Part of a lecture about photosynthesis, the process by which plants

Section 4. You will hear part of a lecture about photosynthesis, the process by which plants make their food. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. WOMAN: Good morning. Today's subject is photosynthesis, which is, without exaggeration, the process that feeds almost all life on Earth. Every meal you have ever eaten can be traced back to a plant capturing sunlight. Let's look at how it actually works, and why it matters so much. WOMAN: The word itself is a good starting point. Photosynthesis means, roughly, building with light. What a plant does is take three simple, cheap ingredients, and turn them into food. Those ingredients are water, drawn up from the soil through the roots; carbon dioxide, taken in from the air; and light, usually from the sun. From these, the plant manufactures a sugar called glucose, which is its fuel. WOMAN: The reaction takes place mainly in the leaves, and it depends on a remarkable green substance called chlorophyll. Chlorophyll is what makes leaves green, and its job is to absorb the energy in sunlight, that trapped energy is what powers the whole process. This is why most leaves are broad and flat, to catch as much light as possible, and why plants on a windowsill grow towards the window. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. WOMAN: Now, there's a by-product, and it is one of the most important substances on the planet. When the plant splits the water molecules, it releases oxygen, which it doesn't need and simply lets go into the air. In other words, the oxygen we breathe is essentially a waste product of plants making their food. It's worth pausing on that: our every breath depends on their leftovers. WOMAN: The gases move in and out of the leaf through tiny openings on the underside, called stomata. These pores can open and close, which lets the plant control water loss, on a hot, dry day it can close them to avoid drying out, though that also slows photosynthesis down. WOMAN: Finally, why does all this matter beyond the biology lesson? Two reasons. First, photosynthesis is the base of almost every food chain, the energy from sunlight, captured by plants, is what ultimately feeds the animals that eat them, and the animals that eat those. And second, by removing carbon dioxide from the atmosphere and locking it away, plants, and especially forests, play a central role in regulating our climate. That's where we'll finish. That is the end of Section 4. You now have some time to check your answers.