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 with their environment
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:
Diffusion | Osmosis | Active Transport | |
|---|---|---|---|
Description | Movement of substances in a fluid down a concentration gradient. | Diffusion of water between two solutions separated by a partially permeable membrane. Water moves from a dilute to concentrated solution. | Movement of substances across a membrane 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
Isotonic sports drinks contain similar concentrations of salt and glucose to human blood plasma; this is useful for:
replacing water lost in sweat to maintain hydration; because the drink has a similar concentration to the blood, it does not draw water out of the blood into the gut by osmosis (as a much more concentrated drink would)
absorption of glucose for respiration in muscle cells
absorption of salts in order to replace salts lost during sweating
Some sports drinks contain a higher concentrations of glucose and salt than the blood
This provides a steep concentration gradient for absorption in the small intestine, supplying glucose and replacing salts lost in sweating during intense physical exercise, e.g. during a marathon
Worked Example
Marathon runners often drink sports drinks during a race.
Explain why.
Answer
Note that the question does not specify the type of sports drink consumed, so the answers can be general rather than referring specifically to concentration gradients
Sports drinks ensure that enough water is absorbed, so the runner can replace water lost during sweating
Sports drinks contain sugar, supplying glucose for respiration
Sports drinks contain salt, replacing ions / salts lost during sweating
Examiner Tips and Tricks
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.
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