师：同学们好，今天我们要学习的内容是人教版生物七年级上册第三单元第五章第一节“光合作用吸收二氧化碳、释放氧气”的第一课时。全世界的生物呼吸和物质燃烧都要消耗大量氧气，而大气层中氧气仅占21% ，但大气中氧气含量并未减少，这是为什么呢？相信大家已经找到了答案，那就是绿色植物能够吸收二氧化碳、释放氧气，从而维持了大气中的碳氧平衡。
本节课，我们将通过学习科学家们的经典实验，了解光合作用的探究历程，进而归纳出光合作用的概念及反应式。光合作用的发现历程漫长而曲折，下面请同学们准备好本节课的学习任务单，跟随老师一起回到2000多年前，追寻数代科学家的足迹，回顾光合作用的发现之旅。
As a botanist, I'm fascinated by what makes plants grow. For instance, to produce all this color and diversity, you need just a few minerals and three basic ingredients: water, sunlight and carbon dioxide, the stuff that I'm breathing out right now. And that's all, nothing else. Plants turn these ingredients into food for growth and a waste product we find very useful, oxygen. It sounds simple, but this process is one of the most fascinating and complicated in the whole of science. It's called photosynthesis. It took the pioneers of botany over 400 years to work out why a leaf needs sunlight, what role water plays, and why a plant can't exist without carbon dioxide. Today, photosynthesis is at the forefront of scientific research. If we get this right, if we're able to learn from it, we might be able to produce very quickly a liquid fuel that we could then use in our cars. So no more gasoline, no more diesel, no more fossil fuel. Photosynthesis is taking place right now in every leaf of every plant. I find that amazing.
2000 years ago, people believed plants grew by “eating” soil. Van Helmont wanted to know if this was true, so he designed an experiment, one that hopefully wouldn't attract the attention of the Spanish Inquisition. Van Helmont used a willow tree and a wagon full of soil. I'm using a bay tree and less soil, but the principle is the same. The first thing Van Helmont did was weigh his tree and note its weight.
Next, he weighed the dry soil, because he didn't want water to affect its weight.
Van Helmont then planted his tree, watered it, and his experiment was ready to go.
Each of my bay trees represents a year in the growth of the willow tree that Van Helmont planted. He watched it grow for not one or two years, but tended the tree for five years and then reweighed it. After five years, the tree had gained a hefty 12 stone. Then Van Helmont dried and weighed the soil. It weighed almost exactly the same as it did five years ago. He concluded the tree had grown not by “eating” soil, but by “drinking” water.
After all this effort, Van Helmont decided not to publish his results. He was scared, and with good reason. His experiment relied on evidence, not faith. He didn't want to risk getting on the wrong side of the authorities again, so his results were only published after his death. For all his personal sacrifice, Van Helmont was wrong.
Water is important for plant growth, but it's far from the whole story. He missed something fundamental, and he wasn't the only one. I found a document at my botanic garden that shows how little people in the 17th century knew about plants. Look at this plan of the oxybotanic garden in 1675. Up in the top - right corner is a new addition, a house for plants. This was the pride and joy of the director back then, his big 17th - century project. But if you look closely, you can see why this wasn't a great success. There's something missing from this house. Virtually no windows, and those that are there are tiny. Hardly any windows and no glass in the roof, so plants were never going to grow much in here.
What's really interesting about this is that it clearly shows 17th - century botanists hadn't made the connection between plant growth and light. It sounds obvious to us today, but back then, many people believed that leaves grew by God's will. So suggesting sunlight played a part was pretty radical. It was an important step on the road to understanding photosynthesis.
In the spring of 1779, a brilliant Dutch physician took a carriage from London to enjoy the air in the English countryside. He didn't know it yet, but this pioneering doctor was about to open a new chapter in the story of how plants grow. His name was Jan Ingenhousz.
As a young man, he had a gift for science and medicine. Inspired by his father's work as an apothecary making remedies for ailments, his leap to fame didn't come from studying plants. He was a smallpox inoculator, a well - paid job. So Ingenhousz could afford to rent a plush villa. He exchanged the hustle and bustle of the city for the tranquility of the countryside, planning to write a book about smallpox. But it wasn't long before the countryside became a distraction.
It wasn't long before Ingenhousz put his book on the back burner. Instead, he turned his attention to the countryside and the plants that flourished all around him. He embarked on a series of experiments that would revolutionize our understanding of plants. In the late 18th century, it was fashionable among scientists to investigate gases. One eminent scientist suggested that plants gave off gas. Ingenhousz set up an experiment to find out if this was true. His test was simple. He collected leaves from his garden and put them in water.