Plasma Membranes-2023 PDF

PLASMA MEMBRANES-2023 INTRODUCTION Plasma membrane forms the outer most layer in animal cell and located immediately beneath the cell wall in plant cell. The plasma membrane separates the intracellular components (cytoplasm) from extracellular components (outside the cytoplasm). It is a ph...

PLASMA MEMBRANES-2023 INTRODUCTION Plasma membrane forms the outer most layer in animal cell and located immediately beneath the cell wall in plant cell. The plasma membrane separates the intracellular components (cytoplasm) from extracellular components (outside the cytoplasm). It is a phospholipid bilayer in which protein molecules are either partially (extrinsic) or wholly embedded (intrinsic). It is an amphipathic molecule with both hydrophillic region (water loving) and hydrophobic region (water fearing).  The polar heads face outside where water is found and the non polar tail face each other in the interior of the bilayer. A plasma membrane is asymmetrical with carbohydrates chains attached to the outside surface and project into the extracellular matrix. Cytoskeleton filaments are on the inside surface. FLUID-MOSAIC MODEL The membrane is said to be a fluid-mosaic because of the variety of macromolecules that make up its structure. The principal component of plasma membrane are phospholipids, integral proteins, peripheral proteins, glycoproteins, glycolipids, cholestrol and sometimes lipoprotein. The proportion of protein, lipids and carbohydrates vary with cell type. In a typical human cell protein account for 50% of the Plasma membrane mass is protein , lipids about 40% and carbohydrates about 10 % PHOSPHOLIPIDS Provides the principle physical properties of a cell membrane. They are arranged in two layer (bilayers) with hydrophilic head of phospholipids molecules directed away from each other. The hydrophobic lipid tails are located in the interior. Unsaturated fatty acids (kink nature) in the phospholipid molecule helps to maintain the fluidity of plasma membrane. CARBOHYDRATES CHAINS They occur on the surface of the membrane as complexes of carbohydrates and proteins called glycoprotein ; carbohydrates and lipids called glycolipids. Glycoprotein and glycolipids are collectively referred to as Glycalyx (sugar coating). Glycalyx is highly hydrophilic and aids in interaction of cells with watery environment and also in the cells’ ability to obtain dissolved substances in water. It also facilitates adhesion between cells, reception of signaling molecules and cell to cell recognition. PROTEINS There are two major classes of membrane protein; transmembrane (Integral) and peripheral protein Transmembrane proteins are embedded in the phospholipid bilayer. They partially penetrate through the bilayers Integral protein that spans the entire width of the bilayer are called transmembrane protein. Peripheral protein are not embedded in the lipid bilayer. The are located on the inner or outer surface of the plasma membrane. They are usually attached to a transmembrane protein or phospholipids. They can be easily removed without disrupting the structure of the phospholipid bilayer. FUNCTIONS OF PLASMA MEMBRANE PROTEIN Anchoring; some membrane protein such as integrin anchor the cell to the extracellular matrix. They also connect to microfilament within the cell. Passive transport; Certain proteins form channels for selective passage of ions or polar molecules. Active transport; Some transport protein pump solutes across the membrane which requires direct input of energy. Enzymatic activity; Many membrane bound enzymes catalyse reactions that take place within or along the membrane surface. Signal transduction; Some receptors bind with signal molecules such as hormones and transmit information to the cell.  Cell recognition; Some glycoprotein function as identification tags. For example, bacterial cells have surface protein or antigens that human cell recognise as foreign material. Intercellular junction; Cell adhesion protein attach membrane of adjacent cells. PERMEABILITY OF PLASMA MEMBRANE The major function of plasma membrane in eukaryotes and prokaryotes is to regulates the passage of molecules into and out of the cell. This function is important because the life of the cell depends on maintenance of its normal composition. The plasma membrane can carry out this function because it is selectively (differentially) permeable meaning certain substances can move across the membrane while others cannot.  Permeability of a membrane is how easy it is for molecules to pass through it. Permeability depends on electric charges. In general, small non charged (non polar) such as carbon dioxide, oxygen, glycerol and alcohol freely cross the membrane. They are able diffuse passively across the membrane by slipping between the hydrophilic heads of phospholipids passing through the hydrophobic tails of the membrane Plasma membrane serves as barrier to polar molecules and ions. Polar molecules such as water, glucose and amino acids carry a charge on them and can not pass through a lipid bilayer.  Most organic molecules of biological importance have polar functional groups and hence fail to diffuse freely through the hydrophobic lipid bilayer. Several mechanisms exist for getting both hydrophilic molecules and ions across the cell membrane. PASSIVE TRANSPORT DIFFUSION Molecules move across a plasma membrane by the processes of diffusion. Diffusion involves the movement of molecules of the same kind from a region of high concentration to a region of lower concentration. Gases can diffuse through a lipid bilayer; this is the mechanism by which oxygen enters cells and carbon dioxide exits cells. Factors which affect the rate diffusion include temperature, size of molecule, concentration gradient, distance travelled , pressure etc.  Concentration: Diffusion of molecules is entirely dependent on moving from an area of higher concentration to an area of lower concentration. It occurs down the concentration gradient of the molecule in question. If the difference in concentration is higher, then the molecules will go down the concentration gradient faster. If there is not as great of a difference in concentration, the molecules will not move as quickly and the rate of diffusion will decrease.  Temperature: Particles move due to the kinetic energy associated with them. As temperature increases, the kinetic energy associated with each particle also increases. As a result, particles will move faster and can also diffuse faster. Conversely, when the kinetic energy associated with the molecules decreases so does their movement. As a result, the rate of diffusion will be slower.  Mass of Particle: Heavier particles will move more slowly and so will have a slower rate of diffusion. Smaller particles on the other hand will diffuse faster because they can move faster. As is key with all factors affecting diffusion, movement of the particle is paramount in determining if diffusion is slowed down or sped up.  Solvent Properties: Viscosity and density greatly affect diffusion. If the medium that a given particle has to diffuse through is very dense or viscous, then the particle will have a harder time diffusing through it. So the rate of diffusion will be lower. If the medium is less dense or less viscous, then the particles will be able to move more quickly and will diffuse faster. FACILITATED DIFFUSION It is passive transport of substance that otherwise would not cross the membrane readily. Polar molecules such as glucose, amino acids, ions and water cannot easily cross the plasma membrane. These molecules can still enter the cell by diffusion through specific membrane proteins; channel protein and carrier protein. This diffusion mediated by membrane protein is called facilitated diffusion.  Facilitated diffusion just like simple diffusion is also dependent on concentration gradient across the membrane. Facilitated diffusion makes the membrane permeable to ions and polar molecules. CHANNEL PROTEIN Have hydrophilic passage that provides a aqueous channel through which polar molecules can pass. (i) Non gated channel - Cell membrane has permanent pores (porins) through which substances of specific size may pass. - Some channel proteins have large tunnels through which water molecules and solutes pass. - Most protein channels form narrow channels that transport specific ions. - Ions diffuse into or out of the cell via ion channel depending on relative concentration across the membrane. - Each channel is specific to a particular type of ion such as Ca2+, Na+ , K+ and Cl-. (ii) Gated channel - These are special protein channel which can be stimulated to open or close. - Stimulation can be through hormone or electrical impulse. - These channel can be stimulated at specific time depending on the nutritional needs of a cell. - For example some ion channels can be gated to regulate their concentrations in the cytoplasm. CARRIER PROTEIN They transport ions and other solutes such as glucose and amino acids across the cell membrane. Carrier protein binds to the solute on one side of the membrane and then change shape pulling the molecule through the membrane to the other side. Transportation by carrier protein across the membrane occurs along the concentration gradient..  Example of facilitated diffusion by a carrier protein is a protein known as glucose transporter 1 (GLUT 1) which transport glucose into red blood cells. GLUT 1 facilitates diffusion along the concentration from the plasma into the red blood cell thousands of time faster than simple diffusion. Hence, facilitated diffusion helps to speed up the diffusion of molecules into and out of the cell. DIFFERENCES BETWEEN CARRIER AND CHANNEL PROTEINS CARRIER PROTEINS CHANNEL PROTEINS Can change shape Fixed shape Transport large molecules Transport small molecules Exist in ping and pong Specific shape Transport water-soluble and insoluble substances Transport only water-soluble substances Moves across cell membrane Cannot move Synthesized on free ribosomes Synthesized on ribosomes attached to the endoplasmic reticulum Can transport against concentration gradient Transport down a concentration gradient Involved in passive and active transport Involved in passive transport only Glycoprotein by nature (protein+ carbohydrate Lipoprotein (protein that combine & transport attached to polypeptide) fat/other lipids) Transport substances slowly (about 1000 solutes per Transport substances very fast second) Bind solutes on one side & release it on another side. Solutes diffuse through channel pores Transport few solutes (100 to 1000) molecules at a Millions of ions can pass through membrane per time second Have binding sites Have pores Either inner gate open or outer gate is open Both sides can open at the same time OSMOSIS Osmosis is a process by which the molecules of a solvent pass from a solution of low concentration to a solution of high concentration through a semi-permeable membrane. Movement of water molecules across the plasma membrane occurs by a diffusion called osmosis. Water diffuse across the plasma membrane via channel proteins. Water flow in living cells is facilitated by aquaporins which are the specialised channels proteins for water. The movement of water molecules into and out of the cell is dependent of the difference in concentration gradient between cytoplasm and the surrounding solution. Types of Osmosis: Osmosis is of two types: Endosmosis– When a substance is placed in a hypotonic solution, the solvent molecules move inside the cell and the cell becomes turgid or undergoes deplasmolysis. This is known as endosmosis. Exosmosis– When a substance is placed in a hypertonic solution, the solvent molecules move outside the cell and the cell becomes flaccid or undergoes plasmolysis. This is known as exosmosis. OSMOTIC SOLUTIONS There are three different types of solutions: An isotonic solution is one that has the same concentration of solutes both inside and outside the cell. A hypertonic solution is one that has a higher solute concentration outside the cell than inside. A hypotonic solution is the one that has a higher solute concentration inside the cell than outside.  ISOTONIC SOLUTION - Occurs when the concentration of the solution surrounding the cell is equal to the concentration of the cytoplasm. - Under such conditions, the rate of movement of water molecules across the plasma membrane will be equal. - This is because the concentration of two solutions on either side of the membrane are at equilibrium.  HYPOTONIC SOLUTION - When solution surrounding a cell has a lower concentration (diluted) than cytoplasm it is said to be hypotonic. - Under such condition water molecules will move across the plasma membrane from the hypotonic solution into the cytoplasm along the concentration gradient. - Animal cells will under go lysis (burst) if kept in a hypotonic solution for too long whereas plant cells undergo turgidity.  HYPERTONIC SOLUTION - When solution surrounding a cell has a higher concentration (less diluted) than cytoplasm it is said to be hypertonic. - Under such condition water molecules will move across the plasma membrane from the cytoplasm to the surrounding hypertonic solution along the concentration gradient. - Animal cells will undergo crenation if kept in a hypertonic solution for too long whereas plant cells undergo plasmolysis. IMPORTANT FACTS ABOUT OSMOSIS 1. The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation the cell is in an isotonic solution (iso = equal or the same as normal). A cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell. 2. The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). In this situation, a cell will appear to shrink as the water flows out of the cell and into the surrounding environment. 3. The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). A cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell. ACTIVE TRANSPORT Active transport is an energy consuming transport of molecules or ions across a membrane against a concentration gradient. Transporting molecules against their concentration gradient require the use of energy. The energy supplied is in the form of Adenosine Triphosphate (ATP). Enzyme ATPase converts ATP to Adenosine diphosphate releasing an inorganic phosphate (Pi) and energy in the process. Active transport is achieved by carrier proteins. There are three types of carrier proteins or transporter for active transport; uniporter, symporter and antiporter.  Uniporter carries one specific ion or molecule. Symporter carries two different ions or molecules in the same direction. Antiporter carries two different ions or molecules in different direction The carrier protein binds with specific molecules to transported across a membrane. Energy is used to change the shape of the carrier protein as it binds with the passenger molecule and twists around moving the molecule from extracellular position to the cytoplasm. A good example of active transport is the sodium potassium pump.  Pumps active in all animals cells spans the whole width of plasma membrane. This transport system pumps ions against steep concentration gradient. Sodium ion concentration movement is high outside the cell and low inside while potassium ion concentration is low outside the cell and high inside. The sodium-potassium pump maintains this electrochemical gradient. It moves 2 potassium ions into the cell for every 3 sodium ions moved out pumped out of the cell. THE PROCESS OF THE FOLLOWING 6 STEPS; 1. 3 Sodium ions binds to the carrier protein. 2. Phosphate group is transferred from ATP to carrier protein 3. Phosphorylation causes the carrier protein to change shape releasing 3 sodium ions outside the cell. 4. 2 potassium ions bind to carrier protein. 5. Phosphate is released from the carrier. 6. Phosphate released causes carrier protein to return to its original shape releasing 2 potassium ions inside the cell. SECONDARY ACTIVE TRANSPORT (COTRANSPORT) A cotransport is system in which the electrochemical gradient, created by primary active transport is being used to move other substances against their concentration. An energy source such as ATP is required to power the pump that produces the concentration gradient. For example the sodium-potassium pump uses energy to generate electrochemical gradient. The energy of this gradient drives the active transport of required substances such as glucose and amino acids.  A carrier protein cotransports a solutes against its concentration gradient while sodium, potassium or hydrogen ions move down their gradient. In glucose cotransport, a carrier protein transport both sodium and glucose. As sodium moves into the cell along its gradient, the carrier protein captures the energy released and use it to transport glucose into the cell. BULK TRANSPORT Bulk transport involves transportation of large molecules such polypeptides, polysaccharides or polynucleotide including small cells across the plasma membrane into or out of the cell. Like active transport processes that move ions and small molecules via carrier proteins, bulk transport is energy requiring( and infact energy-intensive) process They are transported by exocytosis and endocytosis. Exocytosis involves the exit of substances from the cell. Endocytosis involves the entry of substances into the cell EXOCYTOSIS During exocytosis, a vesicle fuses with the plasma as secretion occurs. Hormones, neurotransmitters and digestive enzymes are secreted from cells using these vesicles. The golgi body often produces the vesicles that carry these cell products to the membrane. During exocytosis the membrane of the vesicle becomes part of the plasma membrane. Content of vesicles adhere to the cell surface or become part of extracellular matrix.  Cells of particular organs are specialized in producing and exporting materials. For example pancreatic cells produce digestive enzymes and pituitary cells produce growth hormones. The secretory vesicle accumulate near the plasma membrane only release their content when the stimulated by a signal. For example a rise in blood sugar signals pancreatic cells to release the hormone insulin. This is regulated secretion. ENDOCYTOSIS During endocytosis, cells take in substances by vesicle formation. A portion of the plasma membrane inviginates to envelope a substance and then a membrane pinches off to form an intracellular vesicle. Endocytosis occurs in three ways; Phagocytosis, Pinocytosis and Receptor mediated endocytosis. 1. PHAGOCYTOSIS Involves the taking in of large food particles or small cells i.e virus or bacteria. Phagocytosis is common in unicellular organism such the ameoba. Human white blood cells i.e phagocytes exhibits phagocytosis. They are mobile and engulf bacteria or viruses that invade the body and then digest it. Phagocytosis is a methods of immune protection against bacteria infection 2. PINOCYTOSIS Occur when vesicles form around a liquid or around very small particles. Blood cells that line the kidney, intestine walls and plants roots use pinocytosis to ingest substances. Cell may use pinocytosis to take in water from extra cellular fluids. The vesicle usually does not have to merge with lysosome. 3. RECEPTOR MEDIATED ENDOCYTOSIS Its similar to pinocytosis except it is quite specific receptor protein shaped in such way that vitamin, peptide hormone or lipoprotein can bind to it. The receptor for a specific is found on one location the plasma membrane called the coated pit. Once formed, the vesicle is uncoated and may fuse with lysozyme. When the vesicle fuses with endosome the receptor are returned the plasma membrane.