Revision notes for Biology AS Unit 1 – Prokaryote and Eukaryote cells and Cholera

Prokaryotic cells

Eukaryotic cells

No true nucleus, only a diffuse area of nuclear material with no nuclear envelope

Distinct nucleus, with a nuclear envelope

No nucleolus

Nucleolus present

Circular strands of DNA but no chromosomes

Chromosomes present, in which DNA is located

No membrane-bounded organelles

Membrane-bounded organelles, such as mitochondria, are present

No chloroplasts, only photosynthetic regions in some bacteria

Chloroplasts present in plants and algae

Ribosomes are smaller

Ribosomes are larger

No endoplasmic reticulum or associated Golgi apparatus and lysosomes

Endoplasmic reticulum present along with Golgi apparatus and lysosomes

Cell wall made of peptidoglycan

Where present, cell wall is made mostly of cellulose (or chitin or fungi)


The sequence of events that enable the cholera bacterium to bring about the symptoms of cholera  

Main symptoms of cholera: diarrhoea and consequently dehydration.

Vibrio cholera is transmitted by the ingestion (intake) of water or food that has been contaminated with faeces containing this pathogen. Once ingested, it causes symptoms in the following ways:

  • Almost all the Vibrio cholera ingested are killed by the acidic conditions in the human stomach however, some may survive especially if the pH is above 4.5
  • When the surviving bacteria reach the small intestine they use their flagella to propel themselves through the mucus lining of the intestinal wall
  • They start to product a toxic protein. This protein has 2 parts. One part binds to specific carbohydrate receptors on the cell-surface membrane. (only epithelial cells in the small intestine have these specific receptors which is why the cholera toxin only affects this part of the body). The other, toxic, part enters the epithelial cells
  • This causes the ion channels of the cell-surface membrane to open, so that the chloride ions within the epithelial cells flood into the lumen of the intestine
  • The loss of chloride ions from the epithelial cells raises their water potential, while the increase of chloride ions in the lumen of the intestine lowers its water potential. So water flows from the cells into the lumen
  • The loss of ions from the epithelial cells establishes a concentration gradient and so the ions diffuse into the epithelial cells from the surrounding tissues, including the blood
  • This in turn establishes a water potential gradient that causes water to move by osmosis from the blood and other tissues into the intestine
  • It is the loss of water from the blood and other tissues into the intestine that causes the symptoms, diarrhoea and severe dehydration


How are people with diarrhoea treated?

Cholera is treated by restoring the water and ions that have been lost using oral rehydration therapy

It is vital to rehydrate the patient. There is more than one type of carrier protein in the cell-surface membranes of the epithelial cells that absorb sodium ions. You have to develop a rehydration solution that uses these alternative pathways. As sodium ions are absorbed, the water potential of the cells decrease and water enters the cells by osmosis. A rehydration solution needs to contain:

  • Water to rehydrate the tissues
  • Sodium to replace the sodium ions lost from the epithelium of the intestine and to make optimum use of the alternative sodium-glucose carrier proteins
  • Glucose to stimulate the uptake of sodium ions and to stimulate appetite
  • Potassium to replace lost potassium ions and to stimulate appetite
  • Other electrolytes such as chloride and citrate ions to help prevent electrolyte imbalance



The development and improvement of the oral re-hydration therapy

  • Early rehydration solutions led to side effects, caused by excess sodium ions
  • Mixtures with lower sodium content but more glucose were tested but the additional glucose lowered the water potential in the lumen of small intestine so much that it started to draw more water from epithelial cells which made dehydration worse
  • Lowering glucose content reduced this effect but it reduced the energy being supplied to the patient (glucose acted as a respiratory substrate). How do you supply glucose without it having an osmotic effect?
  • Use starch instead of glucose because starch is a large and insoluble molecule so it has no osmotic effect. It is however easily broken down by amylase and maltase in the small intestine
  • By experimenting with different starch concentrations, a rehydration solution was developed that released glucose at optimum rate so it was taken up as it was produced without affecting water potential.



                Testing new drugs and oral re-hydration solutions

Initial testing can be done on animals but ultimately, drugs must be tested on humans for its safety.

  1. Small group of healthy people given small amount of drug to test for side-effects rather than to see if the drug is effective. Dose may be increased gradually over the 6 months in a series of such trials
  2. Drug then given to larger group who have the condition the drug is designed to treat. This is to check that the drug works and to look at any safety issues. This stage usually takes 2 years
  3. Large scale trial of thousands of patients is given a dummy drug called placebo. Often the none of the scientists and patients know who has taken the real drug and who has taken the placebo until after the trial and this trial takes many years (double-blind trial)
  4. If drug passes these stages it may be granted a license, but its use and effects are still monitored over many years to check for long-term effects

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