Homeostasis, Response, and Regulation in Living Organisms

Posted on Mar 19, 2024 in Biology

Homeostasis

Homeostasis is the maintenance of a constant internal environment to ensure optimum conditions for enzyme and cellular function. It controls body temperature, blood sugar concentration, water levels, etc. Nervous and hormonal communication is involved in the automatic control system. All control systems have receptors (cells that detect stimuli), coordination centers (to process information), and effectors (the thing that brings about the response – muscle or gland).

Human Nervous System

Normal Nervous System

1. Stimuli
2. Receptor cells convert the stimulus into an electrical impulse
3. Sensory neurons
4. CNS (spine and brain), where it is processed
5. Motor neuron
6. Effector

The reflex arc is slightly different:

1. Stimuli
2. Receptor cells convert the stimulus into an electrical impulse
3. Sensory neuron
4. Relay neuron – loops around vertebrae
5. Motor neuron
6. Effector

They are done instantly, to have a response as soon as possible to avoid harm (e.g., touching a hot object). Synapses are gaps between two neurons. When an electric signal reaches the end of the pre-synaptic neuron, it becomes a chemical that diffuses across the synapse and when it reaches the post-synaptic neuron, it turns back into an electrical impulse that goes down the neuron (think – psych neurotransmitters). Your reaction is how long it takes you to respond to a stimulus.

Reaction Times (Ruler Drop) RP

1. Person 1 drinks caffeinated coke and holds a ruler vertically by the 0 cm mark.
2. They then drop the ruler and catch it again as fast as they can
3. Person 2 records the measurement that is level with the top of person 1’s thumb when they catch it.
4. Repeat test three times and find an average. The closer the number is to 0, the faster reaction time they have.
5. Repeat the experiment with person 2, who has drunk decaf coke. It should be done as a double-blind placebo test, so any psychological effect is eliminated.

The Brain

Cerebral Cortex – controls consciousness, intelligence, memory, and language; it is the outer 3 mm of the brain.

Cerebellum – controls fine movement of muscles; rounded structure towards the bottom/back of the brain.

Medulla – controls unconscious actions such as breathing and heart rate, found in the brain stem in front of the cerebellum.

Investigating brain function and treating brain damage and disease is difficult because it is complex and delicate, and easily damaged. Also, drugs given to treat diseases cannot always reach the brain because of the membranes that surround it, and it’s not fully understood which part of the brain does what.

Neuroscientists can map out the regions of the brain using a number of methods, such as studying patients with brain damage and observing the changes in an individual following damage on a certain area of the brain can provide information on the role this area has. Electrically stimulating different parts of the brain by pushing an electrode onto the brain, which results in a mental or physical change that the patient can describe (think Penfield), or using MRI scanning. A magnetic resonance imaging scanner can be used to create an image of the brain. This can be used to show which part of the brain is affected by a tumor, or which part is active during a specific task.

The Eye

Retina – layer of light-sensitive cells found at the back of the eye. When light hits it, the cells are stimulated, impulses are sent to the brain which interprets the information to create an image.

Optic nerve – nerve that connects the eye and the brain. It carries the impulses from the retina to the brain to create an image.

Sclera – white outer layer which supports the structures inside the eye. It is strong to prevent damage to the eye.

Cornea – see-through layer at the front of the eye. It allows light through and the curved surface bends and focuses light onto the retina.

Iris – Muscles that surround the pupil. They contract or relax to alter the size of the pupil. In bright light, the circular muscles contract and radial muscles relax to make the pupil smaller which avoids damage to the retina. In dim light, the circular muscles relax and the radial muscles contract to make the pupil larger so more light can enter to create a better image.

Ciliary muscles and suspensory ligaments – Hold the lens in place, they control its shape. To focus on a near object – the ciliary muscles contract, the suspensory ligaments loosen, the lens is then thicker and more curved – which refracts the light more. To focus on a distant object – the ciliary muscles relax, the suspensory ligaments tighten, the lens then becomes thinner so light is refracted less.

Eye defects occur when light cannot focus on the retina. Short-sightedness (myopia) is when the lens is too curved, so distant objects appear blurry. Long-sightedness (hyperopia) is when the lens is too flat, so it cannot refract light enough.

Treatments for the Eye

Spectacle lenses (glasses) – concave lenses to spread out the light to treat myopia and convex lenses to bring the rays together to treat hyperopia.

Contact lenses – work in the same way as glasses, but sit on the eye, hard or soft contact lenses last for different lengths of time.

Laser eye surgery – lasers can be used to either reduce the thickness of the cornea (so it refracts light less) to treat myopia or change its curvature (so it refracts light more strongly) to treat hyperopia.

Replacement lens – Hyperopia can be treated by replacing the lens with an artificial one made of clear plastic (or adding the plastic on top of the natural lens). The risks include damage to the retina or cataracts developing.

Thermoregulation

Human body temperature is 37.5°C. If it becomes too high then sweat secretes from the skin, which evaporates off the skin leaving the skin cooler. Vasodilation also occurs, where blood vessels expand (makes skin go red, causes blushing). Hairs become flat to stop any air from being trapped. If it becomes too low, then skeletal muscles contract rapidly to generate heat from respiration (shivering). Hairs stand on end to trap warm air. Vasoconstriction happens which means that blood vessels constrict/become smaller (results in paler skin).

Human Endocrine System

The endocrine system sends hormones (chemical messengers) around the body. When they reach the target tissue, they produce a response.

Pituitary gland – aka master gland, secretes hormones into the blood to act on other glands to stimulate them to produce hormones.

Pancreas – secretes insulin, controls blood sugar levels.

Thyroid – secretes thyroxine, controls metabolic, heart rate and temperature. Adrenal gland – secretes adrenaline, involved in fight or flight response. Ovary – secretes oestrogen, involved in menstrual cycle. Testes – secretes testosterone. Blood sugar control: controlled by hormones. Too much sugar – INSULIN is released from pancreas, which travels through bloodstream to liver. Liver then recieves the hormone, and in response releases GLYCOGEN for the sugar to be stored in, which lowers the concentration of sugar in the bloodstream. Not enough sugar – GLUCAGON released from pancreas which goes to liver. In response, liver releases sugar from the GLYCOGEN stores to bring up the blood sugar level. Negative feedback loop –  once the hormone has done the desired task, another hormone is released to inhibit the first hormone, and so the hormone level is stable. Diabetes: Type 1 – Pancreas can’t produce enough insulin. Blood sugar can rise to a fatal amount. Glucose is secreted with urine, and lots of urine is produced leaving the individual very thirsty. Treated with insulin injections at meal times. Type 2 – Body cells no longer respond to insulin. Blood sugar can rise to a fatal amount. Obesity is a risk factor. Treatments include lifestyle change (increase in excerice and diet change). Drugs can be taken to make insulin more effective on body cells. Maintaining water levels: Too much water – body cells take up water (due to osmosis). They expand and can burst. Not enough water – water moves out the cells (osmosis), cells shrink as they do so. If cells loose or gain too much water, they wont be able to function properly. The Kidneys – As blood moves through the body, it makes urine by filtering out the waste products and selectively reabsorbing useful substances such as glucose and water. Waste products produced –  Water – can also leave the body through exhalation or sweat. Depending on the concentration of water in the blood, a certain amount of water is lost as urine. Ions – taken in via food. If ion concentration is incorrect, then incorrect amount of water may enter the cells. In the kidneys, some ions are reabsorbed into the bloodafter it has been filtered to ensure the concentration in the blood is maintained They can aslo be lost through sweat. Urea – amino acids are deaminated in the liver to form ammonia (which is toxic) and converted into urea. Anti diuretic hormone (ADH) is a hormone involved in the control of the loss of water as urine. It is released into the pancreas by pituitary gland when a receptor in the brain detects that the blood is too concentrated. It travels in the bloodstream to the kidney tubules. An increased amount of ADH reaching the tubules increases their permeability to water, so more moves out of the tubule and back into the bloodstream, this results in a smaller volume of more concentrated (yellow) urine and the blood becoming less concentrated as more water moves into it. (example of negative feedback). Problems with the kidney: Kidney failure is when your kidneys stop working, which can result in a build up of waste product, which can be life threatning. There 2 ways to treat it 1) Dialysis – the function of the kidneys is carried out using an artificial membrane . Blood moves between semi permeable membranes surrounded by dialysis fluid (has the same concentrations of ions and glucose as healthy blood). Useful ions and glucose are not lost from the blood but urea, excess ions and water diffuse across the membrane. It has to be done three times a week, with the process itself taking three to four hours. S – It acts as an artificial kidney and keeps people alive while they wait for a transplant. W – possibility of blood clots forming, takes a long time and you have to follow a strict diet. It is also an unpleasant experience, and the build up of waste products between sessions can leave you feeling ill. 2) Kidney Transplants – currently the only cure, transplanted from people who have died suddenly or a healthy person with both kidneys functioning. S – new finctioning kidney. W – doesnt last forever, could be rejected (can be surpressed with immunosuppressant drugs). Menstrual Cycle: begins with uterus lining breaking down, then woman has her period, then the layer builds up again until ovulation (day 14) occurs – egg is released from ovary and moves down to the uterus via the fallopian tube. The events are controlled by 4 hormones – 1) follicle stimulating hormone (FSH) causes maturation of the egg. produced in pituitary gland and stimulate ovaries to produce oestrogen 2) oestrogen – causes lining of uterus to thicken. produced in ovarties. stimulates production of LH and inhibits production of FSH 3) LH – produced in pituitary gland and stimulates ovulation (release of the egg). Inhibits oestrogen production 4) progesterone – released from empty follicle (holds egg) maintains lining of the womb. The woman will then either get pregnant or menstruate which will continue the cycle. Contraception:  hormonal methods – contraceptive pill – contains oestrogen and progesterone, which inhibits FSH and stops development of the womb lining. must be taken daily, and has many side effects (depression, breast pain, mood swings, increase in blood pressure). patch – same as pill, but applied as a patch stuck on the skin and lasts for a week. Implant – releases continuous amounts of progesterone and lasts for 3 years. Injection – same effect as implant, lasts for 2 to 3 months. IUD – inserted in uterus and has same effect as implant. can also be made of copper which stops any fertilised embryos from implanting in uterus lining. non hormonal methods – chemical methods such as spermicides kill/disable sperm, but are only 70% – 80% effective. barrier methods such as condoms or diaphragms, these are the only ones that can also prevent STDs. however, they can be unconfortable. a vasectomy (surgery) can also be done, but is irreversible. abstaning is the only one that has 100% effect. IVF: fertility drugs can also be used to help. IVF (In vitro fertilisation) – egg and sperm extracted and develop into embryos in a lab. Once the embryos form, it is inserted back into the woman’s womb (usually about 5 inserted). S – provides a way for an infertile couple to have a child W – 26% success rate, physically stressful, can lead to multiple births, can be expensive. Plant Hormones: Tropism is growth in response to a stimuli. Phototropism – plants grow in response to light. positive phototropism grows towards light (leaves/shoots) negative phototropism grows away from light (roots). Geotropism – plants grow in response to gravity, positive towards gravity (roots), negative away from gravity (leaves/shoots). both are controlled by auxins. they Stimulate growth in Shoots, and Restrict growth in Roots. Gibberellins are important to stimulate seed germination. Ethene is involved in cell division and fruit ripening. Uses of plant hormones: Auxins – used as weed killers. many weeds are broad leaved, and weedkillers contaning auxin are sunthesized so they only affect broad leaved plants. increased amount of auxin causes the cells to grow too rapidly, resulting in the weed dying. As rooting powders – Plants with desirable features are cloned to make more plants with the same feature, this can be done by using a cutting from the orgininal plant. Rooting powder containing auxin is applied to it and it is placed in the ground. Roots grow and the new plant begins to grow very quickly. To promote growth in tissue culture – Another way to clone a plant is to use tissue culture. Cells from the plant are taken are placed in a growth medium containing lots of nutrients. Hormones such as auxins are added. The cells begin to form roots and shoots. As ethene controls ripening, it is used in the food industry. Fruit is picked when it is not ripe. It is firm which means that during transport it gets less bruised and damaged. When it is needed to be sold, it is exposed to ethene and warmer temperatures. This reduces wastage as more fruit is suitable to be sold and it does not ripen too early. Gibberellins – Ending seed dormancy. In the brewing industry, the germination rate of barley seeds is increased to make malt. Promoting flowering – Instead of requiring certain conditions such as longer days and low temperatures to flower, applying this hormone allows it to flower in any conditions and with bigger flowers.Increasing fruit size – The seeds in fruit produce gibberellins to increase fruit size. This means that seedless fruit is generally smaller. Seedless fruit can be sprayed with gibberellins to increase their size.