Plasma Membrane, Diffusion, and Tonicity Review

Barriers function to prevent something from coming in or prevent something from going out that shouldn’t be.
The plasma membrane is the outermost component of the cell (when there isn’t a cell wall).
Key role: regulate what enters and exits the cell to maintain internal stability.
The barrier is created by the phospholipid bilayer.

A phospholipid is a lipid with two sections: a hydrophilic head (water-loving) and a hydrophobic tail (water-fearing).
In a bilayer, two rows of phospholipids orient themselves so that the hydrophobic tails face inward toward each other, and the hydrophilic heads face outward to the aqueous environments (outside and inside the cell).
The bilayer forms the core barrier of the membrane, governing what can pass through based on polarity and solubility.

Proteins are embedded among the phospholipids and act as channels or carriers to aid passage of specific substances across the membrane.
Some proteins are bound to carbohydrates; carbohydrates are typically on the exterior surface and contribute to specific functions.
Carbohydrates on the exterior tend to be involved in immune response and cell recognition.
Not everything can freely pass through the phospholipid layer; proteins supplement and regulate transport of diverse molecules.

The plasma membrane is not a rigid wall; it’s a fluid mosaic of lipids and proteins that can move laterally within the plane of the membrane.
Think of the membrane as a pool of floating components that can move, but remain in a general organized layer.
This fluidity provides both structure and dynamic movement; however, it has physical limits akin to a balloon—excessive pressure can cause failure.

The membrane is semipermeable/selectively permeable: it allows some substances to cross more easily than others.
Because of the hydrophobic interior, the membrane interacts well with nonpolar and lipid-soluble (lipophilic) molecules and is less permissive to polar or charged species.
Nonpolar molecules pass through the phospholipid bilayer more easily; polar molecules and ions often require protein channels or carriers.
Diffusion: spontaneous spreading of molecules through a liquid or gas, from regions of high concentration to regions of low concentration.
- Everyday examples: scent from a candle diffuses through a room; tea diffuses from a tea bag into hot water.
- Diffusion rate depends on several factors, including the concentration difference (gradient) and temperature; larger gradients speed diffusion, while smaller gradients slow it down.
Illustrative note: outside of the membrane are various solutes (e.g., sugars represented as blue hexagons in the illustration) and inside the cell there may be different concentrations; these differences drive diffusion across the membrane.
A concise way to describe diffusion across a membrane is using Fick’s first law, which provides a quantitative framework:
- J = -D \, \frac{dC}{dx}
- where J is the flux (amount crossing per unit area per unit time), D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Tonicity refers to the relative solute concentration of two solutions, typically described in relation to a cell.
In a solution with low solute concentration relative to the cell (hypotonic solution), water tends to move into the cell.
In a solution with higher solute concentration (hypertonic solution), water tends to move out of the cell.
An isotonic solution has equal solute concentrations across the membrane, resulting in no net water movement.
For context, a 0.9% solute solution is often described as isotonic to many cells; pure water is hypotonic relative to cells and can cause water influx.
Plant cells differ from animal cells in this regard due to the presence of a cell wall:
- The cell wall provides structural support and helps resist osmotic swelling, contributing to turgor pressure.
- This turgor pressure counteracts osmotic water movement and helps maintain plant cell shape and rigidity.

Red blood cells (RBCs) provide a common visualization for tonicity:
- In isotonic solutions (e.g., around 0.9% solute), there is no net water movement; the cell remains stable.
- In hypotonic solutions (lower solute concentration outside), water moves into RBCs, causing swelling; the plasma membrane and cytosolic components resist rupture up to a limit.
- In hypertonic solutions (higher solute outside), water exits the RBCs, causing shrinkage.
Plant cells versus animal cells:
- Plant cells have a cell wall that maintains structural integrity under osmotic pressure and supports the cell via turgor pressure when in hypotonic environments.
- The cell wall prevents the cell from bursting, allowing plant tissues to maintain rigidity in favorable environments.

The plasma membrane’s selective permeability is fundamental to homeostasis in all cells and organisms.
Transport across membranes underlies nutrient uptake, waste removal, nerve impulse transmission, and many signaling pathways.
The fluid mosaic model explains the dynamic nature of membranes, including the mobility of proteins and lipids that enables rapid responses to environmental changes.
Understanding tonicity and osmosis is essential in medical contexts (e.g., IV solutions), plant physiology (water management, wilting), and cellular biology.

Diffusion is a passive process; it does not require energy input from the cell.
The presence and arrangement of proteins in the membrane enable selective transport for molecules that cannot diffuse directly through the lipid bilayer.
Temperature and concentration gradients are key determinants of diffusion speed; higher temperatures increase molecular motion and diffusion rates.
The membrane’s structure supports a balance between stability and flexibility, enabling cells to adapt to varying external conditions without losing essential internal conditions.