A-Level Biology

16:26 Topic 1 How We Study Cells You are built from about thirty trillion cells, yet you have never seen a single one. Cells are simply too small for the eye. A grain of sand is about a… 19:33 Topic 2 Biological Molecules Suppose you have an unknown food sample. How do you tell what it is made of? Biology has a beautiful trick: add the right chemical, and the colour tells you… 13:52 Topic 3 Enzymes Inside every cell, thousands of chemical reactions happen every second — far faster than they ever could on their own. The secret is a special kind of protein… 14:03 Topic 4 Cell Membranes Every cell must keep the right things in and the wrong things out. That job falls to a single, astonishingly thin layer — the cell surface membrane, just two… 7:19 Topic 5 Mitosis Right now, inside you, millions of cells are dividing. A cell grows, copies everything it contains, and splits into two — and the two daughter cells are not… 9:34 Topic 6 DNA & Protein Synthesis Inside almost every one of your cells sits a molecule about two metres long, folded up so tightly you cannot see it. It is DNA, and it is written in an… 8:36 Topic 7 Transport in Plants A giant redwood lifts water more than a hundred metres into the air, every single day — and it has no heart, no muscle, no pump of any kind. So what does the… 9:17 Topic 8 The Circulatory System Your heart beats about a hundred thousand times a day, and it never gets a day off. But here is the clever part. It is not one loop. It is two. Blood leaves… 6:58 Topic 9 The Gas Exchange System Spread the inside of your lungs out flat and it would cover about seventy square metres — roughly half a tennis court. That whole area is folded up inside your… 8:28 Topic 10 Infectious Disease The animal that kills the most people every year is not the shark or the snake. It is the mosquito. And the strange thing is that the mosquito itself does… 8:37 Topic 11 Immunity Catch measles once and you will almost certainly never catch it again. Your body has not become tougher, and the virus has not become weaker. What has changed… 9:49 Topic 12 Respiration At any moment your body holds only about fifty grams of ATP. Yet over a single day you will make and spend roughly your own body weight of it. The same… 8:58 Topic 13 Photosynthesis Here is something strange about a tree. Almost none of it came out of the ground. The wood, the leaves, the fruit — nearly all of that mass was pulled out of… 9:42 Topic 14 Homeostasis Look at a day's worth of blood sugar. Three big meals go in — and the level hardly stirs. It wanders a little, then comes straight back. That is not luck, and… 9:38 Topic 15 Nerves and Muscles Touch something hot and your hand is already moving before you feel the pain. The signal never went to your brain to ask permission. It ran into the spinal… 9:35 Topic 16 Genetics You have twenty three pairs of chromosomes. When a gamete is made, each pair lines up independently — either way round — so the number of different gametes you… 8:23 Topic 17 Variation and Evolution In the early eighteen hundreds, almost every peppered moth in England was pale and speckled, which hid it beautifully on lichen-covered bark. Then the… 7:58 Topic 18 Biodiversity About one point seven million species have been described and given a name. The best estimates say there are probably eight to ten million out there. So most… 8:31 Topic 19 Genetic technology Two cotton plants. Same field, same insects, same week. The one on the left is barely touched. The one on the right has been eaten to bare stalks. The…