Section 6 - Organisms respond to changes in their environments

14 Response to stimuli

15 Nervous coordination and muscles

16 Homeostasis

Internal and external stimuli

Receptors

Nervous coordination

Skeletal muscle

Homeostasis

Homeostasis

Heart rate

External stimuli

Myogenic - contraction is initiated from within heart itself

Autonomic nervous system

Chemoreceptors and baroreceptors

Sino-atrial node

Electrical wave is transmitted throughout heart

CO2, pH, and pressure changes

Parasympathetic - decreases heart rate

Sympathetic - increases heart rate

Pacinian corpuscle

Rods and cones in retina

Distribution in retina (periphery and fovea)

Cone cells are sensitive to wavelength and colour

Retinal convergence with rods

Different light sensitives

Visual activity

Layers of connective tissue surrounding a neurone ending

Specific to detecting mechanical pressure

Converts mechanical energy into a generator potential

Stretch-mediated sodium channels are deformed when pressure is applied

Influx of sodium ions depolarises the membrane of the neurone

Responses

Plants

Stimulus - a change that is detected and leads to a response

Motile organisms

Kinesis

Change in speed of movement or rate of turning

Taxis

Movement (not growth) of an organism in response to a stimulus

Increases chance of survival

Three neurone reflex

Stimulus - receptor - sensory neurone - intermediate neurone - motor neurone - effector - response

IAA, an auxin, controls plant cell elongation

Gravitropism

Phototropism

Negative gravitropism in shoots - grow away from force of gravity

Positive gravitropism in roots

IAA is produced in tip of root an moves to the lower side of the root

IAA inhibits cell elongation in roots

Negative phototropism in roots

IAA synthesised in the tip moves down to the shaded side of the shoot

Shoots grow and bend towards light - positive phototropism

Nerve impulses

Synaptic transmission

Synapse

Neuromuscular junction - between motor neurone and muscle end plate

Junction between 2 neurons

Inhibitory synapses

Chloride ion enter postsynaptic neurone

Hyperpolarisation

Summation

Spatial - multiple presynaptic neurones

Temporal - one presynaptic neurone firing many times

Undirectionality

Always from presynaptic neurone to postsynaptic neurone

Action potential

Resting membrane potential

Neuron = nerve cell

Stimulus opens voltage-gated sodium channels

Influx of sodium ions cause depolarisation of axon, which reaches a threshold

All-or-nothing principle

Action potential - wave of depolarisation

At +40mV, sodium channels close and potassium channels open

Repolarisation

Hyperpolarisation - temporary overshoot of outward potassium ions

Sodium-potassium channel returns membrane potential to -65mV

Refractory period

Discrete impulses

Limits number of impulses

Unidirectionality

Around - 65mV, in a polarised state

Sodium-potassium pump in axon membrane

3 sodium ions out, 2 potassium ions in

Protein channels allow ions to diffuse in and out

Membrane is more permeable to K+ ions, so some K+ ions diffuse back out

Structure

Adapted to carry rapid nerve impulses

Schwann cells

Cell body

Dendrites - carry nerve impulses towards cell body

Axon - single fibre that carries impulses away from cell body

Axons can be myelinated

Myelin sheath and nodes of Ranvier between

Muscle fibres

Muscle contraction

Structure

Fast twitch

Slow twitch

Calf and back muscles

Slow, sustained contractions

Store of myoglobin to provide oxygen

Aerobic respiration

Mitochondria

e.g. bicep

Thicker myosin filaments

Glycogen

Strong, fast contractions for a short period of time

Phosphocreatine to generate ATP rapidly

Sliding filament mechanism

Sarcomeres shorten

Neuromuscular junction

Motor unit = group of muscle fibres supplied by a single motor neurone

Action potential enters T-tubules

Release of calcium ions into sarcoplasm

Calcium binds to tropomyosin

Uncovers actin binding sites

Myosin heads bind to actin

Cross-bridge formaation

Myosin heads pull on actin

ADP released

ATP molecule attaches to myosin head

Myosin head detaches from actin

ATPase hydrolyses ATP to ADP

Calcium ions activate ATPase

Releases energy

Myosin head returns to original position, cycle repeats

Actin slides past myosin

Sarcomere - unit of muscle

Muscle contains bundles of muscle fibres

Actin (thin) and myosin (thick) protein filaments

Skeletal muscle is responsible for voluntary movement of the skeleton

Sarcoplasm around muscle fubres

Contains mitochondria and endoplasmic reticulum

Muscle fibres contain many myofibrils

Myofibrils consist of sarcomeres

Z-line - boundary of sarcomere

I-band - length of actin filament

Lighter than the A-band

A-band - length of myosin filament

Darker due to overlap of filaments

H-zone - in the centre of the A-band

Myosin heads

Blood glucose

Osmoregulation

Liver

Insulin

Diabetes

Glycogenesis

Type 1 and type 2

Insulin-dependent or insulin independent

Gluconeogenesis

Glycogenolysis

Regulates channel proteins and glucose uptake in cells

Pancreas

Activates enzymes to convert glucose into glycogen

Decreases blood glucose concentration

Glucagon

Activates enzymes to convert amino acids and glycerol into glucose

Increases blood glucose concentration

Activates enzymes to convert glycogen into glucose

Adrenaline

Second-messenger model of hormone action

Blood glucose is controlled to ensure the cells have enough respiratory substrates

Kidney

Cortex and medulla

Water potential of blood detected in hypothalamus

Nephron = functional unit of kidney

Glomerular filtrate - forms in Bowman's capsule

Glucose and water are reabsorbed in the proximal convoluted tubule

Loop of Henle - maintenance of sodium ion gradient

Counter-current multiplier

Water is reabsorbed in the distal convoluted tubule and collecting duct

ADH from posterior pituitary

Maintaining an internal environment within set limits

Temperature

Detected by hypothalamus

Too hot - vasodilation, sweating, seeking shade

Too cold - shivering, vasoconstriction, raising hairs on skin

pH

Need to control to maintain enzyme activity

Changes in water potential in blood and cells can lead to shrinkage or bursting

Need to control to maintain enzyme activity

Negative feedback

Positive feeback

The change detected leads to a further deviation away from the normal value

The change detected leads to a reduction in the stimulus, turning the system off or restoring to original level

Example - control of blood glucose