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A short class with Sarah and her students. The other ways are under More.
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A red blood cell has one main job. What is it?
Carrying oxygen around the body.
So what would make a cell good at that?
Lots of haemoglobin, and plenty of surface for oxygen to get in and out.
Exactly. It has no nucleus, which leaves more room for haemoglobin, and its dished shape gives a large surface area for its volume.
But without a nucleus, can it divide?
No. That is the trade-off: it lasts about 120 days, and stem cells in the bone marrow keep making replacements.
A nerve cell must carry a signal from your spinal cord to your toes. What shape would help?
A very long one, with a long axon.
Right, some axons are about a metre long. The myelin sheath around them insulates them and speeds up the impulse. And the branches at the other end?
Dendrites, to pick up signals from lots of other nerve cells.
Good. Now muscle cells must contract. What do they need?
Protein filaments that slide together, and lots of energy, so lots of mitochondria.
Exactly. Each feature you can name links back to the job the cell does.
Plants have specialised cells too. Why might a root hair cell have a long, thin extension?
To make the surface area bigger, so it can absorb more water and minerals.
Yes, and it has no chloroplasts. Why not?
Because it's underground, where there's no light for photosynthesis.
Exactly. Now, what is the advantage of having so many different specialised cells?
Each cell does one job really well, like workers in a factory.
That is the division of labour. The cost is that the cells depend on each other and many can no longer divide.
A specialised cell's shape and contents suit the job it does.
Haemoglobin, no nucleus and a biconcave shape make them efficient oxygen carriers.
A long axon, branched dendrites and a myelin sheath carry impulses quickly over long distances.
Sliding protein filaments and many mitochondria allow powerful contraction.
Specialised cells form tissues and organs, but they depend on each other and many cannot divide.
A red blood cell's biconcave shape gives it a large surface area for its volume, so oxygen diffuses in quickly in the lungs and out quickly in the tissues.
A nerve cell running from the spinal cord to a toe carries an impulse about a metre in a single cell, with its myelin sheath speeding the signal along.
Leg muscle cells contain many mitochondria, which release the large amounts of energy needed for running.
Shape for the job and parts for the task, structure fits function is all that we ask.
In the city of Body, the delivery vans (red blood cells) have no driver's cab, so they can carry more oxygen; the telephone lines (nerve cells) stretch across the whole city; and the builders (muscle cells) carry their own power generators, the mitochondria.
Red cells Carry, Nerve cells Notify, Muscle cells Move.
H.N.D. - Haemoglobin, No nucleus, Disc shape: the three key features of a red blood cell.
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Why do mammalian red blood cells have no nucleus?
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How does the biconcave shape help a red blood cell?
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Myelin sheath
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Why do muscle cells have many mitochondria?
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Root hair cell adaptation
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A cell whose structure is adapted to carry out a particular function.
The red, iron-containing protein in red blood cells that binds and carries oxygen.
Curved inwards on both sides, like the dished disc shape of a red blood cell.
The long fibre of a nerve cell that carries impulses away from the cell body.
A short, branched extension of a nerve cell that receives signals from other cells.
A fatty, insulating layer around many axons that speeds up nerve impulses.
The organelle where aerobic respiration releases energy; plural mitochondria.
The sharing out of tasks among specialised cells, so that each does one job efficiently.
Specialised cells have a shape and contents suited to one particular job: red blood cells carry oxygen, nerve cells carry electrical impulses and muscle cells contract, and in each case the cell's structure fits its function.
This lesson examines how the structure of a specialised cell suits its function. It looks in detail at three animal cells, the red blood cell, the nerve cell and the muscle cell, and then more briefly at sperm cells, ciliated cells, root hair cells and palisade cells. It shows how shape, size and contents (such as haemoglobin, a long axon or many mitochondria) make each cell efficient, uses a surface-area-to-volume calculation to explain the red blood cell's shape, and explains the division of labour that specialisation makes possible, along with its cost.
A single-celled organism must carry out every life process in one cell. In a multicellular organism, cells share out the work: each type of cell becomes specialised, with a structure suited to one job. The key idea is that structure fits function: if you know what a cell does, you can usually explain its shape and contents, and the other way round.
The price of these features is that a red blood cell cannot divide or repair itself. Each one lasts about 120 days and is replaced by new cells made from stem cells in the bone marrow.
Compare two shapes with the same volume, 64 cubic units. A cube with sides of 4 has a surface area of 6 × 16 = 96, so its surface-area-to-volume ratio is 96 ÷ 64 = 1.5. A flat box measuring 8 × 8 × 1 has a surface area of 2 × (64 + 8 + 8) = 160, so its ratio is 160 ÷ 64 = 2.5. Flattening the same volume gives much more surface for substances to diffuse across, and brings every part closer to the surface. The red blood cell's thin, dished shape works in the same way.
Specialised cells of the same type group together as tissues, tissues form organs, and organs work together in organ systems. Because each cell does one job very well, the organism as a whole can be larger and more complex. The cost is dependence: a red blood cell cannot feed itself, and most nerve cells cannot divide, so damage to them, as in a spinal cord injury, is very hard for the body to repair. The process by which cells become specialised, differentiation, is the subject of the next lesson.