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How animals and plants grow, why cells differentiate, how to measure growth, and how percentile charts are used to monitor children.
Specification references this topic covers: AQA 4.1.1; Edexcel B2.5, B2.6, B2.7, B2.8.
You started life as a single cell. Today you are made of billions of cells. Growth is an increase in size or mass. For living things it happens by making more cells, by making cells bigger, or both.
This matters because doctors check that children are growing normally, and farmers and gardeners want plants to grow well. You also need to know how specialised cells such as nerve cells and root hair cells arise.
By the end you will be able to describe growth in animals and plants, explain differentiation, compare ways of measuring growth, and read a percentile chart.
Growth needs new cells. Cells divide by mitosis, which makes two identical daughter cells from one. Each new cell then grows bigger before it divides again.
Growth is not the same in every part of an organism or at every age. A baby grows quickly. By adulthood, humans have stopped growing taller, but cells still divide to repair damage and to replace cells that wear out, such as skin cells and blood cells.
To grow, a cell needs raw materials. They enter by diffusion, osmosis and active transport, and the cell makes new proteins and other molecules from them.
In animals, growth happens by cell division and differentiation. A fertilised egg divides by mitosis to make a ball of identical cells. These cells are embryonic stem cells, which can become any type of cell.
As the embryo develops, most cells differentiate. They change to become specialised cells such as muscle, nerve or blood cells. Different genes are switched on, so the cells make different proteins and take on different shapes and jobs.
Animals grow in a set pattern and usually stop growing when they reach adult size. Most adult cells have already differentiated, so they cannot become other cell types. Adults keep some adult stem cells, for example in the bone marrow, to make new blood cells.
Plants grow in a different way. Cell division happens in areas called meristems, which are found at the tips of roots and shoots. Meristem cells divide by mitosis to make new cells.
The new cells then elongate, which means they get longer. They take in water and the vacuole swells, which makes the cell bigger. Finally the cells differentiate into specialised cells such as xylem, phloem and root hair cells.
Many plant cells keep the ability to differentiate throughout life, so plants can keep growing for all of their lives. This is one reason why a cutting can grow into a new plant.
Differentiation is the process by which a cell becomes specialised for a particular job. Without it, all cells would be the same and an organism could not have tissues, organs or organ systems.
Specialised cells are adapted to their jobs. A sperm cell has a tail to swim. A red blood cell has no nucleus so it can carry more oxygen. A root hair cell has a long projection to give a large surface area for absorbing water.
Stem cells are unspecialised. Embryonic stem cells can become any type of cell. Adult stem cells can become only some types. Plant meristems contain stem cells that can become any plant cell type.
There are three common ways to measure growth: length or height, fresh mass and dry mass. Height is easy to measure but does not show whether the plant is also getting thicker. Fresh mass is the mass of the living organism, but it includes water, and the amount of water can change from hour to hour.
Dry mass is the mass left after all the water has been removed by drying the organism in an oven. It is the best measure of growth because it shows only the mass of the cells.
To compare growth, calculate the change or the percentage change. Percentage change = (new value − old value) ÷ old value × 100. A seedling that grows from 4.0 cm to 10.0 cm has grown by (10.0 − 4.0) ÷ 4.0 × 100 = 150%.
Doctors monitor the growth of babies and children with percentile charts. The chart has lines for different percentiles. A percentile shows the percentage of children of the same age and sex that are smaller. A child on the 50th percentile line is average: half of children are heavier and half are lighter.
A child on the 25th percentile is heavier than 25% of children. Curves for mass, length (or height) and head size are plotted against age. The usual pattern is that a healthy child follows roughly the same percentile curve as they grow.
A doctor may be concerned if a child crosses several percentile lines, or if the measurements are very high or very low. A single point is less useful than a trend. Charts are an average, so they do not tell you everything about an individual.
Animals grow by cell division and differentiation. Plants grow by cell division in meristems, cell elongation and differentiation.
Differentiation makes specialised cells. Stem cells are unspecialised.
Dry mass is the best measure of growth. A percentile chart compares a child with others of the same age. A child on the 50th percentile is average.
Name the part of a plant where cell division happens in a root or shoot tip.
Answer: The meristem
A seedling is 4.0 cm tall on day 1 and 10.0 cm tall on day 7. Calculate its percentage increase in height.
Answer: 150%
Explain how a plant root grows longer, using the words meristem, mitosis, elongation and differentiation.
Answer: Meristem cells divide by mitosis, the new cells elongate, and then they differentiate
Growth, differentiation and percentile charts practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
What type of cell division is used for growth?
Answer: mitosis
Where does cell division take place in a plant shoot?
Answer: meristem
What is the name of the process in which a cell becomes specialised?
Answer: differentiation
What word describes a plant cell getting longer?
Answer: elongation
What is the mass of an organism called after all the water has been removed?
Answer: dry mass
A plant grows from 4.0 cm to 10.0 cm tall. Calculate the percentage increase in height.
Answer: 150%
A baby has a mass of 3.2 kg at birth and 6.4 kg at six months. Calculate the percentage increase in mass.
Answer: 100%
A baby is on the 50th percentile for mass. What fraction of babies of the same age are heavier?
Answer: half
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
(a) Describe how a root grows longer. [3 marks] (b) Give one reason why dry mass is a better measure of growth than fresh mass. [1 mark]
Answer: (a) cells in the meristem divide by mitosis, the new cells elongate, then the cells differentiate (b) fresh mass includes water which varies
(a) A child's mass is on the 25th percentile on a growth chart. Explain what this means. [1 mark] (b) A baby has a mass of 3.2 kg at birth and 6.4 kg at six months. Calculate the percentage increase in mass. [2 marks] (c) Explain why a doctor looks at several measurements over time rather than one. [1 mark]
Answer: (a) the child is heavier than 25% of children of the same age (b) 100% (c) a trend shows whether growth is normal but a single point may not
Saying plants grow everywhere at once
Plant cells divide in meristems, at the tips of roots and shoots.
Saying elongation is cell division
Elongation is a cell getting longer as it takes in water. Division is mitosis.
Using fresh mass as the best measure of growth
Dry mass is best because it does not include the changing amount of water.
Saying the 50th percentile means 50% of the normal mass
It means half of children of that age are heavier and half are lighter.
Dividing by the new value in a percentage change
Divide by the original value.
Growth, differentiation and percentile charts is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 5.
Saying plants grow everywhere at once. Plant cells divide in meristems, at the tips of roots and shoots.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
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