3.1.3 Substances are exchanged between organisms and their environment by passive or active transport across exchange surfaces. The structure of plasma membranes enables control of the passage of substances across exchange surfaces.
Cholera

• The cholera bacterium as an example of a prokaryotic organism.
• The structure of prokaryotic cells to include cell wall, cell-surface membrane, capsule, circular DNA, flagella and plasmid.
• Cholera bacteria produce toxins which increase secretion of chloride ions into the lumen of the intestine. This results in severe diarrhoea.
• The use of oral rehydration solutions (ORS) in the treatment of diarrhoeal diseases.
• Candidates should be able to discuss the applications and implications of science in developing improved oral rehydration solutions and ethical issues associated with trialling improved oral rehydration solutions on humans.

Cholera is a serious water-borne disease that you will need to learn for GCE AQA Biology. It is caused by a prokaryotic organism, the cholera bacterium. 

Prokaryotic organisms: cholera bacterium
The cholera bacterium (Vibrio cholerae) is a classic example of a prokaryote 🦠. Prokaryotic cells have the following organelles: 

• Cell wall: prevents bursting due to excess entry of water by osmosis 
• Cell-surface membrane: control the entry and exit of substances
• Capsule: a slimy layer that protects the cell and helps with cell to cell adhesion 
• DNA: prokaryotic DNA is circular, and holds the genetic information of the cell 
• Flagella: allows the cell to move around independently  
• Plasmid: a circular loop of DNA, may contain genes that help the bacteria to survive (e.g. antibacterial resistance, may be transferred from cell to cell
• Cytoplasm: a jelly-like substance that holds the organelles 
• Ribosomes: 80S in size, assemble amino acids to form a polypeptide during protein synthesis 

Why is the cholera bacterium harmful? 🧫

1. The bacterium produces a toxin 
2. The toxin causes the release of chloride ions into the lumen of the intestine 
3. As the concentration of ions increases, the water potential decreases
4. This causes water to enter the lumen via osmosis 
5. Excess amounts of water moved into lumen cause severe diarrhoea 

Treating cholera
Cholera, like other diarrhoeal diseases, is treated using oral rehydration solutions (ORS) 

• The main ingredients (often packaged as a powder) contain: 
  • Water - for rehydration 
  • Sodium ions - to make up for the lost sodium
  • Sodium is also important because the body cannot absorb water on its own via normal osmosis pathways (because the cholera toxin causes water loss)
  • The sodium added is thus to use alternative carrier proteins not affected by cholera 
  • This decreases the water potential of the epithelial cells, allowing for water uptake to occur via osmosis again 
  • Glucose - stimulate sodium ion uptake, provide energy 
  • Potassium - to make up for the lost potassium, encourage appetite 
  • Other electrolytes - prevent electrolyte imbalance  
• The ingredients can be mixed with water and swallowed 

Ethical issues associated with developing ORS 
History of the development of ORS 

• Earliest ORS had too much sodium, leading to side effects 
• Newer solutions with more glucose and less sodium caused even more dehydration, as the increased glucose caused water to enter the intestinal lumen via osmosis (it lowered water potential) 
• The next generation with less glucose reduced how much energy the patient could get 
• Finally, starch replaced some glucose 

Implications 

• Improved medicines require testing in stages so that scientists can continually improve on past medicines 🥼
• Trialling different forms of the same medicine means that some patients may be subjected to a version that is more inferior, or may even make the condition even worse (e.g. with the ORS development version 2) - this is why patient consent in trials is so important 🧪

Wanna Boost Up Your GCE AQA Biology? Check out our study tips here! 

Written by Justine | Biology Specialist @ Tuttee

References
Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.
Images from 

1. https://thealevelbiologist.co.uk/pathogens/
2. https://www.scientificamerican.com/article/satellites-predict-a-cholera-outbreak-weeks-in-advance/