Cell level systems Study Guide

Electron microscopy has revolutionised our understanding of sub-cellular structures by providing vastly superior magnification and resolution compared to traditional light microscopy. Light microscopes use visible light and glass lenses to magnify specimens, but their resolution is limited by the wavelength of light. This means that details smaller than about 200 nanometres cannot be distinguished, making many internal cell organelles invisible.

Electron microscopes, on the other hand, use a beam of electrons instead of light and electromagnetic lenses to focus the image. Electrons have a much shorter wavelength than visible light, allowing electron microscopes to achieve resolutions up to 0.1 nanometres and magnifications of over 1,000,000x. This enhanced capability means that scientists can now visualise the intricate internal structures of cells that were previously unseen or poorly resolved. For example, the detailed internal architecture of mitochondria, the double membrane of the nucleus with its pores, the structure of ribosomes, and the network of the endoplasmic reticulum have all been revealed with clarity by electron microscopy. This ability to see finer details has been crucial for understanding the function of these organelles, as structure is intimately linked to function. Without electron microscopy, our knowledge of how cells work at a molecular level would be significantly constrained, as many fundamental cellular processes rely on the precise arrangement of these tiny components.

Photosynthesis is the process by which green plants and some other organisms convert light energy into chemical energy, in the form of glucose. This vital process requires carbon dioxide and water, and releases oxygen as a byproduct. The equation is: carbon dioxide + water --(light energy)--> glucose + oxygen. Several environmental factors can limit the rate at which photosynthesis occurs. These are known as limiting factors. The main limiting factors are temperature, light intensity, and carbon dioxide concentration.

Temperature directly affects the rate of photosynthesis because the reactions involved are controlled by enzymes. Like all enzyme-controlled reactions, the rate of photosynthesis increases with temperature up to an optimum point. Beyond this optimum, usually around 40-45°C, the enzymes begin to denature, causing the rate of photosynthesis to decrease rapidly and eventually stop. Denaturation means the active site of the enzyme changes shape, so it can no longer bind to the substrate.

Light intensity is crucial because light energy is absorbed by chlorophyll and used to power the conversion of carbon dioxide and water into glucose and oxygen. As light intensity increases, the rate of photosynthesis generally increases, assuming other factors are not limiting. At very high light intensities, another factor might become limiting, and the rate will plateau. If there is insufficient light, even with adequate temperature and carbon dioxide, the plant cannot photosynthesise efficiently.

Carbon dioxide concentration is another key limiting factor. Carbon dioxide is one of the raw materials for photosynthesis. As the concentration of carbon dioxide increases, the rate of photosynthesis also increases, provided that light intensity and temperature are not limiting. Eventually, if the carbon dioxide concentration continues to rise, the rate will plateau, indicating that another factor (like light or temperature) has become limiting.