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e-Learning

Chapter 1

Communication

This photograph shows a slime mould, growing on a decaying tree. It is sometimes known as the dog vomit fungus.

Safety in numbers

In fact, slime moulds are not fungi at all, although they were for a long time classifi ed in that kingdom. They have an amazing life history, which makes it di� cult to classify them into any of the fi ve kingdoms. They are now placed in the Protoctista.

Slime moulds spend most of their lives as single cells, each like an amoeba. They crawl slowly around in damp leaf litter or rotting wood, capturing and digesting other microorganisms, such as bacteria, by phagocytosis.

But when food begins to run short, their behaviour drastically changes. A hungry slime mould cell secretes a chemical called cyclic AMP (cAMP), which acts as a signal to other slime mould cells in the vicinity. These other cells also begin to secrete cAMP, and to move towards areas where the cAMP concentration is greatest. Gradually, more and more of them collect up together in a heap.

Once the pile of slime mould cells has around 100 000 members, they begin to behave like a multicellular organism. This organism crawls slowly along, secreting slimy mucus as it goes – hence the name. After a few days, the cells begin to specialise to form a ‘fruiting body’. Some of the cells form a disc that holds the slime mould organism fi rmly to a surface, such as a piece of rotting wood. Some of them form a hollow tube leading upwards from the disc, and yet others move up through the tube and form a bulge at the top (see photograph below).

The cells at the top of the tube now round themselves up individually, and each produces a cellulose cell wall. They are now spores, and can remain in this state for a long time, even in di� cult conditions such as extreme temperatures or drought. Eventually, the fruiting body bursts, propelling the spores up into the air, where they are carried away by the wind. Any spore that lands in a suitable place ‘hatches’ into a single-celled slime mould, and the life cycle begins again.

Objectives

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• amount of water – lack of water in the tissue fl uid causes water to move out of cells by osmosis, which can cause metabolic reactions in the cells to slow down or even stop; too much water in the tissue fl uid causes water to move into cells by osmosis, which can cause the cell to swell and burst

• concentration of glucose – glucose is the fuel for respiration in many cells, so a lack of it causes respiration to slow down or halt, as the cells now have no energy source; too much glucose in the tissue fl uid can cause water to move out of cells by osmosis, having the same e3 ect as described above.

SAQ2 Suggest the advantages to an

organism of having a relatively constant core temperature.

Coordinating cell activitiesMost animals and plants are complex organisms, made up of many millions of cells. They are said to be multicellular. Di3 erent parts of the organism perform di3 erent functions. It is important that information can pass between these di3 erent parts, so that their activities are coordinated.

Sometimes, the purpose of this information transfer is to regulate the levels of some substance within the organism, such as the control of blood glucose levels in mammals. Or it may be to change the activity of some part of the organism in response to a stimulus, such as salivating when you smell good food cooking, or moving away from someone you do not want to talk to.

In both plants and animals, chemicals called hormones (in plants they are sometimes known as plant growth regulators) help to transfer information from one part to another. In many animals, including mammals, nerves transfer information in the form of electrical impulses. Both of these methods, hormonal and nervous, involve information from one cell being passed on to another, a process called cell signalling. We will look in detail at how cell signalling occurs in the nervous system in Chapter 2, and the hormonal (endocrine) system in Chapter 3.

HomeostasisOne of the most crucial functions of the nervous and hormonal systems in mammals is to control internal conditions, maintaining a stable internal environment. This is called homeostasis.

‘Internal environment’ means the conditions inside the body, in which cells function. For a cell, its immediate environment is the tissue fl uid that surrounds it. Many features of this environment a3 ect the activities of the cell. They include:

• temperature – low temperatures slow down metabolic reactions; high temperatures can cause proteins to denature, causing damage to enzymes and to cell membranes

SAQ1 Explain why low temperatures

slow down metabolic reactions.

The principles of homeostasisIt is easiest to understand how homeostasis works by looking at one particular example. Humans are mammals, and we share one of the most distinctive mammalian characteristics – the ability to control core body temperature. (Birds are the only other group of organisms able to do this to the same extent.) Whatever the temperature of the environment outside the body, the cells within it are kept at a temperature close to 37 °C. This ensures that metabolic reactions taking place inside the cells do not speed up or slow down just because the external temperature changes.

Hint

Answer

Hint

Answer

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Figure 1.1 Negative feedback.

Receptor senses change in parameter.

Effector receives information from receptor.

Effector acts to increase parameter.

Effector acts to decrease parameter.

Parameter rises too high.

Parameter falls too low.

The control of core temperature, like water content and glucose concentration, involves a mechanism called negative feedback. The general principles of negative feedback are shown in Figure 1.1. A negative feedback system requires a receptor and an e� ector and an e� cient means of communication between them. The receptor monitors the factor that is being controlled – in this case, the temperature of the blood. If the value of this factor is not within a suitable range, the receptor communicates with the e3 ector. The e3 ector then causes an action that brings the factor back towards normal.

In the control of temperature, the communication between receptor and e3 ector involves both the nervous system and hormonal system. In general, the nervous system brings about rapid, short-term changes, whereas hormones act more slowly but have longer-term e3 ects.

Because there is inevitably a short time delay between a change in the factor, its detection by the receptor, communication with the e3 ector and the action of the e3 ector, the control does not happen instantly. This results in an oscillation around a set value, rather than an absolutely unchanging one.

There are a few examples of positive feedback in living organisms. For example, if a person breathes air that has a very high carbon dioxide content, this produces a high concentration of carbon dioxide in the blood. This is sensed by carbon dioxide receptors, which cause the breathing rate to increase. So the person breathes faster, taking in even more carbon dioxide, which stimulates the receptors even more, so the person breathes faster and faster. You can see that positive feedback does not have a role to play in homeostasis. It pushes a parameter further and further away from the norm, rather than keeping it constant.

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Temperature regulation in endothermsAll mammals are able to generate heat within their bodies, and so are said to be endothermic. (‘Endo’ means ‘inside’.) Body temperature is controlled by ensuring a balance between heat input and heat output (Figure 1.2 and Figure 1.3).

We are perhaps better at keeping our bodies warm in a cold environment than we are at keeping them cool in a hot one. We have several di3 erent ways of keeping warm, but if the external temperature rises above our internal one, the only physiological way to keep cool is by sweating.

Temperature receptorsThe hypothalamus (right in the middle of the head, at the base of the brain) never stops monitoring the temperature of the blood fl owing through it. The temperature it monitors is our

Figure 1.2 Heat is transferred by conduction, convection and radiation.

Conduction – Heat passes from one particle to another if they are next to each other. The speed of transfer depends on how good a conductor of heat the material is. Air is quite a poor conductor of heat. Water is a better heat conductor.

Radiation – This is infrared radiation (e.g. the main heat of an electric bar fi re or the Sun). All objects emit infrared radiation. The hotter they are, the more they emit. This radiation travels in straight lines through the air. It is absorbed by solid objects in its path, warming them. Black objects absorb most; silvered surfaces refl ect it. Black objects also radiate most and silvered surfaces radiate least.

Convection – If a fl uid (i.e. a gas or liquid) such as air is warmed (e.g. by contact with warm skin) it becomes less dense and so will rise through surrounding cooler air.

Figure 1.3 Heat inputs and heat outputs.

Heat inputs generated by chemical reactions inside the body absorption of radiation from the Sun or a fi re conduction from warmer substances

Heat outputs radiation from the body conduction to colder substances conduction and convection in sweating

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core temperature – the temperature inside the body. This should remain at around 37 °C, although this value varies between di3 erent people. It also depends on the level of activity and the time of day and – for women – the stage of the menstrual cycle.

The hypothalamus receives information about temperature from other sources as well. The skin contains receptors that monitor the changes in skin temperature, which is the fi rst to change if there is a change in environmental temperature. These peripheral (‘around the outside’) receptors are able to give ‘early warning’ about a possible change in core temperature. When you walk from a warm room into a cold one and ‘feel cold’, it is nerve impulses from these peripheral temperature receptors in the skin that you are responding to. Your core temperature will not have changed at all.

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•

Body temperature is determined by the balance between inputs and outputs.

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When core temperature might fallIf core temperature falls, or if skin temperature receptors signal that environmental temperature has fallen, this information is picked up by the hypothalamus. The hypothalamus sends electrical impulses along motor neurones (nerve cells) to some of your muscles and to the skin.

The nerve impulses from the hypothalamus cause some of the muscles repeatedly to contract and relax very rapidly, which we call shivering. This involves respiration inside the cells, as glucose is oxidised to release energy that is used to make ATP. Some of the energy, however, is lost as heat. Shivering therefore generates heat inside the muscles. The heat is transferred into the blood as it passes through the muscles, and is then carried around the rest of the body in the bloodstream.

The nerve impulses also a3 ect the smooth muscle in the walls of arterioles that supply blood to capillaries near the skin surface (Figure 1.4). This muscle contracts, narrowing the lumen of the arterioles and shutting o3 blood supply to the surface capillaries. This is known as vasoconstriction. It ensures that less heat is lost

to the surroundings by radiation from the warm blood at the surface of the skin.

Sweat glands in the skin reduce their output of sweat. Erector muscles, each attached to the base of a hair follicle, contract and pull the hairs up on end. We are not very hairy, so this does nothing to keep us warm. In other mammals, it increases the thickness of the layer of air trapped next to the skin. Air is a poor conductor of heat, so this insulates the body from changing temperatures around it. We just get goose bumps.

We can also change our behaviour to prevent body temperature falling too low. The obvious thing is to put on more clothes, increasing insulation of the body surface. We can move into a warmer place, drink hot drinks or eat hot food. Jumping up and down increases the rate of respiration, and therefore heat generation, in the muscles. Wrapping your arms around yourself and huddling up reduces your surface area, so that less heat leaves your body by radiation.

dermis

hairsebaceous gland

adipose tissue

epidermis

sweat duct

capillaries swollen with blood

sweat gland

dilated arteriole constricted shunt

arteriole

venule

temperature receptor

erector muscle

capillaries

constricted arteriole

Most blood is shunted from arteriole to venule before it enters skin arterioles.

When the body is warm

When the body is cold

Figure 1.4 Section through skin to show vasodilation and vasoconstriction.

Extension

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