Cell membrane - definition, structure, function and biology (2023)

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What is a cell membrane? A quick overview

Our cell is literally a balloon filled with water (70% of our body weight is water). This soft but strong balloon is made of the cell membrane (also called the plasma membrane). The cell membrane is a thin biological membrane that separates the inside of the cell from the outside and protects the cells from the environment.

The cell membrane consists of two layers of lipid films (oil molecules) into which many types of proteins are inserted. These proteins control the movement of molecules such as water, ions, nutrients, and oxygen in and out of the cell.

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[In this figure]The anatomy of an animal cell with labeled organelles.

Enclosed by this cell membrane are the components of the cell, including the cellorganellesand gelatinous fluids called cytosols that contain water-soluble molecules such as proteins, nucleic acids, carbohydrates, and substances involved in cellular activities.

So, the cell membrane has two key functions:

1. Being a barrier that keeps cellular components in and unwanted substances/toxins out.

2. Being a gateway that allows the transport of essential nutrients into the cells and waste products out of the cells.

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[In this figure]The cell membrane defines the interior and exterior spaces of a cell.
The cell membrane is a phospholipid bilayer membrane with many proteins. These proteins could insert into or associate with the membrane and act as channels (controlling the entry and exit of molecules) or receptors (receiving signals from the outside world).
The image was created with BioRender.com.

The structure of the cell membrane.

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[In this figure]The liquid mosaic model of the cell membrane showing membrane proteins assembled with a lipid bilayer.

Phospholipid bilayer as a versatile biological barrier

The main structure of the cell membrane is a thin polar membrane composed of two layers of lipid molecules, calledlipid bilayer(oDoppelschicht of phospholipids). This bilayer is formed by the amphiphilic phospholipids thathydrophilic(preferably water) phosphate head and ahydrophobe(who prefers to stay away from water) Tail made up of two chains of fatty acids.

(Video) Cell Membrane Structure and Function

In an aquatic environment, the hydrophobic ends of many phospholipids naturally stick together with their hydrophilic phosphate heads facing the outer water molecules. The lipid bilayer forms spontaneously through self-assembly.

Another type of lipid molecule, calledsteroidsIt is also an important component of the cell membrane.cholesterolit is the most common steroid and cholesterol levels can potentially alter membrane fluidity and function.

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[In this figure]The cell membrane consists of two lipid films called the lipid bilayer.

Selective cell membrane permeability.

Lipid bilayers are ideal for preventing ions, proteins, and other charged molecules from diffusing across the membrane, despite being only a few nanometers wide. At the same time, uncharged molecules and gases can easily pass through the cell membrane. This property is calledsemipermeabilityodirected patency.

Selective permeability prevents the free diffusion of molecules, allowing membranes to form compartments that maintain different internal and external environments. This is because the hydrophobic cores of lipid bilayers (created by fatty acid chains) are impermeable to most water-soluble (hydrophilic) molecules. Small molecules without electrical charges such as CO2, norte2, o2, and molecules with high fat solubility, such as ethanol, can pass through membranes almost unimpeded.

This property allows cells to regulate salt concentrations and pH levels within cells. The transport of ions across their membranes requires special permission from proteins called ion pumps or channels.

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[In this figure]Selective Cell Membrane Permeability: The size and electrical properties of molecules determine their ability to cross cell membranes.
Small molecules with no electrical charge (for example, gases) and oil-soluble molecules can pass through membranes almost unhindered. The permeability is lower for uncharged molecules such as water and glycerin. The ability of large uncharged molecules, such as glucose, to cross membranes is low. Membranes are highly impermeable to ions and charged molecules.

Phospholipid bilayers are commonly used for all membrane-bound organelles.

Because lipid bilayers are large enough to create compartments for biochemical reactions, membrane-bound organelles (such as the nucleus, endoplasmic reticulum, mitochondria, chloroplasts, Golgi apparatus, lysosomes, peroxisomes). and vacuoles) use the same lipid bilayers for their functions. membranes We even created artificial spherical vesicles made up of lipid bilayer membranes calledliposomesto help us deliver medicines and vaccines.

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[In this figure]The three main structures that form phospholipids in solution: the liposome (a closed bilayer), the micelle, and the bilayer.
Author of the photo:wiki

membrane protein

There are different types of proteins that associate with the cell membrane to perform many essential biological functions. These proteins can make up approximately 50% of the membrane volume. About a third of the genes in multicellular organisms specifically encode membrane-related proteins.

Membrane proteins are of three main types: transmembrane proteins, lipid-anchored proteins, and peripheral proteins.

transmembrane proteinThese proteins insert into lipid bilayers with one or two portions facing the extracellular or intracellular space.Ion channels, G protein-coupled receptors
lipid-anchored proteinsThe protein itself is not in contact with the membrane. Instead, they are anchored to the cell membrane via covalently bound lipid molecules.G protein
peripheral proteinsBound to integral membrane proteins or associated with peripheral regions of the lipid bilayer. These proteins tend to have only transient interactions with cell membranes for specific reactions.Some enzymes and hormones
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[In this figure] Schematic representation of transmembrane, peripheral and lipid-anchored membrane proteins.
Some transmembrane proteins have carbohydrate chains (also called glycoproteins) on the outer surface of the cell.

Membrane proteins allow cell membranes to perform various functions. We will talk about these features later.

Fluid mosaic model: What makes the cell membrane fluid?

The cell membrane is flexible and fluid. To better describe the properties of the cell membrane, scientists explain the appearance and functions of the cell membrane using theliquid mosaic model.

If you zoom in on the cell membrane, you can see the ocean of lipid molecules adorned with membrane proteins, cholesterol, and carbohydrates. These molecules are constantly moving in two dimensions in a fluid fashion, like icebergs floating in the ocean. There is no uniform pattern or arrangement of these molecules; They are more like a mosaic.

(Video) Cell Membrane: Definition, Function, and Structure
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[In this figure]The fluid mosaic model of the cell membrane describes the cell membrane as a fluid combination of phospholipids, cholesterol, and proteins.
photo source:LibreTexts Biology

The fluid property of the cell membrane is essential for many cellular activities. There are 3 main factors that affect the fluidity of the cell membrane:

(1)Temperature: Temperature can affect how phospholipids move and how close they stay. When it's cold they lie closer together and when it's hot they move further apart.

(2)cholesterol: Cholesterol molecules are randomly distributed in the lipid bilayer and help the membrane stay fluid. Cholesterol keeps phospholipids at a reasonable distance, not too close and not too far. Without cholesterol, the phospholipids begin to separate from each other, leaving large gaps in warm temperatures. On the other hand, without cholesterol, the phospholipids in your cells will begin to build up when exposed to cold, making it difficult for small molecules like gases to pass through the phospholipids as they normally would.

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[In this figure]Cholesterol is a key component of the cell membrane. The presence of cholesterol molecules can stabilize cell membrane properties over a range of temperatures.

(3)Saturated and unsaturated fatty acids:Fatty acids are tails of phospholipids. Saturated fatty acids are chains of carbon atoms that only haveeinzelties between them. This keeps the strings straight and easy to pack well.
Unsaturated fatty acids are chains of carbon atoms that havedobleBonds between some of the carbons. Double bonds create kinks within the chains, making it difficult for the chains to pack tightly. These folds play a role in the fluidity of the membrane, as they increase the distance between the phospholipids and make it more difficult for the molecules to freeze at lower temperatures. In addition, the increase in space allows certain small molecules such as CO2to cross the membrane quickly and easily.

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[In this figure]The composition of saturated and unsaturated fatty acids influences the fluidics of the cell membrane.

What does the cell membrane do? – The function of the cell membrane

Like our skin, the cell membrane serves as a cellular barrier.

The cell membrane encloses the cytoplasm of living cells and physically separates the intracellular components from the extracellular environment. Plant cells have cell walls outside the cell membrane for added protection and support.

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The cell membrane defines the cell shape and provides the sites for cell attachment.

The cell membrane plays a role in anchoring the cytoskeleton to give the cell its shape (especially in animal cells without cell walls). The cell membrane also provides the sites to interact with the extracellular matrix and other cells. These contact points can transmit extracellular mechanical stimuli (such as pressure or shear forces) to the cytoskeleton, resulting in a change in cell behavior by altering gene expression in the cell nucleus.

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[In this figure]The cell membrane and transmembrane proteins serve as junction points to link the intracellular cytoskeleton and the extracellular matrix (ECM).

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[In this illustration] Amoeba movement: An amoeba moves by stretching out its pseudopods.
Beneath the plasma membrane of pseudopods are organized cytoskeletons that generate the force to drive cell shape change.

Membrane proteins control the transport of biomolecules into and out of cells.

The cell membrane is selectively permeable and can regulate what enters and leaves the cell, facilitating the transport of materials necessary for cellular activities. The movement of substances across the membrane can be “passive', which occurs without cellular power supply, or 'asset' so the cell has to expend energy to transport it.

(Video) PLASMA MEMBRANE structure and function: Phospholipid bilayer for A-level Biology. Fluid-mosaic model

1.Passive Diffusion and Osmosis: Some uncharged molecules such as carbon dioxide (CO2) and oxygen (O2), can move across the cell membrane by diffusion, which is a passive transport process. Diffusion occurs when small molecules move freely from a high concentration to a low concentration to balance the membrane. Some proteins can facilitate passive diffusion by serving as channels or transporters. Water also flows across the cell membrane through water channels (called aquaporin) by osmosis to make up for the difference in salt concentration.

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[In this figure]Depending on the nature of the molecules, cells transport them passively or actively. Passive transport (protein-mediated diffusion or facilitated diffusion) only moves molecules from a high concentration to a lower one. Active transport requires special transport proteins and consumes energy. Active transport can move substances in both directions.

2.Active transport: The lipid bilayer effectively repels many large water-soluble molecules and electrically charged ions, which require cells to import or export them to survive. The transport of these vital substances is carried out by certain classes of transmembrane proteins, which act as "pumps", pushing solutes across the membrane when they are not sufficiently concentrated to diffuse spontaneously. To transport these substances, cells must expend energy, or ATP. Adenosine triphosphate (ATP) is the biochemical energy "currency" of cells.

Cells eat and excrete by changing the cell membrane.

Particles too large to diffuse or pump are often swallowed or spit out whole by opening and closing the membrane. To carry materials into cells, the cell membrane surrounding the particles is invaginated and engulfed by the formation of a vesicle (endocytosis). On the other hand, to remove unwanted materials from cells, the membrane of a vesicle fuses with the cell membrane and extrudes its contents into the surrounding medium, a process called vesicle formation.Exocytosis.

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[In this figure]Endocytosis is the process by which cells take in molecules by engulfing them. Exocytosis is a form of active transport in which a cell transports molecules out of the cell by secreting them through the fusion of the vesicle and cell membrane.

Cells communicate with each other through direct or indirect contacts on their cell membranes.

In a multicellular organism, cells must communicate with each other. Cell-to-cell communication requires special proteins in cell membranes. The cell that wants to send a message has "ligand" proteins secreted or expressed on its cell membrane. The recipient cells then have appropriate "receptor" proteins to receive these messages (by binding to the ligand proteins).

Recipient cells can respond immediately but temporarily by changing the shape of the cell or by releasing specific ions. Alternatively, they can make a slow but permanent change by relaying messages to the nucleus to turn certain genes on or off. When two cells are close enough, they can also establish a direct exchange of molecules through protein channels (called gap junctions) that twist the cell membranes of both cells.

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[In this figure]Cell-cell communication through (a) direct contact and (b) gap junctions.
photo source:Top-hat

Signal transduction along the cell membrane of nerve cells

Because the membrane acts as a barrier to charged molecules and ions, these can occur in different concentrations on either side of the membrane. The total charge difference between the inside and outside of the cell is calledMembrane potential.

For the nervous system to function, neurons (or nerve cells) must be able to send and receive signals. These signals are electrical currents generated by changes in membrane potential. The membrane potential can change in response to neurotransmitter molecules released by other neurons and environmental stimuli.

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[In this figure] Voltage-gated ion channels open in response to changes in membrane potential.
(a) The resting membrane potential results from different concentrations of Na+ and K+ ions inside and outside the cell. A nerve impulse causes Na+ to enter the cell, leading to (b) depolarization. At the highest action potential, the K+ channels open and the cell becomes (c) hyperpolarized.
photo source:Lumenaprendizaje

What does a cell membrane look like under a microscope?

Under a compound light microscope, the cell membrane (only 5-10 nm) may be too thin to see. However, you can easily see the edges of the cells if you stain them with the appropriate dyes. That's where the cell membrane is.

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[In this figure]Cheek cells stained with methylene blue..
The image on the left is a low magnification. You can see the nuclei stained dark blue (because methylene blue strongly stains DNA). The cell membrane acts like a balloon and contains all the parts of a cell inside it, such as the nucleus, cytosol, and organelles.

Plant cells have a layer of cell membrane below the cell wall. Most of the time it is difficult to see the cell membrane. However, under hypertonic conditions it may be noted that the cell membrane has separated from the cell wall.

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The cell membrane can be stained with lipid-binding fluorescent dyes. This provides a useful tool for visualizing cell boundaries and morphology in multicolor staining experiments.

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(Video) Structure and Composition of Cell Membrane | Biology

[In this figure](A) Human epithelial cells stained for cell membrane (red) and nuclei (green); (B) Baker's yeast cells (S. cerevisiae) stained with three-color membrane dyes (red, purple, and green); (C) bacteria,E. colistained with violet membrane dyes.
photo source:ABP Life Sciences

Electron microscopes provide high-resolution images of cell membranes. Many important scientific discoveries have been made using electron microscopy.

Here are some examples:

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[In this figure]Transmission electron microscopy (TEM) image of the cell membranes of two cells in close proximity.
photo source:cytochemistry.

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[In this figure]TEM image of coated vesicle formation during endocytosis.
photo source:An introduction to biological membranes.

To study the structure of the cell membrane, the scientists used a "freeze fracture" technique to break the lipid bilayers and analyze them separately under a scanning electron microscope (SEM).

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[In this figure] Freeze fracture is a technique that can be used to visualize membrane structure and protein distribution.
First, a cell is quickly frozen. It is then cleaved along the fracture plane that divides the lipid bilayer. Separation along this plane exposes membrane-embedded transmembrane proteins. Right: SEM images show bumps on the surface of the cleaved bilayer, which are actually transmembrane proteins. This confirmed that the membrane proteins are randomly distributed throughout the phospholipid bilayer. There are also integral transmembrane proteins that span the entire membrane.
photo source:Wikilibros

Special cell membrane structures in special cell types

To carry out specific cellular functions, some cells possess unique cell membrane structures. Here are some examples:

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[In this figure] TEM image of the epithelial surface of the small intestine.Microvilli are microscopic projections from the cell membrane that increase the surface area to maximize nutrient uptake.
photo source:Atlas of Plant and Animal Histology.

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[In this figure]T-tubules (transverse tubules) are extensions of the cell membrane that enter the center of skeletal and cardiac muscle cells. The T tubules allow the action potential to be rapidly transmitted into the cell, allowing the heart muscle cells to contract more.
photo source:wiki

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[In this figure]Electron micrograph showing the membrane surface of endothelial cells coated with a thick layer of carbohydrate componentsglycocalyx. Endothelial cells are the types of cells that line the inner lumen of blood vessels.
photo source:altered physiology


  • The cell membrane is a biological membrane that separates the inside of the cell from the outside and protects it from its environment.
  • The cell membrane consists of two layers of lipid films (oil molecules) with many types of membrane proteins.
  • The cell membrane controls the movement of molecules such as water, ions, nutrients, and oxygen in and out of the cell.
  • Proteins in the cell membrane are also involved in cell movement and cell-to-cell communication. For example, cells receive signals from the outside through different types of receptor proteins in the cell membrane, which act like tiny antennae.



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