AS Workshop – SAQ Revision

Topic C – Enzymes

Learning outcomes

a) explain that enzymes are globular proteins that catalyse metabolic reactions

b) explain the mode of action of enzymes in terms of active site, enzyme/substrate complex, lowering of activation energy and

enzyme specificity

Example: Question 1

Describe how enzymes take part in chemical reactions. [4]

Enzymes are proteins which will speed up a chemical reaction. Enzymes

have active site which is very specific, thus it only binds with its specific

complementary substrate to form enzyme substrate complex, then

product.

Mould around substrate

:: Notes ::

3M

Example: Question 2

Enzymes are globular proteins that catalyse specific reactions.

Explain how enzymes catalyse specific reactions. [3]

Enzymes have specific active site which binds with its complementary

substrate, thus it provides an alternative pathway for for the substrates to

react with a lower activation energy. More substrates will be able to

obtain sufficient energy to overcome the activation energy, products are

formed in a faster rate.

Induced fit.

“Lock and Key” mechanism

Forms enzyme substrate complex

:: Notes ::

3M

1

Learning outcomes

c) follow the time course of an enzyme-catalysed reaction by measuring rates of formation of products or rates of disappearance

of substrate

d) investigate and explain effects of temperature, pH, enzyme concentration and substrate concentration on the rate of enzyme-

catalysed reactions

Example: Question 3

Enzymes catalyse reactions in which substrate molecules are converted to

products.

There are two main approaches to investigation of the activity of an

enzyme. State the two ways in which the activity of an enzyme can be

found. [2]

(i) appearance of product

(ii) disappearance of substrate

:: Notes ::

2M

Example: Question 4

A student investigated the activity of catalase by measuring the release of oxygen from hydrogen peroxide. The

reaction occurs as follows.

2 H2O2 2 H2O + O2

The student used a solution of catalase that was mixed with a 5% hydrogen peroxide solution and placed in the

apparatus shown in Fig. 2.1. The total volume of gas collected was recorded every 15 seconds. The results are

shown in Fig. 2.2.

Figure 2.1

2

Figure 2.2

a) Explain why the total volume of gas collected after 210 seconds

remains constant. [1]

All substrate, hydrogen peroxide has been reacted with with the

catalyse to formits product, water and oxygen gas.

:: Notes ::

1M

Catalase and hydrogen peroxide were kept separately at 10 °C for 30

minutes before they were added together. The reaction mixture was then

kept at 10 °C.

b) Sketch, on Fig. 2.2, the results you would expect when the

experiment was repeated at 10 °C. [1]

1M

c) Describe and explain the results you would expect if the catalase

solution was pretreated by being kept in a water bath at 70 °C before

being added to the hydrogen peroxide. [3]

The rate of reaction will be very slow or even nil. This is because

enzyme kept at high temperature, 70 °C will denature. Bonds

forming in the enzymes will be disrupted, causes the active site of the

enzyme to alter. Therefore, enzyme can no longer bind with its

complementary substrate, to form enzyme substrate complex, so no

or less product will be formed. Hydrogen/ Ionic bond

3M

3

d) Explain how you would use this apparatus to investigate the effect

of changing the substrate concentration on the activity of catalase. [4]

Temperature is fixed at 20°C. Repeat the experiment using 5 different

concentration of substrate such as 0.5 Mol, 1.0, 1.5, 2.0, 2.5. Record

volume of oxygen gas collected at every 50 seconds. Use fresh

solution of catalase each time. Repeat readings. Method of

calculation: volumein 15 seconds/concentration

3M

***

Topic F – Genetic Control

Learning outcomes

a) describe the structure of RNA and DNA and explain the importance of base pairing and hydrogen bonding

Example: Question 1

Fig. 1.1 shows the replication of one strand of a DNA double helix.

4

Figure 1.1

5

a) Name W to Y. [3]

W cytosine,

X pentose sugar (deoxyribose)

Y nucleotide

:: Notes ::

2M

Learning outcomes

b) explain how DNA replicates semi-conservatively during interphase

Example: Question 2

a) Explain how the structure of DNA enables it to replicate semi-

conservatively. [3]

It is double stranded, and coiled to each other by hydrogen bonds

formed between the complementary pairing of bases such as adenine

with thymine and guanine with cytosine. During semi-conservative

replication, two strands of DNA will uncoil by helicase, each act as a

template then, DNA polymerase will bring the free nucleotides to

bind together to form a new strand of DNA. Two new DNA produced

are identical to each other.

:: Notes ::

3M

b) Explain why it is important that an exact copy of DNA is made

during replication. [2]

So that there is no mutation. Genetic information can be maintained.

Number of chromosomes can be maintained in each generation.

2M

6

Learning outcomes

c) state that a gene is a sequence of nucleotides as part of a DNA molecule, which codes for a polypeptide

d) describe the way in which the nucleotide sequence codes for the amino acid sequence in a polypeptide

e) describe how the information on DNA is used to construct polypeptides, including the role of messenger RNA,

transfer RNA and the ribosomes

Example: Question 3

During an immune response, B-lymphocytes become plasma cells and begin to make

polypeptides that are assembled into antibodies.

Fig. 3.1 is a diagram showing the formation of a polypeptide at a ribosome in a plasma cell.

Figure 3.1

a) State the sequence of bases at J. [1]

______________________CAG_____________________________

:: Notes ::

1M

b) Use the information in Fig. 3.1 to describe the role of transfer

RNA molecules in translation. [5]

TRNA will bind with an amino acid to form amino-acyl tRNA with

aid of amino acyl tRNA synthase. Trna carries anticodon with is

complementary to the triplet codon in the Mrna. Anticodon of the trna

will bind with the codon. Each time 2 trna are available to ind with a

mrna. Hydrogen bond will form in between acino acids. Specificity

3M

***Topic G – Transport in plants

7

Learning outcomes

f) relate the structure of xylem vessel elements, sieve tube elements and companion cells to their functions

Example: Question 1

Describe the role of companion cells in translocation in the phloem. [2]

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

:: Notes ::

Learning outcomes

g) explain the movement of water between plant cells and between them and their environment, in terms of water potential (no calculations involving water potential will be set)

Example: Question 2

Explain, using the term water potential, how water moves from the

vascular tissue to the atmosphere. [4]

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

:: Notes ::

Learning outcomes

j) explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between the leaves (sources) and other parts of the plant (sinks)

k) explain the translocation of sucrose using the mass flow hypothesis

Example: Question 3

Explain how the structure of sieve tube elements helps the translocation

of substances in the phloem. [3]

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

:: Notes ::

8

Example: Question 4

The liquid extracted from the phloem of C. pepo contains sucrose.

Explain how sucrose is transported in the phloem along the stem from the

leaf to the fruit. [4]

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

___________________________________________________________

:: Notes ::

Example: Question 5

Fig. 4.1 shows the movement of sucrose from source to sink through the

phloem in a plant.

a) With reference to Fig. 4.1,

i) name an example of a source and a sink. [1]

source _______________________________________________

sink _______________________________________________

ii) name cells C and D. [1]

cell C _______________________________________________

cell D _______________________________________________

:: Notes ::

b) With reference to Fig. 4.1, explain how sucrose travels from,

the source to cell C. [2]

________________________________________________________

________________________________________________________

________________________________________________________

________________________________________________________

cell C to the sink. [2]

________________________________________________________

________________________________________________________

________________________________________________________

________________________________________________________

9

Figure 4.1

10