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How Does Glucose Transport Across The Cell Membrane?

Passive Transport And Active Transport Across A Cell Membrane Article

Passive Transport And Active Transport Across A Cell Membrane Article

The cell membrane is one of the great multi-taskers of biology. It provides structure for the cell, protects cytosolic contents from the environment, and allows cells to act as specialized units. A membrane is the cell’s interface with the rest of the world - it’s gatekeeper, if you will. This phospholipid bilayer determines what molecules can move into or out of the cell, and so is in large part responsible for maintaining the delicate homeostasis of each cell. Some cells function best at a pH of 5, while others are better at pH 7. The steroid hormone aldosterone is made in the adrenal gland, but affects mostly the kidney. Sodium is more than ten times more concentrated outside of cells rather than inside. If our cells couldn’t control what crossed their membranes, either no molecules would make it across, or they’d be traveling willy-nilly and the internal environment would always be in flux. It’d be like taking every item on a menu and blending it together before serving (not the tastiest idea). So how do cells maintain different concentrations of proteins and molecules despite the pressures on them to be homogenous? Cell membranes are semipermeable, meaning they have control over what molecules can or cannot pass through. Some molecules can just drift in and out, others require special structures to get in and out of a cell, while some molecules even need an energy boost to get across a cell membrane. Each cell’s membrane contains the right mix of these structures to help that cell keep its internal environment just right. There are two major ways that molecules can be moved across a membrane, and the distinction has to do with whether or not cell energy is used. Passive mechanisms like diffusion use no energy, while active transport requires energy to g Continue reading >>

Membrane Transport Mechanisms

Membrane Transport Mechanisms

Diffusion The hydrophobic layer of the plasma membrane creates a barrier that prevents the diffusion of most substances. Exceptions are small molecules such as gases like nitric oxide (NO) and carbon dioxide (CO2), and nonpolar substances such as steroid hormones and fatty acids. Even though fatty acids can diffuse across the plasma membrane, this occurs slowly. Recent work indicates that a substantial amount of fatty acid transport is via carrier proteins. Channels Channels are large proteins in which multiple subunits are arranged in a cluster so as to form a pore that passes through the membrane. Each subunit consists of multiple transmembrane domains. Most of the channels that we will consider are ion channels. Another important type of channel protein is an aquaporin. Aquaporins are channels that allow water to move rapidly across cell membranes. Movement through a channel does not involve specific binding (see facilitated diffusion below). The two factors that affect the flow of ions through an open ion channel are the membrane potential and the concentration gradient. Note that when ions move through a channel across a membrane, this changes the membrane potential (depolarization or hyperpolarization). Changes in membrane potential are used to code information, particularly in the nervous system. See the web page on Membrane Potentials. Properties of Ion Channels For any ion channel, there are two important properties to consider: selectivity and gating. Selectivity refers to which ion (Na+, K+, Ca++, or Cl-) is allowed to travel through the channel. Most ion channels are specific for one particular ion. Gating refers to what opens or closes a channel. Below we classify different ion channels according to the type of gating. Ungated A few types of ion channels ar Continue reading >>

Transport Across Cell Membranes

Transport Across Cell Membranes

Facilitated Diffusion of Ions Ligand-gated ion channels. External Ligands Internal Ligands Mechanically-gated ion channels Voltage-gated ion channels The Patch Clamp Technique Facilitated Diffusion of Molecules Active Transport Direct Active Transport The Na+/K+ ATPase The H+/K+ ATPase The Ca2+ ATPase of skeletal muscle ABC Transporters Indirect Active Transport Symport Pumps Antiport Pumps Some inherited ion-channel diseases Osmosis Hypotonic Solutions Isotonic Solutions Hypertonic Solutions All cells acquire the molecules and ions they need from their surrounding extracellular fluid (ECF). There is an unceasing traffic of molecules and ions in and out of the cell through its plasma membrane Examples: glucose, Na+, Ca2+ In eukaryotic cells, there is also transport in and out of membrane-bounded intracellular compartments such as the nucleus, endoplasmic reticulum, and mitochondria. Examples: proteins, mRNA, Ca2+, ATP 1. Relative concentrations Molecules and ions move spontaneously down their concentration gradient (i.e., from a region of higher to a region of lower concentration) by diffusion. Molecules and ions can be moved against their concentration gradient, but this process, called active transport, requires the expenditure of energy (usually from ATP). 2. Lipid bilayers are impermeable to most essential molecules and ions. The lipid bilayer is permeable to water molecules and a few other small, uncharged, molecules like oxygen (O) and carbon dioxide (CO). These diffuse freely in and out of the cell. The diffusion of water through the plasma membrane is of such importance to the cell that it is given a special name: osmosis. Lipid bilayers are not permeable to: ions such as K+, Na+, Ca2+ (called cations because when subjected to an electric field they migrate towa Continue reading >>

Can Glucose Diffuse Through The Cell Membrane By Simple Diffusion?

Can Glucose Diffuse Through The Cell Membrane By Simple Diffusion?

Glucose is a six-carbon sugar that is directly metabolized by cells to provide energy. The cells along your small intestine absorb glucose along with other nutrients from the food you eat. A glucose molecule is too large to pass through a cell membrane via simple diffusion. Instead, cells assist glucose diffusion through facilitated diffusion and two types of active transport. Cell Membrane A cell membrane is composed of two phospholipid layers in which each molecule contains a single phosphate head and two lipid, or fatty acid, tails. The heads align along the inner and outer boundaries of the cell membrane, while the tails occupy the space in between. Only small, nonpolar molecules can pass through the membrane through simple diffusion. The lipid tails reject polar, or partially charged, molecules, which include many water-soluble substances such as glucose. However, the cell membrane is peppered with transmembrane proteins that provide passage to molecules that the tails would otherwise block. Facilitated Diffusion Facilitated diffusion is a passive transport mechanism in which carrier proteins shuttle molecules across the cell membrane without using the cell’s energy supplies. Instead, the energy is provide by the concentration gradient, which means that molecules are transported from higher to lower concentrations, into or out of the cell. The carrier proteins bind to glucose, which causes them to change shape and translocate the glucose from one side of the membrane to the other. Red blood cells use facilitated diffusion to absorb glucose. Primary Active Transport The cells along the small intestine use primary active transport to ensure that glucose only flows one way: from digested food to the inside of cells. Active transport proteins use adenosine triphospha Continue reading >>

Transport Across Epithelia

Transport Across Epithelia

Go to: The Intestinal Epithelium Is Highly Polarized An epithelial cell is said to be polarized because one side differs in structure and function from the other. In particular, its plasma membrane is organized into at least two discrete regions, each with different sets of transport proteins. In the epithelial cells that line the intestine, for example, that portion of the plasma membrane facing the intestine, the apical surface, is specialized for absorption; the rest of the plasma membrane, the lateral and basal surfaces, often referred to as the basolateral surface, mediates transport of nutrients from the cell to the surrounding fluids which lead to the blood and forms junctions with adjacent cells and the underlying extracellular matrix called the basal lamina (Figure 15-23). Extending from the lumenal (apical) surface of intestinal epithelial cells are numerous fingerlike projections (100 nm in diameter) called microvilli (singular, microvillus). Often referred to collectively as the brush border because of their appearance, microvilli greatly increase the area of the apical surface and thus the number of transport proteins it can contain, enhancing the absorptive capacity of the intestinal epithelium. A bundle of actin filaments that runs down the center of each microvillus gives rigidity to the projection. Overlying the brush border is the glycocalyx, a loose network composed of the oligosaccharide side chains of integral membrane glycoproteins, glycolipids, and enzymes that catalyze the final stages in the digestion of ingested carbohydrates and proteins (Figure 15-24). The action of these enzymes produces monosaccharides and amino acids, which are transported across the intestinal epithelium and eventually into the bloodstream. Go to: Transepithelial Movement Continue reading >>

Transport Across Cell Membranes

Transport Across Cell Membranes

Three-dimensional structure of a recombinant cardiac gap junction membrane channel determined by electron crystallography. These channels allow the direct exchange of ions and small molecules between adjacent cells. Each channel is formed by association of six connexin subunits, each of which contains four α helices, in one plasma membrane, with a similar structure in the plasma membrane of an adjacent cell. [From V. Unger et al., 1999, Science 283:1176; courtesy of Mark Yeager.] The plasma membrane is a selectively permeable barrier between the cell and the extracellular environment. Its permeability properties ensure that essential molecules such as glucose, amino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell. In short, the selective permeability of the plasma membrane allows the cell to maintain a constant internal environment. In several earlier chapters, we examined the components and structural organization of cell membranes (see Figures 3-32 and 5-30). The phospholipid bilayer — the basic structural unit of biomembranes — is essentially impermeable to most water-soluble molecules, such as glucose and amino acids, and to ions. Transport of such molecules and ions across all cellular membranes is mediated by transport proteins associated with the underlying bilayer. Because different cell types require different mixtures of low-molecular-weight compounds, the plasma membrane of each cell type contains a specific set of transport proteins that allow only certain ions or molecules to cross. Similarly, organelles within the cell often have a different internal environment from that of the surrounding cytosol, and organelle membranes contain specific transport proteins that maintain this di Continue reading >>

C2006/f2402 '11 -- Outline For Lecture #6

C2006/f2402 '11 -- Outline For Lecture #6

Handouts: 6A-- Transport of glucose through body (gif) 6A-- pdf 6B -- RME (gif) 6B --RME (pdf) 6C -- Structure of Capillaries & Transcytosis (Posted on Courseworks). Here are links for a diagram of a capillary, a diagram of transcytosis, and an electron micrograph of a capillary. I. Putting all the Methods of Transport of Small Molecules Together or What Good is All This? A. How glucose gets from lumen of intestine → muscle and adipose cells. An example of how the various types of transport are used. (Handout 6A) Steps in the process: 1. How glucose exits lumen. Glucose crosses apical surface of epithelial cells primarily by Na+/Glucose co-transport. (2o act. transport). 2. Role of Na+/K+ pump. Pump in basolateral (BL) surface keeps Na+ in cell low, so Na+ gradient favors entry of Na+. (1o act. transport) 3. How glucose exits epithelial cells. a. Glucose (except that used for metabolism of epithelial cell) exits BL surface of cell by facilitated diffusion = carrier mediated transport. b. Transporter protein = GLUT2 (more details on GLUT family of proteins below). c. When glucose leaves cells it enters the interstitial fluid = IF = fluid in between body cells. 4. How glucose enters and leaves capillaries -- by simple diffusion through spaces between the cells. Cells surrounding capillaries in most of body are not joined by tight junctions. a. Material does NOT enter capillaries by diffusion across a membrane. Material diffuses through liquid in spaces (pores) between the cells. b. For structure of capillaries, see handout 6C, bottom. (Also see links at start of lecture.)Pictures are provided on handout since function is hard to understand without the anatomy. Picture shows how endothelial cells surround capillary lumen, forming pores between cells. Pores allow diffusio Continue reading >>

Glucose Uptake

Glucose Uptake

Method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose. The two ways in which glucose uptake can take place are facilitated diffusion (a passive process) and secondary active transport (an active process which depends on the ion-gradient which is established through the hydrolysis of ATP, known as primary active transport). Facilitated diffusion[edit] There are over 10 different types of glucose transporters; however, the most significant for study are GLUT1-4. GLUT1 and GLUT3 are located in the plasma membrane of cells throughout the body, as they are responsible for maintaining a basal rate of glucose uptake. Basal blood glucose level is approximately 5mM (5 millimolar). The Km value (an indicator of the affinity of the transporter protein for glucose molecules; a low Km value suggests a high affinity) of the GLUT1 and GLUT3 proteins is 1mM; therefore GLUT1 and GLUT3 have a high affinity for glucose and uptake from the bloodstream is constant. GLUT2 in contrast has a high Km value (15-20mM) and therefore a low affinity for glucose. They are located in the plasma membranes of hepatocytes and pancreatic beta cells (in mice, but GLUT1 in human beta cells; see Reference 1). The high Km of GLUT2 allows for glucose sensing; rate of glucose entry is proportional to blood glucose levels. GLUT4 transporters are insulin sensitive, and are found in muscle and adipose tissue. As muscle is a principal storage site for glucose and adipose tissue for triglyceride (into which glucose can be converted for storage), GLUT4 is important in post-prandial uptake of excess glucose from the bloodstream. Moreover, several recent papers show that GLUT 4 is present in the brain also. The drug Metformin phosphor Continue reading >>

Movement Of Substances Across Cell Membranes

Movement Of Substances Across Cell Membranes

Diffusion Diffusion is the spontaneous movement of a substance down its concentration gradient from higher to lower concentration. If you have a bottle of perfume and take the cap off. The perfume molecules will waft out and diffuse into the air where there is a lower concentration of them. All atoms and molecules are in motion, the only way you could stop all motion is to cool things down to absolute zero (-273.15 C or 0K). On the bottom of b25 , look at the diffusion thing and read it. Factors that affect the RATE of diffusion 1. Difference in concentration between the inside and outside of the cell. The bigger the difference between concentrations, the diffusion will be faster. 2. The size of the chemical substance. O2 is two atoms. Glucose is 24 atoms big. Protein is massive. Oxygen can easily diffuse across a cell membrane. Sugar can kind of, that’s why it’s assisted by a transporter protein to facilitate it. The proteins don’t move at all. 3. Temperature. Higher temps = molecules move faster. ‘Nuff said. 4. Whether the chemical substance is water-soluble or lipid soluble. The lipid soluble goes through faster because the cell membrane is phospholipids and can easily diffuse through a fatty membrane. The effect of osmosis on cells Osmosis is simply the diffusion of water. Anytime water flows in or out, it’s called osmosis. The water flows to wherever there is more solute. 1. The cytoplasm of a cell is 80% water, but not completely. Anything that’s not water are called solutes, such as electrolytes. If we surround the cell with a fluid that’s similarly 80% water and 20% “other stuff” on the outside of the cell. The “other stuff” don’t even have to be the same things as the inside of the cell. With osmosis we only care about the concentration Continue reading >>

Facilitated Diffusion And Active Transport Of Glucose

Facilitated Diffusion And Active Transport Of Glucose

Concept 4 Review Whether a cell uses facilitated diffusion or active transport depends on the specific needs of the cell. For example, the sugar glucose is transported by active transport from the gut into intestinal epithelial cells, but by facilitated diffusion across the membrane of red blood cells. Why? Consider how different these two environments are. Epithelial cells lining the gut need to bring glucose made available from digestion into the body and must prevent the reverse flow of glucose from body to gut. We need a mechanism to ensure that glucose always flows into intestinal cells and gets transported into the bloodstream, no matter what the gut concentration of glucose. Imagine what would happen if this were not so, and intestinal cells used facilitated diffusion carriers for glucose. Immediately after you ate a candy bar or other food rich in sugar, the concentration of glucose in the gut would be high, and glucose would flow "downhill" from the gut into your body. But an hour later, when your intestines were empty and glucose concentrations in the intestines were lower than in your blood and tissues, facilitated diffusion carriers would allow the glucose in blood and tissues to flow "downhill," back into the gut. This would quickly deplete your short-term energy reserves. Because this situation would be biologically wasteful and probably lethal, it is worth the additional energy cost of active transport to make sure that glucose transport is a one-way process. By contrast, erythrocytes (red blood cells) and most other tissues in your body move glucose by facilitated diffusion carriers, not by active transport. Facilitated diffusion makes sense in this context because the environment is different for red blood cells and the gut. Whereas the gut experiences Continue reading >>

Sodium And Glucose Transport Across Cell Membrane | Science 2.0

Sodium And Glucose Transport Across Cell Membrane | Science 2.0

Sodium and glucose transport across cell membrane Cell membrane composed of lipids is impermeable to glucose which is a polar compound. Transport of glucose across the cell membrane requires a carrier protein located in cell membrane. In plant system it is triose phosphate which is transported across the chloroplast . Availability or lack of Pi determines the transport of metabolites across chloroplast besides other factors. Cell membrane also has Na anti-porters. ATPase have some important role to play in it. Recently I came across a review in nature ( Chao and Henry (2010: Nature review Drug discovery Nature.com/nrd/collections/type2diabetes pp 30 which describes SGLT2 mediated reabsorption in the kidney. Sodium glucose co transporter 2 (SGLT2) catalyses active transport of glucose (against a concentration gradient ) across the luminal membrane by coupling it with the downhill transport of Na +., Earlier Sopory from ICGEB has reported role of Glyoxylase I in salinity tolerance . I was wondering in what way basic mechanisms of plants and animals are related to each other . Do we require a better understanding for sodium resistance in glucose transport systems in plants . Continue reading >>

Glucose Transport

Glucose Transport

The oxidation of glucose represents a major source of metabolic energy for mammalian cells. Because the plasma membrane is impermeable to polar molecules such as glucose, the cellular uptake of this important nutrient is accomplished by special carrier proteins called glucose transporters[1][2][3][4][5][6][7]. These are integral membrane proteins located in the plasma membrane that bind glucose and transfer it across the lipid bilayer. The rate of glucose transport is limited by the number of glucose transporters on the cell surface and the affinity of the transporters for glucose. There are two classes of glucose carriers described in mammalian cells: the Na+-glucose cotransporters (SGLTs) and the facilitative glucose transporters (GLUTs)[1-7]. There are two families of glucose transporters The Na+-glucose cotransporter or symporter is expressed by specialized epithelial (brush border) cells of the small intestine and the proximal tubule of the kidney and mediates an active, Na+-linked transport process against an electrochemical gradient[1-3] . It actively transports glucose from the lumen of the intestine or the nephron against its concentration gradient by coupling glucose uptake with that of Na+, which is being transported down its concentration gradient. The Na+ gradient is maintained by the active transport of Na+ across the basolateral (antiluminal) surface of the brush border cells by membrane-bound Na+-K+- ATPase[1-3,7]. The second class of glucose carriers is the facilitative glucose transporters (GLUTs) of which there are 14 genes in the human genome[1,4-7] . These proteins mediate a bidirectional and energy-independent process of glucose transport in most tissues and cells where glucose is transported down its concentration gradient by facilitative diffusio Continue reading >>

16 3.1 The Cell Membrane

16 3.1 The Cell Membrane

Structure and Composition of the Cell Membrane The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions. A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 1). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.” A hydrophobic molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away. The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interi Continue reading >>

Transport Of Small Molecules

Transport Of Small Molecules

Go to: Passive Diffusion The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion. During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane. No membrane proteins are involved and the direction of transport is determined simply by the relative concentrations of the molecule inside and outside of the cell. The net flow of molecules is always down their concentration gradient—from a compartment with a high concentration to one with a lower concentration of the molecule. Passive diffusion is thus a nonselective process by which any molecule able to dissolve in the phospholipid bilayer is able to cross the plasma membrane and equilibrate between the inside and outside of the cell. Importantly, only small, relatively hydrophobic molecules are able to diffuse across a phospholipid bilayer at significant rates (Figure 12.15). Thus, gases (such as O2 and CO2), hydrophobic molecules (such as benzene), and small polar but uncharged molecules (such as H2O and ethanol) are able to diffuse across the plasma membrane. Other biological molecules, however, are unable to dissolve in the hydrophobic interior of the phospholipid bilayer. Consequently, larger uncharged polar molecules such as glucose are unable to cross the plasma membrane by passive diffusion, as are charged molecules of any size (including small ions such as H+, Na+, K+, and Cl-). The passage of these molecules across the membrane instead requires the activity of specific transport and channel proteins, which therefore control the traffic of most biological molecules into and out of the cell. Go to: Facilitated Diffusion and Carrier Proteins Facilitated diffusion, Continue reading >>

How Do Sugar Molecules Cross The Cell Membrane?

How Do Sugar Molecules Cross The Cell Membrane?

Sugar molecules cannot cross the cell membrane on their own. Special proteins embedded in the cell membrane are required to transport sugar across the cell membrane. Read on to learn more about this process and take a quiz. Why Cells Need Sugar A cell is kind of like a city. It has several moving parts and jobs that need to be done. And just like a city, a cell needs energy to function. But instead of gas or electricity, cells need sugar. Sugar is typically present outside the cell in the form of glucose, a sugar molecule used by most living things for energy, and it must get into the cell to be used to generate energy. However, the cell membrane is kind of like the wall of a medieval city. It's difficult to cross without special permission, and even a molecule as important as glucose needs help getting across. How the Cell Membrane Works Like a city wall, the cell membrane marks the borders of the cell and protects it from invasion. The city wall is studded with towers and gates to allow merchants, messengers and farmers to come and go so that the city can survive. Similarly, the cell membrane also controls what comes in and out of the cell. One of the ways materials can enter the cell is through special proteins that are embedded in the membrane. These proteins act like gates to allow large molecules, like glucose, to get across the membrane. If glucose tried to cross the membrane without the protein gate, it would take a very long time. The cell membrane is made of a double layer of lipids, called a bilayer. Lipids are molecules with a hydrophilic head and hydrophobic tail. The hydrophobic tails stick together to create the bilayer, so the hydrophilic heads line the interior and exterior of the cell, but in between is a hydrophobic region. In addition to being a rela Continue reading >>

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