Early Experiments: von Sachs, Engelmann and van Niel
Before photosynthesis became a molecular story, three nineteenth-century investigators turned a vague phenomenon into a measurable, chemically precise process. This quiz explores how Julius von Sachs linked glucose production to starch storage and located chlorophyll in chloroplasts, how T.W. Engelmann used a prism and aerobic bacteria to sketch the first action spectrum of photosynthesis, and how Cornelius van Niel used purple and green sulphur bacteria to identify the true hydrogen donor and the source of released oxygen.
What you will practise
- Von Sachs and the evidence for glucose, stored as starch, in the green parts of plants.
- Engelmann’s Cladophora experiment and bacterial accumulation in blue and red light.
- Van Niel’s hydrogen-donor reasoning, including ##\ce{H2O}## versus ##\ce{H2S}##.
- The empirical and corrected overall equations of photosynthesis.
Work carefully with the chemical equations, written in MathJax, and aim for conceptual clarity rather than memorisation.
Q1. Julius von Sachs, working around 1854, provided evidence for the production of which substance when plants grow?
Von Sachs provided evidence for the production of glucose when plants grow, and he noted that this glucose is usually stored as starch.
Q2. According to von Sachs, the glucose produced during growth is usually stored in the plant in the form of:
The lesson states clearly that glucose is usually stored as starch, an insoluble and compact reserve.
Q3. Von Sachs's later studies showed that the green substance in plants, now called chlorophyll, is located in:
His later studies showed that chlorophyll is located in special bodies within plant cells, structures later called chloroplasts.
Q4. Which statement best summarises von Sachs's conclusion about where glucose is made?
Von Sachs found that the green parts of plants are where glucose is made, and that the glucose is usually stored as starch.
Q5. The working lifespan of T.W. Engelmann is given in the lesson as:
Engelmann is dated 1843 to 1909 in the source material.
Q6. In his experiment, Engelmann used a prism in order to:
Using a prism he split light into its spectral components and then illuminated the alga with this split spectrum.
Q7. Which green alga did Engelmann place in a suspension of aerobic bacteria?
Engelmann illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria.
Q8. In Engelmann's experiment, the aerobic bacteria were used to:
The bacteria were used to detect the sites of ##\ce{O2}## evolution, since they gather where oxygen is released.
Q9. Engelmann observed that the bacteria accumulated mainly in which regions of the split spectrum?
He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum.
Q10. What did Engelmann's bacterial accumulation pattern allow him to describe?
A first action spectrum of photosynthesis was thus described, and it resembles roughly the absorption spectra of chlorophyll a and b.
Q11. Engelmann's action spectrum resembles roughly the absorption spectra of:
The lesson states that the first action spectrum resembles roughly the absorption spectra of chlorophyll a and b.
Q12. In the mid-nineteenth-century empirical equation ##\ce{CO2 + H2O ->[light] [CH2O] + O2}##, what does ##\ce{[CH2O]}## represent?
The symbol ##\ce{[CH2O]}## represented a carbohydrate, for example glucose, a six-carbon sugar.
Q13. Cornelius van Niel, whose working lifespan is given as 1897 to 1985, based his milestone contribution mainly on studies of:
Van Niel was a microbiologist who based his conclusions on studies of purple and green bacteria.
Q14. Van Niel demonstrated that photosynthesis is essentially a light-dependent reaction in which:
He showed that hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates, summarised as ##\ce{2H2A + CO2 ->[light] 2A + [CH2O] + H2O}##.
Q15. When ##\ce{H2S}## is the hydrogen donor for purple and green sulphur bacteria, the oxidation product is sulphur or sulphate rather than ##\ce{O2}##. From this, van Niel inferred that the ##\ce{O2}## evolved by green plants comes from:
Because no ##\ce{O2}## is released when ##\ce{H2S}## is the donor, van Niel inferred that the oxygen evolved by green plants comes from ##\ce{H2O}##, not from carbon dioxide; this was later proved using radioisotopic techniques.