Download Phototropism Reading

Phototropism
Essential Knowledge 2C2: Organisms respond to changes in their
external environment.
Phototropism is directional growth in which the direction of
growth is determined by the direction of the light source. In
other words, it is the growth and response to a light stimulus.
Phototropism is most often observed in plants, but can also
occur in other organisms such as fungi. The cells on the plant
that are farthest from the light have a chemical called auxin
that reacts when phototropism occurs. This causes the plant
to have elongated cells on the farthest side from the light.
Phototropism is one of the many plant tropisms or
movements which respond to external stimuli. Growth
towards a light source is a positive phototropism, while
growth away from light is called negative phototropism.
Most plant shoots exhibit positive phototropism, while roots usually exhibit negative
phototropism, although gravitropism may play a larger role in root behavior and growth. Some
vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid
objects and climb them.
Phototropism describes the process by which plants respond the light in their environment.
The light-sensing hormone responsible for the curvature of plant shoots towards light is auxin.
Upon release from the cells in the apical meristem, auxin collects primarily on the darker side
of the stem and stimulates cell elongation. Thus, the cells on the side not directly exposed to
light will grow much faster than the opposing side, and the stem will curve towards the light
source.
Phototropism in plants such as Arabidopsis thaliana is directed by blue light receptors called
phototropins. Other photosensitive receptors in plants include phytochromes that sense red
light and cryptochromes that sense blue light. Different organs of the plant may exhibit
different phototropic reactions to different wavelengths of light. Stem tips exhibit positive
phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue
light. Both root tips and most stem tips exhibit positive phototropism to red light.
Phototropism is enabled by auxins. Auxins are plant hormones that have many functions. In
this respect, auxins are responsible for expelling protons (by activating proton pumps) which
decreases pH in the cells on the dark side of the plant. This acidification of the cell wall region
activates enzymes known as expansins which break bonds in the cell wall structure, making
the cell walls less rigid. In addition, the acidic environment causes disruption of hydrogen
bonds in the cellulose that makes up the cell wall. The decrease in cell wall strength causes
cells to swell, exerting the mechanical pressure that drives phototropic movement.
**Thank you Wikipedia for the information**
Phototropism Analysis
Directions: Answer the questions below using complete
sentences.
1) What is phototropism?
2) Compare positive and negative phototropism.
3) What hormone is responsible for phototropism.
4) Make a sketch of the cell elongation of a plant stem bending towards the light.
5) What chemicals direct phototropism? What colors do they absorb?
6) Specifically, describe how auxins contribute to the growth of the stem when bending towards the light.