AQA A Level Biology Topic 6: Organisms Respond to Changes in Environments

Practice vocabulary flashcards covering stimuli, nervous coordination, skeletal muscles, and homeostasis based on AQA A Level Biology Topic 6 notes.

Stimulus

A change in an organism's internal or external environment.

Tropism

The growth of a plant in response to a directional stimulus.

Indoleacetic acid (IAA)

A specific growth factor (auxin) that stimulates cell elongation in shoots and inhibits cell elongation in roots.

Gravitropism

A plant's growth response to gravity, where IAA accumulates on the lower side of the shoot or root to cause bending.

Phototropism

A plant's growth response to light, where IAA moves to the shaded side, causing shoots to bend towards light and roots to bend away.

Taxes (tactic response)

A directional response of a mobile organism involving movement towards or away from a directional stimulus.

Kinesis (kinetic response)

A non-directional response where the speed of movement or rate of direction change of an organism changes in response to the intensity of a stimulus.

Pacinian corpuscle

A receptor that responds to mechanical pressure; pressure deforms lamellae and opens stretch-mediated $Na^+$ channels to establish a generator potential.

Generator potential

The depolarisation of a receptor membrane (e.g., in a Pacinian corpuscle) that, if it reaches a certain threshold, triggers an action potential.

Rod cells

Photoreceptors in the retina that are highly sensitive to light due to spatial summation but provide lower visual acuity and monochromatic vision.

Cone cells

Photoreceptors in the retina that provide high visual acuity and color vision but are less sensitive to light because each connects to a single neurone.

Myogenic

Describes cardiac muscle's ability to contract and relax without receiving electrical impulses from external nerves.

Sinoatrial node (SAN)

The pacemaker of the heart located in the right atrium that sends regular waves of electrical activity to initiate contraction.

Atrioventricular node (AVN)

A node that receives electrical impulses from the SAN and delays them to allow the atria to fully empty before the ventricles contract.

Resting potential

The potential difference across an axon membrane when not conducting an impulse, established by the active transport of $3 \text{ Na}^+$ out and $2 \text{ K}^+$ in.

All-or-nothing principle

The principle stating that an action potential is only produced if depolarisation exceeds the threshold potential, and all action potentials produced are of the same magnitude.

Saltatory conduction

The rapid propagation of a nerve impulse along a myelinated axon, where depolarisation occurs only at the nodes of Ranvier.

Refractory period

The time following an action potential during which the axon membrane cannot be stimulated, ensuring discrete, unidirectional impulses.

Cholinergic synapse

A synapse that uses the neurotransmitter acetylcholine (ACh) to transmit impulses between neurones.

Spatial summation

The effect where multiple pre-synaptic neurones release enough neurotransmitter together to reach the threshold in a single post-synaptic neurone.

Temporal summation

The effect where a single pre-synaptic neurone releases neurotransmitter many times over a short period to reach the threshold in the post-synaptic neurone.

Inhibitory synapses

Synapses that release neurotransmitters to open $Cl^-$ or $K^+$ channels, hyperpolarising the post-synaptic membrane to reduce the likelihood of an action potential.

Neuromuscular junction

A specialized synapse between a motor neurone and a muscle fiber that is always excitatory.

Antagonistic pairs

Pairs of muscles, such as the biceps and triceps, that work in opposite directions to produce movement at a joint.

Sarcomere

The functional unit of a myofibril, consisting of the section between two Z-lines.

Actinomyosin crossbridge

The structure formed when a myosin head binds to an exposed binding site on an actin filament.

Phosphocreatine

A chemical stored in muscle fibers that provides a source of inorganic phosphate ($P_i$) to rapidly phosphorylate ADP into ATP during vigorous exercise.

Homeostasis

The maintenance of a stable internal environment within restricted limits via physiological control systems.

Negative feedback

A primary mechanism of homeostasis where a change from the optimum level is detected by receptors and corrected by effectors to return the system to normal.

Glycogenesis

The process activated by insulin that converts glucose into glycogen for storage in the liver.

Glycogenolysis

The hydrolysis of glycogen into glucose, activated by glucagon or adrenaline when blood glucose is low.

Gluconeogenesis

The conversion of non-carbohydrate sources, such as amino acids or glycerol, into glucose.

Second messenger model

The mechanism where a hormone (like adrenaline) binds to a receptor, activating adenylate cyclase to produce cyclic AMP (cAMP), which then activates enzymes inside the cell.

Type I diabetes

A condition where the $\beta$ cells in the islets of Langerhans produce insufficient insulin, typically due to an autoimmune response.

Type II diabetes

A condition where target cell receptors lose responsiveness to insulin, often linked to obesity.

Ultrafiltration

The process in the Bowman's capsule where high hydrostatic pressure forces small molecules out of the blood into the nephron to form glomerular filtrate.

Selective reabsorption

The process in the proximal convoluted tubule (PCT) where useful substances like glucose are reabsorbed from the filtrate back into the blood.

Osmoregulation

The homeostatic control of blood water potential, involving the hypothalamus, posterior pituitary gland, and ADH.

Antidiuretic hormone (ADH)

A hormone produced by the hypothalamus and released by the posterior pituitary that increases the permeability of the distal convoluted tubule and collecting duct to water.

Aquaporins

Channel proteins that are inserted into the cell-surface membranes of the collecting duct in response to ADH to increase water reabsorption.