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Transcript
الحمد هلل رب العالميه الذي هداوا لهذا وما كىا لىهتدي لىال أن هداوا اهلل والصالة والسالم على أشزف األوبياء 1 222Cell Biology Lecture 5 222Cell Biology 2 The plasma membrane Each cell must interact with its environment in a number of ways. Each cell needs to obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules, mineral ions, etc.) from the environment, maintain water balance with its surroundings, and remove waste materials from the cell. The Plasma membrane separates a cell from its environment. 222Cell Biology 3 The plasma membrane 222Cell Biology 4 The plasma membrane functions 1. 2. 3. 4. 5. The phospholipid bilayer provides the cell with a structure that separates the outside from the inside of the cell. The integrity of the membrane is necessary for life functions. Because of the nature of the phospholipid, many molecules cannot move across the membrane without help. Maintains the cell's environment by regulating materials that enter or leave the cell. The plasma membrane is differentially, or selectively, permeable. Some materials enter and leave easily through the membrane, some with the assistance of membrane molecules, and some prohibited. Provides mechanisms for cell-to-cell communication. Provides mechanisms for a cell to recognize "self" versus "nonself" (foreign materials), important to the immune system, development and defense of the organism through genetically unique cell recognition markers 222Cell Biology 5 The Fluid Mosaic Membrane Structure 1. 2. 3. 4. 5. 6. The structure and function of a membrane depends of its molecular composition The foundation of the membrane is its phospholipid bilayer, with a number of associated proteins. Membranes also contain carbohydrates (glycoproteins and proteoglycans) and glycolipids. The resultant membrane structure (proteins scattered throughout the fluid phospholipid layers) resembles a mosaic, hence the name “Fluid Mosaic Membrane" Membrane molecules are manufactured in the endoplasmic reticulum and distributed by Golgi vesicles The orientation of membranes is determined at the manufacturing site. Molecules on the inside of the ER and Golgi vesicles become exterior membrane molecules. 222Cell Biology 6 Membrane Phospholipids 1. 2. 3. Phospholipids have both hydrophilic (polar) and hydrophobic (non polar) regions (in other words, they are amphipathic). The fatty acid "tails" of the two phospholipid layers are oriented towards each other so that the hydrophilic "heads", which contain the "charged" phosphate portion, face out to the environment as well as into the cytoplasm of the cell's interior, where they can form hydrogen bonds with surrounding water molecules. The phospholipid molecules of a membrane provide for its physical integrity Exterior Cytoplasm Phospholipid Bilayer 222Cell Biology 7 Phospholipid Movements A membrane is held together, for the most part, by hydrophobic interactions within the phospholipid bilayer. Because individual phospholipid molecules are not bonded to each other, a membrane is flexible, or "fluid", particularly to lateral movement of the fatty acids. Phospholipid molecules easily move along the plane of the membrane; reversing exterior – interior position (or flip-flopping) is less common. Unsaturated/Saturated 222Cell Biology 8 Phospholipid Movements Cholesterol, found in membranes of many animal cells, reduces fluid movement of the phospholipids, helping to maintain membrane integrity. The saturation of fatty acids affects membrane fluidity – the more saturated, the less movement Membranes will also solidify as temperature decreases, reducing function. The saturation of fatty acids will affect the temperature at which the membrane "solidifies" (just as it does with fats and oils). 222Cell Biology 9 Membrane Proteins Interspersed throughout a membrane's phospholipid layer are a number of amphipathic proteins. The hydrophobic regions of the proteins are within the fatty acid regions of the phospholipids and hydrophilic regions of the proteins are at the interior and exterior aqueous interfaces of the membrane. This orientation is important to how the membrane proteins function. The membrane is also associated with a network of supporting cytoskeletal filaments, some of which help shape the cell and some help anchor proteins within the membrane. 222Cell Biology 10 Protein Mobility Many proteins within the membrane are mobile; studies of fused mouse and human cells show that proteins from the two cells are intermixed within an hour of fusion Membrane proteins are divided into two categories, integral and peripheral, location depending on their 222Cell Biology 11 Membrane Proteins Integral (Transmembrane) Proteins o Proteins that go through the membrane are called integral or transmembrane proteins. They have hydrophobic (non-polar amino acids with alpha helix coiling) regions within the interior of the membrane and hydrophilic regions at either membrane surface o 222Cell Biology 12 Peripheral Proteins Are attached to the surface of the membrane, often to the exterior hydrophilic regions of the transmembrane proteins. On the interior surface, peripheral proteins typically are held in position by the cytoskeleton. On the exterior, proteins may attach to the extracellular matrix. Peripheral proteins help give animal cell membranes strength. The different proteins contribute to the "sidedness" of membranes so that the interior and exterior sides of membranes have different properties that affect membrane function. 222Cell Biology 13 Anchoring proteins Other proteins have non-polar α helix regions that fix the protein into specific regions of the phospholipid bilayers. Such proteins are called anchoring proteins The protein receptors at neuromuscular junctions on muscle cells are anchored proteins Anchor proteins can attach to the fibrous network of the cytoskeleton to give shape and strength to some cells. Some membrane lipid regions, called lipid rafts, are also specialized to help anchor proteins within a specific region of a membrane. 222Cell Biology 14 Membrane Protein Functions 1-Transport Proteins Transport Proteins are transmembrane proteins that serve as carriers for specific substances that need to pass through the membrane by providing a hydrophilic channel or pore. Transport proteins have binding sites that attract specific molecules. Most of our ions, amino acids, sugars and other small nutrient molecules are moved through transport proteins. When a molecule binds to the carrier protein, the protein shape changes moving the substance through the membrane. This process may require energy (ATP), and the ATP complex is then a part of the transport protein. 222Cell Biology 15 Membrane Protein Functions 2- Enzymatic Proteins Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface. The active site of the enzyme will be oriented in the membrane for the substrate to bind Enzymes needed for metabolic pathways can be aligned adjacent to each other to act like an assembly line for the reactions, minimizing the need for intermediates to diffuse through the cytoplasm of the cell. 222Cell Biology 16 Membrane Protein Functions 3- Signal Transduction (Receptor) Proteins: Signal transduction proteins have attachment sites for chemical messengers, such as hormones. The signal molecule, when it attaches to the receptor promotes a conformational change that relays the message into the cell to trigger some cell activity 222Cell Biology 17 Membrane Protein Functions 4-Attachment Proteins -Attachment proteins attach to the cytoskeleton or extracellular matrix to help maintain cell shape (particularly for animal cells) 5-Recognition (Identity) Proteins -Glycoproteins serve as surface receptors for cell recognition and identification. They are important to the immune system. 6-Cell Adhesion (Intercellular Joining) Proteins - Special membrane proteins are responsible for the cell junctions (tight junctions, desmosomes and gap junctions. - They permit cells to adhere to each other 222Cell Biology 18 Membrane Carbohydrates Glycoprotein complex with long Polysacchari de معقد جاليكىبزوتيه مع سلسلت طىيلت مه السكاكز المتعددة Collagen fiber ليفت كىالجيه Intgrine اوتيجزيه 222Cell Biology Glycoproteins and glycolipids are also important to membrane structure and function. Glycolipids function as recognition signals for cell-to-cell interactions. Glycoproteins, with their oligosaccharides portions, are critical for a cell to be recognized by other cells and by protein molecules, and for cell-to-cell adhesion 19 Functions of cell membranes: Cell membranes are selectively permeable membranes i.e allow passage of certain substances but not others 1. Transport Passive transport : a) Do not require energy b) Cells do not perform work c) Driving force is the concentration gradient. Types of passive transport 1. Simple diffusion : movement of molecules from an area of higher concentration to an area of lower concentration untill equilibrium e.g diffusion of gases (O2 and CO2) in our lungs and RBCs. 2. Facilitated diffusion : a type of passive diffusion provides a channel for sugars and H2O. Binds its passenger, changes shape and release it to the other side. 222Cell Biology 20 Facilitated Diffusion Most molecules cannot move freely through the membrane, but can pass through membranes if a gradient exists, with the help of membrane transport proteins. No energy is involved, so it is still a passive process. Transport proteins are specific, and are limited in number in membranes. The rate of movement of materials is dependent on the availability of transport proteins as well as the concentration of the substance to be moved. Amino acids, monosaccharides and ions move through membranes via facilitated diffusion. It move through a hydrophilic protein channel or pore of the transport protein Ion channel proteins are common in membranes. Much water movement through membranes also involves facilitated diffusion. There are special channel proteins, called aquaporins that facilitate the movement of water at a rate needed for cell activities. 222Cell Biology 21 Some transport proteins have channels with gates. The gate opens to let the target molecule pass through when it receives an electrical or chemical signal. For example, neurotransmitter chemicals serve as signal molecules to open the gates for sodium to flow into the nerve cell. Facilitated diffusion also occurs with carrier molecules, substances to which the target molecule to be transported temporarily binds, resulting in a conformational change that moves the target substance through the membrane Potassium Ion Channel 222Cell Biology Protein Glucose Carrier Proteins 22 Active transport of a solute across a membrane Solute binding 222Cell Biology Phosphorylation Transport Protein reversion 23 Moving Materials Through Membranes There are a number of ways to move materials through membranes: 1. Simple diffusion 2. Facilitated diffusion 3. Active transport - These ways are used to move small quantities of substances - Simple and facilitated diffusion are means of passive transport - Active transport uses energy to move substances against a gradient. - Larger volumes are moved by exocytosis or endocytosis - Endocytosis and exocytosis, which involve extensive membrane rearrangements, are also energy consuming. 222Cell Biology 24 1- Active Transport Active transport requires energy and uses a variety of transport mechanisms. - Uniports move one substance in one direction. - Symports move two substances in the same direction - Antiports move two substances in opposite directions. Symports and antiports are also known as coupled transporters 222Cell Biology 25 Membrane Interactions with the Environment Larger substances, including macromolecules, require changes in membrane shape and/or the fusion of membranes to move into or out of cells. Such substances move into the cell by endocytosis and from the cell by exocytosis 1- Exocytosis Materials can be exported from the cell by fusing vesicles with the plasma membrane, a process called exocytosis Materials for export are packaged in a Golgi body and the vesicles formed travel along the cytoskeleton until they reach the plasma membrane. Once the vesicle membrane and plasma membrane fuse, the contents of the vesicle are freed from the cell for example : insulin, 222Cell Biology 26 2. Endocytosis Substances that enter the cell using membrane modifications move by endocytosis. There are three endocytosis processes: phagocytosis, receptor-mediated endocytosis and pinocytosis A. Phagocytosis Phagocytosis occurs when membrane pseudopodia surround and engulf particulate objects, packaging them in a membrane-bounded vacuole. Phagocytosis is used for solid large objects, such as prey engulfed by Amoeba, and bacteria by white blood cells. B. Pinocytosis (cell drinking) In pinocytosis, the plasma membrane invaginates, substances "fall" in cavity, the membrane seals over and the molecules in the fluid will be moved into the cell enclosed vesicle. Once in the cytoplasm, the vesicle membrane degrade, the substances release into the cytoplasm 222Cell Biology 27 Membrane Interactions with the Environment C. Receptor mediated endocytosis: Highly specific receptor proteins in the membrane attract the substance to be moved into the cell. The receptor proteins are attached to specific substances in the membrane creating a membrane depression in that area called a coated pit The cytoplasmic side of the coated pit is coated with specific proteins, called clathrins When sufficient target molecules have been attracted, the pocket will be pinched off forming a clathrin-coated vesicle in the cytoplasm. Molecules that bind to receptor sites are called ligands. (It’s a general term that simply means something that attaches to a receptor.) Receptor-mediated endocytosis is an effective means of moving desired materials into cells. e.g absorption of cholesterol. 222Cell Biology 28 Three kinds of endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis 222Cell Biology 29 Plasma membrane markers Special proteins or enzymes found in plasma membrane like: - Adenosine triphosphate transporting enzymes (Na+/K+ transporting ATPase ) , which found in all plasma membranes - Endoplasmic reticulum membrane contains glucose 6-phosphorylase - Inner mitochondrial membranes contains succinate dehydrogenase Transport through membrane occurs via: Rotation Crossing Conformation change Pore formation 222Cell Biology 30 222Cell Biology 31