Stem senses
SIGHT
Despite having no eyes, plants have adaptations to still be able to "see" their surrounding environment. These adaptations are crucial, as they are needed to detect sunlight, an important component of photosynthesis. Photosynthesis, is the process by which plants produce food to grow and reproduce. Just as humans have photoreceptors in the back of their eye to detect light, plants have photoreceptors on the surface of their leaves and stems, to detect the intensity, direction and wavelength of light. All of these are factors which will then affect the development of plants.
How plants see different kinds of light
To detect and respond to different kinds of light, plants have a special kind of cell on their surface called a photoreceptor. Photoreceptors are bound to light absorbing pigments called chromophores. Chromophores absorb the light which hits the plants surface. Using this light energy chromophores then regulate cell activity, and can cause a change in plant behaviour, such as triggering photosynthesis to occur.
Key definitions
Photosynthesis: The process by which plants convert light energy into chemical energy, producing organic compounds rich in energy
Photoreceptor: a type of protein that is specialised to perceive and respond to light.
Chromophore: a pigment molecule which absorbs light
Phototropism: plant growth in response to a directional light source.
Hormone: chemical messengers that regulate growth and development.
Auxin: plant hormones that control cell elongation
Plants will respond to changes in light by growing towards or away from a light source. This pattern of growth is known as phototropism. Phototropism can be either negative or positive.
Phototropism
Fact!
Plants are green because a chromophore called chlorophyll absorbs only red and blue light. Therefore all that can be seen is reflected green light.
Fact!
(Hover over me)
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Positive phototropism: growth towards a source of light
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Negative phototropism: growth away from a source of light
How a plant moves in response to light
Plant growth towards a light source is controlled by a hormone called an Auxin. This hormone promotes cell elongation. Auxin is found in the tip of the plants stem. Sunlight reduces the amount of auxin in the plant tip. Therefore when light shines onto the plant stem there is more auxin on the side of the stem not exposed to light. On the shaded side auxin continues to promote cell elongation and so the shaded side of the stem continues to grow, causing it to bend towards the light.
Watch this video to see phototropism in action...
The light spectrum
Light lies on a spectrum known as the electromagnetic spectrum. Humans can only see part of this spectrum with the naked eye. This portion is known as the visible light spectrum. The sun is able to deliver wavelengths of light across the spectrum, from UV-A to far red-light. Some of these wavelengths of light at the far ends of the spectrum whilst they are not visible to humans, they can be "seen" and absorbed by plants. However not all wavelengths of light are absorbed and some wavelengths of light are more useful to plants than others.
The effects of different kinds of light on plant development:
Blue light and red light: in combination they largely increase the efficiency of photosynthesis and the development of the tissues and apparatus used for photosynthesis.
Far-red light: increases the growth rate of plants through stem elongation and by increasing leaf area.
White light: increases the rate of photosynthesis and produces a regular leaf shape.
Green light: helps to increase the efficiency of photosynthesis.
Summary
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Plants absorb different wavelengths of light for photosynthesis to aid their growth
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Plants use specialised cells, including photoreceptors and chromophores to absorb and respond to light
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Plants respond to light by growing either towards or away from a light source (phototropism)
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Certain wavelengths of light are more easily absorbed by plants than others
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Different wavelengths of light have different effects on plant development