Chapter 14: Homeostasis

Homeostasis, any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that is optimal for survival. Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function. Some of physiological factors that are controlled by homeostasis in mammals include:

  • Core body temperature
  • Metabolic waste (eg. carbon dioxide and urea)
  • Blood pH
  • Concentration of glucose in the blood
  • Water potential of the blood

Homeostasis in mammals relies on two different coordination systems to transfer information between different parts of the body:

    • Nervous system – information is transmitted as electrical impulses that travel along neurons
    • Endocrine system – information is transmitted as chemical messengers called hormones that travel in the blood

Deamination of Amino Acids

The liver is responsible for the breakdown of excess amino acids coming from digestion of protein. The reason for excess amino acids need to be excreted is because nitrogenous substance are damaging to the body, therefore they must be excreted.

The first step of amino acid excretion is deamination. Deamination is the removal of an amino group from a molecule. The amino group is removed from the amino acid and converted to ammonia. The rest of the amino acid is made up of mostly carbon and hydrogen, and is recycled or oxidized for energy (ATP). Ammonia is converted to urea by addition of carbon dioxide. Finally, the urea is released from liver into the blood and subsequently filtered out by Kidney to produce urine.


The kidneys are two bean-shaped organs, each about the size of a fist. They are located just below the rib cage, one on each side of your spine. Healthy kidneys filter about a half cup of blood every minute, removing wastes and extra water to make urine.

A tough, fibrous renal capsule surrounds each kidney and provides support for the soft tissue inside. Beyond that, two layers of fat serve as further protection. The adrenal glands lie on top of the kidneys. The adrenal glands sit on top of each kidney and are also called the suprarenal glands.

The kidneys are responsible for carrying out two very important functions:

    • As an osmoregulatory organ – they regulate the water content of the blood (vital for maintaining blood pressure)
    • As an excretory organ – they excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)

Kidney Function

  • Blood flows into your kidney through the renal artery. This large blood vessel branches into smaller and smaller blood vessels until the blood reaches the nephrons. In the nephron, your blood is filtered by the tiny blood vessels of the glomeruli and then flows out of your kidney through the renal vein.
  • As blood flows into each nephron, it enters a cluster of tiny blood vessels—the glomerulus. The thin walls of the glomerulus allow smaller molecules, wastes, and fluid—mostly water—to pass into the tubule. Larger molecules, such as proteins and blood cells, stay in the blood vessel.
  • As blood flows into each nephron, it enters a cluster of tiny blood vessels—the glomerulus. The thin walls of the glomerulus allow smaller molecules, wastes, and fluid—mostly water—to pass into the tubule. Larger molecules, such as proteins and blood cells, stay in the blood vessel.


  • The control of the water potential of body fluids is known as osmoregulation
  • Osmoregulation is a key part of homeostasis
  • Specialized sensory neurons, known as osmoreceptors, monitor the water potential of the blood (these osmoreceptors are found in an area of the brain known as the hypothalamus)
  • If the osmoreceptors detect a decrease in the water potential of the blood, nerve impulses are sent along these sensory neurons to the posterior pituitary gland (another part of the brain just below the hypothalamus)
  • These nerve impulses stimulate the posterior pituitary gland to release antidiuretic hormone (ADH)
  • ADH molecules enter the blood and travel throughout the body
  • ADH causes the kidneys to reabsorb more water
  • This reduces the loss of water in the urine

Blood Glucose Regulation

If the level of one hormone is higher or lower than the ideal range, blood sugar levels may spike or drop. Together, insulin and glucagon help maintain a state called homeostasis in which conditions inside the body remain steady. When blood sugar is too high, the pancreas secretes more insulin.

If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and may not be able to function normally. If the concentration of glucose in the blood increases above a certain level, this can also disrupt the normal function of cells, potentially causing major problems. The control of blood glucose concentration by glucagon can be used to demonstrate the principles of cell signaling.

In case blood glucose concentration is high:

  • The rise in glucose concentration is detected by beta cell in pancreas
  • Insulin is secreted by beta cells, inhibiting the action of alpha cells
  • Insulin travels to target cells known as hepatocytes in liver and muscle cells
  • Binding of insulin to the receptor on plasma membrane causes adenyl cyclase to convert ATP into cyclic AMP(cAMP)
  • cAMP acts as secondary messenger and activates enzyme controlled reaction thus causing more glucose to enter cell which is then converted to glycogen or fats and used for respiration.

In case bold glucose concentration is low:

  • Alpha cells detect change and secrete hormone called glucagon
  • Glucagon secretion inhibits beta cell action
  • Glucagon binds to receptors on cell surface membrane which causes a conformational change
  • This activates G-protein which activates adenylyl cyclase enzymes
  • cAMP formation is initiates and this activates protein kinases which tends to initiation of cascade of enzymes
  • The final enzyme that is activated is glucagon which stimulates the hepatocytes to convert glycogen to glucose.

Homeostasis in Plants

Plants carry out homeostasis – just like animals they need to maintain a constant internal environment. Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves.

This means stomata control the entry of carbon dioxide into leaves

Opening & Closing of Stomata

The opening and closing of stomata is governed by increases or decreases of solutes in the guard cells, which cause them to take up or lose water, respectively. In general, stomata open by day and close at night. During the day, photosynthesis requires that the leaf mesophyll be exposed to the air to get CO2.Stomata open and close in a daily rhythm. Even when the plant is kept in constant light or constant darkness, the daily rhythm of opening and closing of the stomata continues.

  • Opening of stomata during the day:
    • maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen
    • allows the outward diffusion of water vapour in transpiration
  • Closing of stomata at night when photosynthesis cannot occur:
    • reduces the rate of transpiration
    • conserves water

Guard cells control the opening and closing of the stomata by either inflating to allow water and gas exchange or deflate to prevent water loss.

Stomata inflates when the turgidity caused by increase in potassium ions occurs, thus decreasing the water potential hence causing water to enter guard cells. Stomata close following an excess water loss, usually in response to drop in light levels and lower rate of photosynthesis.

Abscisic Acids and Stomata Closure

Abscisic acid is produced in roots of plant when the water potential decreases in response to stress. The abscisic acid then activates a secondary messenger; calcium ions. Guard cells are sensitive to change in calcium ions concentration which leads to the closing of stomata. During times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata.

Guard cells have ABA receptors on their cell surface membranes. ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells. ABA also causes calcium (Ca2+) ions to move into the cytoplasm of the guard cells through the cell surface membranes

The calcium ions act as second messengers. They cause channel proteins to open that allow negatively charged ions to leave the guard cells. This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells. The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells. This loss of ions increases the water potential of the guard cells thus, water leaves the guard cells by osmosis. The guard cells become flaccid, causing the stomata to close.