A-Level Biology - Ocr Study Guide Rated A+

Define the term diffusion. {Ans: The net movement of a substance from an area of high concentration to an area of low concentration. It is passive, does NOT require ATP.}Describe three features of an effective transport system {Ans: A fluid to carry nutrients/oxygen/wastes around the body (blood); a pump to create pressure that will push the fluid around the body (heart); exchange surfaces (capillaries)}Describe how an antigen presenting cell leads to large numbers of T helper cells {Ans: APC binds specifically to a Th cell (clonal selection). This selected Th cell then proliferates by mitosis (clonal expansion)}How do prokaryotes divide? {Ans: Binary fission}Why is there a delay before the AVN depolarises the ventricular walls? {Ans: To allow the atria to finish contracting}Describe the role of mucous membranes as a primary non-specific defence {Ans: Mucus traps pathogens and contains lysozymes; phagocytes engulf and digest pathogens in mucus}Describe two adaptations of roots that could help a plant survive in arid conditions {Ans: Roots could be very long to reach water deep in soil; roots could be very widespread to absorb water from a large area when it does rain.}Explain the orientation of phospholipids within the