Lessons 7–8 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology Grade 11, 1st edition. Sharq Publishing and Printing Joint-Stock Company, Tashkent, 2018
8
Light as an ecological factor
Textbook: pp. 47–52
GoalExplain the importance of light as an ecological factor, the parts of the solar spectrum, adaptations of plants and animals to light and photoperiodism.
New words
photoperiodism · fotoperiodizmphototropism · fototropizmlight-loving plants · yorug‘sevar o‘simliklarbiological clock · biologik soat
Explanation
Solar energy is the main source of life on Earth: it powers photosynthesis and sets the heat regime and water exchange. The solar spectrum has three biologically important parts: ultraviolet rays (below ~400 nm; the short-wave part is absorbed by the ozone layer, the longer part takes part in making vitamin D and melanin, and an excess damages skin and eyes); visible light (about 400–750 nm, for photosynthesis and vision); and infrared rays (above 750 nm, providing heat). In plants light causes phototropism (bending toward light) and photonasty (for instance, a tulip opening during the day). By their light needs plants are light-loving (saxaul, maize), shade-tolerant (strawberry) and shade-loving (moss, fern), while animals are diurnal, crepuscular and nocturnal. The response to seasonal change of day length is photoperiodism: it regulates flowering, leaf fall, moulting, migration and reproduction; hence plants are long-day, short-day or day-neutral. The «biological clock» in organisms controls daily rhythms.
Worked examples
A short-day plant (chrysanthemum) flowers in autumn when days are shorter than about 12 h; if the dark period is interrupted by an artificial lamp, flowering is delayed. So flowering is set by the length of the dark period, not by light intensity.
Swallows prepare to migrate in autumn from the signal of shortening days – an example of photoperiodism in animals. A bat is crepuscular – it hunts at dusk – whereas an owl is active at night.
Class activity
Plant two identical bean seeds in two cups, grow one by a window and the other in a closed box (1–2 weeks). Compare the effect of light and record it. Wash hands after working with soil; do not put seeds in the mouth.
Practice
1
Name the three parts of the solar spectrum and their biological roles.
Ultraviolet – vitamin D, melanin, harmful in excess; visible – photosynthesis and vision; infrared – heat.
2
How does phototropism differ from photoperiodism?
Phototropism – growth of an organ toward light; photoperiodism – a physiological response to day length (flowering, migration, etc.).
3
Two plants: one grows in an open field, the other on a forest floor. Whose leaf is likely thicker and why?
The field plant’s – it is light-loving and needs a thick leaf, wax layer and many stomata to withstand strong sun and hold many chloroplasts.
4
Why is protection from ultraviolet rays (clothing, sunscreen) important?
Excess UV causes sunburn, DNA damage and increases skin-cancer risk.