Summary
Highlights
This second part of the mini-review focuses on how substances enter and exit cells. It will cover the cell membrane, aqueous solutions, and two key processes: diffusion and osmosis. The cell membrane acts as a border, separating the internal and external environments of the cell, controlling what enters and exits. It exhibits selective permeability, meaning not all particles can pass through it.
The human body is largely composed of water, around 60% of its mass. Water is essential as a solvent for all solutions in our body, inside and outside cells, making them aqueous solutions. An aqueous solution is a homogeneous mixture where water is the solvent, and it contains one or more solutes.
For cells and the organism to function correctly, concentrations of substances must be balanced between the inside and outside of the cell. Any concentration change triggers cellular exchanges to restore equilibrium. Two main processes facilitate these exchanges: diffusion and osmosis.
Diffusion occurs when the cell membrane is permeable to a solute. In diffusion, the solute particles move from an area of higher concentration to an area of lower concentration until equilibrium is reached. In the human body, diffusion enables gas exchanges, such as oxygen entering cells for cellular respiration and carbon dioxide exiting cells to be expelled during exhalation.
Osmosis occurs when solute particles cannot cross the cell membrane, or when the membrane is impermeable to the solute. In this process, the solvent (water in our body) moves from an area of lower solute concentration to an area of higher solute concentration to balance the concentrations. For instance, in plants, osmosis allows roots to absorb water from the soil because nutrient concentrations are higher inside the root cells, drawing water in. Plants maintain this concentration difference by losing water through their leaves via transpiration.
This mini-review covered cellular exchanges. We established what cellular inputs and outputs are, referring to cellular respiration and photosynthesis. In this second part, we explored the cell membrane's role in facilitating exchanges and detailed the two crucial processes: diffusion and osmosis, which enable substances to enter and exit the cell.