Define cell membrane Describe the characteristic of the cell membrane with the help membrane with the help of the diagram Explain types of transport of the cell
All living cells example prokarytic and eukaryotic have a plasma membrane that encloses their contents and serves as semi – pronous barrier to the outside environtment The membrane acts as boundary. It is because membrane holding the cell constituents together and keeping other substances from entering. Plasma membrane also called the Cell Membrane or plasmalemma. Plasma membrane is one of the biological membrane that separating the interior of a cell from the outside environment Cell membrane or plasma membrane are surrounds all cell and it is selectively permeable. Plasma membrane controlling the movement of substance in and out of the cell. Plasma membrane is permeable to specific molecules and allows nutrients other essential elements to enter the cell and waste materials to leave the cell. For example oxygen, carbon dioxide, and water are able to pass freely across the membrane , but the passage of large molecules, such as amino acids and sugar is carefully regulated Plasma membrane contains a wide variety of biological molecules, primarily proteins and also lipid. 
The plasma membrane also serve as the attachment point for the intracellular cytoskeleton and also teh extracellular cell wall
Figure 1 Plasma Membrane
It has lipid bilayer
Two back-to-back layers made up three types of lipid molecules – phospholipid , cholesterol , and glycolipids About 75% of the membrane lipids are phospholipids, lipids that contain phosphorus Present in smaller amount are cholesterol (about 20%) and various glycolipids ( about 5%) , lipid with attached carbohydrate groups The bilayer arrangement occurs because the lipids are amphiphatic molecules, which means that they have both polar and nonpolar parts. In phospholipid the polar parts is the phosphate containing “head”, which is hydrophilic and hydrophobic The phospholipid molecules orient themselves in the bilayer with their hydrophilic heads facing outward. The heads face a watery fluid on either side – cytosol on the inside and extracellular fluid on the outside The hydrophobic fatty acids tails in each half of the bilayer point towards one another, forming nonpolar, hydrophobic region in the membrane interior
It contains proteins
That can act as pumps, channels, receptors, enzymes or structural components Integral proteins act as carriers, selectively moving a polar substance or ion from one side of the membrane to the other Most integral proteins are transmembrane proteins, which means they span the entire lipid bilayer and protrude into both the cytosol and extacellular fluid. A few integral protein are tightly attached to one side of the bilayer by covalent bonding to fatty acids. Like membrane lipids, integral membrane proteins are amphitatic. Their hydrophilic regions protrude into either the watery extracellular fluid or the cytosol, and their hydrophobic regions extend among the fatty acids tails Peripheral proteins are not as firmly embedded in the membrane. They associate more loosely with the polar head of membrane lipids or with integral proteins at the inner or outer surface of the membrane Many membrane proteins are glycoproteins, proteins with carbohydrate groups attached to the ends that protrude into the extracellular fluid. In addition, peripheral proteins help support the plasma membrane, anchor integral proteins, and participate in mechanical activities such as moving materials and organelles within cells, changing cell shape in dividing and muscle cells, and attaching cells to anotherIt is semipermeable
Some particles can pass through it easily based on concentrations gradients or other factors The factors that determine the permeability of a molecule across the plasma membrane are polarity , charge and size The permeability of the plasma membrane to different substance varies. Plasma membrane permit some substance to pass more readily than other. The lipid bilayer portion of the membrane is permeable to nonpolar, uncharged molecules. Such as oxygen, carbon dioxide, and steroids But is impermeable to ions and large, uncharged polar molecules such as glucose The slight permeability to water and urea is an unexpected property since they are polar moleculesIt has potential
Which means a voltage – the difference in charge between the extracellular fluid and intracellular fluid 
Transport of the material across the plasma membrane is essential to the life of a cell Certain substance must move into the cell to support metabolic reactions Substance generally move across cellular membrane via transport process that can be classified as passive or active, depending on whether they require cellular energy In passive process, a substance moves down it concentration or electrical gradient to cross the membrane using only its own kinetic energy Kinetic energy is intrinsic to the particles that are moving There is no input of energy from the cell, example is simple diffusion In active process, cellular energy is used to drive the substance against its concentration or electrical gradient The cellular energy used is usually in the form of ATP, example is active transport.PASSIVE PROCESSES
THE PRINCIPLE OF DIFFUSION
Diffusion is a passive process in which the random mixing of particle in asolution occurs because of the particle in a solution occurs because of the particles kinetic energy If a particular solute is present in high concentration in one area of a solution and in low concentration in another area, solute molecules will diffuse toward the area of lower concentration Some time, the particles become evenly distributed throughout the solution is said to be at equilibrium The particle continue to move about randomly due to their kinetic energy, but their concentration do not change.SIMPLE DIFFUSION
Simple diffusion is passive process in which substance move freely through the lipid bilayer of the plasma membrane of cell without the help of membrane transport proteins Nonpolar, hydrophobic molecules move across the lipid bilayer through the process of simple diffusion
some molecules include oxygen, carbon dioxide, and nitrogen gases and fat-soluble vitamins ( A,D,E and K) small, uncharged polar molecules such as water, urea and small alcohols also pass through the lipid bilayer by simple diffusion. simple diffusion through the lipid bilayer is important in the movement of oxygen and carbon dioxide between blood and body cell and between blood and air within the lungs during breathing it also is the route for absorption of some nutrients and excretion of some waste by body cellsFACILITATED DIFFUSION
Solute that are too polar or highly charged to move through the lipid bilayer by simple diffusion can croos the plasma membrane by a passive process called facilitated diffusion In this process, an integral membrane protein assists a specific substance across the membrane The integral membrane protein can be either a membrane channel or a carrierCHANNEL – MEDIATED FACILITATED DIFFUSION IN CHANNEL MEDIATED FACILITATED DIFUSSION
A solute moves down its concentration gradient across the lipid bilayer through a membrane channel Most membrane channels are ion channels integral transmembrane proteins that allow passage of small, inorganic ions that are too hydrophilic to penetrate the nonpolar interior of the lipid bilayer Diffusion of ions through channels is generally slower than free diffusion though the lipid bilayer because channel occupy a smaller fraction of the membrane total surface area than lipids. A channel is said to be gated when part of the channel protein acts as a “plug” or “gate” changing shape in one way to open the pore and in another way to close it Some gated channels randomly alternate between the open and closed position other are regulated by chemical or electrical changes inside and outside of the cell. The plasma membrane of different types of cells may have different number of ion channels and thus display different permeabilities to various ionsCHANNEL – MEDIATED FACILITATED DIFFUSION IN CARRIER MEDIATED FACILITATED DIFUSSION
A carrier is used to move a solute down its concentration gradient across the plasma membrane The solute binds to a specific carrier undergoes in change a shape The solute binds more often to the carrier on the side of the membrane with a higher concentration of solute. The rate of carrier – mediated facilitated diffusion is determine by the steepness of the concentration gradient across the membrane Substance that move across the plasma membrane by carrier-mediated facilitated diffusion include glucose, fructose, galactose and some vitamins. Glucose enters many body cells by carrier-mediated facilitated diffusion The selective permeability of the plasma membrane is often regulated to achieve homeostasis. With more glucose carrier available, body cells can pick up glucose from the blood more rapidly An inability to produce or utilize insulin is called diabetes mellitus OSMOSIS
Osmosis is the type of diffusion in which there is net movement of a solvent through a selectively permeable membrane. Osmosis is passive process Passive movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration Osmosis occurs only when a membrane is permeable to water but is not permeable to certain solutes The solution with the impermeable solute also exerts a force called the osmotic pressure The osmotic pressure of a solution is proportional to the concentration of the solute particles that cannot cross the membrane- the higher the solute concentration, the higher the solution osmoticACTIVE PROCESSES
Such solutes may be able to cross the membrane by a process called active transport Active transport is consider an active process because energy is required for carrier proteins to move solute across the membrane against a concentration gradient Solute actively transported across the plasma membrane include several ionPRIMARY ACTIVE PROCESSES
In primary active transport, energy derived from hydrolysis of ATP changes the shape of a carrier protein, which ‘pumps’ a substance across a plasma membrane against its concentration gradient Carrier proteins that mediate primary active transport are often called pumps. A typical body cell expends about 40% of the ATP it generates on primary active transport The most prevalent primary active transport mechanism expels sodium ions (Na+) from cells and brings potassium ions (K+) in. The different concentrations of Na+ and K+ in cytosol and extracellular fluid are crucial for maintaining normal cell volume and for ability of some cell to generate electrical signals such as action potentials By helping to maintain normal tonicity on each side of the plasma membrane, the sodium-potassium pumps ensure that cells neither shrink nor swell due to the movement of water by osmosis out of or into cellsSECONDARY ACTIVE PROCESSES
In secondary active transport, the energy stored in a Na+ or H+ concentration gradient is used to drive other substance across the membrane against on their own concentration gradients. Because a Na+ or H+ gradient is established by primary active transport, secondary active transport indirectly uses energy obtained from the hydrolysis of ATP Secondary active transport proteins harness the energy in the Na+ concentration gradient by providing routes for Na+ to leak into cells If these transporters move two substances in the same direction they are called symporters; antiporters in contrast, move two substances in opposite direction across the membraneTRANSPORT IN VESICLES
A vesicle as noted earlier, is a small, spherical sac. A variety of substance are transported in vesicle from one structure to another within cells Vesicles also import materials from and release materials into extracellular fluid. During endocytosis, material move into a cell in a vesicle formed from the plasma membrane In exocytosis, materials move out of a cell by the fusion with the plasma membrane of vesicles formed inside the cell Thus, transport in vesicles is an active process Phagocytosis is a form of endocytosis in which the cell engulfs large solid particles, such as worn-out cell Only a few body cells, termed phagocytes, are able to carry out phagocytosis. Most body cells carry out bulk-phase endocytosis, also called pinocytosis a form of endocytosis in which tiny droplets of extracellular fluid are taken up.EXOCYTOSIS
In contrast with endocytosis, which brings materials into a cell, exocytosis releases materials from a cell. All cells carry out exocytoses, but it is especially important in two types of cells(1) Secretory cells thet liberate digestive enzymes, hormones, mucus, or other secreation
(2) Nerve cell that release substances called neutransmitters
In some case waste are also released by exocytosis Segments of the plasma membrane lost through endocytosis are recovered or recycled by exocytosis Membrane exchange is quite extensive in certain cellsEXOCYTOSIS
Transport in vesicles may also be used to successively move a substance into, across, and out of a cell This active process called transcytosis, vesicles undergo endocytosison one side of a cell, move across the cell, and then undergo exocytosis on the opposite sideTRANSPORT OF MATERIALS INTO AND OUT OF CELLS
| TRANSPORT PROCESS | DESCRIPTION | SUBSTANCE TRANSPORTED |
| Passive processes | movement of substance down a concentration gradient until equilibrium is reached | |
| Diffusion | The movement of molecules or ions down a concentration gradient due to their kinetic energy until they reach equilibrium | |
| Simple diffusion | Passives movement of a substance down its concentration gradient through the lipid bilayer of the plasma membrane without the help of membrane transport proteins | Nonpolar, hydrophobic solute, polar molecules, charged solute |
| Facilitated diffusion | Passive movement of a substance down its concentration gradient through the lipid bilayer by transmembrane proteins that function as channels or carriers | Polar or charged solutes solvent |
| Osmosis | Passive movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration | Solvent: water in living systems |
| Active processes | Movement of substances against a concentration gradient requires cellular energy in the form of ATP | |
| Active transport | Active process in which a cell expends energy to move a substance across the membrane against its concentration gradient by transmembrane proteins that function as carriers | Polar or charged solutes |
| Primary active transport | Active process in which a substance moves across the membrane against its concentration gradient by pumps that use energy supplied by hydrolysis of ATP | Na+, K+, H+ and other ions |
| Secondary active transport | Coupled active transport of two substances across the membrane using energy supplied | Antiport: h+ out of cell, Ca2+ Symport: glucose, amino acids |
| Transport in vesicles | Active process in which substance move into or out of cells in vesicles that bud from the plasma membrane | |
| endocytosis | Movement of substances into a cell in vesicles | |
| Receptor-mediated endocytosis | Ligand-receptor complexes trigger infolding of a clathrin-coated pit that forms a vesicle containing ligands | Ligands: transferring, low density lipoproteins |
| phagocytosis | “cell eating” movement of asolid particle into a cell after pseudopods engulf it to form a phagosome | Bacteria, virus and aged or dead cell |
| Bulk-phase endocytosis | “cell drinking” movement of extracellular fluid into a cell by infolding of plasma membrane to form a vesicle | Solute in extracellular fluid |
| Exocytosis | Movement of substances out of a cell in secretory vesicles that fuse with the plasma membrane and release their contents into the extracellular fluid | Neurotransmitter, hormones, and digestive enzymes |
| transcytosis | Movement of asubstance through a cell as a result of endocytosis on one side and exocytosis on the opposite side | antibodies |
No comments:
Post a Comment