AQA Biology [homeostasis]

in humans

Homeostatic responses:

homeostatic responses help organisms to keep their body conditions near constant

three key homeostatic responses are:

1. thermoregulation
2. osmeoregulation
3. the control of blood glucose

there are two types of coordination systems:

1. nervous system
2. endocrine system

the nervous system:

the human nevous system consists of:

1. the central nervous system [brain and spinal cord]
2. the peripheral nervous system [all of the nerves in the body]

it allows us to make sense of our surroundings and respond to them to regulate and control our body responses

information is sent as nerve impulses - electrical signals that pass along nerve cells known as neurones

a bundle of neurones is a nerve

Neurones coordinate the activities of:

• sensory receptors (eg. those in the eye)
• decision-making centres in the central nervous system
• effectors (such as muscles and glands)

the endocrine system:

a hormone is a chemical substance produced by an endocrine gland and secreted by the blood

they are chemicals which transmit information from one part of the organism to another and bring a change

they alter the activity of one or more specific target organs

hormones do not control functions that need instant responses

a gland is a group of cells that release and produce substances [this is called secretion]

• pituitary gland [master gland] base of the brain
• thyroid gland produces thyroxine
• pancreas produces insulin
• adrenal glands produce adrenalin
• testes produce testosterone
• ovaries produce oestrogen

insulin regulates blood glucose

endocrine glands have a good blood supply as the blood plasma transports the hormones

hormones are transported to the target organs

hormones only affect cells with receptors

receptors are cells that hormones can bind to[these are found on the cell surface or inside the membrane and have to be complementary for the hormones to affect them]

this is the lock and key theory

homeostasis

Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function

Sensory cells can detect information about the conditions inside and outside of the body

Examples of physiological factors that are controlled by homeostasis in mammals include:

• Core body temperature
• Metabolic waste (eg. carbon dioxide and urea)
• Blood pH
• Concentration of glucose in the blood
• Water potential of the blood
• Concentration of the respiratory gases (carbon dioxide and oxygen) in the blood

negative response:

negative feedback maintains balance

Negative feedback control loops involve:

• A receptor (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
• A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
• An effector (muscles and glands) – to carry out a response

Outcome of a negative feedback loop:

• The factor / stimulus is continuously monitored
• If there is an increase in the factor, the body responds to make the factor decrease
• If there is a decrease in the factor, the body responds to make the factor increase

maintain of blood glucose

If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and may not be able to function normally

 concentration of glucose in the blood increasing above a certain level, can disrupt the normal function of cells, potentially causing major problems

The control of blood glucose concentration is a key part of homeostasis

Blood glucose concentration is controlled by two hormones secreted by endocrine tissue in the pancreas

decrease in blood glucose

If a decrease in blood glucose concentration occurs, it is detected by the α and β cells in the pancreas:

• The α cells respond by secreting glucagon
• The β cells respond by stopping the secretion of insulin

The decrease in blood insulin concentration reduces the use of glucose by liver and muscle cells

Glucagon binds to receptors in the cell surface membranes of liver cells

This binding causes a conformational change in the receptor protein that activates a G protein

This activated G protein activates the enzyme adenylyl cyclase

Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes

This process is known as glycogenolysis

The enzyme cascade described above amplifies the original signal from glucagon and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level

in plants

stomata

plants carry out homeostasis to maintain a constant internal environment

• for example, mesophyll cells require a constant supply of carbon dioxide

stomata control the diffusion of gases in and out of the cells

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