4.3 Co-ordination And Control Flashcards
- Coordination and Control in Plants
Learning Objectives:
•Understand the role of phytochromes in the control of flowering in long-day and short-day plants
•Understand the role of plant growth substances (hormones) in stem elongation
Phytochromes
Phytochromes are pigment molecules in plants that can detect light in the red sprectrum. It is found in the leaves of plants than can flower and that are involved in the process of flowering.
Phytochromes exist in two interchangeable forms:
•Phytochrome 660 (P660 or PR)
•Phytochrome 730 (P730 or PFR)
P660 is able to absorb red light and will rapidly convert to P730 - this happens in daylight.
P730 absorbs far-red light and converts to P660 - this happens in darkness.
Note:
P730 is the active form
The plants response to light is referred to as phototropism.
Diagram of Phytochrome Conversion
Types of Plants
Long-day Plants (LDP)
-Only flower when the period of light exceeds a critical minimum length. Examples of LDP include radish, clover and barley.
They need a long period of light and therefore needs high amounts of P730.
Short-day Plants (SDP)
-Only flowers when the period of darkness exceeds a critical minimum. Examples of SDP include chrysanthemum and tobacco.
They need a long period of darkness and therefore need high amounts of P660.
Note:
The presence or absence P730 is critical to SDP and LDP as it determines whether flowering is stimulated or inhibited.
•P730 inhibits SDP
•P730 stimulates LDP
It is the length of the dark period which is crucial in determining flowering.
Short day plants require a long dark period to convert to P730 into P660 so they can flower. Long day plants require a short dark period as non-removal of P730 is needed for flowering.
Florigen
As the flowering response takes place in the flower buds, the plant must have some means of transmitting information from leaves to flower buds. A chemical messenger called florigen has been suggested, which is thought to be transported in the phloem.
Florigen Diagram
Period of Light/Dark Determining Flowering
The Role of Plant Hormones
Hormones interact with each other and often need certain levels of one for another to have an effect.
There are three main hormones involved in the growth of plant:
- Auxin - Promotes cell elongation at the shoot tip
- Cytokinin - Promotes cell division
- Gibberellin - Promotes elongation of the internodial regions
Auxin in Cell Elongation
Auxin causes phototropism in plants:
-more light absorbed by chlorophyll
-therefore more photosynthesis
-therefore more glucose
-therefore more respiration
-therefore more energy
And therefore more growth
Auxin Elongation Diagram
Plant Diagram
- Coordination and Control in Animals
Learning Objectives:
•The structure of a neurone
•The generation and transmission of nerve impulses
•The structure and function of a synapse
•The structure and function of the mammalian eye
•The structure and function of voluntary muscle
Neurones
Neurones
Neurones are nerve cells that are specialised for the transmission of impulses to other neurons, muscle cells and gland cells. Within the mammalian nervous system, there are three types of neurons - these are:
•Sensory (afferent) Neurones
Transmits impulses from receptors towards the CNS (brain and spinal cord)
•Association Neurones
Transmits impulses between sensory and motor neurones and are located within the CNS
•Motor (efferent) Neurones
Transmits impulses away from the CNS to effectors (muscles and glands)
Nerve Diagram
Neurone Structure
Diagram
Structure and Function
Structure and Function:
•Dendrites - Thin strands of cytoplasm that extend from the cell body. All but one of the extensions are short. They conduct impulses towards the cell body. This is called the Axon.
•Axon - Can be very long and contains cytoplasm. The axon carries electrical impulse along the length of the neurone
•Cell Body - Contains all the usual organelles, eg the mitochondria, ER and Golgi apparatus
•Schwann Cells - Wrap along the length of the axon enclosing it in many layers of its plasma membrane. The membrane contains fatty substances called myelin and so the covering is called the Myelin Sheath. It insulates the neurone so impulses can be conducted faster along the Nodes of Ranvier.
•Nodes of Ranvier - Small areas of the axon that aren’t covered by the sheath. They occur every 1-3 mm in human neurones. These are important to allow nerve impulses to be generated.
Neurone Structure
Relay Neurone Structure
Sensory Neurone Structure
Generation and Transmission of Nerve Impulses
During a resting potential the neurone is not conducting an impulse and the inside of the neurone is negatively charged compared with the outside. The potential different is -70mV.
Potential Difference
This is caused by by a difference in the electrical charge of the inside of the plasma membrane compared to the outside. This is a result of the concentration of Na+ and K+ either side of the membrane. With the concentration of Sodium on the outside higher than the inside.
Cells that exhibit a membrane potential are said to be polarised.
The presence of a stimulus, at a point on the neuron cell membrane, triggers and action potential:
An action potential is a temporary reversal of the membrane potential; the inside of the neurone membrane becomes positive (+40mV) relative to the outside and then returns to the resting potential (-70mV).
This temporary reversal of charge across the membrane of a ‘resting’ neurone is described as depolarisation. The restoration of the resting membrane potential is described as re-polarisation.
Depolarisation and Re-polarisation of a Neuron
How an Action Potential is Generated
- Many charged particles on the inside and outside of the neurone (Na+ and K+ are in their highest concentration)
- Unstimulated neurone is polarised with a resting potential of -70mV
- Na+ ions move across membrane into the neurone in the region of the stimulus (causing further depolarisation)
- ** Na+** ions continue moving in until the membrane potential is +40mV - this is the height of an action potential
- K+ ions move out of the neurone, re-establishing the rooting potential - this is repolarisation
- The membrane is unresponsive to any further stimulus. This refractory period lasts until the relatively slow flow of K+ ions return the membrane potential to its resting value of -70mV
All of Nothing Law
~All or Nothing Law~
Nerve impulses are all or nothing phenomena, i.e. provided the stimulus exceeds a certain value, called the threshold value, an action potential of a fixed value proceeds along the whole length of the neurone. There are no intermediate types- the potential is either the full value or it is nothing.
Transmission of a Nerve Impulse
This is due to localised circuits being set up which lead to further depolarisation of the membrane ahead of them and repolarisation behind them (this happens in sections).
- At resting potential there is a high concentration of sodium ions outside and a high concentration of potassium ions inside it.
- When the neurone is stimulated, sodium ions rush into the axon along a concentration gradient. This causes depolarization of the membrane.
- Localized electrical circuits are established which cause further influx of sodium ions and so progression of the impulse. Behind the impulse, potassium ions begin to leave the axon (repolarisation) along a concentration gradient
- As the impulse passes the outflow of potassium ions causes the neurone to become repolarised behind the impulse.
- After the impulse has passed and the neurone is repolarised, **sodium is once again actively expelled against a concentration gradient in order to increase the external concentration and so allow the passage of another impulse.
Speed of Impulse
Two factors are important in determining the speed of conduction:
1.Myelin Sheath - Acts as an electrician insulator and prevents depolarisation in that part of the neurone. APs jump from node of Ranvier to another Node of Ranvier through a process called saltatory conduction.
2.Diameter of the axon – the thicker the axon, the faster the impulse
3. Temperature causes more kinetic energy and therefore ions will move more rapidly
The myelin sheath, which is produced by Shwann cells, is absent at points called nodes of Ranvier, which arrive every mm or so along the neurones length.
As a fatty myelin acts as an electical insulator, an AP cannot form in the part of the axon covered with myelin. They can form at the nodes. The AP therefore jump from node to node through saltatory conduction, increasing the speed with which they are transmitted.
Inhibitory Synapses
• These make it more difficult for synaptic transmission to take place. The neurotransmitter they release make it more difficult for an EPSP to form.
• The inhibitory neurotransmitters (e.g. GABA) lead to an influx of negative ions in the post-synaptic membrane, making the inside of the membrane even more negative (lower than -70mV), creating an inhibitory post-synaptic potential (IPSP).
• This hyperpolarisation makes it even more difficult than normal for excitatory synapses to produce an EPSP that reaches threshold level.
Why have inhibitory synapses?
Reduce input of background stimuli that would clutter up the nervous activity in the brain or may prevent some reflex actions.
Prevents hypersensitivity.
GABA
Another neurotransmitter thag is released at inhibitory synapses. It is the main inhibitory neurotransmitter in animals causing hyperpolarisation.
It causes ions to flow into the postsynaptic neurones, causing hyperpolarisation.
Helps to reduce anxiety and panic attacks by ‘damping’ down nerve pathways.
Transmission across a Synapse
• AP cannot jump across synapse so instead a signal is passed across by a chemical called a transmitter substance.
• AP arrives along the plasma membrane of the presynaptic neurone and causes the membrane to become permeable to Ca ions that diffuse into it
• Ca ions stimulate movement of synaptic vesicles towards the presynaptic membrane
• Vesicles fuse with the presynaptic membrane and release the transmitter substance acetylcholine into the synaptic cleft through exocytosis
• Substances diffuse across the cleft and bind to the receptors on the postsynaptic membrane- causing the opening of Na ion channels in the membrane of the postsynaptic neurone. As Na+ ions diffuse in the membrane becomes gradually depolarised and an excitatory post synaptic potential (EPSP) is generated.
• if there is sufficient depolarisation, the EPSP will reach the threshold intensity required to produce an AP in the postsyntactic neurone
•If acetylcholine (ACh) remained bound to the postsynaptic receptors the Na+ channels would remain open and an AP would fire continuously - to prevent this the ACh is recycled
• The synaptic cleft contains enzyme acetylcholinesterase which splits the acetylcholine knot acetate and choline
• The choline is taken back into the presynaptic membrane to form ACh again which is then transported into the presynaptic vesicles ready for the next AP
Mark scheme for 16 Marker
The Eye
Pupils in Dim vs. Bright light
Radial and Circular Muscles
Focusing of Light Rays onto the Retina
• Light rays entering the eye must be refracted (bent) in order to focus them onto the retina and so give a clear image.
• Most refraction is achieved by the cornea.
• However, the degree of refraction needed to focus light rays onto the retina varies according to the distance from the eye of the object being viewed.
• Light rays from objects close to the eye need more refraction to focus them on the retina than do more distant ones.
• The cornea is unable to make these adiustments and so the lens has become adapted to this purpose. The actions of the ciliary muscles and suspensory ligaments allow the lens to change shape and thus to accommodate for near and distant objects.
The Retina
• The retina possesses the photoreceptor cells.
• These are of two types, rods and cones.
• Both act as transducers in that they convert light energy into the electrical energy of a nerve impulse.
• Both cell types are partly embedded in the pigmented epithelial cells of the choroid.
• The basic structure of rods and cones is similar. However, there are both structural and functional differences and these are detailed in the diagram below:
Comparison of Rods and Cones
The Human Retina
Section Through the Human Retina
Retinal Convergence, Summation and Amplification - RODS
~Rods~
• The response of many rods may be summed by the anatomical arrangement of the bipolar cells
• They synapse with several rods but only a single ganglion cell.
• This synaptic or retinal convergence permits great visual sensitivity
• Rods are of great value for night vision
• However, because several rods share a ganglion cell, they offer poor visual acuity
• i.e. the resolution of night vision is poor
Trichromatic Theory of Colour Vision
There are about six million cones in the human retina and these are the photoreceptors responsible for our perception of colour. There are *three types of cone (red, green and blue) each possessing a different form of the pigment iodopsin. The degree of stimulation of each type of cone determines the colour perceived. The blue, green and red cones absorb and respond to a range of wavelengths of light with maximum absorption of 420m, 530m and 560m respectively.
The Trichromatic Theory of Colour Vision proposes that there are three varieties of the colour-sensitive pigment iodopsin with each different pigment being located in separate cone cells (red, green and blue) in the retina; different colours and shades are perceived by the degree of stimulation of each type of cone.
Muscles as an Effector
~Types of Muscle~
There are three types of muscle fibre: smooth muscle, cardiac muscle and skeletal muscle.
Structure of Skeletal Muscle
• An individual muscle is an effector which brings a response to particular stimulus and is made up of hundreds of muscle fibres.
• Each fibre has many nuclei (multi nucleus) and a distinctive pattern of bands or cross striations.
• It is bounded by a membrane - the sarcolemma.
• The fibres are composed of numerous myofibrils arranged parallel to one another.
Each repeating unit of cross striations is called a sarcomere.
• The cytoplasm of the myofibril is known as sarcoplasm and possesses a system of membranes called the sarcoplasmic reticulum.
• The myofibril has alternating dark and light bands known as the A and I bands.
• Each isotropic (light) band possesses a central line called the Z line
• The distance between adjacent Z lines is a sarcomere
• Each A (dark) band has at its centre a lighter region called the H zone
• This may itself have a central dark line - the M line
This pattern of bands is the result of the arrangement of the two types of protein g found in a myofibril:
- Myosin is made up of thick filaments, which is held together by m line
- Actin is made up of thin filaments
• Where the two types overlap, the appearance of the muscle fibre is much darker
• A bands are therefore made up of both actin and myosin filaments
• I bands are made up solely of actin filaments
Muscle Structure
Electron Micrographs
Arrangement of the Myofilaments
The Sliding Filament Hypothesis
- Impulse reaches the neuromuscular junction (end plate).
- Synaptic vesicles fuse with the end-plate membrane and release a transmitter (e.g. acetylcholine).
- Acetylcholine depolarizes the sarcolemma.
- Acetylcholine is hydrolysed by acetylcholinesterase.
5.Provided the threshold value is exceeded, an action potential (wave of depolarization) is created the muscle fibre and travels through the T-tubules.
6.Calcium ions (Ca2+) are released from the sarcoplasmic reticulum.
7.Calcium ions unblock the binding sites on the actin filaments.
8.The myosin heads now become attached to the actin filament.
- The myosin head changes position, causing the actin filaments to slide past the stationary myosin ones.
10.An ATP molecule becomes fixed to the myosin head, it needs to become detached from the actin.
11.Hydrolysis of ATP provides energy for the myosin head to be “cocked“ (change shape).
12.The myosin head becomes reattached further along the actin filament.
13.The muscle contracts by means of this ratchet mechanism.
14.The following changes in the muscle fibre occur:
• I band shortens;
• Z lines move closer together (i.e. sarcomere shortens);
• H zone shortens.
• Calcium ions are actively absorbed back into the T- system.
• The actin filament becomes blocked against the myosin heads causing relaxation.
• ATP is regenerated.
Muscle Contraction and Relaxation
BioFactsheet - Eyes
Markscheme for Generation of AP and Transmission of Synapse
Diagram
