Organism level systems Study Guide

Living organisms, including humans, must maintain a stable internal environment to survive and function correctly. This process is called homeostasis. Cells and organs in the body operate best within narrow ranges of conditions, such as temperature, pH, and water potential. If these conditions deviate significantly from their optimum, enzymes can denature, metabolic reactions can slow down or stop, and cells can be damaged. For example, human body temperature must be kept around 37 °C for enzymes to function efficiently. If the temperature rises too high, enzymes lose their specific three-dimensional shape (denature), reducing or stopping their activity. If the temperature falls too low, enzyme activity decreases, slowing down vital metabolic processes. Similarly, blood glucose levels need to be regulated to provide a constant energy supply to cells without damaging them due to excessively high or low concentrations. Maintaining a constant internal environment is crucial to ensure that all cellular processes, from respiration to protein synthesis, can occur effectively, allowing the organism to respond appropriately to both internal changes (like increased metabolic activity) and external changes (like changes in ambient temperature). Without this precise control, organisms could not adapt or survive in fluctuating conditions, making homeostasis fundamental to life.

Hormones play a crucial role in many methods of contraception by mimicking the natural hormonal changes that occur during pregnancy, thereby preventing ovulation, thickening cervical mucus, or thinning the uterine lining. The primary hormones used are oestrogen and progesterone, or synthetic versions of them. Oestrogen, typically combined with progesterone, works by inhibiting the release of FSH (follicle-stimulating hormone) and LH (luteinising hormone) from the pituitary gland. FSH is responsible for stimulating egg maturation, and LH triggers ovulation. By suppressing these, ovulation is prevented. Progesterone, or progestogen-only contraception, primarily works by thickening the cervical mucus, making it difficult for sperm to pass through, and by thinning the lining of the uterus, making it unsuitable for implantation of a fertilised egg. Some progestogen-only methods also suppress ovulation.

Contraceptive methods can be broadly categorised into hormonal and non-hormonal types, each with advantages and disadvantages. Hormonal methods include combined oral contraceptives (the pill), progestogen-only pills, contraceptive patches, injections, implants, and hormonal IUDs. Their advantage is high effectiveness in preventing pregnancy, and some can regulate menstrual cycles or reduce heavy bleeding. Disadvantages can include potential side effects like mood changes, weight gain, headaches, or in rare cases, increased risk of blood clots. Non-hormonal methods include barrier methods like condoms and diaphragms, spermicides, copper IUDs, natural family planning, and permanent methods like sterilisation. Advantages of non-hormonal methods often include no hormonal side effects, and condoms also protect against sexually transmitted infections (STIs). Disadvantages can be lower effectiveness rates (especially for natural methods), user-dependence (e.g., remembering to use a condom every time), or invasive procedures for IUDs and sterilisation. Evaluation of these methods involves considering their effectiveness, side effects, convenience, cost, and protection against STIs. The most suitable method depends on an individual's health, lifestyle, and preferences.