Science Paper on Tropism

Topics: DNA, Gene, Genetics Pages: 5 (1711 words) Published: January 22, 2013
A 'tropism' is a growth in response to a stimulus. Plants grow towards sources of water and light, which they need to survive and grow. Auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weedkillers, rooting powder and to control fruit ripening. Tropisms

The direction of plant growth
Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water. There are different types of tropisms:

Positive phototropism in plant stems
* Tropism – growth in response to a stimulus
* Positive tropism – towards the stimulus
* Negative tropism – away from the stimulus
* Phototropism – growth in response to the direction of light * Geotropism – growth in response to the direction of gravity Responses of different parts of the plant
Response| Part of plant| Direction of growth| Advantage| Positive phototropism| Stem tip| Growth towards light| To get maximum light for photosynthesis| Negative phototropism| Root tip| Growth away from light| Less chance of drying out| Positive geotropism| Root tip| Towards gravity| More chance of finding moisture| Negative geotropism| Stem tip| Away from gravity| More chance of finding light| Controlling the direction of growth

Auxin is a plant hormone responsible for controlling the direction of growth of root tips and stem tips in response to different stimuli including light and gravity. Auxin is made at the tips of stems and roots. It's moved in solution to older parts of the stem and root where it changes the elasticity of the cells. More elastic cells absorb more water and grow longer, causing bending in the stem or root. It's thought that light and gravity can interfere with the transport of auxin causing it to be unevenly distributed. Auxin experiment

3 groups of seeds are grown in a cardboard box.
A - when the tips are removed, no auxin is made so the stems do not grow B – when the tips are covered, auxin moves to all parts of the stem causing all parts to grow C - when the tips are lit from one side only auxin accumulates on the shaded side causing it to grow more than the illuminated side Nervous System And Nerves

Function: To transmit messages from one part of your body to another

Neurons: Messenger cells in your nervous system

Nerve impulses: Electrical signals carrying messages

Neurotransmitters:Chemicals released by one neuron to excite a neighbouring one Millions of messengers
Your nervous system contains millions of nerve cells, called neurons. Neurons are highly specialised to transmit messages from one part of your body to another. All neurons have a cell body and one or more fibres. These fibres vary in length from microscopic to over 1 metre. There are two different kinds of nerve fibres: fibres that carry information towards the cell body, called dendrites, and fibres that carry information away from it, called axons. Nerves are tight bundles of nerve fibres. Teamwork

Your neurons can be divided into three types:
* Sensory neurons, which pass information about stimuli such as light, heat or chemicals from both inside and outside your body to your central nervous system * Motor neurons, which pass instructions from your central nervous system to other parts of your body, such as muscles or glands * Association neurons, which connect your sensory and motor neurons Electrical and chemical signals

Your neurons carry messages in the form of electrical signals called nerve impulses. To create a nerve impulse, your neurons have to be excited. Stimuli such as light, sound or pressure all excite your neurons, but in most cases, chemicals released by other neurons will trigger a nerve impulse. Although you have millions of neurons that are densely packed within your nervous system, they never actually touch. So when a nerve impulse reaches the end of one...
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