Diffusion
Chemical substances must be able to move from one place to another in order to keep living
organisms alive and growing.
Imagine a multicellular organism like yourself. Food substances that you have absorbed
must:
- move from one cell to another;
- move in and out of the cell;
- move from one part of the cell to another.
Diffusion is an important process where substances are moved without use of energy. It is the movement of particles (or molecules; or ions) from a region
where they are in a higher concentration to a region of lower concentration. Thus the
movement is down a concentration gradient.
It is important to bear in mind that:
- the movement is random
- the steeper the concentration gradient (ie. the bigger the difference between the
higher
concentration and lower concentration), the faster will be the movement.
Too see the process visually, clip on the site below. See only the first 2 slides of the
tutorial.
http://edtech.clas.pdx.edu/osmosis_tutorial/default.html
Examples of diffusion are (you will discover more on these topics later):
- movement of carbon dioxide during photosynthesis
- transpiration
- movement of oxygen and carbon dioxide in animals
Osmosis
What happens when the membrane separating the solutions has pores so small that only the
very tiny particles can pass through?
Using the same solutions, but this time with a partially permeable membrane, only the
water particles can pass through but not the sugar. Water molecules continue to diffuse
from the left side (where they are in higher concentration) to the right side. Sugar
molecules cannot pass through the membrane and so cannot diffuse from the left side to the
right side.
This is a special type of diffusion called osmosis. Osmosis is the movement of water molecules from a region of their
higher concentration to a region of their lower concentration, through a partially
permeable membrane.
You must also understand the term water potential. The solution with a higher
concentration of water particles (ie. the dilute sugar solution) has a higher water
potential than the solution with lower concentration of water particles (concentrated
sugar solution).
Uptake of water in animal and plant cells
Water potential and osmosis are very important to the uptake (or loss) of water in plant
and animal cells.
Please find out:
1. The changes that take place when a plant cell is placed in a
solution with higher water potential
(ie a dilute sugar solution).
2. Why is this (question 1) so important to plants?
3. What happens to animal cells when they are placed into such a solution
(higher water potential)?
Check out the following site: http://www.mun.ca/biology/Osmosis_Diffusion/tutor2.html#Diffusion
Active transport
If we examine the root hair cells of plants, we will realise that
these cells are able to absorb mineral salts from the soil even though they (the cells)
have a much higher concentration of these mineral salts. The mineral salts actually moved
from a region of their low concentration to a region of high concentration.
Active transport is the process where energy is consumed to transport
substances against a concentration gradient (ie. from a region of their low concentration
to a region of high concentration).
Examples of active transport:
- uptake of ions (or mineral salts) by root hair cells.
- absorption of digested food by cells in the small intestine.
Define
diffusion as the movement of molecules from a region of their higher concentration to a
