Cell Membrane Structure

Cell surface membranes are structures that surround and encapsulate each cell. They separate the cell from its extracellular environment. Membranes can also surround organelles within the cell, such as the nucleus and the Golgi body, to separate it from the cytoplasm.

• Cell communication

• Compartmentalization

• Regulation of what enters and exits the cell

The cell membrane contains components called glycolipids and glycoproteins, which we will discuss in the later section. These components can act as receptors and antigens for cell communication. Specific signalling molecules will bind to these receptors or antigens and will initiate a chain of chemical reactions within the cell.

Cell membranes keep incompatible metabolic reactions separated by enclosing the cell contents from the extracellular environment and the organelles from the cytoplasmic environment. This is known as compartmentalization. This ensures that each cell and each organelle can maintain the optimal conditions for their metabolic reactions.

The passage of materials entering and exiting the cell is mediated by the cell surface membrane. Permeability is how easily molecules can pass through the cell membrane - the cell membrane is a semipermeable barrier, meaning only some molecules can pass through. It is highly permeable to small, uncharged polar molecules such as oxygen and urea. Meanwhile, the cell membrane is impermeable to large, charged nonpolar molecules. This includes charged amino acids. The cell membrane also contains membrane proteins that allow the passage of specific molecules. We will explore this further in the next section.

The cell membrane structure is most commonly described using the 'fluid mosaic model'. This model describes the cell membrane as a phospholipid bilayer containing proteins and cholesterol which are distributed throughout the bilayer. The cell membrane is 'fluid' as individual phospholipids can flexibly move within the layer and 'mosaic' because the different membrane components are of different shapes and sizes.

Phospholipids contain two distinct regions - a hydrophilic head and a hydrophobic tail. The polar hydrophilic head interacts with water from the extracellular environment and the intracellular cytoplasm. Meanwhile, the nonpolar hydrophobic tail forms a core inside the membrane as it is repelled by water. This is because the tail is comprised of fatty acid chains. As a result, a bilayer is formed from two layers of phospholipids.

The fatty acid tails can be either saturated or unsaturated. Saturated fatty acids have no double carbon bonds. These results in straight fatty acid chains. Meanwhile, unsaturated fatty acids contain at least one carbon double bond and this creates 'kinks'. These kinks are slight bends in the fatty acid chain, creating space between the adjacent phospholipid. Cell membranes with a higher proportion of phospholipids with unsaturated fatty acids tend to be more fluid as the phospholipids are packed more loosely.

Channel proteins provide a hydrophilic channel for polar molecules, such as ions, to travel across the membrane. These are usually involved in facilitated diffusion and osmosis. An example of a channel protein is the potassium ion channel. This channel protein allows the selective passage of potassium ions across the membrane.

Carrier proteins change their conformational shape for the passage of molecules. These proteins are involved in facilitated diffusion and active transport. A carrier protein involved in facilitated diffusion is the glucose transporter. This allows the passage of glucose molecules across the membrane.

Glycoproteins are proteins with a carbohydrate component attached. Their main functions are to help with cell adhesion and act as receptors for cell communication. For example, receptors that recognize insulin are glycoproteins. This aids in glucose storage.

Glycolipids are similar to glycoproteins but instead, are lipids with a carbohydrate component. Like glycoproteins, they are great for cell adhesion. Glycolipids also function as recognition sites as antigens. These antigens can be recognized by your immune system to determine if the cell belongs to you (self) or from a foreign organism (non-self); this is cell recognition.

Cholesterol also prevents the cell membrane from being destroyed when temperatures become too high or low. At higher temperatures, cholesterol decreases membrane fluidity to prevent large gaps from forming between individual phospholipids. Meanwhile, at colder temperatures, cholesterol will prevent the crystallization of phospholipids.

Cells function best at the optimal temperature of 37 ° c. At higher temperatures, cell membranes become more fluid and permeable. This is because the phospholipids have more kinetic energy and move more. This enables substances to pass through the bilayer more easily.

Below are the steps:

1. Cut 6 pieces of beetroot using a cork borer. Make sure each piece is of equal size and length.

2. Wash the beetroot piece in water to remove any pigment on the surface.

3. Place the beetroot pieces in 150ml of distilled water and place in a water bath at 10ºc.

4. Increase the water bath in 10 ° C intervals. Do this until you reach 80ºc.

5. Take a 5ml sample of the water using a pipette 5 minutes after each temperature is reached.

6. Take the absorbance reading of each sample using a colourimeter that has been calibrated. Use a blue filter in the colourimeter.

7. Plot the absorbance (Y-axis) against temperature (X-axis) using the absorbance data.

From the example graph below, we can conclude that between 50-60ºc, the cell membrane was disrupted. This is because the absorbance reading has notably increased, meaning that there is betalain pigment in the sample that has absorbed the light from the colourimeter. As there is betalain pigment present in the solution, we know that the cell membrane structure has been disrupted, making it highly permeable.