• outline the need for communication systems within multicellular organisms, with reference to the need to respond to changes in the internal and external environment and to co-ordinate the activities of different organs

All living things to maintain a certain limited set of conditions inside their cells as cellular activity relies on the action of enzymes which only work efficiently at optimum conditions:

  • a suitable temperature – optimum for enzyme.
  • a suitable pH – optimum for enzyme.
  • an aqueous environment that keeps the substrates and products in solution – enzyme-substrate complexes can easily form as the molecules are quite close together.
  • freedom from toxins and excess inhibitors – inhibit the enzyme.

  • state that cells need to communicate with each other by a process called cell signalling

Cells communicate with each other by the process of cell signalling. This is where one cell releases a chemical that is detected by another cell and will respond to the signal released by the first cell.

  • state that neuronal and hormonal systems are examples of cell signalling

There are two major systems of communication that work by cell signalling:

  • Neuronal system – an interconnected network of neurones that signal to each other across synapse The neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly.
  • Hormonal system – uses the blood to transport its signals. Cells in an endocrine organ release the signal (a hormone) directly into the blood, where it is carried all over the body but only recognised by specific target cells. The hormonal system enable longer-term responses to be coordinated.

  • define the terms negative feedback, positive feedback and homeostasis
  • explain the principles of homeostasis in terms of receptors, effectors and negative feedback

Negative feedback – a process that brings about the reversal of any change in conditions. It ensures that an optimum steady state can be maintained, as the internal environment is returned to its original set of conditions after any change. It is essential for homeostasis.

For negative feedback to work effectively there must be a complex arrangement of structures that are all coordinated through cell signalling.

There are a number of structures required for this pathway to work:

  • Sensory receptors – e.g. temperature receptors or glucose concentration receptors. They are internal and monitor conditions inside the body. If they detect a change they will be stimulated to send a message.
  • A communication system – e.g. nervous system and hormonal system. This acts by signalling between cells. It is used to transmit a message from the receptor cells to the effector cells. The message may or may not pass through a coordination centre such as the brain.
  • Effector cells – e.g. liver cells or muscle cells. These cells will bring about a response that reverses the change detected by the receptor cells.

Positive feedback – a process that increases any change detected by the receptors. It tends to be harmful and does not lead to homeostasis.

When positive feedback occurs the response is to increase the original change. This destabilises the system and is usually harmful, e.g. when the body gets cold – below a certain core body temperature, enzymes become less active meaning the exergonic reactions that release heat are slower and release less heat, allowing the body to cool further and slows down the enzyme-controlled reactions even more, so that the temperature spirals downwards.

Positive feedback can also be beneficial. It can be used to stimulate an increase in a change, e.g. at the end of pregnancy to bring about dilation of cervix – as the cervix begins to stretch the change is signalled to the anterior pituitary gland, stimulating the secretion of the hormone oxytocin, leading to increased uterine contractions which stretch the cervix more, causing the secretion of more oxytocin, until the cervix is fully dilated and the baby can be born.

Homeostasis – the maintenance the internal environment in a constant state despite external changes, so enzymes can work at an optimum.

  • describe the physiological and behavioural responses that maintain a constant core body temperature in endotherms and endotherms, with reference to peripheral temperature receptors, the hypothalamus and effectors in skin muscles

Ectotherms – organisms that relies on external sources of heat to regulate its body temperature.

Endotherms – organisms that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature.

There are 4 ways that heat moves from one place to another:

  1. Conduction – when heat transfers from one surface tp another by direct contact (solids).
  2. Convection – when hot objects heat the particles around them, causing the heat energy to be carried up and away (liquids and gases).
  3. Radiation – where rays of infrared radiation transfer heat from any hot object to a cooler one (waves – vacuum).
  4. Evaporation – where heat energy is used to evaporate a liquid. The gaseous molecules then carry the energy away with them as they disperse.

The thermoregulatory centre in the hypothalamus monitors blood temperature and detects any changes in the core body temperature. An early warning of the body temperature could help the hypothalamus respond more quickly and avoid too much variation in the core body temperature.

The peripheral temperature receptors in the skin monitor the temperature in the extremitiesearly warning for the core body temperature. This information is fed to the thermoregulatory centre in the hypothalamus – negative feedback.