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2130-Module Three Summary

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Physiology 3120
Anita Woods

Weekly Thoughts: MODULE 3 – The Human Cell Basic Cell Organelles For the basic cell organelles, make yourself a list of all the organelles and state their function. e.g.: Mitochondria (the powerhouse) - produces ATP … etc. I will NOT ask you an exam question based on cell organelles (with the exception of the cell membrane). But, you should have a good understanding of what each does so, when you see their names throughout the course, you will know what they do. The Cell Membrane For our purposes in this module, the most important element of the cell, is the cell membrane. This is the part of the cell that you will want to know about! An important thing to keep in mind is that the cell membrane is selectively permeable – some items can cross very easily, while others cross with difficulty and some items can't cross at all. The lipid or fatty acid region of the cell membrane is what creates the "barrier" to water- soluble molecules such as ions (Na , K , Cl and water). It is this selective permeability of the membrane that helps to establish the chemical concentration differences between the inside and outside of the cell for all the ions. Proteins are also found in the cell membrane and perform a variety of functions. The most important function for our current purpose is to act as membrane transport mechanisms. You should be familiar with diffusion, diffusion through the lipid bilayer, diffusion through a protein lined pore, facilitated diffusion/transport and active transport. Please note the small mistake in the animation on page 3.5. The phospholipid tails are HYRDROPHOBIC!! Not hydrophilic as it shows when you click on the number 2. Membrane Transport Mechanisms I would recommend that you make a list of the all transport mechanisms and list their similarities and differences!! For example: all the transport mechanisms rely upon the concentration gradient as the driving force to move the molecule across the membrane EXCEPT active transport. Diffusion down the concentration gradient as like a ride on a toboggan down a hill (this doesn't take any energy on your part) but pulling the toboggan back up the hill does require you to expend energy - much like active transport. Osmosis Osmosis is simply the movement of water down its concentration gradient - from high water concentration to low water concentration - just like diffusion. The important point to take note of is - what determines the water concentration? If you have a very concentrated solution (high solute concentration) then that solution has a low water concentration and visa versa. Be sure you know the difference between the terms solute, solvent and solution. The most difficult part of osmosis is determining the concentration (osmolality) of a solution. How do you know when one solution is more concentrated than another? - It all depends upon the number of solute particles in solution! The normal concentration of body fluids is 300 mOsmoles (that's 300 milli - osmoles). The Osmol is the unit used to describe the number of particles in solution that cause osmosis (we call these osmotically active particles). Units of concentration: Osmolality - this is the number of Osmoles (number of particles that will cause osmosis) per kg Water Osmolarity is the number of Osmoles per liter of solution. These units are slightly different but we will consider them to be the same for this course. However, I will generally use the term Osmolality. Eg. What is the osmolality of a 1 molar solution of NaCl? If you were to take this 1 mole of NaCl and put it into 1 Kg (1 litre) of water, what would happen? What would be the concentration/osmolality of the resulting solution? + The answer -o th+ first -uestion is - the NaCl would dissociate into 1 mole of Na and 1 mole of Cl. Na and Cl are now ions due to their charges (+, -). Each of these ions can cause osmosis by themselves therefore they are considered osmotically active particles. Well, according to our definition above, we have 2 osmotically active particles in solution. Therefore, we have 2 osmoles in one Kg of water. Therefore, the concentration/osmolality of this solution is 2 osmoles! Try the following (assume that each is in 1 kg of water): 1. What is the osmolality of 1.5 molar solution of NaCl? 2. What is the osmolality of 1 molar solution of CaCl 2 3. What is the osmolality of 1 molar solution of glucose (C 6 O12 6 (the answers are at the bottom of this review) Tonicity Tonicity is the ability of a solution to cause osmosis across a biological cell membrane. In order to know whether you're going to have osmosis or not you have to know the concentration of the fluid inside a typical biological/human cell. The concentration inside a typical cell is roughly 300 mOsm/kg water (mOsm = milli-osmoles or 1/1000 of an osmole). There are 3 types of solutions in the animation: isotonic (solution with the same concentration as the cell = no osmosis will occur), hypotonic (solution with a lower conc. = osmosis will occur into the cell) or hypertonic (higher concentration = osmosis will occur out of the cell) Membrane Potentials You've seen how things move across the cell membrane - by simple diffusion, diffusion through protein-lined pores, facilitated diffusion and active transport. You must + - remember the concentration gradients for each ion as shown on page 3.23 (Na , Cl, and Ca ++ in high concentration outside the cell and K in high concentration inside the cell - You will see this again and again and again…). Given the opportunity, these ions will move down their concentration gradients, that is, from high concentration to low concentration. What keeps these ions from moving down their concentration gradien
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