Exercises: Bioelements and inorganic biomolecules

Explain the difference between primary bioelements, secondary bioelements and trace elements, and give a specific example of each of the three groups.

Look at the bar chart showing the abundance of the main bioelements in living matter. State which group of bioelements the four most abundant elements belong to, approximately what percentage of living matter they represent as a whole, and why they are considered essential for forming biomolecules.

You can do it on paper and send a photo with your answer.

State two functions of water in living organisms that are a direct consequence of its polarity, and briefly explain each of them.

A cell accumulates $\text{K}^+$ ions inside itself and maintains $\text{Na}^+$ ions on the outside. Explain what form these mineral salts are in within the cellular medium (dissolved or precipitated) and name two functions they can perform.

Relate the high specific heat capacity of water to thermoregulation in organisms and give an example showing this function in a specific living organism.

The human skeleton contains calcium salts in precipitated form. Name a specific mineral salt that forms it and explain what advantage is gained by it being precipitated rather than dissolved.

In an experimental hydroponic culture, the effect of bioelement deficiency on tomato seedlings is studied. Batch 1 grows in a medium lacking nitrogen and batch 2 in a medium lacking iron. After two weeks, both batches suffer severe chlorosis and growth arrest. A student claims that the consequences in batch 2 should be insignificant compared to batch 1, arguing that nitrogen accounts for around 3% of the dry mass of the plant (primary bioelement), while iron accounts for less than 0.01% (trace element).

1. a) Refute the student's argument by explaining the fundamental difference between the biological role of primary bioelements and that of trace elements.
2. b) Justify at a biochemical level why the deficiency of such a minuscule amount of iron blocks chlorophyll synthesis and photosynthesis with a severity comparable to the lack of nitrogen.

The elemental composition (% dry mass) of two unknown samples is analysed in the laboratory:

• Sample 1: Oxygen 62.4%, Carbon 19.8%, Hydrogen 9.5%, Nitrogen 3.1%, Calcium 1.4%, Iron 0.005%, Zinc 0.003%.

• Sample 2: Oxygen 46.6%, Silicon 27.7%, Aluminium 8.1%, Iron 5.0%, Calcium 3.6%, Carbon 0.03%.

1. a) Reasonably deduce which of the samples comes from living matter and which from the Earth's crust, justifying your choice based on the physicochemical properties of carbon that make it more suitable than silicon for forming biomolecules.
2. b) From the data of the biological sample, classify its elements into primary, secondary, and trace elements according to their relative abundance.
3. c) Justify why a trace element like zinc is just as essential for maintaining cellular life as carbon, despite being present in a proportion thousands of times lower.

Unlike almost all liquids, whose density increases continuously as they cool until solidifying, water reaches its maximum density at 4 °C (1.000 g/cm³) and decreases to 0.917 g/cm³ when freezing at 0 °C.

1. a) Explain the molecular cause of this thermal anomaly of water, describing how hydrogen bonds are reorganised in the transition from liquid water to ice.
2. b) Imagine a hypothetical scenario in which water did not present this anomaly and ice were denser than liquid water. Reasonably predict the thermal and physical evolution that a 20 m deep mountain lake would undergo during a winter with temperatures of -10 °C, and justify how it would affect the survival of benthic and nektonic aquatic organisms.