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Which of the following statements about phloem transport in plants is correct?
\(C\)
Consider each option.
Option A: Incorrect. While sugar concentration plays a role, phloem transport is more complex and can move against concentration gradients.
Option B: Incorrect. Gravity is not the determining factor as phloem transport can occur both up and down the plant.
Option C: Correct. Phloem transport is an active process requiring energy, and it can move substances bidirectionally based on the plant’s needs.
Option D: Incorrect. The rate of phloem transport varies depending on factors such as photosynthetic activity and the plant’s growth needs.
\(\Rightarrow C\)
Van Helmont's willow tree experiment showed that over five years, the tree gained 164 pounds while the soil only lost 2 ounces. What conclusion can be drawn from this experiment?
\(D\)
→ Van Helmont’s experiment challenged the prevailing belief that plants gained all their mass from soil.
→ While his conclusion that water alone was responsible for the growth was incorrect, the experiment was crucial in prompting further investigations into plant nutrition and growth.
\(\Rightarrow D\)
Describe a key experiment that demonstrated photosynthesis occurs as a multi-step process. (3 marks)
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Blackman and Mathgel’s experiment:
→ This experiment demonstrated that photosynthesis involves both light-dependent and light-independent reactions.
→ They showed that when light intensity increased, the rate of photosynthesis only increased up to a certain point, after which it levelled off even with more light.
→ This led to the discovery of limiting factors in photosynthesis, such as carbon dioxide concentration and temperature.
→ Their work was crucial in establishing that photosynthesis is a complex process with multiple stages, each potentially limited by different factors.
Robin Hill experiment (1937):
→ Robin Hill demonstrated that isolated chloroplasts could produce oxygen in the presence of light but without carbon dioxide.
→ This showed that the oxygen released during photosynthesis comes from water, not carbon dioxide as previously thought.
→ Hill’s experiment provided evidence that photosynthesis occurs in two main stages: a light-dependent reaction that splits water and produces oxygen, and a light-independent reaction that fixes carbon dioxide.
→ This discovery was crucial in developing our current understanding of photosynthesis as a complex, multi-step process.
Blackman and Mathgel’s experiment:
→ This experiment demonstrated that photosynthesis involves both light-dependent and light-independent reactions.
→ They showed that when light intensity increased, the rate of photosynthesis only increased up to a certain point, after which it levelled off even with more light.
→ This led to the discovery of limiting factors in photosynthesis, such as carbon dioxide concentration and temperature.
→ Their work was crucial in establishing that photosynthesis is a complex process with multiple stages, each potentially limited by different factors.
Robin Hill experiment (1937):
→ Robin Hill demonstrated that isolated chloroplasts could produce oxygen in the presence of light but without carbon dioxide.
→ This showed that the oxygen released during photosynthesis comes from water, not carbon dioxide as previously thought.
→ Hill’s experiment provided evidence that photosynthesis occurs in two main stages: a light-dependent reaction that splits water and produces oxygen, and a light-independent reaction that fixes carbon dioxide.
→ This discovery was crucial in developing our current understanding of photosynthesis as a complex, multi-step process.
Explain van Helmont's willow tree experiment and discuss its significance in the early understanding of plant growth and nutrition. (2 marks)
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→ Van Helmont grew a willow tree in a measured amount of soil for five years, adding only water.
→ He found that while the tree gained significant mass, the soil lost very little, leading him to conclude that the tree’s substance came from water alone.
→ Although his conclusion was incorrect, this experiment was significant because it challenged the prevailing belief that plants gained their mass from the soil and laid the groundwork for future investigations into plant nutrition and photosynthesis.
→ Van Helmont grew a willow tree in a measured amount of soil for five years, adding only water.
→ He found that while the tree gained significant mass, the soil lost very little, leading him to conclude that the tree’s substance came from water alone.
→ Although his conclusion was incorrect, this experiment was significant because it challenged the prevailing belief that plants gained their mass from the soil and laid the groundwork for future investigations into plant nutrition and photosynthesis.
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i. Scientists could include one of:
→ Van Helmont, Hales, Priestley, Ingen-Housz, Senebier, Saussure
ii. Priestly’s Experiment
→ In 1771, Joseph Priestly ignited a candle in a jar with some mint, in which he ignited the candle and then it went out. After a few trials, the candle wouldn’t ignite as all the oxygen in the air had been used (note that oxygen was not yet discovered). However after 27 days, by igniting the candle with a mirror and sunlight as to not open the jar, it reignited.
→ Similarly, by putting mice into a jar with and without a sprig of mint, the jar with the mint allowed the mouse to survive much longer.
→ These observations lead to questions about plant structure, and how they were somehow able to ‘restore air’ that is used by animals breathing and combustion reactions.
i. Scientists could include one of:
→ Van Helmont, Hales, Priestley, Ingen-Housz, Senebier, Saussure
ii. Priestly’s Experiment
→ In 1771, Joseph Priestly ignited a candle in a jar with some mint, in which he ignited the candle and then it went out. After a few trials, the candle wouldn’t ignite as all the oxygen in the air had been used (note that oxygen was not yet discovered). However after 27 days, by igniting the candle with a mirror and sunlight as to not open the jar, it reignited.
→ Similarly, by putting mice into a jar with and without a sprig of mint, the jar with the mint allowed the mouse to survive much longer.
→ These observations lead to questions about plant structure, and how they were somehow able to ‘restore air’ that is used by animals breathing and combustion reactions.
'Over the past 400 years, the development of our knowledge of the chemical transformations occurring both inside and outside plants has led to our current understanding of photosynthesis.'
Evaluate this statement with reference to the experiments of TWO named scientists. (7 marks)
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→ In 1774 Joseph Priestley made observations about changes to the air around a living plant. He observed a candle would burn longer within a bell jar of air if a plant were included under the bell jar.
→ He deduced that the burning of the candle was using up something in the air and thus had a limited burning time in a fixed volume of air. He further deduced the plant was reversing that change and restoring the air to allow the candle to burn longer.
→ Priestley later described a gas (oxygen) used up from air in combustion of the candle, yet released by plants. Adding oxygen to the outside air mixture can be termed a chemical transformation of air done by plants. This led to the understanding that photosynthesis produces oxygen.
Data collected inside the plant
→ The experiments by Calvin in the 1940s involved an illuminated flattened glass vessel, wide but thin, containing microscopic algae in solution. The algae were given a pulse of radioactive carbon dioxide \(\ce{(^{14}CO2)}\), injected into a stream of air into the vessel.
→ Samples of Chlorella were then released at intervals (3, 5, 10 seconds and then 15 second periods) into boiling alcohol to kill the algae and stop the progress of biochemical reactions containing the \(\ce{(^{14}CO2)}\).
→ Compounds that the radioactive carbon had reached at a particular moment were determined by two-dimensional paper chromatography and autoradiography after the chlorella cells were broken up.
→ New chemicals formed were deduced with the same results with chromatography of standard chemicals. For example, phosphoglycerate was identified as the first metabolite in the carbon cycle that was then changed into the glyceraldehyde phosphate.
→ In this way the series of chemical transformations of carbon compounds in the light-independent reactions of photosynthesis could be followed.
→ The work of scientists studying both the external and internal environments was essential in developing our understanding of photosynthesis.