Pre-lab Exercise 8: The Plasma Membrane

Pre-lab Exercise 8: The Plasma Membrane

An adult human is composed of nearly 100 trillion cells of approximately 200 different types. Despite their varied forms and functions, each of these cells has the same basic needs for oxygen, nutrients and removal of waste products. The boundary of each cell, called the plasma membrane, separates the internal metabolism of the cell from the external environment and controls the movement of materials into and out of the cell. This movement of molecules across the plasma membrane is integral to many functions of the cell, from the transport of oxygen and removal of waste carbon dioxide from cells, to the removal of waste products from our bodies by our kidneys.

The significance of the plasma membrane becomes obvious when one sees examples of what happens when components of the membrane do not function correctly. In this lab you will look at several examples of inherited diseases that affect plasma membrane functions, and address some of the potential implications not only for individuals, but also for society.

The plasma membrane is the outermost boundary of a cell that allows needed oxygen and other nutrients to enter while waste products are released or kept out of the cell. This membrane is very selective about which solutes are allowed to pass in either direction and is described as a semi-permeable membrane.

The plasma membrane is composed of a double layer of phospholipids, with the non-polar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer surfaces of the membrane. All of the cell’s membranes have this fundamental structure, but there are other components of the membrane that make different areas of membrane unique and allow different functions to be served.

Molecules in the form of gases, water, or molecules dissolved in water can be moved across a membrane by either passive transport or active transport. Diffusion is the mechanism behind passive transport, and only allows molecules to be moved from an area of higher concentration to an area of lower concentration, described as “down a concentration gradient.”  Diffusion is driven by the intrinsic kinetic energy of molecules; no additional energy needs to be supplied by the cell. Although molecules are moving randomly and in all directions, the net diffusion is down the molecules concentration gradient, or, from high concentration to low. Thus, simple diffusion moves molecules across a membrane without expenditure of additional energy (the intrinsic kinetic energy of the molecules drives the process), and without the assistance of proteins in the membrane.

Osmosis is the term used to describe the diffusion of water across a semi-permeable membrane. Animals in all environments must maintain the correct concentration of solutes and water in their body fluids. This is accomplished by removal of excess materials through organs such as the skin and kidneys. Keeping the water and dissolved solutes in balance is called osmoregulation.

Water moves down its concentration gradient (from where it is at highest concentration to where it is lower), and as it moves into a cell, may build up pressure inside of it. Osmosis will occur if a container is separated into two compartments by a semi-permeable membrane, both compartments are filled to the same level with a solvent (in this case, water), and solute is added to one side. The level of the liquid on the side containing the solute will rise as the water flows from the side where it is at higher concentration to the side of lower concentration. If external pressure is exerted on the side containing the solute, the transfer of solvent can be stopped and even reversed (reverse osmosis).

Small, uncharged molecules can easily diffuse across cell membranes by simple diffusion, but water-soluble molecules and ions cannot due to the hydrophobic interior of the cell membrane. These molecules are helped across the membrane by transport proteins that are specific for them. Facilitated diffusion describes the diffusion of these solutes through special channel proteins in the plasma membrane. The process does not require any energy expenditure by the cell, and also carries molecules down their concentration gradients.

Some solutes have a higher concentration inside the cell than outside, so they have to cross the membrane against the concentration gradient. This means they can’t get into the cell by passive transport. Active transport requires that the cell expend energy, but allows solutes to move against a concentration gradient.. Active transport is achieved in one of two general ways:

  • Through the use of transport proteins in the plasma membrane that are able to transfer solutes such as small ions (Na+, K+, CL-, H+), amino acids, monosaccharides and other small molecules.
  • Through the formation of membrane vesicles which are formed from the plasma membrane and bring large molecules into the cell, or which are formed in the cell, fuse with the plasma membrane and release their contents to the outside of the cell.

Transport proteins found in the plasma membrane have binding sites that pick up specific molecules. An example of an active transport protein in cells is the sodium-potassium pump. This transporter is present in essentially all cells, and is found in particularly high concentrations in the membranes of nerve cells. The ability of the pump to transport three Na+ ions out of a cell and two K+ ions into a cell allows the cell to generate a difference in electrical potential across the membrane; the exterior of the cell becomes positively charged relative to the interior of the cell. This allows the propagation of electrical impulses along nerve cells.

Practical Implications of Diffusion and Osmosis

The flow of ions and water across cell membranes is important in the regulation of fluid balance in all living organisms. This balance is key to the transport of fluids across the renal tubules in the kidney, to the secretion and absorption of water within the gastrointestinal tract, and to fluid balance across the membrane of red blood cells and within the human eye. Many disease states are influenced by abnormalities in fluid transport including sickle cell anemia, cholera and glaucoma.

Osmoregulation in Humans

A major way animals have evolved to osmoregulate is by controlling the amount of water excreted through the excretory system. Osmosis, diffusion and active transport are the basic processes upon which the actions of the kidney are based. These processes involve solute molecules, water molecules and a biological membrane. The nephron is the functional unit of the kidney that utilizes these processes to remove waste products from the blood and to maintain a proper level of water in the body. The internal structure of the kidney includes an outer cortex and an inner medulla. The ureter divides into branches, the ends of which envelop medullary tissues called renal pyramids. Each human kidney contains about a million nephrons.

In vertebrates, four basic processes occur in the nephron:

Filtration: the fluid portion of the blood (the plasma) is filtered from the nephron structure through a structure known as the glomerulus into Bowman’s capsule and flows down the proximal convoluted tubule to a “u-turn” called the Loop of Henle in the medulla portion of the kidney.

Reabsorption: most of the viscous glomerular filtrate is returned to blood vessels which surround the convoluted tubules

Secretion: the remaining fluid becomes urine, which travels down collecting ducts to the medullary region of the kidney

Excretion: the urine is stored in the urinary bladder and exits via the urethra 

Dialysis

In biochemistry, dialysis is the process of separating molecules in solution by the difference in their rates of diffusion through a semi-permeable membrane. Typically a solution of several different types of molecules is placed into a semi-permeable dialysis bag, made of a cellulose membrane that contains pores (very small holes) of a specific size. The bag is sealed and placed into a container that holds a different solution or water. Molecules small enough to pass through the pores in the bag will move into or out of the bag in the direction of decreasing concentration. Larger molecules that have dimensions greater than the pore diameter will be retained inside the bag. The technique is often used as a step in the purification of molecules such as proteins. The technique also can be used to demonstrate how a cell’s plasma membrane functions as a semi-permeable membrane as you will do in this week’s lab exercise.

Hemodialysis is a method for removing waste products such as potassium and urea from the blood when the kidneys are incapable of doing so. It also removes free water when the kidneys have failed and can no longer produce urine. Hemodialysis involves diffusion of solutes across a semi-permeable membrane. It utilizes counter- current flow, where the dialysis fluid flows in the opposite direction to the blood flow. This counter-current exchange maintains the concentration gradient across the membrane at a maximum level in increases the efficiency of the process.

The hemodialysis machine performs the function of pumping the patient’s blood and the dialysis fluid through the dialyzer. The dialyzer, or artificial kidney, is the part of the equipment that actually filters the blood. One common type is a hollow fiber dialyzer, in which the blood is run through a bundle of very thin tubes that are made of a synthetic semi-permeable membrane. The dialysis solution that is used is a sterilized solution of mineral ions. Urea and other cellular waste products diffuse into the dialysis solution. Proteins and blood cells are too large to move through the membrane, and thus remain in the blood. The process is generally very efficient, allowing dialysis to occur only 3-4 times per week for 3-4 hours per session.

Pre-Lab Video Exercise

Cell Homeostasis Virtual Lab

Your goals is to work through the Cell Homeostasis Virtual Lab found at https://video.esc4.net/video/assets/Science/Biology/Gateway%20Resources/cell%20homeostasis%20virtual%20lab%20-%20activity/index.html Use the Skill Check Worksheet to collect the data as requested during the virtual exercise. Then complete the data calculations and analysis as set up on the Skill Check Worksheet.

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