Beyond the answers - interesting background to this quiz

The gas that keeps us breathing

Photosynthesis is often summarised as plants "breathing in" carbon dioxide and "breathing out" oxygen, and that is close enough for a quiz answer. In reality the process runs in two linked stages: light-dependent reactions that split water molecules and release oxygen, and the Calvin cycle, which uses carbon dioxide to build sugars. Essentially all of the oxygen in Earth's atmosphere is a by-product of this water-splitting, which makes photosynthesis the reason complex animal life is possible at all.

A capital with a layered history

Antananarivo sits in the highlands of Madagascar, roughly in the centre of the island, and its name means "city of a thousand" in Malagasy. It grew around a royal court in the seventeenth century and later became the seat of the Merina kingdom before French colonial rule reshaped the city. Today it remains the political and cultural heart of Madagascar, an island whose wildlife evolved in near isolation for tens of millions of years.

The organelle with its own DNA

Calling mitochondria the "powerhouse of the cell" is a useful shorthand for their role in producing ATP, the molecule cells use as fuel. What makes them unusual is that they carry their own small loop of DNA, separate from the DNA in the nucleus, and they are inherited almost entirely from the mother. This has led to the widely accepted idea that mitochondria descend from free-living bacteria that were absorbed into an early ancestor cell.

A river of many names and many countries

The Niger is one of Africa's great rivers, rising in the highlands of Guinea and curving through Mali, Niger and Nigeria before emptying into the Gulf of Guinea through a vast delta. Its course puzzled early European geographers, who long assumed such a large river must flow to the sea by a shorter route. The delta it forms is one of the largest in the world and supports dense mangrove forests and a major oil-producing region.

Vitamin D and the calcium connection

Vitamin D's best-known job is helping the intestine absorb calcium, which is why a deficiency can weaken bones even when calcium intake is adequate. Unlike most vitamins, the body can make it itself: sunlight on the skin triggers a chain of reactions that produces the active form. This also explains why deficiency is more common at high latitudes and during winter, when sun exposure drops.