IGCSE Biology Paper-4: Specimen Questions with Answers 199 - 200 of 279

Question 199

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Shows a computer model of the enzyme alcohol dehydrogenase, which is the enzyme responsible for breaking down alcohol.

Enzyme Alcohol Dehydrogenase

Enzymes have a specific three-dimensional shape.

Explain why the shape of an enzyme is important.


Proteins consist of combinations of amino acids. Body proteins include structural proteins, enzymes, hormones, and antibodies. The shape of an enzyme allows it to speed up a biological reaction. Enzymes act on substrates by attaching to them. The part of an enzyme that one or more substrates molecules can attach to is called the active site. Enzymes can also catalyze the synthesis (joining) of two substrates to form one product. Each different type of enzyme will usually catalyze one biological reaction. Enzymes are specific because different enzymes have different shaped active sites.

The shape of an enzyme՚s active site is complementary to the shape of its specific substrate or substrates. This means they can fit together.

Enzyme Type 1,2 and 3


Soon after starting physical activity the concentration of carbon dioxide in the blood increases.

Question 200 (1 of 3 Based on Passage)


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Explain how this effect on breathing is coordinated.


The respiratory rhythm is regulated by the neural or nervous system which consists of various respiratory centers present in the brain. The respiratory center is composed of group of neurons located in the medulla oblongata and pons. The respiratory system regulates the rate and depth of the breathing.

There are two centers present in the brain for regulation of respiration:

  • Respiratory rhythm center: It is a specialized center located in the medulla region of hind brain. It can either cause expiration or inspiration according to the neurons which are activated and thereby, regulate the respiratory rhythm according to the need of the body.
  • Pneumotaxic center: This center is present in the pons region of hind brain. It moderates the function of respiratory rhythm center. The neural signal from this center can reduce the duration of inspiration and thereby alter the respiratory rate.
  • A chemosensitive area, i.e.. , sensitive for chemicals is located adjacent to the rhythm center which contains chemoreceptors which are sensitive for and ions. The respiratory center is stimulated by concentration or partial pressure of and in blood and body fluids. Chemosensitive area gets activated due to an increase in and ions concentration which in turn activates the respiratory rhythm center for altering the rate of respiration. For example, increase of in blood increases the rate and depth of respiration and decrease in depresses rate and depth of respiration.

and ions sensitive receptors (chemoreceptors) are:

  • Aortic bodies: These are located on the aortic arch.
  • Carotid bodies: These are present in carotid arteries which supply blood to brain. Increased concentration of in blood lowers its and low concentration of raises the of blood. The low of blood increases the rate of respiration.

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