Gas exchange in animals and plantsEdexcel A-Level Biology B: Subtopic test

1. 1
   A locust is an insect that can fly long distances. Air enters its body through paired openings along the thorax and abdomen. From these openings a branching network of air-filled tubes carries air towards the tissues, and the finest tubes end in thin-walled, fluid-filled tips lying against the flight muscle cells. The locust has no oxygen-carrying pigment in its body fluid.

   (a)

   Which structure of the locust can be opened and closed to reduce water loss from the tracheal system?

   [1 mark]

   • ATrachea
   • BTracheole
   • CSpiracle
   • DOperculum

   (b)

   Which feature of tracheoles best explains how the flight muscle cells receive oxygen quickly?

   [1 mark]

   • AThey are thin-walled and end against the muscle cells, so the diffusion distance is very short
   • BThey contain haemoglobin that carries oxygen to the muscle
   • CThey have rings of chitin that keep the tubes open
   • DThey are lined with cilia that move air along the tube

   (c)

   Explain how the structure of the tracheal system allows the flight muscle cells to be supplied with oxygen rapidly.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Trout live in cold, fast-flowing streams, where the water contains far less oxygen than air does. A trout takes water in through its mouth and passes it over its gills before it leaves beneath a bony flap. Each gill has many filaments, and each filament bears many thin plate-like lamellae containing blood capillaries. Blood flows through each lamella in the opposite direction to the water. Blood leaving a lamella of a trout is about 80% saturated with oxygen, whereas if blood and water flowed in the same direction the blood could not exceed about 50% saturation.

   (a)

   How does blood flow through a gill lamella in relation to the flow of water?

   [1 mark]

   • AIn the same direction as the water
   • BIn the opposite direction to the water, which is counter-current flow
   • CAt right angles to the water
   • DBlood is stationary in the lamella while the water moves

   (b)

   Which is the function of the opening and closing movements of the mouth and the bony flap?

   [1 mark]

   • ATo absorb oxygen directly into the blood through the flap
   • BTo reduce the surface area of the gills when the fish is inactive
   • CTo make the water flow in the same direction as the blood
   • DTo maintain a continuous one-way flow of water over the gills, which keeps up the concentration gradient

   (c)

   Explain why the counter-current flow of blood and water in the lamellae allows the trout to take up more oxygen than parallel flow would.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Marram grass grows on exposed sand dunes, where it is windy, hot and dry. In dry weather its leaves roll up into a tube. Its stomata are found only on the inner surface of the leaf, where they sit in pits lined with hairs, and the outer surface of the leaf is covered by a thick waxy cuticle.

   (a)

   Explain how these features of marram grass reduce the loss of water.

   [3 marks]

   (b)

   The stomata of the grass must be open for part of each day. Explain why, and describe the pathway taken by carbon dioxide from the air to a mesophyll cell.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is studying gas exchange in different animals. She is given a freshly killed locust, a dissecting board, mounting pins, fine scissors, forceps, saline solution and a hand lens, with the task of exposing the tracheal system. She then examines a model of mammalian lung tissue so that she can compare the two gas exchange systems.

   (a)

   Describe how the student should dissect the locust to show its tracheal system, including how she should take account of safe and ethical working.

   [6 marks]

   (b)

   Compare how the structure of the mammalian lung and the insect tracheal system allow rapid gas exchange.

   [6 marks]

   Total for question 4: 12 marks

End of questions