responding to the environment

• plants can respond to biotic and abiotic stimuli
• tropism- a directional growth response of a plant
• phototropism is when plants grow towards light
• geotropism allows plants to grow toward/away from gravity
• auxin is a plant hormone that is made in the shoot tips that causes cell elongation
• When light falls directly above the stem auxin diffuses down evenly into the lower tissues of the stem, causing cell elongation and growth directly upwards.
• Directional light causes auxin to diffuse towards the darker side of the stem. High auxin concentration on the darker side of the stem increases cell elongation, and therefore growth, on this side, causing the plant stem to bend towards light.
• shoot bends toward light
• Shoots show negative geotropism (they grow away from gravity) and roots show positive geotropism (they grow towards gravity, benefiting the plant as the roots will grow into soil where they act as an anchor, increasing stability (MP1) and ensuring water and mineral ions can be taken up by root hair cells (MP2).
• . The nervous system has two components: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS is composed of the brain and spinal cord,
• whilst the PNS is composed of the nerves (bundles of nerve cells called neurons) that link the CNS to sense organs and effectors.
• When a sense organ is stimulated a nervous impulse is sent along nerves into and out of the central nervous system then to an effector(s) causing a rapid response
• The neurons in the nerves that send impulses towards the CNS are called sensory neutrons, whilst the neurons in the nerves that send impulses away from the CNS are called motor neurons
• Stimulus → Receptor → PNS (sensory neurons) → CNS → PNS (motor neurons) → Effector → Response
• The neurons found in the spine linking sensory neurons and motor neurons are known as relay neurons
• Junctions between neurons where the electrical signal is converted into a chemical signal to “cross the gap” are called synapses.
• (MP1) Receptors detect a stimulus • (MP2) and generate a nervous impulse • (MP3) in a sensory neuron. • (MP4) The nervous impulse travels to the spinal cord in the CNS • (MP5) Where the impulse is transferred to a relay neuron • (MP6) via a synapse • (MP7) The impulse is then transferred to a motor neuron • (MP8) Which leads to an effector, which causes a response
• The impulse that travels along the sensory neurone, relay neurone and motor neurone cannot “jump” over the synapses between these neurones.
• In order for an impulse to fire in the adjacent neurone, chemicals called neurotransmitters are released that travel across the synapse.
• • Vesicles (little packages) containing the neurotransmitter are found at the end of the pre-synaptic neuron. • When an impulse arrives at the end of the presynaptic neuron, these vesicles move towards and fuse with the cell membrane. • The neurotransmitter is released into the synapse and diffuse across the gap. • The neurotransmitter binds to specific receptors on the post-synaptic knob membrane. • The causes an impulse to fire in the adjacent neurone and it travels down the axon.
• impulses travel faster when the neurone is mylenated
• The problem is that impulses will continue to be fired as along as there are neurotransmitters attached to the receptors in the post-synaptic neurone. This would result in the effector (muscle or gland) to be continually stimulated.
• For this reason, an enzyme breaks down the neurotransmitter, preventing over stimulation of the post-synaptic neurone and effector. The products of this reaction are then reabsorbed back into the pre-synaptic neuron. They are then turned back into neurotransmitter, ready to be used again
• label all of the parts

  1. cornea

  2. iris

  3. lens

  4. sclera

  5. retina

  6. choroid

  7. fovea

  8. blind spot

  9. optic nerve

  10. vitreous humour

  11. sensory ligaments

  12. ciliary muscles

  13. pupil

  
• Conjunctiva -Protects the surface of the eye
• Sclera -The visible, white part of the eye which is opaque
• Cornea- Allows light to pass through the sclera at the front of the eye
• Pupil -Allows light to pass through the eye to the lens;
• Iris -Contains radial muscles and circular muscles that contract/relax in response to changing light intensity, altering the size of the iris and the pupil.
• Lens Refracts light to form an image at the retina
• Suspensory ligaments Fibres that hold the lens in place
• Ciliary muscles Contract/relax to alter the length of suspensory ligaments, and therefore the shape of the lens when focussing on near/distant objects
• Retina Light sensitive layer at the back of the eye containing rods and cones. Detects light and generates nervous impulses, which are passed to the optic nerve.
• Fovea Part of the retina where cones (colour sensitive cells) are concentrated
• Choroid Absorbs light to prevent light reflecting around the eye
• Optic nerve Carries nervous impulses generated by rods and cones to the brain
• If the eye is to function effectively when light intensity decreases then the pupil must widen so that more light can enter the eye. This occurs by radial muscles in the iris contracting and circular muscles in the iris relaxing (MP1) this causes the iris to get smaller, and the pupil to widen (MP2) so more light can enter (MP3).
• the process for accommodation is:

  • (MP1) Ciliary muscles • (MP2) contract • (MP3) causing the suspensory ligaments • (MP4) to slacken [not relax – they are ligaments not muscles] • (MP5) This causes the lens to become fatter • (MP6) and so refracts light at a greater angle, allowing a focussed image to be presented on the retina.
• The converse is true when focussing on a distant object: ciliary muscles relax causing the suspensory ligaments to tighten. This causes the lens to become thinner, reducing the angle at which light is refracted, allowing a focussed image to be presented on the retina.