast gradually separates from the cell wall, leading to what is known as plasmolysis. When the concentration of cell sap is higher than that of the external solution, water from the external solution passes through the protoplast into the cell sap, gradually restoring the entire protoplast to its original state and causing the gradual recovery of plasmolysis in plant cells.

Substances like water molecules enter and exit cells through diffusion, without consuming energy released by chemical reactions inside the cell. This type of transmembrane transport of substances is called passive transport. Passive transport is further divided into free diffusion and facilitated diffusion.

Free Diffusion and Facilitated Diffusion

Some small molecule substances can easily pass through the phospholipid bilayer of the cell membrane, such as oxygen and carbon dioxide. When the concentration of oxygen in the alveoli is higher than that inside the alveolar cells due to respiratory action, oxygen diffuses into the alveolar cells. When the concentration of carbon dioxide inside the cell increases due to respiratory action, carbon dioxide is expelled from the cell through diffusion into the extracellular fluid. Lipid-soluble small molecule organic substances such as glycerol, ethanol, and benzene also pass easily through simple diffusion into and out of cells.

Ions and some small molecule organic substances like glucose and amino acids cannot freely pass through the cell membrane. Special proteins embedded in the membrane assist these substances in crossing the membrane along the concentration gradient; these proteins are called transport proteins. This diffusion of substances into and out of cells facilitated by transport proteins is called facilitated diffusion (Figure 4-4), also known as facilitated diffusion.

Transport proteins can be divided into carrier proteins and channel proteins. Carrier proteins only allow molecules or ions that fit their binding sites to pass through, and their conformation changes with each transport event; channel proteins only allow molecules or ions of appropriate size and charge to pass through their channel, without requiring direct binding to the protein (Figure 4-5).

In the past, it was generally believed that water molecules entered and exited cells through free diffusion, but later research showed that water molecules predominantly use aquaporins on the cell membrane to facilitate diffusion into and out of cells.

Since both free diffusion and facilitated diffusion transport substances across the membrane along the concentration gradient without consuming energy produced by chemical reactions inside the cell, the size of the concentration gradient inside and outside the membrane directly affects the rate of substance transport. However, facilitated diffusion requires transport proteins, so the rate of transport for certain substances also depends on the quantity of transport proteins.