The cells of plants, fungi and animals consist of three components such as the nucleus, the cytoplasm with the organoids and inclusions located in it, and the plasma membrane. The nucleus is responsible for storing genetic material recorded on DNA, and also controls all the processes of the cell. The cytoplasm contains organoids, each of which has its own functions, such as, for example, the synthesis of organic substances, cellular respiration, cellular digestion, etc. And we will talk about the last component in more detail in this article.
What is a membrane in biology?
In simple terms, this is a shell. However, it is not always completely impenetrable. Almost always allowed the transport of certain substances through the membrane.
In cytology, membranes can be divided into two main types. The first is the plasma membrane that covers the cell. The second is the membranes of organelles. There are organelles that have one or two membranes. Golgi complex, endoplasmic reticulum, vacuoles, lysosomes belong to single-membrane ones. Plastids and mitochondria belong to two-membranes.
Membranes can also be inside organelles. Usually these are derivatives of the inner membrane of two-membrane organoids.
How are membranes of two-membered organoids arranged?
Plastids and mitochondria have two membranes. The outer membrane of both organoids is smooth, but the inner one forms the structures necessary for the functioning of the organoid.
So, the mitochondrial membrane has protrusions inward - cristae or crests. A cycle of chemical reactions necessary for cellular respiration takes place on them.
The derivatives of the inner membrane of chloroplasts are disk-shaped sacs - thylakoids. They are collected in piles - grains. Separate grains are combined with each other using lamellae - long structures also formed from membranes.
The structure of membranes of single-membrane organoids
Such organelles have one membrane. It usually is a smooth membrane composed of lipids and proteins.
Structural features of the plasma membrane of the cell
The membrane consists of substances such as lipids and proteins. The structure of the plasma membrane provides for its thickness of 7-11 nanometers. The bulk of the membrane are lipids.
The structure of the plasma membrane provides for the presence of two layers in it. The first is a double layer of phospholipids, and the second is a layer of proteins.
Plasma membrane lipids
The lipids that make up the plasma membrane are divided into three groups: steroids, sphingophospholipids and glycerophospholipids. The molecule of the latter contains the residue of the trihydric alcohol glycerol, in which the hydrogen atoms of two hydroxyl groups are replaced by chains of fatty acids, and the hydrogen atom of the third hydroxyl group is replaced by a phosphoric acid residue, to which, in turn, the residue of one of the nitrogen bases is attached.
The glycerophospholipid molecule can be divided into two parts: the head and tails. The head is hydrophilic (i.e., soluble in water), and the tails are hydrophobic (they repel water, but dissolve in organic solvents). Due to this structure, the glycerophospholipid molecule can be called amphiphilic, i.e., hydrophobic and hydrophilic at the same time.
Sphingophospholipids are similar in chemical structure to glycerophospholipids. But they differ from those mentioned above in that in their composition, instead of the glycerol residue, they have a sphingosine alcohol residue. Their molecules also have heads and ponytails.
In the picture below, the structure of the plasma membrane is clearly visible.
Proteins of the plasma membrane
As for the proteins that make up the plasma membrane, these are mainly glycoproteins.
Depending on the location in the shell, they can be divided into two groups: peripheral and integral. The first are those that are on the surface of the membrane, and the second are those that penetrate the entire thickness of the membrane and are inside the lipid layer.
Depending on the functions that proteins perform, they can be divided into four groups: enzymes, structural, transport and receptor.
All proteins that are in the structure of the plasma membrane are not chemically bound to phospholipids. Therefore, they can freely move in the main layer of the membrane, assemble into groups, etc. This is why the structure of the plasma membrane of the cell cannot be called static. It is dynamic, as it changes all the time.
What role does the cell membrane play?
The structure of the plasma membrane allows it to cope with five functions.
The first and main is the restriction of the cytoplasm. Due to this, the cell has a constant shape and size. This function is ensured by the fact that the plasma membrane is strong and flexible.
The second role is the provision of intercellular contacts. Due to their elasticity, the plasma membranes of animal cells can form outgrowths and folds at their junctions.
The next function of the cell membrane is transport. It is provided by special proteins. Thanks to them, the necessary substances can be transported into the cell, and unnecessary ones can be disposed of from it.
In addition, the plasma membrane performs an enzymatic function. It is also carried out thanks to proteins.
And the last function is the signal. Due to the fact that proteins under the influence of certain conditions can change their spatial structure, the plasma membrane can send cells signals.
Now you know everything about membranes: what is a membrane in biology, what are they, how are the plasma and organoid membranes arranged, and what functions do they perform.