KS3 Biology › Cells
KS3 Biology › Cells
These are the points the worksheet questions are built around.
Every person starts life as a single cell. During sexual reproduction, a sperm cell from the father fuses with an egg cell from the mother in a process called fertilisation, producing a single, unspecialised cell called a zygote — essentially a blank canvas with no specific function yet.
The zygote divides repeatedly through mitosis, the process where one cell splits into two identical cells: one becomes two, two becomes four, four becomes eight, and so on. After several days, this forms a bundle of cells called an embryo. In the very early embryo, around the eight-cell stage, all of these cells are unspecialised and identical — these are embryonic stem cells, sometimes called master cells because of their unlimited potential to become any type of cell the body needs.
Since the human body contains over 200 different types of specialised cells, something has to turn these identical stem cells into all that variety — a process called differentiation. Every cell in the body contains the same DNA, since they all descended from the original zygote, but not all of that DNA is switched on in every cell. During differentiation, chemical changes switch some genes on and others off, and the specific pattern of which genes are active determines what type of cell forms — a nerve cell has different genes switched on compared to a muscle cell, despite sharing identical DNA. Through this process, embryonic stem cells can become neurons (which transmit electrical signals), muscle cells (packed with proteins that allow contraction), epithelial cells (like those lining the small intestine), red and white blood cells, fat cells, and more.
Not every embryonic stem cell differentiates during development. Some remain unspecialised even after birth — these are called adult stem cells (a name that applies to children too, since "adult" here just means "after the embryonic stage"). Unlike embryonic stem cells, which can become any cell type, adult stem cells can only differentiate into a limited number of related cell types specific to where they're found. Bone marrow stem cells, for example, can become red blood cells, white blood cells, and platelets, but not nerve or muscle cells. Adult stem cells are found in specific regions of the body, including bone marrow, the brain, skin, muscles, liver, intestines, and eyes — each type only producing the cell types relevant to its own region.
Adult stem cells have two essential jobs. The first is self-renewal: dividing by mitosis to make more stem cells, maintaining a reserve so the supply never runs out. The second is differentiation into specialised cells, replacing damaged or worn-out ones — red blood cells, for instance, only live around 120 days, so bone marrow stem cells are constantly differentiating to replace them, producing roughly 2 million new red blood cells every second.
Bone marrow is one of the most important sources of adult stem cells — soft, jelly-like tissue inside bones, packed with stem cells that produce red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which help blood clot). This is put to real medical use in bone marrow transplants, where healthy stem cells from a donor are given to a patient with conditions like leukaemia, allowing them to start producing healthy blood cells again.
Once an embryonic stem cell has differentiated, it normally can't turn back into a stem cell — differentiation permanently switches certain genes off, and reversing that would be dangerous, since specialised cells need to stay specialised to keep the body functioning properly. However, scientists have found a way around this using induced stem cells: taking an ordinary specialised cell, such as a skin cell, and using chemicals to switch its genes back on so it behaves like an embryonic stem cell again. This is significant for medicine because it avoids some of the ethical issues involved in using stem cells taken directly from embryos.
In short: embryonic stem cells, formed early in development, can become any cell type but lose that flexibility once they differentiate. Adult stem cells persist throughout life in specific body regions, with the more limited but essential jobs of self-renewal and replacing worn-out cells. Both types are being researched for treating conditions such as diabetes, Parkinson's disease, and spinal cord injuries, though clinical, ethical, and social questions remain.
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The 3 questions from the video, one at a time. See if you caught them.
1. What do we call the single cell formed when a sperm fertilizes an egg?
Why: A zygote is the single cell formed when a sperm fertilises an egg. It divides to form an embryo.
2. Which of these is TRUE about adult stem cells?
Why: Adult stem cells are more limited than embryonic stem cells. They can only turn into a few related types of cell, for example bone marrow stem cells make different kinds of blood cell.
3. What process do adult stem cells use to make more stem cells?
Why: Stem cells make more stem cells by mitosis, which is cell division. Differentiation is when a cell becomes specialised.
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