Molecules-transport-by-endocytosis

Fluid Mosaic Model-Definition and Overview

Introduction

The fluid mosaic model is an interactive way to visualize the structure of a cell. In this article, we will learn about what it is and how it works. The model aimed to introduce a new way to illustrate molecular transport within cells’ membranous organelles. This model also shows the interactions between these organelles and their structures.

According to the model, there are four main components of the cell membrane. These are carbohydrates, Phospholipids, cholesterol and proteins. This membrane is the boundary that separates a cell’s interior from its environment.


Image: The fluid mosaic model

Why is the Cell Membrane Considered Fluid?

The cell membrane is considered fluid because it is made up of phospholipids. These structures are mobile and can change shape easily to accommodate changes in the cell’s environment.

According to the fluid mosaic model, the molecules in the plasma membrane constantly move. They appear as though they are floating on water. The membrane fluidity makes the cell hard to penetrate.

As the molecules constantly move in a fluid motion, they form a barrier hence protecting the cell. Three factors influence the fluidity of the cell membrane:

  • Fatty Acids-Saturated and Unsaturated
  • Temperature change
  • The Cholesterol Molecules
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1. Fatty Acids

Fatty acids make the phospholipid tails. They determine the fluidity or rigidity of a cell membrane. There are two types: saturated and unsaturated. Saturated fatty acids have no double bonds, while unsaturated have one or more.

Unsaturated fatty acids are less rigid and move faster than saturated ones, making the plasma membrane more fluid. The cell membrane will have a higher percentage of unsaturated fatty acids if the temperature is cold and saturated ones when it’s warm.

Saturated fatty acids have a chain of carbons with single bonds, while unsaturated fatty acids have a chain with double bonds. The number of carbons can also affect the fluidity of the cell membrane, and more carbon chains mean a more rigid structure.

For instance, a saturated fatty acid will be less fluid than an unsaturated one with 12 carbons because the single bonds make it less mobile. The cholesterol molecules can also change the fluidity of a cell membrane, as they are located in the phospholipid bilayer.

You can expand your knowledge of fatty acids and other molecules by checking our exclusive post on monomers!

2. Temperature Changes

Temperature also affects the fluidity of the plasma membrane. When a temperature change is sudden, it takes time for the cells to adjust.

When cells do not have enough time to adjust, the cell membrane moves to a less fluid state. This can happen when going from an environment warm such as outdoors, to one where the temperature is cool such as indoors.

The cell membrane will stiffen when a person is cold due to the decrease in heat from the environment. When it is warm outside, the plasma membrane will be more fluid because of higher temperatures in an environment and higher heat from inside the cell.

The phospholipids are far apart during hot weather, while in cold weather, they are closer together. When the cell membrane is too rigid, it will hold its shape, but it’s at the danger of breaking. A more fluid and flexible membrane will not hold a definite shape but is less likely to break.

3. The Cholesterol Molecules

Image: cholesterol structure

Cholesterol affects the fluidity of the plasma membrane by providing the necessary support for it. Cholesterol is a type of lipid that makes up about 25% to 35% of membrane lipids in the cell.

It is a hydrophobic molecule that can’t dissolve in the water-based environment of the membrane and thus attaches to lipids.

Without cholesterol molecules in the phospholipid bilayer, it would be impossible for a cell to maintain its shape. The reason being, cholesterol molecules have hydrophobic properties and can’t dissolve in water, so they bind to the lipids.

Without a plasma membrane, the water will seep in and make it difficult for cells to maintain their shape.

We have an exclusive article that shows the structures of all the macromolecules!

Why The Need for Plasma Membrane Fluidity?

The fluidity of a cell is very important to its survival and function. A more fluid membrane will change shape for the cell to adapt when needed, such as during dehydration.

It will also be able to absorb nutrients and exchange waste with the outside environment. A less fluid membrane won’t have this ability, so molecules need to flow freely within it.

Membrane fluidity is also important because it protects the cell from dehydration. When a person is dehydrated, their cells will draw water and become more rigid to maintain hydration.

The cell membrane will be less fluid and won’t allow water to flow in as easily. The more rigid a person becomes, the harder it is for them to hydrate themselves. The rigidity can lead to death if a person’s fluidity doesn’t change and doesn’t get enough water.

The fluid mosaic model believes that cells are not static structures but rather amoeba-like and constantly in flux.

Arrangement of the Plasma Membrane Components

This model is also called the model of cell membranes, and it shows how the four membrane components are arranged. Carbohydrates form the outermost layer of a plasma membrane, and they serve as an attachment site for proteins.

Phospholipids form the next layer, and they are composed of a lipid tail and two phosphate groups. Cholesterol is also found beneath the phospholipid layer, and it is important for membrane fluidity.

Proteins are found in the centre of a plasma membrane. The proteins are made in the cell’s nucleus and then transported to the membrane.

The Phospholipid Molecules

The fluid mosaic model describes the phospholipid bilayer as the main part of the membrane. It comprises the hydrophilic and hydrophobic areas. The hydrophilic side is the outermost layer and consists of water-soluble molecules, making it easier for nutrients to move in and out of the cell.

The hydrophobic side is found inside the membrane, and it consists of non-polar molecules which are water-hating. The phospholipid molecule is made of a 3-carbon molecule called glycerol and two phosphate groups.

One of the phosphates is attached to one end of the molecule with a bond that can easily break, and one of them is attached to the other end. The head group (phosphoethanolamine) will be either a choline group or a serine group.

The phospholipid molecule is also polar, meaning that each end of the head will have an opposite charge. The phosphate end is negatively charged, and the head group positively charged.

Image: phospholipid

The Hydrophilic End of a Phospholipid

The hydrophilic end is the outermost layer, and it consists of water-soluble molecules. It means that these molecules are good at dissolving in water and will be found on the outermost layer of a plasma membrane.

The Hydrophobic End of a Phospholipid

The hydrophobic end is found inside the membrane and consists of non-polar molecules. These types of molecules are water-hating, meaning that they will not dissolve in water and are found inside a plasma membrane.

Fatty Acids

Image: fatty acids

According to the fluid mosaic model, fatty acids are mainly found in the hydrophobic end of a plasma membrane. However, they can also be found in the hydrophilic end.

The fatty acid molecule comprises a long chain of carbon atoms with an even number of hydrogen molecules attached. The fatty acid is attached to the plasma membrane by a single bond. It has both hydrophobic portions (hydrogen atoms) and hydrophilic portions (single bond).

Cholesterol

Cholesterol is a hydrophobic molecule found in the centre of a plasma membrane. It can also be found on the hydrophilic side of a plasma membrane. The cholesterol molecule is attached to an outer layer of phospholipids by one bond, and it is not made from any sugar.

The cholesterol molecule has a hydrophobic tail (hydrogen atoms) and an oxyethylene group at the other end. The oxyethylene bond is hydrophilic, allowing cholesterol to be found on both sides of a plasma membrane.

Proteins

The fluid mosaic model recognizes that proteins are the second main component of the cell membrane after phospholipids. They are made in the cell’s nucleus and then transported to the membrane.

Proteins are found on both sides of a plasma membrane because they have a hydrophilic and a hydrophobic end. Proteins are composed of long chains of amino acids that form the central core, carrying out different functions.

The protein that makes up the plasma membrane has a central core of alpha-helices connected by non-polar amino acids. The non-polar amino acid ensures that the protein will not dissolve in water because of its hydrophobic properties.

Some proteins help decide how much fluidity the cell membrane will have. For example, when there is a signal to make more fluidity, proteins will change shape and rearrange themselves on the membrane.

One protein (claudin) is in charge of making sure that nutrients and other molecules cannot move back and forth across the cell membrane.


Image : claudins

Carbohydrates

According to the fluid mosaic model, carbohydrates come third in the composition of the plasma membrane. They are found on the outermost layer of a plasma membrane and are composed mainly of sugar molecules.

Carbohydrates can also be made up to form a cell’s skeleton. They form long, thin threads linked together to create an overall structure for the cell. The carbohydrate molecule is composed of many monosaccharide molecules which have a hydrophilic head and a hydrophobic tail.

The sugar molecule is linked to the plasma membrane by a single bond, and it has both hydrophilic (head) and hydrophobic (tail). The carbohydrates in the membrane contain up to 60 monosaccharide units. They take either a linear or branched structure and can be found on either side of a plasma membrane.

Image: Sugar molecule attached to the cell membrane

The carbohydrates help determine the thickness, rigidity and porosity of a cell’s outermost layer. Carbohydrates on the outer part of the membrane(glycolipids) team with proteins (glycoproteins) to form glycocalyx. The glycocalyx is hydrophilic, so it helps the cell to interact with watery environments.

Signals and Communication in The Cell Membrane

Image: cell receptors

Another important aspect of the fluid mosaic model is how elements in the membrane communicate. The signalling process is initiated by a receptor that is found on the membrane.

The signalling process starts with an extracellular stimulus, such as hormone or chemical, and it is then transmitted by the receptor, which will trigger a response. The stimulus can also be found on the cell membrane, and it would have to bind with the receptor for a response to happen.

Proteins then propagate the signal until it reaches its destination, either an organelle or the nucleus. In other words, a receptor will bind with an extracellular stimulus and then release it to propagate in the cell membrane.

The signal will continue propagating until it reaches the nucleus, where a transcription factor is waiting. That factor can also be found on the membrane, and if the stimulus binds with it, then a response will happen.

The transcription factor can initiate DNA synthesis or RNA production to respond to the stimulus.

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Receptor Inhibition

Viruses such as HIV can disrupt the cell membrane by binding with the receptor. This will prevent other signals from being propagated, and it can disrupt many functions of a healthy cell, such as the ability to replicate.

The virus can then infect a cell by inserting its DNA into it, which will result in the replication of more viruses. This is a problem for cells with the cell membrane because it disrupts their normal functions.

Plasma Membrane Transport Methods

The fluid mosaic model explains the different transport mechanisms of the cell membrane. These include:

Endocytosis


Image : Molecules’ transport by endocytosis

It is a process where the cell membrane invaginates and creates a small sac that eventually pinches off. Endocytosis is used for many processes like taking in:

  • Molecules that are too large to diffuse across the plasma membrane
  • Hormones and vitamins that need a lipid coat to allow for passage into the cell
  • Viruses or bacteria that have breached a plasma membrane.

Endocytosis brings these molecules and cells into the cell’s cytoplasm.

Transmembrane protein transport

Image: Transmembrane protein transport

Transmembrane protein transport involves proteins that have both a hydrophilic and a hydrophobic end.

These molecules depend on other transport mechanisms to move across the plasma membrane. For example, a protein can be linked with a transport vesicle that will carry it from one part of the cell to another.

When proteins need to be transported, they will bind with a transport vesicle through non-covalent bonds. Protein transport can also happen through endocytosis.

Exocytosis

Image : Exocytosis

Exocytosis is a process in which the cell membrane secretes the following substances:

  • Hormones and enzymes
  • Insulin from the pancreas
  • Neurotransmitters
  • Digestive enzymes and bile from the liver and gall bladder
  • Antibodies

Exocytosis prefers to use larger vesicles than endocytosis

Passive Diffusion and Osmosis

Image: passive diffusion

The fluid mosaic model also explains how passive diffusion and osmosis work. Passive Diffusion is the movement of molecules down their concentration gradient. For example, in this process, glucose will diffuse across the plasma membrane to a region of higher concentration.

Osmosis is when water diffuses across the plasma membrane according to its concentration gradient. What does this mean? In osmosis, water will move from an area of high to an area of low solute concentration.

Image: Osmosis in cells

The water will continue to move until the concentrations are equal on both sides. Osmosis can be used to measure how well a cell has been hydrated.

Osmosis happens when a cell is in an environment with water, and the membrane contains small pores to let water molecules through. It can also occur due to damage on the plasma membrane’s lipid bilayer is damaged

The cell transports Oxygen, Carbon dioxide and ions through passive Diffusion, while osmosis ensures that water can continue to flow.

You may be interested in Biological Translation

Categories of Molecules in The Cell Environment

There are four main categories of molecules found in the cell environment. These are:

  • Large polar molecules, which include ions and sugar molecules
  • Small polar molecules like water
  • Large non-polar molecules like lipids.
  • Small non-polar molecules like oxygen and carbon dioxide.
Image: molecules in the cell environment

Bottom Line

The fluid mosaic model is a great way to understand how cells are structured and function. To learn more about this scientific perspective, explore our other blogs. You’ll get an explanation of the components that make up the cell membrane and the interactions between molecules on both sides of it.

The content also includes transport mechanisms like osmosis and Diffusion that occur in order for chemical reactions to occur. We hope you’ve enjoyed learning about what goes into making a living organism! Remember that our professionals writers for hire are ready to ace that assignment for you in case you feel stuck!

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