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 - the lipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes, lipids are located with heads outward, tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are 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 passes the necessary substances into the cell and removes metabolic products, and also provides communication between cells.

The barrier, delimiting function of the membrane provides a double layer of lipids. It does not allow the contents of the cell to spread, mix 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 due to the proteins immersed in it. With the help of receptor proteins, it can perceive various irritations on its surface. Transport proteins form the thinnest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins - provide vital processes in itself.

Large food particles that are not able to pass through thin membrane channels enter the cell by phagocytosis or pinocytosis. The common 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 in this place. Then the particle, surrounded by a membrane, enters the cell, a digestive one is formed, and digestive enzymes penetrate into the formed vesicle.

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

In the case of pinocytosis, the invagination of the membrane does not capture solid particles, but droplets of liquid with substances dissolved in it. This mechanism is one of the main pathways for the penetration of substances into the cell.

Plant cells covered over the membrane with a solid layer of the cell wall are not capable of phagocytosis.

The reverse process of endocytosis is exocytosis. Synthesized substances (for example, hormones) are packed into membrane vesicles, approach, are embedded in it, and the contents of the vesicle are ejected from the cell. Thus, the cell can also 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 main physicochemical properties of membranes, in particular, their fluidity at body temperature. Proteins are embedded in this lipid double layer.

They are subdivided 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).

At the same time, protein molecules are located in the lipid bilayer mosaically and can "swim" 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 structure of the membrane), receptor(to form receptors for biologically active substances), transport(carry out the transport of substances through the membrane) and enzymatic(catalyze certain chemical reactions). This is currently the most recognized fluid mosaic model The biological membrane was proposed in 1972 by Singer and Nikolson.

Membranes perform a delimiting function in the cell. They divide the cell into compartments, compartments in which processes and chemical reactions can proceed independently of each other. For example, the aggressive hydrolytic enzymes of lysosomes, which are able to break down most organic molecules, are separated from the rest of the cytoplasm by a membrane. In the event of its destruction, self-digestion and cell death occur.

Having a common 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.

The cytolemma is the biological membrane that surrounds the outside of the cell. This is the thickest (10 nm) and complexly organized cell membrane. It is based on a universal biological membrane, covered 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 sections 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 sites of which are located in the supramembrane zone (for example, immunoglobulins). The glycocalyx contains histocompatibility receptors, receptors for many hormones and neurotransmitters.

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

Cytolemma fulfills a bunch of 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;

provides 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, signal 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 the receptor, cause a cascade of biochemical changes in the cell, while transforming into a specific physiological response (change in cell function).

All receptors have a common structural plan and consist of three parts: 1) supramembrane, which interacts with a 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 provide close interaction between adjacent cells. Distinguish simple And complex intercellular connections. IN simple At intercellular junctions, the cytolemmas of cells approach each other at 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 depression of the neighboring cell, forming serrated and finger-like connections (connections "like a lock").

Complex intercellular connections are of several types: locking, fastening And communication(Fig. 2-3). TO locking compounds include tight contact or blocking zone. At the same time, the integral proteins of the glycocalyx of neighboring cells form a kind of mesh network along the perimeter of neighboring epithelial cells in their apical parts. Due to this, intercellular gaps are locked, delimited from the external environment (Fig. 2-3).

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

Simple connection.
Tight connection.
Adhesive band.
Desmosome.
Hemidesmosome.
Slotted (communication) connection.
Microvilli.

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

TO linking, anchoring compounds include adhesive belt And desmosomes. Adhesive band located around the apical parts of the cells of a single-layer epithelium. In this zone, the integral glycocalyx glycoproteins of neighboring cells interact with each other, and submembrane proteins, including bundles of actin microfilaments, approach them from the cytoplasm. Desmosomes (adhesion patches)– paired structures about 0.5 µm in size. 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 refer gap junctions (nexuses) and synapses. Nexuses have a size of 0.5-3 microns. In them, the cytolemmas of neighboring cells converge up 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 the nervous tissue and are found between nerve cells, as well as between nerve and effector cells (muscle, glandular). They have a synaptic cleft, where, when a nerve impulse passes from the presynaptic part of the synapse, a neurotransmitter is released that transmits a nerve impulse to another cell (for more details, see the chapter "Nervous tissue").

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

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

Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not linked by covalent bonds and are able to move quite quickly in the plane of the membrane. Due to this, the 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.

The membranes of different cell types differ significantly both in chemical composition and in the relative content of proteins, glycoproteins, and lipids in them, and, consequently, in the nature of the receptors present in them. Each cell type is therefore characterized by an individuality that is determined mainly glycoproteins. Branched chain glycoproteins protruding from the cell membrane are involved in factor recognition external environment, as well as in the mutual recognition of related cells. For example, an egg and a sperm cell recognize each other by cell surface glycoproteins that 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 recognizing sections of the plasmalemma correctly orient themselves 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 informational molecules (similar to proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, enzymatic proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, carry out the structural connection of the cytoskeleton with 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 their passage through the membrane. This property defines the plasma membrane as osmotic barrier. Water and the gases dissolved in it have the maximum penetrating power; ions pass through the membrane much more slowly. The diffusion of water across a membrane is called osmosis.

There are several mechanisms for the transport of substances across the membrane.

Diffusion- penetration of substances through the membrane along the concentration gradient (from the area where their concentration is higher to the 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 carrier proteins selectively bind to one or another ion or molecule and carry them across the membrane along a concentration gradient.

active transport is associated with energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins, which form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, actively pumping out Na + ions, while absorbing K - ions. Due to this, a large concentration of K - and a lower Na + in comparison with the environment are maintained in the cell. This process consumes the energy of ATP.

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

In 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. The transport of macromolecules, their complexes and particles into the cell occurs in a completely different way - through endocytosis. At endocytosis (endo...- inside) a certain section of the plasmalemma captures and, as it were, envelops the extracellular material, enclosing it in a membrane vacuole that has arisen as a result of the invagination of the membrane. Subsequently, such a vacuole is connected to a lysosome, the enzymes of which break down macromolecules to monomers.

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

bubbles. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. How digestive enzymes, hormones, hemicellulose, etc. are excreted.

Thus, biological membranes, as the main structural elements of the cell, serve not just as physical boundaries, but as dynamic functional surfaces. On the membranes of organelles, numerous biochemical processes are carried out, 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 the organelles from the cytoplasm.

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

They play the role of receptors (receiving and converting signals from the environment, recognition of cell substances, etc.).

They are catalysts (providing membrane chemical processes).

Participate in the transformation of energy.

The cell membrane, also called the plasmalemma, cytolemma, or plasma membrane, is a molecular structure that is elastic in nature and is made up of various proteins and lipids. It separates the content of any cell from the external environment, thereby regulating its protective properties, and also provides an exchange between the external environment and the directly internal contents of the cell.

The plasmalemma is a septum located inside, directly behind the shell. 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.

Basic information

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 an animal or plant cell. The sections, which are called the head, face the inside of the cell, and the tails face the outside. Plasmalemms are structurally invariable and very similar in different organisms; the most common exception may be archaea, in which the partitions consist of various alcohols and glycerol.

Plasmalemma thickness approximately 10 nm.

There are partitions that are on the outside or outside of the part adjacent to the membrane - they are called superficial. Some types of protein can be a kind of contact points for the cell membrane and shell. Inside the cell is the cytoskeleton and the 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 get data on the basis of which you can build 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 musculoskeletal apparatus of the cytoplasm, which will have a submembrane part.

This apparatus can be attributed to the cytoskeleton of the cell. The cytoplasm with organelles and the 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 "shell". The term was proposed more than 200 years ago and was more often called 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 plasmalemma was put forward in 1940 by a scientific institute in Great Britain. Already in 1960, William Roberts proposed to the world the hypothesis "On the elementary membrane". She assumed that all plasma membranes of a cell consist of certain parts, in fact, they are formed according to a general principle for all kingdoms of organisms.

In the early seventies of the XX 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.

The structure of the plasma membrane

The 1972 model is universally recognized to this day. That is, in modern science, various scientists working with the shell rely on the theoretical work "The structure of the biological membrane of the fluid-mosaic model."

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

The shell in the composition has 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 lipid molecules are called glycosides. Carbohydrates are located on the outer side of the membrane and act as receptors in animal cells.

Glycoprotein - are a complex of supra-membrane functions. It is also called glycocalyx (from the Greek words glik and kalyx, which means "sweet" and "cup"). The complex promotes cell adhesion.

Functions of the plasma membrane

Barrier

Helps to separate the internal components of the cell mass from those substances that are outside. Protects the body from the ingress of various substances that will be alien 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 works in the following processes:

endocytosis;
exocytosis;
sodium and potassium metabolism.

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

Passive transport A process in which a substance passes through a membrane without the expenditure of 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- inherent in small neutral molecules H2O, CO2 and O2 and some hydrophobic organic substances with a low molecular weight and, accordingly, pass through membrane phospholipids without problems. These molecules can permeate the membrane until the concentration gradient is stable and unchanged.
Facilitated diffusion- characteristic of various molecules of the hydrophilic type. They can also pass through the membrane following a concentration gradient. However, the process will be carried out with the help of various proteins that 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 a transfer requires a significant expenditure of energy resources in the cell. Most often, it is active transport that is the main source of energy consumption.

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

Sodium-potassium pump. Obtaining the 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- the process by which certain particles are released from the 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, receptacle). The process characterizes the capture of the external composition by the cell and is carried out during the production of membrane vesicles. This term was proposed in 1965 by Belgian professor of cytology Christian Bales, who studied the absorption 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 a process in which liquid droplets are captured by the cell. Phagocytosis (from the Greek words "devourer" and "receptacle") is the process by which very small objects of wildlife are captured and consumed, as well as solid parts of various unicellular organisms.

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

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

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

adhesion - adhesion of bacteria to the cell membrane;
absorption;
the formation of a vesicle with a bacterial cell;
bubbling of the bubble.

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

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

Full or partial digestion can be observed.

In the case of partial digestion, most often a residual body is formed, which will remain inside the cell for some time. Those residues that will not be digested are withdrawn (evacuated) from the cell by exocytosis. In the course of evolution, this phagocytic propensity function gradually separated and moved from various single-celled cells to specialized cells (such as digestive in coelenterates and sponges), and then to special cells in mammals and humans.

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

pinocytosis

Pinocytosis is the capture by the surface of a cell of a liquid in which various substances are located. 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 by which macromolecular compounds enter the cell, for example, various glycoproteins or soluble proteins. 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, in vessels, renal tubules, and also in growing oocytes. In order to depict the process of pinocytosis, which will be carried out with the help of human leukocytes, a protrusion of the plasma membrane can be made. In this case, the parts will be laced and separated. The process of pinocytosis requires the expenditure of energy.

Steps in the process of pinocytosis:

Thin outgrowths appear on the outer cellular plasmalemma, which surround the drops of liquid.
This section of the outer shell becomes thinner.
Formation of a membranous vesicle.
The wall breaks through (fails).
The vesicle travels in the cytoplasm and can fuse with various vesicles and organelles.

Exocytosis

The term comes from the Greek words "exo" - external, external and "cytosis" - a vessel, a bowl. The process consists in the release of certain particles by the cellular part into the external environment. The process of exocytosis is the opposite of pinocytosis.

In the process of ecocytosis, bubbles of intracellular fluid leave the cell and pass to the outer membrane of the cell. The contents inside the vesicles can be released to the outside, and the cell membrane merges with the shell of the vesicles. Thus, most macromolecular compounds 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 an increase in the area of the membrane, for example, certain proteins or phospholipids;
release or connection of various parts;
excretion 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 that occurs at the nerve level, inside the neuron membrane;
synthesis of polypeptides, as well as lipids and carbohydrates of the rough and smooth network of the endoplasmic reticulum;
change in light energy and its conversion into chemical energy.

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Biological membranes form the basis of the structural organization of the cell. The plasma membrane (plasmalemma) is the membrane that surrounds the cytoplasm of a living cell. Membranes are made up of lipids and proteins. Lipids (mainly phospholipids) form a double layer in which the hydrophobic "tails" of the molecules face inside the membrane, and the hydrophilic tails - to its surfaces. Protein molecules can be located on the outer and inner surface of the membrane, they can be partially immersed in the lipid layer or penetrate it through. Most of the immersed 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 contain carbohydrates in the form of glycolipids and glycoproteins (glycocalix) located on the outer surface of the membrane. The set of proteins and carbohydrates on the membrane surface of each cell is specific and is a kind of cell type indicator.

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 (secretory function) are excreted into the external environment.
Transport. Transport across the membrane can take place in different ways. Passive transport is carried out without energy expenditure, by simple diffusion, osmosis or facilitated diffusion with the help of carrier proteins. Active transport is by carrier proteins and requires energy input (eg 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 the cell. The phenomenon was first described by I.I. Mechnikov. First, substances adhere to the plasma membrane, to specific receptor proteins, then the membrane sags, forming a depression.

A digestive vacuole is formed. It digests the substances that have entered the cell. In humans and animals, leukocytes are capable of phagocytosis. Leukocytes engulf bacteria and other solid particles.

Pinocytosis is the process of capturing and absorbing liquid droplets 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 - the release by the cell of substances synthesized in the cell into the external environment. Hormones, polysaccharides, proteins, fat droplets are enclosed in membrane-bound vesicles and approach the plasmalemma. The membranes merge, and the contents of the vesicle are released into the environment surrounding the cell.
Connection of cells in tissue (due to folded outgrowths).
Receptor. There are a large number of receptors in membranes - special proteins, the role of which is to transmit signals from the outside to the inside of the cell.

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