Cell membrane Definition Function,Structure
The cell membrane is an essential component of the cell, providing isolation between intracellular and extracellular environments. In this article, we will consider the main functions of the cell membrane, the structure of the membrane, and the clinical conditions in which a part of the cell membrane is abnormal.
According to cell theory, cells are the main unit of organization in biology. Whether you are a single cell or a blue whale with trillions of cells, you are still made of cells. All cells are contained by a cell membrane that holds the fragments inside. When you think of a membrane, imagine that it is like a big plastic bag with a few small holes. That bag keeps all cell pieces and liquids inside the cell and any kind of dirty things outside the cell. There are holes to move some things in and out of the cell.
The cell membrane is not a solid structure. It is made up of millions of small molecules that form a flexible and porous container. Proteins and phospholipids make up most of the membrane structure. Phospholipids form the basic bag. Proteins are found around the pores and help move molecules in and out of the cell. There are also proteins associated with the inner and outer surfaces of the membrane.
Scientists use the fluid mosaic model to describe the organization of phospholipids and proteins. The model shows you that phospholipid molecules are shaped with a head and a tail region. The head portion of the molecule prefers water (hydrophilic) while the tail (hydrophobic) does not. Because the tails want to avoid water, they cling to each other and allow the head to face watery (aquatic) areas inside and outside the cell. The two surfaces of the molecules form lipid bilayers.
Definition of Cell membrane
The cell membrane, also known as the plasma membrane, is a dual layer of lipids and proteins that surround a cell. It separates the cytoplasm (cell contents) from the external environment. It is a characteristic of all cells, prokaryotic and eukaryotic.
The function of the Cell Membrane
The cell membrane gives its structure to the cell and controls the substances that enter and leave the cell. It is a selectively permeable barrier, meaning that it allows some substances to pass through, but not others. Like the drawbridge intended to protect a castle and keep enemies out, the cell membrane only allows certain molecules to enter or exit.
Cell membranes are important for the normal functioning of all cells in our body. His main works include:
Producing a continuous, highly selectively permeable barrier – around both cells and intracellular compartments.
- Forming a continuous, highly selectively permeable barrier – is important for maintaining ion gradients.
- Communication – with both external and extra-organelle space.
- Recognition – including recognition of signaling molecules, adhesion proteins, and other host cells (very important in the immune system).
- Signal generation – in response to stimuli that produce changes in membrane potential.
In a cell, different parts of the membrane have different functions and hence their structure is special for it. An example of this specialization can be seen in different parts of a nerve; The cell membrane in the axon is specialized for electrical conduction while the nerve endings are specialized for synapse, which means that the membrane structure is different.
Crossing the Membrane
Small molecules, such as oxygen, which require cells to carry out metabolic functions such as cellular respiration, and carbon dioxide, a by-product of these functions, can easily enter and exit through the membrane. Water can also cross the membrane independently, although it does so at a slower rate.
However, highly charged molecules, such as ions, cannot pass directly, nor can large macromolecules such as carbohydrates or amino acids. Instead, these molecules must pass through proteins that are inherent in the membrane. In this way, the cell can control the rate of diffusion of these substances.
Another way that cell membranes can bring molecules into the cytoplasm is through endocytosis. The reverse process, where the cell distributes material outside the membrane barrier, is called exocytosis.
Endocytosis includes phagocytosis (“cell eating”) and pinocytosis (“cell drinking”). During these processes, the cell membrane forms a depression, around the particle to which it is attached. It is then “pinched off”, forming a small circle of membrane called vesicle that contains the molecule and carries it to wherever it will be used in the cell.
Cells can also transport substances across the cell membrane to the external environment via exocytosis, which is the opposite of endocytosis. During exocytosis, vesicles form in the cytoplasm and migrate to the surface of the cell membrane. Here, they merge with the membrane and release their contents outside the cell. Exocytosis removes the waste products of the cell, which are parts of molecules that are not used by the cell, including older organelles.
Signaling at the Cell Membrane
The cell membrane also plays an important role in cell signals and communication. Membranes contain many embedded proteins that can bind molecules found outside the cell and pass messages inside the cell.
Importantly, these receptor proteins on the cell membrane can bind to substances produced in other areas of the body, such as hormones. When a molecule binds its target receptor to the membrane, it initiates a signal transduction pathway inside the cell that transmits the signal to appropriate molecules.
As a result of these often complex signaling pathways, the cell can perform actions specified by the signaling molecule, such as producing or inhibiting a certain protein.
How does the structure of the cell membrane allow these functions to be performed?
Structure of the Cell Membrane
E.g. phospholipid molecules and cholesterol
E.g. channel proteins and carrier proteins
Often found attached to proteins/lipids on the outside of the cell membrane – a coat of carbohydrate surrounding a cell is often called the glycocalyx
The cell membrane is made up of a phospholipid bilayer. Phospholipids are lipid molecules made up of a phosphate group head and two fatty acid tails. Importantly, the properties of phospholipid molecules allow them to spontaneously form two-layered membranes.
The phosphate group of phospholipid is the predominant hydrophilic, while the phospholipid tail is hydrophobic. This means that the phosphate group is attracted to water, while the tail is removed from the water.
When hydrophobic heads of phospholipids in water or an aqueous solution (including inside the body) will orient themselves as far away from water as possible, inward. In contrast, the hydrophilic head will be outward, making contact with water.
The result is that a dual layer of phospholipids is formed, with the hydrophobic head clustering together in the center, and the hydrophilic tail formed outside the structure. The technical term for this dual layer of phospholipids that form cell membranes is a phospholipid bilayer.
Membranous billers contain many phospholipid molecules.
These include a head molecule, a phosphate molecule, a glycerol, and two fatty acid chains.
- Head group– It is a polar group, e.g. A sugar or choline – meaning that the head end of a phospholipid is hydrophilic.
- The tail of 2 fatty acid chains – typically between 14–24 carbons (but the most common carbon lengths are 16 and 18). If the chain has a cis double bond the chain is kinked – hence reducing the tight packing of the membrane and therefore increasing its movement. As the tail is composed of fatty acids, it does not form hydrogen bonds with water and is therefore hydrophobic and non-polar.
Phospholipid molecules are therefore amphipathic – both hydrophilic and hydrophobic. They spontaneously form boilers in water, with protruding forward groups and exposed tail groups.
In the bilayer, there are van der Waal forces between hydrophilic groups and the fatty acid tail of phospholipid with electrostatic and hydrogen bonds between water.
Cholesterol in the cell is important for many functions, including very important, a major component of the cell membrane.
Cholesterol itself consists of a polar head, a planar steroid ring, and a non-polar hydrocarbon tail. Cholesterol in the membrane is important because it helps to maintain the stability of the cell membrane at different temperatures.
Cholesterol binds to neighboring phospholipid molecules via hydrogen bonds and therefore at low temperatures, reduces their packing. Overall this means that when the rate of motion is lowest at low temperatures, a fluid state remains.
At high temperatures, cholesterol helps prevent the formation of crystalline structures and the rigid planar steroid ring inhibits intrachain vibration and therefore makes the membrane less fluid.
As shown in the table above, a typical cell membrane contains about 60% protein. Proteins have such a high proportion because they are very important in almost every process within a cell. A list of only a few functions of membrane proteins may include:
- Catalysts – enzymes.
- Transporters, pumps, and ion channels.
- Receptors for hormones, local mediators, and neurotransmitters.
- Energy transducers.
More active cells or organelles e.g. Mitochondria, containing more proteins, again show that the specialization of the function determines the structure.
As part of the cell membrane, the protein can either be embedded deep within the bilayer (integral) or be associated with the cell surface (peripheral).
In addition to phospholipid billers, cell membranes also contain lipid molecules, specifically glycolipids, and sterols. An important sterol is a cholesterol, which controls the fluidity of cell membranes in animal cells. When there is less cholesterol, the membrane becomes more liquid, but also more permeable to the molecules. The amount of cholesterol in the membrane helps to maintain its permeability so that the right amount of molecules can enter the cell at once.
Cell membranes also contain many different proteins. Proteins make up about half of the cell membrane. Many of these proteins are transmembrane proteins, which are embedded in membranes, but stick to both sides (ie, they spread throughout the lipid bilayer).
Some of these proteins are receptors that bind signal molecules. There are other ion channels, which are the only means of allowing ions into or out of the cell. Scientists use the fluid mosaic model to describe the structure of the cytoplasm. The cell membrane has fluid stability due to a large amount of phospholipids being formed, and because of this, the protein moves freely on its surface. A multitude of different proteins and lipids in the cell membrane gives it the form of a mosaic.