A specific hormone can only affect cells if the cells have a receptor for it. The diagram below shows two cells targeted by two different hormones. Hormone one cannot affect the cell on the right ...
Watch on BBC One, listen on 5 Live and follow text updates as the Netherlands face Sweden at the Fifa World Cup.
The USA take a huge step towards winning their World Cup group as they beat Australia 2-0 with a dominant display in Seattle.
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