Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 509–517 of the course textbook. Image credits are on slide 14.
Grow seedlings in the dark, then light them from one side. Record drawings at intervals and measure the angle of curvature with a protractor. Use seedlings lit from above and seedlings kept in the dark as controls, and find the mean of many seedlings.
Phototropins in the shoot tip detect light from one side. Auxin is moved to the shaded side, where cells elongate more, so the shoot bends towards the light.
Plants use many signalling chemicals: auxin (elongation, apical dominance), cytokinin (cell division, side buds), gibberellin (stem growth, germination), ethylene (ripening), abscisic acid (stomatal closure) and jasmonic acid (defence).
Auxin diffuses into cells but cannot diffuse out. It leaves through efflux carriers placed on one side of each cell. When neighbouring cells use the same side, auxin is pumped through the tissue in one direction and builds up a concentration gradient.
Auxin makes cells pump H+ into their walls. The acid loosens cross-links between cellulose molecules, so turgor stretches the cell. Unequal auxin causes unequal growth, which bends the shoot.
Shoot tips make auxin, which moves to the roots; root tips make cytokinin, which moves to the shoots. Their interaction keeps root and shoot growth in proportion, for example in apical dominance.
Ethylene stimulates ripening, and ripening fruit makes more ethylene: positive feedback. Ripening is fast and synchronized, so many fruits are ready together to attract seed dispersers.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all seven statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Cress seeds, damp cotton wool, dishes, box with a side slit, ruler
Safety: Keep lamps away from water. Wash hands after handling seedlings.
Carry this out with your teacher's supervision and follow your school's lab rules.
Charles Darwin and his son Francis covered shoot tips to study bending towards light. In 1928 Frits Went isolated the signal, later named auxin.
Each step depended on the one before. Consider how a question can stay productive for decades.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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