Physiology Lecture 1&2 PDF

Physiology Doctor Hussam Hadi Kadhum Physiology: is the science that seeks to explain the physical and chemical mechanisms that are responsible for the origin, development, and progression of life. Human Physiology: The science of human physiology attempts to explain the specifi...

Physiology Doctor Hussam Hadi Kadhum Physiology: is the science that seeks to explain the physical and chemical mechanisms that are responsible for the origin, development, and progression of life. Human Physiology: The science of human physiology attempts to explain the specific characteristics and mechanisms of the human body that make it a living being. The fact that we remain alive is the result of complex control systems. Hunger makes us seek food, and sensations of cold make us look for warmth.  The cells The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. The entire body contains about 100 trillion cells. Although many cells of the body often differ markedly from one another, all of them have certain basic characteristics that are alike.  For instance, oxygen reacts with carbohydrate, fat, and protein to release the energy required for all cells to function.  Further, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver products of their chemical reactions into the surrounding fluids.  Almost all cells also have the ability to reproduce additional cells of their own kind. Fortunately, when cells of a particular type are destroyed, the remaining cells of this type usually generate new cells until the supply is replenished. Body Fluids  Body Fluids  About 60 percent of the adult human body is fluid, mainly a water solution of ions and other substances.  Although most of this fluid is inside the cells and is called intracellular fluid(ICF), about one third is in the spaces outside the cells and is called extracellular fluid(ECF).  This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and the tissue fluids by diffusion through the capillary walls. Ions and nutrients needed by the cells to maintain life are in the extracellular fluid. Thus, all cells live in essentially the same environment (the extracellular fluid) environment.  For this reason, the ECF is also called the internal environment of the body.  Cells are capable of living and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment.  The ECF contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion.  The ICF differs significantly from the extracellular fluid; for example, it contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids.  Homeostasis (Steady State)  Homeostasis refers to the body’s ability to maintain a stable internal environment (regulating hormones, body temp., water balance, etc.) (i.e. the maintenance of nearly constant conditions in the internal environment).  Essentially all organs and tissues of the body perform functions that help maintain these relatively constant conditions.  For instance, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients.  Homeostasis (Steady State)  Powerful control systems exist for maintaining the concentrations of sodium and hydrogen ions as well as for most of the other ions, nutrients, and substances in the body at levels that permit the cells, tissues, and organs to perform their normal functions despite wide environmental variations and challenges from injury and diseases.  Normal body functions require the integrated actions of cells, tissues, organs, and the multiple nervous, hormonal, and local control systems that together contribute to homeostasis and good health.  Disease is often considered to be a state of disrupted homeostasis.  Homeostasis  Maintaining homeostasis requires that the body continuously monitors its internal conditions.  Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point.  Control centers in the brain play roles in regulating physiological parameters and keeping them within the normal range.  As the body works to maintain homeostasis, any significant deviation from the normal range will be resisted and homeostasis restored through a process called a feedback loop  Types of feedback loop  Negative feedback is a mechanism in which the effect of the response to the stimulus is to shut off the original stimulus or reduce its intensity.  Negative feedback loops are the body’s most common mechanisms used to maintain homeostasis.  For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream.  These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream.  The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream.  Negative feedback  As glucose concentration in the bloodstream drops, the decrease in concentration is detected by pancreatic alpha cells, and insulin release stops.  This prevents blood sugar levels from continuing to drop below the normal range.  Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain. Negative feedback  When the brain’s temperature regulation center receives data from the sensors indicating that the body’s temperature exceeds its normal range, it stimulates a cluster of brain cells referred to as the “heat-loss center.” This stimulation has three major effects: 1. Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin allowing the heat to radiate into the environment. 2. As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it. 3. The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.  In contrast, activation of the brain’s heat-gain center by exposure to cold  Reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins.  This arrangement traps heat closer to the body core and restricts heat loss.  If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering.  The muscle contractions of shivering release heat while using up ATP..  The brain triggers the thyroid gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells throughout the body.  The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose also results in increased metabolism and heat production.  Positive feedback  Positive feedback intensifies a change in the body’s physiological condition rather than reversing it.  A deviation from the normal range results in more change, and the system moves farther away from the normal range.  Positive feedback in the body is normal only when there is a definite end point.  Childbirth is an example of positive feedback loops that is normal but it activated only when needed.  Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired.  Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy. And the events of childbirth, once begun, must progress rapidly to a conclusion or the life of the mother and the baby are at risk.  The extreme muscular work of labor and delivery are the result of a positive feedback system Positive feedback Transport of Substances through Cell Membranes  Transport of Substances through Cell Membranes  One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell.  These substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), which must leave the cell.  The membrane’s lipid bilayer structure provides the first level of control.  The phospholipids are tightly packed together, and the membrane has a hydrophobic interior.  This structure causes the membrane to be selectively permeable.  Transport of Substances through Cell Membranes  A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided.  In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar).  Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol.  However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. Transport of Substances through Cell Membranes  The cell membrane consists of a lipid bilayer with cell membrane transport proteins many of which penetrate all the way through the membrane.  The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease. Transport of Substances through Cell Membranes  The molecular structures of proteins interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane.  Many of these penetrating proteins can function as transport proteins.  Different proteins function differently.  Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called channel proteins. Transport of Substances through Cell Membranes  Other proteins, called carrier proteins, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane.  Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane.  Difference between channel proteins and carrier proteins  Channel proteins provide water filled pores for charged ions to pass through  Carrier proteins bind to larger molecules, and change their shape, so molecules can diffuse through  All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. 1. Passive transport: is the movement of substances across the membrane without the expenditure of cellular energy. 2. Active transport: is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).  Passive Transport  In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion.  A concentration gradient is the difference in concentration of a substance across a cell membrane.  Molecules (or ions) will diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space.  When molecules move in this way, they are said to move down their concentration gradient.  Three common types of passive transport include simple diffusion, facilitated diffusion and osmosis.  Simple Diffusion  Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane.  It is the movement of particles from an area of higher concentration to an area of lower concentration.  Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2.  O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them.  Neither of these examples requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane.  Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell.  Simple Diffusion  On the other hand, because cells produce CO2 as a byproduct of metabolism, CO2 concentrations rise within the cytoplasm; therefore, CO2 will move from the cell through the lipid bilayer and into the interstitial fluid, where its concentration is lower.  This mechanism of molecules spreading from where they are more concentrated to where they are less concentration is a form of passive transport called simple diffusion.  The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move Simple Diffusion  Facilitated diffusion  The diffusion process used for those substances that cannot cross the lipid bilayer due to their size and/or polarity.  A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP.  Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.  There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell.  Facilitated diffusion  Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins.  Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge.  Carrier proteins are more selective, often only allowing one particular type of molecule to cross.  Difference between simple and Facilitated diffusion mechanism Simple diffusion Facilitated diffusion Rate of diffusion Concentration gradient Concentration gradient through carrier protein Saturation continued according to Reach saturation when all concentration gradient carrier proteins are difference occupied Example O2 and CO2 Glucose  Diffusion Versus Active Transport  Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes: diffusion or active transport.  Diffusion means random molecular movement of substances molecule either through intermolecular spaces in the membrane or in combination with a carrier protein, where the movement from a high concentration of molecules to a low concentration of molecules. The energy that causes diffusion is the energy of the normal kinetic motion of matter.  In contrast, active transport means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a high-concentration state. This movement requires an additional source of energy besides kinetic energy. Diffusion of Lipid-Soluble Substances through the Lipid Bilayer  An important factor that determines how rapidly a substance diffuses through the lipid bilayer is the lipid solubility of the substance.  For instance, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution.  The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist. Diffusion of Water and Other Lipid-Insoluble Molecules through Protein Channels  Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane.  Many of the body’s cell membranes contain protein “pores” called aquaporins that selectively permit rapid passage of water through the membrane.  The rapidity with which water molecules can diffuse through most cell membranes is astounding  For example, the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell itself.  Other lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water soluble and small enough.  However, as they become larger, their penetration falls off rapidly. For instance, the diameter of the urea molecule is only 20 percent greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes. Diffusion through Protein Pores and Channels—Selective Permeability and “Gating” of Channels  Substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other.  Pores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through.  For example, protein pores that called aquaporins or water channels, permit rapid passage of water through cell membranes but exclude other molecules.  Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file and its too narrow to permit passage of any hydrated ions.  The protein channels are distinguished by two important characteristics: (1) They are often selectively permeable to certain substances, and (2) many of the channels can be opened or closed by gates that are regulated by electrical signals (voltage-gated channels) or chemicals that bind to the channel proteins (ligand-gated channels).  Selective Permeability of Protein Channels  Many of the protein channels are highly selective for transport of one or more specific ions or molecules.  This selectivity results from the characteristics of the channel, such as its diameter, its shape, and the nature of the electrical charges and chemical bonds along its inside surfaces for example Potassium channels permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions.  One of the most important of the protein channels, the sodium channel, is only 0.3 to 0.5 nanometer in diameter, but more important, the inner surfaces of this channel are lined with amino acids that are strongly negatively charged, as shown by the negative signs inside the channel proteins.  These strong negative charges can pull small dehydrated sodium ions into these channels, actually pulling the sodium ions away from their hydrating water molecules. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for passage of sodium ions.  Gating of Protein Channels Gating of protein channels provides a means of controlling ion permeability of the channels. It is believed that some of the gates can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule itself. The opening and closing of gates are controlled in two principal ways: 1. Voltage gating. In the case of voltage gating, the molecular conformation of the gate or of its chemical bonds responds to the electrical potential across the cell membrane.  For instance, a strong negative charge on the inside of the cell membrane could presumably cause the outside sodium gates to remain tightly closed; conversely, when the inside of the membrane loses its negative charge, these gates would open suddenly and allow sodium to pass inward through the sodium pores. This process is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals.  Gating of Protein Channels  Conversely potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential. 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein, which causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate.  One of the most important instances of chemical gating is the effect of acetylcholine on the so-called acetylcholine channel.  This gate is important for the transmission of nerve signals from one nerve cell to another and from nerve cells to muscle cells to cause muscle contraction. 3. Mechanically gated channels. Where physical stretching of the membrane may affect the conformation of some channel proteins.  factors affecting rate of diffusion 1- Difference in concentration Higher difference in concentration gradient lead to gradient higher rate of diffusion 2-Size of molecules or The smaller the size, the higher the rate ions/molecular weight 3- temperature the higher the temperature, the higher the rate 4- Surface area The larger the surface area, the higher the rate 5-Solubility of solute in the higher the solubility, the higher the rate transporter membrane 6-Thickness of membrane inversely proportional  Osmosis  Osmosis is the diffusion of water through a semipermeable membrane.  Water can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself.  However, it is concentration of solutes within the water that determine whether or not water will be moving into the cell, out of the cell, or both.  Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient.  If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration).  Osmosis  In the beaker on the left, the solution on the right side of the membrane is hypertonic. Solutes within a solution create osmotic pressure, a pressure that pulls water.  Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell.  The more solute a solution contains, the greater the osmotic pressure that solution will have.  A solution that has a higher concentration of solutes than another solution is said to be hypertonic. Water molecules tend to diffuse into a hypertonic solution because the higher osmotic pressure pulls water.  Osmosis  If a cell is placed in a hypertonic solution, the cells will shrink as water leaves the cell via osmosis.  In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic.  Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting, a process called lysis.  A critical aspect of homeostasis in living things is to create an internal environment in which all of the body’s cells are in an isotonic solution, an environment in which two solutions have the same concentration of solutes (equal osmotic pressure).  When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, so water flows both in and out and the cells maintain their normal shape (and function).  Various organ systems, particularly the kidneys, work to maintain this homeostasis.  Osmosis  A hypertonic solution has a solute concentration higher than another solution.  An isotonic solution has a solute concentration equal to another solution.  A hypotonic solution has a solute concentration lower than another solution.  Active transport  At times, a large concentration of a substance is required in the intracellular fluid even though the extracellular fluid contains only a small concentration.  This situation is true, for instance, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell even though their concentrations in the extracellular fluid are great.  This situation is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane.  Active transport  Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells.  When a cell membrane moves molecules or ions “uphill” against a concentration gradient (or “uphill” against an electrical or pressure gradient), the process is called active transport.  Primary Active Transport and Secondary Active Transport  Active transport is divided into two types according to the source of the energy used to facilitate the transport:  primary active transport and secondary active transport.  In primary active transport, the energy is derived directly from breakdown of adenosine triphosphate (ATP) or some other high- energy phosphate compound.  In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport.  Primary Active Transport and Secondary Active Transport  In both instances, transport depends on carrier proteins that penetrate through the cell membrane, as is true for facilitated diffusion.  However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient.  Among the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions.  Primary active transport  The active transport mechanism that has been studied in greatest detail is the sodium-potassium (Na+-K+) pump which is a transport process that pumps sodium ions outward through the cell membrane of all cells and at the same time pumps potassium ions from the outside to the inside.  This pump is responsible for maintaining the sodium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells.  Essential for oxygen utilization by the kidneys  Primary active transport  This pump is also considered the basis of nerve function, transmitting nerve signals throughout the nervous system. When two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated.  Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy.  This liberated energy is then believed to cause a chemical and conformational change in the protein carrier molecule, extruding the three sodium ions to the outside and the two potassium ions to the inside.  The Na+-K+ Pump Is Important for Controlling Cell Volume  One of the most important functions of the Na+-K+ pump is to control the volume of each cell.  Without function of this pump, most cells of the body would swell until they burst.  The mechanism for controlling the volume is as follows: Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell.  Most of these proteins and other organic molecules are negatively charged and therefore attract large numbers of potassium, sodium, and other positive ions as well. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this process is checked, the cell will swell indefinitely until it bursts.  The Na+-K+ Pump Is Important for Controlling Cell Volume  The normal mechanism for preventing this outcome is the Na+- K+ pump.  Note again that this device pumps three Na+ ions to the outside of the cell for every two K+ ions pumped to the interior.  Also, the membrane is far less permeable to sodium ions than it is to potassium ions, and thus once the sodium ions are on the outside, they have a strong tendency to stay there.  This process thus represents a net loss of ions out of the cell, which initiates osmosis of water out of the cell as well. If a cell begins to swell for any reason, the Na+-K+ pump is automatically activated, moving still more ions to the exterior and carrying water with them.  Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume.  Electrogenic Nature of the Na+-K+ Pump  The fact that the Na+-K+ pump moves three Na+ ions to the exterior for every two K+ ions that are moved to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior for each cycle of the pump.  This action creates positivity outside the cell but results in a deficit of positive ions inside the cell; that is, it causes negativity on the inside.  Therefore, the Na+-K+ pump is said to be electrogenic because it creates an electrical potential across the cell membrane and this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals. Electrogenic Nature of the Na+-K+ Pump  Secondary active transport—co-transport and counter-transport  In secondary active transport, the movement of an ion down its electrochemical gradient is coupled to the transport of another molecule, such as a nutrient like glucose or an amino acid.  Thus, transporters that mediate secondary active transport have two binding sites, one for an ion typically but not always Na and another for the co- transported molecule.  When sodium ions are transported out of cells by primary active transport, a large concentration gradient of sodium ions across the cell membrane usually develops, with high concentration outside the cell and low concentration inside.  This gradient represents a storehouse of energy because the excess sodium outside the cell membrane is always attempting to diffuse to the interior.  Secondary active transport—co-transport and counter- transport  Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane.  This phenomenon, called co-transport (Symport), is one form of secondary active transport that involve the transport of Na+ via its concentration gradient coupled to the transport of other substances in the same direction.  Example sodium/ glucose co-transport from the lumen of intestine and renal tubules into the lining epithelial cells.  Secondary active transport—co-transport and counter- transport  Once they both are attached to a carrier protein, the energy gradient of the sodium ion causes both the sodium ion and the other substance to be transported together to the interior of the cell.  In counter-transport (Antiport), the transport of Na+ via its concentration gradient is coupled to the transport of other substance in the opposite direction.  Secondary active transport—co-transport and counter- transport  The sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient.  However, this time, the substance to be transported is on the inside of the cell and must be transported to the outside.  Therefore, the sodium ion binds to the carrier protein where it projects to the exterior surface of the membrane, while the substance to be counter- transported binds to the interior projection of the carrier protein.  Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior. This transport occurs in Sodium-Hydrogen counter transport in the proximal tubule of the kidneys, and Sodium-Calcium exchange in the cardiac cells. THANK YOU