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Diagram explaining ATP's role in powering cellular activities, with arrows showing ATP produced by mitochondria, chloroplasts, and glycolysis, and used for making biomolecules, transporting substances, and driving energy-requiring processes; includes a structural diagram of ATP with labeled adenine, ribose, and phosphate groups. Early experiments on blood circulation Diagram showing the steps of glycolysis, beginning with glucose and ATP, progressing through intermediates like glucose 6-phosphate and fructose 6-phosphate, and ending with the production of pyruvate, ATP, and NADH. A detailed inset highlights the enzymatic reactions and energy transfers from glyceraldehyde 3-phosphate to pyruvate. Diagram showing the knee jerk reflex: a reflex hammer strikes the patellar tendon below the knee, activating a stretch receptor in the quadriceps muscle. This sends a signal through a sensory neuron to the spinal cord, where it connects to a motor neuron. The motor neuron then signals the muscle to contract, causing the lower leg to kick forward. The diagram also illustrates the pathway of sensory and motor neurons and their connection in the spinal cord. Diagram showing the organization of the autonomic nervous system, with labeled pathways for parasympathetic and sympathetic nerves, highlighting synapses, neurotransmitters like acetylcholine and norepinephrine, and their receptor types. steroid hormones Neurohormones are released from neurosecretory nerve cells. These nerve cells are considered true endocrine cells because they produce and secrete hormones that enter the circulation to reach their target cells. Detailed diagram of a plasma membrane's lipid bilayer showing two layers of phospholipid molecules with hydrophilic heads facing outward and hydrophobic tails inward, embedded cholesterol, integral (intrinsic) proteins spanning the membrane, peripheral (extrinsic) proteins attached to the surface, and sugar side chains extending from the outer face.
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greylag. Flock of Greylag geese during their winter migration at Bosque del Apache National Refugee, New Mexico. greylag goose (Anser anser)
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Muscles of facial expression, human anatomy, (Netter replacement project - SSC). Human face, human head.
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Diagram explaining ATP's role in powering cellular activities, with arrows showing ATP produced by mitochondria, chloroplasts, and glycolysis, and used for making biomolecules, transporting substances, and driving energy-requiring processes; includes a structural diagram of ATP with labeled adenine, ribose, and phosphate groups.
adenosine triphosphate; physiology Adenosine triphosphate (ATP) is the power source of many biochemical reactions. It is produced in the cell structures and system listed at the left to energize the important life processes listed on the right. An abbreviated chemical formula of the structure of ATP is also shown. The two high-energy P―O―P bonds are responsible for its power.

Physiology

biology
Written by
Bradley Titus Scheer
Emeritus Professor of Biology, University of Oregon, Eugene. Author of Animal Physiology.
Bradley Titus Scheer
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Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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Physiology, study of the functioning of living organisms, animal or plant, and of the functioning of their constituent tissues or cells. The word physiology was first used by the Greeks around 600 bce to describe a philosophical inquiry into the nature of things. The use of the term with specific reference to vital activities of healthy humans, which began in the 16th century, also is applicable to many current aspects of physiology. In the 19th...

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