Membranes serve different functions in a cell. They are responsible for keeping unwanted particles out of the cell while letting important macromolecules enter the cell. They are also important in partitioning the cell into segregated and functional compartments. Given emphasis in this article is the fluid mosaic model for cellular membranes, the different proteins embedded in the membranes and the different roles they play in cell activity. Cytoskeleton and extracellular matrix are also discussed in this article.

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Fluid mosaic model of the cell membrane

Image: “Fluid mosaic model of the cell membrane” by OpenStax – https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface. License: CC BY 4.0


Fluid Mosaic Model

Proposed in 1972 by S. J. Singer and G. L. Nicholson, the fluid mosaic model is an accepted theory, which describes the cell structure as containing phospholipid molecules, that have a hydrophobic and hydrophilic ends.

phospholipid bilayer

Image: “Phospholipid bilayer.” by OpenStax – https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface. License: CC BY 4.0

The hydrophobic end of the molecule repels water from the inside and outside of the cell, while the hydrophilic heads interact freely with water in the inner and outer surface of the membrane. In this way a phospholipid bilayer is formed where the hydrophilic ends are exposed to water, while the hydrophobic tails are sandwiched by the two hydrophilic heads. The structure of the phospholipid bilayer is shown in the figure.

The arrangement of the phospholipid molecules to form the membrane is said to be fluid because of having various functional macromolecules embedded in its matrix. It is not considered as solid, as it allows passage of material through various channels in it.

On the other hand, the model is said to be mosaic as each membrane may be composed of different parts consisting of proteins, carbohydrates and lipids. Depending on the functions, which the membrane will serve, proteins may be embedded to provide as channels for molecules to pass through.

Membrane Fluidity Changes – Role of Cholesterol and Fatty Acid Solution

The viscosity of the lipid bilayer of the cell membrane is referred to as membrane fluidity. This is primarily affected by how lipids are packed in the membrane. Membrane fluidity is important because it affects the diffusion and rotation of proteins and other biomolecules within the membrane, which also affects the functioning of these molecules.

Membrane fluidity is affected by a number of factors, one of which is temperature. When you increase the temperature of a cellular system, lipids acquire thermal energy making it move around more, rearranging themselves making the membranes more fluid. Low temperature enables lipids to organize themselves laterally leading to good packing.

Membrane composition also affects membrane fluidity. Phospholipids in membranes incorporate fatty acids of varying length and saturation.

Lipid chains with double bonds are more fluid because it makes the lipids less likely to pack orderly because of presence of kinks due to the unsaturation sites. Membranes composed of this type of lipid have lower melting points which means less thermal energy is needed to make the membranes behave like a membrane composed of saturated lipid chains.

Cholesterol acts as a bidirectional regulator of membrane fluidity. At high temperatures, it stabilizes the membrane by raising its melting point. On the other hand, at lower temperatures, it intercalates between phospholipids to prevent them from clustering together.

Role of Cell Membrane Proteins

Channel and carrier proteins

Image: “Channel and carrier proteins” by LadyofHats Mariana Ruiz Villarreal – Own work. Image renamed from Image:Facilitated_diffusion_in_cell_membrane.svg. License: Public Domain

  • Cell membrane proteins can play different roles in cellular activity. There are transport proteins, which span the membrane and form channels that can allow movement of specific molecules into and out of the cell.
  • They may also serve as adhesion proteins responsible of anchoring the cell in position in the extracellular matrix or may anchor other materials inside the cell.
  • They also serve important roles in communication between cells. Gap junctions are communication proteins that hook up or connect to other gap junctions in neighboring cells. They serve for chemical and electrical signalling.
  • There are also receptor proteins, which bind to specific molecules causing changes in cell activity.
  • Recognition proteins are present in cellular membranes to recognize if a cell belongs to the host or not.

Cytoskeleton

The cytoskeleton is an interior network of proteins composed of interlinked filaments and tubules extending throughout the cytoplasm, from the nucleus to the plasma membrane. In eukaryotic cells, cytoskeleton may be in the form of three types of filaments: actin filaments, intermediate filaments, and microtubules.

cytoskeleton

Image: “Endothelial cells under the microscope. Nuclei are stained blue with DAPI, microtubles are marked green by an antibody bound to FITC and actin filaments are labelled red with phalloidin bound to TRITC. Bovine pulmonary artery endothelial cells.” by http://rsb.info.nih.gov/ij/images/. License: Public Domain

Actin filaments are composed of linear polymers of G-actin proteins that can generate force when the growing end of the filament pushes itself against a barrier. They are also called microfilaments because of having the smallest diameter of the three types of filaments. They typically have a diameter of 7 nm.

Intermediate filaments have diameters averaging typically 10 nm. They are responsible for maintaining the cell shape. They organize internal structures of cell by anchoring organelles. They can also participate in cell-cell and cell-matrix junctions. Some common intermediate filaments include keratin, lamin, vimentins and desmin.

Microtubules have diameters of 25 nm. They are made up of proteins called tubulin. They are particularly important in transport and mobility. Inside the cell, microtubules play important roles in intracellular transport. They are also present in cilia and flagella that are used by some eukaryotic cells for movement. They are also formed in mitotic spindles during mitosis.

Two important proteins involved in cellular movement are kinesins and dyneins. These proteins utilize energy from ATP hydrolysis in the process causing movement. Dyneins and kinesin-2 are linked to vesicles and organelles to be transported by a protein complex called dynactin.

Dynactin aids in bidirectional intracellular transport by modulating binding of dynein to cell organelles. It also contributes to mitotic spindle pole focusing through binding to NuMA (nuclear mitotic apparatus).

Extracellular Matrix

Extracellular matrix (ECM) is a collection of extracellular molecules produced by cells to provide biochemical and structural support to the surrounding cells.

Common functions of extracellular matrix include cell adhesion, cell-to-cell communication, and differentiation. Animal ECM includes interstitial matrix and the basement membrane.

The interstitial matrix contains polysaccharide gels and fibrous proteins that act as a compression buffer against stress placed on the ECM.

Basement membranes are sheet-like depositions where various epithelial cells rest. Some examples of ECM are the collagen fibers and bone minerals in bone tissues, reticular fibers and ground substance in loose connective tissues, and the blood plasma in the blood.

Review Questions on Cell Membranes

The correct answers can be found below the references.

1. Which of the following is not a role of membrane proteins in cell activity?

  1. Catalysis
  2. RNA synthesis
  3. Transport
  4. Receptor

2. Which of the following filaments have the smallest diameter?

  1. Actin
  2. Intermediate filament
  3. Microtubules
  4. None of the above
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