GSCE  Biology  Week  3.  Unit  B2  (B2  2.3,  2.20  – 2.21)

B2  2.3  DiffusionB2  2.20  OsmosisB2  2.21  Investigate  osmosis

Molecules  in  liquids  and  gases  are  in  constant  random  motion.

Molecules  spread  from  areas  of  high  

concentration  to  low  concentration.  Hence  

there  is  a  net movement  of  molecules.  This  is  called  diffusion.

What  is  Diffusion?

The  rate  of  diffusion  depends  on  several  factors…

1. The  concentration  gradient.  The  higher  it  is  the  faster  the  rate  of  diffusion 2.  The  thickness  of  the  exchange  

surface  – the  thinner  the  exchange  surface  (I.e.  the  shorter  the  distance  the  particles  have  to  travel),  the  faster  the  rate  of  Diffusion.

3.  The  surface  area  – the  larger  the  surface  area  (e.g.  of  the  cell  membrane),  the  faster  the  rate  of  diffusion.

The  greater  the  difference  between  the  regions  of  high  concentration  and  low  concentration,  the  faster  the  substances  rate  of  diffusion.

Substances  move  into  and  out  of  cells  by  diffusion.

A  good  example  of  this  gas  exchange.  The  process  by  which  oxygen  enters  the  lungs  and  carbon  dioxide  leaves.  This  exchange  takes  place  across  the  cell  membranes  of  the  alveoli  (air  sacs)  in  the  lungs.

Gas  exchange  Gases  crosses Respiratory  surfaces  byDIFFUSION

The  surfaces  are:  -­‐

Large surface area (efficient diffusion)

Thin (faster diffusion rate – short diff path)

Moist (product of respiration) Gas  exchange  in  an  alveolus

Structure  and  function  of  the  alveoli

The  greater  the  difference  in  these  regions  the  greater  the  

concentration  gradient,  resulting  in  an  increased  rate  of  

diffusion.

Concentration  gradient.This  is  the  difference  in  concentration  between  the  substance  in  the  high  and  low  regions

Active  Transport – movement  across  a  cell  membrane  against  its  concentration  gradient  – thus,  

energy  is  needed.  

What is osmosis?Osmosis is the movement of water (H2O) from a region of high waterconcentration to low water concentration across a semi permeablemembrane. A semi permeable membrane only allows H20 across (not thesolute – i.e. salts and sugars that are dissolved in water (H20 is the solvent).

Remember!!A solution that is highlyconcentrated, e.g. has a high sugarcontent, will have a low waterconcentration!

Example:  Root  hair  cells.Substances  in  the  soil  are  taken  up  by  plant  root  hair  cells.The  diagram  shows  the  direction  of  movement  of  two  substances  A  and  B  across  the  cell  membrane  of  a  root  hair  cell.

(i) Name  process  X   (ii)  Name  process  Y.