Transport Through The Cell Membran

Transport of Substances through the Cell Membrane

Necessity for Transport

Cells are surrounded by fluid. The fluid that circulates around and between the cells is called interstitial fluid. This fluid constitutes the internal environment of the body. Everything that the cell requires must be obtained from this environment. The cytoplasm of the cell is separated from this environment by the cell membrane. Materials that are needed by the cell must enter the cell by being transported through the cell membrane. Waste products must move across the membrane in the opposite direction. For example, the cell requires oxygen in order to survive. Oxygen diffuses into the cell. The cell also needs water. This enters the cell by diffusion through the cell membrane. The cell requires sugars, which it breaks down to produce energy. They must also pass through the cell membrane. The cell requires amino acids, which serve as building blocks for protein. Similarly, fatty acids and glycerol are needed for producing fats. Vitamins are needed. All of these must be transported through the cell membrane into the cell. Waste products must be excreted from the cell. Waste products include carbon dioxide, which is a waste product of cellular respiration. This must diffuse out of the cell. Nitrogenous waste products such as urea must also diffuse out of the cell. The cell membrane controls the transport of these substances into or out of the cell.

Outline of Processes Involved in Transport

Transport processes may be divided into 1) Passive Transport Processes and 2) Passive Transport Processes. Passive Transport Processes do not require the expenditure of energy by the cell (breakdown of ATP). Active Transport Processes do require the expenditure of energy by the cell (breakdown of ATP).

I. Passive Transport Processes

A. Diffusion

B. Facilitated Diffusion

C. Osmosis

II. Active Transport Processes

A. Active Transport Mechanism

B. Phagocytosis

C. Pinocytosis

Diffusion

Diffusion is the movement of molecules of a substance from a region of higher concentration to a region of lower concentration.

Examples of Diffusion

If a crystal of a dye such as potassium permanganate is placed in a beaker of water, the purple dye will gradually spread or diffuse throughout the liquid. It diffuses from an area of higher concentration, in which it was first placed, to the area of lower concentration elsewhere in the beaker.

Kinetic Energy

The force that powers diffusion is kinetic energy.

Kinetic energy is energy of motion.

All molecules are in constant motion. This is because they possess kinetic energy, or energy of motion.

Law of Diffusion

The Law of Diffusion states that molecules diffuse from a region of higher concentration to a region of lower concentration.

Another way of expressing this idea is to say that substances diffuse down their concentration gradients.

An analogy can be made between a concentration gradient and an incline or a hill. Just as a ball placed at the top of an incline or a ramp will automatically roll down the hill from high to low, a molecule of a substance will diffuse from an area of high concentration to an area of low concentration.

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Example of diffusion using specific concentrations

Assume that you have a U-shaped glass tube. A membrane separates a 20% NaCl solution on side A from a 10% NaCl solution on side B. The membrane is permeable to both water and salt. Salt will pass through the membrane from the region of higher salt concentration (20% on side A) to the region of lower salt concentration (10% side B). Water will also move from the area of its high concentration to the area of its low concentration. If the concentration of salt on Side A is 20%, this means that the concentration of water there is 80%. If the salt concentration on side B is 10%, the water concentration is 90%. Water will move from side B to side A. As water diffuses from B to A and salt diffuses from side A to side B, the concentrations of the two solutions on side A and side B will become equal. A state of equilibrium has been reached.

Will the diffusion of water from B to A or of salt from side A to side B stop? No the movement of molecules never stops (except at absolute zero). Will water continue to diffuse from side B to side A? Yes. However, because the concentration of water is now equal on both sides of the membrane, the movement of water from side B to side A will be balanced by the movement of an equal amount of water from side A to side B. By the same reasoning the diffusion of salt does not stop, but because the salt concentrations on both sides of the membrane are equal the diffusion of salt from one side of the membrane is balanced by the movement of salt from the opposite direction.

Factors that Affect Diffusion

1) Concentration gradient

The greater the difference in concentration from the area in which diffusion begins to where it proceeds, the faster the rate of diffusion.

2) Molecular Weight of the substance

The lower the molecular weight of the substance, the faster the rate of diffusion.

A demonstration of this principle was seen in a laboratory exercise. A crystal of the dye potassium permanganate was placed on the surface of agar on one Petri dish. Potassium permanganate has a molecular weight of 158 g mole-1. It is a purple dye. A crystal of the dye Malachite green was placed in the center of a second Petri dish. Malachite green has a molecular weight of 929 g mole-1. The time at the beginning of the experiment is noted and the materials are allowed to diffuse. Some time later the plates are examined. As the dyes diffuse, they create a colored ring, which spreads out. The ring created by each dye is measured. The dye with the lower molecular weight, potassium permanganate has the wider ring. Therefore it has diffused at a faster rate as compared to Malachite green.

3) Temperature

The higher the temperature, the faster the rate of diffusion. This is because the higher the temperature, the greater is the kinetic energy of the molecules. Molecules with more kinetic energy move more rapidly.

Facilitated Diffusion

Facilitated diffusion is the diffusion of molecules through the cell membrane with the aid of a carrier molecule.

In this process, a molecule to be transported binds to a specific carrier in the membrane. By itself, the molecule cannot diffuse across the membrane. However, by combining with the carrier, the substance is able to cross the membrane. The carrier facilitates the diffusion of the substance across the membrane.

Facilitated Diffusion does not require the expenditure of energy by the cell (breakdown of ATP). Facilitated Diffusion can only move substances from a region of high concentration to a region of low concentration, that is, in the same direction as ordinary diffusion.

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Osmosis

Osmosis is the diffusion of water through a selectively permeable membrane.

If a membrane is permeable to a certain substance, it means that that substance can pass through the cell membrane. If a membrane is selectively permeable, it means that the membrane allows certain substances to pass through the membrane but excludes others.

Several terms are used that are synonymous to selectively permeable. The terms semi-permeable, and differentially permeable also mean the same thing.

Example of Osmosis

Assume that a membrane separates a pure water solution on one side from a salt (NaCl) solution on the other. The membrane is selectively permeable; it allows water to pass through it but not salt. Therefore, salt will not move across the membrane. Water will move from the area of its higher concentration (pure water side) and pass across the membrane to the area of lower water concentration (salt solution side).

The key to understanding osmosis is to first determine which substance the membrane is permeable to and therefore can move through the membrane. This is usually water. The next step is to determine where the concentration of the substance is higher and where it is lower. The direction of movement in osmosis is always from the region of high concentration to the region of lower concentration.

Example of osmosis using specific concentrations

Assume that you have a U-shaped glass tube. A selectively permeable membrane separates a 20% NaCl solution on side A from a 10% NaCl solution on side B. The membrane is permeable to water but not to salt. Because the membrane is not permeable to salt, it will not pass through the membrane. The membrane is permeable to water. Water will move across the membrane from the region of its high concentration to the region of its low concentration. If the concentration of salt on Side A is 20%, this means that the concentration of water there is 80%. If the salt concentration on side B is 10%, the water concentration is 90%. Because the concentration of water on side B is higher, water will move from side B to side A. Over a period of time, water will diffuse from B to A and the liquid in side A will rise while that in side B will drop. A true equilibrium will not result because NaCl cannot cross the membrane. Therefore although the concentration of the salt on side A will decrease as water moves into it, its concentration will never become equal to the salt concentration on side B. This is because the salt cannot move across the membrane to equalize its concentration on both sides.

Will the diffusion of water from B to A stop? If so why? Although a true equilibrium will not be achieved, the net movement of water from B to A will eventually stop. This is because as the level of the liquid on side A rises, it exerts a downward pressure due to the force of gravity. This force eventually counteracts the force of osmosis that draws water from B to A. This force is called osmotic pressure, the force that is just great enough to stop further osmosis.

Another Example of Osmosis

A glass thistle tube (a tube which has an open expanded end) is filled with a 20% sucrose solution. The end is covered with a selectively permeable membrane. The membrane is permeable to water but impermeable to sucrose. The tube is lowered into a beaker of pure water and held in place with a clamp and support stand. After allowing a few minutes, the level of liquid in the tube is marked with a wax pencil and the starting time is noted. At intervals of time (10 or 15 minutes) readings of the level of liquid in the tube are taken. The level of liquid in the tube rises as time goes on. This is an example of osmosis. A selectively permeable membrane separates a 20% sucrose solution inside the tube from a pure water (100% water) concentration on the outside. The membrane allows water to pass through the membrane but prevents the movement of sucrose. Because the sucrose cannot pass through the membrane, it is described as a nondiffusable solute. The substance that will diffuse across the membrane is water. It will pass from the region of higher concentration to the region of lower concentration. The region of higher concentration is obviously the outside (you cannot have a higher concentration than 100%). The concentration of water inside the tube is 80%. If the concentration of sucrose is 20%, subtracting 20% from 100% leaves a concentration of 80% for the water. Therefore water flows from the outside into the tube. As water enters the tube, the level of water within the tube will rise. Will the level of liquid inside the tube eventually stop rising? If so, why? The level of liquid in the tube will eventually stop rising. However, it is not because the concentration of the solutions inside the tube and outside become equal and equilibrium becomes established. This cannot happen because the sucrose cannot pass through the membrane. Although the solution inside the tube becomes more and more dilute as water enters, it will never become equal to the concentration outside. Why then does the liquid stop rising? It is because as the solution in the tube rises, it exerts a downward force due to gravity. Eventually this force becomes great enough to counteract the force of osmosis responsible for the inward flow of water. At this point the force exerted by the column of liquid in the tube is equivalent to the osmotic pressure, the force that is just great enough to stop further osmosis.

Osmotic pressure is affected primarily by the number of particles of the non-diffusible solute in solution. For example, if the solution inside the tube were replaced with a solution of 40% sucrose, the liquid would rise higher and at a faster rate. This would occur because there is a steeper concentration gradient for the water in this example using 40% sucrose than there was in the example that used 20% sucrose. In the previous example using 20% sucrose, the concentration of water outside was 100% and that inside was 80% giving a difference of 20%. In the example using 40% sucrose, the difference between the solution outside (100% water) and inside (60% water) was 40%. Because there is a greater difference in concentration of the water in this example, the diffusion of water is faster. Another way of saying this is that the concentration gradient is steeper. What is the role of the sucrose (non-diffusible solute) in these examples? It did not move by osmosis across the membrane since it is not permeable in the membrane. However the presence of sucrose on one side of the membrane creates a concentration gradient for water and this leads to the diffusion of water across the membrane.

Factors which Affect Osmosis

1) The same factors which affect diffusion, plus

2) Solubility of the substance in the membrane

3) Osmotic pressure

Isotonic, Hypertonic, and Hypotonic Solutions

The terms isotonic, hypertonic, and hypotonic refer to solutions that are found outside of cells. They are defined with respect to the osmotic concentration found within the cell. The solution could contain a higher osmotic concentration than the cell, in which case it would be hypertonic. It could contain a lower solute concentration than the cell, in which case it would be hypotonic. Or it could contain a solute concentration that is equivalent to that within the cell, in which case it would be isotonic. Depending upon which type of solution is placed outside the cell, water can either move into the cell, move out of the cell, or equal amounts can move in both directions.

Solute – the solid that is dissolved in a liquid

Solvent – the liquid in which a solid is dissolved

Solution – a liquid containing a dissolved substance

Concentration – check

Examples of Isotonic, Hypertonic, and Hypotonic Solutions

Hypertonic Solution

Lets assume that red blood cells are placed in a 5% NaCl solution. Lets also assume that the cytoplasm inside the red blood cell has a concentration that is equivalent to that of a 0.9% NaCl solution. A selectively permeable membrane surrounds the red blood cell. This membrane is permeable to water but impermeable to salt (does not allow salt to pass through it). Osmosis will occur in this situation. In which direction will there be a net diffusion of water? To determine this, we must figure out the concentration of water on each side of the membrane. If there is the equivalent of 0.9% salt solution inside the cell, there is 99.1% water (100% - 0.9% = 99.1%). If the outside solution contains 5% salt, it contains 95% water (100% - 5% = 95%). Water will move from the region of higher concentration to the region of lower concentration. In this case water moves from inside the cell where the concentration is 99.1% to the outside, where the concentration is 95%.

The solution outside the cell is a hypertonic solution.

Hypertonic solution – a solution that when placed outside of a cell causes osmosis of water out of the cell into the surrounding, more highly concentrated (solute concentration) solution.

Note that any solution that has a solute concentration greater than that inside the cell (in this case 0.9%) is an example of a hypertonic solution. Examples: solutions of 1%, 2%, 3%, 7%, 10% etc. are all hypertonic solutions.

As water moves out of the red blood cell into the hypertonic solution, it shrinks. The shrinkage of a red blood cell when placed in a hypertonic solution is known as crenation.

Hypotonic Solution

Assume that a red blood cell is placed in pure water. Now the concentration of the water is 100% (the highest concentration). The concentration inside the cell is 99.1%. In this case, water will move into the cell. The solution outside the cell (pure water) is an example of a hypotonic solution.

Hypotonic solution – a solution that when placed outside of cells causes osmosis of fluid into the cell from the surrounding, more dilute (solute concentration) solution.

As water moves into the red blood cell from a hypotonic solution, the cell swells and may burst open, releasing its hemoglobin and other contents to the outside. The bursting of a red blood cell when placed in a hypotonic solution is called hemolysis. When the red blood cell bursts, intact cells are no longer visible when the cells are viewed microscopically. Instead the solution becomes clear and tiny fragments of the cell membrane that surrounded the cells, known as red blood cell ghosts, are all that remain.

Note that any solution that has a solute concentration lower than the solute concentration inside the cell (in this case 0.9% NaCl) would be hypotonic. For example, solutions with concentrations of 0.8%, 0.7%, 0.5% etc. are all hypotonic.

Isotonic Solution

Assume that the red blood cell is placed in a 0.9% salt solution. In this case, the solution outside the cell and the solution inside the cell are equivalent in terms of solute concentration. Will water move into the cell? Yes. Will water move out of the cell? Yes. However, the amount moving in and the amount moving out will be equal. As a result the cell will not shrink. Neither will it swell. It will retain its normal shape and volume. The solution outside the cell is an example of an isotonic solution.

Isotonic solution – a solution that when placed outside of a cell causes no net osmosis (osmosis that is greater in one direction than in another) in either direction.

If a patient requires an injection or intravenous fluids, these fluids must be isotonic with respect to the patient’s cells.

Osmosis and Preservation of Food

In order to preserve foods, they are often placed in concentrated salt or sugar solutions. For example, fish or pickles are preserved in brine, a concentrated (hypertonic) solution of salt. Jams and jellies are preserved in concentrated sugar solutions. How does this preserve the food? The concentrated solutions surrounding the food prevent the growth of bacteria. Bacteria are also cells. When placed in hypertonic solutions, they become dehydrated and are unable to grow. Spoilage is prevented.

Plasmolysis

Elodea is an aquatic plant. It is commonly sold for use in aquariums. If we were to examine an elodea cell under the microscope, in a drop of water, its normal environment, we would see a cell that has a large central vacuole filled with water. The water would press against the cell membrane and the cell wall. The chloroplasts and other organelles would appear as they normally do in the cytoplasm next to cell membrane and cell wall. You may see the chloroplasts circling around the inside the cell. This process is called cyclosis, the streaming of cytoplasm. In the cell in its normal environment, the cell is said to be turgid, the water pressure caused by osmosis moving water inside the cell is pressing the cytoplasm and its organelles against the inside of the cell. The pressure is called turgor pressure. Turgor pressure is the force within the plant cell that presses the cytoplasm against the cell wall.

If a plant cell, such as an Elodea cell is placed in a hypertonic solution, for example 10% NaCl, water will flow out of the cell. Such cells contain a large central vacuole that contains a lot of water, and this water will diffuse through the vacuolar membrane and cell membrane to the outside. As the cell loses water, the cytoplasm will shrink away from the cell wall. The plant cell is surrounded by a rigid cell wall. This retains its shape. As the cytoplasm shrinks, it usually forms a ball in the center of the cell. This process is known as plasmolysis.

Plasmolysis – the shrinking of the cytoplasm of a plant cell away from the cell wall when placed in a hypertonic solution.

Active Transport

Active transport is the movement of molecules of a substance against a concentration gradient with the expenditure of energy by the cell in the form of the breakdown of ATP.

Active transport can move molecules of a substance against a concentration gradient. This means that active transport can move substances from a region of low concentration to a region of high concentration. “It enables a cell to take up additional molecules of a substance that is already present in its cytoplasm in concentrations higher than in the extracellular fluid. Without active transport, for example, liver cells would be unable to accumulate glucose molecules from the blood plasma, as the glucose concentration is often higher inside the liver cells than it is in the plasma. Active transport also enables a cell to move substances from its cytoplasm to the extracellular fluid despite higher external concentrations (Raven and Johnson, 2002 p.118)”. This is unlike ordinary diffusion. Ordinary diffusion can only move molecules from a region of high concentration to a region of low concentration. This direction is described as down the concentration gradient. The energy comes from the kinetic energy of the molecules. The cell does not have to expend energy to power the process. That is, it does not have to break down ATP to supply the energy. An analogy can be made between ordinary diffusion and a ball rolling down an incline. A ball placed at the top of an incline will roll down. No additional energy has to be added. The ball rolls from high to low. In the same way, molecule that move by diffusion move from a region of high concentration to a region of low concentration, that is, down their concentration gradient. The cell does not have to break down ATP to supply the energy.

Active transport can move molecules of a substance against a concentration gradient. This means that active transport can move molecules from a region of low concentration to a region of high concentration. Using the analogy of an incline, this would be like moving the ball up the incline that is in an uphill direction. Obviously, moving the ball up the incline would require energy. Similarly, in order to move a substance against its concentration gradient would require energy. The cell supplies this energy by breaking down ATP.

The Sodium Potassium Pump

“Most animal cells have a low internal concentration of Na+, relative to their surroundings, and a high internal concentration of K+. They maintain these concentration differences by actively pumping Na+ out of the cell and K+ in. The remarkable protein that transports these two ions across the cell membrane is known as the sodium potassium pump. The cell obtains the energy it needs from adenosine triphosphate (ATP) –“More than one-third of all of the energy expended by an animal cell that is not actively dividing is used in the active transport of sodium (Na+) and potassium (K+) ions.”

“The important characteristic of the sodium-potassium pump is that it is an active transport process, transporting Na+ and K+ from areas of low concentration to areas of high concentration. This transport up their concentration gradients is the opposite of the passive transport in diffusion; it is achieved only by the constant expenditure of metabolic energy. the sodium-potassium pump works through a series of conformational changes in the transmembrane protein:

Step 1. Three sodium ions bind to the cytoplasmic side of the protein, causing the protein to change its conformation (shape).

Step 2. In its new conformation, the protein binds a molecule of ATP and cleaves it into adenosine diphosphate and phosphate (ADP + Pi). ADP is released, but the protein is now phosphorylated.

Step 3. The phosphorylation of the protein induces a second conformational change in the protein. This change translocates the three Na+ across the membrane, so they now face the exterior. In this new conformation, the protein has a low affinity for Na+, and the three bound Na+ dissociate from the protein and diffuse into the extracellular fluid.

Step 4. The new conformation has ha high affinity for K+, two of which bind to the extracellular side of the protein as soon as it is free of the Na+.

Step 5. The binding of the K+ causes another conformational change in the protein, this time resulting in the dissociation of the bound phosphate group.

Step 6. Freed of the phosphate group, the protein reverts to its original conformation, exposing the two K+ to the cytoplasm. This conformation has a low affinity for K+, so the two bound K+ dissociate from the protein and diffuse into the interior of the cell. The original conformation has a high affinity for Na+; when these ions bind, they initiate another cycle.

Three Na+ leave the cell and two K+ enter in every cycle. The changes in protein conformation that occur during the cycle are rapid, enabling each carrier to transport as many as 300 Na+ per second. The sodium potassium pump appears to be ubiquitous in animal cells, although cells vary widely in the number of pump proteins they contain.

endocytosis

Endocytosis is the ingestion of particles by invagination of the cell membrane.

Phagocytosis and pinocytosis use the same mechanism to bring particles into the cell. However, in the case of phagocytosis, it is solid particles that are being ingested and it pinocytosis, it is liquid particles.

Phagocytosis is the ingestion of solid particles by invagination of the cell membrane.

Pinocytosis is the ingestion of liquid particles by invagination of the cell membrane.

Mechanism of Phagocytosis and Pinocytosis

Insert Diagram

Phagocytosis is carried out by white blood cells as they engulf bacteria. This is an important part of our protection against disease.

Exocytosis

Exocytosis is a form of active transport in which particles enclosed in a membrane-bound vesicle are moved to the surface of the cell and are released to the outside of the cell as the membrane of the vesicle opens up and fuses with the cell membrane.

A more technical definition is: Exocytosis is the egestion of particles by evagination of the cell membrane. Egestion is the process of discharging material from the cell. Evagination is the process of turning inside out.

Exocytosis is the opposite of endocytosis. In endocytosis, particles were captured at the cell surface as the membrane evaginated or dipped downward and formed a membrane-enclosed vesicle. The vesicle containing the particles then moved downward (descended) into the cytoplasm of the cell. In exocytosis, the process begins in the cytoplasm with a membrane-enclosed vesicle containing particles destined for export. The vesicle with its particles then moves toward the surface of the cell. The vesicles can move through the cytoplasm on tiny tracks created by microtubules and are propelled along by motor proteins. When the vesicle reaches the surface of the cell, the membrane of the vesicle breaks open and pushes the contained particles to the outside as it turns inside out while it fuses with the cell membrane.

Functions of Exocytosis

Secretion of Neurotransmitters from Neurons

In nerve cell neurotransmitters such as acetylcholine are stored within vesicles concentrated in the nerve endings. When a nerve impulse reaches the nerve ending, it causes the vesicles to move to the presynaptic membrane (the membrane before a gap known as the synapse) and break open to release the acetylcholine into the synapse. The membrane of the synaptic membrane fuses with the presynaptic membrane during this process of exocytosis. The acetylcholine then diffuses across the synapse to the postsynaptic membrane (the membrane after the synapse) where it combines with an acetylcholine receptor. This process transmits the nerve impulse to the postsynaptic cell.

Secretion of Hormones

The pancreas secretes the hormones insulin and glucagon by exocytosis.

Secretion of Digestive Enzymes

The pancreas also secretes the digestive enzymes (proteases, lipases, and amylases) by exocytosis.

Excretion of Excess Water from Contractile Vacuoles in Protists

In Paramecium, there are a pair of contractile vacuoles. They collect excess water from the cytoplasm and then contract to expel it through a pore to the outside.

Supply of Materials Used to Construct Cell Walls

Exocytosis in used in plant cells to supply materials needed to construct the cell wall.

Transport through the Cell Membrane Study Guide Revised 1-12-2021