Cell level systems Study Guide

Enzymes are biological catalysts, meaning they speed up the rate of biochemical reactions without being used up themselves. They are proteins with a specific three-dimensional shape, crucial for their function. Each enzyme has an 'active site', which is a unique region on its surface that perfectly complements the shape of a particular molecule called a 'substrate'. This complementarity is often described by the 'lock and key' model. Just like a specific key fits into a specific lock, a specific substrate fits into the active site of its corresponding enzyme. When the substrate binds to the active site, an 'enzyme-substrate complex' is formed. This binding can slightly alter the enzyme's shape to fit the substrate even more closely, a concept known as the 'induced fit' model. Within this complex, the enzyme facilitates the chemical reaction – either breaking down the substrate into smaller products or joining smaller substrates together to form a larger product. Once the reaction is complete, the products are released from the active site, and the enzyme is free to bind to another substrate molecule and catalyse the same reaction again. This explains why enzymes are efficient and can be reused many times. Factors like temperature and pH can affect the enzyme's shape and thus its active site, leading to denaturation if conditions are outside the optimal range, which reduces or stops its activity.

Photosynthesis is a fundamental biological process carried out by photosynthetic organisms, primarily plants, algae, and some bacteria. These organisms are unique because they can convert light energy, usually from the sun, into chemical energy in the form of glucose (a sugar). This process uses carbon dioxide from the atmosphere and water absorbed from the environment. Oxygen is produced as a by-product and released into the atmosphere. The chemical energy stored in glucose powers the organism's life processes, such as growth and reproduction. More importantly, photosynthetic organisms form the base of almost all food webs on Earth. They are known as producers because they create their own food and are not reliant on consuming other organisms. All other organisms, directly or indirectly, depend on these producers for their food and energy requirements. Herbivores consume plants directly, while carnivores consume herbivores, and omnivores consume both. The energy originally captured by photosynthesis is transferred through these feeding relationships. Therefore, photosynthetic organisms are crucial for generating the biomass (total mass of living organisms) that sustains nearly all life forms on our planet, making them the primary producers of food and energy globally.