This results in swelling of the cell and potential hemolysis bursting of the cell. In an isotonic solution, the flow of water in and out of the cell is happening at the same rate. Osmosis is the diffusion of water molecules across a semipermeable membrane from an area of lower concentration solution i. Water moves into and out of cells by osmosis. A red blood cell will swell and undergo hemolysis burst when placed in a hypotonic solution. When placed in a hypertonic solution, a red blood cell will lose water and undergo crenation shrivel.
Animal cells tend to do best in an isotonic environment, where the flow of water in and out of the cell is occurring at equal rates. Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell. The three main kinds of passive transport are diffusion or simple diffusion , osmosis, and facilitated diffusion. Simple diffusion and osmosis do not involve transport proteins.
Facilitated diffusion requires the assistance of proteins. Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. For cell transport, diffusion is the movement of small molecules across the cell membrane. The difference in the concentrations of the molecules in the two areas is called the concentration gradient.
The kinetic energy of the molecules results in random motion, causing diffusion. In simple diffusion, this process proceeds without the aid of a transport protein. It is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration. Diffusion will continue until the concentration gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient".
The end result is an equal concentration, or equilibrium , of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop.
At equilibrium, there is equal movement of materials in both directions. Not everything can make it into your cells. Pinocytosis : In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off.
A variation of endocytosis is called pinocytosis. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome. Potocytosis, a variant of pinocytosis, is a process that uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin.
The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae singular caveola are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.
Receptor-Mediated Endocytosis : In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane. A targeted variation of endocytosis, known as receptor-mediated endocytosis, employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances.
In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material 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 the failure of 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 LDL particles from their blood. Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site.
Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells. Privacy Policy. Skip to main content. Organization at the Cellular Level.
Search for:. Transport Across Membranes. Diffusion Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane. Key Takeaways Key Points Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients.
After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as dynamic equilibrium.
Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled.
Key Terms diffusion : The passive movement of a solute across a permeable membrane concentration gradient : A concentration gradient is present when a membrane separates two different concentrations of molecules. Osmosis Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration. Learning Objectives Describe the process of osmosis and explain how concentration gradient affects osmosis. Key Takeaways Key Points Osmosis occurs according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.
Osmosis occurs until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure.
Osmosis occurs when there is a concentration gradient of a solute within a solution, but the membrane does not allow diffusion of the solute. Key Terms solute : Any substance that is dissolved in a liquid solvent to create a solution osmosis : The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane semipermeable membrane : A type of biological membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally by specialized facilitated diffusion.
Tonicity Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow. Learning Objectives Define tonicity and describe its relevance to osmosis. Key Takeaways Key Points Osmolarity describes the total solute concentration of a solution; solutions with a low solute concentration have a low osmolarity, while those with a high osmolarity have a high solute concentration. Water moves from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water.
In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell; water enters the cell. In a hypertonic solution, the extracellular fluid has a higher osmolarity than the fluid inside the cell; water leaves the cell.
In an isotonic solution, the extracellular fluid has the same osmolarity as the cell; there will be no net movement of water into or out of the cell. Key Terms osmolarity : The osmotic concentration of a solution, normally expressed as osmoles of solute per litre of solution. Examples Tonicity is the reason why salt water fish cannot live in fresh water and vice versa. Facilitated transport Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
Learning Objectives Explain why and how passive transport occurs. Key Takeaways Key Points A concentration gradient exists that would allow ions and polar molecules to diffuse into the cell, but these materials are repelled by the hydrophobic parts of the cell membrane. Facilitated diffusion uses integral membrane proteins to move polar or charged substances across the hydrophobic regions of the membrane.
Channel proteins can aid in the facilitated diffusion of substances by forming a hydrophilic passage through the plasma membrane through which polar and charged substances can pass. Channel proteins can be open at all times, constantly allowing a particular substance into or out of the cell, depending on the concentration gradient; or they can be gated and can only be opened by a particular biological signal.
Carrier proteins aid in facilitated diffusion by binding a particular substance, then altering their shape to bring that substance into or out of the cell. Key Terms facilitated diffusion : The spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins. Examples Channel-mediated facilitated diffusion functions much like a bridge over a river that must raise and lower in order to allow boats to pass.
The Role of Passive Transport Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes. Learning Objectives Indicate the manner in which various materials cross the cell membrane. Key Takeaways Key Points Plasma membranes are selectively permeable; if they were to lose this selectivity, the cell would no longer be able to sustain itself.
In passive transport, substances simply move from an area of higher concentration to an area of lower concentration, which does not require the input of energy. Concentration gradient, size of the particles that are diffusing, and temperature of the system affect the rate of diffusion.
Some materials diffuse readily through the membrane, but others require specialized proteins, such as channels and transporters, to carry them into or out of the cell.
Key Terms concentration gradient : A concentration gradient is present when a membrane separates two different concentrations of molecules. Primary Active Transport The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell.
Learning Objectives Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. When the sodium-potassium- ATPase enzyme points into the cell, it has a high affinity for sodium ions and binds three of them, hydrolyzing ATP and changing shape.
As the enzyme changes shape, it reorients itself towards the outside of the cell, and the three sodium ions are released. The enzyme changes shape again, releasing the potassium ions into the cell. After potassium is released into the cell, the enzyme binds three sodium ions, which starts the process over again. Key Terms electrogenic pump : An ion pump that generates a net charge flow as a result of its activity. Learning Objectives Define an electrochemical gradient and describe how a cell moves substances against this gradient.
Key Takeaways Key Points The electrical and concentration gradients of a membrane tend to drive sodium into and potassium out of the cell, and active transport works against these gradients. To move substances against a concentration or electrochemical gradient, the cell must utilize energy in the form of ATP during active transport.
Primary active transport, which is directly dependent on ATP, moves ions across a membrane and creates a difference in charge across that membrane. Secondary active transport, created by primary active transport, is the transport of a solute in the direction of its electrochemical gradient and does not directly require ATP. Secondary Active Transport In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
Facilitated diffusion can increase the rate of diffusion of particular molecules at low concentrations. However, the rate of facilitated diffusion levels off with increasing solute concentration. Additional increases in external solute concentration cannot increase the rate of diffusion once carrier proteins are saturated.
Conclusion: Passive diffusion of solute into a cell is linearly related to the concentration of solute outside the cell. Carrier proteins increase the rate of diffusion by allowing more solute to enter the cell. Facilitated diffusion, however, approaches a maximum rate as the carrier proteins become saturated with solute. What is the slope of this line? What do increases or decreases in the slope mean biologically?
Now assume the concentration gradient is a constant. Look at the equation for facilitated diffusion and find the horizontal asymptote. Try graphing this equation with different values for K. From Wikipedia The diffusion continues until the colour is evenly distributed. At this point, the process is at equilibrium. Diffusion across a membrane A substance may be present in unequal concentrations on either side of a cell membrane, as shown in the left-hand side of the figure below. At that point, equilibrium is reached.
Related questions How does temperature affect dynamic equilibrium? Why is dynamic equilibrium important for living organisms? What is dynamic equilibrium? What are some common mistakes students make with dynamic equilibrium?
0コメント