Lecture 4 - Cell transport mechanisms-AM.pdf

THE CORE OF HEALTHCARE EDUCATION TOPIC: Cell transport mechanisms- L4 ANP 101 (Presented by: A. Mogibelo) c 2021  LEARNING OUTCOMES By t...

THE CORE OF HEALTHCARE EDUCATION TOPIC: Cell transport mechanisms- L4 ANP 101 (Presented by: A. Mogibelo) c 2021  LEARNING OUTCOMES By the end of this session the student should be able to: ▪Describe the structure of the plasma membrane. ▪Explain the functions of the different components of the plasma membrane. ▪Explain the differences between active and passive transport. ▪Describe the different active and passive transport mechanisms that occur in the cell. THE CORE OF QUALITY HEALTH CARE EDUCATION 2 PLASMA MEMBRANE STRUCTURE THE CORE OF QUALITY HEALTH CARE EDUCATION 3  MEMBRANE STRUCTURE ▪Plasma membrane is an extremely thin layer of lipids and proteins. ▪It forms the outer boundary of every cell and encloses intracellular contents. ▪Phospholipids are the most abundant membrane lipids, with a small amount of cholesterol. ▪Phospholipids have a polar (electrically charged) head containing a negatively charged phosphate group and two non-polar (electrically neutral) fatty-acid chain tails. THE CORE OF QUALITY HEALTH CARE EDUCATION 4 CONT… THE CORE OF QUALITY HEALTH CARE EDUCATION 5  CONT… ▪The polar end is hydrophilic (“water loving”) because it interacts with water molecules. ▪The non-polar end is hydrophobic (“water fearing”) and will not mix with water molecules. THE CORE OF QUALITY HEALTH CARE EDUCATION 6  PHOSPHOLIPID BILAYER ▪In water, phospholipids self-assemble into a lipid bilayer (a double layer of lipid molecules). ▪The hydrophobic tails face the interior or centre of the lipid bilayer away from water. ▪The hydrophilic heads line up on outer and inner surfaces in contact with water. THE CORE OF QUALITY HEALTH CARE EDUCATION 7 CONT… THE CORE OF QUALITY HEALTH CARE EDUCATION 8  FUNCTIONS OF LIPID BILAYER The lipid bilayer serves three important functions: 1. It forms the basic structure of the plasma membrane 2. The hydrophobic interior serves as a barrier to passage of water- soluble substances between ICF and ECF 3. It is responsible for fluidity of membrane THE CORE OF QUALITY HEALTH CARE EDUCATION 9  CHOLESTEROL ▪The lipid bilayer is fluid with a consistency more like cooking oil. ▪Cholesterol contributes to both the fluidity and stability of the membrane. ▪Membrane fluidity allows cells to change shape. THE CORE OF QUALITY HEALTH CARE EDUCATION 10 CONT… THE CORE OF QUALITY HEALTH CARE EDUCATION 11  PLASMA MEMBRANE PROTEINS ▪Plasma membrane contains two types of proteins: oTransmembrane proteins oPeripheral proteins ▪Transmembrane proteins extend through entire thickness of the membrane ▪Peripheral proteins do not penetrate the membrane and are attached on either the outer or inner surface of the membrane THE CORE OF QUALITY HEALTH CARE EDUCATION 12 CONT… THE CORE OF QUALITY HEALTH CARE EDUCATION 13 MEMBRANE TRANSPORT THE CORE OF QUALITY HEALTH CARE EDUCATION 14  MEMBRANE TRANSPORT ▪The plasma membrane is selectively permeable in that it permits certain particles to pass through while excluding others. ▪If a substance can cross the membrane, the membrane is said to be permeable to that substance. ▪If a substance cannot pass, the membrane is impermeable to it. THE CORE OF QUALITY HEALTH CARE EDUCATION 15  FACTORS THAT INFLUENCE PERMEABILITY Two properties of particles influence whether they can permeate the plasma membrane without assistance or not: 1. Relative solubility of particle in lipid 2. Size of particle THE CORE OF QUALITY HEALTH CARE EDUCATION 16  ACTIVE VS PASSIVE TRANSPORT 1. Passive transport – does not require cell to use energy to transport a substance across the membrane 2. Active transport – requires cell to use energy (in the form of ATP) to transport a substance across the membrane THE CORE OF QUALITY HEALTH CARE EDUCATION 17  PASSIVE FORCES ▪Molecules or ions that can penetrate the plasma membrane on their own are passively driven across the membrane by two forces: 1. Diffusion down a concentration gradient 2. Movement along an electrical gradient THE CORE OF QUALITY HEALTH CARE EDUCATION 18  DIFFUSION DOWN A CONCENTRATION GRADIENT ▪All molecules and ions are in continuous random motion at temperatures above absolute zero as a result of thermal energy. ▪Molecules within a particular space tend to become evenly distributed over time. ▪The uniform spreading out of molecules from an area of high concentration to an area of low concentration by random molecular movement is known as simple diffusion (diffusere means “to spread out”). THE CORE OF QUALITY HEALTH CARE EDUCATION 19 THE CORE OF QUALITY HEALTH CARE EDUCATION 20  SOLUTIONS ▪Solutions are homogenous mixtures containing a relatively large amount of one substance called the solvent (the dissolving medium, which in the body is water) and smaller amounts of one or more dissolved substances called solutes. ▪The concentration of a solution refers to the amount of solute dissolved in a specific amount of solution. THE CORE OF QUALITY HEALTH CARE EDUCATION 21 ▪The concentration gradient is the difference in concentration of a solute between two areas that next to each other. THE CORE OF QUALITY HEALTH CARE EDUCATION 22  FICK’S LAW OF DIFFUSION Several factors in addition to concentration gradient influence the rate of net diffusion across a membrane The effects of these factors collectively makes up Fick’s law of diffusion: 1. Magnitude (or steepness) of concentration gradient -Rate of simple diffusion is directly proportional to concentration gradient of substance 2. Surface area of membrane across which diffusion is taking place - The larger the surface area available, the greater the rate of diffusion THE CORE OF QUALITY HEALTH CARE EDUCATION 23 3. Lipid solubility of substance -The greater the lipid solubility of a substance, the more rapidly the substance can diffuse through membrane’s double layer down its concentration gradient 4. Molecular weight of substance -As molecular weight increases, rate of diffusion decreases 5. Distance through which diffusion must take place - The greater the distance, the slower the rate of diffusion 6. Temperature- The higher the temperature, the faster the rate of diffusion. THE CORE OF QUALITY HEALTH CARE EDUCATION 24  ELECTRICAL GRADIENT ▪An electrical gradient refers to a difference in charge between two areas that are next to each other. This promotes movement of ions toward the area of opposite charge ▪When an electrical gradient exists between the ICF and ECF, only ions that can permeate the plasma membrane can move along this gradient. THE CORE OF QUALITY HEALTH CARE EDUCATION 25  ELECTROCHEMICAL GRADIENT ▪Both an electrical and concentration (chemical) gradient may be acting on a particular ion at the same time. ▪The net effect of simultaneous electrical and concentration gradients on this ion is called an electrochemical gradient. THE CORE OF QUALITY HEALTH CARE EDUCATION 26 THE CORE OF QUALITY HEALTH CARE EDUCATION 27 PASSIVE TRANSPORT THE CORE OF QUALITY HEALTH CARE EDUCATION 28  PASSIVE TRANSPORT Passive transport involves the movement of particles down a concentration gradient with no energy being used. Passive transport across the plasma membrane can occur directly through the plasma membrane or through transmembrane proteins. There are three types passive transport: ▪Simple diffusion ▪Facilitated diffusion ▪Osmosis THE CORE OF QUALITY HEALTH CARE EDUCATION 29  SIMPLE DIFFUSION ▪Nonpolar and lipid-soluble substances diffuse directly through the plasma membrane. Examples include oxygen, carbon dioxide and lipid-soluble vitamins. THE CORE OF QUALITY HEALTH CARE EDUCATION 30  FACILITATED DIFFUSION ▪Large water-soluble molecules (e.g. ions, proteins, glucose & amino acids) cannot cross the plasma membrane on their own no matter what forces are acting on them. ▪Facilitated diffusion uses a carrier protein to transfer a particular substance across the membrane from high to low concentration. ▪This process is passive and does not require energy because movement occurs naturally down a concentration gradient. THE CORE OF QUALITY HEALTH CARE EDUCATION 31 THE CORE OF QUALITY HEALTH CARE EDUCATION 32  OSMOSIS ▪Osmosis is the net diffusion of water down its concentration gradient through a selectively permeable membrane. ▪Water specific channels of transmembrane proteins called aquaporins also allow water to move freely. ▪Osmolarity: Is the total concentration of all solute particles in a solution. Water pressure that develops as a result of osmosis is called osmotic pressure. THE CORE OF QUALITY HEALTH CARE EDUCATION 33  TONICITY ▪The tonicity of a solution is the effect the solution has on cell volume when the solution surrounds the cell. Or Measure of the solutions ability to change the volume of cells by altering their water content (Tortora and Derrickson, 2011). ▪The tonicity of a solution is a reflection of its concentration of non-penetrating solutes relative to the cell’s concentration of non-penetrating solutes. ▪An isotonic solution (iso means “equal”) has the same concentration of non- penetrating solutes as normal body cells do. ▪When a cell is bathed in an isotonic solution, no water enters or leaves the cell by osmosis, so cell volume remains constant. THE CORE OF QUALITY HEALTH CARE EDUCATION 34  CONT… ▪A hypotonic solution (hypo means “below”) is a solution with a below- normal concentration of non-penetrating solutes and therefore a higher concentration of water. ▪As a result, water enters the cells by osmosis, causing them to swell or even to rupture. ▪A hypertonic solution (hyper means “above”) is a solution with an above-normal concentration of non-penetrating solutes and therefore a lower concentration of water. ▪As a result, the cells shrink as they lose water by osmosis. THE CORE OF QUALITY HEALTH CARE EDUCATION 35 THE CORE OF QUALITY HEALTH CARE EDUCATION 36 ACTIVE TRANSPORT THE CORE OF QUALITY HEALTH CARE EDUCATION 37  ACTIVE TRANSPORT Active transport uses a carrier protein to transfer a specific substance against its concentration gradient with the use of energy There are two types of active transport: 1. Primary active transport 2. Secondary active transport THE CORE OF QUALITY HEALTH CARE EDUCATION 38  PRIMARY ACTIVE TRANSPORT ▪Primary active transport uses energy directly from ATP to move a substance against its concentration. ▪Carrier protein splits ATP into ADP and inorganic phosphate plus free energy. ▪This free energy enables the substance to move from an area of low concentration to an area of high concentration. THE CORE OF QUALITY HEALTH CARE EDUCATION 39 THE CORE OF QUALITY HEALTH CARE EDUCATION 40  SECONDARY ACTIVE TRANSPORT ▪With secondary active transport, the carrier does not directly split ATP to move a substance against its concentration ▪Instead, energy is provided by the electrochemical gradient created by the Na⁺-K⁺ pump THE CORE OF QUALITY HEALTH CARE EDUCATION 41 THE CORE OF QUALITY HEALTH CARE EDUCATION 42  VESICULAR TRANSPORT ▪The two types of vesicular transport are endocytosis and exocytosis. ▪Large polar molecules, e.g. hormones secreted by endocrine glands, are too big to pass through channels or carrier proteins. ▪They are transported across the plasma membrane by vesicular transport in which they are wrapped by a membrane enclosed vesicle. ▪Vesicular transport requires energy and so is an active method of membrane transport. ▪Energy is required for vesicle formation and vesicle transport within the cell. THE CORE OF QUALITY HEALTH CARE EDUCATION 43  EXOCYTOSIS ▪This is the release of substances originating from within the cell to outside the cell. THE CORE OF QUALITY HEALTH CARE EDUCATION 44  ENDOCYTOSIS ▪This refers to the internalization of extracellular material within a cell. ▪Endocytosis can be accomplished in three ways: -Pinocytosis -Receptor-mediated endocytosis -Phagocytosis THE CORE OF QUALITY HEALTH CARE EDUCATION 45  PINOCYTOSIS ▪In pinocytosis (“cell drinking”), a droplet of extracellular fluid is taken up non-selectively. ▪Plasma membrane dips inwards, forming a pouch that contains a bit of ECF. ▪Plasma membrane then seals surface of pouch, trapping contents in a small, intracellular endocytic vesicle. THE CORE OF QUALITY HEALTH CARE EDUCATION 46 THE CORE OF QUALITY HEALTH CARE EDUCATION 47  RECEPTOR-MEDIATED ENDOCYTOSIS ▪Highly selective process that enables a cell to import specific large molecules that it needs from its environment. ▪Triggered by binding of specific target molecule to a surface membrane receptor specific for that molecule. ▪This binding causes the plasma membrane at that site to pocket inward, then seal at the surface, trapping the bound molecule inside the cell. THE CORE OF QUALITY HEALTH CARE EDUCATION 48 THE CORE OF QUALITY HEALTH CARE EDUCATION 49  PHAGOCYTOSIS ▪During phagocytosis (“cell eating”), large multi-molecular particles are internalized. ▪The cell forms surface projections known as pseudopods (‘false feet’) that completely engulf the particle and trap it within an internalized vesicle. THE CORE OF QUALITY HEALTH CARE EDUCATION 50  SUMMARY OF TRANSPORT MECHANISMS Passive Transport Active Transport Simple diffusion Primary active transport Facilitated diffusion Secondary active transport Osmosis Exocytosis Endocytosis THE CORE OF QUALITY HEALTH CARE EDUCATION 51  SUMMARY (STUDENTS LEAD) ▪The plasma membrane structure. ▪Functions of the different components of the plasma membrane. ▪Distinguish between active and passive transport. ▪Examples of active and passive transport mechanisms THE CORE OF QUALITY HEALTH CARE EDUCATION 52  REFERENCES 1. Sherwood, L. (2009) Human Physiology: From Cells to Systems. 7th Edition. Brookes Cole. 2. Marieb, EN & Hoehn, K (2007). Human Anatomy and Physiology. 9th Ed. Pearson, Boston. 3. Drake RL, Vogl AW, Mitchell AWM (2010). Gray’s Anatomy for Students. 2nd Ed. Churchill Livingstone, Elsevier. 4. Tortora, G. J., Derrickson, B. (2013). Principles of Anatomy and Physiology. 14th Edition. John Wiley and Sons, Inc. THE CORE OF QUALITY HEALTH CARE EDUCATION 53