Ch 14 Growth and development

Question 1

Measurement of growth by change in size

Answer 1

Advantages:
1. The organism remains alive after the measurement, its growth can be measured easily
2. Easy and convenient

Disadvantages:
- Growth in all dimensions may not be taken into account

Question 2

Measurement of growth by fresh mass

Answer 2

Advantages:
1. The organism remains alive after the measurement, its growth can be measured easily
2. Easy and convenient

Disadvantages:
- The fresh mass of an organism is affected by its water content

Question 3

Measuring dry mass

Answer 3

Heating an organism in an oven at around 100C until a constant mass is obtained upon repeated weighing

Question 4

Measurement of growth by dry mass

Answer 4

Advantages:
- Accurate

Disadvantages:
1. The organism is killed by heating and dehydration, continuous measurement of growth of the same individual is impossible
2. A large sample size of specimens is needed to measure growth
3. Not suitable for large organisms
4. Time-consuming

Question 5

Conditions necessary for seed germination

Answer 5

Water
1. Softens seed coat and makes the cotyledons swell, breaks the seed coat so that the radicle can emerge
2. Activates the enzymes in the seed, enzymes catalyse the conversion of insoluble stored food into soluble food substances
3. Acts as a reactant in hydrolysis of stored food for used in respiration to provide the energy needed
4. Acts as a medium for transporting soluble food substances to the growing regions of the embryo

Warmth
- Allows the enzymes in the seed to work efficiently

Oxygen
- Needed for the seed to carry out respiration which provides energy for the growth of the embryo

Question 6

Process of seed germination

Answer 6

1. The seed absorbs water and swells up. Enzymes in the seed are activated. The seed coat softens.
2. The seed coat breaks. The radicle emerges and grows down into the soil to form the root
3. The plumule grows up to form the shoot. The hypocotyl lifts the cotyledons up above the soil. The cotyledons protect the delicate plumule tip on their way out
4. The shoot straightens after emergence from the soil. The first leaves sprout and begin making food by photosynthesis
5. The roots and shoot continue to grow and develop. The cotyledons will fall off later

Question 7

Interpreting a graph of changes in mass of a seedling during germination

Answer 7

Dry mass
- Whole seedling: Dry mass decreases first as stored food is used to provide energy for growth. When green leaves are developed, the rate of food production by photosynthesis is higher than the rate of food consumption by respiration.
- Radicle / Plumule: Dry mass increases as new cells are formed. The radicle grows faster and emerges from the seed first. It has a greater dry mass than the plumule.
- Cotyledons: Dry mass decreases as stored food in the cotyledons is broken down to release energy for growth

Fresh mass
- Cotyledons: Fresh mass of the cotyledons increases at the beginning of germination. A large amount of water is absorbed by the seed at this stage

Question 8

Primary growth at root tip

Answer 8

Region of cell division
- Contains the apical meristem. The cells are small and have thin cell walls. The cytoplasm is dense and there are no vacuoles. The cell multiply by mitotic cell division.

Region of elongation
- The cells come from the new cells produced in the region of cell division. The cells take in water by osmosis to form many small vacuoles. Their cell walls are stretched when vacuoles are formed. The cells elongate, pushing the apical meristem downwards. The root grows down into the soil.

Region of differentiation
- Small vacuoles in the cells fuse to form a large vacuole. When the cells elongate, more cellulose fibres are added to the cell walls. The cell walls become thicker and more rigid. The cells differentiate into different types of specialized cells to perform particular functions.

Question 9

Secondary growth at stems and roots

Answer 9

Cells of the vascular cambium divide to form a ring. Cells of the cambium ring divide and differentiate to form secondary xylem and secondary phloem.

Question 10

Annual rings

Answer 10

• In spring and summer, water is plentiful. The xylem cells produced by the vascular cambium are larger and have thinner cell walls. These cells are lighter in colour
• In autumn and winter, water becomes scarcer. The xylem cells produced are smaller and have thicker cell walls. These cells give a darker colour

Question 11

Stages of growth in annual plants

Answer 11

1. Dry mass decreases at the early stage of seed germination as the food stored in the seed is broken down to provide energy for growth.
2. Dry mass increases when green leaves have developed to make food by photosynthesis. The rate of photosynthesis is higher than the rate of respiration, there is a net gain in the amount of organic matter.
3. Dry mass decreases due to the dispersal of food and seeds.

Question 12

Stages of growth in humans

Answer 12

1. In infancy, growth occurs rapidly
2. Growth slows down in childhood
3. Growth becomes rapid again in adolescence
4. When a person reaches the maximum size in adulthood, there is no net growth but cell division still occurs to replace worn-out and damaged cells
5. Growth becomes negative during the old age

Question 13

Design an experiment to investigate whether pesticide X affects germination of the wheat grains

Answer 13

• Sample size: Soak a sufficiently large number of wheat grains in water added with pesticide X
• Control: Soak equal number of wheat grains in water without pesticide X
• Controlled variables: Allow the grains in the two set-ups to germinate under identical environmental conditions for the same period of time
• Measurement: Compare the success rate of germination to see if there is any significant difference