Organisms Response to change in their environment


Survival and Response

Responding to their environment helps organisms survive 

Animals are able to increase their chances of survival by responding to changes in their external environment such as avoiding harmful environments like places which are too hot or cold. They also are enabled to respond to changes in their internal environment to make sure that the conditions are always optimal for their metabolism (chemical reactions inside the body). Plants are also able to increase their chances of survival by responding to changes in their environment. Any change in the internal or external environment is called a stimulus. 

 Receptors detect stimuli and effectors produce a response

Receptors detect the stimuli such as cells or proteins on cell surface membranes - different receptor types detect different stimuli - Receptors are specific to one type of stimulus. Effectors are cells that bring about a response to a stimulus, to produce an effect e.g. muscle cells and cells found in glands, the pancreas. Receptors communicate with effectors via the nervous system or the hormonal system, or sometimes both. 

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The Nervous System

The nervous system is made up of a complex network of cells called neurones. These are the three main types: 

Sensory Neurones : Transmit electrical impulses from receptors to the central nervous system (CNS) - the brain and spinal cord. 

Motor Neurones: Transmit electrical impulses from the CNS to effectors 

Relay Neurones: Trasmit electrical impulses between sensory neurones and motor neurones. 

A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone. 

When an electrical impulse reaches the end of a neurone, chemicals called neurotransmitters take the information across to the next neurone, which then sends an electrical impulse. The CNS (the coordinator) proscesses the information and sends impulses along motor neurones to an effector. 

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Sympathetic and Parasympathetic nervous system

The Central Nervous System (CNS) - made up of the brain and spinal cord 

The peripheral nervous system- made up of the neurones that connect the CNS to the rest of the body. It also has two different systems 

Somatic Nervous System 

Controls concious activities such as running and playing video games. 

Autonomic Nervous System 

Controls unconcious activities e.g. digestion. It has two divisions that have opposite effects on the body; 

Sympathetic nervous system: This gets the body ready for action, the 'flight or fight' system 

Parasympathetic nervous system: This calms the body down, the 'rest and digest' system 

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Reflexes are rapid, automatic responses to stimuli 

  • A reflex is where the body responds to a stimulus without making a concious decision to respond. 
  • This is because often the individual does not have time to spend time deciding how to respond, information travels really fast from receptors to effectors 
  • So simple reflexes help organisms to protect the body because they are rapid 
  • The pathway of neurones linking receptors to effectors in a reflex is called a reflex arc. 

Example of reflex arc - the hand-withdrawal response to heat

  • 1.) Thermoreceptors in the skin detect the heat stimulus 
  • 2.) The sensory neurone carries impulses to the relay neurone 
  • 3.) The relay neurone connects to the motor neurone 
  • 4.) The motor neurone sends impulses to the effector (the biceps muscle) 
  • 5.) Your muscle contracts to withdraw your hand and stop it being damaged 


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Nervous system communication

Nervous system communication is localised, short-lived and rapid

1.) When an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto target cells (e.g. muscle cells) - so the nervous response is localised. 

2.) Neurotransmitters are quickly removed once they've done their job, so their response is short-lived. 

3.) Electrical impulses are really fast, so the response is rapid- this allows animals to react quickly to stimuli. 

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