What is the plasma membrane involved in? Semi-integral membrane proteins

It has a thickness of 8-12 nm, so it is impossible to examine it with a light microscope. The structure of the membrane is studied using an electron microscope.

The plasma membrane is formed by two layers of lipids - a bilipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes the lipids are located with their heads outward and tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are located on the surface of the membrane (external or internal), others penetrate the membrane.

Functions of the plasma membrane

The membrane protects the contents of the cell from damage, maintains the shape of the cell, selectively allows necessary substances into the cell and removes metabolic products, and also ensures communication between cells.

The barrier, delimiting function of the membrane is provided by a double layer of lipids. It prevents the contents of the cell from spreading, mixing with the environment or intercellular fluid, and prevents the penetration of dangerous substances into the cell.

A number of the most important functions of the cytoplasmic membrane are carried out by proteins immersed in it. With the help of receptor proteins, it can perceive various irritations on its surface. Transport proteins form the finest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins provide vital processes in the body itself.

Large food particles that are unable to pass through thin membrane channels enter the cell by phagocytosis or pinocytosis. The general name for these processes is endocytosis.

How does endocytosis occur - the penetration of large food particles into the cell?

The food particle comes into contact with the outer membrane of the cell, and an invagination forms at this point. Then the particle, surrounded by a membrane, enters the cell, a digestive vesicle is formed, and digestive enzymes penetrate into the resulting vesicle.

White blood cells that can capture and digest foreign bacteria are called phagocytes.

In the case of pinocytosis, the invagination of the membrane captures not solid particles, but droplets of liquid with substances dissolved in it. This mechanism is one of the main ways for substances to enter the cell.

Plant cells covered with a hard layer of cell wall on top of the membrane are not capable of phagocytosis.

The reverse process of endocytosis is exocytosis. Synthesized substances (for example, hormones) are packaged in membrane vesicles, approach the membrane, are built into it, and the contents of the vesicle are released from the cell. In this way, the cell can get rid of unnecessary metabolic products.

Universal biological membrane formed by a double layer of phospholipid molecules with a total thickness of 6 microns. In this case, the hydrophobic tails of the phospholipid molecules are turned inward, towards each other, and the polar hydrophilic heads are turned outward of the membrane, towards the water. Lipids provide the basic physicochemical properties of membranes, in particular their fluidity at body temperature. Embedded within this lipid bilayer are proteins.

They are divided into integral(permeate the entire lipid bilayer), semi-integral(penetrate up to half of the lipid bilayer), or surface (located on the inner or outer surface of the lipid bilayer).

In this case, protein molecules are located in a mosaic pattern in the lipid bilayer and can “float” in the “lipid sea” like icebergs, due to the fluidity of the membranes. According to their function, these proteins can be structural(maintain a certain membrane structure), receptor(form receptors for biologically active substances), transport(transport substances across the membrane) and enzymatic(catalyze certain chemical reactions). This is currently the most recognized fluid mosaic model biological membrane was proposed in 1972 by Singer and Nikolson.

Membranes perform a demarcation function in the cell. They divide the cell into compartments, in which processes and chemical reactions can occur independently of each other. For example, the aggressive hydrolytic enzymes of lysosomes, capable of breaking down most organic molecules, are separated from the rest of the cytoplasm by a membrane. If it is destroyed, self-digestion and cell death occurs.

Having a general structural plan, different biological cell membranes differ in their chemical composition, organization and properties, depending on the functions of the structures they form.

Plasma membrane, structure, functions.

Cytolemma is a biological membrane that surrounds the cell from the outside. This is the thickest (10 nm) and most complexly organized cell membrane. It is based on a universal biological membrane coated on the outside glycocalyx, and from the inside, from the side of the cytoplasm, submembrane layer(Fig. 2-1B). Glycocalyx(3-4 nm thick) is represented by the outer, carbohydrate regions of complex proteins - glycoproteins and glycolipids that make up the membrane. These carbohydrate chains play the role of receptors that ensure that the cell recognizes neighboring cells and intercellular substance and interacts with them. This layer also includes surface and semi-integral proteins, the functional regions of which are located in the supramembrane zone (for example, immunoglobulins). The glycocalyx contains histocompatibility receptors, receptors for many hormones and neurotransmitters.

Submembranous, cortical layer formed by microtubules, microfibrils and contractile microfilaments, which are part of the cell cytoskeleton. The submembrane layer maintains the shape of the cell, creates its elasticity, and ensures changes in the cell surface. Due to this, the cell participates in endo- and exocytosis, secretion, and movement.

The cytolemma performs many functions:

1) delimiting (the cytolemma separates, delimits the cell from the environment and ensures its connection with the external environment);

2) recognition by this cell of other cells and attachment to them;

3) recognition by the cell of the intercellular substance and attachment to its elements (fibers, basement membrane);

4) transport of substances and particles into and out of the cytoplasm;

5) interaction with signaling molecules (hormones, mediators, cytokines) due to the presence of specific receptors for them on its surface;

ensures cell movement (formation of pseudopodia) due to the connection of the cytolemma with the contractile elements of the cytoskeleton.

The cytolemma contains numerous receptors, through which biologically active substances ( ligands, signaling molecules, first messengers: hormones, mediators, growth factors) act on the cell. Receptors are genetically determined macromolecular sensors (proteins, glyco- and lipoproteins) built into the cytolemma or located inside the cell and specialized in the perception of specific signals of a chemical or physical nature. Biologically active substances, when interacting with a receptor, cause a cascade of biochemical changes in the cell, transforming into a specific physiological response (change in cell function).

All receptors have a general structural plan and consist of three parts: 1) supramembrane, which interacts with the substance (ligand); 2) intramembrane, carrying out signal transfer and 3) intracellular, immersed in the cytoplasm.

Types of intercellular contacts.

The cytolemma is also involved in the formation of special structures - intercellular connections, contacts, which ensure close interaction between adjacent cells. Distinguish simple And complex intercellular connections. IN simple At intercellular junctions, the cytolemmas of cells come closer to a distance of 15-20 nm and the molecules of their glycocalyx interact with each other (Fig. 2-3). Sometimes the protrusion of the cytolemma of one cell enters the recess of an adjacent cell, forming jagged and finger-like connections ("lock-type" connections).

Complex There are several types of intercellular connections: locking, interlocking And communication(Fig. 2-3). TO locking compounds include tight contact or locking zone. In this case, the integral proteins of the glycocalyx of neighboring cells form a kind of cellular network along the perimeter of neighboring epithelial cells in their apical parts. Thanks to this, the intercellular gaps are closed and delimited from the external environment (Fig. 2-3).

Rice. 2-3. Various types of intercellular connections.

Simple connection.
Tight connection.
Adhesive belt.
Desmosome.
Hemidesmosoma.
Slot (communication) connection.
Microvilli.

(According to Yu. I. Afanasyev, N. A. Yurina).

TO cohesive, anchoring connections include adhesive belt And desmosomes. Adhesive belt located around the apical parts of single-layer epithelial cells. In this zone, the integral glycoproteins of the glycocalyx of neighboring cells interact with each other, and submembrane proteins, including bundles of actin microfilaments, approach them from the cytoplasm. Desmosomes (adhesion spots)– paired structures with a size of about 0.5 microns. In them, the glycoproteins of the cytolemma of neighboring cells closely interact, and from the side of the cells in these areas, bundles of intermediate filaments of the cell cytoskeleton are woven into the cytolemma (Fig. 2-3).

TO communication connections include gap junctions (nexuses) and synapses. Nexuses have a size of 0.5-3 microns. In them, the cytolemmas of neighboring cells come closer to 2-3 nm and have numerous ion channels. Through them, ions can pass from one cell to another, transmitting excitation, for example, between myocardial cells. Synapses characteristic of nervous tissue and occur between nerve cells, as well as between nerve and effector cells (muscle, glandular). They have a synaptic cleft, where, when a nerve impulse passes, a neurotransmitter is released from the presynaptic part of the synapse, transmitting the nerve impulse to another cell (for more details, see the chapter “Nerve Tissue”).

Plasma membrane , or plasmalemma,- the most permanent, basic, universal membrane for all cells. It is a thin (about 10 nm) film covering the entire cell. The plasmalemma consists of protein molecules and phospholipids (Fig. 1.6).

Phospholipid molecules are arranged in two rows - with hydrophobic ends inward, hydrophilic heads towards the internal and external aqueous environment. In some places, the bilayer (double layer) of phospholipids is penetrated through and through by protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules penetrate the lipid bilayer halfway on one side or the other (semi-integral proteins). There are peripheral proteins on the surface of the membranes of eukaryotic cells. Lipid and protein molecules are held together due to hydrophilic-hydrophobic interactions.

Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not interconnected by covalent bonds and are able to move quite quickly in the plane of the membrane. Thanks to this, membranes can change their configuration, i.e., they have fluidity.

Membranes are very dynamic structures. They quickly recover from damage and also stretch and contract with cellular movements.

Membranes of different types of cells differ significantly both in chemical composition and in the relative content of proteins, glycoproteins, lipids in them, and, consequently, in the nature of the receptors they contain. Each cell type is therefore characterized by an individuality, which is determined mainly glycoproteins. Branched chain glycoproteins protruding from the cell membrane are involved in factor recognition external environment, as well as in mutual recognition of related cells. For example, an egg and a sperm recognize each other by cell surface glycoproteins, which fit together as separate elements of a whole structure. Such mutual recognition is a necessary stage preceding fertilization.

A similar phenomenon is observed in the process of tissue differentiation. In this case, cells similar in structure, with the help of recognition areas of the plasmalemma, are correctly oriented relative to each other, thereby ensuring their adhesion and tissue formation. Associated with recognition transport regulation molecules and ions through the membrane, as well as an immunological response in which glycoproteins play the role of antigens. Sugars can thus function as information molecules (like proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, and enzyme proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, provide a structural connection between the cytoskeleton and cell membranes, or serve as receptors for receiving and converting chemical signals from the environment.

The most important property of the membrane is also selective permeability. This means that molecules and ions pass through it at different speeds, and the larger the size of the molecules, the slower the speed at which they pass through the membrane. This property defines the plasma membrane as osmotic barrier. Water and gases dissolved in it have the maximum penetrating ability; Ions pass through the membrane much more slowly. The diffusion of water through a membrane is called by osmosis.

There are several mechanisms for transporting substances across the membrane.

Diffusion- penetration of substances through a membrane along a concentration gradient (from an area where their concentration is higher to an area where their concentration is lower). Diffuse transport of substances (water, ions) is carried out with the participation of membrane proteins, which have molecular pores, or with the participation of the lipid phase (for fat-soluble substances).

With facilitated diffusion special membrane transport proteins selectively bind to one or another ion or molecule and transport them across the membrane along a concentration gradient.

Active transport involves energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins that form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, which actively pumps Na + ions out while absorbing K - ions. Due to this, a higher concentration of K - and a lower concentration of Na + is maintained in the cell compared to the environment. This process requires ATP energy.

As a result of active transport using a membrane pump in the cell, the concentration of Mg 2- and Ca 2+ is also regulated.

During the process of active transport of ions into the cell, various sugars, nucleotides, and amino acids penetrate through the cytoplasmic membrane.

Macromolecules of proteins, nucleic acids, polysaccharides, lipoprotein complexes, etc. do not pass through cell membranes, unlike ions and monomers. Transport of macromolecules, their complexes and particles into the cell occurs in a completely different way - through endocytosis. At endocytosis (endo...- inward) a certain area of the plasmalemma captures and, as it were, envelops extracellular material, enclosing it in a membrane vacuole that arises as a result of invagination of the membrane. Subsequently, such a vacuole connects with a lysosome, the enzymes of which break down macromolecules into monomers.

The reverse process of endocytosis is exocytosis (exo...- out). Thanks to it, the cell removes intracellular products or undigested residues enclosed in vacuoles or pu-

zyryki. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. This is how digestive enzymes, hormones, hemicellulose, etc. are removed.

Thus, biological membranes, as the main structural elements of a cell, serve not just as physical boundaries, but are dynamic functional surfaces. Numerous biochemical processes take place on the membranes of organelles, such as active absorption of substances, energy conversion, ATP synthesis, etc.

Functions of biological membranes the following:

They delimit the contents of the cell from the external environment and the contents of organelles from the cytoplasm.

They ensure the transport of substances into and out of the cell, from the cytoplasm to organelles and vice versa.

Act as receptors (receiving and converting chemicals from the environment, recognizing cell substances, etc.).

They are catalysts (providing for near-membrane chemical processes).

Participate in energy conversion.

The cell membrane, also called plasmalemma, cytolemma or plasma membrane, is a molecular structure, elastic in nature, which consists of various proteins and lipids. It separates the contents of any cell from the external environment, thereby regulating its protective properties, and also ensures the exchange between the external environment and the immediate internal contents of the cell.

The plasmalemma is a partition located inside, directly behind the membrane. It divides the cell into certain compartments, which are directed to compartments or organelles. They contain specialized environmental conditions. The cell wall completely covers the entire cell membrane. It looks like a double layer of molecules.

Basics

The composition of the plasmalemma is phospholipids or, as they are also called, complex lipids. Phospholipids have several parts: a tail and a head. Experts call hydrophobic and hydrophilic parts: depending on the structure of the animal or plant cell. The areas called the head face the inside of the cell, and the tails face the outside. Plasmalemmas are invariable in structure and are very similar in different organisms; Most often, the exception may be archaea, whose partitions consist of various alcohols and glycerol.

Plasmalemma thickness approximately 10 nm.

There are partitions that are located on the outside or outside the part adjacent to the membrane - they are called superficial. Some types of protein can be unique contact points for the cell membrane and membrane. Inside the cell there is a cytoskeleton and an outer wall. Certain types of integral protein can be used as channels in ion transport receptors (in parallel with nerve endings).

If you use an electron microscope, you can obtain data on the basis of which you can construct a diagram of the structure of all parts of the cell, as well as the main components and membranes. The upper apparatus will consist of three subsystems:

complex supramembrane inclusion;
the supporting-contractile apparatus of the cytoplasm, which will have a submembrane part.

This apparatus includes the cytoskeleton of the cell. Cytoplasm with organelles and a nucleus is called the nuclear apparatus. The cytoplasmic or, in other words, plasma cell membrane is located under the cell membrane.

The word "membrane" comes from the Latin word membrum, which can be translated as "skin" or "sheath". The term was proposed more than 200 years ago and was more often used to refer to the edges of the cell, but during the period when the use of various electronic equipment began, it was established that plasma cytolemmas make up many different elements of the membrane.

Elements are most often structural, such as:

mitochondria;
lysosomes;
plastids;
partitions.

One of the first hypotheses regarding the molecular composition of the plasma membrane was put forward in 1940 by a British scientific institute. Already in 1960, William Roberts proposed the “Elementary Membrane” hypothesis to the world. She assumed that all cell plasmalemmas consist of certain parts and, in fact, are formed according to a general principle for all kingdoms of organisms.

In the early seventies of the 20th century, a lot of data was discovered, on the basis of which in 1972, scientists from Australia proposed a new mosaic-liquid model of cell structure.

Structure of the plasma membrane

The 1972 model is generally recognized to this day. That is, in modern science, various scientists working with the shell rely on the theoretical work “Structure of the biological membrane of the liquid-mosaic model.”

Protein molecules are associated with the lipid bilayer and completely penetrate the entire membrane - integral proteins (one of the common names is transmembrane proteins).

The shell contains various carbohydrate components that will look like a polysaccharide or saccharide chain. The chain, in turn, will be connected by lipids and protein. Chains connected by protein molecules are called glycoproteins, and by lipid molecules - glycosides. Carbohydrates are located on the outside of the membrane and function as receptors in animal cells.

Glycoprotein - represent a complex of supra-membrane functions. It is also called glycocalyx (from the Greek words glyk and kalix, which means “sweet” and “cup”). The complex promotes cell adhesion.

Functions of the plasma membrane

Barrier

Helps separate the internal components of the cell mass from those substances that are external. It protects the body from the entry of various substances that would be foreign to it, and helps maintain intracellular balance.

Transport

The cell has its own “passive transport” and uses it to reduce energy consumption. The transport function operates in the following processes:

endocytosis;
exocytosis;
sodium and potassium metabolism.

On the outer side of the membrane there is a receptor, at the site of which mixing of hormones and various regulatory molecules occurs.

Passive transport- a process in which a substance passes through a membrane without expending energy. In other words, the substance is delivered from an area of the cell with a high concentration to the side where the concentration will be lower.

There are two types:

Simple diffusion- is inherent in small neutral molecules H2O, CO2 and O2 and some hydrophobic organic substances with low molecular weight and, accordingly, pass through membrane phospholipids without problems. These molecules can penetrate the membrane until the concentration gradient is stable and unchanged.
Facilitated diffusion- characteristic of various hydrophilic molecules. They can also pass through the membrane according to a concentration gradient. However, the process will be carried out with the help of various proteins, which will form specific channels of ionic compounds in the membrane.

Active transport- this is the movement of various components through the membrane wall as opposed to a gradient. Such transfer requires significant expenditure of energy resources in the cell. Most often, active transport is the main source of energy consumption.

There are several varieties active transport with the participation of carrier proteins:

Sodium-potassium pump. Receipt of necessary minerals and trace elements by the cell.
Endocytosis- a process in which the cell captures solid particles (phagocytosis) or various drops of any liquid (pinocytosis).
Exocytosis- a process in which certain particles are released from a cell into the external environment. The process is a counterbalance to endocytosis.

The term "endocytosis" comes from the Greek words "enda" (from within) and "ketosis" (cup, container). The process characterizes the capture of external compounds by the cell and is carried out during the production of membrane vesicles. This term was coined in 1965 by Christian Bayles, a professor of cytology in Belgium, who studied the uptake of various substances by mammalian cells, as well as phagocytosis and pinocytosis.

Phagocytosis

Occurs when a cell captures certain solid particles or living cells. And pinocytosis is the process by which droplets of liquid are captured by a cell. Phagocytosis (from the Greek words "devourer" and "receptacle") is the process by which very small living objects are captured and absorbed, as well as solid parts of various single-celled organisms.

The discovery of the process belongs to the physiologist from Russia - Vyacheslav Ivanovich Mechnikov, who determined the process itself, while he conducted various tests with starfish and tiny daphnia.

The nutrition of unicellular heterotrophic organisms is based on their ability to digest and also capture various particles.

Mechnikov described the algorithm for the absorption of bacteria by amoeba and the general principle of phagocytosis:

adhesion - sticking of bacteria to the cell membrane;
absorption;
formation of a vesicle with a bacterial cell;
uncorking the bottle.

Based on this, the process of phagocytosis consists of the following stages:

The absorbed particle is attached to the membrane.
Surrounding the absorbed particle with a membrane.
Formation of a membrane vesicle (phagosome).
Detachment of a membrane vesicle (phagosome) into the interior of the cell.
Combination of phagosome and lysosome (digestion), as well as internal movement of particles.

Complete or partial digestion can be observed.

In the case of partial digestion, a residual body is most often formed, which will remain inside the cell for some time. Those residues that are undigested are removed (evacuated) from the cell by exocytosis. During the process of evolution, this phagocytosis predisposition function gradually became separated and passed from various single-celled cells to specialized cells (such as the digestive cell in coelenterates and sponges), and then to specialized cells in mammals and humans.

Lymphocytes and leukocytes in the blood are predisposed to phagocytosis. The process of phagocytosis itself requires large amounts of energy and is directly combined with the activity of the outer cell membrane and lysosome, where digestive enzymes are located.

Pinocytosis

Pinocytosis is the capture of a fluid containing various substances by the cell surface. The discovery of the phenomenon of pinocytosis belongs to the scientist Fitzgerald Lewis. This event took place in 1932.

Pinocytosis is one of the main mechanisms in which high-molecular compounds, for example, various glycoproteins or soluble proteins, enter the cell. Pinocytotic activity, in turn, is impossible without the physiological state of the cell and depends on its composition and the composition of the environment. We can observe the most active pinocytosis in amoeba.

In humans, pinocytosis is observed in intestinal cells, blood vessels, renal tubules, and also in growing oocytes. In order to depict the process of pinocytosis, which will be carried out using human leukocytes, a protrusion of the plasma membrane can be made. In this case, the parts will be unlaced and separated. The process of pinocytosis requires energy.

Stages of the pinocytosis process:

Thin growths appear on the outer cellular plasmalemma, which surround droplets of liquid.
This section of the outer shell becomes thinner.
Formation of a membrane vesicle.
The wall is breaking through (failing).
The vesicle moves in the cytoplasm and can merge with various vesicles and organelles.

Exocytosis

The term comes from the Greek words “exo” - external, external and “cytosis” - vessel, cup. The process involves the release of certain particles by the cell into the external environment. The process of exocytosis is the opposite of pinocytosis.

During the process of ecocytosis, bubbles of intracellular fluid emerge from the cell and move to the outer membrane of the cell. The contents inside the vesicles can be released out, and the cell membrane merges with the membrane of the vesicles. Thus, most macromolecular connections will occur in this way.

Exocytosis performs a number of tasks:

delivery of molecules to the outer cell membrane;
transportation throughout the cell of substances that will be needed for growth and increase in membrane area, for example, certain proteins or phospholipids;
releasing or connecting various parts;
removal of harmful and toxic products that appear during metabolism, for example, hydrochloric acid secreted by the cells of the gastric mucosa;
transport of pepsinogen, as well as signaling molecules, hormones or neurotransmitters.

Specific functions of biological membranes:

generation of an impulse occurring at the nerve level, inside the neuron membrane;
synthesis of polypeptides, as well as lipids and carbohydrates of the rough and smooth reticulum of the endoplasmic reticulum;
change in light energy and its conversion into chemical energy.

Video

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The basis of the structural organization of the cell is biological membranes. The plasma membrane (plasmalemma) is the membrane surrounding the cytoplasm of a living cell. Membranes are composed of lipids and proteins. Lipids (mainly phospholipids) form a double layer, in which the hydrophobic “tails” of the molecules face the inside of the membrane, and the hydrophilic ones face its surfaces. Protein molecules can be located on the outer and inner surface of the membrane, can be partially immersed in the lipid layer or penetrate through it. Most of the buried membrane proteins are enzymes. This is a fluid-mosaic model of the structure of the plasma membrane. Protein and lipid molecules are mobile, which ensures the dynamism of the membrane. The membranes also include carbohydrates in the form of glycolipids and glycoproteins (glycocalyx), located on the outer surface of the membrane. The set of proteins and carbohydrates on the surface of the membrane of each cell is specific and is a kind of indicator of the cell type.

Membrane functions:

Dividing. It consists in the formation of a barrier between the internal contents of the cell and the external environment.
Ensuring the exchange of substances between the cytoplasm and the external environment. Water, ions, inorganic and organic molecules enter the cell (transport function). Products formed in the cell are released into the external environment (secretory function).
Transport. Transport through the membrane can occur in different ways. Passive transport occurs without energy expenditure, by simple diffusion, osmosis, or facilitated diffusion with the help of carrier proteins. Active transport is carried out using carrier proteins and requires energy (for example, the sodium-potassium pump). Material from the site

Large molecules of biopolymers enter the cell as a result of endocytosis. It is divided into phagocytosis and pinocytosis. Phagocytosis is the capture and absorption of large particles by a cell. The phenomenon was first described by I.I. Mechnikov. First, substances adhere to the plasma membrane, to specific receptor proteins, then the membrane bends, forming a depression.

A digestive vacuole is formed. Substances that enter the cell are digested in it. In humans and animals, leukocytes are capable of phagocytosis. White blood cells absorb bacteria and other solid particles.

Pinocytosis is the process of capturing and absorbing droplets of liquid with substances dissolved in it. Substances adhere to membrane proteins (receptors), and a drop of solution is surrounded by a membrane, forming a vacuole. Pinocytosis and phagocytosis occur with the expenditure of ATP energy.

Secretory. Secretion is the release by a cell of substances synthesized in the cell into the external environment. Hormones, polysaccharides, proteins, fat drops are contained in vesicles bounded by a membrane and approach the plasmalemma. The membranes fuse and the contents of the vesicle are released into the environment surrounding the cell.
Connection of cells in tissue (due to folded outgrowths).
Receptor. Membranes contain a large number of receptors - special proteins whose role is to transmit signals from outside to inside the cell.

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