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How does glucose pass through the phospholipid bilayer?
Although ions and most polar molecules cannot diffuse across a lipid bilayer, many such molecules (such as glucose) are able to cross cell membranes. These molecules pass across membranes via the action of specific transmembrane proteins, which act as transporters.
Can glucose pass directly through the plasma membrane?
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-).
Where does glucose cross the membrane?
There are two mechanisms for glucose transport across cell membranes. In the intestine and renal proximal tubule, glucose is transported against a concentration gradient by a secondary active transport mechanism in which glucose is cotransported with sodium ions.
How does glucose enter and exit the cell membrane?
Glucose enters most cells by facilitated diffusion. There seem to be a limiting number of glucose-transporting proteins. The rapid breakdown of glucose in the cell (a process known as glycolysis) maintains the concentration gradient.
Why can't glucose cross the plasma membrane?
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, and therefore, repelled by the phospholipid membrane.
Why can glucose pass through a membrane but not starch?
Starch does not pass through the synthetic selectively permeable membrane because starch molecules are too large to fit through the pores of the dialysis tubing. In contrast, glucose, iodine, and water molecules are small enough to pass through the membrane. Diffusion results from the random motion of molecules.
How does glucose enter cells?
As it travels through your bloodstream to your cells, it's called blood glucose or blood sugar. Insulin is a hormone that moves glucose from your blood into the cells for energy and storage. People with diabetes have higher-than-normal levels of glucose in their blood.
How does glucose transport from the blood to the cell?
Glucose is transported across the cell membranes and tissue barriers by a sodium-independent glucose transporter (facilitated transport, GLUT proteins, and SLC2 genes), sodium-dependent glucose symporters (secondary active transport, SGLT proteins, and SLC5 genes), and glucose uniporter—SWEET protein ( SLC50 genes).
How does glucose enter the body cells?
After food is digested, glucose is released into the bloodstream. In response, the pancreas secretes insulin, which directs the muscle and fat cells to take in glucose. Cells obtain energy from glucose or convert it to fat for long-term storage.
How is glucose transported into a cell?
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.
Can glucose pass through semipermeable membrane?
The membrane is selectively permeable because substances do not cross it indiscriminately. Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot.
How does a molecule cross the plasma membrane?
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?
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 >>
Why does a lipid soluble cell go through a fatty membrane?
The lipid soluble goes through faster because the cell membrane is phospholipids and can easily diffuse through a fatty membrane. Osmosis is simply the diffusion of water. Anytime water flows in or out, its called osmosis. The water flows to wherever there is more solute.
What is a hydrophilic molecule?
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.”.
What is 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 ...
What is the phosphate group on a phospholipid?
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.”.
Which part of a phospholipid is attracted to water?
A phospholipid molecule has one part that is attracted to water (it's said to be hydrophilic ) and one part that repels water (it's said to be hydrophobic). In the bi-layer, the hydrophilic parts are on the outside, attracted by the water in the cell and water in the fluid surrounding the cell.
Why Can't Glucose Pass Directly Through The Plasma Membrane?
As such glucose cannot pass through the bilayer wirhout the assistance of transport channels called glucose transports or GLUTs. Some of which activate in the presence of insulin and others of which are part of a sodium potassium pump system. Ask New Question Continue reading >>
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 >>
Why does osmosis work faster?
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.
How does glucose enter and leave capillaries?
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.
How do erythrocytes move glucose?
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.
Where are glucose transporters located?
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.
Where are the hydrophilic parts of a phospholipid bilayer located?
The hydrophobic parts are in the middle of the membrane. Proteins There are many different types of proteins associated with the phospholipid bi-layer.
How does glucose transport across the cell membrane?
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 >>
What would happen if glucose tried to cross the membrane without the protein gate?
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.
How does a molecule cross the plasma membrane?
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 does glucose uptake occur?
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 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 >>
What is the purpose of the cardiac gap junction?
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 >>
How do cells assist glucose 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, ...
What is the oxidation of glucose?
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].
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 >>
How Does Glucose Get Into The Cells?
Glucose in the GI tract can also enter the cell through secondary active transport where sodium gradient inside the cell drives a trans-membrane protein to import glucose with it. Red blood cells contain primarily GLUT 1, allowing them to absorb glucose from the bloodstream to make energy through glycolysis. Muscle and fat cells contain a lot of GLUT 4, a protein that is fond main in vesicles in cell cytoplasm. Insulin allows GLUT 4 vesicles to fuse with the cell membrane to increase the number of GLUT transporters found on the cell membrane, thus increasing glucose uptake into fat and muscle (as well as other ) cells. Glucose molecules are simply diffused across the cell membrane due to its low molecular weight. But the proper question is “How do they stay there inside the cell?” Its blocking from existing the cell is due to a reaction of phosphorylation carried out by an enzyme called hexokinase, which add a phosphate group to the 6th carbon converting it into glucose-6-phosphate which is now negatively charged and let’s be aware of that lipid bi-layer cell membrane have phospholipid also making it negatively charged also. Now the same charges pushes each other away keeping the phosphorylated glucose molecule inside the cell. Continue reading >>
How does glucose uptake occur?
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 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 >>
What is the cell membrane made of?
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 >>
What is the function of the cell membrane?
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 >>
What is the hydrophobic layer of the plasma membrane?
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 >>
How does passive transport work?
But...it also doesn't work in every situation. For instance, suppose the sugar glucose is more concentrated inside of a cell than outside. If the cell needs more sugar in to meet its metabolic needs, how can it get that sugar in? Here, the cell can't import glucose for free using diffusion, because the natural tendency of the glucose will be to diffuse out rather than flowing in. Instead, the cell must bring in more glucose molecules via active transport. In active transport, unlike passive transport, the cell expends energy (for example, in the form of ATP) to move a substance against its concentration gradient. Here, we’ll look in more detail at gradients of molecules that exist across cell membranes, how they can help or hinder transport, and how active transport mechanisms allow molecules to move against their gradients. We have already discussed simple concentration gradients, in which a substance is found in different concentrations over a region of space or on opposite sides of a membrane. However, because atoms and molecules can form ions and carry positive or negative electrical charges, there may also be an electrical gradient, or difference in charge, across a plasma membrane. In fact, living cells typically have what’s called a membrane potential, an electrical potential difference (voltage) across their cell membrane. Image depicting the charge and ion distribution across the membrane of a typical cell. Overall, there are more positive charges on the outside of the membrane than on the inside. The concentration of sodium ions is lower inside the cell than in the extracellular f Continue reading >>
How Does Glucose Get Into The Cells?
Glucose in the GI tract can also enter the cell through secondary active transport where sodium gradient inside the cell drives a trans-membrane protein to import glucose with it. Red blood cells contain primarily GLUT 1, allowing them to absorb glucose from the bloodstream to make energy through glycolysis. Muscle and fat cells contain a lot of GLUT 4, a protein that is fond main in vesicles in cell cytoplasm. Insulin allows GLUT 4 vesicles to fuse with the cell membrane to increase the number of GLUT transporters found on the cell membrane, thus increasing glucose uptake into fat and muscle (as well as other ) cells. Glucose molecules are simply diffused across the cell membrane due to its low molecular weight. But the proper question is “How do they stay there inside the cell?” Its blocking from existing the cell is due to a reaction of phosphorylation carried out by an enzyme called hexokinase, which add a phosphate group to the 6th carbon converting it into glucose-6-phosphate which is now negatively charged and let’s be aware of that lipid bi-layer cell membrane have phospholipid also making it negatively charged also. Now the same charges pushes each other away keeping the phosphorylated glucose molecule inside the cell. Continue reading >>
How does glucose transport?
Active transport is the movement of molecules or ions against their concentration gradient, using energy in the form of ATP, across a plasma membrane. In glucose absorption, there is an initially high concentration of glucose in the lumen of the gut as carbohydrates break down. This means there is a concentration gradient allowing the diffusion of glucose into the cells. Once the glucose is at equilibrium, it then needs to be taken up by active transport: 1) Sodium ions (Na+) are actively pumped out of the cells of the small intestine and into the blood via Sodium/Potassium (Na+/K+) pumps. 2) This creates an Na+ concentration gradient, where there is a higher concentration of Na+ in the lumen of the small intestine than inside the cells. 3) The Na+ then re-enters the cells of the small intestine via diffusion through a sodium-glucose transporter protein (alongside glucose). 4) The glucose concentration inside the cell increases and a concentration gradient is created between the inside of the cells and the blood. This allows glucose to move via facilitated diffusion into the blood. Continue reading >>
How does glucose uptake occur?
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 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 >>
What is the main source of energy for cells?
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 >>
What is the outer membrane of eukaryotic cells?
As you have certainly learned earlier, the outer membrane of eukaryotic cells has a lipid bilayer structure. Refer to a standard text book for a review of this. I will emphasize just one important point here; most metabolically active water-soluble materials are effectively hindered from crossing these membranes. Small channels are found in these membranes and these do allow low-molecular compounds (MW < 80) to diffuse into and out of cells. However, "simple" compounds such as sugars and amino acids are much larger. Carriers are necessary if these materials are to gain access to cells. These are usually large proteins that span the cell membrane. They are specific, transporting only the molecules they recognize. This is illustrated in the cartoon shown to the left (from Trends in Biochemical Sciences, ca 1980). As the reader surely understands, transport of glucose is essential for life and is carefully regulated. Five proteins with a high degree of homology are involved in concentration-driven transport of glucose over cellular membranes. Each of these has special physiological functions and tissue distribution. Properties of Glucose Transport Proteins Transporter Tissue distribution Special properties GLUT 1 Most cells. High capacity, relatively low Km (1-2mM). GLUT 2 Liver, beta cells, hypothalamus, basolateral membrane small intestine. High capacity but low affinity (high Km, 15-20mM) part of "the glucose sensor" in ß-cells. Carrier for glucose and fructose i liver and intestine! GLUT 3 Neurons, placenta, testes. Low Km (1mM) and high capacity. GLUT 4 Skeletal and cardiac muscle, fat. Activated by insulin. Km 5mM. GLUT 5 Mucosal surface in small intestine, sperm. Primarily fructose carrier in intestine. These transport proteins mediate facilitated transport, that i Continue reading >>
What is the function of the cell membrane?
Its main function is as a permeability barrier that regulates the passage of substances into and out of the cell. The plasma membrane is the definitive structure of a cell since it sequesters the molecules of life in the cytoplasm, separating it from the outside environment. The bacterial membrane freely allows passage of water and a few small uncharged molecules (less than molecular weight of 100 daltons), but it does not allow passage of larger molecules or any charged substances except when monitored by proteins in the membrane called transport systems. Transport of Solutes The proteins that mediate the passage of solutes through membranes are referred to variously as transport systems, carrier proteins, porters, and permeases. Transport systems operate by one of three transport processes as described below in Figure 22. In a uniport process, a solute passes through the membrane unidirectionally. In symport processes (also called cotransport) two solutes must be transported in the same direction at the same time; in antiport processes ( also called exchange diffusion), one solute is transported in one direction simultaneously as a second solute is transported in the opposite direction. Figure 22. Transport processes in bacterial cells. Solutes enter or exit from bacterial cells by means of one of three processes: uniport, symport (also called cotransport) and antiport (also called exchange diffusion). Transport systems (Figure 23 below) operate by one or another of these processes. Types of Transport Systems Bacteria have a variety of types of transport systems which can be used alternatively in various environmental situations. The elab Continue reading >>
Is postprandial blood glucose a predictor of cardiovascular events?
Postprandial Blood Glucose Is a Stronger Predictor of Cardiovascular Events Than Fasting Blood Glucose in Type 2 Diabetes Mellitus, Particularly in Women: Lessons from the San Luigi Gonzaga Diabetes Study
