Transport Across Cell Membranes PDF

# Transport across cell membrane ## General Physiology - The plasma membrane acts as a "fence" between the extracellular fluid (ECF) and the cytosol in our cells. - Substances are continuously being exchanged between the cytosol and ECF. The ECF contains substances that the cell needs, including n...

# Transport across cell membrane ## General Physiology - The plasma membrane acts as a "fence" between the extracellular fluid (ECF) and the cytosol in our cells. - Substances are continuously being exchanged between the cytosol and ECF. The ECF contains substances that the cell needs, including nutrients such as glucose, amino acids, and fatty acids, as well as water, ions, oxygen, vitamins, and other macromolecules. Within the cytosol we find substances that have to leave the cell, including waste products such as carbon dioxide. We also find molecules that the cell has manufactured that have to enter the ECF to perform their functions. So both the cell and the body as a whole depend for survival on the transport of substances into and out of the cell. -Obtaining and eliminating substances across the plasma membrane occur through several different processes that are collectively called membrane transport. These processes are organized into two major categories Passive processes & active processes based upon the requirement for expending cellular energy. ### A) Passive processes 1. **Diffusion** - Simple diffusion - Facilitated diffusion 2. **Osmosis** ### B) Active processes 1. **Active transport** - Primary active transport - Secondary active transport 2. **Vesicular transport** - Endocytosis - Exocytosis ## Passive processes - Passive diffusion needs no energy. - Occurs down an electrochemical gradient ("downhill"). ### Simple diffusion - Solutes that are small and nonpolar move into or out of a cell down their concentration gradient by simple diffusion. - These molecules do not require a transport protein. They simply pass between the phospholipid molecules that form the plasma membrane. - Lipid soluble molecules e.g. O2 & CO2 and steroid hormones. ### Facilitated Diffusion - Small solutes that are charged or polar are effectively blocked from passing through the plasma membrane by the nonpolar phospholipid bilayer. Their transport either into or out of the cell must be assisted by plasma membrane proteins in a process called facilitated diffusion. #### I) Channel-mediated diffusion - It is the movement of small ions e.g. Na* or K⁺ across the plasma membrane through protein channels. - Each channel is typically specific for one type of ion. - The channel is either a leak channel, which is continuously open, or a gated channel which is usually closed, opens only in response to a stimulus. #### II) Carrier-mediated diffusion - In carrier-mediated facilitated diffusion, a carrier (also called a transporter) moves a solute down its concentration gradient across the plasma membrane. Since this is a passive process, no cellular energy is required. 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. **• Example of carrier-mediated diffusion:** - An example of a facilitated diffusion carrier protein is the glucose transporter 4 (GLUT4), which is found in muscle cells and adipocytes. - Insulin increases glucose transport into these cells by inducing the insertion of GLUT4 proteins into the plasma membrane. GLUT4 molecules are synthesized in the rough endoplasmic reticulum, packaged in the membrane of secretory vesicles, and stored in the cytosol until insulin triggers their insertion into the plasma membrane. - In diabetes mellitus, the lack of insulin action means fewer GLUT4 transporters in the plasma membrane of these cells, so glucose uptake is decreased, the cells have less energy resources, and glucose stays in the blood where it can have deleterious effects. ## Differences between simple and facilitated diffusion: 1. **Specificity** - The carrier proteins are highly specific for different molecules. A glucose carrier, for example, cannot transport fructose. 2. **Competition** - When two molecules, say A and B are carried by the same protein there occur a competition between the two molecules for the transport. Thus, an increase in the concentration of 'A' molecule will decrease the transport of molecule 'B' and vice versa. No such competition is known to occur in simple diffusion. 3. **Saturation** - In simple diffusion the rate of diffusion increases proportionately with the increase in the concentration of the substance and there is no limit to it. However, in facilitated diffusion the rate of diffusion increases with increase in concentration gradient to reach a limit beyond which a further increase in the diffusion cannot occur. This is called saturation point and here all the binding sites on the carrier proteins are occupied and the system operates at its maximum capacity. ## Types of carrier proteins: 1. **Uniport system** A carrier that transports one substance in one direction e.g. D-glucose. 2. **Co-transport or Symport system:** - A carrier that transports two substances simultaneously in the same direction. e.g. Na*-sugar transporters (glucose, mannose, galactose). 3. **Countertransport or Antiport system:** A carrier that transports one substance in one direction and another substance in the opposite direction. - e.g. Na* in & Ca** out in nerve cell. ## Active transport ### I) Primary active transport **Characteristics of primary active transport:** - Occurs against an electrochemical gradient (uphill). - Requires direct input of metabolic energy in the form of adenosine triphosphate (ATP) and so it is active. - Is carrier-mediated & so exhibits specificity, saturation, & competition. **Example of primary active transport:** **Na*-K+ ATPase (or Na*-K* pump):** -Na*-K+ ATPase in cell membranes transports Na* from intracellular to extracellular fluid & K+ from extracellular to intracellular fluid. -Both Na* & K+ are transported against their electrochemical gradients. -Na⁺ is pumped out of the cell & K* into the cell in a ratio of 3:2. **Functions of the Na*-K* pump:** 1. **Secondary active transport.** - It maintains a Na* concentration gradient across the membrane. Just as water held back by a dam can do work as it flows downward (to generate electricity, for instance), this gradient is a source of potential energy that can be tapped to do other work e.g. uptake of glucose into kidney cells. 2. **Regulation of cell volume.** - Certain anions are confined to the cell and cannot penetrate the plasma membrane. These "fixed anions," such as proteins and phosphates, attract and retain cations. If there were nothing to correct for it, the retention of these ions would cause osmotic swelling and possibly lysis of the cell. Cellular swelling, however, elevates activity of the Na*-K+ pumps. Since each cycle of the pump removes one ion more than it brings in reduces intracellular ion concentration controls osmolarity → prevents cellular swelling. ### II) Secondary active transport **Characteristics of secondary active transport:** a) The transport of two or more solutes is coupled. b) One of the solutes (usually Na*) is transported "downhill" & provides energy for the "uphill" transport of the other solute (s). c) Metabolic energy is not provided directly, but indirectly from the Na* gradient, which is maintained across cell membranes via primary active transport (Na*-K+ pump). **Example of secondary active transport:** **Na*-glucose cotransport:** - This occurs in intestinal mucosal & renal proximal tubule cells. 1) **At the basolateral membrane (A Na*-K* pump creates a concentration gradient of sodium ions):** From the cytosol, sodium ions bind to the Na*-K+ primary active transport pump. - ATP is hydrolyzed, and sodium ions are transported out of the cell, into the ECF, against their concentration gradient. 2) **At the luminal membrane (A carrier protein uses the potential energy of the sodium ion gradient to power the transport of glucose):** - From the lumen, both a sodium ion and a glucose molecule bind to another carrier protein (which is a symporter). - The carrier protein transports the sodium ion and glucose molecule into the cell-the sodium ion with its concentration gradient and the glucose molecule against its gradient. **NB:** - Poisoning the Na*-K+ pump decreases the transmembrane Na* gradient & consequently inhibits Na*-glucose cotransport. ## Vesicular Transport - In vesicular transport, the traffic of materials into or out of the cell takes place in vesicles, small membranous sacs that form at, or fuse with, the plasma membrane. - Because these vesicles move tiny droplets of fluid and solutes rather than single molecules, this process is also known as bulk transport. - The two major types of vesicular transport are endocytosis and exocytosis. - Vesicular transport requires the energy from ATP hydrolysis to fuel several steps of the process, including vesicle formation. ### Endocytosis **• Definition:** -It is the uptake (internalization) of material by cells, includes phagocytosis, pinocytosis, & receptor-mediated endocytosis. **• Types of endocytosis:** a) **Phagocytosis: for particles ("cell eating")** - It is a nonspecific process that occurs when a cell engulfs or captures a large particle (such as a bacterium or a bit of cell debris) external to the cell by forming membrane extensions that are called pseudopodia to surround the particle. Once the particle is engulfed by the pseudopodia, it is enclosed within a membrane sac. When the sac is internalized, its contents are broken down chemically (digested) after it fuses with a lysosome. - Phagocytosis is carried out especially by white blood cells and macrophages. b) **Pinocytosis: for droplets of fluid ("cell drinking")** - This process occurs when the cell internalizes droplets of ECF that contain dissolved solutes. - Multiple, small vesicles are formed. - This process is considered nonspecific because all solutes dissolved within the droplet are taken into the cell. - Most cells perform this type of membrane transport. c) **Receptor-mediated endocytosis:** - It is more selective. It enables a cell to take in specific molecules from the ECF with a minimum of unnecessary fluid. - Specific molecules in the ECF bind to specific receptor proteins on the plasma membrane. The membrane sinks in at this point, creating a pit. The pit soon pinches off to form a vesicle in the cytoplasm. - One use of receptor-mediated endocytosis is the absorption of insulin from the blood. - Hepatitis, polio, and AIDS viruses “trick" cells into admitting them by receptor mediated endocytosis. ### Exocytosis - It is the process by which cells release their secretions to the exterior. - This process is often triggered by an increase in intracellular calcium. **● Release molecules have 3 fates:** 1- Attached to the outer surface of the cell & become peripheral proteins. 2- From part of the extracellular matrix e.g. collagen. 3- Enter ECF → blood stream e.g. hormones. ## The body fluids - Water is the most important molecule in the human body because it is the solvent for all living matter. As we look for life in distant parts of the solar system, one of the first questions scientists ask about a planet is, "Does it have water?" Without water, life as we know it cannot exist. **• Distribution of total body water (TBW):** - The total body water (TBW) is approximately 60% of body weight (42liters). - The percentage of TBW is highest in newborns (80%) and adult males (70%) and lowest in adult females (60%). ## TBW is distributed as follows: (1) **Intracellular fluid (ICF): [28 liters]** - This constitutes about 2/3 of the TBW (about 40% of the body weight). - The major cations of ICF are K+ and Mg2+. - The major anions of ICF are protein and organic phosphates ( [ATP], [ADP], and [AMP]). (2) **Extracellular fluid (ECF): [14 liters]** - This constitutes about 1/3 of TBW (about 20% of the body weight). - The major cation of ECF is Na*. - The major anions of ECF are Cl and HCO3. - It includes the following subdivisions: a) **Intravascular fluid (the plasma): [3.5 liters]** This constitutes about 5% of the body weight. b) **Interstitial fluid: [10.5 liters]** This includes water resides outside the vasculature and occupies spaces between cells (the interstitium). - It constitutes about 15% of the body weight. ## Forces affecting exchange of body fluids ### I) Osmosis -This is the movement of water through a selectively permeable membrane from an area of higher water concentration (lower solute concentration) into an area of lower water concentration (higher solute concentration), either by crossing the plasma membrane directly or by moving through a channel protein (aquaporin channels). **The osmotic pressure of a solution:** - It is the pressure that must be exerted on the side containing the higher solute concentration to prevent the diffusion of water (by osmosis) from the side containing the lower solute concentration. - The osmotic pressure of solution (side 2 in the figure) is equal to the amount of hydrostatic pressure required to stop the osmotic flow. - Osmosis is driven by a force called osmotic pressure, which is the "pulling" force that solutes exert on water molecules. ### II) Filtration - Filtration is the transfer of water and dissolved substances from a region of high pressure to a region of low pressure; the force behind it is hydrostatic pressure. - The filtration rates depends on the amount of hydrostatic pressure. - Hydrostatic pressure is the pressure exerted by a fluid against a wall or membrane. In the body, filtration is powered by blood pressure. ### III) Donnan effect - If selective semipermeable membrane separates two solutions, one of them containing a non-diffusible ion, Donnan and Gibbs showed that that in the presence of a non-diffusible ion, the diffusible ions distribute themselves in such a way that they are at equilibrium: a) Each ionized solution should be electrically neutral. b) The product of diffusible ions on one side of membrane should equalize the other side. - This can be represented diagrammatically as follows: **• Physiological significance of Donnan's effect:** 1. The cells contain non-diffusible anions (prot.) ⇒ exert donnan's equilibrium ⇒ ↑ number of osmotically active particles inside the cell ⇒ draw water into the cell ⇒ cell swelling ⇒ malfunction. Therefore, the cells continually pump Na* out by Na*-K* pump. ↓ osmotically active particles ⇒ no cell swelling ⇒ proper function. In summary, the oncotic force created by a high concentration of protein within the cell is offset by the osmotic force generated by the movement of sodium outside the cell. The pump depends on ATP. Depletion of ATP for any reason ⇒ cell swelling. 2. In the capillaries, blood contains non diffusible prot. anions ⇒ exert Donnan's equilibrium ⇒ ↑ osmotically active ions mainly Na* in the capillaries than in the interstitial fluid ⇒ draw water into the capillaries by osmosis, but this is prevented under healthy conditions by the hydrostatic capillary blood pressure. 3. Because of Donnan's equilibrium, there is asymmetric (unequal) distribution of the permeant ions across the membrane (higher concentration of K+ inside) ⇒ electric difference exists across the membrane whose magnitude can be determined by the Nernst equation (refer to nerve) ⇒ resting membrane potential (RMP). ## Homeostasis - Cells are surrounded by the interstitial fluid which constitutes the “internal environment" for these cell. Life is compatible within narrow limits of change in the in the chemical or physical properties of the internal environment. - Homeostasis means keeping the composition of the internal environment constant, physically & chemically, in response to changing internal or external conditions, such that the cells can survive. - By contrast, the “external environment" of the body is the space that surrounds the entire body. ## Basic components of homeostatic mechanisms: 1. **Receptors = detectors = sensors.** 2. **Afferent pathways.** 3. **Control center "decision-maker".** 4. **Efferent pathways.** 5. **Effector organs.** - 1- The receptors detect changes in the environments both outside & inside the body, and they give signal information to the control center. - 2-The afferent pathway is the component through which signal information are sent from the sensor to the control center. - 3- The control center receives the signal information from the sensors about the change, and it generates the signal commands necessary for correction of the error. The control center is usually a part of the brain, that includes a set point, which is a particular value [normal range the body tries to stay within] e.g. body temperature = (36.5 - 37.5) 37 °C. Control center depends on comparing the received information (the change that occur in the internal environment) to the set value. - 4- Efferent pathways: - This is the components through which the signal commands pass from the center to the effector organs. Efferent pathway may be neural or hormonal. - 5- Effector organs: - They respond to signals from the center, to correct the error. - They are mainly muscles & glands. ## Feedback control of the homeostatic mechanism: - Homeostatic control systems are separated into two broad categories based on whether the system maintains the variable within a normal range by moving the stimulus in the opposite direction, or amplifies the stimulus in the same direction. These two types of feedback control are called negative feedback and positive feedback, respectively - [I] **Negative feedback systems:** - It opposes change & maintain stability. (The effect is opposite to the change). They are very common in mammals & in the human body, and play an important role in many physiological functions: 1- Regulation of body temperature. 2- Regulation of body water & electrolytes. 3- Regulation of arterial blood pressure. 4- Regulation of blood glucose level. 5- Regulation of hormones. - [II] **Positive feedback:** - The initial disturbance in a system sets off a train of events that increases the deviation from the desired level even further. - It magnifies the effect of a disturbance & results in instability. **Some important examples for physical phenomena utilizing positive feedback are:** 1- In blood clotting mechanisms. 2- During excitation of membranes. 3- During birth of baby. As the fetus head pushes into the cervix of uterus, nerve impulses pass from cervix to hypothalamus causing release of oxytocin hormone from the pituitary gland leading to stronger uterine contractions causing further stretch of cervix & further release of oxytocin and so on. ## Cell Communication - Cell communication is essential to orchestrate the activities of cells in the body. - Our bodies are made up of approximately 100 trillion (1014) cells, and each depends on the others for survival, because they must work together to maintain homeostasis. To do so, cells must be able to communicate with each other to carry out coordinated activities such as the maintenance of body temperature...etc. - Communication occurs not only between neighboring cells but also between cells at different locations within the body. **● Types of Cell Communication:** ### I-Direct contact communication: 1. **Gap junctions:** - Adjacent cells have direct channels (gap junctions) linking their cytoplasms. - The main role of gap junctions is to synchronize metabolic activities or electrical signals between cells in a tissue. - For example, gap junctions play a key role in spreading electrical signals from one cell to the next in cardiac muscle. 2. **Cell-cell recognition**. - In this process, animal cells with particular membrane-bound cell-surface molecules dock with one another, initiating communication between the cells. - For example, cell-cell recognition of this kind activates particular cells in the immune system in order to mount an immune response. ### II- Local signaling communication: - In local signaling, a cell releases a signal molecule that diffuses through the aqueous fluid surrounding and between the cells and causes a response in nearby target cells. - The signal molecule is called a local regulator &the process is called paracrine signaling. In some cases the local regulator acts on the same cell that produces it; this is called autocrine signaling. **For example, many of the growth factors that regulate cell division are local regulators that act in both a paracrine and autocrine fashion.** **Another, more specialized type of local signaling called synaptic signaling occurs in the nervous system. An electrical signal along a nerve cell triggers the secretion of neurotransmitter molecules carrying a chemical signal. These molecules diffuse across the synapse, the narrow space between the nerve cell and its target cell (often another nerve cell), triggering a response in the target cell.** ### III-Long-distance signaling communication: - In this form of communication, a controlling cell secretes a long-distance signaling molecule called a hormone which produces a response in target cells that may be far from the controlling cell. - This method is the most common means of cell communication.. - Hormones secreted by controlling cells enter the circulatory system where they travel to target cells elsewhere in the body. **● Stages of Cell Signaling:** 1. **Reception.** - Reception is the target cell detection of a signaling molecule coming from outside the cell. - A chemical signal is detected when the signaling molecule binds to a receptor protein located at the cell surface or inside the cell. **NB: Regulation of reception;** - An important mechanism that cells use to regulate reception is increasing or decreasing the number of each type of receptor. - Depending on the needs of the cell, receptors are synthesized or degraded. - When a ligand (e.g. hormone) is present in excess, the number of the receptors for this ligand decreases and the sensitivity to the ligand also decreases. This is called "Down Regulation" of the receptor sensitivity. This may explain the decreased sensitivity to insulin in some cases of diabetes and the "tolerance" to morphine. - Receptor down regulation often involves transporting receptors to lysosomes, where they are destroyed. 2. **Transduction:** - The binding of the signaling molecule changes the receptor protein in some way, initiating the process of transduction. - The transduction stage converts the signal to a form that can bring about a specific cellular response. - Transduction sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules a signal transduction pathway. 3. **Response:** - The transduced signal finally triggers a specific cellular responses. - The responses depends on the signal and the receptors on the target cell. - Most of these responses fall into three categories: ion channels open or close; enzyme activity is altered, leading to metabolic changes and other effects; and specific gene activity may be turned on or off. **• Types of receptors:** ### I- Cell membrane receptors: - Most signal receptors are plasma membrane proteins. Their ligands are water-soluble and generally too large to pass freely through the plasma membrane. - There are three major types of cell-surface receptors: 1. **G protein-linked receptors [also called G protein-coupled receptor (GPCR)]: ** - It is a cell-surface transmembrane receptor that works with the help of a G protein, a protein that binds guanosine triphosphate (GTP). 2. **Enzyme-linked receptors [Receptor tyrosine kinases (RTKs)]: ** - The part of the receptor protein extending into the cytoplasm functions as a tyrosine kinase, an enzyme that catalyzes the transfer of a phosphate group from ATP to the amino acid tyrosine on a substrate protein. - Receptor tyrosine kinases bind insulin and growth factors. 3. **Ion channel-linked receptors (ligand-gated Channels):** - It is a type of membrane receptor containing a region that can act as a gate. - When a signaling molecule (ligand) binds to the receptor protein, the gate opens or closes, allowing or blocking the flow of specific ions, such as Na* or Ca2+ through a channel in the receptor. ### II- Intracellular Receptors: - Intracellular receptor proteins are found in either the cytoplasm or nucleus of target cells. - The signaling molecules that bind with intracellular receptors are small, hydrophobic molecules that can diffuse across the membranes of target cells. - Signaling molecules that bind with these receptors are steroid hormones and thyroid hormones. **• Signal Transduction Pathways:** - The sequences of events by which the binding of a chemical messenger (hormone, neurotransmitter, or paracrine/ autocrine agent) to a receptor causes the cell to respond. - The "signal" is the receptor activation, and “transduction" denotes the process by which a stimulus is transformed into a response. - During signal transduction, the original signal is amplified. #### I- Pathways Initiated by Lipid-Soluble Messengers: - Lipid-soluble messengers generally act on cells by binding to intracellular receptor proteins. - Lipid-soluble messengers include steroid hormones & the thyroid hormones. - Steroid hormone diffuse through the plasma membrane & binds with its specific cytoplasmic receptor forming hormone -receptor complex. - Hormone -receptor complex enters the nucleus of the cell ⇒ binds to DNA ⇒ activation of specific genes to form mRNA . - mRNA diffuse back into the cytoplasm ⇒ translated in the ribosomes ⇒ formation of proteins with enzymes activity ⇒ hormonal effect. #### II- Pathways Initiated by Water-Soluble Messengers: - Water-soluble messengers exert their actions on cells by binding to receptor proteins on the extracellular surface of the plasma membrane. - Water-soluble messengers include most hormones, neurotransmitters, and paracrine/autocrine compounds. - The intercellular chemical messengers that reach the cell from the extracellular fluid and bind to their specific plasma membrane receptors are often referred to as first messengers. Second messengers, then, are substances that are generated in the cytoplasm as a result of receptor activation by the first messenger. **• The commonest 2nd messengers are:** (1) **Cyclic AMP:** - Binding of hormone (first messenger) to its receptor ⇒ activation of G protein (GDP is replaced by GTP) ⇒ activation of adenyl cyclase (AC) enzyme ⇒ formation of cAMP (second messenger) from ATP = activate protein kinase A (PKA), which catalyze phosphorylation of enzymes or other proteins ⇒ cellular response. (2) **Inositol triphosphate (IP3) & diacylglycerol (DAG):** - Binding of hormone to its receptor ⇒ activation of G protein (GDP is replaced by GTP) ⇒ activation of phospholipase C⇒ hydrolysis of membrane "phosphatidyl inositol diphosphate-PIP2" to inositol triphosphate (IP3) & Diacyl glycerol (DAG). - IP3 causes release of Ca** from endoplasmic reticulum (ER) or sarcoplasmic reticulum (SR) into the cytosol ⇒ intracellular effects. - DAG activate protein kinase C (PKC) ⇒ phosphorylates Proteins ⇒ alters cell functions. (3) **Ca**-calmodulin system: - Many hormones act by ↑ cytoplasmic Ca**: a) Activation of ligand gated Ca** channels ↑ Ca** influx. b) Stimulation of Ca** release from endoplasmic reticulum & mitochondria. - Ca** binds with a protein called calmodulin → Ca**-calmodulin complex ⇒ activation of protein kinases ⇒ physiologic effects. **• Signal amplification:** - The ability of relatively small changes in the concentration of a chemical messenger to elicit marked responses in target cells, a phenomenon known as signal amplification. - To take an actual example, one molecule of the hormone epinephrine can cause the liver to generate and release 10⁹ molecules of glucose. - In a war, the general gives the signal to attack, and thousands of soldiers carry out the order. The general alone could not neutralize thousands of enemies. Likewise, one hormone could not single-handedly produce millions of final products within a few seconds. However, with amplification, one hormone has an army of molecules working simultaneously to produce the final products. **• Signal termination:** **• Definition:** - It is the process of inactivating the receptor and each component of the signal transduction pathway once they have done their jobs. **• Mechanisms:** 1. After a G protein is activated, GTPase catalyzes the hydrolysis of GTP to GDP. This action inactivates the G protein. 2. In the cyclic AMP pathway, any increase in cAMP concentration is temporary. Cyclic AMP is rapidly inactivated by a phosphodiesterase, which converts it to adenosine monophosphate (AMP). **• Significance:** - Signal termination allows molecules in the system to respond to new signals. - Failure to terminate signals can lead to dire consequences. For example, the cholera bacterium releases a toxin that activates proteins in the epithelial cells lining the intestine. The toxin chemically changes the G protein so that it no longer switches off. As a result, the G protein continues to stimulate adenylyl cyclase to make cAMP ⇒ allowing a large flow of chloride ions into the intestine. Water and other ions follow severe watery diarrhea.