Module 5: Section 5 - Plant and animal responses

Like animals, plants must also need to respond to external stimuli. This is important to:

• Avoid predation
  
• Avoid abiotic stress
  
• Maximise photosynthesis
• Obtain more light, water and minerals
• Obtain more light, water and minerals
• Ensure germination in suitable conditions

Types of stimuli

• Tannins - toxic to microorganisms and larger herbivores
• Alkaloids - scientists think they are a feeding deterrent to animals, tasting bitter
• Pheromones - chemicals which are released by one individual and which can alter the behaviour or physiology of another 

Types of response

• Tropism - a directional growth response in which the direction of the response is determined by the direction of the external stimulus - may be positive (a growth response towards the stimulus) or negative (a growth response away from the stimulus)
• They include:
  • Phototropism (light) – shoots grow towards light – they are positively phototropic
  • Geotropism (gravity) – roots grow towards the pull of gravity
  • Chemotropism (chemicals) – on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place
  • Thigmotropism(touch) – shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support
• Non-directional responses to external stimuli are called nastic reponses eg. thigmonasty - a non-directional response to the stimulation of contact

Cytokinins

• Promote cell division
• Delay leaf senescence
• Overcome apical dominance
• Promote cell expansion

Abscisic acid

• Inhibits seed germination and growth
• Causes stomatal closure when the plant is stressed by low water availability

Auxins eg. IAA 

• Promote cell elongation
• Inhibit growth of side shoots
• Inhibit leaf abscission (leaf fall)

Gibberellins 

• Promote seed germination and growth of stems

Ethene

• Promotes fruit ripening

Auxins are produced at the tips of shoots in flowing plants

IAA stimulates cell elongation

IAA is moved around the plant to control tropisms - moves by diffusion and active transport

This results in different parts of the plant having different amounts of IAA. The uneven distribution of IAA menas there's uneven growth of the plant eg:

• Phototropism - IAA moves to the more shaded parts of the shoots and roots - cells elongate in shoots so they bend towards the light, and growth is inhibited in roots so they bend away from the light 
• Geotropism - IAA moves to the underside of shoots and roots - cells elongate in shoots so they bend towards the light, and growth is inhibited in roots so they bend away from the light 

Stimulate the stems of plants to grow by stem elongation - helps plants to grow very takk

If a dwarf variety of a plant is treated with gibberellin, it will grow to the same height as the tall variety

Don't inhibit plant growth in any way (unlike auxins)

Stimulate seed germination by triggering the breakdown of starch into glucose in the seed

The plant embryo in hte seed can then use the glucose to begin respiring and release the energy it needs to grow

Giberellins are inhibited (and so seed germination is prevented) by abscisic acid)

The shoot tip at the top of a flowering plant is called the apical bud

Auxins stimulate growth of the apical bud and inhibit the growth of side shoots from lateral buds - apical dominance 

Apical dominance prevents side shoots from growing, saving energy and preventing side shoots from the same plant competing with the shoot for light

If you remove the apical but then the plant won't produce auxins, so the side shoots will begin growing by cell division and cell elongation

If you replace the tip with a source of auxin, side shoot development is inhibited - shows side shoot development is controlled by auxon

Auxins become less concentrated as they move away from the apical bud - tall plants will have a low auxin concentration near the bottom of the plant so side shoots will start to grow

Auxins

• Used in weedkillers
• Used as rooting power when taking cuttings
• Prevent fruit drop 
• Producing seedless fruit

Cytokinins

• Can delay leaf senescence
• Used in tissue culture to help mass produce plants - promote shoot and bud growth from small pieces of tissue taken from a parent plant

Gibberellins

• Can delay senescence in citrus fruit
• Acting with cytokinins they can make apples elongate, improving their shape
• Elongating sugar canes and grape stalks
• Brewing

Ethene

• Speeds up ripening in apples, tomatoes and citrus fruits
• Promotes fruit drop in cotton, cherry and walnut
• Promote female sex expression in cucumbers, reducing the chance of self-pollination (pollination makes cucumbers taste bitter) and increasing yield
• Promoting lateral growth in some plants, yielding compact flowering stems
• Storing fruits at a low temperature, with little oxygen and high carbon dioxide inhibits ethene synthesis so prevents fruit ripening

The nervous system is split into two main structural systems: the CNS (made up of the brain and the spinal chord) and the PNS (made up of neurones that connect the CNS to the rest of the body)

The PNS can be divided into the SNS (controls conscious activities eg. running) and the ANS controls unconscious activities eg. digestion.

The ANS can be divided into the sympathetic branch (gets the body ready for action, 'fight or flight', neurones release noradrenaline) and the parasympathetic branch (calms the body down, 'rest and digest', neurones release acetylcholine)

Cerebellum: region of the brain coordinating balance and fine control of movement

Cerebrum: region of the brain dealing with higher functions such as conscious thought; it is divided into two cerebral hemispheres

Hypothalamus: the part of the brain that coordinates homeostatic responses

Medulla oblongata: region of the brain that controls physiological processes (eg. heart rate and breathing rate)

Pituitary gland: endocrine gland at the base of the brain, below but attached to the hypothalamus; the anterior lobe secretes many hormones; the posterior lobe stores and releases hormones made in the hypothalamus

A reflex is where the body responds to a stimuli without making a conscious decision to respond, because the pathway of communication does not involve conscious parts of the brain - rapid

The blinking reflex

• Sensory nerve endings in the cornea are stimulated by touch
• A nerve impulse is sent along the sensory neurone to a relay neurone in the CNS
• The impulse is then passed from the relay neurone to the motor neurones
• The motor neurones send impulses to effectors - the orbicularis oculi muscles contract, causing the eyelid to close quickly and prevent the eye from being damaged

The knee jerk reflex

• Helps maintain posture and balance
• Strech receptors in the quadriceps muscle detect that the muscle is being stretched
• A nerve impulse is passed along a sensory neurone, which communicates directly with a motor neurone in the spinal cord
  The motor neurone carries the nerve impulse to the effector, causing it to contract so the lower leg quickly moves forward

1. Nerve impulses from sensory neurones arrive at the hypothalamus, activating both the hormonal system and the sympathetic nervous system

2. The pituitary gland is stimulated to release ACTH, which causes the cortex of the adrenal gland to release steriodal hormones eg. cortisol

3. The sympathetic nervous system is activated, triggering the release of adrenaline from the medulla of the adrenal fland

Effects:

• Heart rate is increased so blood is pumped around the body faster
• The muscles around the bronchioles relax, so breathing is deeper
• Glycogen is converted to glucose, so more glucose is avaliable for muscles to respire
• Muscles in the arterioles supplying the skin and gut constrict, and muscles in the arterioles supplying the heart, lungs and skeletal muscles dilate - blood is diverted from the skin and gut to the heart, lungs and sketal muscles
• Erector pili muscles in the skin constrict, making hairs stand on end so the animal looks bigger

1. SAN sends an electrical impulse over the atrial walls
2. Causes atria to contract
3. Conducted to AVN, where the electrical wave of excitation is conducted down the Purkinje fibres
4. Causes the ventricles to contract

Altering heart rate:

Increase - the receptor will send impulses to the medulla, which will send impulses down the accelerans nerve. This causes the release of noradrenaline, which binds to receptors at the SAN

Decrease - the receptor will send impulses to the medulla, which will send impulses down the vagus nerve. This causes the release of acetylcholine, which binds to receptors at the SAN

pH = chemoreceptors in the carotid arteries, the aorta and the brain (more carbon doixide produced during exercise - carbonic acid - decreases blood pH)

Blood pressure = stretch receptors in the carotid sinus

Movement of limbs = stretch receptors in muscles

Cardiac muscle

• Cell structure: myogenic, rows of cells connected through intercalated disks, striated
• Function: consists of atrial and ventricular muscle, which contract like skeletal muscle and excitatory and conductive muscle fibres, which conduct electrical impulses and control the heartbeat
• Some cardiac muscle is myogenic - can contract without nervous input
• Cross-bridges help ensure even electrical stimulation over the walls of the chambers of the heart 

Skeletal muscle 

• Cell structure: form fibres about 100 μm in diameter,multinucleate, many mitochondria, contain contractile elements called myofibrils, cell membrane called sarcolemma
• Location: all skeletal muscle
• Function: allow for the movement of the skeleton at joints

Smooth (involuntary) muscle

• Cell structure: spindle-shaped, around 500 μm in length when relaxed and 5 μm wide
• Location: walls of intestine, iris of eye, walls of arteries and arterioles 
• Function: contracts slowly and regularly, does not tire quickly, controlled by the ANS

1. When an action potential stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T tubules into the sarcoplasmic reticulum

2. This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm

3. Calcium ions bind to troponin, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament

4. This exposes the binding site, which allows the myosin head to bind - an actin-myosin cross bridge has formed

5. Calcium ions also activate ATPase which breaks down ATP into ADP + Pi to provide the energy for muscle contration

6. The energy released moves to the myosin head, which pulls the actin filament along

7. ATP also provides the energy to break the actin-myosin cross bridge so the myosin head detaches from the actin filament after its moved

8. The myosin head then reattaches to a different binding site further along the actin filament, a new actin-myosin cross bridge is formed and the cycle will be repeated

9. Many cross bridges form and break very rapidly, shortening the sarcomere and causing the muscle to contract

10. When the muscles stop being stimulated, the calcium ions will leave their binding sites on the troponin, so the myosin head can no longer bind to the binding site as it has been blocked by the tropomyosin

11. No myosin heads are attached to actin filaments, so the acin filaments will slide back to their relaxed position, lengthening the sarcomere

Aerobic respiration

• Most ATP is generated by oxidative phosphorylation in the cell's mitochondria
• Only works when there is oxgen - good for long periods of low intensity exercise eg. jogging

Anaerobic respiration

• ATP is made rapidly by glycolysis
• End product = pyruvate, which is converted to lactate, which can cause muscle fatique (only good for short periods)

Creatine phosphate

• CP in the sarcoplasm acts as a reserve store of phosphate groups
• The phosphate can be transferred from the creatine phosphate to ADP molecules, rapidly creating molecules of ATP
• Sufficient for 2 - 4 seconds

1. Action potentials arriving at the end of the axon open calcium ion channels in the membrane.

2. Calcium ions flood into the end of the axon

3. Vesicles of acetylcholine move towards and fuse with the end membrane 

4. Acetylcholine molecules diffuse across the gap and fuse with receptors in the sarcolemma

5. This opens sodium ion channels, which allow sodium ions to enter the muscle fibre, causing depolarisation of the sarcolemma

6. A wave of depolarisation spreads along the sarcolemma and down transverse tubules into the muscle fibre