Transport Summary & Applications (AQA GCSE Combined Science: Trilogy: Biology): Revision Note
Exam code: 8464
Comparing transport processes
In order to survive, cells must exchange substances between themselves and their environment
In summary, the transport of substances occurs into and out of cells by diffusion, osmosis and active transport
The table below describes some of the differences between the three processes:
Transport summary and applications
Diffusion | Osmosis | Active Transport | |
|---|---|---|---|
Summary | Movement of substances in a fluid down a concentration gradient. | Special type of diffusion of water; occurs between two solutions separated by a partially permeable membrane down a water potential gradient (or from a dilute to concentrated solution). | Movement of substances actively across a membrane (usually against a concentration gradient). |
Energy from respiration required | No | No | Yes |
Examples of substances moved | Oxygen, carbon dioxide, glucose, amino acids, urea | Water | Glucose, mineral ions (e.g. sodium, potassium, magnesium, nitrates) |
Important places it occurs | In the lungs and leaves (gas exchange), in the digestive system between the lumen of the small intestine and intestinal cells. | Between all cells and their immediate environment (but particularly between cells and the bloodstream). | Between the lumen of the small intestine and intestinal cells; between roots and the soil. |
Application: Sports drinks
During sport and exercise, the body’s temperature increases and this leads to the loss of water and salts through sweat (and water vapour through the lungs from increased breathing rate)
Physical activity also requires greater muscle activity, so the rate of aerobic respiration in muscle cells increases during exercise
As a result, more glucose is used up in muscle cells and must be replaced
Athletes opt to drink sports drinks over bottled water after competing
These drinks are designed to help replace water, salts (ions/electrolytes) and glucose lost from the body during exercise
It is very important that the balance between ions and water is correct between the cells of the body and the bloodstream
If there is an imbalance then there could be negative effects on the body’s cells leading to them working inefficiently:
If there is too little water in the blood or the blood ion concentration is too high: cells will lose too much water by osmosis and shrink/dehydrate
If the concentration of water in the blood is too high or the ion concentration is too low, cells will absorb too much water, swelling and possibly bursting
Isotonic sports drinks contain similar concentrations of salt and sugar (glucose) as the human body and are primarily used for hydration and fluid replacement
Hypertonic sports drinks contain concentrations of salt and sugar higher than typical blood levels
This provides high salt and sugar levels for absorption from the small intestine - these drinks are suitable for supplying glucose in particular during intense physical exercise (eg. during a marathon)
Hypotonic sports drinks contain concentrations of salt and sugar lower than typical blood levels
Drinking this type of drink creates a concentration gradient between the bloodstream and small intestine with the water potential in the small intestine being higher - these drinks are suitable for rapid rehydration as water is drawn into the bloodstream by osmosis
Examiner Tips and Tricks
Glucose is broken down in aerobic respiration to release cellular energy which can be used for muscle contraction. Never, ever say that glucose allows energy to be produced or that it provides energy for respiration. Energy cannot be created or destroyed.
Expect to see tables of sports drinks with different concentrations of nutrients and ions in them in the exam. You may be asked to decide which drink is the best or worst option for an athlete to consume during recovery.
Also, be conscious that sports drinks are often high in sugar (for obvious reasons!) but that for individuals with diabetes or other health problems they can be unhealthy and make them ill.
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