- a suitable temperature
- a suitable pH
- an aqueous environment
- freedom from toxins and excess inhibitors
The environment for cells is tissue fluid. To prevent the accumulation of waste toxins in the blood, they must be removed by excretion.
A good communication system will:
- cover the whole body
- enable cells to communicate with each other
- enable specific communication
- enable rapid communication
- allow for both short term and long term responses
Cell signalling works through the neuronal system and aslo the hormonal system.
Negative and positive feedback
Homeostasis is the maintenance of the internal environment despite external changes, keeping the environment constant and within narrow limits.
- Negative feedback reverses the change that has occured and returns the state back to the optimum. It is essential for homeostasis.
- Positive feedback amplifies and increases the change away from the optimum. This is often harmful and does not lead to homeostasis.
Stimulus ---> receptor ---> cell signalling ---> effector ---> response
Positive feedback may be beneficial in cases such as the end of pregnancy, in regards to the hormone oxytocin. Oxytocin increases uterine contractions, and also causes mother's tendency to pacify or protect their offspring.
Endotherms and ectotherms
Enzymes are globular proteins and their structure and efficiency will change depending on the temperature. An ectotherm is an organism that relies on external sources of heat to regulate its body temperature. An endotherm is an organism that uses internal sources of heat (eg. generated from metabolism in the liver) to maintain its body temperature.
Ectotherms use less of their food in respiration, meaning more of the energy from it can be used for growth. They can survive for longer periods of time without eating.
HOWEVER ...They're less active in colder climates which puts them at a greater risk of predation, and need to sufficient stores of energy to survive the winter without eating.
Endotherms need to eat more food, and even more so if it's needed for growth. A significant amount of the energy consumed is needed to maintain its body temperature. They monitor the temperature of their blood in the hypothalamous. If the temperature drops, the hypothalamous sends signals to reverse the change. This is done through increasing the rate of metablosim by the liver cells, which generates more heat by respiration, and the heat is transferred to the blood( exergonic reactions). Also by muscle contractions, and decreasing the loss of heat to the environment. This is all negative feedback.
The thermoregulatory centre in the hypothalamou monitors the blood temperature. The peripheral temperature receptors in the skin monitor the temperature in the extremeties.
Sensory receptors are energy transducers that convert one form of energy to another. They convert it into a form of electrical energy called a nerve impulse.
- Depolarisation is caused by the influx of sodium ions into a neurone (nerve cell) which causes a change in potential difference (charge) across the membrane. The inside of the cell becomes less negative than usual, resulting in depolarisation.
- Generator potentials are caused by one or two sodium channels opening, which results in a small change in potential.
- If enough sodium ions enter the cell then the potential difference increases and initiates an action potential.
- An action potential is achieved when the membrane is depolarised to about +40mV (an all or nothing response).
Different types of neurone:
- Sensory neurones (these carry action potentials from a sensory receptor to the CNS).
- Motor neurones (carry an action potential from the CNS to the effector eg. muscle/gland. Their cell bodies are in the CNS).
- Relay neurones (connect the sensory and motor neurones).