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Our book explains how glucose from the blood plasma gets inside red blood cells via facilitated transport.
It states here in the book that the glucose will be transported inside by a carrier protein. Then the glucose will be phosphorylated by ATP. The phosphorylation of glucose molecules maintains the concentration gradient. This prevents the glucose from diffusing back to the blood plasma.
Based from my current understanding, concentration only change when the number of dissolved molecules changes.
My question is, "How can phosphorylation maintain the concentration gradient if it doesn't change the number of molecules inside the red blood cell?"
Sort of a "magic trick" that biology does.
With facilitated transport, the movement is passive. That is, you have a protein that certain molecules/ions can pass through, but you aren't doing any pumping or using energy to move molecules. That means that the molecules are free to move in either direction, according to concentration (and electrical) gradients.
Therefore, assuming no influence of electrical gradients, the highest concentration a passively transported molecule can reach is equal concentration inside and out.
Chemically, phosphorylated glucose is not glucose. It is phosphorylated glucose. If you phosphorylate every glucose molecule that comes in, your internal concentration of glucose would stay at zero. If phosphorylated glucose can't leave via the same facilitated diffusion pathway, then glucose will effectively only come in via that pathway, it can't go back out. The result is that, although glucose inside can never be higher than glucose outside, the sum total of phosphorylated glucose + glucose inside can continue to rise above the concentration of glucose outside.
Of course, this step isn't exactly free, because it costs the energy of phosphorylation.
Simple diffusion is an example of passive transport. Where particles, atoms, or ions move from an area of higher concentration to lower concentration area by the assistance of a concentration gradient. When equilibrium reaches between the two regions, the movement slows down to the natural motion of particles. This process regularly runs between the intercellular fluid and external side of a human cell, exchanging both useful and waste products at the same time.
Osmosis and concentration gradient
Osmosis is very similar to diffusion as both involve the movement of particles along a concentration gradient. The actual difference is, which particle is moving. In osmosis, the solvent moves through the membrane while in diffusion, the solute moves through the membrane. An osmotic gradient is a pressure that forces solvent particles to move from a higher concentration to lower concentration. Since a water molecule is a polar molecule, it requires a transport protein to travel through the membrane.
No chemical energy is required to carry out this process as the movement is downhill.
Active transport and concentration gradient
In active-transport, molecules travel against the concentration gradient. The movement of particles from a higher concentration area to a lower concentration region demands chemical energy. Primary active-transport like adenosine triphosphate requires chemical energy. A secondary active-transport utilizes both electrical and chemical gradient. An electrochemical gradient is the gradient of an ion that defuses in the cell passing through a cell membrane. The diffusion of ions affects the electric potential of the cell membrane. If a charge gradient has occurred, the charged particles then follow a downhill movement.
3.1 The Cell Membrane
Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. Just as the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.
Structure and Composition of the Cell Membrane
The cell membrane is an extremely pliable structure composed primarily of two layers of phospholipids (a “bilayer”). Cholesterol and various proteins are also embedded within the membrane giving the membrane a variety of functions described below.
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 3.1.1). 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 interior of the cell and one layer exposed to the exterior (Figure 3.1.1).
Figure 3.1.1 – Phospholipid Structure and Bilayer: A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.
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. 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 the wash water while the hydrophobic portion can trap grease in stains that then can be washed away. A similar process occurs in your digestive system when bile salts (made from cholesterol, phospholipids and salt) help to break up ingested lipids.
Since the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane (see above Figure). Since the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. In addition to phospholipids and cholesterol, the cell membrane has many proteins detailed in the next section.
The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral protein and peripheral protein (Figure 3.1.2). As its name suggests, an integral protein is a protein that is embedded in the membrane. Many different types of integral proteins exist, each with different functions. For example, an integral protein that extends an opening through the membrane for ions to enter or exit the cell is known as a channel protein. Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein.
Figure 3.1.2- Cell Membrane: The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.
Some integral proteins serve as cell recognition or surface identity proteins, which mark a cell’s identity so that it can be recognized by other cells. Some integral proteins act as enzymes, or in cell adhesion, between neighboring cells. A receptor is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. Some integral proteins serve dual roles as both a receptor and an ion channel. One example of a receptor-channel interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell. Peripheral proteins are often associated with integral proteins along the inner cell membrane where they play a role in cell signaling or anchoring to internal cellular components (ie: cytoskeleton discussed later).
Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has carbohydrate molecules attached, which extend into the extracellular environment. The attached carbohydrate tags on glycoproteins aid in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. The glycocalyx is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s trillions of cells the “identity” of belonging in the person’s body. This identity is the primary way that a person’s immune defense cells “know” not to attack the person’s own body cells, but it also is the reason organs donated by another person might be rejected.
Transport Across the Cell Membrane
One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca ++ , Na + , K + , and Cl – , nutrients including sugars, fatty acids, and amino acids, and waste products, particularly carbon dioxide (CO2), which must leave the cell.
The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).
In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance across a space. Molecules (or ions) will spread/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration gradient, from high concentration to low concentration.) Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed room. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the room, and this diffusion would go on until the molecules were equally distributed in the room. Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures.
Visit this link to see diffusion and how it is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why?
Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as cell membranes, any substance that can move down its concentration gradient across the membrane will do so. If the substances can move across the cell membrane without the cell expending energy, the movement of molecules is called passive transport. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and carbon dioxide (CO2). These small, fat soluble gasses and other small lipid soluble molecules can dissolve in the membrane and enter or exit the cell following their concentration gradient. This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion. O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them.
Before moving on, it is important to realize that the concentration gradients for oxygen and carbon dioxide will always exist across a living cell and never reach equal distribution. This is because cells rapidly use up oxygen during metabolism and so, there is typically a lower concentration of O2 inside the cell than outside. As a result, oxygen will diffuse from outside the cell directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO2 as a byproduct of metabolism, CO2 concentrations rise within the cytoplasm therefore, CO2 will move from the cell through the lipid bilayer and into the extracellular fluid, where its concentration is lower. (Figure 3.1.3).
Figure 3.1.3 – Simple Diffusion Across the Cell (Plasma) Membrane: The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.
Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size, charge, and/or polarity but do so down their concentration gradients (Figure 3.1.4). As an example, even though sodium ions (Na + ) are highly concentrated outside of cells, these electrolytes are charged and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins that form sodium channels (or “pores”), so that Na+ ions can move down their concentration gradient from outside the cells to inside the cells. A common example of facilitated diffusion using a carrier protein is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar, and therefore, repelled by the phospholipid membrane. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion. The difference between a channel and a carrier is that the carrier usually changes shape during the diffusion process, while the channel does not. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes.
Figure 3.1.4 – Facilitated Diffusion: (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.
A specialized example of facilitated transport is water moving across the cell membrane of all cells, through protein channels known as aquaporins. Osmosis is the diffusion of water through a semipermeable membrane from where there is more relative water to where there is less relative water (down its water concentration gradient) (Figure 3.1.5).
Figure 3.1.5 – Osmosis: Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic.
On their own, cells cannot regulate the movement of water molecules across their membrane, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be isotonic (equal tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).
Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure 3.1.6). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body’s cells are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.
Figure 3.1.6 – Concentration of Solution: A hypertonic solution has a solute concentration higher than another solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute concentration lower than another solution.
For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During primary active transport, ATP is required to move a substance across a membrane, with the help of membrane protein, and against its concentration gradient.
One of the most common types of active transport involves proteins that serve as pumps. The word “pump” probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, against their concentration gradients (from an area of low concentration to an area of high concentration).
The sodium-potassium pump, which is also called Na + /K + ATPase, transports sodium out of a cell while moving potassium into the cell. The Na + /K + pump is an important ion pump found in the membranes of all cells. The activity of these pumps in nerve cells is so great that it accounts for the majority of their ATP usage.
Figure 3.1.7 The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.
Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell. In this way, the action of an active transport pump (the sodium-potassium pump) powers the passive transport of sodium ions by creating a concentration gradient. When active transport powers the transport of another substance in this way, it is called secondary active transport.
Symporters are secondary active transporters that move two substances in the same direction. For example, the sodium-glucose symporter uses sodium ions to “pull” glucose molecules into the cell. Since cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside however, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.
Conversely, antiporters are secondary active transport systems that transport substances in opposite directions. For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H + ) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell’s interior.
Other Forms of Membrane Transport
Other forms of active transport do not involve membrane carriers. Endocytosis (bringing “into the cell”) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (Figure 3.1.8). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. Phagocytosis (“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles.
Figure 3.1.8 – Three Forms of Endocytosis: Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in large particles into larger vesicles known as vacuoles. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.
Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane which contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.
In contrast with endocytosis, exocytosis (taking “out of the cell”) is the process of a cell exporting material using vesicular transport (Figure 3.1.9). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases its contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane.
Specific examples of exocytosis include cells of the stomach and pancreas producing and secreting digestive enzymes through exocytosis (Figure 3.1.10) and endocrine cells producing and secreting hormones that are sent throughout the body.
The addition of new membrane to the plasma membrane is usually coupled with endocytosis so that the cell is not constantly enlarging. Through these processes, the cell membrane is constantly renewing and changing as needed by the cell.
Figure 3.1.9 – Exocytosis: Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space. Figure 3.1.10 – Pancreatic Cells’ Enzyme Products: The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Diseases of the Cell: Cystic Fibrosis
Cystic fibrosis (CF) affects approximately 30,000 people in the United States, with about 1,000 new cases reported each year. The genetic disease is most well-known for its damage to the lungs, causing breathing difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines. Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical treatment, many CF patients live into their 30s.
The symptoms of CF result from a malfunctioning membrane ion channel called the Cystic Fibrosis Transmembrane Conductance Regulator, or CFTR. In healthy people, the CFTR protein is an integral membrane protein that transports Cl– ions out of the cell. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell.
The CFTR requires ATP in order to function, making its Cl– transport a form of active transport. This puzzled researchers for a long time because the Cl– ions are actually flowing down their concentration gradient when transported out of cells. Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule.
In normal lung tissue, the movement of Cl– out of the cell maintains a Cl–-rich, negatively charged environment immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. A cilium (plural = cilia) is one of the hair-like appendages found on certain cells. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the mucus cannot be too viscous, rather, it must have a thin, watery consistency. The transport of Cl– and the maintenance of an electronegative environment outside of the cell attracts positive ions such as Na+ to the extracellular space. The accumulation of both Cl– and Na+ ions in the extracellular space creates solute-rich mucus, which has a low concentration of water molecules. As a result, through osmosis, water moves from cells and extracellular matrix into the mucus, “thinning” it out. In a normal respiratory system, this is how the mucus is kept sufficiently watered-down to be propelled out of the respiratory system.
If the CFTR channel is absent, Cl– ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions. The absence of ions in the secreted mucus results in the lack of a normal water concentration gradient. Thus, there is no osmotic pressure pulling water into the mucus. The resulting mucus is thick and sticky, and the ciliated epithelia cannot effectively remove it from the respiratory system. Passageways in the lungs become blocked with mucus, along with the debris it carries. Bacterial infections occur more easily because bacterial cells are not effectively carried away from the lungs.
The cell membrane provides a barrier around the cell, separating its internal components from the extracellular environment. It is composed of a phospholipid bilayer, with hydrophobic internal lipid “tails” and hydrophilic external phosphate “heads.” Various membrane proteins are scattered throughout the bilayer, both inserted within it and attached to it peripherally. The cell membrane is selectively permeable, allowing only a limited number of materials to diffuse through its lipid bilayer. All materials that cross the membrane do so using passive (non-energy-requiring) or active (energy-requiring) transport processes. During passive transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient. Water passes through the membrane in a diffusion process called osmosis. During active transport, energy is expended to assist material movement across the membrane in a direction against their concentration gradient. Active transport may take place with the help of protein pumps or through the use of vesicles.
Interactive Link Questions
Visit this link to see diffusion and how it is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why?
Higher temperatures speed up diffusion because molecules have more kinetic energy at higher temperatures.
Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created vacuole that is formed from the plasma membrane.
Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the invader through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure 3).
A variation of endocytosis is called pinocytosis. This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this process takes in solutes that the cell needs from the extracellular fluid (Figure 3).
Figure 3. Three variations of endocytosis are shown. (a) In one form of endocytosis, phagocytosis, the cell membrane surrounds the particle and pinches off to form an intracellular vacuole. (b) In another type of endocytosis, pinocytosis, the cell membrane surrounds a small volume of fluid and pinches off, forming a vesicle. (c) In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds at the receptor on the external cell membrane. (credit: modification of work by Mariana Ruiz Villarreal)
A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances (Figure 3). The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by a failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear the chemical from their blood.
Concept in Action
See receptor-mediated endocytosis in action and click on different parts for a focused animation to learn more.
This uprise in glucose molecule will eventually bring up your blood glucose level to its normal range. This two biological mechanism, insulin, and glycogen, .
Increased liver gluconeogenesis uses up available oxaloacetate, which results in the buildup of acetyl CoA. At the same time, increased beta-oxidation of fat.
 Figure 1 – Insulin Secretion  The above image shows insulin secretion stimulated by glucose. There is an uptake of glucose by the carri.
The glucose, fructose and galactose are usually stored in the liver and muscle cells in the form of glycogen. When there is a shortage of glucose in the bloo.
The first is the proximal convoluted tubule, this is where the majority of water and solutes (including glucose) from the tubular fluid (once the fluid leave.
From the interstitium there is an Anti-porter that moves PAH into the tubule (secreting it) and allows another molecule into the interstitium. Another mechan.
* Excretion - The kidneys eliminate unwanted substances from the body as urine. * Reabsorption - Previously filtered substances (water and sodium) needed by .
In the medical article “What Is Diabetes”, Dr. Ananya Mandal states that, “Type 1 diabetes is called insulin-dependent diabetes mellitus (Mandal 1). Mandal a.
Facilitated diffusion 13. In this transport process, the energy from hydrolysis of ATP is used to drive substances across the membrane against their o.
ARTIFICIAL PANCREAS DEVICE SYSTEM The artificial pancreas device system is a system of devices that closely imitates the glucose regulating function of a hea.
Diffusion and Concentration Gradients
The direction of the passage of particles through the channel is also dependent on concentration gradients. A concentration gradient exists whenever a concentrated solution is in contact with a less concentrated solution. Because the solutions are in contact, particles may flow between the two solutions (or between two regions of the same solution) by the process known as diffusion. Diffusion is a term used to describe the mixing of two different substances that are placed in contact. The substances may be gases, liquids, or solids. Diffusion is the migrating by random motion of these different particles.Although particles move in every direction, there is a net flow from the more concentrated solution to the less concentrated solution ("down the concentration gradient"). As the number of particles in the more concentrated solution diminishes and the number of particles in the less concentrated increases, the difference in concentration between the two solutions decreases. Hence, the concentration gradient is said to get smaller (Movie 1). All else being equal, the concentrations of the solutions change more rapidly when the difference in their concentrations is greater. This diffusion process continues until the concentrations of the two solutions are equal. This state is known as dynamic equilibrium. When the two solutions are in dynamic equilibrium, particles continue to move between the two solutions, but there is no net flow in any one direction, i.e., the concentrations do not change.
The graph at the top of this figure plots the time course of the changes in concentration that occur after a solution (A) with a 1.0 M concentration of some particle is placed in contact (via a semipermeable membrane) with another solution with a 0.0 M concentration of the particle. The blue line represents the concentration of the particle in solution A, and the magenta line represents the concentration of the particle in solution B. Over time, the concentrations become equal and no longer change at this point, the solutions are said to be in dynamic equilibrium.
The schematic at the bottom shows the two solutions approximately 2 seconds after the solutions are placed in contact with one another.
To view a QuickTime movie showing the movement of the particles by diffusion between these two solutions, please click on the pink button below. Click the blue button below to download QuickTime 4.0 to view the movie.
In biological systems such as the kidney, the two solutions are often separated by a membrane. Protein channels in the membrane allow particles to cross the membrane, flowing "down the concentration gradient" until equilibrium is reached. Sometimes these channels may be closed, so that particles will not travel across the membrane, even if there is a strong concentration gradient. (In effect, the two solutions are no longer in contact when the channels are closed.) In other cases, the proteins in the membranes act like "pumps," using energy to move particles "against the concentration gradient" (i.e., so the more concentrated solution becomes even more concentrated) examples are the light-driven proton pump that occurs in the photosynthetic thylakoid membrane discussed in the introduction to the Experiment, the proton pumps used in the synthesis of ATP, the body's energy currency (which you will encounter in the tutorial entitled "Energy for the Body: Oxidative Phosphorylation"), and the sodium pumps discussed below.
Questions on Diffusion and Concentration Gradients
- Look at Figure 6, and the movie showing the diffusion of particles between two solutions, separated by a membrane.
- At what point is the rate of change in the concentrations of the two solutions greatest?
- Briefly, explain why the rate of concentration change is greatest at this point.
- A solution of 0.10 M NaCl is separated from another solution of 0.10 M NaCl by a membrane that is permeable to Na + and Cl - ions.
- Does diffusion occur across the membrane? Briefly, explain your answer.
- Does the concentration of Na + or Cl - change in neither, either, or both of the two solutions? Briefly, explain your answer.
How does 'phosphorylation of glucose' maintain concentration gradient in membrane transport (facilitated diffusion)? - Biology
Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.
Passive transport does not require the input of metabolic energy the net movement of molecules is from high concentration to low concentration. Passive transport plays a primary role in the import of resources and the export of wastes. Membrane proteins play a role in facilitated diffusion of charged and polar molecules through a membrane. External environments can be hypotonic, hypertonic or isotonic to internal environments of cells. (examples - glucose transport, Na+/K+ transport)
Activity transport requires free energy to move molecules from regions of low concentration to regions of high concentration. Active transport is a process where free energy (often provided by ATP) is used by proteins embedded in the membrane to "move" molecules and/or ions across the membrane and to establish and maintain concentration gradients. Membrane proteins are necessary for active transport.
The processes of endocytosis and exocytosis move large molecules from the external environment to the internal environment and vice versa, respectively. In exocytosis, internal vesicles fuse with the plasma membrane to secrete large macromolecules out of the cell. In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles derived from the plasma membrane.
Loop of Henle
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Loop of Henle, long U-shaped portion of the tubule that conducts urine within each nephron of the kidney of reptiles, birds, and mammals. The principal function of the loop of Henle is in the recovery of water and sodium chloride from urine. This function allows production of urine that is far more concentrated than blood, limiting the amount of water needed as intake for survival. Many species that live in arid environments such as deserts have highly efficient loops of Henle. Anatomically, the loop of Henle can be divided into three main segments: the thin descending limb, the thin ascending limb, and the thick ascending limb (sometimes also called the diluting segment).
The liquid entering the loop of Henle is the solution of salt, urea, and other substances passed along by the proximal convoluted tubule, from which most of the dissolved components needed by the body—particularly glucose, amino acids, and sodium bicarbonate—have been reabsorbed into the blood. The first segment of the loop, the thin descending limb, is permeable to water, and the liquid reaching the bend of the loop is much richer in salt and urea than the blood plasma is. As the liquid returns through the thin ascending limb, sodium chloride diffuses out of the tubule into the surrounding tissue, where its concentration is lower. In the third segment of the loop, the thick ascending limb, the tubule wall can, if necessary, effect further removal of salt, even against the concentration gradient, in an active-transport process requiring the expenditure of energy.
In a healthy person the reabsorption of salt from the urine exactly maintains the bodily requirement: during periods of low salt intake virtually none is allowed to escape in the urine, but in periods of high salt intake the excess is excreted.
The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers, Senior Editor.
Secondary lysosomes digest extracellular material of which is engulfed into the cell by a process called endocytosis. A phagocytic vesicle is pinched off fro.
The electrically charged head of these layers face toward the water as the uncharged tails face each other. This makes it easier for small, neutrally-charged.
It transports cholesterol across the membrane. “These mechanisms involve enclosing the substances to be transported in their own small globes of membrane, wh.
If the solute concentration is greater in the solution on the inside of the cell, compared to the solute concentration of the solution that is outside the ce.
Cellular Respiration is a cycle that can be found in all living organisms. The significance of Cellular Respiration is that it creates energy by breaking dow.
Cellular Respiration takes place on the level of the cell, inside the mitochondria. This is the process of breaking down food in the presence of oxygen to fo.
The actual membrane is created up of phospholipids which naturally form a bilayer, these phospholipids have a head that are hydrophilic and two fatty acid ta.
Intrinsic proteins are known as carrier proteins, these help certain substances pass through the membrane, specifically the ones that cannot do it alone, for.
ATP hydrolysis powers cellular work since ATP hydrolysis leads to a change in a protein’s shape and its ability to bind to another molecule. When ATP is bond.
INTRODUCTION Trypsin is a proteolytic enzyme, important for the digestion of proteins. Enzymes are biological catalysts for metabolic process in cells. A cat.
Concentration Gradient Definition
A concentration gradient occurs when a solute is more concentrated in one area than another.
The concentration gradient refers to the gradual change in the concentration of solutes present in a solution between two regions. In biology, a gradient results from an unequal distribution of ions across the cell membrane. When this happens, solutes move along a concentration gradient.
Concentration gradient and Electrical Potential
To understand how the flow of ions contribute to the RMP, the formation of a concentration gradient and electrical potential must first be understood.
. a difference in the concentration of a substance across a distance.
Covalent . A compound where atoms are shared.
Cytokinesis . The final stage of the cell cycle, in which the cell's cytoplasm divides, distributing the organelles into each of the two new cells.
Cells have a pretty sophisticated cell membrane, which acts as a barrier to the outside world. We've described this membrane as selectively permeable, meaning not just anything can get through it. The key to this phrase is that the cell membrane is selective, but not impermeable.
/GRADE-ee-ənt/ A condition in which the concentration of a solute varies continuously from one position to another within a solution.
The difference in concentration in two parts of a system.
concentricycloidea The class of echinoderms whose members are characterized by two concentric water-vascular rings encircling a disklike body no digestive system and internal brood pouches. Sea daisies.
The concentration of most molecules inside a cell is different than the concentration of molecules in the surrounding environment.
is the difference in the concentration of a substance between two regions.
itself represents potential energy and drives diffusion.
s of both ions are restored by the sodium-potassium pump. Sodium is pumped out of the cell while potassium is pumped in. The resting potential is restored and the neuron is ready to conduct another nerve impulse.
equilibrium particle movement
In Biology and in Science overall diffusion is one of the basic principles that once you get it, it is usable or applicable in a bunch of different places.
These molecules will move from where they are at a high concentration to where they are at a lower concentration. i.e. they diffuse down a
surface area across within diffusion occurs (larger)
thickness of surface (thinner)
Fick's law = (surface area x difference in conc gradient) / thickness of surface .
Complementary Matching. Complementary bases are those that pair up in DNA and RNA, such as cytosine (C) with guanine (G).
The membrane proteins then grab one molecule and shift their position to bring the molecule into the cell. That's an easy situation of passive transport because the glucose is moving from higher to lower concentration. It's moving down a
Coordinate genes such as bicoid lay down the grand plan, so to speak, upon which the genes downstream will act. The pattern of the developing embryo arises as these downstream genes are activated or repressed.
The bcd protein is distributed in an exponential
with a maximum at the anterior tip, reaching background levels in the posterior third of the embryo. The gradient is probably generated by diffusion from the local mRNA source and dispersed degradation.
^ Carlson, Bruce M.
Morphogens are soluble molecules that can diffuse and carry signals that control cell differentiation via
s. Morphogens typically act through binding to specific protein receptors.
of ions, it can be called an electrochemical potential gradient of ions across membranes. Ionophores are important for ion gradients.
A kind of transport wherein ions or molecules move against a
, which means movement in the direction opposite that of diffusion - or - movement from an area of lower concentration to an area of higher concentration.
Some of these proteins can move materials across the membrane only when assisted by the
Simple diffusion can only move material in the direction of a
Simple diffusion is not saturable facilitated diffusion rates are limited by the number of functional membrane proteins and can be saturated .
The energy equivalent of the proton (H+)
, and movement of bacterial flagella.
Full glossary .
Chemotaxis Directed movement of a motile cell up or down a
of a chemical resulting in movement towards or away from the chemical's source. Watch the video of Neutrophil Chemotaxis.
Chloroplast The chlorophyll-containing organelle in green plant cells where photosynthesis occurs.
of charged protons on one side of a membrane is a great store of potential energy and is referred to as the proton motive force.
Concept 3: Movement of Molecules in Solution
Concept 4: Movement of Molecules and Cells
Concept 5: Types of Solutions Based on Solute Concentration
Concept 6: Water Potential
Concept 7: Calculating Water Potential
Concept 8: Factors that Affect Water Potential .
The sole job of pump proteins is to move molecules from one side of a cell's membrane to another, against their
s. There are different kinds of pumps, each of which moves specific ions, such as sodium (Na+), potassium (K+), or protons (H+). A calcium (Ca++) pump is shown below.
A transport mechanism that moves compounds or ions down a
, and requires no energy also known as accelerated diffusion or mediated transport. (see also active transport passive diffusion)
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The majority of biological solutes are charged organic or inorganic molecules and hydrophobic cellular membranes establish
s through the use of transporters.
Active Transport the movement of substances from where they are less concentrated to where they are more concentrated (against a
Adaptation any feature of the structure or physiology of an organism that makes it well suited to its environment.
passive transport (facilitated diffusion) - movement of a molecule across a membrane down its
peripheral membrane proteins-proteins that can be released from the membrane .
Hydrogen Ion Pump [H+] - a membrane bound protein that transports hydrogen ions against
and creates a positive charge on one side of a membrane .
energy-expanding process in which cells transport materials across the cell membrane against a
Energy in the form of ATP is required when the cell is pumping molecules in or out against the
. This is called active transport. A dialysis machine which is used for people with kidney failure cannot differentiate between the useful and toxic molecules in the blood filtrate.
Through a series of winding, crazy-straw-like tubules and confusing
s, our bodies-via the kidneys-excrete just enough water and dissolved nutrients to make sure we can function properly.
Many nutrients move through the soil and into the root system as a result of
s, moving by diffusion from high to low concentrations. However, some nutrients are selectively absorbed by the root membranes, enabling concentrations to become higher inside the plant than in the soil.
Homeostasis : The ability of an organism to maintain a constant internal environment.
Ectotherms : Gain/Lose heat from or to their external environment.
Endotherms : Generate their own heat from metabolic reactions.
Active transport : Energy is used to move molecules against a