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 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 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