Biotech Basics: The Cell Membrane

As we mentioned in an earlier blog post, a cell is essentially a drop of a thick, watery substance (cytoplasm) surrounded by a layer of fats (cell membrane).  The cell membrane separates the inside of the cell from the outside environment, creating a compartment suitable for the chemistry of life.  However, most cells need to bring water, ions, and other important molecules into the cell, and to move waste out of the cell.  So, how does the cell membrane work?  Let’s explore this structure in depth.

What makes up the cell membrane?

The main part of the cell membrane is the phospholipid.  We can imagine that a phospholipid looks like a lollipop with two sticks.  The lollipop ‘head’ is a phosphate group, which carries a negative charge.  This charged part of the molecule is attracted to water, or hydrophilic.  The lollipop ‘sticks’ are composed of fatty acid lipid chains linked to the phosphate head.  Lipid chains are hydrophobic, meaning that they repel water to associate with other similar molecules.

When we add phospholipids to water, something interesting happens based on the chemical properties of the molecule.  Individual molecules self-arrange into a continuous sheet with the water-loving hydrophilic heads in contact with water, and the hydrophilic tails facing in towards one another and away from the aqueous surroundings.  The sheet bends onto itself to form a sphere with a tiny watery pocket in the center called a vesicle.  The environment within the vesicle is separate from the environment outside the vesicle, concentrating nutrients and important molecules within the cell and removing waste to the outside the cell.

What is the fluid mosaic model of a cell membrane?

After many studies and observations, scientists identified different proteins and molecules found within and associated with the cell membrane.  These molecules are important for a cell’s normal function. Their observations led them to propose the fluid mosaic model.

There are other molecules in the membrane that contribute to its function.

• Cholesterol – you may think of cholesterol as being something to avoid in your diet, but it is an important part of the cell membrane! This steroid molecule interacts with the hydrophobic tails of the phospholipids to keep the membrane fluid at different temperatures.
• Integral Proteins – the cell membrane is embedded with proteins that pass through the membrane and move ions and other molecules into and out of the cell.
• Peripheral protein – these proteins attach to the cell membrane or to integral proteins. These proteins perform a wide variety of functions, from enzymatic activity, to cell signaling and electron transport.
• Glycoproteins and glycolipids – these proteins or lipids are bonded to short, branched carbohydrates present on the extracellular side of the cell membrane. The carbohydrates serve as markers to help cells recognize one another, as glue to help stick together, and also as targets by pathogens to infect cells.

How do molecules travel across the cell membrane?

The plasma membrane is semi-permeable, meaning that the cell limits which ions and molecules pass into and out of the cell.  There are three main ways that move molecules across the cell membrane – passive transport, active transport, and bulk transport.

Passive transport occurs when molecules pass through the cell membrane without the cell using energy to enable the process.

• Diffusion – movement of molecules from high to low concentration.
• Osmosis – movement of water molecules across a membrane.
• Facilitated transport – molecules pass through membrane protein channels that only transport specific molecules.

Active transport uses cellular energy to move molecules and ions across the cell membrane.

• Ion pumps – membrane protein channels that use energy to move molecules across a membrane regardless of the concentration.
• Cotransport — membrane protein channels that use the favorable movement of a molecule across a membrane to drive the movement of a second molecule across the membrane.

Bulk transport uses cellular energy to move large molecules or large volumes of ions across the cellular membrane

• Endocytosis – a cell creates a vesicle around the extracellular environment to bring food, ions, or other biological molecules into the cell.
• Exocytosis – a cell creates a vesicle within the cell that is used to transport a molecule to the cell membrane, where it is then moved outside the cell.

We offer several experiments where you can explore the cell membrane in your classroom laboratory:

• Cell Membranes Model – Teach your students about structure and function of cell membranes by making a working paper model! A great way to show cell membrane structure covering phospholipids, cholesterol, glycolipids, glycoproteins and membrane proteins.
• Principles and Practice of Diffusion and Osmosis – Materials and dialysis tubing are provided to perform procedures involving diffusion, dialysis, water potential and osmotic potential.
• Osmosis and Diffusion Model – This easy-to-make model gives your students a new way to visualize what happens during diffusion and osmosis. The model lets students explore how molecules behave during diffusion and osmosis in a simple way.
• Investigation 4: Diffusion and Osmosis – In this experiment, students use artificial cells to study the relationship of surface area and volume. Then they will create models of living cells to explore osmosis and diffusion, and observe osmosis in living cells.