In this lesson, we’ll discuss what a cell membrane is and its location. We’ll also examine the cell membrane’s functions, its components and structure, and its characteristics.
What Is a Cell Membrane?
Picture a night of camping in the woods. You start a fire and then pitch your tent by sliding the tent poles into its fabric and anchoring the tent to the ground with small spikes. The tent’s air vents keep you cool but keep the bugs out.
Your tent resembles a cell membrane, a thin, flexible barrier that covers a cell.A cell membrane serves three important functions for a cell:
- Protecting the cell
- Regulating the transportation of molecules and ions
- Communicating with other cells
A cell membrane surrounds a cell like a tent, shutting out the environment. This protects a cell from its surroundings. The cell membrane also regulates cellular transport. The membrane can do so because it is selectively permeable.
That means it only allows certain molecules to enter and exit a cell, the same way a tent can keep out bugs but let in breezes.Cell membranes help cells communicate with other cells through intercellular communication, accomplished through markers, receptors, or activators. Markers identify the cell’s type for other cells, receptors are protein molecules that receive signals from other cells and bind certain substances, and activators send signals to other cells.
Components and Structure
A cell membrane is composed mainly of fats called phospholipids. The head of a phospholipid binds to water molecules while its tail repels water molecules. Both a cell’s extracellular (exterior) and intracellular (interior) environments contain water. So, when phospholipids are immersed in water, they form a bilayer, or a structure made of two molecular layers in which the phospholipid’s heads face both the cell’s extracellular and intracellular environments.
The cytoplasm occupying a cell’s intracellular environment is 65-95% water. The hydrophobic tails lie in the bilayer’s interior and are thus shielded from water.The bilayer serves as a great protective barrier due to its chemical properties. Because the bilayer’s phospholipids are packed tightly together, only tiny molecules or ions may move without any problems between the inside and outside of the cell.
These molecules or ions must be slightly hydrophobic (or water repellent) to move through the cell membrane.Proteins, or macromolecules composed of amino acids, also comprise a large part of a cell membrane’s components. Channel or transport proteins regulate the movement of molecules and ions, including nutrients, in and out of a cell. Surface proteins float laterally like boats on water and may serve as markers that enable cells to recognize each other or as receptors of signals from other cells or molecules.Also playing important roles in the cell membrane are carbohydrates, or sugars made of carbon, hydrogen, and oxygen.
Sugars on a cell membrane’s outer surface attached to phospholipids or proteins enable cells to engage in intercellular communication. These markers act as name tags that announce to other cells that they’re part of the same organism and what job they do. For example, the blood types A, B, AB, and O are a consequence of sugars on the cell membrane.
Each blood type has a different sugar, which is what makes the blood types unique.White blood cells roll around the blood and lymphatic vessels like they’re on patrol, looking out for infectious agents (bacteria and viruses). These cells use proteins with sugars, or lectins, attached on their cell membranes to stick to the vessels as they roll.
The cell membrane’s last component is cholesterol. This small hydrophobic molecule sits in between the phospholipid tails to help the membrane remain flexible, yet stable. The cholesterol is also involved in signaling between cells.These components come together to create a flexible membrane. When the cell moves, the membrane moves. This characteristic is especially important for our white blood cells since they protect us from pathogens that enter our bodies.
They must extend their cell membranes to engulf and then devour the pathogens.
The Fluid Mosaic Model
With all these different components, it can be hard to visualize how components of a cell fit together. To help meet this challenge, scientists have developed the fluid mosaic model, a picture scientists use to represent the structure of a cell membrane. The fluid mosaic model shows how the phospholipids, proteins, carbohydrates, and cholesterol join together to create a cell membrane. A mosaic is a type of art in which small pieces of glass are arranged to make a larger picture.
The same is true for a cell membrane. Although the individual components are small, they form a remarkable barrier when put together.
A cell membrane is a fluid, flexible barrier on the outside of a cell that protects it. A cell membrane’s phospholipids and proteins arranged in a bilayer give it semi-permeability, allowing some molecules into the cell but not all. Cholesterol keeps the membrane flexible and stable.
The cell membrane is also responsible for the regulation of the transportation of substances and for intercellular communication, which proteins, carbohydrates, and cholesterol mediate. Combined, these components create a structure scientists refer to as the fluid mosaic model.