5.1 COMMUNICATION AND HOMEOSTASIS

5.1.1

THE NEED FOR COMMUNICATION SYSTEMS

Cell signalling

The way in which cells communicate with each other

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.

What is a stimulus and what is a response?

The environmental change is a stimulus and how the organism changes its behaviour or physiology is its response.

The internal environment of organisms

Most multicellular organisms have a range of tissues and organs. Many of the cells and tissues are not exposed to the external environment - they are protected by epithelial tissues and organs such as skin or bark. In many animals the internal cells and tissues are bathed in tissue fluid.

How do the activities of the cells alter their own environment?

As cells undergo their various metabolic activities, they use up substrates and create new products. Some of these compounds may be unwanted or even toxic. These substances move out of the cells into the tissue fluid.

How is the composition of tissue fluid maintained?

by blood

blood flows throughout the body and transports substances to and from the cells

any wastes or toxins accumulating in tissue fluid likely to enter blood and be carried away, and are excreted so don't build up in blood

A good communication system will:

- Cover whole body

- Enable cells to communicate with each other

- Enable specific communication

- Enable rapid communication

- Enable both short-term and long-term responses

What are the two major systems of communication that work by cell signalling?

- Neuronal system

- Hormonal system

What is the neuronal system?

An interconnected network of neurones that signal to each other across synapse junctions.

The neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly.

What is the hormonal system?

A system that uses blood to transport its signals. Cells in an endocrine organ release the signal (a hormone) directly into the blood. The hormone is transported throughout the body but is only recognised by specific target cells. The hormonal system enables long-term responses to be coordinated.

5.1.2

HOMEOSTASIS

Effector

A cell, tissue or organ that brings about a response

Homeostasis

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

Negative feedback

The mechanism that reverses a change, bringing the system back to the optimum

Positive feedback

Mechanism that increases change taking system further away from 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

Aspects maintained by homeostasis may include:

- Body temperature

- Blood glucose concentration

- Blood salt concentration

- Water potential of the blood

- Blood pressure

- CO2 concentration

The mechanism of homeostasis

stimulus - receptor - communication pathway - effector - response

A number of specialised structures are required for this pathway to work:

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

- A communication system such as the neuronal system or the hormonal system. This acts by signalling between cells. It is used to transmit a message from the receptor cells to the effector cells via a coordination centre which is usually in the brain. The message from the receptor to the coordination centre are known as the input. The message sent to the effectors are known as the output.

- Effector cells such as liver cells or muscle cells. These cells will bring about a response.

Example of negative feedback

body temperature regulation

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

1. A change to the internal environment must be detected.

2. The change must be signalled to the other cells.

3. There must be an effective response that reverses the change in conditions.

Example of positive feedback

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 secretion of more oxytocin.

5.1.3

TEMPERATURE CONTROL IN ECTOTHERMS

Ectotherm

An organism that relies on external sources of heat to maintain body temperature

Endotherm

An organism that uses heat from metabolic reactions to maintain body temperature

Why must temperature be controlled in the body?

To prevent denaturing of enzymes

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

- Move into a sunny area

- Lie on a warm surface

- Expose a larger surface area to the sun

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

- Move out of the sun.

- Move underground.

- Reduce the body's surface exposed to the sun.

Advantages of ectothermy

Less food used in respiration.

More energy and nutrients from food can be used in growth.

Need to find less food.

Can survive long periods without food.

Disadvantages of ectothermy

Less active in cooler temperatures, meaning they are at risk from predators and cannot take advantage of food.

5.1.4

TEMPERATURE CONTROL IN ENDOTHERMS

Hypothalamus

The part of the brain that coordinates homeostatic responses

What is the response of the skin if the body is too hot?

- Sweat glands secrete fluid onto the skin surface; as this evaporates it used heat from the blood as the latent heat of vaporisation.

- Hairs and feathers lie flat to reduce insulation and allow greater heat loss.

- Vasodilation of arterioles and precapillary sphincters directs blood to the skin surface so more heat can be radiated away from the body.

What is the response of the skin if the body is too cold?

- Less sweat is secreted, so less evaporation means less heat is lost.

- Hairs and feathers stand erect to trap air, which insulates the body.

- Vasoconstriction of arterioles and precapillary sphincters leading to skin surface. Blood is diverted away from the surface of the skin and less heat is lost.

What is the response of the gaseous exchange system if the body is too hot?

- Some animals pant, increasing evaporation of water from the surface of the lungs and airways. Evaporation uses heat from the blood as the latent heat of vaporisation.

What is the response of the gaseous exchange system if the body is too cold?

Less panting, so less heat is lost.

What is the response of the liver if the body is too hot?

Less evaporation takes place, so less heat is released.

What is the response of the liver if the body is too cold?

Increased respiration in the liver cells means that more energy from food is converted to heat.

What is the response of the skeletal muscles if the body is too hot?

Fewer contractions mean that less heat is released.

What is the response of the skeletal muscles if the body is too cold?

Spontaneous muscle contraction (shivering) release heat.

What is the response of the blood vessels if the body is too hot?

Dilation to direct blood to the extremities so that more heat can be lost.

What is the response of the blood vessels if the body is too cold?

Constriction to limit blood flow to the extremities, so that blood is not cooled too much - this can lead to frostbite in extreme conditions.

What is the behavior of ectotherms when its too hot?

- Hide away from the sun in the shade or in a burrow.

- Orientate body to reduce surface area exposed to sun.

- Remain inactive and spread limbs out to enable greater heat loss.

- Wet skin to use evaporation to help cool the body. Cats lick themselves and elephants spray water over their bodies.

What is the behavior of ectotherms when its too cold?

- Lie in the sun.

- Orientate body towards sun to increase surface area exposed.

- Move about to generate heat in the muscles or, in extreme conditions, roll into a ball shape to reduce surface area and heat loss.

Advantages of endotherms

- maintain a fairly constant temperature despite the external temp

- activity possible most of the time

- ability to inhabit colder parts of the planets

- more likely to survive global warming

Disadvantages of endotherms

- 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 the energy and nutrients gained from food

- May overheat in hot weather

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.

What is an exergonic reaction?

A chemical reaction that releases energy

What is the role of peripheral temperature receptors?

The thermoregulatory centre in the hypothalamus monitors blood temperature and detects changes in the core body temperature. Peripheral temperature receptors in the skin monitor the temperature in the extremities. This information is then fed to the thermoregulatory centre in the hypothalamus. If the thermoregulatory centre signals to the brain that the external environment is very cold or very hot, the brain can initiate behavioral mechanisms for maintaining the body temperature.