What Can Get Across A Cell Membrane Animation

3.one The Cell Membrane

Learning Objectives

Past the stop of this section, you will be able to:

• Describe the molecular components that make up the cell membrane
• Relate structures of the cell membrane to its functions
• Describe how molecules cross the jail cell membrane based on their backdrop and concentration gradients
• Compare and contrast unlike types of passive send with active transport, providing examples of each

Despite differences in construction and function, all living cells in multicellular organisms have a surrounding cell membrane. Merely as the outer layer of your peel separates your body from its environment, the cell membrane (besides known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective bulwark around the prison cell and regulates which materials tin can pass in or out.

Structure and Composition of the Cell Membrane

The cell membrane is an extremely pliable structure composed primarily of 2 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 grouping on ane end, called the "head," and ii side-by-side chains of fatty acids that make up the lipid "tails" (Figure 3.1.1). The lipid tails of one layer face up the lipid tails of the other layer, meeting at the interface of the ii layers. The phospholipid heads face outward, i layer exposed to the interior of the cell and 1 layer exposed to the exterior (Effigy iii.i.1).

Figure three.one.1 – Phospholipid Structure and Bilayer: A phospholipid molecule consists of a polar phosphate "head," which is hydrophilic and a not-polar lipid "tail," which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails. The phospholipid bilayer consists of ii 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 prison 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 h2o molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or "h2o 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, lather works to remove oil and grease stains because it has amphipathic backdrop. The hydrophilic portion can dissolve in the wash h2o while the hydrophobic portion tin trap grease in stains that then tin can be washed abroad. 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 as well attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment exterior the enclosure of the prison cell membrane (come across 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 infinite. In improver to phospholipids and cholesterol, the jail cell membrane has many proteins detailed in the next section.

Membrane Proteins

The lipid bilayer forms the basis of the cell membrane, only information technology is brindled throughout with diverse proteins. 2 different types of proteins that are commonly associated with the cell membrane are the integral protein and peripheral protein (Figure 3.ane.ii). As its name suggests, an integral protein is a poly peptide that is embedded in the membrane. Many different types of integral proteins exist, each with dissimilar functions. For example, an integral protein that extends an opening through the membrane for ions to enter or get out the cell is known as a aqueduct protein. Peripheral proteins are typically institute on the inner or outer surface of the lipid bilayer only can likewise be fastened to the internal or external surface of an integral protein.

Figure 3.ane.ii- Jail cell Membrane: The jail 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 every bit prison cell recognition or surface identity proteins, which mark a cell'south identity and then that it can exist recognized by other cells. Some integral proteins human action 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 bounden 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-aqueduct interaction is the receptors on nerve cells that demark neurotransmitters, such every bit dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane poly peptide opens to allow certain ions to flow into the cell. Peripheral proteins are often associated with integral proteins along the inner prison cell membrane where they play a role in cell signaling or anchoring to internal cellular components (ie: cytoskeleton discussed afterwards).

Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has sugar molecules fastened, which extend into the extracellular environment. The attached carbohydrate tags on glycoproteins assist in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively grade the glycocalyx. The glycocalyx is a fuzzy-appearing coating around the jail cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx tin can have various roles. For example, it may take molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to suspension downwardly 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 trunk. This identity is the primary way that a person's allowed defense cells "know" non to assail the person'south own trunk cells, but information technology as well is the reason organs donated by some other person might be rejected.

Transport Beyond the Cell Membrane

One of the smashing wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such equally Ca⁺⁺, Na⁺, Yard⁺, and Cl^–, nutrients including sugars, fatty acids, and amino acids, and waste products, particularly carbon dioxide (CO₂), which must leave the cell.

The membrane'due south 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 exist selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through information technology unaided. In the example of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (recall, 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—similar glucose, amino acids, and electrolytes—demand some assist to cantankerous the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane practice and then by one of two general methods, which are categorized based on whether or non energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In dissimilarity, agile transport is the motility of substances across the membrane using energy from adenosine triphosphate (ATP).

Passive Transport

In guild to understand how substances move passively across a cell membrane, it is necessary to sympathize concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance beyond a space. Molecules (or ions) will spread/diffuse from where they are more than concentrated to where they are less full-bodied until they are equally distributed in that space. (When molecules move in this style, they are said to move downwards their concentration gradient, from high concentration to depression concentration.) Diffusion is the motility of particles from an area of higher concentration to an area of lower concentration. A couple of common examples volition aid 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 along until the molecules were every bit distributed in the room. Some other example is a spoonful of sugar placed in a cup of tea. Eventually the carbohydrate will diffuse throughout the tea until no concentration slope remains. In both cases, if the room is warmer or the tea hotter, improvidence occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures.

External Website

Visit this link to see diffusion and how it is propelled past the kinetic free 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 equally jail cell membranes, any substance that can motion downwards its concentration slope across the membrane will do so. If the substances tin can motility across the cell membrane without the jail cell expending energy, the movement of molecules is called passive transport. Consider substances that can easily lengthened through the lipid bilayer of the cell membrane, such as the gases oxygen (O_two) and carbon dioxide (CO_ii). These small-scale, fat soluble gasses and other small lipid soluble molecules can deliquesce in the membrane and enter or leave the cell following their concentration slope. This machinery of molecules moving beyond a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a grade of passive transport called unproblematic improvidence. O_ii generally diffuses into cells because it is more concentrated exterior of them, and CO₂ typically diffuses out of cells because it is more concentrated within of them.

Earlier 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 considering cells apace use upwardly oxygen during metabolism and then, at that place is typically a lower concentration of O₂ inside the cell than exterior. As a result, oxygen will diffuse from outside the cell directly through the lipid bilayer of the membrane and into the cytoplasm within the jail cell. On the other hand, because cells produce CO₂ every bit a byproduct of metabolism, CO_ii concentrations rise within the cytoplasm; therefore, CO₂ will move from the jail cell through the lipid bilayer and into the extracellular fluid, where its concentration is lower. (Figure 3.i.3).

Effigy three.ane.3 – Unproblematic Improvidence Across the Jail 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 laissez passer through the cell membrane, down their concentration gradient, by elementary improvidence.

Large polar or ionic molecules, which are hydrophilic, cannot hands cross the phospholipid bilayer. Charged atoms or molecules of any size cannot cross the cell membrane via elementary diffusion every bit the charges are repelled past the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in h2o on either side of the jail cell membrane will tend to diffuse down their concentration gradients, but because near substances cannot pass freely through the lipid bilayer of the cell membrane, their motility is restricted to protein channels and specialized send mechanisms in the membrane. Facilitated diffusion is the diffusion procedure used for those substances that cannot cross the lipid bilayer due to their size, charge, and/or polarity but practice so downward their concentration gradients (Figure iii.1.iv). As an example, fifty-fifty 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 course sodium channels (or "pores"), so that Na+ ions can move down their concentration gradient from outside the cells to within 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 tin can be more full-bodied outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar, and therefore, repelled past the phospholipid membrane. To resolve this, a specialized carrier poly peptide chosen the glucose transporter volition transfer glucose molecules into the cell to facilitate its inward diffusion. The difference between a aqueduct and a carrier is that the carrier usually changes shape during the diffusion process, while the channel does not. At that place are many other solutes that must undergo facilitated diffusion to move into a cell, such every bit amino acids, or to move out of a cell, such as wastes.

Figure three.1.4 – Facilitated Diffusion: (a) Facilitated improvidence of substances crossing the cell (plasma) membrane takes identify with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and ordinarily mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more than selective, often merely assuasive one detail type of molecule to cantankerous.

Osmosis

A specialized example of facilitated ship 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 at that place is more relative water to where there is less relative h2o (down its water concentration gradient) (Effigy 3.i.5).

Figure 3.1.5 – Osmosis: Osmosis is the diffusion of h2o 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 h2o concentration (and thus the side of college 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, then 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 h2o molecules is the aforementioned outside and inside the cells, and the cells maintain their normal shape (and part).

Osmosis occurs when at that place is an imbalance of solutes outside of a cell versus inside the prison cell. A solution that has a higher concentration of solutes than some other solution is said to exist hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure iii.ane.half-dozen). Cells in a hypertonic solution will shrivel every bit 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 also much water and swell, with the adventure of eventually bursting. A critical attribute of homeostasis in living things is to create an internal surround in which all of the body'southward cells are in an isotonic solution. Various organ systems, particularly the kidneys, piece of work to maintain this homeostasis.

Figure iii.1.6 – Concentration of Solution: A hypertonic solution has a solute concentration college than some other solution. An isotonic solution has a solute concentration equal to some other solution. A hypotonic solution has a solute concentration lower than another solution.

Active Send

For all of the transport methods described above, the cell expends no energy. Membrane proteins that assist in the passive transport of substances do so without the utilize of ATP. During master agile transport, ATP is required to move a substance across a membrane, with the aid of membrane protein, and against its concentration slope.

1 of the almost common types of active transport involves proteins that serve as pumps. The word "pump" probably conjures up thoughts of using energy to pump upwardly 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 surface area of low concentration to an area of high concentration).

The sodium-potassium pump, which is besides called Na⁺/K⁺ ATPase, transports sodium out of a prison 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 not bad that it accounts for the majority of their ATP usage.

Figure iii.1.seven The sodium-potassium pump is establish in many prison cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in reverse directions, each against its concentration gradient. In a single bicycle of the pump, three sodium ions are extruded from and 2 potassium ions are imported into the cell.

Active transport pumps can also piece of 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 information technology 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 manner, the action of an active send 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 agile send.

Symporters are secondary agile transporters that move two substances in the aforementioned management. For example, the sodium-glucose symporter uses sodium ions to "pull" glucose molecules into the cell. Since cells shop glucose for free energy, glucose is typically at a college concentration inside of the prison cell than outside; notwithstanding, due to the activity 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, confronting its concentration gradient.

Conversely, antiporters are secondary active ship systems that transport substances in opposite directions. For example, the sodium-hydrogen ion antiporter uses the free energy from the inward flood of sodium ions to motion hydrogen ions (H⁺) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell'southward interior.

Other Forms of Membrane Send

Other forms of active ship do non involve membrane carriers. Endocytosis (bringing "into the prison cell") is the process of a cell ingesting fabric by enveloping it in a portion of its cell membrane, and so pinching off that portion of membrane (Figure 3.1.8). Once pinched off, the portion of membrane and its contents becomes an contained, 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 trunk tissues for unwanted matter, such equally invading bacterial cells, phagocytize them, and digest them. In dissimilarity to phagocytosis, pinocytosis ("jail cell drinking") brings fluid containing dissolved substances into a jail cell through membrane vesicles.

Figure 3.ane.8 – Iii Forms of Endocytosis: Endocytosis is a grade of agile 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 demark 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 jail cell membrane which contains many receptors that are specific for a certain substance. One time the surface receptors have bound sufficient amounts of the specific substance (the receptor's ligand), the jail cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed past crimson blood cells in this manner. Atomic number 26 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 prison cell") is the process of a cell exporting material using vesicular transport (Figure three.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 so becomes function of the cell membrane.

Specific examples of exocytosis include cells of the tum and pancreas producing and secreting digestive enzymes through exocytosis (Figure three.1.x) 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 and then that the cell is not constantly enlarging. Through these processes, the cell membrane is constantly renewing and irresolute equally 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 within the prison cell. The membrane of the vesicle fuses with the prison cell membrane, and the contents are released into the extracellular space.

Figure 3.1.x – Pancreatic Cells' Enzyme Products: The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny blackness 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 xxx,000 people in the United States, with about one,000 new cases reported each year. The genetic affliction is most well-known for its damage to the lungs, causing animate difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines. Only well-nigh fifty 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 chosen the Cystic Fibrosis Transmembrane Conductance Regulator, or CFTR. In good for you 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 articles a lacking channel poly peptide that typically is non incorporated into the membrane, but is instead degraded by the cell.

The CFTR requires ATP in order to function, making its Cl– transport a course of active transport. This puzzled researchers for a long time considering the Cl– ions are actually flowing downwardly their concentration gradient when transported out of cells. Active send mostly pumps ions confronting their concentration gradient, but the CFTR presents an exception to this dominion.

In normal lung tissue, the movement of Cl– out of the jail cell maintains a Cl–-rich, negatively charged environment immediately outside of the cell. This is especially important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, leaner, and other droppings. A cilium (plural = cilia) is 1 of the pilus-like appendages plant 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 club to be effectively moved upward, the mucus cannot be also viscous, rather, it must accept a thin, watery consistency. The transport of Cl– and the maintenance of an electronegative surroundings outside of the prison cell attracts positive ions such as Na+ to the extracellular space. The aggregating of both Cl– and Na+ ions in the extracellular infinite creates solute-rich mucus, which has a depression concentration of water molecules. Equally a effect, through osmosis, h2o moves from cells and extracellular matrix into the mucus, "thinning" it out. In a normal respiratory arrangement, this is how the mucus is kept sufficiently watered-downwardly to be propelled out of the respiratory system.

If the CFTR channel is absent-minded, Cl– ions are not transported out of the cell in adequate numbers, thus preventing them from cartoon positive ions. The absence of ions in the secreted fungus results in the lack of a normal water concentration slope. Thus, there is no osmotic pressure pulling h2o into the mucus. The resulting mucus is thick and gummy, and the ciliated epithelia cannot effectively remove it from the respiratory system. Passageways in the lungs become blocked with fungus, forth with the droppings information technology carries. Bacterial infections occur more easily because bacterial cells are not effectively carried abroad from the lungs.

Affiliate Review

The prison cell membrane provides a barrier around the cell, separating its internal components from the extracellular environs. 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 express number of materials to lengthened through its lipid bilayer. All materials that cantankerous the membrane do so using passive (non-energy-requiring) or active (free energy-requiring) transport processes. During passive transport, materials movement by uncomplicated diffusion or past facilitated improvidence through the membrane, downwards their concentration slope. Water passes through the membrane in a diffusion process called osmosis. During agile transport, energy is expended to assist material movement beyond the membrane in a direction against their concentration slope. Active transport may accept place with the help of protein pumps or through the utilize of vesicles.

Interactive Link Questions

Visit this link to see diffusion and how information technology is propelled by the kinetic energy of molecules in solution. How does temperature affect diffusion rate, and why?

Higher temperatures speed upwards diffusion because molecules have more than kinetic energy at higher temperatures.

Review Questions

Critical Thinking Questions

What materials tin can easily diffuse through the lipid bilayer, and why?

Only materials that are relatively small-scale and nonpolar can hands diffuse through the lipid bilayer. Large particles cannot fit in between the individual phospholipids that are packed together, and polar molecules are repelled by the hydrophobic/nonpolar lipids that line the inside of the bilayer.

Why is receptor-mediated endocytosis said to exist more selective than phagocytosis or pinocytosis?

Receptor-mediated endocytosis is more than selective because the substances that are brought into the cell are the specific ligands that could demark to the receptors beingness endocytosed. Phagocytosis or pinocytosis, on the other hand, have no such receptor-ligand specificity, and bring in whatever materials happen to be close to the membrane when it is enveloped.

What do osmosis, improvidence, filtration, and the movement of ions abroad from like charge all have in mutual? In what manner do they differ?

These 4 phenomena are similar in the sense that they depict the movement of substances down a detail blazon of slope. Osmosis and diffusion involve the movement of water and other substances down their concentration gradients, respectively. Filtration describes the motility of particles downwardly a pressure level gradient, and the motion of ions away from a like accuse describes their movement downward their electric slope.

Source: https://open.oregonstate.education/aandp/chapter/3-1-the-cell-membrane/

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