OCR A2 UNIT F214 COMMUNICATION

Specification:

1. Outline the need for communication systems within multicellular organisms, with reference to the need to respond to changes in the internal and external environment and to co-ordinate the activities of different organs

2. State that cells need to communicate with each other by a process called cell signalling

3. State that neuronal and hormonal systems are examples of cell signalling

4. Define the terms negative feedback, positive feedback and homeostasis

5. Explain the principles of homeostasis in terms of receptors, effectors and negative feedback

6. Describe the physiological and behavioural responses that maintain a constant core body temperature in ectotherms and endotherms, with reference to peripheral temperature receptors, the hypothalamus and effectors in skin and muscles

Why Multicellular Organisms need Communication Systems

The majority of animals and plants are multicellular with different organs and tissues carrying out specific functions

To ensure that these organisms are efficient, it is vital that the activities of different cells, tissues and organs are co-ordinated

Cell signalling and interactions between cells, tissues and organs was referenced in unit F211

In this section, the emphasis is on cells detecting changes in the external or internal environment (stimuli) so that a response can be brought about

In multicellular animals, the endocrine system (hormone production) and the nervous system are responsible for co-ordinating the activities of different cells, tissues and organs

In multicellular plants there is no nervous system but plant cells do produce chemicals with hormone type effects called plant growth regulators (this is covered in F215)

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Co-ordination between cells, tissues and organs depends upon cell signalling. This is the communication between cells so that they work together to coordinate the activities of different organs to bring about a response

In multicellular animals, coordination brought about by the nervous system and the endocrine system works by cell signalling

Definitions:

TERM DEFINITIONHomeostasis Maintenance of a constant internal

environment of an organism within narrow limits, despite greater fluctuations in the external environment

External environment This refers to the air, water or soil environment that living organisms are exposed to. A change in this environment may cause a living organism stress eg temperature changes

Internal environment In multicellular animals, the blood and tissue fluid bathe cells and are the immediate environment around cells

Negative feedback A mechanism to ensure that any change from the optimum condition in the internal environment is detected, corrected and returned to the optimum, constant/steady state condition

Positive feedback This mechanism is less common. It occurs when a change in the optimum condition is detected and moved further away from the constant/steady state condition

Stimulus A change in the internal or external environment that causes a response

Receptor Receptors may be cells or molecules within plasma membranes that detect a change from the internal optimum steady state condition (detect a stimulus)

Response A change in behaviour or physiology as a result of the environmental change (stimulus)

Effector Effectors are cells, tissues or organs that respond to cell signalling and bring about a change to maintain the optimum condition (this response would involve the negative feedback mechanism)

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Features of an Efficient Communication System in Multicellular Animals

Cover the whole organism

Allow cell to cell communication

Allow specific communication

Allow rapid communication

Result in both short term and long term responses

Some Physiological Factors that must be controlled in Mammals

Physiological Factor

Reason for Control

Organs detecting Changes

Effector Organs

Co-ordinating Systems

Feedback Mechanism

Core body temperature

Blood plasma glucose concentration

Blood plasma water potential

Blood pressure

Blood plasma CO2 and urea concentrations

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Principles of Negative Feedback

This feedback mechanism involves the following cell signalling processes:

Stimulus Receptor Communication Pathway Effector Response (nervous or hormonal systems)

Principles of Positive Feedback

Positive feedback may be beneficial or harmful. Some examples are detailed below

Beneficial Positive Feedback Mechanisms - Effect of Oxytocin in Childbirth and Breast Feeding

Oxytocin is a hormone secreted from the posterior pituitary gland

Oxytocin causes contractions of the uterus muscles that start labour, at the end of pregnancy

Although oxytocin causes the uterine muscle contractions, these muscle contractions (response) stimulate the release of more oxytocin

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and this increased hormone release stimulates more uterine contractions

Oxytocin also stimulates the release of milk from the nipples during breast feeding. The stimulus for this hormone release is the baby sucking the nipples. The more the baby sucks (stimulus) the more oxytocin is released and the more milk is ejected from the nipples

Harmful Positive Feedback Examples

1. Too Low Body Temperature

Changes in body temperature change enzyme activity

If body temperature is too low, enzyme activity will fall. The exergonic reactions (respiration) that release thermal energy will be slower and less heat will be released. The body will cool still further and enzyme reactions will be far too slow to support life

2. Too High Body Temperature

If body temperature is too high, enzyme activity increases. The exergonic reactions in respiration that release thermal energy will be faster and more heat will be released. The body will heat up still further

3. Breathing in Air containing a High Concentration of CO2

Breathing in air containing a high concentration of CO2 increases the blood plasma CO2 concentration. Chemoreceptors detect the high plasma CO2 concentration. The response is to breathe more rapidly to remove the CO2 at a faster rate. In turn, more CO2 enters the blood causing the breathing to be even more rapid

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Importance of Body Temperature Control

Changes in body temperature affect the structure and activity of globular proteins including enzymes

Comparison of Endotherms and Ectotherms – two groups of animals with different body temperature control mechanisms

Comparative Feature Endotherms Ectotherms

Types of animal Mammals and birds All animals except mammals and birds

Are physiological methods used?

Yes – many mechanisms are used

Less likely

Is change of metabolic rate a major part of temperature control?

Yes No

Are behavioural methods used?

Yes Yes – these are the main mechanisms for controlling body temperature. Ectotherms aim to maximise heat exchange directly with their environment

A constant body temperature is maintained within narrow limits

Yes No – body temperature fluctuates with changes in external environmental temperatures

Body temperature is maintained above external temperature

Yes No

Temperature Regulation (Thermoregulation) in Endotherms

Mammals and birds are endotherms that maintain a constant body temperature within narrow limits

Endotherms control their body temperature by both physiological and behavioural methods. Unlike ectotherms, they can alter their metabolic rate as a major part of their temperature control

Endotherms balance heat input and heat output – as shown on page 7

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Summary of Radiation, Conduction and Convection as Mechanisms of Heat Transfer

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Physiological Mechanisms to Control Body Temperature of Endotherms

Body Structure Involved

Response if Core* Body Temperature is too High

Response if Core* Body Temperature is too Low

Sweat glands in skin

Secrete more sweat onto skin

Water in sweat evaporates using heat from blood to supply latent heat of vaporisation

Less sweat is secreted onto skin

Less evaporation of water Less loss of latent heat

Hairs on skin Erector muscles relax to flatten hairs against skin to reduce the insulating air layer (important in hairy mammals)

More heat loss from skin by radiation and convection

Erector muscles contract to raise hairs to trap a layer of insulating air close to the skin (important in hairy mammals)

Reduces heat loss from the skin

Arterioles leading to capillaries in skin

Vasodilation of arterioles allows more blood to flow into skin capillaries

More heat radiated from skin

Vasoconstriction of arterioles allows less blood to flow into skin capillaries

Less heat is radiated from skin

Liver cells Rate of metabolism is reduced

Less heat generated from exergonic reactions such as respiration

Rate of metabolism is increased (stimulated by adrenaline and thyroxine)

More heat generated from exergonic reactions such as respiration and transferred to the blood

Skeletal muscles

No spontaneous contractions Spontaneous contractions (shivering) generates heat as muscle cells respire more

Lungs, mouth and nose

Panting increases evaporation of water from lungs, tongue and other moist surfaces, using latent heat of vaporisation

No panting therefore less loss of latent heat

Core body temperature is the temperature in body organs and tissues. The temperature control centre is in the hypothalamus and this monitors core body temperature

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Behavioural Mechanisms to Control Body Temperature of Endotherms

Behaviour if Too Hot Behaviour if Too Cold

Move to shade or hide in a burrow Move into a warmer place– bask in the sun

Orientate body to decrease surface area exposed to sun

Orientate body to increase surface area exposed to sun and heat gain from radiation

In shade, spread out limbs to increase surface area for heat loss by conduction and radiation

In extreme cold, reduce body surface area by rolling into a ball to reduce heat losses by radiation and conduction. Humans can wrap their arms around them and huddle up

Remain inactive to reduce internal heat generation

Move about to generate heat in muscles

Humans can wear fewer clothes Humans can put on more clothes to increase insulation of body surface

Humans eat cool salads and drink iced drinks

Humans can eat hot food or drink hot drinks

Advantages and Disadvantages of Endothermy

Advantages of Endothermy Disadvantages of Endothermy

Maintains a constant body temperature despite fluctuating external temperatures

A significant part of energy intake is needed to keep the body warm in colder climates

Activity possible regardless of external temperatures

Animal must consume more food to supply energy needs

Enables mammals and birds to inhabit colder parts of the planet

Less of the food consumed is used for growth

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Control of Temperature Regulation

The thermoregulatory control centre is in the hypothalamus, at the base of the brain

Temperature receptors in the hypothalamus monitor blood temperature as blood flows through the organ and detect changes in core body temperature (for humans this should be 37oC)

The hypothalamus also receives impulses from peripheral temperature receptors in the skin (see skin diagram page 9). This indicates to the hypothalamus that the external environment is very cold or very hot and is a warning that core body temperature may change shortly

The hypothalamus sends nerve impulses along motor neurones to the muscles and liver to control internal heat input. Nerve impulses are also sent to the skin to control heat output

Temperature Regulation in Ectotherms

What types of animals are ectotherms? .............................................

……………………………………………………………………………….

Ectotherms obtain most of their body heat from the external environment. They do not generate large amounts of heat inside the body

The body temperature of an ectotherm fluctuates with the external temperature.

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Ectotherms mainly use ……………………. mechanisms to control their body temperature

Temperature regulation relies upon increasing the exchange of heat with their environment

Advantages of being an Ectotherm Disadvantages of being an Ectotherm

Use less food in respiration Less active in cooler temperatures. Need to warm up in the morning to be active

Need to find less food. Can survive for long periods without eating

At greater risk of predation when less active

More energy from food used for growth

Will need energy stores to survive in the winter without eating

Adaptation How it regulates temperature

Example

Expose body to sun/ lie on a warm surface to warm up

More heat is absorbed by radiation/ conduction

Snake

Orientate body to sun to warm up

Exposes larger surface area for increased heat absorption

Locust, Lizard

Orientate body away from sun to cool down

Less surface area exposed and less heat absorbed

Locust

Hide in a burrow to cool down

Reduces heat absorption by keeping out of the sun

Lizard

Alter body shape by contracting or expanding its rib cage

Exposes more or less surface area to the sun to increase or decrease heat absorption

Horned lizard

Increased breathing movements

Evaporates more water. More heat loss through latent heat of evaporation

Locust

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