OCR GCSE Biology A Gateway revision notes
Organism level systems Revision Notes
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Osmotic Changes in Body Fluids and Their Effect on Cells Revision Notes
What is Osmosis?
Osmosis is defined as the net movement of water molecules across a partially permeable membrane. This movement occurs from a region where there is a higher concentration of water molecules (a dilute solution) to a region where there is a lower concentration of water molecules (a more concentrated solution). This passive process does not require energy and is fundamental for maintaining water balance in living organisms.
Effect of Osmosis on Animal Cells
Animal cells do not have a cell wall. In a dilute external solution (hypotonic), water moves into the cell by osmosis, causing it to swell and potentially burst (lysis). In a concentrated external solution (hypertonic), water moves out of the cell, causing it to shrink and shrivel (crenation). Both lysis and crenation are damaging and can lead to cell death.
Effect of Osmosis on Plant Cells
Plant cells have a rigid cell wall outside the cell membrane. In a dilute external solution, water enters the plant cell, and the cell becomes turgid as its membrane presses against the cell wall, preventing bursting. Turgidity is vital for plant support. In a concentrated external solution, water leaves the plant cell, causing the cell membrane to pull away from the cell wall (plasmolysis). This makes the plant cell flaccid and the plant wilts.
Importance of Maintaining Body Fluid Concentration
Maintaining the correct water concentration in body fluids is essential for proper cell function and overall organism health. The human body has regulatory mechanisms to ensure that the fluid surrounding cells remains isotonic, preventing excessive water movement into or out of cells. Disruptions to this balance can lead to severe health issues as cells cannot function correctly under extreme osmotic conditions.
Kidneys and Water Balance Revision Notes
The Role of Kidneys in Water Balance
The kidneys are vital organs responsible for osmoregulation, which is the maintenance of water balance in the body. They achieve this by filtering the blood and adjusting the amount of water reabsorbed back into the bloodstream. This fine-tuning ensures that the body's cells have the correct fluid environment to function properly, preventing them from becoming too dilute or too concentrated. This process is essential for overall homeostasis.
Producing Dilute or Concentrated Urine
When the body has excess water, the kidneys produce a large volume of dilute urine. This means less water is reabsorbed from the filtered blood back into the bloodstream, and more is excreted. Conversely, when the body needs to conserve water (e.g., due to dehydration), the kidneys reduce the volume of urine produced, making it more concentrated. This involves actively reabsorbing more water back into the blood, ensuring water is conserved.
Hormonal Control of Water Reabsorption
The regulation of water reabsorption is largely controlled by Antidiuretic Hormone (ADH). When the body is dehydrated, the pituitary gland releases more ADH. ADH increases the permeability of the kidney tubules to water, allowing more water to be reabsorbed into the blood. If the body is over-hydrated, less ADH is released, making the tubules less permeable to water, and therefore less water is reabsorbed, leading to more dilute urine.
Importance of Maintaining Water Balance
Maintaining the correct water balance is critical for health. If the body has too much water, cells can swell and burst. If there is too little water, cells can shrink due to dehydration, impairing their function. The kidneys, through their precise control over water reabsorption, prevent these extreme conditions, ensuring the internal environment remains stable for all bodily processes.
Hormones in Reproductive Technologies for Infertility (Higher) Revision Notes
The Role of Hormones in IVF (Higher)
In Vitro Fertilisation (IVF) is a common reproductive technology used to treat infertility. Hormones are crucial in several stages of IVF to manipulate the female reproductive cycle. The primary goal is to retrieve multiple mature eggs for fertilisation outside the body. This involves a carefully timed sequence of hormone administration to achieve specific physiological responses that would not naturally occur in a single menstrual cycle.
Follicle Stimulating Hormone (FSH) for Superovulation (Higher)
Follicle Stimulating Hormone (FSH) is widely used in IVF therapies. Women receive daily injections of FSH to stimulate their ovaries to produce multiple mature eggs. In a natural cycle, typically only one follicle matures and releases an egg. High doses of FSH encourage the development of several follicles, increasing the number of eggs that can be retrieved. This 'superovulation' significantly enhances the chances of successful fertilisation and embryo creation in the laboratory.
Luteinising Hormone (LH) Trigger and Progesterone Support (Higher)
After FSH treatment, when a sufficient number of follicles have matured, a hormone resembling Luteinising Hormone (LH), usually human chorionic gonadotropin (hCG), is administered. This 'trigger shot' induces the final maturation of the eggs and promotes their release from the follicles, precisely timing egg retrieval. Post-fertilisation and embryo transfer, progesterone supplementation is often prescribed. Progesterone helps to thicken and maintain the uterine lining, making it more receptive for embryo implantation and supporting early pregnancy development, thus reducing the risk of early miscarriage.
Addressing Male Infertility with Hormones (Higher)
While often associated with female fertility, hormonal treatments can also be used for male infertility, particularly in cases of low sperm count or poor sperm motility caused by hormonal imbalances. Hormones can stimulate the testes to produce more sperm or improve sperm quality. However, the application of hormone therapy in male infertility is generally less complex and less frequent compared to female infertility treatments, where multiple controlled steps are required for egg retrieval.
Hormones and the Menstrual Cycle Revision Notes
Introduction to Hormones in Reproduction
Hormones are chemical messengers produced by endocrine glands that regulate various bodily functions, including reproduction. In human females, specific hormones control the menstrual cycle, which prepares the body for potential pregnancy. Understanding the roles of these hormones is key to grasping reproductive biology. This complex interplay ensures the precise timing of egg release and uterine preparation.
Key Hormones of the Menstrual Cycle
There are four main hormones that orchestrate the menstrual cycle: Follicle-Stimulating Hormone (FSH), Luteinising Hormone (LH), oestrogen, and progesterone. FSH, produced by the pituitary gland, stimulates follicle development in the ovaries. LH, also from the pituitary, triggers ovulation. Oestrogen is produced by developing follicles and plays roles in uterine lining repair and LH release. Progesterone, from the corpus luteum, maintains the uterine lining after ovulation.
Stages of the Menstrual Cycle and Hormonal Actions
The menstrual cycle begins with menstruation (shedding of the uterine lining). FSH then rises, stimulating follicle growth. As follicles grow, they release oestrogen, which rebuilds the uterine lining and inhibits FSH. A peak in oestrogen then causes an LH surge, leading to ovulation around day 14. After ovulation, the empty follicle becomes the corpus luteum, producing progesterone to maintain the uterine lining. If no pregnancy, progesterone drops, leading to menstruation.
Interactions and Feedback Mechanisms
The hormones in the menstrual cycle work through feedback loops. High levels of oestrogen initially stimulate LH release (positive feedback), but later inhibit FSH release (negative feedback). Progesterone also provides negative feedback, inhibiting both FSH and LH, which prevents further follicle development and ovulation once an egg has been released and the uterus is prepared for implantation. This coordinated action ensures efficient reproductive control.
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