A LEVEL: Biology (F215), Responding to the Environment

Biology: F215

Responding to the Environment

PLANTS: RESPONSE TO THE ENVIRONMENT

tropism: directional growth response in which the direction of response is determined by the direction of the external stimulus

Phototropism: shoot growth towards light 

Geotropism: roots grow towards the pull of gravity

Chemotropism: movement determined by chemicals, - pollen tubes grow down the style, attracted by the chemicals, towards the ovary for fertilisation

Thigmotropism: shoots that wind around other plants for support.

PLANT HORMONES

Hormones coordinate plant responses to environmental stimuli. 

• produced by cells

Hormones can move around the plant by: 

• active transport
• diffusion
• mass flow in the phloem sap/xylem vessels

hormones influence: 

• cell differentiation
• cell elongation
• cell division

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PLANT GROWTH

The cell wall inhibits the ability for a plant cell to divide, so cell division can only happen in certain parts of the plant:

They are called MERISTEMS:

• Apical meristems: Tips/apices of roots and shoots - they make the roots/shoots grow longer
• Lateral bud meristems: In the buds - side shoots grow
• Lateral meristems: cylinder near the outside of the roots/shoots, responsible for making them wider
• Intercalary meristems: located between the nodes, responsible for shoot becoming longer

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GROWTH REGULATOR HORMONES: EFFECTS

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PHOTOTROPISM

The redistribution of auxin stimulated by light:  

1. Phototropin 1
2. Phototropin 2 

SHEDDING LEAVES 

CYTOKININS - Stops leaves of deciduous trees from ageing (sensescing) 

Q: HOW DOES IT DO THIS?

A: The leaves becomes a sink for phloem transport (leaf guaranteed to get a good supply of nutrients)

• so less production of cytokinins means that supply of nutrients lessens and senescence occurs. Leaves fall : abscission 
• auxin can prevent this

BUT

1. Senescence causes auxin production at the leaf tip to decrease
2. Cells in the abscission area are more susceptible to other growth regulators
3. Such as ethene, (low auxin production = high ethene production)
4. Stimulates production of cellulase - breaks down the cellulose cell wall
5. Occurs in the abscission zone, so petiole* separates from the plant    

*the stalk that joins a leaf to a stem.

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CONTROL OF PLANT GROWTH

Apical Dominace: when the growing apical bud inhibits the growth of lateral buds along down the shoot.

IMPORTANT:

• breaking the shoot tip (apex) means that the side shoots begin to grow
• auxin prevents the lateral buds from growing
• removed apex = decrease in auxin concentration

Hypothesis: 

normal levels of auxin prevents growth in the lateral buds

low level concentrations of auxin promotes lateral shoot growth

Other explanations:

• abscisic acid inhibits bud growth - high concentrations of auxin may help keep concentration of abscisic acid high, so when the apex is removed the reduced concentration of auxin may stimulate a decrease in abscisic acid concentration and the buds start to grow
• cytokinins are able to override the inhibition committed by auxin, high concentration of auxin makes the apex a sink for cytokinins when the apex is removed, the cytokinins spreads more evenly around the plant, so there is more growth in the buds.

GIBBERELLINS AND STEM ELONGATION 

GA1 + GA3 - compounds which stimulate cell elongation and cell division

able to occur due to a cascade reaction whereby the enzymes found in a Le allele break down G20 -G1

Commercial use of plant hormones

AUXINS

• prevents fruit drop
• taking cuttings, encourages root growth
• seedless fruit - unpollinated plants can be placed into a medium of the growth hormone
• herbicides - artificial auxins can kill weeds, over stimulated cell elongation makes the stem weak so that plants buckle and die

GIBBERELLINS 

Fruit production: 

delay in senescence means that fruits can be left unpicked for an extended amount of time - available in shops for longer

giberrellins can act with cytokinins to make apples look nicer

gibberellins allows stalks to grow

Brewing:

barley seeds have an aleurone layer that produced amylase which breaks down the starch into maltase - gene switched on by gibberellins, adding more increases speed of brewing. 

Sugar Production:

sugar canes store their sugar in the internodes, gibberellins stimulate cells elongation between the nodes, more storage space. 

Plant Breeding

gibberellins increase seed formination, so that plant breeder’s are able to produce with desired characteristics at a faster rate. 

seed companies spray gibberellins to increase rate of seed production by inducing the process. 

ps. inhibition of gibberellin means that plants can remain shorts and helps with lodging to prevent the stem of plants bending and buckling

CYTOKININS

• used to prevent leaf senescence, or yellowing of plants 
• used in tissue culture to help mass produce plants, promote bud and shoot growth so that lots of cultures can be taken from the the side branches

ETHENE

• Is a gas -> converted -> 2-chloroethylphophonic acid
• speeds up fruit ripening
• promotes fruit drop
• sex expression in cucumbers (female) reduces self pollination
• promotes lateral growth for plants

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THE BRAIN

Cerebrum: the largest and most recognisable part of the brain, responsible for the elements of the nervous system that are associated with being human, including thought, imagination and reasoning.  

• Cerebrum is divided into two hemispheres
• Joined by the corpus callosum
• Covered by a thin layer of nerve cell bodies known as the cerebral cortex.
• Conscious thought and emotional response
• Ability to override some reflexes
• Features associated with intelligence

CEREBRAL CORTEX

• Sensory areas: receive impulses indirectly from the receptors
• Association areas: compare input with previous experiences in order to interpret what the input is and then create an appropriate response. 
• Motor areas: send impulses to effectors

neurones from the cerebellum carry impulses in the motor area so that motor output to effectors are judged appropriately to the relation of the stimuli.

Medulla Oblongata:

• Non skeletal muscles - in control of the autonomic nervous system
• cardiac cycle
• respiratory centre

Hypothalamus:

• homeostatic mechanisms 
• endocrine functions, 

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ORGANISING THE NERVOUS SYSTEM 

The nervous system works alongside the endocrine system.

• CNS (spinal cord & brain)
• made of grey matter (billions of non-myelinated neurones)
• and of white matter (long myelinated axons and dendrons)
• peripheral nervous system (neurones that carry impulses in and out of the CNS)

SENSORY & MOTOR SYSTEM OF THE PERIPHERAL NERVOUS SYSTEM

Sensory neurones ——> The CNS ——> Motor neurones

nerves: lots of neurones bundled up together

somatic: motor neurones carry impulses away from the CNS to skeletal muscles - VOLUNTARY 

autonomic: motor neurones carry impulses away from the CNS to the cardiac muscle/smooth muscle in the gut wall/glands - NOT VOLUNTARY

Autonomic Nervous System- ‘self governing’, responsible for controlling majority of the homeostatic mechanisms, - can also control the heightened responses to do with stress response.

AUTONOMIC NERVOUS SYSTEM	SOMATIC NERVOUS SYSTEM
Non-myelinated	myelinated
always consist of at least two neurones (connections to effectors) - two connect at a GANGLION	somatic connections to effectors only require of one neurone
neurones: 1. parasympathetic 2. sympathetic	/

the sympathetic and parasympathetic neurone are antagonistic, action of one opposes the other.

PARASYMPATHETIC	SYMPATHETIC
active at rest (eg. sleep)	active at times of stress
neurones are connected at a ganglion in the target tissue - pre-ganglionic neurones vary in length	neurones are linked a ganglion outside the spinal cord, pre-ganglionic neurones are very short
post-ganglionic neurones secrete acetylcholine as the neurotransmitter at the synapse between the neurone and effector	post-ganglionic neurones secrete noradrenaline at the synapse between the neurone and the effector
effects of action: decreased heart rate pupil constriction decreased ventilation rate sexual arousal	effect of action: increased heart rate pull dilation increased ventilation rate orgasm

  

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COORDINATED MOVEMENT 

skeletal muscles —— connected by tendons to ——> bones

bones —— connected by ligaments to ——> bones

tendons: made of tough, inelastic collagen ball and socket joints - movement in all directions

gliding joints - movement in a wide rage of directions, lots of small bones sliding over each other

hinge joints - movement in one plane only (up and down)

muscles are antagonistic

CONTROL OF CONTRACTION

• controlled by the nervous system
• motor neurones are connected to muscle cells at a neuromuscular junction (specialised synapse)

The Motor Unit: the more motor units stimulated, the greater force of contraction. 

Also known as gradation of response.

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THREE TYPES OF MUSCLE

All muscles are only able to contract because they contain filaments containing the proteins: actin & myosin.   

1. involuntary muscle: smooth muscle

Can be found in the:

• walls of the intestine: circular and longitudinal bundles, peristalsis (moves food along)
• Iris of the eye: circular and radical bundles, controls intensity of entering light. Contraction of the radial muscles for pupil dilation and contraction of circular bundles for pupil constriction  

2. Cardiac muscle

There are three types of cardiac muscle:

• atrial muscle
• ventricular muscle
• specialised excitatory and conductive muscle fibres

Atrial and ventricular muscle have a longer duration of contraction. excitatory and conductive fibres are able to contract feebly but allow conduct of electrical impulses and the control the rhythmic heartbeat. 

Can contract without nerve stimulation, they are myogenic. 

Sympathetic stimulation: increase in heart rate

Parasympathetic stimulation: decrease in heart rate.

3. voluntary muscle: striated/skeletal muscle

leads to movement of the skeleton at the joints

muscles cells form a lot of fibres, of about 100 micrometers (μm)

each fibre is surrounded by a cell surface membrane: sarcolemma

muscle cell cytoplasm is known as sarcoplasma

muscle cells have a lot of organelles:

• many mitochondria
• extensive sarcoplasmic reticulum
• number of myofibrils with chain of smaller units called sarcomeres.

Two types of microfilaments, 

1. action
2. myosin

• notice how it is striated.

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THE SLIDING FILAMENT MODEL

Area from one Z line to the next is approx. 2.5micrometers in length. 

Z lines are closer during contractions as I-Band and H-Zone are reduced, the length of the A-band does not change.

TWO TYPES OF PROTEIN FILAMENT USED IN MUSCLE CONTRACTION 

THIN FILAMENT 

• Thin filament made of F-actin, coiled around each other
• The strand are made up of small globular protein subunits: G-actin
•   Tropomyosin coil around the F-actin, this reinforces it.
•   Troponin, attaches to the tropomyosin. 
• It has three polypeptides: actin, tropomyosin, and the Ca2+. 

THICK FILAMENT

• Bundles of protein myosin
• Myosin has a tail and 2 protruding heads

THE POWER STROKE

1. Myosin heads attach to the actin - forming a cross bridge
2. The head group bends backwards, thin filament is pulled along and overlaps with the thick filament. This is the power stroke, ADP and Pi are released
3. Cross bridge is broken when a new ATP molecule attaches to the myosin head
4. Head group moves backwards as ATP is hydrolysed into ADP and Pi. New cross-bridge is formed with the thin filament further along, and then the cycle occurs again. 

ROLE OF THE CALCIUM IONS 

actin-myosin binding sites are covered by the tropomyosin subunits.

->  So binding cannot occur nor can cross bridges so muscle contraction is not viable. 

->  Instead:

• action potential arrives as neuromuscular junction, 
• calcium ions released from the sarcoplasmic reticulum in the sarcomeres
• calcium ions diffuse through the sarcoplasm, and bind to the troponin molecules
• binding changes the shape of the troponin
• tropomyosin therefore moves away from the binding sites on the actin
• actin-myosin binding sites are uncovered
• cross bridges form
• myosin heads bind to the actin 
• power stroke and muscle contraction can occur 
• nervous stimulation stops - Ca2+ are actively transported back into the sarcoplasmic reticulum by carrier proteins 
• leads to muscle relaxation

ROLE OF ATP

• energy from ATP, is needed to break the cross-bridge connection

but how do we maintain this supply of ATP?

• Aerobic respiration, this is dependent on the supply of oxygen to the muscle cells and the availability of respiratory substrates 
• Anaerobic respiration, in muscle cell, sarcoplasm, is toxic (lactic acid), 
• Creatine phosphate, is able to donate a Pi, to ADP, by the enzyme: Creatine phosphotransferase.

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MUSCLES, NERVES AND HORMONES

Fight or Flight Response:

physiological:

• pupil dilation
• sweat production increases
• increase/decrease in heart rate
• metabolic rate increases

COORDINATION OF PHYSIOLOGICAL CHANGES

CEREBRAL —> HYPOTHALAMUS —> INCREASED ACTIVITY IN THE SYMPATHETIC 

PATHWAY —> ADRENAL MEDULLA —> ADRENALINE RELEASED

HYPOTHALAMUS ————————————> PITUITARY GLAND 

ANTERIOR PITUITARY GLAND: RELEASES ADRENO-CORTITROPIC HORMONE (ACTH)

ACTH then stimulates the release of CORTICOSTEROID hormones from the adrenal cortex - helps the body to resist stressors

stressors: stimulus that causes a stress response, wear or tear on the body’s physical or emotional resources.  

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INNATE BEHAVIOUR

Innate behaviour is any animal response that occurs without the need for learning. It is an inherited response, similar in all members of the same species and is always performed in the same way in response to the same stimulus. 

organisms are able to detect a change in environment (stimuli) and carry out an appropriate response through the operation of various effectors in order to adjust. (synoptic link to homeostasis)

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LEARNED BEHAVIOUR

Learned behaviour refers to animal responses that change or adapt with experience. There is a range of learned behaviour identified, from simply learning not to respond to a repeated stimulation to the ability to consider a problem and formulate a solution. 

CLASSIFYING LEARNED BEHAVIOUR

Habituation: being able to ignore certain stimuli because of repeated exposure to the stimulus results in neither a reward nor a punishment. 

avoids wasting energy in making escape responses to non-harmful stimuli. 

Imprinting: young animals becoming associated with another organism. Only occurs in the sensitive period.

Classical Conditioning: Pavlov’s dogs, animals or organisms react and respond to a pair of events, they respond to the first in anticipation of the second, this learning is passive and involuntary. 

Operant Conditioning: B. F. Skinner: specific behavioural reaction to a stimulus by including the prospect of award. Behaviour by association, active learning, and to an extent voluntary, in natural circumstances this is trial and error learning. 

Latent(exploratory) Learning: information gained through experience and retaining knowledge through exploration of the surroundings, and the beneficial and disadvantages for survival. 

Insight Learning: seen most highly, ability to think and reason in order to solve problems. the solution is then remembered. 

Wolfgang Kohler’s research involved using chimpanzees. 

PRIMATE BEHAVIOUR 

Organisation of groups: reveal that there are hierarchies which lead to social control within the group that protects all members. 

Primates have a more highly developed cerebral cortex. This is linked to:

• social development 
• social interaction 
• all social behaviours are derived from the extended dependency period of the offspring. (primates)

Social Behaviours in Gorillas:

• grooming (reinforces group hierarchy)
• care of offspring by the mother suckling the young
• imitational behaviour, young will copy the parents
• variety of calls, grunts and displays, (for danger, threats, fighting displays)
• facial expressions (for recognition)

Advantages to Social Behaviour

• birthing to a few young, means more maternal care, and group protection, improves chances of survival
• learned behaviour is possible
• security of a group results in protective and security for the survival of the next generation
• knowledge and food can be shared
• greater ability to deter predators away. 

HUMAN BEHAVIOUR

Dopamine:

• is a neurotransmitter and a hormone.
• precursor molecule for adrenaline and noradrenaline
• low levels of dopamine = Parkinson’s - treatment with L-dopa
• high levels of dopamine = schizophrenia 
• dopamine increases arousal and decreases inhibition = more creativity in connection with cerebral activity
• five different types of dopamine receptors
• each coded by a separate gene

The DRD4 receptor gene

DRD4: one of five genes that code for dopamine receptor molecules, dopamine can bind to each of these receptor molecules but they cause differing effects because they lead to different cellular responses. 

50 known variants of the DRD4 gene. 

Variants differ in a specific sequence known as variable number tandem repeat. 

Some of the variants of the DRD4 gene are inherited and this affects the behaviour and level and action of dopamine in the brain. 

Attention-deficit hyperactivity disorder (ADHD)

Drugs like methylphenidate (Ritalin), is used to treat ADHD, but this also affects the dopamine levels in the brain, some studies suggest that the particular receptor variant of DRD4 is what is more frequently found in those with ADHD. 

Addictive and risk behaviours

Some variants of the DRD4 gene may increase likelihood of gambling and addictive behaviours.

Giving L-dopa to one group and haloperidol to another group indicated that a difference in general arousal, and decrease n the risk taking levels of the individual.