Hormonal communication, plant responses and homeostasis definitions

Hormonal communication and negative feedback

Exocrine gland

A gland that secretes its product into a duct, which carries it to a surface.

Endocrine gland

A ductless gland that secretes hormones directly into the blood.

Hormone

A chemical messenger, secreted by an endocrine gland into the blood, that acts on target cells carrying a complementary receptor.

Target cell

A cell with receptors complementary to a particular hormone, and therefore the only kind of cell that responds to it.

Hypothalamus

The region of the brain that monitors the blood and controls the pituitary gland.

Posterior pituitary

The lobe that stores and releases ADH and oxytocin, both made in the hypothalamus.

Anterior pituitary

The lobe that makes its own hormones, under the control of releasing factors from the hypothalamus.

Homeostasis

The maintenance of a stable internal environment within narrow limits, despite changes in the external environment.

Negative feedback

A control mechanism in which a deviation from the norm triggers a response that reverses the deviation.

Positive feedback

A mechanism in which a deviation from the norm triggers a response that increases the deviation.

Antagonistic

Describes two effectors or hormones whose effects on a variable are opposite.

Regulation of blood glucose and diabetes mellitus

Glycogenesis

The synthesis of glycogen from glucose, in liver and muscle cells. Promoted by insulin.

Glycogenolysis

The breakdown of glycogen to glucose. Promoted by glucagon and by adrenaline.

Gluconeogenesis

The production of glucose from non-carbohydrate sources such as amino acids and glycerol, in the liver. Promoted by glucagon.

Biosensor

A device that uses a biological molecule, usually an immobilised enzyme, to detect a specific substance and convert its concentration into an electrical signal.

Chromogen

A colourless substance that is converted into a coloured product, so that a reaction can be followed by eye.

Osmoregulation and kidney function

Nephron

The functional unit of the kidney: a renal capsule and its tubule, running from the cortex into the medulla and back.

Ultrafiltration

Filtration under pressure through a partially permeable barrier, separating small molecules from large ones.

Selective reabsorption

The return of useful substances from the filtrate to the blood, by transport proteins in the tubule wall.

Osmoregulation

The control of the water potential of the blood and body fluids.

Fibrous capsule

The tough outer layer enclosing a kidney, which protects it and holds its shape.

Renal pelvis

The funnel-shaped space at the centre of a kidney, into which the collecting ducts drain and which narrows into the ureter.

Antidiuretic hormone (ADH)

A hormone released by the posterior pituitary when the water potential of the blood falls. It makes the collecting duct wall more permeable to water by adding aquaporins, so more water is reabsorbed.

Aquaporin

A channel protein that lets water molecules cross a membrane by osmosis.

Osmoreceptor

A receptor cell in the hypothalamus that detects changes in the water potential of the blood.

The liver: detoxification, deamination and the making of urea

Excretion

The removal from the body of the waste products of metabolism, such as carbon dioxide and urea.

Egestion

The removal of undigested food and other material that has never taken part in the body's metabolism.

Deamination

The removal of the amino group from an amino acid, producing ammonia and a keto acid.

Detoxification

The conversion by the liver of a toxic substance into one that is less toxic and can be excreted.

Lobule

The repeating structural unit of the liver: a roughly hexagonal block of hepatocytes with a central vein at its axis.

Sinusoid

A wide, leaky capillary running from the edge of a lobule to its central vein, carrying mixed blood from the hepatic artery and hepatic portal vein.

Hepatocyte

A liver cell, carrying out detoxification, deamination, storage and the secretion of bile.

Kupffer cell

A macrophage fixed in a sinusoid, which engulfs bacteria and breaks down old red blood cells.

Thermoregulation in endotherms and ectotherms

Thermoregulation

The control of body temperature within narrow limits, despite changes in the temperature of the surroundings.

Endotherm

An animal that maintains its body temperature chiefly by heat released from its own metabolism, and controls it physiologically.

Ectotherm

An animal whose body temperature depends mainly on heat gained from its surroundings, and which controls it chiefly by behaviour.

Core temperature

The temperature of the blood and of the deep organs, as distinct from that of the skin and extremities.

Plant responses: tropisms, auxins and the classic experiments

Tropism

A growth response of a plant in which the direction of growth is determined by the direction of the stimulus.

Auxin

A plant growth substance, in practice indole-3-acetic acid, that promotes cell elongation in shoots.

Apical dominance

The inhibition of lateral bud growth by the apical bud.

Statolith

A dense, starch-filled organelle that settles to the lower side of a cell and so detects gravity.

Phytochrome

A photoreceptor protein in plants that exists in two interconvertible forms, Pr and Pfr, and controls responses to the presence, duration and colour of light.

Pr

The form of phytochrome that absorbs red light, about 660 nm, and is converted by it into Pfr.

Pfr

The active form of phytochrome, which absorbs far-red light, about 730 nm, and is converted by it, and slowly in darkness, into Pr.

Etiolation

The growth of a plant in darkness, with long, thin internodes and small, folded leaves that lack chlorophyll.

Photomorphogenesis

The control of a plant's growth and development by light, such as the de-etiolation of a seedling that reaches light.

Photoperiodism

The response of a plant to the relative lengths of day and night, such as the timing of flowering.

Short-day plant

A plant that flowers only when the night is longer than its critical night length.

Long-day plant

A plant that flowers only when the night is shorter than its critical night length.

Critical night length

The length of uninterrupted darkness that decides whether a photoperiodic plant flowers.

Auxin response factor

A transcription factor that activates auxin-responsive genes once IAA has caused the repressor proteins bound to it to be broken down.

Starch agar assay

A test for amylase in which tissue is placed on agar containing starch; after iodine is added, the size of the clear zone where the starch has been hydrolysed measures the amylase produced.

Aleurone layer

The layer of living cells around the starchy endosperm of a cereal grain, which makes amylase when gibberellin reaches it during germination.