GCSE Biology B5: Homeostasis

Homeostasis

• regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes

• maintains optimal conditions for enzyme action and all cell functions

• in the body: homeostatis controls: blood glucose conc, body temp, water levels

• nervous and hormonal communication is involved in the automatic control systems which detect changes and respond to them

all control systems have:

• receptors: cells that detect stimuli (changes in the environment)

• coordination centres: process the information received from the receptors e.g. brain, spinal chord, pancreas

• effectors: bring about responses to bring the conditions in the body back to optimum levels e.g. muscles or glands

Human Nervous System

• receptor cells in the skin detect a stimulus and convert it into an electrical impulse

• this electrical impulse travels along the sensory neuron

• The electrical impulse gets converted to a chemical neurotransmitter to diffuse through the synapse to the sensory neurons in the central nervous system (CNS: either the brain or spinal cord) if at the brain, the info is processed, and an appropriate response is coordinated

• goes to the brain for complex, conscious actions such as (touch, pain, needs thinking) and to the spinal cord for immediate automatic, unconscious reflex actions (pulling hand from heat)

• then diffuses through the synapse again as a neurochemical transmitter to the motor neuron and back into an electrical impulse to be sent to the efforctors which carry out the response

• this can be muscles contracting or glands secreting hormones

reflex actions

• automatic and rapid actions that do not involve the conscious part of the brain

• important to prevent individuals from getting hurt

• information travels down the reflex arc, allowing vital responses to take place quickly

diffusion through the synapse

• nerve impulse arrives at the synpase

• chemical messenger (a neurotransmitter) is released into the synpase. impulses causes vesicles to fuse with neuron membrane

• neutransmitter diffuses across the synpase

• neurotransmitter binds with a receptor on the membrane of the post synaptic neuron

• binding of neurotransmitter to receptor stimulates impulse in postsynaptic neuron

• There are junctions between neurones

• Each time the electrical impulses meets the end of a neurone, a chemical transmitter is released

• This diffuses across the gap, binds to receptors and starts another impulse in the next neurone

reaction time

• how long it takes you to respond to a stimulus. can be measured with ruler drop test

The brain

• controls complex behavior

• made of billions of interconnected neurones and has different regions that carry out different functions

Name the parts of the brain

• cerebral cortex
  control consciousness, intelligence, memory and language

• Hypothalamus
  helps releases hormones and regulates body temp

• pituitary gland
  takes messages from the brain (via hypothalamus) to produce hormones that affects many parts of the body

• medulla (oblongata)
  controls unconsciousness activity such as heartbeat, movement of the gut and breathing

• cerebellum
  controls muscle activity and balance

• spinal chord
  a long, thin tubular structure made up of neuron tissue connected to the medulla

investigating brain function and treating brain damage and disease is difficult because:

• it is complex and delicate

• it is easily damaged

• drugs given to treat disease cannot always reach the brian because of the membranes that surround it

• it is not fully understood which part of the brain does what

neuroscientists can map out the regions of the brain using a number of methods:

• studying patients with brain damage
  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 which can be done by pushing an electrode into the brain. the stimulation may result in a mental or physical change in the individual, providing information on the role this area of the brain has

• using MRI scanning techniques
  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 tumour, or which part is active during a specific task

The eye

Parts of the eye

• Retina: layer of light sensitive cells found at the back of the eye

• Optic Nerve: a nerve that leaves the eye and leads to 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 some damage to the eye

• Cornea: The see-through layer at the front of the eye which allows light through and the curved surface bends and focuses light onto the retina

• Iris: Muscles that surround the pupil. In bright light, the circular muscle contract and radial muscles relax to make the pupil smaller, avoiding 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 and they control its shape

To focus on a near and far object

• near:
  - ciliary muscles contract
  - suspensory ligaments loosen
  - lens is then thicker and more curved this refracts light more

• far:
  - ciliary muscles relax
  - suspensory ligaments tighten
  - the lens then becomes thinner so light refracts less

Eye defects

• oxxur when light cannot focus on the retina

• short sightedness: myopia: lens is too curved and thick or eyeball is too long, distant objects appear blurry

• long sightedness: hyperopia: lens is too flat or thin or eyeball is too short so it can not refract light enough

treatment methods for eye defects

• spectacle lenses: concave lenses to spread out the light for myopia and convex lenses to bring the rays together to treat hyperopia

• contact lenses: work in the same way as glasses but allow acitivites such as sport, hard or soft contact lenses last for different amounts 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 retina or cataracts developing

Control of body temperature

• thermoregulatory centre monitors and controls body temp is found in the brain

• has receptors that monitor the temp of the blood

• has receptors in the skin that sends impulses to the thermoregulatory centre

• human body temp is 37.5 degrees celsius

when body temp is too high

• sweat (evaporates from the skin surface resulting in increased energy transfer away from the body) is produced from sweat glands

• vasodilation means more blood flows closer to the surface of the skin, resulting in increased energy transfer from the body

• hairs lay flat which means less nsulating air is trapped next to the skin

when body temp is too low

• skeletal muscles contract rapidly (shivering) to generate heat from respiration

• hairs stand on end to create an insulating layer trapping warm air

• vasoconstriction means blood does not flow so close to the surface, reuslting in less ehat loss

Endocrine system

• composed of glands which secrete chemicals called hormones directly into the bloodstream

• the blood carries the hormone to a target organ where it produces an effect

• the effects are slower but act for longer compared to nervous system

glands

• pituitary gland:
  - master gland
  - secretes hormones into the blood to either have an effect on the body or act on other glands to stimulate them to produce different hormones

• pancreas:
  - secretes insulin
  - controls blood glucose levels

• thyroid
  - secretes thyroxine
  - controls metabolic rate, heart rate and temp

• adrenal gland:
  - secretes adrenaline
  - involved in fight or flight response

• ovary:
  - secretes oestrogen
  - involved in the menstrual cycle and development of female secondary characteristics

• testes:
  - secretes testosterone
  - is involved in production of sperm and development of male secondary sexual characteristics

Control of blood glucose concentration

• controlled by pancreas

• eating foods that contain carbohydrates increases the glucose levels in the blood

if glucose levels are too high

• pancreas produces the hormone insulin

• insulin binds to cell in target organs (muscles and liver) causing:
  - glucose to move from the blood into muscle cells for respiration
  - excess glucose to be converted into glycogen which is stored in the liver

• blood glucose concentration is reduced

if blood glucose is low

• rigorous acitivity (exercise) uses glucose for respiration and therefore there is less in the blood

• if glucose levels decrease, the pancreas produces a hormone glucagon

• glucagon binds to the liver cells causing glycogen to be broken down into glucose

• glucose is released into the blood, increasing the blood glucose conc

negative feedback loop

• when blood glucose levels increase/decrease, a hormone is secreted to oppose the change

• the action of this hormone cannot occur continually because when the blood arrives at a certain glucose concentration the other hormone is produced resulting in the opposite effect

type 1 diabetes

• heridatory

• pancreas cannot produce enough insulin

• blood glucose level can rise to a fatal amount

• glucose is excreted with urine and lots of urine is produced leaving individual very thirsty

• it is treated with insulin injections at meal times, which results in glucose being taken up from the bloodstream

• it is advised to limit intake of simple carbs which contain lots of glucose

• doctors are attempting to cure diabetes with pancreas and pancreatic cell transplants, and genetically engineering pancreatic cells from mice to make insulin

type 2 diabetes

• the body cells no longer respond to insulin

• blood glucose levels can rise to a fatal amount

• obesity can cause this

• treatments include reducing number of simple carbs in diet, losing weight and inreasing exercise

• there are also drugs to make insulin more effective on body cells, help the pancreas make more insulin or reduce the amount of glucose absorbed from the gut

if water conc of the blood increases

• hypothalamus sends signal to pituitary gland

• Gland produces little to no hormones (ADH)

• The kidney tubules are then less permeable to water and more water is then released into the bladder to be urinated

if water conc of the blood decreases

• If there is too little water in the blood, there will be low water potential which is dangerous.

• so the hypothalamus sends a signal to the pituitary gland. The gland will produce hormones such as ADH

• This travels via the blood to the kidney tubules. They tell the kidney tubule to reabsorb more water back into the blood.

• They do this by making the kidney tubules more permeable to water (increases the permeability to water).

• More water is reabsorbed back into the blood

• example of negative feedback loop

Kidneys

• very important to maintain balance of water and other substances in the body

• The kidney filters out the blood

• ultrafiltration of glucose, ions, water and urea in the nephron (kidney tubules)

• selectively reasborbs all of the glucose, some ions, some water, release urea to the bladder

Why is it hard to cool down in humid countries?

• Because there is a high concentration in the atmosphere the water doesn't want to evaporate from your skin as evaporation happens from a low to high concentration but there is already a high concentration of water in the atmosphere

   

  Humidity can reduce the rate of sweat evaporation making it difficult to regulate the body's temperature

water

• can also leave the body at the lungs as a result of exhalation or from the skin in sweat

• can also be lost as urine depending on the conc of water in the blood

Ions

• taken into the body via food

• if the ion conc is incorrect, then too little or too much water may enter body cells. this is because the water potential of the blood would be altered

• they are lost in sweat

• in the kidneys, certain amounts of ions are reabsorbed into the blood after it has been filtered to ensure the conc in the blood is maintained

Urea

• amino acids are the products of digestion of proteins. Amino acids are deaminated in the liver to form ammonia. As ammonia is toxic it is converted immediately to urea and excreted safely

• it is lost in sweat

• in the kidneys it is filtered out of the blood

Kidney failure

• waste products build up which can be harmful for the body, eventually results in death

• Dialysis: function of the kidneys is carried out using an artificial membrane:
  - blood moves between partially permeable membranes surrounded by dialysis fluid (has the same conc 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 3-4 hours
  - it acts as an artificial kidney and keeps people alive while they wait for a transplant

• kidney transplants: provides individual with healthy kidney
  - currently the only cure
  - they are transplanted from people who have died suddenly, or from people still alive
  - mean that the person does not have to visit the hospital as regularly as they did when undergoing dialysis and it is cheaper than the overall cost of dialysis

Problem for dialysis treatments (5)

• posibility of blood clots forming

• amount of time it takes

• have to follow strict diet

• unpleasant experience

• build up of waste products between sessions can make you feeling ill

Problems of kidney transplant (4)

• rejected from the body as a result of immune system recognising the antigens on the donor organ as foreign

• change of this happening can be reduced by using immunosuppressant drugs

• however this means that the person can be left vulnerable to other diseases

• transplanted kidneys do not last forever

puberty

• reproductive hormones begin to be released

• causes secondary sexual characteristics to develop in men and women and eggs to mature in women

main male reproductive hormone

• testosterone

• produced by testes

• stimulates sperm production

main female reproductive hormone

• oestrogen

• produced in ovary

• produces physcal changes and is involved in the menstrual cycle

Menstrual cycle

• process the body undergoes each month to prepare for a potential pregnancy

• day 1-5: uterus lining breaks down and the woman is on her period

• day 6-10: repair of uterus lining

• day 11-18: uterus lining grows and thickens

• day 6-14: maturation of egg in follicle

• day 14: ovulation occurs - an egg is released from the ovary and moves to the uterus via the fallopian tube. if a fertilised egg has not been embedded in the lining after 28 days, it begins to break down and the cycle continues

• day 14-17: most likely to fertilise

• day 19-28: uterus lining thickens futher and becomes more vascular, preparing for implantation, if egg has not been embedded, it begins to break down and cycle continues

• when an egg is fertilised, it embeds into the uterus lining which provides it with nutrients and helps cushion the fetus

• days 0-13 she is on her period

hormone

• chemical messenger produced by endocrine glands and released into the bloodstream to the target organ

Hormones involved in the cycle

• Follicle stimulating hormone (FSH):
  - causes maturation of an egg in ovary, within a follicle
  - produced in pituitary gland
  - stimulates ovaries to produce oestrogen

• Oestrogen: causes lining of uterus to grow again
  - produced in ovaries
  - secreted as a result of FSH
  - stimulates production of LH and inhibits secretion of more FSH

• Luteinising hormone (LH)
  - produced in pituitary gland
  - its release results in ovulation

• progesterone
  - produced in ovaries and secreted from egg follicle
  - maintains lining of uterus
  - supports a pregnancy if egg is fertilised
  inhibits the release of both FSH and LH

once egg is released from mature follicle

• leftover mass of cells forms corpus luteum

• this produces high levels of progesterone which is important to maintain uterus lining in event that fertilisation occurs

• after ovulation this occurs which is called luteal phase

Hormonal methods of contraception (5)

• pill

• patch

• implant

• injection

• IUD

Contraceptive Pill

• must be taken regularly or the bodies own hormones will be released, leading to egg maturing

• contains oestrogen and progesterone

• this means oestrogen levels are constantly high, inhibiting FSH so no eggs mature

• lining also stops developing and the mucus in cervix becomes thick so sperm cannot move through

• possible side effects: changes in mood, breast enlargement, increased blood pressure

• you can get progesterone only pill which has less side effects

Contraceptive Patch

• contains oestrogen and progesterone

• small and is stuck on the skin

• lasts for one week

Contraceptive Implant

• releases a continuous amount of progesterone

• this prevents the ovaries from releasing the egg, thickens the mucus in cervix so sperm cannot swim and stops fertilised eggs from embedding in the uterus

• lasts for 3yrs

Contraceptive Injection

• made up of progesterone

• same effect as implant

• lasts for 2-3 months

Plastic intrauterine device (IUD)

• releases progesterone

• same effect as implant

• T - shaped, inserted into uterus

Non - hormonal methods of contraception

• chemical methods: spermicides: kill or disable sperm but only 70-80% effective

• barrier methods: condoms and diaphragms
  - condoms are either worn over the penis or are inside the vagina. They also prevent the individual from contracting sexually transmitted diseases. A problem is that it can tear and therefore let sperm through
  - a diaphragm is a plastic cup positioned over the cervix, used with spermicide

• surgical methods: male and female sterilisation, cutting and tying the fallopian tube or sperm duct. this lasts forever

• abstaining from intercourse ensures that an egg is not fertilised. others may only abstain during ovulation

Copper Intrauterine device

• works by killing sperm in the uterus and stopping any fertilised embryos from implanting in the uterus lining

• copper ions act as a spermicide hindering sperm movement so they cant reach the egg

Hormones to treat infertility

• fertility drugs: increase chance of pregnancy
  - main hormone used is FSH and LH because they stimulate maturation and release of the egg
  - woman can then become pregnant normally

• In Vitro Fertilisation (IVF):
  - mother is given FSH and LH to encourage maturation eggs
  - these are extracted from the mother and fertilised in the lab using sperm
  - fertilised eggs develop into embryos (tiny balls of cells) and then one or two are inserted in the uterus

Bnefits and Cons of hormones of IVF

• Pros: provides a way for infertile couple to have a child

• Cons: - physically stresfful as women may jave reactions to the hormones, such as feeling sick
  - it is emotionally stressful as it may not work: success rate for IVF is 26%
  - can lead to multiple births - unexpected and may be a risk to the babies and the mother
  - can be expensive if process needs to be repeated

Thyroxine

• regulates metabolic rate (how quickly reactions occur). also important in growth and development

• released by the thyroid gland

• its released is stimulated by the thyroid stimulating hormone

• levels of thyroxine is controlled by negative feedback

• when levels increase, it is detected by receptors in the brain

• inhibits the release of TSH

• this inhibits the release of thyroxine so levels fall

Adrenaline

• produced in times of stress, stimulates fight or flight response

• released by adrenal glands (on top of the kidneys)

• stimulated by fear or stress, detected in the brain

• effects of adrenaline include:

• increased heart and breathing rate to deliver more oxygen and glucose to the brain and muscles for respiration

• glycogen stored in the liver is converted to glucose for respiration

• pupils dilate to let in more light

• blood flow to muscles is increased

• blood flow to digestive system is decreased

Plant hormones (tropisms)

• needed to cordinate and control growth e.g. tropisms

• phototropism: response to light

• gravitropism

• geotropism

positive phototropism

• most plants show this because they grow towards the light source

• the plant is exposed to light on one side

• auxin moves to the shaded side of the shoot

• auxin stimulates cells to grow more there

• this means the shoot bends towards the light

• the plant recieves more light, meaning photosynthesis can occur at a faster rate

Negative gravitropism

• most shoots show this as they grow away from gravity, if a shoot is horizontal:

• auxin moves to the lower side

• the cells of the shoot grow more on the side with the most auxin, so it stimulates cells to grow more here

• this makes the shoot bend and grow away from the ground

• this is beneficial as light levels are likely to be higher further away from the ground

Positive gravitropism

• most roots show this as they grow towards gravity

• if a root is horizontal:

• auxin moves to the lower side

• the cells of the root grow more on the side with less auxin, so it stimulates cells to grow on the upper side

• this makes the root bend and grow downwards

• this is beneficial as there are more likely to be increased levels of water and nutrients lower down, and it provides stability for the plant

what happens when auxin distribution becomes equal on both sides

• it grows straight up

effect of light or gravity on newly germinated seedlings in varying conditions RP

• place germinated seeds in a cardboard box and shining light from one side and one in no light, the seeds growing in no light will grow more as the shoots grow more in order to find light

• attaching a petri dish containing seedlings to a wall (effects of gravity

other plant hormones

• gibberellins: important to stimulate seed germination

• ethene: involved in cell division and ripening of fruits

uses of plant hormones

• increase yield

• obtain desirable features and to lower costs

Auxin uses

• as weed killers:
  - many weeds are broad-leaved
  - weedkillers, containing auxin have been synthesised so they only affect broad-leaved plants
  - the increased amount of auxin causes the cells to grow too rapidly
  - leading to the weed dying

• as rooting powders:
  - plants with desirable features are cloned to make more plants with the same feature
  - one way to clone a plant is to take a cutting from the original 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 tissue culture
  - another way to clone a plant is tissue culture
  - cells from the plant are taken and placed in a growth medium containing lots of nutrients (petri dish)
  - hormones such as auxins are added
  - the cells begin to form roots and shoots

Ethene

• as it controls ripening, it is used in 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 temps

• ethene is involved in cotnrolling cell division and stimulates enzymes that result in fruit ripening

• this reduced wastages as more fruit is suitabloe to be sold and it does not ripen too early

Gibberellins

• ending seed dormancy:
  - in the brewing industry, germination rate of barely seeds is increased to make malt

• promoting flowering:
  - instead of requiring certain conditions such as longer days and low temps 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