I’m sure you know of someone, maybe even yourself, that believes they can’t cut out the sugar from their diet.
The concept of “food addiction” materialized in the diet industry on the basis of subjective reports, clinical accounts and case studies described in self-help books. Do you crave, lose control, and eat more than you plan?  You just may be addicted to sugar.  Now that you know the problem, you can make a decision, take action, and live in the solution. Be careful of BBQ sauces, ketchup, salad dressings, baked beans and processed foods!  They are usually loaded with sugar.
For Type 1 diabetes sufferers, constant monitoring of insulin and blood sugar levels is both inconvenient and time consuming. MIT researchers have created a type of nanoparticle—for those of us not enrolled at MIT, that’s an extremely tiny particle often used in biomedical research—that can determine when glucose levels in the blood are off and immediately trigger the secretion of enough insulin (which breaks down glucose and gets blood sugar levels under control) to stop the problem. With diabetes becoming an ever-growing issue for our society, we’re in support of any research that could provide a safe, long-term treatment. All cells acquire the molecules and ions they need from their surrounding extracellular fluid (ECF).
In eukaryotic cells, there is also transport in and out of membrane-bounded intracellular compartments such as the nucleus, endoplasmic reticulum, and mitochondria. Molecules and ions can be moved against their concentration gradient, but this process, called active transport, requires the expenditure of energy (usually from ATP).
Transmembrane proteins create a water-filled pore through which ions and some small hydrophilic molecules can pass by diffusion. Active transportTransmembrane proteins, called transporters, use the energy of ATP to force ions or small molecules through the membrane against their concentration gradient.
Link to a quantitative treatment of the free energy changes involved in facilitated diffusion and active transport. Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. External ligands (shown here in green) bind to a site on the extracellular side of the channel. ATP is needed to open the channel that allows chloride (Cl-) and bicarbonate (HCO3-) ions out of the cell.
Sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound.
Mechanical deformation of the cells of stretch receptors opens ion channels leading to the creation of nerve impulses. In so-called "excitable" cells like neurons and muscle cells, some channels open or close in response to changes in the charge (measured in volts) across the plasma membrane.
Example: As an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane. Some 7000 sodium ions pass through each channel during the brief period (about 1 millisecond) that it remains open. Such measurements reveal that each channel is either fully open or fully closed; that is, facilitated diffusion through a single channel is "all-or-none". This technique has provided so much valuable information about ion channels that its inventors, Erwin Neher and Bert Sakmann, were awarded a Nobel Prize in 1991. Some small, hydrophilic organic molecules, like sugars, can pass through cell membranes by facilitated diffusion. Another example: The plasma membrane of human red blood cells contain transmembrane proteins that permit the diffusion of glucose from the blood into the cell. Whether all cases of facilitated diffusion of small molecules use channels is yet to be proven. In either case, the interaction between the molecule being transported and its transporter resembles in many ways the interaction between an enzyme and its substrate.
Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. The cytosol of animal cells contains a concentration of potassium ions (K+) as much as 20 times higher than that in the extracellular fluid. It helps establish a net charge across the plasma membrane with the interior of the cell being negatively charged with respect to the exterior.
The accumulation of sodium ions outside of the cell draws water out of the cell and thus enables it to maintain osmotic balance (otherwise it would swell and burst from the inward diffusion of water). The gradient of sodium ions is harnessed to provide the energy to run several types of indirect pumps.


In resting skeletal muscle, there is a much higher concentration of calcium ions (Ca2+) in the sarcoplasmic reticulum than in the cytosol. ABC transporters that pump chemotherapeutic drugs out of cancer cells thus reducing their effectiveness.
Indirect active transport uses the downhill flow of an ion to pump some other molecule or ion against its gradient. In this type of indirect active transport, the driving ion (Na+) and the pumped molecule pass through the membrane pump in the same direction. Link to an animation of the process produced by the father and son team of John and John Giannini. Sodium-driven symport pumps also return neurotransmitters to the presynaptic neuron [More]. In antiport pumps, the driving ion (again, usually sodium) diffuses through the pump in one direction providing the energy for the active transport of some other molecule or ion in the opposite direction.
Antiport pumps in the vacuole of some plants harness the outward facilitated diffusion of protons (themselves pumped into the vacuole by a H+ ATPase) to the active inward transport of sodium ions. A growing number of human diseases have been discovered to be caused by inherited mutations in genes encoding channels.
Although water is a polar molecule, it is able to pass through the lipid bilayer of the plasma membrane. Water is never transported actively; that is, it never moves against its concentration gradient. Example: the reabsorption of water from the kidney tubules back into the blood depends on the water following behind the active transport of Na+.
If the concentration of water in the medium surrounding a cell is greater than that of the cytosol, the medium is said to be hypotonic. Plant cells and bacterial cells avoid bursting in hypotonic surroundings by their strong cell walls. How the kidneys of freshwater fishes and amphibians permit their owners to live in their hypotonic surroundings. When red blood cells are placed in a 0.9% salt solution, they neither gain nor lose water by osmosis.
If red cells are placed in sea water (about 3% salt), they lose water by osmosis and the cells shrivel up.
Similarly, if a plant tissue is placed in sea water, the cell contents shrink away from the rigid cell wall.
Marine birds, which may pass long periods of time away from fresh water, and sea turtles use a similar device.
A report in the 23 April 1998 issue of The New England Journal of Medicine tells of the life-threatening complications that can be caused by an ignorance of osmosis. Large volumes of a solution of 5% human albumin are injected into people undergoing a procedure called plasmapheresis. The albumin is dissolved in physiological saline (0.9% NaCl) and is therefore isotonic to human plasma (the large protein molecules of albumin have only a small osmotic effect).
If 5% solutions are unavailable, pharmacists may substitute a proper dilution of a 25% albumin solution.
The rise in obesity, coupled with the emergence of scientific findings of parallels between drugs of abuse and palatable foods has given credibility to this idea. But now there is some good news: An MIT project currently underway could allow the body to do it automatically. In essence, the particles, which are used to create a toothpaste-consistency gel, are mimicking the role of the pancreas, which is the organ that malfunctions in those who suffer from diabetes.
With this system of extended release, the amount of drug secreted is proportional to the needs of the body,” says Daniel Anderson, an associate professor of chemical engineering and member of MIT’s Koch Institute for Integrative Cancer Research and Institute for Medical Engineering and Science. A single injection of the gel controlled blood sugar levels in the mice for about 10 days without incident before safely dissolving into the body.
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These transmembrane proteins form a water-filled channel through which the ion can pass down its concentration gradient. Many ion channels open or close in response to binding a small signaling molecule or "ligand".
The binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction.


This allows the influx of Na+ into the neuron and thus the continuation of the nerve impulse. Perhaps some molecules are passed through the membrane by a conformational change in the shape of the transmembrane protein when it binds the molecule to be transported. Some transporters bind ATP directly and use the energy of its hydrolysis to drive active transport. Conversely, the extracellular fluid contains a concentration of sodium ions (Na+) as much as 10 times greater than that within the cell.
This resting potential prepares nerve and muscle cells for the propagation of action potentials leading to nerve impulses and muscle contraction. These cells transport protons (H+) from a concentration of about 4 x 10-8 M within the cell to a concentration of about 0.15 M in the gastric juice (giving it a pH close to 1). Activation of the muscle fiber allows some of this Ca2+ to pass by facilitated diffusion into the cytosol where it triggers contraction.
This is done by another Ca2+ ATPase that uses the energy from each molecule of ATP to pump 2 Ca2+ ions. The ATP-binding domains in archaea, eubacteria, and eukaryotes all share a homologous structure, the ATP-binding "cassette".
This symporter pumps iodide ions into the cells of the thyroid gland (for the manufacture of thyroxine) and also into the cells of the mammary gland (to supply the baby's need for iodide). Inadequate sodium transport out of the kidneys, because of a mutant sodium channel, leads to elevated osmotic pressure of the blood and resulting hypertension (high blood pressure).
Note that this refers to the concentration of water, NOT the concentration of any solutes present in the water.
However, the concentration of water can be altered by the active transport of solutes and in this way the movement of water in and out of the cell can be controlled. This balance must be actively maintained because of the large number of organic molecules dissolved in the cytosol but not present in the ECF.
They, too, drink salt water to take care of their water needs and use metabolic energy to desalt it. Mixing 1 part of the 25% solution with 4 parts of diluent results in the correct 5% solution of albumin. The reviewed evidence supports the theory that, in some circumstances, intermittent access to sugar can lead to behavior and neurochemical changes that resemble the effects of a substance of abuse. The scientists believe the technology could be transferred to humans, and are currently working to fine tune speed of reaction, dosage, and delivery method. The diffusion of water through the plasma membrane is of such importance to the cell that it is given a special name: osmosis. Although the energy liberated by the hydrolysis of ATP is needed to open the channel, this is not an example of active transport; the ions diffuse through the open channel following their concentration gradient. Small wonder that parietal cells are stuffed with mitochondria and uses huge amounts of ATP as they carry out this three-million fold concentration of protons. The activity of these pumps helps to maintain the ~20,000-fold concentration gradient of Ca2+ between the cytosol (~ 100 nM) and the ECF (~ 20 mM). The sodium ions flow down their concentration gradient while the glucose molecules are pumped up theirs. These organic molecules exert an osmotic effect that, if not compensated for, would cause the cell to take in so much water that it would swell and might even burst. In the herring gull, shown here, the salt is extracted by two glands in the head and released (in a very concentrated solution — it is saltier than the blood) to the outside through the nostrils. According to the evidence in rats, intermittent access to sugar and chow is capable of producing a “dependency”. When this is relieved by facilitated diffusion, the energy released can be harnessed to the pumping of some other ion or molecule. This was operationally defined by tests for binging, withdrawal, craving and cross-sensitization to amphetamine and alcohol.
That is, if the pumped ions are allowed to diffuse back through the membrane complex, ATP can be synthesized from ADP and inorganic phosphate.



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Comments

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