Ibn Sina University First Year Biology - Cell Membrane & Transport Lecture Notes PDF

IBN SINA UNIVERSITY COLLEGE OF MEDICINE FIRST YEAR BIOLOGY  In animal cell, Plasma Membrane consists of: 1. Phospholipid bilayer. 2. Proteins. 3. Carbohydrates. 4. Cholesterol. 1. The Phospholipid bilayer: The phospholipid is an amphipathic molecule, meaning that it...

IBN SINA UNIVERSITY COLLEGE OF MEDICINE FIRST YEAR BIOLOGY  In animal cell, Plasma Membrane consists of: 1. Phospholipid bilayer. 2. Proteins. 3. Carbohydrates. 4. Cholesterol. 1. The Phospholipid bilayer: The phospholipid is an amphipathic molecule, meaning that it has both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region. The amphipathic nature of phospholipids gives the membrane the property of being selectively permeable.  The hydrophilic molecules represent the phosphate heads of the molecules. The hydrophobic molecules represent the lipid tails of the molecules.  The heads are composed of polar phosphate groups, and located in the outside surface and the inside surface of the membrane. The tails are composed of non-polar fatty acids, and face each other in the interior region of the membrane. 2. The Proteins: The proteins are scattered throughout the membrane in an irregular pattern. Either partially or wholly embedded in the phospholipid bilayer. The embedded proteins are termed integral proteins, and the other proteins are termed peripheral proteins.  Integral proteins span the phospholipid bilayer of with their hydrophobic region, while their hydrophilic regions protrude from both surfaces of the bilayer. Peripheral proteins. Found only on the cytoplasmic side of the membrane. These proteins can be held in place by attachments to protein fibers of the cytoskeleton (inside the cell) and fibers of the extracellular matrix (ECM) (outside the cell). Proteins Functions: 1. Channel proteins. 2. Carrier proteins. 3. Cell recognition proteins. 4. Receptor proteins. 5. Enzymatic proteins. 6. Junction proteins. 1. Channel proteins: Form channels within the phospholipid bilayer, which allows small water-soluble molecules to pass through. Aquaporins are important water channels that facilitate the movement of water through membranes. 2. Carrier proteins: Have binding sites that attract specific molecules. When a molecule binds to the carrier protein, the protein changes shape and moves the substance through the membrane. 3. Cell recognition proteins: Glycoproteins (carbohydrate-protein hybrids) and some glycolipids serve as surface receptors for cell recognition and identification. They are important to the immune system so that immune system cells can distinguish between one’s own cells and foreign cells. 4. Receptor proteins: Receptor proteins serve as binding or attachment sites, especially for hormones. Once activated, they trigger certain cell responses, often using signal transduction pathways. 5. Enzymatic proteins: Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface. Enzymes needed for metabolic pathways can be aligned adjacent to each other to act like an assembly line for the reactions. 6. Junction proteins (Cell Adhesion Proteins) : Some proteins are responsible for the cell junctions such as tight junctions that permit cells to adhere to each other (Intercellular Joining). 3. The Carbohydrates: The carbohydrate chains are attached to the outside surface and project into the extracellular matrix (ECM). Only animal cells have an extracellular matrix (ECM), which contains various protein fibers and also very large and complex carbohydrate molecules.  The ECM has various functions, from lending support to the plasma membrane to assisting communication between cells. Both phospholipids and proteins can have attached carbohydrate (sugar) chains. These molecules are called glycolipids and glycoproteins, respectively.  In animal cells, the carbohydrate chains of proteins give the cell a “sugar coat,” more properly called the glycocalyx. The glycocalyx protects the cell and has various other functions. 4. The Cholesterol: Cholesterol is another lipid found in the animal plasma membrane. The presence of cholesterol molecules maintains the stability of the plasma membrane and affects its fluidity.  Fluid Mosaic Model The model currently in use to describe the plasma membrane is called the fluid-mosaic model. The fluidity of the membrane is due to its lipid component.  At body temperature, the phospholipid bilayer of the plasma membrane has the consistency of olive oil. The greater the concentration of unsaturated fatty acid residues, the more fluid is the bilayer.  At higher temperatures, cholesterol stiffens the membrane and makes it less fluid than it would otherwise be. At lower temperatures, cholesterol helps prevent the membrane from freezing by not allowing contact between certain phospholipid tails.  The mosaic nature of the plasma membrane is due to its protein content. The number and kinds of proteins can vary in the plasma membrane and in the membrane of the various organelles.  For the survival of the cell some materials need to be able to enter and leave the cell. These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes. The movement of these materials occurs throgh the plasma membrane.  Types of Transport 1. Passive Transport A- Simple Diffusion B- Facilitated diffusion C- Osmosis 2. Active Transport 3. Bulk Flow A- Exocytosis B- Endocytosis 1. Passive Transport: Molecules can passively cross a membrane (no energy required). The molecules move according to their concentration gradient from high to low concentration. A. Simple Diffusion: Diffusion is a passive process (no energy is required). Movement of molecules from a higher to a lower concentration (down their concentration gradient) until equilibrium is achieved and they are distributed equally. The molecules across the membrane through the phospholipid – bi-layer directly.  Includes the movement of small, non-polar, non- charged, and lipid soluble molecules, such as carbon dioxide, oxygen, glycerol, and alcohol. The rate of diffusion can be affected by; temperature, pressure, electrical currents and molecular size. B. Osmosis : It is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane. So it is special type of diffusion involving water molecules only. The "force" to move water through membranes is called osmotic pressure.  Concentration of water Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water  Hypertonic Solutions: Contain a high concentration of solute relative to another solution. When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel. Plant cells wilt due to plasmolysis.  Hypotonic Solutions: Contain a low concentration of solute relative to another solution. When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. Turgid  Isotonic Solutions: contain the same concentration of solute as another solution. When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic. c. Facilitated diffusion: Large polar molecules such as glucose and amino acids, cannot diffuse across the phospholipid bilayer. Also ions such as Na+ or Cl- cannot pass. These molecules pass through protein channels or carrier instead. Movement of molecules is still passive.  Active transport ❖ Means the movement of ions or other substances in combination with a carrier protein from a low concentration gradient to high concentration gradient. ❖ (energy required). Example:- Na-K Pump. ❖Active transport goes against the concentration.  Bulk Flow ❖Movement of large molecules in cells. Vesicles and vacuoles are used to transport large particles across the plasma membrane. Requires energy. Types: Exocytosis Endocytosis Phagocytosis, pinocytosis.  Exocytosis Cytoplasmic vesicle merges with the plasma membrane and releases its contents. Example: Golgi body vesicles merge with the plasma membrane and release their contents. eg. insulin. Also the waste products of cell secreted out of the cell by exocytosis. exocytosis  Endocytosis Plasma membrane sinks inward, pinches off and forms a vesicle. Very large particles enter the cells by endocytosis. Vesicle often merges with Golgi body for processing and sorting of its contents. Endocytosis ❖ The principal forms of endocytosis are Pinocytosis (cellular drinking), phagocytosis (cellular eating)  Phagocytosis (Cellular eating) Membrane sinks in and captures solid particles for transport into the cell. Examples: Solid particles often include: bacteria, cell debris, or food. Endocytosis – Phagocytosis Transports Large Particles  Pinocytosis (Cellular drinking) Cell brings in a liquid. “cell-drinking” because it brings into the cell fluids with materials suspended in it Ex. Movement of blood  Endocytosis Vesicle forming Endocytosis can occur in three ways Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis