Gas exchange in the fish - Exam questions 
Q1.	There are many different species of annelid worm. Some are very small, only a few millimetres in length. Others, such as lugworms, are much larger. The drawing shows a lugworm and part of one of its gills.

 (a)	Smaller species of annelid do not have gills. Explain why these small worms do not need gills to obtain sufficient oxygen.
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 (b)	In many of the lugworm gills, the blood flows in the opposite direction to the current of water passing over them. Explain the advantage of this arrangement.
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(c)	Explain two ways, other than that described in part (b), in which the structure of a lugworm gill is adapted for efficient gas exchange.
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 (d)	Explain why water is always lost from the gas exchange surfaces of terrestrial organisms.
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Q2.	The electron micrograph shows a section through a fish gill. The directions of flow of water and of blood are indicated by arrows.

Source: www.ucdavis.edu/mjguinan
 (a)	Calculate the minimum distance that a molecule of oxygen would have to travel from the water to a red blood cell. Give your answer in micrometres and show your working.

Answer .......................................... µm.
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(b)	Explain how the relationship between the direction of flow of water and of blood shown in the micrograph is useful to a fish.
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Q3.	(a)	A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange.
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	Mackerel live in the surface waters of the sea. Toadfish live on the seabed in deep water.
(b)	The concentration of oxygen is higher in the surface waters than it is in water close to the seabed. Suggest why.
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Answers – Gas Exchange in Fish 

 A1.	(a)	Large surface area: volume ratio;
meet requirements by diffusion (over body surface);	2
(b)	Maintains a concentration gradient;
(allowing diffusion of oxygen) across width of gill;	2
 
(c)	Large surface area for diffusion / O2 uptake / gaseous exchange;
short diffusion path;
highly vascularised / many blood capillaries for O2 transport;	max 2
 
(d)	Gas exchange surfaces are permeable (to small molecules);
higher concentration of water molecules inside animal than out /
y gradient;
water will diffuse outwards / evaporation;	max 2
 
A2.	(a)	;
= 1.25 to 1.5;
allow 1 mark if correct working shown	max 2
 
(b)	Maintains concentration gradient (over whole length of gill) / diffusion can
occur over whole gill;
More oxygen enters blood (/ more CO2 leaves);
More (aerobic) respiration / more energy release in muscle / for swimming;
‘more’ needed ONCE only	3
[5]
A3.	(a)	1	Large surface area provided by lamellae/filaments;
Q Candidates are required to refer to lamellae or filaments. Do not penalise for confusion between two
2	Increases diffusion/makes diffusion efficient;
3	Thin epithelium/distance between water and blood;
4	Water and blood flow in opposite directions/countercurrent;
5	(Point 4) maintains concentration gradient (along gill)/equilibrium
not reached;
5 Not enough to say gives steep concentration gradient
6	As water always next to blood with lower concentration of oxygen;
7	Circulation replaces blood saturated with oxygen;
8	Ventilation replaces water (as oxygen removed);	6 max
6-8 Accept answers relating to carbon dioxide
 
(b)	Mixing of air and water (at surface);
	Air has higher concentration of oxygen than water;
	Diffusion into water;
	Plants/seaweeds near surface/in light;
	Produce oxygen by photosynthesis;	2 max