Enzymes Flashcards

All 42 cards in this deck

What is the active site of an enzyme?

The region of the enzyme where the substrate binds; its shape is complementary to the substrate.

What is an enzyme–substrate complex?

The structure formed when a substrate binds into the active site of an enzyme.

What are the steps of the lock and key model of enzyme action?
e.g. amylase acting on starch

1. Substrate binds to the active site: starch enters amylase's active site.
2. An enzyme–substrate complex forms and the reaction takes place: starch is broken down.
3. Products leave the active site and the enzyme is unchanged: sugars are released.

What is meant by enzyme specificity?

Each enzyme catalyses only one reaction, because only a substrate with a complementary shape fits its active site.

True or false? Any substrate can fit into any enzyme's active site.

False. The substrate must have a shape complementary to the active site, so enzymes are specific.

In the reaction in which catalase breaks down hydrogen peroxide, which molecule is the substrate and what are the products?

Hydrogen peroxide is the substrate; water and oxygen are the products.

What is the optimum temperature of an enzyme?

The temperature at which the enzyme works fastest (has its highest rate of reaction).

What effect does increasing the temperature up to the optimum have on the rate of an enzyme-controlled reaction?

The rate increases, because molecules have more kinetic energy so there are more collisions between enzyme and substrate.

What is denaturation of an enzyme?

A permanent change in the shape of the enzyme's active site, so the substrate no longer fits and no enzyme–substrate complexes form.

What effect does increasing substrate concentration have on the rate of an enzyme-controlled reaction?

The rate increases at first, then levels off when all the active sites are occupied (the enzyme is saturated).

True or false? Enzymes are killed by high temperatures.

False. Enzymes are not alive — they are denatured, as the shape of the active site changes.

What are the steps to investigate the effect of pH on amylase activity?
e.g. testing pH 4, pH 7 and pH 10

1. Mix starch solution, amylase and a buffer of set pH in a tube in a water bath.
2. Every 10 s, take a sample and add it to iodine on a spotting tile.
3. Record the time taken for the iodine to stay orange-brown; repeat at each pH (4, 7, 10).

What colour is iodine solution when starch is present, and when it is absent?

Blue-black when starch is present; orange-brown when no starch is present.

In an investigation of the effect of pH on amylase activity, what are the independent and dependent variables?

Independent variable: pH. Dependent variable: time taken for the starch to be broken down (or rate of reaction).

Name three variables that must be controlled in an investigation of the effect of pH on amylase activity.

Temperature, volume and concentration of starch solution, and volume and concentration of amylase.

Why are buffer solutions used in the enzyme and pH practical?

To keep the pH of each mixture at a fixed, known value throughout the reaction.

True or false? In the amylase and iodine practical, the end point is when the iodine stays orange-brown.

True. Orange-brown iodine shows all the starch has been broken down.

What is the formula for the rate of reaction when you know only the time taken for the reaction to finish?

Rate $= 1 \div$ time, with units s$^{-1}$.

What are the steps to calculate the rate of an enzyme reaction from the time taken for it to finish?
e.g. the reaction is complete after 50 s

1. Take the time in seconds: 50 s.
2. Divide 1 by the time: $1 \div 50$.
3. Rate $= 0.020$ s$^{-1}$.

What are the steps to calculate the rate of an enzyme reaction from the amount of product made?
e.g. 20 cm$^3$ of oxygen collected in 40 s

1. Note the amount of product: 20 cm$^3$.
2. Note the time taken: 40 s.
3. Divide amount by time: $20 \div 40 = 0.5$ cm$^3$/s.

How do you convert a rate in cm$^3$/s into cm$^3$/min?
e.g. a rate of 0.5 cm$^3$/s

Multiply by 60: $0.5 \times 60 = 30$ cm$^3$/min.

How do you find the rate of reaction from a straight-line graph of product formed against time?
e.g. a line through (0 s, 0 cm$^3$) and (20 s, 10 cm$^3$)

1. Choose two points on the line.
2. Rate = change in y $\div$ change in x: $10 \div 20$.
3. Rate $= 0.5$ cm$^3$/s.

On a graph of product formed against time, what does a flat (level) part of the line show?

The reaction has stopped — no more product is being made, e.g. because all the substrate has been used up.

What is a biological catalyst?

A substance (an enzyme) that speeds up the rate of a reaction in a living organism without being changed or used up.

What are carbohydrates such as starch broken down into by enzymes?

Simple sugars, e.g. glucose.

What are proteins broken down into by enzymes?

Amino acids.

What are lipids (fats) broken down into by enzymes?

Fatty acids and glycerol.

What role do enzymes play in synthesis reactions?

They catalyse the joining of small molecules into large ones, e.g. glucose molecules joined to make starch or glycogen.

True or false? Enzymes are used up in the reactions they catalyse.

False. An enzyme is unchanged by the reaction and can be used again.

What is the food test for starch and its positive result?

Add iodine solution; it turns from orange-brown to blue-black.

What is the food test for reducing sugars and its positive result?

Add Benedict's solution and heat in a water bath; it turns from blue to brick-red (through green, yellow, orange).

What is the food test for protein and its positive result?

Add Biuret solution; it turns from blue to purple/lilac.

What is the emulsion test for fats and its positive result?

Shake the sample with ethanol, then add water; a cloudy white emulsion forms.

A food sample turns iodine blue-black and Biuret purple. Which food molecules does it contain?

Starch and protein.

True or false? A Benedict's test that stays blue after heating shows no reducing sugar is present.

True. A colour change to green, yellow, orange or brick-red is needed for a positive result.

What is calorimetry used to measure in food?

The energy content of the food, found from the temperature rise of water heated by burning the food.

What are the steps to measure the energy content of a food sample by calorimetry?
e.g. burning a 1 g crisp under a tube of water

1. Measure the mass of food and the mass of water, and the starting temperature.
2. Burn the food under the water until it stops burning; stir and record the highest temperature.
3. Calculate the energy transferred, then divide by the mass of food to get energy per gram.

What is the formula for the energy transferred to the water in a calorimetry experiment?

Energy (J) = mass of water (g) $\times$ temperature rise (°C) $\times$ 4.2

How do you calculate the energy content of a food per gram?
e.g. 4200 J transferred by a 2 g sample

Divide the energy transferred by the mass of food burned: $4200 \div 2 = 2100$ J/g.

True or false? Heat lost to the surroundings makes the measured energy content of a food lower than its true value.

True. Not all the energy released reaches the water, so the calculated value is an underestimate.

What is the optimum pH of an enzyme?

The pH at which the enzyme works fastest (highest rate of reaction).

True or false? Every enzyme has an optimum pH of 7.

False. Different enzymes have different optimum pH values, e.g. stomach protease works best at about pH 2.