Homeostasis Notes

4.1, 4.2 Homeostasis

• Homeostasis: The maintenance of a constant internal environment (or optimum conditions) despite changes in the external or internal environment.

What Organisms Carry Out Homeostasis?

• In a general sense, all living things aim to maintain a stable internal environment for their cells in the face of change.
• Examples:
  • Unicellular organisms such as Amoeba
  • Plants
  • Invertebrates such as insects
  • Vertebrates such as reptiles and birds.
• Simple organisms (e.g., cells) regulate by their cell membrane.
• Complex organisms (e.g., birds) regulate through interrelated systems working together to maintain optimal conditions.

Conditions Maintained within Optimum Limits

• Complex organisms carry out more homeostatic processes to maintain a constant internal environment.
• Conditions within the internal environment are maintained within optimum limits such as:
  • $CO_2$
  • $O_2$
  • Salts
  • Water
  • Temperature
  • Wastes (e.g., nitrogenous wastes)
  • Glucose
  • pH

Cell Requirements Task

• For each cell requirement, explain why it needs to be maintained at an optimum level at all times.
  • $H_2O$
  • Glucose
  • Wastes (e.g., $CO_2$, nitrogenous wastes)
  • Salts
  • Temperature
  • $O_2$

Maintaining a Balance

• The maintenance of a constant or optimum internal environment is called Homeostasis.

Negative Feedback Model

• Biological parameter (e.g., body temperature)
  1. Stress
  2. Corrective mechanism comes into play
  3. Return to optimum
• Steady optimum cellular environment (set value)

Stimulus - Response Model and the Negative Feedback Mechanism

• Feedback mechanisms are triggered by a change in the internal environment:
  • The stimulus is a change in the internal or external environment/conditions, which is detected by a receptor.
  • The receptor communicates the stimulus detected to the “modulator” or “control center”.
  • The modulator interprets the stimulus, processes it, and coordinates the response.
  • The modulator sends a message to the effector, an organ/structure that will carry out the response.
  • The effector carries out the response, which will result in counteracting the stimulus.
  • The counteracting of the stimulus, i.e., the returning of the conditions to optimal, is referred to as Negative feedback.

Negative Feedback Mechanism

• Cancels or counteracts the original stimulus
  • Stimulus (input)
  • Response (output)
  • Receptor
  • Effector
  • CNS (coordinating center)

Maintaining Conditions Within Tolerance Limits

• All organisms have a certain limit or range within which their cells and bodies can function and survive.
• Homeostasis aims to keep the conditions within this limit or range.
• If the conditions move beyond that range, it can be fatal to the cell or organism.
• Tolerance range includes zone of physiological stress and optimum range. Outside of the tolerance range is the zone of intolerance.

Tolerance Limits - Examples

• Examples of abiotic factors:
  • pH (Too acidic/Too alkaline)
  • Temperature (Too cold/Too hot)

Examples of Homeostatic Mechanisms

• Example 1: Organisms regulate their internal conditions despite changing internal and external conditions by constant movement of substances across membranes.
• Example 2: Maintaining the tolerance limits of the organism’s enzymes.
  • Enzymes are critical for every single chemical reaction in the organism’s cells.
  • If the tolerance limits move beyond those of the enzymes, the cell is unable to function.

Additional Examples of Maintaining Optimal Conditions Through Homeostatic Mechanisms

• Changes to metabolic rate (sum of all the chemical reactions in an organism). E.g., increase metabolic rate increases body heat.
• Changes to physiological processes (the way the body operates). E.g., dilation of blood vessels increases blood flow near the skin surface to increase heat loss.
• Changes in behavior (activities an organism carries out). E.g., sheltering in a burrow during the heat of the day.
• Structural features that help to maintain optimum conditions. E.g., fur to trap a layer of air for increased insulation.

Text Questions - Past WACE Questions

• Outline the role of the effector in homeostasis. (2 marks)
  • The effector receives signals from the coordinating centre/modulator (1); brings about change (needed to maintain homeostasis) (1)
• Outline the role of the receptor in homeostasis. (2 marks)
  • Receptor detects stimulus (1); sends signal to the processor/coordinating centre/modulator
• State the defining feature of a negative feedback loop. (1 mark)
  • Response/feedback reduces stimulus
• A stimulus-response model consists of several parts
  • (i) stimulus; (ii) receptor; (iii) coordinating centre/modulator; (iv) effector; (v) Response
• Describe in general terms how an organism maintains its internal environment within its tolerance limits. (10 marks)
  • Homeostasis
  • Internal environment remains (relatively) stable/constant
  • Despite changes in environment
  • Achieved via negative feedback
  • The response reverses the stimulus/the change in the environment
  • (Negative feedback is a type of) stimulus-response model
  • Change in the (internal or external) environment
  • (Change is called) a stimulus
  • Receptor detects stimulus/change
  • Receptor produces a signal (may be chemical or electrical)
  • The signal is sent to a processing center or brain or central nervous system or modulator
  • Processing center or brain or central nervous system or modulator coordinates a response
  • A message is sent to effector (usually a muscle or gland in animals)
  • Effector brings about a response
  • Specific example (e.g., glucose levels in animals, water balance in a plant).