What Growth Is
Definition
Growth
A permanent increase in the size or dry mass of an organism, brought about by cell division, cell enlargement and differentiation.
- Growth is a permanent increase in size or dry mass, and in a multicellular organism it is brought about by three processes.
- Cell division increases the number of cells, and in body cells this is mitosis.
- Cell enlargement, called elongation in plants, makes each cell bigger.
- Differentiation turns unspecialised cells into the specialised cells the organism needs.
- Animals and plants use all three processes, but they use them at different times and in different places.
Growth in Animals
Definition
Differentiation
The process by which an unspecialised cell switches on particular genes and develops the structures it needs to carry out one specific job.
- Animals grow mainly by cell division followed by differentiation.
- Most differentiation happens early, in the embryo and in the young animal, so an adult has very few cells left that can differentiate.
- Growth in animals happens all over the body rather than in a few fixed regions.
- Growth stops once the animal reaches its adult size, and it does not usually restart.
- Cell division continues in the adult, but almost entirely for repair and replacement rather than growth.
Example
A human infant triples its birth mass in the first year, then growth slows, accelerates again during the adolescent growth spurt, and stops when the growth plates in the long bones fuse.
Growth in Plants
Definition
Cell elongation
The stage of plant growth in which a newly made cell absorbs water into its vacuole and gets longer, making the root or shoot grow.
- Plants grow by cell division, then cell elongation, then differentiation.
- Cell division happens only at the meristems, which sit at the tips of roots and shoots and in the cambium.
- Just behind the meristem the new cells elongate, taking water into the vacuole so the cell gets longer.
- Elongation is what actually pushes a root down through the soil and a shoot up towards the light, because it adds far more length than division alone.
- Behind the elongation region the cells differentiate into xylem vessels, phloem, root hair cells and the rest.
- Plants grow throughout their whole lives and keep the ability to differentiate, which is why a cutting can grow into a complete new plant.
Common Mistake
Elongation is not the same as division. Elongation makes an existing cell longer by taking in water, and the number of cells does not change.
Comparing Animal and Plant Growth
- Where. Animal growth happens throughout the body, while plant growth is confined to the meristems.
- When. Animal growth stops at maturity, while plant growth continues for life.
- How. Animals rely on cell division and differentiation, while plants add cell elongation as the main contributor to length.
- Differentiation. Most animal cells lose the ability to differentiate, while plant cells largely keep it.
Measuring Growth
- Length or height is quick and does not harm the organism, but it measures only one dimension and misses growth in width.
- Wet mass includes everything, so it is easy to measure, but it changes with how much water the organism has taken in and is therefore unreliable.
- Dry mass is the mass after all the water has been removed, so it is the most accurate measure of the material the organism has actually made.
- The drawback of dry mass is that the organism must be killed and dried, so growth cannot be followed in the same individual over time.
Investigating the Growth of Seedlings
Practical
Investigation: Measuring the Growth of Seedlings
- Aim: To find how the dry mass of cress seedlings changes over two weeks, and to compare that with the change in height. This is a class investigation rather than one of the eight Edexcel core practicals, but Edexcel sets method, variables and percentage change questions on work of exactly this kind.
- Apparatus: Cress seeds, 15 Petri dishes lined with cotton wool, distilled water, ruler with a millimetre scale, forceps, balance reading to 0.01 g, drying oven set to 80 degrees Celsius, crucibles or foil trays, desiccator, marker pen.
- Method:
- Sow 20 cress seeds evenly on damp cotton wool in each of 15 dishes and label them day 0 to day 14.
- Keep every dish in the same place at the same temperature and light level, and add the same volume of distilled water each day.
- On day 0, and then every day, take one dish and measure the height of each seedling from the cotton wool to the tip of the tallest leaf, then calculate the mean.
- Lift the same seedlings out with forceps, rinse off the cotton wool fibres, and place them in a weighed foil tray.
- Dry the tray in the oven at 80 degrees Celsius, weigh it, return it to the oven for a further hour, and weigh it again.
- Repeat until two consecutive readings agree, because a constant mass tells you all the water has gone and the reading is a true dry mass.
- Plot mean height and mean dry mass against time on the same set of axes.
- Variables: The independent variable is time in days and the dependent variables are mean height and mean dry mass. Keep the seed variety, the number of seeds per dish, the temperature, the light intensity, the volume of water and the drying temperature the same.
- Results: Height increases rapidly at first as the cells elongate. Dry mass often falls slightly in the first two or three days, because the seedling is respiring its stored food before the leaves open, then rises steadily once photosynthesis begins.
- Maths: Growth is usually reported as a percentage change so that seedlings of different starting sizes can be compared. percentage change in mass=final mass−initial massinitial mass×100\text{percentage change in mass} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100percentage change in mass=initial massfinal mass−initial mass×100 A negative answer means a loss of mass.
- Watch out: A single seedling is not representative, so measure every seedling in the dish and use the mean. Returning the tray to the oven until the mass stops changing is the step most often missed, and without it the dry mass still contains water.
- Safety: The oven and the trays are hot, so use tongs and heatproof gloves. Wash your hands after handling seeds and cotton wool.
Exam technique
- If a question asks you to compare growth in animals and plants, answer in matched pairs so each point about the animal has the plant equivalent next to it.
- Name all three processes when asked how an organism grows, because cell division on its own rarely gets full marks.
- In a data question about a growth curve, describe the rate as well as the direction, for example rises steeply then levels off.
- Quote figures from the graph with their units when you describe a trend, since examiners credit correctly quoted data linked to the trend.
Self review
- Name the three processes that bring about growth in a plant.
- Where in a plant does cell division take place?
- Explain why dry mass is a more reliable measure of growth than wet mass.
- State two differences between growth in an animal and growth in a plant.
- A seedling has a dry mass of 0.040 g on day 3 and 0.150 g on day 10. Calculate the percentage increase in dry mass.
