Principles of Organisation & Enzymes

Levels of organisation

• Cells are the basic building blocks of all living organisms
• A tissue is a group of cells with a similar structure and function: muscle tissue, epithelial tissue, xylem
• An organ is a group of different tissues working together to perform a specific function: the stomach has muscle tissue, glandular tissue and epithelial tissue
• An organ system is a group of organs working together: the digestive system. Organ systems work together to form an organism

Enzymes

• Enzymes are proteins that catalyse (speed up) specific reactions in living organisms without being used up. They control the reactions of metabolism, the sum of all the reactions in the body
• Each enzyme has an active site with a specific shape. Only a substrate with a complementary shape fits, which is why each enzyme catalyses only one reaction: the lock and key theory
• The substrate binds to the active site, the reaction happens, and the products are released. The enzyme is unchanged and can be used again

Temperature and pH

• As temperature rises, the rate increases: particles have more energy and collide more often with the active site
• Above the optimum temperature (about 37 °C for human enzymes), the rate falls sharply: the enzyme denatures. The bonds holding it in shape break, the active site changes shape, and the substrate no longer fits
• Each enzyme has an optimum pH. Most work best near pH 7, but pepsin in the stomach works best at about pH 2. Too far from the optimum and the enzyme denatures in the same way

• Rate calculations: rate $=$ amount of substrate broken down (or product made) $\div$ time. If starch is fully digested in 40 s, the rate is $\frac{1}{40} = 0.025$ per second. When a fixed amount is used up, a shorter time means a faster rate