Responding to the environment

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Survival and Responce

Organisms increase their chance of survival by responding to changes in their external and internal environments. Any changes in these environment are stimuli.

Simple organisms have simple responces to keep them in a favourable environment, can be...... TAXIS: Directional movement in responce to a stimulus, direction of stimulus affects the responce. KINESIS: Non-directional movement in responce to a stimulus, intensity of stimulus affects the responce.

Tropisms : Flowering plants increase their chances of survival by responding to changes in their environment. A TROPISM is the responce of a plant to a directional stimulus --> they respond to stimuli by regulating their growth. A + tropism --> towards stimulus. A - tropism --> away from stimulus.

PHOTOTROPISM: Is the responce to light. Shoots are + phototropic as they grow towards the light and roots are - phototropic as they grow away from the light.

GEOTROPISM: Is the responce to gravity. Shoots are - geotropic as they grow upwards and roots are + geotropic as they grow downwards.

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Survival and Responce Continued.

GROWTH FACTORS: Plants respond to the stimuli using growth factors (chemicals that speed up or slow down growth). They are produced in the growing regions and move to where they are needed in other parts of the plant. Auxins stimulate growth of shoots by cell elongation, high concentrations of auxins inhibit growth of the roots.

Reflex Arc                                                                                                                                             A reflex is the bodys responce to a stimulus without making a conscious decision to respond --> information travels really fast from receptors to effectors. Simple refelexes help organisms avoid damage. The pathway of neurones linking the receptors to effectors is a reflex arc; Sensory Neurone, Relay Neurone & Motor Neurone




If there is a relay neurone in a simple reflex arc there can be an override.

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Control of Heart Rate

The Sinoatrial node (SAN) generates electrical impulses causing the cardiac muscles to contract, the rate at which this happens is unconsciously controlled by the medulla in the brain. Animals alter heart rate to resond to an internal stimuli. The stimuli are detected by pressure receptors and chemical receptors.

PRESSURE RECEPTORS: Baroreceptors in the aorta and vena cava. stimulated by high and low blood pressure.

CHEMICAL RECEPTORS: Chemoreceptors in the aorta, Carotid artery and medulla monitor oxygen levels in the blood as well as CO2 and pH.

Electrical impulses from receptors are sent to the medulla along sensory neurones. The medulla processes information and sends it to the SAN along the sympathetic and parasympethetic neurones.

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Nervous System

Sensory Neurones transmit electrical impulses from receptors to central nervous system (CNS)Motor Neurones transmit electrical impulses from the CNS to effectors.                                        Relay Neurones transmit electrical impulses between sensory neurones and motor neurones.

Nervous Communication                                                                                                                  Stimulus is detected by the receptor cells and an electrical impulse is sent along the sensory neurone. When the electrical impule reaches the end of the neurone chemicals called neurotransmitters take information across the gap (SYNAPSE) to the next neurone, which sends an electrical impulse. The CNS processes the information and decides what to do. It then sends impulses along the motor neurones to an effector.

STIMULUS ---> RECEPTORS --(Sensory Neurone)-> CNS --(Motor Neurone)-> EFFECTORS ---> RESPONCE

Nervous Responce: When an electrical impulse reaches the end of a neurone, Neurotransmitters are secreated directly onto cells --> nervous response is localised. Neurotransmitters are quickly removed once they have done their job --> Response is short lived. Electrical impulses are a fast and rapid response allowing animals to react quickly to stimuli.

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Hormonal System

It is made up of glands and hormones. GLAND: group of cells, that secreate useful substances. HORMONES: are chemical messangers, hormones are secreated when a gland is stimulated.

Hormonal Communication; Hormones diffuse directly into the blood and are taken around the body by the circulatiory system. They diffuse out of the blood at different places around the body, each hormone will only bind to specific receptors for that hormone, they are found on the membranes of some cells ---> Target Cells. Hormones trigger a response in target cells.


E.G: Low Blood Sugar level.. STIMULUS: Low blood glucose concentration. RECEPTORS: Receptors on the pancreas cells detect low blood glucose concentration.  HORMONE: Pancreas releases glucagon into the blood.  EFFECTORS: Target cells in the liver detect glucagon and convert glycogen into glucose. RESPONCE: Glucose is released into the blood, and the glucose concentraion increases.

HORMONAL RESPONCE: Hormones aren't released onto target cells they must travel in blood to get there. Chemical communication is slower that electrical. They are not broken down quickly so effects last much longer. Widespread responce if the target cells are widespread.  

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Chemical Mediators

Chemical Messangers work locally. Similar to communications using hormones, cells release chemicals that bind to specific receptors on target cells to cause a responce. Chemical mediators are secreated from cells all over the body. The target cells are right next to where chemical mediators are produced, stimulating a local response. They travel short distances to target cells, meaning a quicker response.

HISTAMINE: Is stored in cells in the immune system. It is released in response to the body being injured or infected. Allows more immune system cells to move out of the blood and into the infected/injured area.

PROSTAGLANDIS: Produced by most cells in the body, Involved in lots of different things e.g. inflammation + fever.

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Receptors Intro...

Receptors are specific, they only detect one stimulus, some are cells and some are proteins on cells surface membranes. Receptors in the nervous system convert engery of a stimulus into electrical energy used by neurones...

RESTING POTENTIAL: When a nervous system receptor is in its resting state, there is a difference in charge between the inside and the outside of the cell. The inside is charged relative to the outside, there is a voltage across the membrane, this voltage is known as potential difference. When a cell is at rest it is resting potential, it is genertated by ion pumps and channels.

GENERATOR POTENTIAL: A stimulus is detected, the cell membrane gets excited and becomes more permeable allowing more ions to move in and out of the cells, altering the potential difference due to a stimulus is called, generator potential. The bigger the stimulus the more excited the membrane becomes, causing a larger movement of ions and a bigger change in potential difference, so a bigger generator potential is produced.

ACTION POTENTIAL: If a generator potential is big enought it'll trigger and action potential, which is an impulse along a neurone. It is only triggered if the generator potential reaches threshold level.

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Pacinian Corpuscles

They are mechanoreceptors, they detect mechanical stimuli. They are found in the skin. They contain the end of a sensory neurone --> sensory nerve ending, which is wrapped in lots of connective tissue called lamellae. When it is stimulated (e.g. tap on arm) the lamellae are deformed, pressing on the sensory nerve ending causing the deformation of stretch-mediated sodium channels in the sensory neurons cell membrane. The sodium ion channels open and sodium ions diffuse into the cell, creating a generator potential. If the generator potential reached the threshold and action potential is generated.







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They are receptors in the eye that detect light. Light enters through the pupil, the amount is controlled by the muscles of the iris. Light rays are focused by the lens onto the retina which lines the inside of the eye. The retina contains photoreceptor cells. The fovea area of the retina is where there are lots of photoreceptors. Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones. Where the optic nerve leaves the eye is a blind spot are there are no photoreceptors, so it isn't sensitive to light.



How Photoreceptor Work: Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive pigments. The light bleaches the pigments causing a chemical change and altering the membrane permiability to sodium. Generator potential is created & if threshold is reached nerve impulse is sent along a bipolar neurone, these connect photoreceptors to optic nerve which taked the impulse to the brain.

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Rods and Cones

There are two types of photoreceptors: RODS & CONES.   RODS: Found in the peripheral part of the retina and only receive information in black and white CONES: are packed in the fovea and they receive information in colour, there are 3 types of cones; Red , Green and Blue-Sensitive.

SENSITIVITY: Rods are very sensitive to light, because many rods join one neurone, weak generator potentials join to reach the threshold and trigger an action potential



Cones are less sensitive (they only fire an action potential in bright light) because one cone joins one neurone, it takes more light to reach the threshold and trigger an action potential.


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Rods and Cones Continued..

Visual Activity; Ability to tell apart points that are close together.

Rods give low visual activity, as many rods join the same neurone, which means light from 2 objects can't be told apart.




Cones give high visual activity, cones are close together and 1 cone joins 1 neurone. When light from 2 points hits 2 cones, 2 action potentials go to the brain, so you can distinguish 2 points close together as 2 seperate points.



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