5.1 Communication and Homeostasis

Abscisic acid (ABA)

​A plant hormone that stimulates stomatal cloning, maintains seed dormancy and triggers cold protective responses.

Abscission

​The shedding of leaves in plants.

Acetylcholine

​A type of neurotransmitter that is used for communication between neurones.

Actin

​A type of protein filament found in myofibrils. It forms thin filaments consisting of two long twisted chains.

Actin-myosin binding site

​A site on actin that is normally blocked by tropomyosin. During muscle contraction, it becomes exposed, allowing a myosin head to attach.

Actin-myosin cross-bridge

​The cross-bridge formed when a myosin head attaches to the myosin binding site on an actin filament.

Adenylyl cyclase

​An enzyme that catalyses the conversion of ATP to cAMP.

Adrenal glands

​Glands which are located on the top of the kidneys and produce adrenaline and steroid hormones.

Adrenaline

​A hormone that is secreted by the adrenal glands under stressful conditions. It serves as a ‘primary messenger’, activating adenylyl cyclase.

Afferent arteriole

​The larger diameter arteriole which carries blood to the glomerulus for ultrafiltration.

Alkaloids

​A group of nitrogenous compounds found in plants. They are bitter-tasting and affect the metabolism of animals or insects that take them in.

All-or-nothing principle

​The principle that describes how any generator potential which reaches or exceeds the threshold potential will produce an action potential of equal magnitude.

Alpha cells (​α cells)

​Cells found in the pancreas which secrete glucagon into the blood to raise glucose concentration.

Anisotropic (A) bands

​ The darker bands in a myofibril, which consist of overlapping actin and myosin filaments.

Anabolic steroids

​A class of performance enhancing drugs that are structurally similar to testosterone and used illegally by athletes to promote muscle growth.

Anterior pituitary gland

​The region of the pituitary gland that produces and secretes hormones.

Antidiuretic hormone (ADH)

​A hormone released from the posterior pituitary gland that increases the reabsorption of water in the kidney tubules.

Aquaporin

​A membrane channel used for the selective transport of water in and out of the cell.

Autonomic nervous system

​A branch of the nervous system that carries nerve impulses to muscles and glands. It controls involuntary activities and has two divisions

Auxins

​A class of plant hormones that control cell elongation, produce tropisms, prevent abscission, maintain apical dominance and stimulate the production of ethene.

Axon

​An extension from a nerve cell that carries impulses away from the cell body.

Beta cells (β cells)

​Cells found in the pancreas which secrete insulin into the blood to lower glucose concentration.

Bile canaliculi

​Vessels which collect the bile produced by hepatocytes.

Blinking reflex

​The involuntary blinking of the eyelid when an object is held close to the eye. It is the last reflex to be lost.

Cardiac muscle

​ A myogenic muscle found in the walls of the heart.

Central nervous system (CNS)

​The brain and spinal cord.

Cerebellum

​The region of the brain that controls muscle coordination and non-voluntary movement (e.g. balance, posture).

Cerebrum

​The largest region of the brain, consisting of two hemispheres, that receives sensory information from receptors and sends information via the motor neurones to effectors. It is responsible for all voluntary and some involuntary responses.

Cholinergic synapse

​A synapse which uses the neurotransmitter acetylcholine.

Creatine phosphate

​A compound stored in muscles that serves as a phosphate reserve, enabling ATP regeneration.

Cutting

​A small section of the root or stem of an adult plant.

Cyclic AMP (cAMP)

​A ‘second messenger’ involved in the action of adrenaline that activates protein kinase.

Dendron

​An extension from a nerve cell that carries impulses towards the cell body.

Depolarisation

​The rapid influx of sodium ions into the cell which cause it to lose its negative charge and the membrane potential to increase.

Ectotherm

​A type of organism which is dependent on its environment to maintain its body temperature.

Effector

​An organ, tissue, or cell that produces a response to a stimulus.

Efferent arteriole

​The smaller diameter arteriole which carries the blood away from the glomerulus after ultrafiltration.

Endocrine signalling

​A type of signalling that uses hormones secreted by endocrine cells into the blood which produce an effect on receptors.

Endotherm

​A type of organism which can regulate its own body temperature without relying on external heat sources.

Ethene

​ A plant hormone that stimulates the ripening of fruit and promotes abscission in deciduous trees.

Excitatory postsynaptic potential (EPSP)

​An impulse which stimulates an action potential in the postsynaptic neurone.

Excretion

​The process of removing metabolic waste from an organism.

Fight or flight response

​The physiological reaction of the body in response to a potentially dangerous situation (perceived threat).

Geotropism

​A plant’s growth response to gravity.

Gibberellins

​Plant hormones that control stem elongation, trigger the growth of the pollen tube during fertilisation and stimulate the mobilisation of food storage reserves during seed germination.

Glomerular filtration rate (GFR)

​A method of measuring kidney function which assesses the amount of blood plasma filtered per unit of time and produces a value which should be above 90mL/min in healthy adults.

Glomerulus

​The bundle of blood vessels at the beginning of a kidney nephron where ultrafiltration takes place.

Haemodialysis

​A type of dialysis used where a patient’s blood is fed into a dialysis machine and passed across an artificial membrane to remove waste.

Hepatic artery

​The main artery which supplies the liver tissue with oxygenated blood from the heart.

Hepatic portal vein

​The main vein which carries blood rich in nutrients from the small intestine and pancreas to the liver.

Hepatic vein

​The main vein which takes deoxygenated blood away from the liver back to the heart.

Homeostasis

​Maintaining a constant internal environment around an optimum despite external change.

Hormonal weedkillers

​Hormones such as synthetic dicot auxins that alter plant metabolism, promoting rapid growth that is unsustainable.

Hormones

​Cell signalling molecules produced by endocrine glands and released into the blood. They travel to target cells and bind to specific receptors, initiating a response. The effects of hormones are usually long-lasting.

Hyperpolarisation

​The drop in membrane potential below the resting potential after repolarization due to open potassium ion channels.

Hypothalamus

​The region of the brain that serves as the control centre for the autonomic nervous system. It is responsible for production of hormones, the regulation of the water potential of body fluids and the control of behavioural patterns.

H-zone

​The lighter region in the centre of each A band.

Inhibitory postsynaptic potential (IPSP)

​An impulse which inhibits the next neurone from generating an action potential.

Islet of Langerhans

​Regions of the pancreas which contain α and β cells.

Isotropic (I) bands

​The lighter bands in a myofibril, which consist of non-overlapping actin and myosin filaments.

Knee-jerk reflex

​A type of spinal reflex commonly tested by doctors. It involves the sudden involuntary movement of the lower leg when the patella is tapped.

Kupffer cell

​A specialised type of macrophage which is located within the sinusoids of the liver and removes pathogens and dead cells.

Loop of Henle

​A large hairpin shaped loop found in the kidney tubule used to regulate the water and salt concentration of the blood.

Negative tropism

​ The growth of a plant away from a stimulus.

Neurotransmitters

​Chemicals that are used for communication between neurones and their target cells. Neurotransmitters are stored in synaptic vesicles in the presynaptic neurone and released into the synaptic cleft.

Medulla oblongata

​The region of the brain that regulates autonomic activities such as ventilation, heart rate and peristalsis. It is made up of regulatory centres of the autonomic nervous system.

Metabolic waste

​Products produced in metabolic reactions which have no benefit to the organism.

Mimosa pudica​

​A plant that exhibits rapid leaf-folding and collapse in response to touch. This serves as a protective mechanism against larger herbivores and small insects.

Monoclonal antibodies

​Antibodies which have been produced by plasma cells that are identical clones of the same parent cell.

Motor neurone​

A neurone that carries nerve impulses from the CNS to the effectors. Myelination

Myofibrils

​Tiny contractile muscle fibres which group together. Numerous myofibril bundles constitute muscles. Myofibrils consist of two protein filaments

Myogenic

​Describes cardiac muscle tissue that initiates its own contraction without outside stimulation from nervous impulses.

Myosin

​A type of protein filament found in myofibrils. It forms thick filaments, consisting of long tails with bulbous heads, positioned to the side.

Negative feedback

​The product of a process that counteracts change to maintain an equilibrium around a normal level.

Neuromuscular junction

​An excitatory synapse formed between a motor neurone and a muscle fibre that uses the neurotransmitter, acetylcholine.

Nodes of Ranvier

​Unmyelinated sections of nerve cells which allow for the propagation of an action potential due to their many ion channels.

Osmoreceptor

​A type of receptor found in the hypothalamus which can detect the water concentration of blood plasma to maintain an appropriate water balance in the body.

Pacinian Corpuscle

​A type of sensory receptor which detects changes in pressure on the skin.

Pancreas

​An organ which has both exocrine and endocrine functions and is involved in both digestion and the homeostatic control of blood glucose levels.

Paracrine signalling

​A type of signalling which occurs between adjacent cells. Parasympathetic nervous system active under normal, resting conditions. It inhibits effectors, slowing down activity.

Peripheral nervous system (PNS)

​Pairs of nerves that originate from the CNS and carry nerve impulses into and out of the CNS. It is divided into the sensory nervous system and motor nervous system.

Peritoneal dialysis

​A type of dialysis where dialysis fluid is placed inside the abdomen and waste substances are passed across the peritoneal membrane from the blood.

Pheromones

​A cell-signalling molecule produced by an organism that affects the social behaviour of other members of the same species.

Photoperiodism

​A plant’s growth response to seasonal changes in the length of daylight. Phototropism

Pituitary gland

​A small, hormone-producing gland located at the base of the brain. It is divided into two regions, the anterior pituitary and the posterior pituitary.

Positive feedback

​A process which causes an increase in change away from the normal Positive tropism

Posterior pituitary gland

​The region of the pituitary gland that stores and secretes hormones produced by the hypothalamus.

Postsynaptic neurone

​The neurone after the synapse which contains specific receptor proteins on its membrane, complementary to the neurotransmitter.

Presynaptic neurone

​The neurone before the synapse which releases neurotransmitters from synaptic vesicles into the synaptic cleft.

Receptor

​Specialised structure that detects a specific type of stimulus.

Reflex

​A rapid, automatic response to a sensory stimulus by the body. It serves as a protective mechanism.

Reflex arc

​The pathway of neurones involved in a reflex action - stimulus --> receptor --> sensory neurone --> relay neurone --> motor neurone --> effector --> response

Relay neurone

​A neurone located in the spinal cord that links the sensory neurone to the motor neurone.

Resting potential

​The potential difference across the cell membrane of a neurone at rest which is typically between -60 and -70 millivolts (mV).

Rooting powder

​A hormone powder applied to cuttings that increases root formation and the chance of successful propagation.

Saltatory conduction

​The setting up of localised circuits between nodes of Ranvier which allows for the rapid propagation of an action potential.

Sarcomere

​Each repeating unit of striations between adjacent Z-lines.

Sarcoplasm

​The cytoplasm shared by muscle fibres. It consists of a high concentration of mitochondria and endoplasmic reticulum.

Schwann cells

​Cells that form the myelin sheath around nerve cells in the peripheral nervous system.

Second messenger model

​The mechanism by which a hormone (e.g. adrenaline or glucagon) has an effect inside a cell by triggering the production of a second messenger such as cAMP.