College papers academic writing service


Diffusion of substances across semi permeable membranes

15.3: Membrane Transport with Selective Permeability

Menu Brownian Motion Diffusion is the movement of particles from high concentration to low concentration in a substance. This process is essential for life on Earth, allowing for the movement of molecular compounds into and out of the cell. All matter in the universe is in motion, because all molecules are vibrating.

This constant vibration is known as Brownian motion, which can be seen as random zig-zag motion in particles. Brownian Motion Simple Diffusion Diffusion is one of several transport phenomena that occur in nature. A distinguishing feature of diffusion is that it results in mixing. Diffusion is the movement of a substance from a region of high concentration to a region of low concentration.

Diffusion flux is proportional to the negative gradient of concentrations. In other words, particles move from higher concentration to regions of lower concentration. Simple diffusion The Cellular Membrane The 'cell membrane' also known as the plasma membrane or cytoplasmic membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.

The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the phospholipid bilayer with embedded proteins.

  1. Carrier proteins are typically specific for a single substance.
  2. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.
  3. As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion. Types of cellular transport Passive osmosis and diffusion Some substances small molecules, ions such as carbon dioxide CO2 and oxygen O2 , can move across the plasma membrane by diffusion, which is a passive transport process.
  4. At the end of the process, the substance will be on the opposite side from which it started. Membrane permeability coefficients Below, a variety of compounds are plotted with respect to their membrane permeability coefficients MPC as measured against a simple biochemical approximation of a real biological membrane.
  5. If a channel protein exists and is open, the sodium ions will be pulled through the membrane.

The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell.

The cell employs a number of transport mechanisms that involve biological membranes: Types of cellular transport Passive osmosis and diffusion Some substances small molecules, ions such as carbon dioxide CO2 and oxygen O2can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water.

Such molecules diffuse passively through protein channels in facilitated diffusion or are pumped across the membrane by transmembrane transporters. Protein channel proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited food group of chemical substances, often even only a single substance.

Endocytosis Endocytosis Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles "cell eating" or phagocytosissmall molecules and ions "cell drinking" or pinocytosisand macromolecules.

Endocytosis requires energy and is thus a form of active transport. Receptor-mediated endocytosis is a process by which cells internalize molecules endocytosis by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized. Coat proteins of the vesicle signals proteins of specific organelles in the cell, which allow the direct transmission of specific internal molecules be delivered directly to the organelles that require them.

Exocytosis Exocytosis Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier.

In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell.

Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino diffusion of substances across semi permeable membranes.

  • The vesicle membrane comes in contact with the plasma membrane;
  • Such molecules diffuse passively through protein channels in facilitated diffusion or are pumped across the membrane by transmembrane transporters;
  • Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane;
  • Receptor-mediated endocytosis is a process by which cells internalize molecules endocytosis by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized.

If the process uses chemical energy, such as from adenosine triphosphate ATPit is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not use cellular energy. Active transport is a good example of a process for which cells require energy.

Why are cells so small? Cells are so small that you need a microscope to examine them. To answer this question we have to understand that, in order to survive, cells must constantly interact with their surrounding environment.

Gases and food molecules dissolved in water must be absorbed and waste products must be eliminated. For most cells, this passage of all materials in and out of the cell must occur through the plasma membrane.

Each internal region of the cell has to be served by part of the cell surface. As a cell grows bigger, its internal volume enlarges and the cell membrane expands.

Unfortunately, the volume increases more diffusion of substances across semi permeable membranes than does the surface area, and so the relative amount of surface area available to pass materials to a unit volume of the cell steadily decreases. Finally, at some point, there is just enough surface available to service all the interior; if it is to survive, the cell must stop growing. The important point is that the surface area to the volume ratio gets smaller as the cell gets larger.

Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. That is why cells are so small. Osmosis across a semi-permeable membrane Osmosis is the diffusion of water from high concentration to low concentration. When you drink water, your cells have a lower concentration of water than the water in your digestive system.

So water flows across the cell membrane from high concentration to low concentration of your cells hydrating you. Thirst is our bodies way of maintaining an osmotic balance of water. This is rare in humans, but has occured most commonly in endurance athletes consuming more water than their body needed to maintain osmotic balance.

We see this in plants that have not received adequate watering. When this happens, water moves from high concentration on the inside of the cell to lower concentrations out of the cell.

Osmotic conditions of cells. In this exercise, you will simulate the diffusion of water across a semi-permeable membrane, similar to how water diffuses across cell membranes. You will simulate the cell membrane with dialysis tubing which is a semi-permeable membrane that allows water through, but not sucrose.