A LEVEL: Biology (F214), Communication & Homeostasis

Communication

&

Homeostasis

(F214)

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 responses

Two systems of communication by cell communication:

1. Neuronal System
2. Hormonal System 

HOMEOSTASIS

Homeostasis can be defined as keeping the internal environment constant despite external changes. 

Negative Feedback: A process which brings about a reversal of any change in conditions. To ensure, that optimum conditions are maintained, and internal environment is returned to its original set of conditions after any change. It is essential for homeostasis.  

STIMULUS -> RECEPTOR -> COMMUNICATION PATHWAY -> EFFECTOR -> RESPONSE

Communication Pathway* (cell signalling)

Positive Feedback: Increases the effect of the change in conditions. Increases any change detected by receptors, can be harmful and does not result in homeostasis. 

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

Ectotherm: An organism that relies on external sources of heat in order regulate its body temperature. Body temperature in an ectotherm tends to fluctuate with the external temperature. 

Temperature Regulation in Ectotherms:

• Expose body to sun: Enables more heat to be absorbed.
• Orientate body to sun:  Exposes larger surface area for more heat absorption
• Orientate body away from sun: Exposes lower surface are so that less heat is absorbed
• Hide in burrow: Reduces heat absorption by keeping out of the sun
• Alter body shape: Exposes more or less surface area to the sun

MAINTAINING BODY TEMPERATURE - ENDOTHERMS

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

Control of Temperature Regulation

The temperature of the blood is monitored by the hypothalamus which is found in the brain!

If core body temperature drops below optimum, the hypothalamus will send signals to reverse the change: 

• Increased rate of metabolism in order to release more heat from exergonic reactions: respiration
• Release of heat through extra muscular contractions 
• Decreased loss of heat to the environment

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

SENSORY RECEPTORS

Sensory receptors detect a change in the environment, they are energy transducers which basically convert one form of energy into another. 

Generating Nerve Impulses

Neurones:

• Specific channel proteins complimentary to either sodium or potassium
• Some channels are permanently open. 
• Diffusion of ions

• Also contains carrier protein, 3Na+ / 2K+ [Sodium Potassium ion pumps]
• Cell membrane is polarised due to difference in charge, negative on the inside, positive on outside.
• Nerve impulses are made by altering the membrane permeability
• Sodium ion channels opening, increases membrane permeability, so sodium ions move down a concentration gradient. 
• Changing the potential difference and causing depolarisation.

SENSORY AND MOTOR NEURONES

Sensory Neurones —> Carry the action potential to the central nervous system (CNS)

Motor Neurones —> Carry the action potential from the CNS to the effector

Relay Neurones —> Connect the sensory and motor neurones

 Resting Potentials & Action Potentials

Resting Potential:

The potential difference across the neurone cell membrane is at rest and about -60 mV inside the cell compared to the outside. 

Even though a neurone is said to be at rest, it is still actively transporting ions across the cell surface membrane. The cell membrane is said to be polarised and so the potential difference across the membrane is -60mV is called the resting potential.

An Action Potential

At rest the gated sodium channels are kept closed. the sodium/potassium pumps use ATP to actively transport three sodium ions for every two potassium ions brought into axon.

TIMELINE

1.  Stimulus

 Excites the neurone, sodium ion channels open and Na+ ions diffuse into the axon. 
 The membrane is more permeable to Na+
 Membrane becomes less negative

2. Depolarisation

 If potential difference reaches -50mV, threshold potential is reached.

Voltage gated sodium channels open.
 More Na+ ions diffuse in

3. Repolarisation

Action potential is reached at +40mV.

Sodium voltage gated channels close
Potassium channels open
Membrane is more permeable to potassium ions
K+ ions diffuse out of the neurone down the concentration gradient
potential difference back to resting potential

4. Hyperpolarisation

Potassium ion channels are slow to shut

Slight overshoot

too many K+ ions have diffused out

Potential difference is more negative than resting potential

 5. Resting Potential

Ion channels are reset 

Na+:K+ ion pump returns membrane to resting potential

maintains resting potential until next stimulus

The Refractory Period

After an action potential, another one cannot be generated straight away.

Ion channels are recovering and cannot be opened. 

Is a time delay- action potentials do not overlap.

Roles of Synapses

Synaptic Divergence: one neurone connected to many neurones, info is transmitted to different parts of the body

Synaptic Convergence: many neurones connect to one neurone information, information can be amplified 

Summation

1. Spatial Summation - Two or more presynaptic neurones release neurotransmitters at the same time. Each small amount of neurotransmitter released from each neurone may be enough to rigger an action potential.

1. Temporal Summation: Two or more nerve impulses arrive in quick succession from the same presynaptic neurone, action potential is more likely to be generated because more is released into the synaptic cleft

Unidirectional Trasmission: 

synapses make sure that impulses only travel in one direction and are unable to go backwards.

THE ENDOCRINE SYSTEM

Ductless glands that consist of a group of cells that produce and release the hormones into the blood capillaries running through the gland. 

EXOCRINE GLAND: Does the opposite.

• Contain a small tube or duct that carries the secretion to a target receptor.
• Secretes molecules into a duct to be transported

Targeting the signal

To receive a hormone signal, it must bind to a complimentary hormone receptor - found on the plasma membrane. Cells which possess the specific receptor required are called target cells. These are then grouped to form target tissue. 

Two types of hormones:

1. Protein/Peptide hormones, derivatives of amino acids. 

2. Steroid hormones

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

ACTION OF ADRENALINE

 Adrenaline is an amino acid derivative and so cannot enter a target cell - is must cause an effect inside the cell without actually entering the cell. 

There is an adrenaline receptor on the surface of the cell membrane that has a complementary active site so that adrenaline can bind to the receptor. 

The receptor is linked to an enzyme inside the cell called adenyl cyclase. 

ADRENALINE PATHWAY

1. Adrenaline binds to receptors on the cell surface membrane
2. It is the FIRST messenger
3. Adrenaline - receptor complex activates the enzyme adenyl cyclase.
4. Adenyl cyclase converts ATP to AMP. 
5. Forming cAMP, this creates a cascade of enzyme action.
6. It is the SECOND messenger
7. Makes active glycogen phosphorylase.
8. Helps the breakdown of glycogen to glucose.

So what releases adrenaline? It is a hormone, so it is released by the endocrine system.

Adrenaline is released by the adrenal glands and they are found just above the kidneys. The adrenal gland can be separated into two parts: 

 Adrenal Medulla 

• Found in the centre
• Releases adrenaline
• ^ In response to stress (fight or flight)

• Quicker heart rate
• Dilated pupils
• Vasoconstriction
• Relax smooth muscles in the bronchioles
• Stimulates breakdown of glycogen to glucose
• Makes body hair erect
• Inhibits the action in the gut

Adrenal Cortex

• Uses cholesterol to produce steroid hormones

• Mineralocorticoids: Aldosterone, controls the concentration of sodium and potassium in the blood stream.
• Glucocorticoids: Cortisol, controls concentration levels of carbohydrates and proteins in the liver.

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

REGULATION OF BLOOD GLUCOSE 

The Pancreas and the Liver are the main players in the regulation of the blood glucose. 

The Pancreas: small organ below the stomach: has endocrine and exocrine glands. 

Islets of Langerhans: secretes insulin (BETA CELLS) & glucagon (ALPHA CELLS)

An example of NEGATIVE FEEDBACK. 

INSULIN

LOWERS GLUCOSE CONCENTRATION WHEN IT IS TOO HIGH

Binds to specific receptors on the liver plasma membrane and increases the permeability of cell membranes to glucose, so that cells take up more glucose. Inhibits the secretion of glucagon.

GLUCOSE —————————— ——> GLYCOGEN

(glycogenesis)

Insulin can also increase the rate of respiration of glucose in muscle cells.

GLUCAGON

RAISES BLOOD GLUCOSE CONCENTRATION WHEN IT IS TOO LOW

Binds to the specific receptors on the liver plasma membrane and activates enzymes that begin the process of glycogen breakdown into glucose.  Inhibits the secretion of insulin.

GLYCOGEN —————————— ——> GLUCOSE

(glycogenolysis)

GLYCEROL  —————————— ——> GLUCOSE

  (glucagoneogenesis)

Glucose also decreases that rate of respiration of glucose in the cells. Inhibits the secretion of insulin. 

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

CONTROLLING INSULIN SECRETION BY THE BETA CELLS

1.  High Concentration of Glucose on the outside of the cell

• glucose enters the cell by facilitated diffusion.
• the glucose is used in respiration and during the process of glycolysis, more glucose means more ATP is produced. 
• Increase in ATP causes the potassium channel to close. 

2. Potassium Ion Channels Close

• K+ ions accumulate within the cell, changing the polarity inside of the cell, it is less negative on the inside. 
• a depolarisation occurs

3. Calcium Ion Channels Open

• the voltage gated calcium channels open due to the depolarisation and this causes the vesicles containing insulin to move towards the plasma membrane and fuse.
• the vesicles release the insulin out of the cell by exocytosis. 

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

DIABETES

Type I Diabetes: Insulin dependent, can start in childhood or young adulthood. The body attacks and destroys the beta cels in the islets of Langerhans. Blood glucose concentration stays high even after food intake which can lead to death. 

Type II Diabetes: Non-insulin dependent. Beta cells do not produce enough insulin. cells do not respire, because liver cell receptors don't work properly.

GM BACTERIA

Insulin can be extracted from animal pancreases, although human insulin can be genetically modified, it is better because:

• Cheaper than extraction from animals
• larger quantities can be made
• Makes human insulin
• for ethical and religious reasons

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

CONTROLLING THE HEART RATE IN HUMANS

The heart pumps blood in the circulatory system.

Blood…

• supplies tissues with oxygen, glucose and nutrients
• removes waste products that may inhibit cell metabolism 

Adapting to the demands on the body: oxygen and glucose

• increased heart rate to pump faster
• increases strength of contractions
• increases volume of blood being transported per beat

Control of Heart Rate

• myogenic heart muscle
• contains the SAN that initiates an action potential to AVN to Purkyne tissue which can then activate a contraction of the ventricles
• nerves from medulla oblongata in Brain
• the medulla oblongata are linked to the SAN 
• action potential down accelerator nerve increases heart rate
• action potential down the vagus nerve decreases heart rate
• responds to the presence of adrenaline

INTERACTION BETWEEN CONTROL MECHANISMS:

In resting conditions there is a set frequency at which the heart beats, controlled by the SAN. 

However the frequency can be changed with influence from the medulla oblongata. (cardiovascular centre)

• exercise may increase heart rate due to increased need for oxygen and glucose. Chemoreceptors in the carotid arteries detect the increase in CO2 levels and this is sent to the medulla oblongata and back down the accelerator nerve to the SAN. 

• Exercise stops? The concentration of CO2 drops and this reduces the activity down the accelerator nerve, so the heart rate declines. 

• Adrenaline is secreted in response to stress. When released, it increases heart rate, and signal is send to the cardiovascular centre, the secretion of adrenaline helps the body prepare for activity.

• Blood pressure is measured by the baroreceptors in the carotid sinus and when the blood pressure is too high —> signals are sent to the cardiovascular centre and signal sent down the vagus nerve and this reduces the heart rate.

▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂

HIGH BLOOD PRESSURE

Detected by baroreceptors.

Sends an impulse along sensory neurone to the cardiovascular centre

Sends impulses along the parasympathetic neurone

Secrete acetylcholine

Binds to receptors on the SAN

Heart rate slows done back to normal

LOW BLOOD PRESSURE

Detected by baroreceptors

Sends impulse along the sensory neurone to the cardiovascular centre

Along the sympathetic neurone

Secretes noradrenaline 

Binds to receptors on the SAN

Heart rate speeds up.

Blood pressure is back to normal

HIGH BLOOD CONCENTRATION OF OXYGEN, LOW CO2, HIGH pH LEVELS

Chemoreceptors detect change

Sends impulse along the sensory neurone to the cardiovascular centre

Along the parasympathetic neurone 

Secretes acetylcholine

Binds to receptors on the SAN

Heart rate slow down

Oxygen, CO2 levels and pH go back to normal

LOW BLOOD CONCENTRATION OF OXYGEN, HIGH CO2, LOW pH LEVELS

Chemoreceptors detect change

Sends impulse along the sensory neurone to the cardiovascular centre

Sends impulses along the sympathetic neurone

Secretes noradrenaline

binds to receptors on the SAN

Heart rate increases

Oxygen, carbon dioxide and pH levels go back to normal.