|MadSci Network: General Biology|
I'm not sure what you heard about the relationship between gravity and growth but it is not correct to say that gravity works against growth. It's more like there is no proper growth without gravity. What seems to be the case is that the lack of gravity (weightlessness) works against the type of growth that allows us to live comfortably in gravity. All developmental systems of organisms on Earth are subjected to gravity. Without gravity, they would not get the cues they need to complete the developmental process. Plants are especially tuned to gravitational responses because they need to tell 'up' from 'down'. That is, most of them have to grow along a vertical axis. Both light and gravity are good ways for a plant to tell 'up' from 'down' and mechanisms are known by which plants can use both for this purpose. The terms 'geotropism' and 'phototropism' have been coined for the responses of plants to gravity and light. Typically, plant roots are positively geotropic, they grow in the direction of gravity. The upward growth of stems is called negative geotropism. Can you think what positive and negative phototropism must be like? With animals the situation is quite different because animals can move. But the big thing is that all of an animal's developing and growing tissues are in a gravitational field all the time. Much of their growth is tuned to compensating for gravity. It's a mistake to think that gravity is only working on our bodies when we are standing up. Gravity affects us all the time. This is especially important with regard to bone growth. Certain parts of bones, like the ends of the long bones in our arms and legs, Are responsive to the stresses that are put on them. They grow in response to these stresses so as to maximize their strength when put under those particular stresses. If the stress goes away, the bone will lose that responsive development and will no longer be able to handle that kind of stress as well. What do you think would happen if you were lying down most of the time? Your bones would not adapt to supporting your vertical weight in a gravitational field and soon it would be difficult for you to stand up. People who are confined to bed for a long time because of a medical condition have to work hard later to regain their strength for standing and walking. This need that bones have to be subjected to the stress produced on our bodies by gravity is an important consideration for living in space. A prolonged time spent in zero gravity would weaken our bones and muscles to the point where it would be difficult (or even impossible, no one is sure) to return comfortably to a life in normal gravity. Gravity has no known effect on how tall one grows in the sense that lying down would increase the growth. Actually, gravity does not inhibit growth but actually stimulates bone and muscle growth in humans normally subjected to daily activity in a gravitational field. The important thing to remember is that growth is not necessarily taller, longer, or bigger. In some cases it takes growth to make something stronger.
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