IELTS Listening Online – Sample 1

1.   16,000

2.   34%

3.   growing conditions

4.   US / USA / United States

5.   dried

6.   softness / strength / resilience

Why is cotton in everything?

Centuries ago, the Inca developed ingenuous suits of armour that could flex with the blows of sharp spears and maces, protecting warriors from even the fiercest physical attacks. These hardy structures were made not from iron or steel, but rather something unexpectedly soft – cotton.

These thickly woven, layered quilts of cotton could distribute the energy from a blow across a large surface area, shielding warriors without restricting their mobility. These seemingly contradictory features – strength and flexibility, softness and durability – have their roots in the intricate biology of the nearly invisible cotton fibre.

These fibres begin life deep within a cotton flower, on the surface of a seed. As many as 16,000 fibres will festoon a single seed, bulging from the seed’s surface like miniature water balloons.

Each cotton fibre, no matter how large it grows, is made of just one cell. That cell has multiple layers of cell wall. After a few days, the sides of the first layer, called the primary cell wall, stiffen, pushing cell growth in one direction and causing the fibre to elongate. The fibre elongates quickly for about 16 days.

Then it begins the next stage; strengthening the cell wall. It does this by making more of the carbohydrate cellulose. Cellulose will make up 34% of the cell wall at this stage and swiftly increases. This new growth also reinforces the cell wall by going against the grain of the existing wall. The strengthened wall is more rigid, restricting further growth. That means if the fibre remodels its walls too early, it will be short, and ultimately make rough, weak fabrics. But if cell wall strengthening begins too late, the wall won’t be sturdy enough – producing fibres that are too weak to hold fabrics together well.

In ideal growing conditions – with the right temperature, water, fertilizer, pest control, and light – a cotton fibre can grow up to 3.6 centimetres long with only a 25 micrometre width. Long, fine fibres can wrap around one another better than shorter, less fine fibres, which means those long, fine fibres make stronger threads that hang together better as fabric. Cotton with these qualities has diverse uses, from soft textiles to the U.S. dollar bill, which is 75% cotton.

The next crucial stage of the cotton fibre’s growth begins as it thickens its secondary cell wall by depositing large quantities of cellulose into the secondary layer. Cellulose goes on to make up over 90% of the fibre’s weight. The more cellulose that gets deposited, the denser that secondary layer becomes and this determines the strength of the final fibre. This stage is essential for developing long-lasting material for the likes of, say, a t-shirt. The garment’s capacity to withstand years of washing and wear is largely determined by the density of that secondary cell wall. On the other hand, its softness is strongly influenced by the length of the fibre, established with the remodelling of the primary wall layer.

Finally, after about 50 days, the fibre is fully grown. The living matter within the cell dies off, leaving behind only the cellulose. The dried cotton seed pod, or boll, that surrounds the fibres cracks open, unveiling a burst of several thousand fibre cells in a fluffy mass. The thread-like fibres we see, thinner than a human hair, are the remains of those dense, dried out walls of cellulose.

Tens of thousands of these fibres spun into yarn will go on to make everything from fabric, to coffee filters, diapers, and fishing nets. And with the help of modern science, cotton might soon be softer, stronger, and more resilient than ever as researchers investigate how to optimise its growth based on nutrients, weather conditions, and genetics.

Source: TED Ed – Created by Michael R. Stiff 

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