Biology high markers (B1)

• Explain, using an example, why the digestive system is considered an 'organ system'. (4 marks)

  • The digestive system is considered an organ system because it consists of a group of organs...

  ...that work together to perform the common function...

  ...of digesting and absorbing our food (this mark is for describing the function of the system)

  For example the pancreas releases digestive enzymes which help break down the food molecules, while the small intestine absorbs the nutrients
• Explain how a sperm cell is adapted for its role. (3 marks)

  It has lots of mitochondria to provide the energy for movement

  It has a flagellum to allow it to swim to reach the egg

  It is streamlined to make swimming easier 
• Steps of stem cell treatment (5 marks)

  1. Extract embryonic stem cells from early embryos.
  2. Grow them in a laboratory.
  3. Stimulate them to differentiate into whichever type of specialised cell that we want.
  4. Give them to the patient to replace their faulty cells.
• Give two examples of exchange surfaces in humans and describe their function.

  Example 1 - Alveoli (or lungs)

  Example 1 - Alveoli allow oxygen and carbon dioxide to diffuse in and out of the body/bloodstream 

  Example 2 - Villi (or small intestines)

  Example 2 - Villi allow nutrients such as glucose, amino acids, mineral ions etc to be absorbed from the small intestines into the bloodstream
• State and explain three features that most exchange surfaces have in common. (6 marks)

  • Feature 1 - Large surface area

  Explanation 1 - Lots of molecules can diffuse across at the same time

  Feature 2 - Surfaces are very thin

  Explanation 2 - Substances only have to diffuse a short distance

  Feature 3 - Surfaces are permeable 

  Explanation 3 - Substances are able to pass through the surface

  Feature 4 - Good blood supply (good supply of 'internal medium')

  Explanation 4 - Maintains a strong concentration gradient as blood is constantly replaced

  Feature 5 - Good supply of 'external medium'

  Explanation 5 - Maintains a strong concentration gradient as the air or food etc is constantly replaced
• State and explain three features of alveoli that make them good exchange surfaces (6 marks)

  Feature 1 - There are lots of alveoli, giving a large total surface area

  Explanation 1 - Lots of molecules can diffuse across at the same time

  Feature 2 - Surfaces are very thin (only 1 alveolar cell and 1 capillary cell thick)

  Explanation 2 - Substances only have to diffuse a short distance

  Feature 3 - Surfaces are permeable

  Explanation 3 - Oxygen and carbon dioxide are able to diffuse across

  Feature 4 - Good blood supply (good supply of 'internal medium')

  Explanation 4 - Maintains a strong concentration gradient as blood is constantly replaced

  Feature 5 - Good supply of air ( good supply of 'external medium')

  Explanation 5 - Maintains a strong concentration gradient as the air in the alveoli is constantly replaced with new oxygen rich air from the atmosphere
• State and explain three features of villi that make them good exchange surfaces. (6 marks)

  Feature 1 - There are lots of villi, and each villi has microvilli, giving a large total surface area

  Explanation 1 - Lots of molecules can diffuse across at the same time

  Feature 2 - Surfaces are very thin (only 1 villi cell and 1 capillary cell thick)

  Explanation 2 - Substances only have to diffuse a short distance

  Feature 3 - Surfaces are permeable 

  Explanation 3 - Glucose, amino acids, vitamins etc are able to pass through the villi cells