Cells of plants, fungi and animals consist of such three components as the nucleus, the cytoplasm with organelles and inclusions located in it, and the plasma membrane. The nucleus is responsible for storing the genetic material recorded on DNA, and also controls all the processes of the cell. The cytoplasm contains organelles, 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, it's a shell. However, it is not always completely impenetrable. Transport of certain substances through the membrane is almost always allowed.
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. Single-membrane include the Golgi complex, endoplasmic reticulum, vacuoles, lysosomes. Plastids and mitochondria belong to the two-membrane ones.
Membranes can also be inside organelles. Usually they are derivatives of the inner membranetwo-membrane organelles.
How are the membranes of two-membrane organelles arranged?
Plastids and mitochondria have two shells. The outer membrane of both organelles is smooth, but the inner one forms the structures necessary for the functioning of the organoid.
Thus, the shell of mitochondria has protrusions inward - cristae or ridges. The cycle of chemical reactions necessary for cellular respiration takes place on them.
Derivatives of the inner membrane of chloroplasts are disk-shaped sacs - thylakoids. They are collected in stacks - grains. Separate grana are combined with each other with the help of lamellae - long structures also formed from membranes.
Structure of membranes of single-membrane organelles
These organelles have only one membrane. It is usually a smooth shell of lipids and proteins.
Features of the structure of the plasma membrane of the cell
The membrane is made up 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 is 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
Lipids that make up the plasma membrane are divided into three groups: steroids, sphingophospholipids and glycerophospholipids. The molecule of the latter contains a trihydric alcohol residueglycerol, 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 nitrogenous bases is attached.
A glycerophospholipid molecule can be divided into two parts: a head and tails. The head is hydrophilic (that is, it dissolves in water), and the tails are hydrophobic (they repel water, but dissolve in organic solvents). Due to this structure, the molecule of glycerophospholipids can be called amphiphilic, i.e. both hydrophobic and hydrophilic at the same time.
Sphingophospholipids are chemically similar to glycerophospholipids. But they differ from those mentioned above in that in their composition, instead of a glycerol residue, they have a sphingosine alcohol residue. Their molecules also have heads and tails.
The picture below clearly shows the structure of the plasma membrane.
Plasma membrane proteins
As for the proteins that make up the plasma membrane, these are mainly glycoproteins.
Depending on their 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 associated with phospholipids. Therefore, they can move freely in the main layer of the membrane, gather in groups, etc. That 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 is the role of the cell wall?
The structure of the plasma membrane allows it to perform five functions.
First and foremost - 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 elastic.
The second role is to ensure 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 also comes from proteins.
And the last function is signal. Due to the fact that proteins under the influence of certain conditions can change their spatial structure, the plasma membrane can send signals to cells.
Now you know everything about membranes: whatsuch a membrane in biology, what they are, how the plasma membrane and membranes of organelles are arranged, what functions they perform.
