Cell membranes are found in ALL types of cells! This is a crucial part of a cell because of the important role it plays. The cell membrane is used to regulate material going in and out of the cells. It also acts as the boundary that sets the inside of the cells apart from the exterior environment. This boundary is semi permeable, meaning the barrier allows for some molecules to pass through but not others.
This semi permeable boundary lets small hydrophobic molecules and small polar uncharged molecules pass through. However, the membrane does not like larger molecules, polar molecules, and ions.
The cell membrane, also known as the plasma membrane, is made of a phospholipid bilayer. The model that shows this is called the fluid mosaic model:
As shown, we can see that the membrane has TWO layers of phospholipids. The phospholipids have hydrophilic heads and hydrophobic tails. Hydrophobic means that they are not friendly with water. That is why the tails are on the inside of the plasma membrane. Hydrophilic means that they like interacting with water, which is why they are on the outside, where the water is present.
There are proteins in plasma membranes. These proteins can be hydrophobic (nonpolar) or hydrophilic (polar). This could determine how the protein is positioned. For instance, the hydrophobic/nonpolar parts of the protein would be in correspondence with the hydrophobic/nonpolar parts of the membrane. The specific position of the protein in the membrane is what helps the membrane carry out the functions needed.
The fluid aspect of the plasma membrane refers to the nature of the membrane. Rather than staying in place, the phospholipids are able to move laterally (side to side). This lets the membrane be fluid and flexible, which is needed in many cell processes.
There are additional parts of the membrane such as cholesterol and carbohydrates.
Cholesterol molecules are in the phospholipid bilayer. This helps regulate membrane fluidity. It prevents the membrane from being too stiff in cold temperatures or too fluid in warmer temperatures. Carbohydrates are usually found attached to proteins or lipids in the bilayer. They play a part in cell recognition by allowing cells to communicate with each other.
More on permeability..
There are different types of transport that can take place. These are:
Simple diffusion
Facilitated diffusion
Active transport
Endo/Exocytosis
Molecules always have the tendency to move from the higher concentration to the lower concentration solution in order to reach equilibrium. So, when the molecule goes from the high to lower concentration, no additional energy is required.
SIMPLE DIFFUSION
Simple diffusion is passive, meaning no energy is required. Small non-polar molecules pass through along with the concentration gradient.
FACILITATED DIFFUSION
Facilitated diffusion is also passive. Small polar molecules move with the concentration gradient. However, they pass through with the help of proteins.
ACTIVE TRANSPORT
Active transport requires energy, since the molecules pass the membrane going against the concentration gradient.
ENDO/EXOCYTOSIS
Endo/Exocytosis requires energy too, since it is when large quantities of molecules move in or out of the cell.
When water facilitates the diffusion, this is called osmosis.
Osmosis works by water being the molecules that move through the membrane. When the membrane is not permeable to the solutes, the water can move instead. To dilute a solution, you can add water to it. This would end up decreasing the percentage of solute there is in the solution as a whole. So, using this knowledge, we can know that water would have the tendency to move to the solution with a higher solute concentration, to dilute it and make it reach equilibrium.
There are ways to describe solutions’ relationships. You can use the vocabulary hypotonic, hypertonic, and isotonic. You describe a solution as ‘hypertonic’ when it has a higher solute concentration. Conversely, ‘hypotonic’ is used to describe a solution that has a lower solute concentration. Isotonic is when the solution has the same solute and water concentration as the other solution. To describe their relationships, you could say:
“Solution A is hypertonic to solution B”.
For instance, if a red blood cell is placed in a hypertonic solution, they would end up shriveling up and dying. Since the concentration of the solute is greater outside of the red blood cell, water will have the tendency to move outside of the cell. This would eventually lead to the shriveling of these cells.
Comments