OCR Biology F214 Module 1

Stimulus --> Any change in the environment that causes a response

Response --> A change in behaviour or physiology as a result of a change in the environment

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 response

Cell signalling --> a process in which one cell will release a chemical that is detected by another cell, that will then respond to the signal.

• Neuronal system - neurones signal to each other across synapse junctions very rapidly, enabling response to a stimulus that is changing quickly.
• Hormonal system - hormones are carried in the blood, and recognised by receptors in the membrane of target cells. It enables coordination of longer-term responses            

--> The maintenance of the internal environment in a constant state despite external changes

Many conditions must be kept constant in the body:

• Body temperature
• Blood glucose concentration
• Blood pressure
• Water potential of the blood
• Carbon dioxide concentration

Negative feedback -->  A process that reverses any change in conditions. It ensures that an optimum steady state is maintained and is essential to homeostasis.

Negative feedback requires:

• Sensory receptors (e.g temperature receptors)
• A communication system (e.g hormonal/neuronal)
• Effector cells (e.g liver/muscle cells)

Endotherms

Can maintain their body temperature within strict limits, independent of external conditions.

Activity is possible in cool weather, and they can live in cool parts of the planet

They need a significant mmount of their energy intake to maintain body temperature, more food is required, and less of the energy is used for growth.

Sweat glands Secrete more sweat when hot - it evaporates, cooling the organism

Lungs, mouth, nose Panting increases evaporation of water

Hairs on skin Hairs stand erect, trapping warm air when it's cold

Arterioles Vasoconstriction/vasodilation warms/cools the organism

Liver cells Reduce the rate of respiration so less heat is generated from exergonic reactions

Negative feedback is the process by which the body regulates temperature. The thermoregulatory centre in the hypothalamus detects the change, and the nervous and hormonal system carry signals to the skin, liver and muscles.

Ectotherms

Their body temperature relies on external sources of heat

They use less food in respiration, use less energy from food in growth, and may hibernate.

They are less active in cooler temperatures, and may not be active in winter as they don't warm up sufficiently

Ectotherms change their behaviour/physiology to decrease/increase heat absorption

Exposing body towards sun allows more heat to be absorbed

Orientating away from the sun exposes less surface area for heat absorption

Hiding in burrows reduces heat absorption

Decreasing breathing movements evaporates less water

Neurones have a number of adaptations to enable them to transmit action potentials

• Long so can transmit it over a great distance
• Gated ion channels in the membrane control the entry/exit of Na, K and Ca ions
• A potential difference is maintained across their plasma membrane
• Na/K ion pumps actively transport Na out, K in
• Schwann cells wrap around the neurone insulates from nearby electrical activity
• Cell body contains the nucleus, many mitochondria and ribosomes
• Motor neurones have their cell body in the CNS and a long axon, carrying the action potential out to the effector.
• Sensory neurones have a long dendron, carrying the action potential from a sensory receptor to the cell body (just outside the CNS). A short axon carries the action potential from the cell body into the CNS.
• Many dendrites connect them to other neurones

At rest the gated channels are closed, and the Na/K pump actively transports 3 Na out for every 2 K in. The gates in a receptor region can be opened by energy changes.

1. The membrane begins in resting state - polarised -60mV.

2. Na channels open and Na diffuses in. The membrane depolarises -50mV

3. Voltage gated Na channels open and Na floods in. The potential difference reaches +40mV.

4. Na channels close and K channels open. Repolarisation occurs. The pd overshoots slightly as K channels are slow to close, and the cell is hyperpolarised -80mV.

5. The potential difference is restored and Na/K pumps restore the ion concentration

Because the potential difference overshoots slightly, there is a period (the refractory period) after every action potential where no more action potentials can be transmitted. This allows the cell to recover, and ensures that action potentials are only sent one way.

The opening of Na channels at a certain point upsets the balance of Na/K ions. This creates local currents in the cycle of the neurone, causing the Na channels further along to open.

- An action potential opens the Na channels at a certain point.

- Na diffuses into the neurone, upsetting the balance of ionic concentrations. The concentration of Na rises at the point where the Na channels are open.

- Na diffuses sideways, away from the region of increased concentration.

- The influx of Na depolarises the membrane. When it reaches the threshold potential -50mV, the voltage gated channels will open, transmitting an action potential.

- This movement of local ions is called a local current.

Cholinergenic synapses use acetylycholine as their neurotransmitter.

• An action potential arrives at the synaptic knob.
• The voltage gated calcium channels open and calcium diffuses in
• The vesicles fuse with the presynaptic membrane and acetylcholine is released by exocytosis
• Acetylcholine difffuses across the cleft and binds to the receptor sites on the sodium ion channels in the postsynaptic membrane, causing the sodium channels to open.
• Sodium ions diffuse into the postsynaptic membrane into the postsynaptic neurone.
• A generator potential or EPSP (excitatory postsynaptic potential) is created. If sufficient EPSPs combine, the threshold potential is reached, and a new action potential is created.

Acetylcholine is broken down by acetylcholinesterase in the synaptic cleft. The products (ethanoic acid and choline) re-enter the synaptic knob and are remade into acetylcholine, using ATP from mitochondria.

Synapses ensure that signals are carried in the right direction. Only the presynaptic knob contains vesicles of acetylcholine.

After repeated stimulation, the synapse is fatigued and no longer responds - this is called acclimatisation. It helps avoid overstimulation of an effector, and repeated unneccessary responses.

Temporal summation --> one action potential in the presynaptic neurone does not create an action potential in the postsynaptic neurone. Many action potentials or EPSPs are needed.

Spatial summaton --> Several presynaptic neurones may converge to one postsynaptic neurone. This means they can all contribute to creating an action potential after the synapse.

The brain can assess the intensity of a stimulus by frequency of impulse transmission. When a stimulus is at higher intensity more action potentials will arrive at the brain.

In myelinated neurones, an action potential is transmitted by saltatory conduction - they are much faster. 

Hormones are molecules released by endocrine glands directly into the blood. They carry a signal to a specific target organ/tissue (that possesses a complementary receptor).

Endocrine glands release hormones into the blood.

Exocrine glands secrete molecules into a duct and they're carried to their target site.

Protein hormones, e.g adrenaline, are unable to directly enter the target cell. Instead, adrenaline (first messenger) binds to a receptor on the membrane of the target cell, activating adenyl cyclase to convert ATP into cAMP (second messenger). The cAMP then stimulates a response inside the cell. 

The Adrenal Glands

Adrenal Medulla --> manufacture and release adrenaline

Adrenal Cortex --> uses cholesterol to make steroid hormones - mineralocorticoids control the Na/K concentration in the blood, while glucocorticoids control the metabolism of carbohydrates and proteins.

If blood glucose rises too high:

- It is detected by B cells in the islets of Langerhans, which secrete insulin into the blood.

- More glucose channels are put into the membrane, glycogenesis (glucose is converted to glycogen), more glucose is used in respiration/converted to fats.

-Blood glusose falls.

If blood glucose falls too low:

-It is detected by A cels in the islets of Langerhans, which secrete glucagon into the blood.

-Glycogenolysis (ocnversion of glycogen to glucose), more fatty acids are respired, gluconeogenesis (production of glucose by conversion from fats/amino acids.

- Blood glucose rises

This is an example of negative feedback.

The following process happens in B cells (where insulin is secreted).

1. When there are high glucose concentrations outside the cell, glucose diffuses in.

2. The glucose is metabolised to ATP, which closes the K channels and K builds up in the cell.

3. The change in potential difference opens the Ca channels.

4. Ca ions cause the release of insulin by exocytosis

Diabetes

A disease where the body cannot control its blood glucose concentration

Type 1 - insulin dependent. The immune system destroys B cells. Requires insulin injections

Type 2 - insulin receptors decline. Risk increased by obesity, a diet high in sugars, and family history. Requires careful monitoring and control of the diet.

Supplying more Oxygen and Glucose

The heart rate, contraction strength and stroke volume can be increased.

Control of Heart Rate

The heart muscle is myogenic and the SAN acts as it's pacemaker. Nerves run from the medulla oblongata to the SAN and vice versa. Action potentials sent down the accelerator nerve increase heart rate, and the vagus nerve does the opposite. The heart also responds to adrenaline.

Many receptors send signals to the cardiovascular centre in the medulla oblongata:

• Stretch receptors in the muscles when extra oxygen is needed.
• Chemoreceptors detect the pH drop when there is excess carbon dioxide.
• Blood pressure is detected by stretch receptors in the walls of the carotid artery. If it rises too high, heart rate is reduced.