Communication and homeostasis

  • Created by: Abi9ai1
  • Created on: 25-09-19 13:25

Enzyme conditions for survival

All living things need to maintain a certain limited set of conditions inside their cells.

These include:

  • A suitable temperature
  • A suitable pH 
  • An aqueous environment that keeps the substrates and products in solution.
  • Freedom from toxins and excess inhibitors.

Without these conditions the cells will become inactive and die. In multicellular organisms, cells are specialised and rely upon one another;therefore they must be able to communicate in order to coordinate their activities.

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Changing external environments

  • All living organisms have an external environment that consists of the air, water or soil around them. This external environment changes, which may place stress on the living organism.  For example, a cooler environment will cause a greater heat loss.
  • If the organism is going to remain active and survive, the changes in the environment must be monitored and the organism must change its behaviour or physiology to reduce the stress.
  • The environment change is a stimulus and the way in which the organism changes its behaviour or physiology is its response.
  • The environment may change slowly as the seasons pass, leading to a gradual response. E.g. The artic fox has a thick white coat in winter and a thin brown coat in summer.
  • However, the environment may change suddenly, e.g. a predator appearing. A stimulus (change) must be monitored and the organism must respond to the change.
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Changing internal environment

  • Most multicellular organisms have a range of tissues and organs. Many of the cells are not exposed to the external environment due to being protected by epithelial tissues and organs such as skin or bark.
  • In many animals, the internal cells and tissues are bathed in tissue fluid. This is the environment of the cells.
  •  As cells undergo various metabolic activities they produce waste products, some of these are toxic. These substances move out of the cells into the tissue fluid, causing a change in their environment.
  • An example of this is a build-up of carbon dioxide. It can alter the pH of the tissue fluid and could disrupt the activity of enzymes and other proteins. The build-up of this waste acts as a stimulus to cause the removal of the product so it doesn't cause the cell to die. It does this by stimulating more breathing to repel the carbon dioxide out of the body.
  • Build-up of waste products in the tissue fluid may also act directly on the cells, which respond by reducing their activities so that less waste is produced. However, this response may not be good for the whole organism.
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Maintaining the internal environment

  • The composition of tissue fluid is maintained by the blood. 
  • Blood flows throughout the body and transports substances to and from the cells.
  • Any wastes or toxins accumulating in the tissue fluid are likely to enter the blood and be carried away to be excreted.
  • It is important that the concentrations of waste products and other substances in the blood are monitored closely.
  • This ensures that the body doesn't excrete too much of useful substances but removes enough of the waste products to maintain good health.
  • It also ensures that the cells in the body are supplied with all the substrates they need.
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Coordinating the activities of different organs

  • A multicellular organism is more efficient than a single-celled organism because its cells are differentiated.
  • Therefore the cells can perform a particular function. Groups of cells specialized to perform specific functions form tissues and organs.
  •  These all perform different functions but need to be able to communicate and work together, so a communication system is needed.

A good communication system will:

  • Cover the whole body 
  • Enable cells to communicate with each other 
  • Enable specific communication
  • Enable rapid communication
  • Enable both short term and long term responses.
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Cell signalling

Cells communicate with each other by the process of cell signaling. This is a process in which one cell will release a chemical that is detected by another cell. The second cell will respond to the signal released by the first cell.

The two significant systems of communication that work by cell signaling are the:

Neuronal system- An interconnected network of neurons that signal to each other across synapse junctions. The neurons can conduct a signal very quickly and enable rapid responses to stimuli which may be changing quickly

Hormonal system- A system that uses the blood to transport its signals. Cells in an endocrine system release the signal directly into the blood. The hormone is transported throughout the body but is only recognized by specific target cells. The hormonal system enables a longer-term response to be coordinated. 

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Homeostasis definitions

Effector - A cell, tissue or organ that brings about a response.

Homeostasis- Maintaining a constant internal environment despite changes in internal and external factors.

Negative feedback- The mechanism that reverses a change, bringing the system back to the optimum.

Positive feedback- The mechanism that increases a change, taking the system further away from the optimum.

Sensory receptors- Cells/ sensory nerve endings that respond to a stimulus in the internal or external environment of an organism and can create action potentials.

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Homeostasis

Homeostasis is used in many living organisms to maintain conditions inside the body, despite changes in internal and external factors.

Aspects maintained by homeostasis may include:

  • Body temperature
  • Blood glucose concentration
  • Blood salt concentration
  • Water potential of the blood
  • Blood pressure
  • Carbon dioxide concentration
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The mechanism of homeostasis

Any response to changes in the environment requires a complex mechanism, which may involve a series of tissues and organs that are coordinated through cell signaling.

The standard response pathway is:

  • Stimulus
  • Receptor
  • Communication pathway (cell signaling)
  • Effector
  • Response
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Specialised structures for homeostasis

Sensory receptors

  • These receptors may be on the surface of the body, such as temperature receptors on the skin.
  • They monitor changes in the external environment.
  • Other receptors are internal to monitor conditions inside the body, for example, temperature receptors in the brain.
  • When one of these receptors detects a change, it will be stimulated to send a message to an effector.

Communication system (hormonal or neuronal)

  • This acts by signalling between cells.
  • It is used to transmit a message fro the receptor cells to the effector cells via a coordination centre which is usually in the brain.
  • The messages from the receptor to the coordination centre are known as the input. The messages sent to the effectors are known as the output

Effector cells- Cells such as liver or muscle cells that bring about a response.

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Feedback

  • When the effectors respond to the output from the coordination center, they bring about a response that will change the conditions inside the body.
  • The receptors will detect such changes.
  • This will have an effect on the response pathway.
  • In effect, the input will change.
  • This effect is known as feedback.
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Negative feedback

In order to maintain a constant internal environment, any change away from optimum conditions must be reversed. In this way, conditions inside the body will be returned to the optimum. This mechanism that brings the conditions back towards the optimum is known as negative feedback.

  • When conditions change, the receptors detect this stimulus and send an input to the coordination centre.
  • The coordination centre sends an output to the effectors and the effectors respond to this output.
  • When the effectors bring about a change that reverses the initial change in conditions, the system moves closer to the optimum and the stimulus is reduced.
  • The receptors detect the reduction in stimulus and reduce the input to the coordination centre.
  • The output from the coordination centre to the effectors is also reduced, so the effectors reduce their activity.
  • As the system gets closer to the optimum, the response is reduced.
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Negative feedback example

If the internal temperature rises too high, the response s to do something that brings the body back towards its optimum temperature. As a result, the stimulus is reduced.

For negative feedback to work, a number of processes must occur:

  • A change in the internal environment must be detected.
  • The change must be signaled to other cells.
  • There must be an effective response that reverses the change in conditions.
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Maintaining a constant internal environment

  • A negative feedback system can maintain a reasonably constant set of conditions. However, the conditions will never remain perfectly constant: there will be some variation around the optimum condition.
  • When a stimulus occurs it may take time to respond and the response may cause a slight "overshoot". However, as long as the variation isn't too great, the conditions will remain acceptable.
  • When negative feedback is applied to living systems, the conditions inside a living organism will remain within a relatively narrow range. The conditions will remain warm enough to allow enzymes to continue functioning efficiently, but cool enough to avoid damage to the body's many other proteins.
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Positive Feedback

  • Positive feedback is less common than negative feedback. When positive feedback occurs, the response is to increase the original change. This destabilizes the system and is usually harmful.
  • For example, below a certain core body temperature enzymes become less active and the exergonic reactions that release heat are slower and release less heat. This causes the body to cool further and slows the enzyme-controlled reactions even more. This means the body temperature spirals downwards.
  • There are also some occasions when positive feedback is beneficial. An example is seen at the end of pregnancy to bring about dilation of the cervix. As the cervix begins to stretch this causes the posterior pituitary gland to secrete the hormone oxytocin. Oxytocin increases the uterine contractions which stretch the cervix more, which causes the secretion of more oxytocin. Once the cervix is fully dilated, the baby can be born. The birth ends the production of oxytocin.
  • The activity of neurons also relies on positive feedback.
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Controlling body temperature

  • Changes in body temperature has a drastic effect on reactions in the cells. An increase in temperature causes an increase in kinetic energy which causes an increase in collisions. This means that when temperature increases reactions are faster, and when temperature is low reactions are slower.
  • An increase in temperature can cause vital proteins, such as enzymes, to become denatured so they cannot perform their function due to not having the correct tertiary structure.
  • If the body temperature gets too low then exergonic reactions happen too slow and release less heat. This is a form of positive feedback and can have very negative results (death).
  • The core temperature is an important factor, as all vital organs are found in the center of the body. Peripheral parts of the body may be allowed to increase or decrease in temperature to some extent without affecting the survival of the organism. 
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Endotherm or Ectotherm?

Endotherms 

  • Control their body temperature within very strict limits.
  • They use a variety of mechanisms to control body temperature and are largely independent of external temperatures.

Ectotherms

  • Are not able to control their body temperature as effectively as endotherms.
  • They rely on external sources of heat and their body temperature fluctuates with the external temperature.
  • However, using various behavioral mechanisms, some ectotherms are able to control their body temperature in all but the most extreme conditions.
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Temperature control in ectotherms

Ectotherms do not use internal energy sources to maintain their body temperature when cold. However, once they are active their muscle contractions generate some heat from increased respiration. Temperature regulation relies upon behavioral responses that can alter the amount of heat exchanged with the environment.

If ectotherms are not warm enough, they try to absorb heat from their environment. They may:

  • Move into a sunny area
  • Lie on a warm surface
  • Expose a large surface area to the sun

If ectotherms are too hot they try to avoid getting more heat and try to increase heat loss to the environment. They may:

  • Move out of the sun
  • Move underground
  • Reduce the body surface exposed to the sun.
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Ectotherm examples

Snake

  • Basks in the sun so it absorbs heat directly from the sun.

Locust

  • In the early morning, locusts sit side on to the sun exposing a large surface area, so it can absorb more heat. But at midday they climb to the top of the plant and face the sun head-on, exposing a small surface area. This means they get less heat from the sun and the soil.
  • Increases both the rate of breathing and the depth of breathing movements when it is hot. Therefore, more water evaporates from the tracheal system, cooling the body.

Lizard

  • Many lizards use burrows or crevices between rocks. They will hide in the burrow during the hottest part of the day and the coolest part of the night. This is because burrows have a more stable temperature.
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Advantages and disadvantages of ectothermy

Advantages

Ectotherms rely on external sources of heat to keep warm. They do not use up energy to keep warm. Therefore:

  • Less of their food is used in respiration.
  • More of the energy and nutrients gained from food can be converted to growth.
  • They need to find less food.
  • They can survive for long periods without food.

Disadvantages

They are less active in cooler temperatures. Therefore:

  • They are at risk of predators 
  • Unable to escape
  • Can't take advantage of food that becomes available
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Temperature regualtion mechanisms

  • Temperature regulation relies on the skin and the muscles. The skin is the organ in contact with the external environment, Therefore, many of the physiological adaptations involve the skin. The changes that take place in the skin alter the amount of heat being lost to the environment.
  • Many chemical reactions are exergonic- they release energy in the form of heat.
  • Endotherms can increase respiration (exergonic reaction) in the liver and muscles just to release heat.
  • They also direct blood towards and away from the skin to alter the amount of heat lost to the environment. 
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Physiological responses by endotherms

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Behavioural responses by endotherms

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Advantages and disadvantages of endothermy

Advantages

Endotherms can:

  • Maintain a fairly constant body temperature whatever the temperature externally.
  • Remain active even when external temperatures are low, which means they can take advantage of prey that may be available or escape from potential predators.
  • Inhabit colder parts of the planet.

Disadvantages

  • They use a significant part of their energy intake to maintain body temperature in the cold.
  • Need more food.
  • Use for growth a lower proportion of energy and nutrients gained from food.
  • May overheat in hot weather.
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Control of temperature regulation

The maintenance of core body temperature is important. If the core body temperature changes this alters the temperature of the blood. Temperature receptors in the hypothalamus of the brain detect this change. The hypothalamus then sends out impulses to cause different responses that will reverse the change. Some responses need to be quick in order to prevent further change in body temperature - the neuronal system transmits the output from the hypothalamus in order to make these responses rapid. Other responses may need to be longer-term; the hormonal system transmits the output to cause these responses.

If the core temperature is  too low, the hypothalamus will bring about:

  •  Changes in the skin to reduce heat loss.
  • Release of heat through extra muscular contraction.
  • Increased metabolism in order to release more heat from exergonic reactions.

If the core temperature rises above the optimum, the hypothalamus will bring about the opposite changes. This is an example of negative feedback.

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