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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. 

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Cell Membrane Structure

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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.

You will come across membrane-bound organelles very often during your A levels. These organelles include the nucleus, Golgi body, endoplasmic reticulum, mitochondria, lysosomes and chloroplasts (in plants only).

What is the purpose of cell membranes?

Cell membranes serve three main purposes:

Cell communication

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.

Compartmentalisation

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.

Regulation of what enters and exits the cell

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.

What is the cell membrane structure?

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.

Let's take a closer look at the different components.

Phospholipids

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.

You might see phospholipids being referred to as amphipathic molecules and this just means they simultaneously contain a hydrophilic region and a hydrophobic region (so exactly what we just discussed)!

Cell membrane structure, structure of a phospholipid, StudySmarterFig. 1 - Structure of a phospholipid

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.

Membrane proteins

There are two types of membrane proteins you will find distributed throughout the phospholipid bilayer:

  • Integral proteins, also called transmembrane proteins

  • Peripheral proteins

Integral proteins span the length of the bilayer and are heavily involved in transport across the membrane. There are 2 types of integral proteins: channel proteins and carrier proteins.

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.

Cell membrane structure, structure of channel proteins, Study SmarterFig. 2 - A channel protein embedded in a cell 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.

Cell membrane structure, carrier proteins, StudySmarterFig. 3 - The conformational change of a carrier protein in a cell membrane

Peripheral proteins are different in that they are only found on one side of the bilayer, either on the extracellular or intracellular side. These proteins can function as enzymes, receptors or aid in maintaining cell shape.

Cell membrane structure, peripheral proteins, StudySmarterFig. 4 - A peripheral protein positioned in a cell membrane

Glycoproteins

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.

Cell membrane structure, glycoproteins, StudySmarterFig. 5 - A glycoprotein positioned in a cell membrane

Glycolipids

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.

Antigens also make up the different blood types. This means whether you are type A, B, AB or O, is determined by the type of glycolipid found on the surface of your red blood cells; this is also cell recognition.

Cell membrane structure, glycolipids, StudySmarterFig. 6 - A glycolipid positioned in a cell membrane

Cholesterol

Cholesterol molecules are similar to phospholipids in that they have a hydrophobic and hydrophilic end. This allows the hydrophilic end of cholesterol to interact with the phospholipid heads while the hydrophobic end of cholesterol interacts with the phospholipid core of tails. Cholesterol serves two main functions:

  • Preventing water and ions from leaking out of the cell

  • Regulating membrane fluidity

Cholesterol is highly hydrophobic and this helps prevent the cell contents from leaking. This means water and ions from inside the cell are less likely to escape.

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.

Cell membrane structure, cholesterol, StudySmarterFig. 7 - Cholesterol molecules in a cell membrane

What factors affect the cell membrane structure?

We previously discussed the cell membrane functions which included regulating what enters and exits the cell. To perform these vital functions, we need to maintain the cell membrane shape and structure. We will explore the factors that can affect this.

Solvents

The phospholipid bilayer is arranged with the hydrophilic heads facing the aqueous environment and the hydrophobic tails forming a core away from the aqueous environment. This configuration is only possible with water as the main solvent.

Water is a polar solvent and if cells are placed in less polar solvents, the cell membrane can be disrupted. For example, ethanol is a nonpolar solvent that can dissolve cell membranes and therefore destroy cells. This is because the cell membrane becomes highly permeable and the structure breaks down, enabling the cell contents to leak out.

Temperature

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.

What's more, the membrane proteins involved in transport can also become denatured if the temperature is high enough. This also contributes to the breakdown of the cell membrane structure.

At lower temperatures, the cell membrane becomes stiffer as the phospholipids have less kinetic energy. As a result, cell membrane fluidity decreases and the transport of substances is hindered.

Investigating cell membrane permeability

Betalain is the pigment responsible for the red color of beetroot. Disruptions to the cell membrane structure of beetroot cells cause the betalain pigment to leak out into its surroundings. Beetroot cells are great when investigating cell membranes so, in this practical, we are going to investigate how temperature affects the permeability of cell membranes.

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.

Cell membrane structure, colorimeter experiment set up, StudySmarterFig. 8 - Experimental set-up for cell membrane permeability investigation, using a water bath and beetroot

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.

Cell membrane structure, graph of temperature against absorbance reading, Study SmarterFig. 9 - Graph displaying absorbance against temperature from the cell membrane permeability experiment

A higher absorbance reading indicates that there was more betalain pigment present in the solution to absorb the blue light. This indicates that more pigment has leaked out and therefore, the cell membrane is more permeable.

Cell Membrane Structure - Key takeaways

  • The cell membrane has three main functions: cell communication, compartmentalization and regulating what enters and exits the cell.
  • The cell membrane structure is comprised of phospholipids, membrane proteins, glycolipids, glycoproteins and cholesterol. This is described as the 'fluid mosaic model'.
  • Solvents and temperature affect the cell membrane structure and permeability.
  • To investigate how temperature affects cell membrane permeability, beetroot cells can be used. Place beetroot cells in distilled water of different temperatures and use a colorimeter to analyze the water samples. A higher absorbance reading indicates more pigment is present in the solution and the cell membrane is more permeable.

Frequently Asked Questions about Cell Membrane Structure

The major components of the cell membrane are phospholipids, membrane proteins (channel proteins and carrier proteins), glycolipids, glycoproteins and cholesterol.

The cell membrane is a phospholipid bilayer. The hydrophobic heads of the phospholipids face the aqueous environments while the hydrophobic tails form a core away from the aqueous environments. Membrane proteins, glycolipids, glycoproteins and cholesterol are distributed throughout the cell membrane. The cell membrane has three important functions: cell communication, compartmentalisation and regulation of what enters and exits the cell.

Membrane proteins allow the passage of small particles across the cell membranes. There are two main types: channel proteins and carrier proteins. Channel proteins provide a hydrophilic channel for the passage of charged and polar particles, like ions and water molecules. Carrier proteins change their shape to allow particles to cross the cell membrane, such as glucose.

More about Cell Membrane Structure

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