The cell membrane is a complex barrier of phospholipids, cholesterol, proteins, and carbohydrates that separates the cell from its external environment. This selectively permeable membrane regulates the movement of ions, organic molecules, and other compounds in and out of the cell. The plasma membrane is also important to cellular function by providing structure and assisting in cell communication.
The permeability of the cell membrane depends on the characteristics of the lipid bilayer and the characteristics of the molecules moving across the membrane. There are many ways in which substances can enter and exit the cell but they are typically divided into two categories, active and passive transport.
Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion.
Learn more about passive transport by watching this Kahn Academy video: Passive and Selective Transport. Be sure to make note of what molecules can move in and out of the cellular membrane without help.
Here’s a Khan Academy video on passive transport and selective permeability.
Diffusion is the net movement of a substance, liquid or gas, from an area of high concentration of that substance to an area of lower concentration of the same substance. Nonpolar molecules like carbon dioxide and oxygen are hydrophobic and can cross the cell membrane rapidly by diffusion as they dissolve in the lipid bilayer. Polar molecules such as glucose pass though the membrane more slowly.
Osmosis is the diffusion of water across a selectively permeable membrane.
Just as in the diffusion of gases, the net movement of water during osmosis is from the less concentrated or hypotonic solution to the more concentrated or hypertonic solution. When the solutions are isotonic, they have equal concentrations of substances. Although there will still be equal amounts of water movement in and out of the cell, the net flow is zero. Osmosis is essential in biological systems and provides the primary means by which water is transported into and out of cells. In fact, many animals have special adaptations for osmoregulation, the balance of water in an organism.
Animal cells thrive in an isotonic solution where water can enter and leave the cell at a constant rate. When placed in a hypertonic solution (more solute less water) it forces water to leave the cell causing it to shrink. When placed in a hypotonic solution (less solute and more water) water will enter the cells (trying to balance the water between the cell and the environment). If too much water enters the cell it will burst like a balloon.
The plant cell thrives in a hypotonic solution. This causes water to osmosis into the cell, pumping it up (like a balloon). The cell wall acts like a carrier preventing the cell from bursting. In this state plant cells are called turgid which allows plants to stand up tall. In an isotonic solution, water will enter and leave the cell at a constant rate. Plants cells are not ‘pumped’ up enough and so they start to go limp or flaccid . In a hypertonic solution water leaves the plant cell. This causes plasmolysis, the shrinking of the cytoplasm away from the cell wall, and the plant cell can die.
Facilitated diffusion is the movement of molecules across a membrane with the use of transport proteins. There are two types of transport proteins, channel proteins and carrier proteins. Learn more about facilitated diffusion transport by watching this Kahn Academy video Passive and Selective Transport. Be sure to make note of what molecules can move in and out of the cellular membrane without help.
Here’s a Khan Academy video on passive transport and facilitated diffusion.
In contrast to passive transport, active transport requires the cell to spend energy, usually in the form of ATP. Often times, cells must move molecules against the concentration gradient to maintain specific internal solute concentrations. Examples of active transport include ion pumps, co-transport, and bulk transport.