The need for communication
Metabolic processes rely on the action of enzymes. Enzymes need a specific set of conditions in which to work efficiently, which include: a suitable temperature, a suitable pH, an aqueous environment and freedom from toxins and excess inhibitors.
Organisms increase their chance of survival by responding to changes in their external and internal environment.
A stimulus is any change in the (external or internal) environment that causes a response.
A response is a change in behaviour or physiology as a result of a change in the environment. Examples of stimuli are sudden bright light or a cooler environment (external) and accumulation of excess waste or toxins (internal).
The internal cells and tissues are bathed in tissue fluid - this is the environment of the cells. It is maintained by the blood and the blood is monitored closely.
A good communication system is needed to ensure that the different organs are coordinated. It should: cover the entire body and enable cells to communicate with each other; enable specific and rapid communication; and allow short term and long term responses.
Cell signalling is the process by which cells communicate with one another. The two main communication systems are the neuronal system and the hormonal system.
Homeostasis is the maintenance of the internal environment in a constant state despite external changes. Living organisms need to keep many things constant: body temperature, blood glucose concentration, blood salt concentration, blood water potential, blood pressure and carbon dioxide concentration.
For homeostasis, the changes must be detected by sensory receptors which signals via neurones or hormones to the effector cells to respond. Negative feedback is the reversal of a change in the internal environment to return to an optimum steady state.
Optimum conditions Change away from optimum
Return to optimum Detected by receptors
Effector reverses change Communicates to effector
Positive feedback is a process which increases any change detected by the receptors. It tends to be harmful. An example of beneficial positive feedback is at the end of a pregnancy. As the cervix stretches oxytocin is released (anterior pituitary gland), causing contractions so the cervix stretches more which releases more oxytocin, until the cervix is fully dilated.
An ectotherm is an organism that relies on external sources of heat to regulate its body temperature.
The advantages of being an ectotherm are that ectotherms use less food for respiration and so they can survive long periods without eating. They can also use a greater proportion of energy from food for growth.
The disadvantages are they need to warm up in the sun before they can be active which puts them at greater risk of predation. They may not be capable of activity in the winter and need sufficient stores of energy to survive over winter without eating.
Ectotherms rely upon heat absorption and heat loss from and to the environment to regulate their body temperature.
When they are too hot, they will stay underground or in the shade and orientate body away from sun so there is less surface area for heat absorption.
When they are too cold, they will expose and orientate body to sun so more heat absorption occurs.
Physiological changes include altering body shape (rib cage or frills) to expose more or less surface area and changing breathing movements to evaporate more or less water.
Examples of ectotherms are snakes, locusts, lizard, insects and frogs.
An endotherm is an organism that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature.
The advantages are that there is a fairly constant body temperature regardless of external temperatures, activity is possible at all times and can inhabit colder parts of the planet.
The disadvantages are that more food is required for more metabolism to occur so that more energy is used to maintain body temperature. Also, less energy from food is used for growth.
Physiological mechanisms to maintain body temperature: more/less sweat (the water in sweat evaporates using heat from the blood to supply latent heat of vaporisation); panting increases/decreases; erector pili muscles contract/relax which causes the hairs on skin to either stand up or lie flat; vasodilation or vasoconstriction; liver cells metabolise more/less and skeletal muscles will shiver when cold.
Behavioural mechanisms: move into/out of sun, change amount of activity and expose more/less surface area to the sun.
The thermoregulatory centre in the hypothalamus monitors blood temperature and detects any change in core body temperature. The peripheral temperature receptors monitors the temperature in the extremities and sends the information to the thermoregulatory centre.