Homeostasis maintains some conditions inside the body at a constant level. Negative feedback mechanisms respond to a change in a condition to help bring the condition back to the normal level.
- Osmoregulation controls how much water is lost in urine, and so controls the amount of water in the body.
- Too much water in the body= lots of watery urine.
- Too little water in the body= small amount of concentrated urine.
- Thermoregulation keeps core body temperature steady at around 37°C. This is important because enzymes in the main organs are the most active at this temperature.
- Body temperature rises= Vasodilation, moresweat released, sebaceous glands produce oil- helps spread sweat over skin.
- Body temperature falls= Vasoconstriction, less sweat, raised body hairs by erector muscles.
The nervous system includes:
- the central nervous system- brain and spinal cord.
- the sense organs, such as the eyes and ears, which contain receptors that can detect a change in the environment (called a stimulus) and produce an electrical impulse.
- the nerves that join the central nervous system to the sense organs and effector organs- these are made up of bundles of nerve cells, or neurones.
Types of neurones
There are three main types of neurones:
- Sensory neurones- carry impulses to the central nervous system.
Responding to stimuli
- Myelin sheath- Fatty layers surrounding the axon. Helps to insulate nuerone from surrounding tissue, especially other nuerones and also allows impulses to be carried faster.
- Nuerotransmitters- chemical substances that transmit impulses across the gaps in synapses. All impulses are slowed down slightly by synapses.
- reflex arc-
- The neurones in the reflex arc go through the spinal cord or through an unconscious part of the brain (you don’t have to think).
- When a stimulus (e.g. pin) is detected by the receptors, impulses are sent along a sensory neurone to the CNS.
- In the CNS the sensory neurone passes on the message to a relay neurone.
- Relay neurones relay the impulses to a motor neurone.
- The impulses then travel along the motor neurone to the effector (e.g. muscle)
- The muscle then contracts and moves the hand away from the pin.
- As you don’t have to think about this response, it’s quicker than a normal response.
- Hormones - chemical messengers that travel in the blood
- the parts of the body respond to their presence
- Endocrine glands - produce and release hormones
- An organ that responds to a certain hormone is a target organ
- Most carbohydrates in food are digested into glucose
- Concentration of glucose in your blood increases after a meal
- pancreas releases insulin hormone when its a certain concentration
- Insulin takes glucose out of the blook and converts in to glycogen
- which is a store of glucose in the liver
- Glucagon is a hormone released when glucose levels drop
- the hormone causes liver cells to turn glycogen back into glucose
- then its released back into the blood to increase levels
- Diabetes - a disease whereby blood glucose levels aren't controlled
- Low blood glucose concentrations - unconsiciousness
- High blood glucose concentrations - tiredness, organ damage e.g. eyes
- Type 1 diabetes - pancreas does not produce insulin
- body cannot bring down blood glucose levels when they get to high
- between 5% and 10% of diabetics have it
- most will inject insulin everyday
- Must be injected into the fat layer beneath the skin - subcutaneous fat layer
- easily absorbs insulin
- spreads to blood vessels to be carried in the blood
- Diabetics must balance the lowering and increasing factors
- more exercise = less insulin
- more insulin needed some days than others
- Type 2 diabetes - when cells become 'resistant'/respond less to well to insulin being produced
- Contributors to aquiring it are:
- high-fat diets
- lack of exercise
- old age
- Often controlled by changing diet and increasing amounts of exercise
- Obesity - if a person's Body Mass Index (BMI) is over 30
- BMI - estimate of how healthy your mass is for your height
- weight in kilograms
- height in metres squared
- Tropism - responding to a stimulus by growing towards of away from it
- Positive tropsism - growing towards a stimulus e.g. plant shoots
- Negative tropism - growing away from a stimulus e.g. plan roots
- Phototropism - a tropisim caused by light
- Geotropism - a tropism towards the direction of gravity (positive gravitropism)
- Photosythesis - positively phototrophic, to get light, to make food
- Plant growth substances = plant hormones
- Auxins - cause positive phototropism and positive gravitropism
- Gibberellins - cause starch in a seed to be converted into sugars for growth energy
- also stimulate flower/fruit production in some species
- Germination - when roots and a shoot start to grow
- gibberellins are released after this process
Plant Hormones continued
- Auxins produced in the tip of a shoot
- cause elongation of cells
- Shoot grows towards light when
- light coming from one direction
- auxins move to shaded side of shoot
- cells on shaded side elongate more
- Auxins produced in root tips - opposite effect
- causes cells to stop elongating
- causes geotropism
- Anchors the plant in place
- and to reach moisture underground
Uses Of Plant Hormones
- Artificial auxin
- used as a seletive weedkiller
- plants with broad leaves grow out of control and die e.g. dandilions, daisies
- narrow leaved plants are unaffected e.g. grass, wheat
- farmers can kill weeds without killing cereal crop
- used in rooting powders
- plant cuttings are dipped in rooting powders to increase root development
- large numbers of plants can be produced quicker than growing seeds
- Seedless fruits
- flowers sprayed with hormones that cause fruits but not seeds to develop
- naturally seedless but small fruits are sprayed with gibberellins to increase size
- Fruit ripening
- fruit trees sprayed with hormones to stop fruit falling and being damaged
- also to speed up ripening to be picked all together
- unripe fruit so it reaches shops in a 'just ripened' condition