communication and homeostasis

Why do multicellular organisms need communication systems?

Cells are specialised and often far apart, so they need to coordinate their activities so the whole organism works properly.

Cells that detect a change are often different from the cells that produce the response, so information must be transferred between them.

Animals have two communication systems-

Nervous system- uses electrical impulses, rapid responses, usually short lasting, signals travel along neurons.

Endocrine system- uses chemical hormones, generally slower, generally long lasting, hormones travek in the bloodstream.

What is cell signalling?- How is it used by hormones?

Communication between cells often involves sending a signal from one cell to another, allowing multicellular organism to coordinate its activities

Hormones are chemical signals that can travel through the bloodstream to target cells with specific receptors for those hormones.

The effects of hormones last a relatively long time because hormone levels can be maintained by endocrine glands.

Homeostasis

Maintaining a constant internal environment despite changes in the external environment.

Cell signalling

occurs between adjactent/near by cells or between cells in different parts of the body

Signalling cell produces a chemical signal, signal is detected by a receptor in another cell, recieving cell produces a response

Most signals are chemical although some neurons can communicate through electrical impulses.

Two types of cell signalling

Paracrine signalling- short distances- between nearby cells

Endocrine signalling- long distances- signalling molecules travel in the bloodstream

Receptors are usually proteins or glycoproteins and can be on the cell surface membrane or inside the cell in the cytoplasm.

principles of homeostasis- temperature

Maintaining a constant internal environment despite changes in the external environment.

Important factors include core body temperature, blood glucose concentration, and water content.

All life processes depend on enzyme-controlled reactions.

Enzymes are affected by temperature.

too low- enzyme activity becomes slow/ inactive

too high- enzymes can denature

Maintaining body temperature is therefore essential for normal metabolic reactions

Principles of homeostasis- water content

Condentartion of body fluids affect water potential gradients

These gradients are important for processes involving movement of water

The body can tolerate small variations but larger changes can be harmful

Increasing water potential can cause cells to burst

Principles of homeostasis- blood glucose

Glucose is important because it is a source of energy for respiration.

It is a solute so it affects water potential

Negative feedback

Homeostatic mechanism where a change from the normal/ optimum level triggers a response that reverses the original change, bringing the internal environment back towards its normal change.

Example:

Metabolic rate decreases below the normal level

The hypothalamus detects the change and releases thyrotropin-releasing hormone (TRH)

TRH stimulates the pituitary gland to release thyroid-stimulating hormone (TSH)

TSH stimulates the thyroid gland to produce thyroxine

Thyroxine increases metabolic rate, helping it return towards normal level

As thyroxine concentration increases, it inhibits the hypothalamus and pituitary gland, reducing the release of TRH and TSH.

Then less thyroxine is produced to prevent metabolic rate from increasing too far.

Positive feedback

Mechanism where a change causes a response that amplifies or increases the original change.

It causes a cycle that continues to increase until a particular event stops it.

Example:

Oxytocin

Uterine contractions begin during childbirth

Contractions stimulate the hypothalamus to release oxytocin from the pituitary gland.

Oxytocin increases the strength of uterine contractions

Stronger contractions cause more oxytocin to be released.

Acts as a positive feedback loop until the baby is born

Thermoregulation

human body temperature is 36+-0.500c

Temperatures of 38 or 35 °C indicate fever or hypothermia.

370C is the human optimum temperature.

Any significant variation from that temperature affects the rate of enzyme-controlled reactions.

Heat loss from the body

Radiation- loss of heat in the form of electromagnetic radiation from hot objects to cooler objects. It is the main way in which the human body loses heat, but it is also the way we gain heat from external sources.

Convection: is the movement of currents of warm air upwards because the density of the air decreases when it is warm.

Conduction: is the transfer of heat energy from a warmer material to a cooler one. The body can lose heat to the air by conduction, but because air is a good insulator, if a layer of air can be trapped around the body, it will reduce further heat loss by radiation. This is why we wear warm clothes in cold conditions.

Evaporation: evaporation of water from the skin cools the surface because the heat necessary to change the water into water vapour is extracted from the skin.

Temperature control in ectotherms

Animals whose body temperature is largely determined by the temperature of their surroundings.

They produce relatively little metabolic heat compared with endotherms.

Can tolerate a wider range of body temperatures.

Behaviour is more restricted by environmental temperature.

Ectotherms can survive better in situations where there is limited food available as they have lower metabolic demand and can go longer periods without eating.

Low body temperatures can reduce the rate of enzyme-controlled reactions, causing:

slower movement, reduced ability to catch prey, reduced ability to escape predators

If body temperature becomes too high, ectotherms may:

move into shade, enter water, reduce exposure to direct sunlight.

aqautic vs terrestial ectotherms

Aquatic ectotherms generally have fewer problems maintaining a stable temperature because:

Water has high specific heat capacity, which means a large amount of energy is required to change its temperature; therefore, water temperatures change relatively slowly compared to temperatures on land, as aquatic animals experience smaller environmental temperature fluctuations.

Terrestrial ectotherms

Ectotherms living on land experience greater temperature fluctuations because air temperatures vary considerably:

between day and night, between seasons, and depending on exposure to sunlight, wind and shade.

Adaptations of ectotherms

Behavioural adaptations: seeking sunlight to warm up, seeking shade or water t cool down, changing location depending on environmental temperature.

Countercurrent heat exchange in fish- cold blood returning from body surfaces passes close to warmer blood coming from internal tissues and muscles, helping to reduce temperature difference.

Dark coloration- dark colouring can alp ectotherms absorb and retain heat.

temperature control in ectotherms

An animal that maintains its body temperature using heat generated within its body tissues

Cooling mechanisms

Vasodilation of skin capillaries- widening of arterioles supplying the skin- widens through dilation and allows more blood to pass through, which increases blood flow through the skin capillaries and increases heat loss because more warm blood is brought close to the body surface and more heat is transferred from blood to surroundings.

Sweating-Skin has sweat glands to produce sweat in hot conditions to cool the skin. The evaporation of the sweat extracts heat from the skin to convert water into water vapour, so the skin is cooled. Meaning sweating is much less effective in humid conditions, as damp air is less effective at evaporating water (reduced conc gradient).

Flattening of the hair- Air trapped between the hairs on the skin forms an insulating layer. The hair erector muscles in the skin can raise hairs and relax them. In warm conditions, these muscles relax so the insulating layer of hair on the skin is thinner, allowing more heat to be lost.

Warming mechanisms

Boosting metabolic rate: the majority of reactions in the body are exothermic; they produce heat, which is why the body is warm. In cold conditions, thyroxine is released, which increases metabolic rate and increases heat production. The liver also plays a role in generating heat for the body.

Shivering: Nervous mechanism when core body temperature drops. Rapid and regular muscle contractions generate heat by metabolic reactions occurring in the muscle, which warms the blood.

Vasoconstriction: arterioles constrict so that not much blood reaches the capillaries near the surface. Blood is diverted through shunt vessels deeper in the skin so we do not lose heat to the surroundings.

Erection of hair: Erector hair muscles contract, meaning hair can trap a thick layer of air, which insulates the skin and reduces heat loss.

Behavioural homeostatic responses

When too hot animals seek shade, move to cooler places

When too cold animals huddle together; humans may wear thick clothing or build fires

To do this, animals must detect changes in external temperature.

Peripheral thermal receptors are found in the skin and mucous membranes.

They detect a change in the external environment and send information to the hypothalamus, they help coordinate behavioural and physiological responses.

Monitoring temperature in endotherms

The hypothalamus is the main temperature control centre.

It monitors the core body temperature as blood passes through it.

The hypothalamus contains:

temperature receptors, heat loss centre, and heat gain centre.

These centres coordinate responses using nervous and hormonal signals.

If body temperature is too high, the heat loss centre causes vasodilation and sweating

If body temperature is too low, the heat gain centres cause vasoconstriction, shivering, and increased thyroxine and adrenaline.

Sweating, shivering, vasoconstriction and vasodilation are controlled by nerve impulses via the autonomic nervous system.

Negative feedback mechanisms reverse these actions when core body temperature returns to normal.