This is a very detailed summary notes of 57 pages for OCR Biology A Module 5, including all the topics (Communication and homeostasis, Excretion, Neuronal communication, Hormonal communication, Animal and plant responses, Photosynthesis, and Respiration). It includes diagrams and photographs to ass...
5.1.1 Communication and Homeostasis
(a) the need for communication systems in multicellular organisms
Responding to change
Living things need to maintain a certain set of conditions inside their cells. Cellular activities rely on the action of
enzymes. Enzymes need a specific set of conditions to work correctly.
These include:
temperature
pH
aqueous environment that keeps substrates and products in solution
freedom from toxins and excess inhibitors
As cells undergo metabolic activities they use up substrates and produce products
The products may be toxic
They diffuse out into tissue fluid
Therefore, the activities of the cells alter their environment.
e.g Carbon dioxide
Builds up changing the pH of the environment around cell
Enzymes may denature
Co-ordination
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
(b) the communication between cells by cell signalling
One cell releases a chemical that is detected by another cell. The second cells will respond to the signal released by
the first cell.
(c) the principles of homeostasis
Homeostasis:
Maintenance of the internal environment in a constant state within set limits despite external changes.
Living organisms need to keep a number of conditions constant such as:
Body temperature
o Low temperatures slow down metabolic reactions; high temperatures can cause proteins to
denature
Blood glucose concentration
o A lack of it causes respiration to slow down or halt, as the cells now have no energy source
o Too much glucose in the tissue fluid can cause water to move out of cells by osmosis
Blood salt concentration
Water potential of the blood
o Lack of water in tissue fluid causes water to move out of cells by osmosis; causing metabolic
reactions in cells to slow down/stop
o Too much water in tissue fluid causes water to move into cells by osmosis; cell may swell/burst
Blood pressure
Carbon dioxide concentration
o Enters red blood cells and combines with water forming carbonic acid
o Dissociates to hydrogencarbonate and hydrogen ions
o Makes blood more acidic
o Haemoglobin absorbs the H+ ions to form haemoglobonic acid, causing it to lose oxygen molecules
they are carrying, reducing oxygen transport
,Negative feedback:
Reversal of any change of conditions ensuring that an optimum steady state can be maintained as the internal
environment is returned to its original set of conditions.
Positive feedback:
A process that increases any change detected by the receptors.
e.g Pregnancy
As cervix stretches the change is signaled to the
anterior pituitary gland
Secretes the hormone oxytocin
Oxytocin increases uterine contractions, which
stretch the cervix more
This causes more secretion of oxytocin
e.g Body getting too cold
Enzymes become less active
Exergonic reactions that release heat are slower and
release less heat
Allows body to cool further
Makes enzymes even less active
e.g Sodium ion channels in depolarisation
Stimulus → Receptor detects change → Communication system transmits message from receptor to effector →
Effector reverses change (through negative feedback) → Back to optimum condition
Structures:
Sensory receptors; such as temperature receptors are stimulated to send a message if they detect a change in
conditions
Communication system; such as nervous/hormone system signal between cells and are used to transmit a
message from receptor to effector cells.
Effector cells; such as liver cells, bring about a response that reverse change detected by receptor cells.
(d) the physiological and behavioural responses involved in temperature control in ectotherms and endotherms.
Ectotherm: an organism that relies on external sources of heat to regulate its body temperature
Advantages to being an ectotherm:
Use less of their food in respiration
They need to find less food and may be able to survive longer periods without eating
A greater proportion of energy obtained from food can be used for growth
Disadvantages:
Less active in cooler temperatures putting them at risk of predation.
May not be capable of activity during the winter meaning they need a sufficient store of energy to survive
over the winter without eating
Temperature regulation in ectotherms
Ectotherms do not use internal energy sources to maintain body temperature when cold. When active their muscle
contractions generate some heat from increased respiration.
Use behavioural/physiological methods
Endotherm:
uses internal sources of heat, such as that generated by metabolism in the liver, to maintain its body
temperature
Advantages of endothermy:
Fairly constant body temperature whatever the temperature is externally
Activity possible when external temperatures are cool - such as night/winter
Ability to inhabit colder parts of the planet
,Disadvantages:
Significant part of energy intake used to maintain body temperature
More food required
Less of the energy from food is used for growth (or more food needed in order to grow)
Temperature regulation in endotherms
Use internal sources of heat. Many chemical reactions in the body are exergonic - they release energy in the form of
heat.
Endotherms can increase the rate of respiration in the liver (an exergonic reaction) simply to release heat.
Vasodilation/constriction
Capillary
sphincters on Alternatively,
arterioles dilate capillary sphincters
allowing more on arterioles
blood to flow constrict, restricting
near to the blood flow into
surface of the capillaries
skin
Sweat glands
Secrete more sweat
Evaporates
Cooling effect
Hairs lie flat; hardly insulate skin; more heat lost by convection and radiation
Sweating kept to minimum
Hairs raise up
Trap a layer of insulating air on the skin surface
Reduce heat lost by convection and radiation
Skeletal muscle contractions
Contractions within the muscles (shivering)
Generate more heat as more respiration occurs
Behavioral mechanisms -e.g moving into shade, orientating body, remaining inactive, changing surface area
Endotherms monitor the temperature of their blood in the hypothalamus of the brain. If the core temperature drops
below optimum the hypothalamus sends signals to effectors to reverse the change.
If core temperature rises above optimum the hypothalamus sends signals that bring about the opposite changes -
negative feedback.
Role of peripheral temperature receptors
Peripheral thermo receptors stimulated
Impulse sent to hypothalamus
Vasoconstriction of arterioles to reduce heat loss
Prevents heat loss by radiation/conduction/convection
Increase metabolic rate to generate heat
Release of adrenaline
, Shivering to generate heat
Erector muscles raise hair to trap heat
5.1.2 Excretion as an example of homeostatic control
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