Organism level systems Study Guide

The human reproductive system, particularly in females, is intricately regulated by a series of hormones. These chemical messengers are produced by endocrine glands and travel through the bloodstream to target organs, where they exert their specific effects. In females, the menstrual cycle is a monthly process that involves the maturation and release of an egg, and the preparation of the uterus for a potential pregnancy. This cycle typically lasts around 28 days and is controlled by four key hormones: Follicle-Stimulating Hormone (FSH), Luteinising Hormone (LH), oestrogen, and progesterone.

FSH is produced by the pituitary gland and stimulates the development of follicles in the ovaries. As these follicles mature, they produce oestrogen. Oestrogen itself has several roles: it promotes the repair and thickening of the uterine lining, inhibits FSH production (a negative feedback mechanism), and stimulates the release of LH from the pituitary gland when it reaches a high level. The surge in LH triggers ovulation, which is the release of a mature egg from the ovary, typically around day 14 of the cycle.

After ovulation, the empty follicle transforms into a structure called the corpus luteum, which then produces progesterone. Progesterone's primary role is to maintain the thickened uterine lining, making it ready for the implantation of a fertilised egg. It also inhibits the release of both FSH and LH, preventing the development of new follicles and further ovulation during a potential pregnancy. If fertilisation and implantation do not occur, the corpus luteum degenerates, causing a drop in progesterone and oestrogen levels. This drop leads to the shedding of the uterine lining, which is menstruation, and the beginning of a new menstrual cycle.

Plant hormones, specifically auxins, play a crucial role in controlling and coordinating plant growth and development. These hormones are chemical messengers produced in one part of the plant and transported to other parts where they elicit a response. Auxins are primarily produced in the tips of shoots and roots. They influence cell elongation, which is fundamental to plant growth movements known as tropisms.

Phototropism is the growth response of a plant towards or away from a light source. In shoots, auxins move away from light to the shaded side. A higher concentration of auxins on the shaded side promotes cell elongation, causing the shaded side to grow faster than the lit side. This differential growth causes the shoot to bend towards the light, an example of positive phototropism. In roots, however, a higher concentration of auxins inhibits cell elongation, so roots tend to grow away from light.

Gravitropism (also known as geotropism) is the growth response of a plant towards or away from gravity. In shoots, gravity causes auxins to accumulate on the lower side. Here, the higher auxin concentration promotes cell elongation, causing the lower side to grow faster and the shoot to grow upwards, exhibiting negative gravitropism. In roots, auxins also accumulate on the lower side due to gravity. However, in roots, this higher auxin concentration inhibits cell elongation, causing the upper side to grow faster. This results in the root bending downwards, displaying positive gravitropism. These coordinated responses, mediated by auxins, ensure that plants can effectively orient themselves for optimal light absorption and anchorage, as well as water and nutrient uptake.