Plasma Membrane - Chapter 2 PDF

Université Saad Dahleb Blida-1 Faculté des sciences biologiques (SNV) Chapter II PLASMA MEMBRANE (CELL MEMBRANE)  I. DEFINITION The plasma membrane is a dynamic structure that separates the intracellular medium (hyaloplasm or cytosol) f...

Université Saad Dahleb Blida-1 Faculté des sciences biologiques (SNV) Chapter II PLASMA MEMBRANE (CELL MEMBRANE)  I. DEFINITION The plasma membrane is a dynamic structure that separates the intracellular medium (hyaloplasm or cytosol) from the extracellular medium.  The currently accepted model for the structure of the plasma membrane, called the fluid mosaic model. The membrane is a complex set of lipids, proteins and sugars that regulate the exchange between the cell and its environment. The key components of the biological membrane are phospholipids.  II. STRUCTURE AND ULTRASTRUCTURE 1. Photonic microscopy the plasma membrane appears as a thin line or as a dense area that separates the intracellular medium from the extracellular medium. 2. Transmission Electron Microscopy (TEM) The observation of thin sections at high magnification shows that the membrane is 75 A◦ thick, formed of three sheets or layers: two dark lipophobic layers separated by a light lipophilic layer.  The dense outer layer is covered by a thin glycoprotein film, the fibrils of which are perpendicular to the membrane: this is the "cell coat" or glycocalyx. This is responsible for the asymmetry of the membrane. 3. Scanning Electron Microscope (SEM) Observation of replicas: after cryostripping, observation of the replica shows the plasma membrane cleaved into two hemi-membranes: an outer exoplasmic hemi-membrane and an inner hemi-membrane on the hyaloplasmic side.  III. CHEMICAL COMPOSITION According to the fluid mosaic model, the cell membrane is made up mainly of lipids, proteins and small proportions of carbohydrates. The lipids (mainly phosphoglycerolipids) are organized in a bilayer. the membrane thus presents a two-dimensional fluid structure, made up of a "mosaic" of various proteins incorporated into or attached to this bilayer. Membrane carbohydrates always exist in the form of residues associated with proteins (glycoproteins) or lipids (glycolipids) and oriented towards the outside of the cell. These proportions vary from cell to cell, but in general the protein/lipid mass ratio is close to 1 for all plasma membranes ; whereas most endo-membranes are richer in protein.  1-Membrane lipids Lipids give the membrane its skeleton and characteristic structure. They have the property of associating via hydrophobic bonds to form a lipid double layer by self-assembly. On average, lipids have a much lower molar mass than proteins. Depending on the type of membrane, there are between 10 and 100 lipid molecules for one protein molecule. Lipids therefore make up the vast majority of membrane molecules.  1-1. Different classes of membrane lipids There are three main categories of membrane lipids: phospholipids, glycolipids and sterols (cholesterol). A: Phospholipids Phospholipids all have a hydrophilic head (phosphate and specialised group) and a hydrophobic tail (glycerol and fatty acids). The most classic example of a phospholipid is phosphatidylcholine.  B: Cholesterol It is only present in the membranes of animal cells; in fact, it is absent from plant cells (it is replaced by other types of sterols: sitosterol and stigmasterol) and bacteria. It represents ¼ of membrane lipids. It is made up of a polar group and a steroid group. Changes in its proportions affect membrane fluidity. C: Glycolipids Glycolipids 5% membrane components, essentially resulting from the esterification or amidification of fatty acids by oses or sugars. Two types: glyceroglycolipid and sphingolipid  1-2.Membrane lipid properties 1. Fluidity The fluidity of the membrane depends on the unsaturation of the phospholipids, the amount of cholesterol and the temperature. Unsaturation of hydrocarbon chains increases membrane fluidity. 2. Self-assembly Phospholipids can be organised or assembled into bilayers thanks to their amphiphilic or bipolar nature (hydrophilic head and hydrophobic tail). 3. Mechanical stability Cholesterol reinforces the solidity of the membrane. It is placed between phospholipid molecules and immobilises neighbouring hydrocarbon chains, making the membrane more rigid. 4. Asymmetry of the lipid bilayer the distribution of lipids between the inner and outer layers is asymmetrical. 1.3 Membrane lipid functions Lipids determine the basic structure (bilayer) that is fundamental to the organisation of all biological membranes. They form an impermeable barrier to water-soluble molecules.  2- Membrane proteins They have an extracellular amino terminus (-NH2) and an intracellular carboxyl terminus (COOH) and a hydrophobic body. 2-1.Membrane proteins type Depending on their location in relation to the lipid bilayer, two types of membrane protein can be described: A-Integral proteins (transmembrane or intramembrane) cross the membrane from one side to the other. Their N-terminal domain often carries glycans located outside the plasma membrane. The transmembrane (hydrophobic) domain consists only of apolar amino acid residues. The hydrophilic parts are exposed to aqueous solutions on either side of the membrane. B-Peripheral, external or internal proteins or proteins associated with an integral protein.  2-2. Membrane proteins properties 1. Modes of organisation Membrane proteins have two modes of organisation, either integrated or peripheral. 2. Fluidity: protein movements are less frequent, due to the large size of these molecules compared with lipid molecules. They are slow and mainly represent lateral diffusion movement within the lipid bilayer. 3. Asymmetry: the distribution of plasma membrane proteins is different on the two sides, which determines their role in relation to the extracellular matrix (ECM) on the one hand and the cytoskeleton on the other.  2-3. Main roles of membrane proteins They act as 1. receptors and transporters (of matter or information), 2. recognition and adhesion between cells attachment to neighbouring cells or the extracellular matrix, 3. capture of physical energy (light), 4. enzymatic catalysts. 3-Membrane carbohydrates 3-1.Membrane carbohydrates type Cell membrane carbohydrates such as glycolipids, glycoproteins, and proteoglycans are found on the outside surface of the cell to form the cell-coat. 1. Glycolipids Glycolipids are membrane carbohydrates linked to lipids. These carbohydrate chains help in cell-to-cell recognition. 2. Glycoproteins Glycoproteins are membrane carbohydrates linked to proteins. These compounds also help with cell-to-cell recognition. 3. Proteoglycane Proteoglycane are long carbohydrate chains linked to a protein that is embedded in the cell membrane. These structures regulate interactions between components in the cell and communication between receptors. They also control the growth and production of cells. 3.2.Glycocalyx functions 1. Cell protection, 2. Adhesion between neighbouring cells and/or between cell and extracellular matrix, 3. Cell specificity: marker for certain cells (e.g. blood group antigens), 4. Recognition between cells for tissue organisation, 5. Contact inhibition: controls cell division.  VI.FUNCTIONS OF PLASMA MEMEBRANE 1. Physical Barrier The plasma membrane surrounds all cells and physically separates the cytoplasm, which is the material that makes up the cell, from the extracellular fluid outside the cell. This protects all the components of the cell from the outside environment and allows separate activities to occur inside and outside the cell. 2. Cell Signaling Another important function of the membrane is to facilitate communication and signaling between cells. It does so through the use of various proteins and carbohydrates in the membrane. Proteins on the cell “mark” that cell so that other cells can identify it. The membrane also has receptors that allow it to carry out certain tasks when molecules such as hormones bind to those receptors.  3.Permeability Generally, ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane; they must go through specific channels or pores in the membrane instead of freely diffusing through. This way, the membrane can control the rate at which certain molecules can enter and exit the cell. Membrane transport can take place : Without membrane movement (passive or active transport) With membrane movements (vesicular traffic).  3.1.Transport without membrane mouvements 1-Passive transport Molecules can cross the double layer by spontaneous movement towards equilibrium without energy input, in the direction of the concentration gradient. This is the movement of molecules from an area where they are in high concentration to an area where they are in low concentration, so it assumes a concentration gradient: a) Simple diffusion b) Facilitated diffusion: ✓Transport through pores ✓Transporters or permeases  1.1-Simple diffusion This type of passage is only possible if the molecule is soluble in the phospholipid membrane, i.e. if it can pass directly through the phospholipid bilayer. ▪The molecule must therefore be hydrophobic. ▪Non-polar substances such as oxygen and waste products such as CO2, urea and fats diffuse through the plasma membrane by binding to its phospholipid compounds.  1.2. Facilitated diffusion This is always passive, but involves proteins. It is a specific and regulated phenomenon. A-Transport through pores The pores that allow more specific ions to pass through are called ion channels (Na+, cl-) whose transport takes place according to their electrochemical gradient and does not require energy.  B-Transporters or permeases/ carriers These are transmembrane proteins that bind the molecule to be transported in a specific way. The "permease" changes conformation and releases the molecule to be transported on the other side of the membrane.  2-Active transport A transport mechanism that involves the movement of molecules across the membrane against the concentration gradient, meaning the transport of substances from low to high concentration. This movement is achieved by expending the energy of ATP (adenosine triphosphate). Two types of active transport are known:  2. 1.Pumps (primary transporters) Use the energy from ATP hydrolysis to transport the molecule across the plasma membrane (ATPases); for example : Sodium-Potassium Pump, also known as the Na+/K+ pump or Na+/K+-ATPase, is a transmembrane protein pump found in animals’ cell (plasma) membrane. Its fundamental purpose is to transport sodium and potassium ions across the cell in the ratio of 3: 2 2.2. The Ca++/ATPase pump Same principle as the Sodium pump, it exists in the plasma membrane of the endoplasmic reticulum.  3- Secondary carriers (or co-transporters) These are transmembrane proteins which couple the passage of the molecule with that of an ion (generally H+ and Na+): the energy, which comes from moving the ion along its electrochemical gradient, causes the other substance to move against its own gradient (ATP is not used). There are 3 types: 2.1. Uniport Carrer prorein that carries only one substance in a single direction is called uniport. It also known as uniport pump  2. 2.Symport or antiport. Symport or antiport is the carrier protein that transports two substances at a time.. carrier protein that transports two different substances in the same direction is called symport pump.. carrier protein that transports two different substances in opposite direction is called antiport or antiport pump.  3.2. Other ways to cross the membrane Some molecules (e.g. proteins) and particles are too large for membrane transport. Transporting these molecules therefore requires movement of the plasma membrane to evacuate/ingestrate them: Exocytosis Endocytosis 1-Endocytosis Process by which a cell absorbs particles or solutes by enclosing them in vesicles through invagination of the plasma membrane. There are several types of endocytosis, depending on the substances ingested and their size: ▪ Phagocytosis ▪ Pinocytosis ▪ Receptor-mediated endocytosis  1.1. Phagocytosis Phagocytosis is a type of endocytosis that involves uptake of large solid particles, often >0.5 mm.  1.2.Pinocytosis Ingestion of suspended molecules taken from the extracellular environment (e.g. lipid droplets). This is a frequent phenomenon in most cells (especially kidneys and intestines).  1.3. Receptor-mediated endocytosis Selective endocytosis requires membrane receptors specific to the molecule to be ingested. The molecule/receptor complex is then endocytosed and localized in a vesicle: the early endosome. 2-Exocytosis Exocytosis is the process by which cells excrete waste and other large molecules from the cytoplasm to the cell exterior and therefore is the opposite of endocytosis. There are two types of exocytosis: ✓Constitutive or renewal exocytosis ✓Regulated exocytosis  2.1. Constitutive or renewal exocytosis Exocytosis moves elements from the inside to the outside. Constitutive exocytosis, which occurs in all cells, during which vesicles continuously transport neo-synthesized molecules to the plasma membrane.  2.2. Regulated exocytosis Functions in specialized cells in response to a stimulus; example of Ca+2 ion-induced exocytosis.  V. INTERCELLULAR INFORMATION TRANSMISSION 1-Hormonal information Although carried by the blood to all parts of the body, hormones only act on specific target cells. They influence their targets by chemically binding to specific receptors, which are proteins. Only the target cells of a hormone have the receptors to recognise and bind to it. Two types of hormone are involved: Lipid-soluble hormones peptide hormones  1.1. Lipid-soluble hormone Iinclude steroid hormones, thyroid hormones and nitric oxide (NO). These hormones are able to diffuse across the membrane of the target cell and directly produce a biological response by acting on gene activity.  1.2. Peptide hormones Include amino hormones, peptide hormones and protein hormones. As they are not fat-soluble, these hormones do not diffuse through the lipid bilayer of the plasma membrane. Instead, these hormones bind to receptors from the surface of target cells. There, they act as a first messenger, triggering the production of a second messenger inside the cell, where the specific hormonal responses take place. Cyclic AMP (cAMP), which is synthesised from ATP, frequently serves as the second messenger.  2-Neurotransmission The presynaptic neuron releases neurotransmitter molecules by exocytosis from the synaptic vesicles. After diffusing across the synaptic cleft, the neurotransmitter molecules bind to receptors in the plasma membrane of the postsynaptic neuron, generating a postsynaptic potential. As action potentials cannot cross the synaptic cleft, an indirect form of communication is established there. In general, chemical synapses function as follows: 1- The action potential arrives in a terminal bouton of a pre-synaptic axon. 2-The depolarisation phase of the action potential opens voltage-dependent Ca2+ channels in the terminal bouton membrane. Because they are more concentrated in the interstitial fluid, calcium ions enter the terminal bouton via the open channels. 3- The increase in Ca2+ concentration inside the terminal bouton acts as a signal that triggers the exocytosis of some synaptic vesicles, which release neurotransmitter molecules into the synaptic cleft. Each synaptic vesicle contains several thousand neurotransmitter molecules. 4- Neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors located in the plasma membrane of the postsynaptic neuron. 5- The binding of neurotransmitter molecules causes ion channels to open, allowing certain ions to flow across the membrane. 6- The membrane voltage changes as the ions pass through the open channels. This change in membrane voltage is a postsynaptic potential. Depending on the type of ions passing through the channels, the postsynaptic potential may be a depolarisation or hyperpolarisation of the postsynaptic membrane. 7- If it reaches the excitation threshold, the depolarising postsynaptic potential triggers an action potential. 3-Neuromuscular junction (motor plate) A neuromuscular junction comprises the terminal boutons of a motor neuron adjacent to the motor plate of a myocyte. When it produces an action potential (or nerve impulse), a motor neuron triggers a muscle action potential in the myocytes to which it is connected. What happens at a terminal button can be summarised as follows.  Release of acetylcholine: The arrival of the action potential at the terminal bouton triggers the release of acetylcholine (ACh), the neurotransmitter contained in the synaptic vesicles. ACh then diffuses into the synaptic gap between the motor neuron and the motor plate. Activation of acetylcholine receptors: At the motor plate, ion channels are inserted into the sarcolemma of the myocyte. ACh receptors are located on these channels. Binding of ACh to the receptors opens the normally closed ion channel, allowing small cations, mainly sodium ions (Na+), to pass through the membrane. Production of the muscle action potential: The arrival of Na+ in the myocyte triggers a muscle action potential. Normally, each action potential gives rise to a muscle action potential which propagates to the surface of the sarcolemma and inside the tubules. In response, the sarcoplasmic reticulum releases stored Ca2+ ions into the sarcoplasm. The diffusion of Ca2+ into the cytosol then causes the myocyte to contract. Degradation of Ach: The effect of ACh is short-lived because this neurotransmitter is rapidly degraded in the synaptic cleft by an enzyme called acetylcholinesterase (AChE).