Transport Across a Cell Membrane
The cell membrane is one of the great multi-taskers of biology. It provides structure for the cell, protects cytosolic contents from the environment, and allows cells to act as specialized units. A membrane is the cell’s interface with the rest of the world - it’s gatekeeper, if you will. This phospholipid bilayer determines what molecules can move into or out of the cell, and so is in large part responsible for maintaining the delicate homeostasis of each cell.
Semi-Permeability
Some cells function best at a pH of 5, while others are better at pH 7. The steroid hormone aldosterone is made in the adrenal gland, but affects mostly the kidney. Sodium is more than ten times more concentrated outside of cells rather than inside. If our cells couldn’t control what crossed their membranes, either no molecules would make it across, or they’d be traveling willy-nilly and the internal environment would always be in flux. It’d be like taking every item on a menu and blending it together before serving (not the tastiest idea).
So how do cells maintain different concentrations of proteins and molecules despite the pressures on them to be homogenous? Cell membranes are semipermeable, meaning they have control over what molecules can or cannot pass through. Some molecules can just drift in and out, others require special structures to get in and out of a cell, while some molecules even need an energy boost to get across a cell membrane. Each cell’s membrane contains the right mix of these structures to help that cell keep its internal environment just right.
Cartoon representing the cell membrane as a shield preventing things from entering the cell which is represented as a castle.
Cartoon representing the cell membrane as a shield preventing things from entering the cell which is represented as a castle.
Movement Across a Membrane and Energy
There are two major ways that molecules can be moved across a membrane, and the distinction has to do with whether or not cell energy is used. Passive mechanisms like diffusion use no energy, while active transport requires energy to get done.
Cartoon representing passive transport as rolling a boulder down a hill and active transport as rolling a boulder up a hill.
Cartoon representing passive transport as rolling a boulder down a hill and active transport as rolling a boulder up a hill.
Diffusion: the Simple and the Facilitated
Diffusion is the movement of particles down their gradient. A gradient is any imbalance in concentration, and moving down a gradient just means that the particle is trying to be evenly distributed everywhere, like dropping food coloring in water. This is what happened when we made our granola - a bunch of separate ingredients came together and spread out across the whole mixture. We call this evening-out moving “downhill”, and it doesn’t require energy. The molecule most likely to be involved in simple diffusion is water - it can easily pass through cell membranes. When water undergoes simple diffusion, it is known as osmosis.
Image showing purple ink diffuse from a tiny drop into a beaker of water.
Image showing purple ink diffuse from a tiny drop into a beaker of water.
Â
