Cell Membrane Structure-1.pptx cell outer layer
- 1. Cell Membrane Structure DPT 3RD BIOCHEMISTRY & GENETICS -1
- 2. Plasma Membrane The plasma membrane is an envelope surrounding the cell. It forms the cell’s flexible outer surface which separates and protects the cell’s internal environment from the external hostile environment. Besides being a protective barrier, plasma membrane provides a connecting system between the cell and its environment. It is a selective barrier that regulates the flow of materials into and out of a cell. This selectivity helps establish and maintain the environment appropriate for normal cellular activities. It Is responsible for maintenance of shape and size of the cell. The subcellular organelles such as nucleus, mitochondria, lysosomes are also surrounded by membranes.
- 3. Chemical Composition The membranes are composed of lipids, proteins and carbohydrates. The actual composition differs from tissue to tissue , but typically it contains, 40 % of the dry weight is lipids, about 60% proteins and 1 to 10 % carbohydrates.
- 4. Fluid Mosaic Model The plasma membrane is best described by using a structural model called the fluid mosaic model. Proposed by Singer and Nicolson, is a more recent and acceptable model for membrane structure. The biological membranes usually have a thickness of 5-8 nm. A membrane is essentially composed of a lipid bilayer. Globular proteins are irregularly embedded in the lipid bilayer. According to this model, the molecular arrangement of the plasma membrane resembles an ever-moving sea of fluid lipids that contains a mosaic of many different proteins. Some proteins float freely like icebergs in the lipid sea, whereas others are anchored at specific locations. Some of the proteins in the plasma membrane allow movement of polar molecules and ions into and out of the cell. Other proteins can act as signal receptors or adhesion molecules. The membrane is asymmetric due to the irregular distribution of proteins. The lipid and protein subunits of the membrane give an appearance of mosaic. The membrane freely changes, hence the structure of the membrane is considered as fluid mosaic.
- 6. Membrane Lipids The basic structural framework of the plasma membrane is the lipid bilayer, two back-to-back layers. The major classes of membrane lipids are: – Phospholipids – Glycolipids – Cholesterol About 75% of the membrane lipids are phospholipids, lipids that contain phosphorus Present in smaller amounts are cholesterol (about 20%) A steroid with an attached OH (hydroxyl) group, and various glycolipids (about 5%), lipids with attached carbohydrate groups
- 7. Membrane Proteins Proteins of the membrane are classified into two major categories: – Integral proteins or intrinsic proteins or transmembrane proteins and – Peripheral or extrinsic proteins. • Integral proteins are either partially or totally immersed in the lipid bilayer. Many integral membrane proteins span the lipid bilayer from one side to the other and are called transmembrane protein whereas others are partly embedded in either the outer or inner leaflet of the lipid bilayer. Transmembrane proteins act as enzymes and transport carriers for ions as well as water soluble substances, such as glucose. • Peripheral proteins are attached to the surface of the lipid bilayer by electrostatic and hydrogen bonds. They bound loosely to the polar head groups of the membrane phospholipid bilayer. Peripheral proteins function almost entirely as enzymes and receptors.
- 8. Membrane Proteins Some integral membrane proteins form ion channels, pores or holes through which specific ions, such as potassium ions (K+), can flow to get into or out of the cell Other integral proteins act as carriers, selectively moving a polar substance or ion from one side of the membrane to the other. Carriers are also known as transporters. Some integral proteins are enzymes that catalyze specific chemical reactions at the inside or outside surface of the cell. Integral proteins may also serve as linkers, which anchor proteins in the plasma membranes of neighboring cells to one another or to protein filaments inside and outside the cell. Peripheral proteins also serve as enzymes and linkers. Integral proteins called receptors serve as cellular recognition sites. Each type of receptor recognizes and binds a specific type of molecule. For instance, insulin receptors bind the hormone insulin. A specific molecule that binds to a receptor is called a ligand. Membrane glycoproteins and glycolipids often serve as cell identity markers. The ABO blood type markers are one example of cell identity markers.
- 9. Glycocalyx The carbohydrate portions of glycolipids and glycoproteins form an extensive sugary coat complex polysaccharides called the glycocalyx. Most of the integral proteins are glycoproteins and about one-tenth of the membrane lipid molecules are glycolipids. The carbohydrate portion of these molecules protrudes to the outside of the cell, dangling outward from the cell surface. Therefore, it acts like a molecular “signature” that enables cells to recognize one another. Many of the carbohydrates act as receptor for hormones. Some carbohydrate moieties function in antibody processing. For example, a white blood cell’s ability to detect a “foreign” glycocalyx is one basis of the immune response that helps us destroy invading organisms. It enables cells to adhere to one another. Makes red blood cells slippery as they flow through narrow blood vessels.
- 11. Membrane Permeability This property of membranes to pass some substances through it and other not, is called selective permeability. 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. Transmembrane proteins that act as channels and transporters increase the plasma membrane’s permeability that cannot cross the lipid bilayer without help.
- 12. Passive Diffusion It is a passive process in which the net movement of a substance is from a region of higher concentration to a region of lower concentration. The substance moves because of its kinetic energy; diffusion continues until equilibrium is reached, that is, the substance becomes evenly distributed. A good example of diffusion in the body occurs in the lungs for intake of oxygen and out take of carbon dioxide.
- 13. Facilitated Diffusion Passive movement of a substance down its concentration gradient through the lipid bilayer by transmembrane proteins that function as carriers In carrier mediated facilitated diffusion, a carrier (also called a transporter) is used to move a solute down its concentration gradient across the plasma membrane. The solute binds to a specific carrier on one side of the membrane and is released on the other side after the carrier undergoes a change in shape. Glucose enters many body cells by carrier-mediated facilitated diffusion as follows; Glucose binds to a specific type of carrier protein called the glucose transporter (GluT) on the outside surface of the membrane. As the transporter undergoes a change in shape, glucose passes through the membrane. The transporter releases glucose on the other side of the membrane. The selective permeability of the plasma membrane is often regulated to achieve .
- 14. Active transport Active transport occurs against a concentration gradient and this is dependent on the supply of metabolic energy (ATP). Active transport is also a carrier mediated process like facilitated diffusion. The most important primary active transport systems are ion-pumps (through the involvement of pump ATPases or ion transporting ATPases). Na+-K+ pump The Na+-K+ pump is responsible for the maintenance of high K+ and low Na+ concentrations in the cells. Na+-cotransport system : The amino acids and sugars are transported into the cells by a Na+- cotransport system. This process essentially consists of the passage of glucose (or amino acid) into the cell with a simultaneous movement of Na+.
- 17. Cotransport system In cotransport, the transport of a substance through the membrane is coupled to the spontaneous movement of another substance.
- 18. Transport of macromolecules The transport of macromolecules such as proteins, polysaccharides and polynucleotides across the membranes is equally important. This is brought about by two independent mechanisms namely endocytosis—intake of macromolecules by the cells (e.g. uptake of LDL by cells) and exocytosis—release of macromolecules from the cells to the outside (e.g. secretion of hormones-insulin, PTH).
- 19. Osmosis It is also a passive process in which the net movement of water molecules through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration occurs. During osmosis, water molecules pass through a plasma membrane in two ways: 1. by moving through the lipid bilayer via simple diffusion, as previously described, 2. by moving through aquaporins(aquawater), Integral membrane proteins that function as water channels.
- 20. Types of Receptors Receptors are protein molecules in the target cell or on its surface that bind ligands. There are two types of receptors: internal receptors and cell-surface receptors 1. Internal receptors Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of the cell and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane. Once inside the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis to mediate gene expression.
- 21. 2- Cell surface receptors Cell-surface receptors, also known as transmembrane receptors, are cell surface, membrane- anchored, or integral proteins that bind to external ligand molecules. This type of receptor spans the plasma membrane and performs signal transduction, converting an extracellular signal into an intracellular signal. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Cell- surface receptors are also called cell specifIc proteins or markers because they are specific to individual cell types. Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three general categories of cell-surface receptors: 1. Ion channel-linked receptors 2. G-protein-linked receptors 3. Enzyme-linked receptors
- 22. Ion channel - linked Receptors Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through. There is a conformational change in the protein’s structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through. Note: A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells, which are cells that are affected by chemical signals; these proteins are called receptors.
- 23. Enzyme - linked Receptors • Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme. In some cases, the intracellular domain of the receptor itself is an enzyme or the enzyme-linked receptor has an intracellular domain that interacts directly with an enzyme. • An example of this type of enzyme-linked receptor is the tyrosine kinase receptor. The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules.
- 24. G - protein linked Receptors • G- protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The activated G-protein then interacts with either an ion channel or an enzyme in the membrane. • All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
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