The entire surface of the body is covered by skin. In man, the skin is about 1 to 2mm thick except the skin of the palm and soles with thickness of about 6mm. The skin is the most extensive organ in the body. The mammalian skin consists of two major layers: the outer epidermis and inner dermis.   Epidermis This is further divided into three layers known as cornified, granular and malpighian layers.   Cornified layer: This is the upper-most layer of the skin and consists of flat dead cells. Cornified layer protects the body against loss of water and entering of foreign bodies. Granular layer: Contains living cells. It lies just beneath the cornified layer. The cells become gradually thickened and longer until they are given up to the cornified cells. Malpighian layer: This is a layer of living cells and where active cell division takes place. This layer contains dark melanin pigments which give the skin its colour. Black people have more melanin pigments than white people. Oriental people have an additional pigment, carotene making their skin yellowish. Melanin pigments are absent in albinos. As the cells of the malpighian layer divide, they are pushed up. As they move up, they become filled with a substance called keratin and become flat and dead, thus forming the cornified layer.   Dermis The dermis is a thick layer of connective tissues. This layer consists of blood vessels, sweat glands, sebaceous glands, hair follicles, erector muscles, sensory cells and fatty cells. Blood vessels: Blood vessels supply food and oxygen to the tissues of the skin. Sweat gland:  is a coiled tube becoming straight as it passes through the dermis and slightly drawn into folds as it goes through the epidermis. It opens on the surface as a pore. The sweat gland is well supplied with blood capillaries. As blood flows through the blood capillaries, waste products such as water, sodium chloride, small amount of urea are absorbed by the sweat gland. From the sweat gland the wastes are secreted to the sweat duct and finally to the sweat pore. It secretes waste products in form of sweat from the body. It is also greatly concerned with temperature regulation. Sebaceous gland: Each hair follicle is supplied with at least two sebaceous glands. Sebaceous glands secrete oily substance (sebum) which lubricates the hair. The sebum contains lysozyme which kills bacteria and viruses. Hair follicle: Each hair lies in a deep pit called hair follicle. The hair projects out of the skin. A hair is made up of keratin. Hair takes part in temperature regulation. The root of each hair is situated deep in the dermis, but the hair is an epidermal structure. Erector muscle: This is a slender muscle which runs from each side of the hair to the base of the Malpighian layer. The contraction of muscle makes the hair to stand up-right and its relaxation flattens the hair. It is concerned with temperature regulation. Fatty cells (adipose tissue): These cells are found below the dermis. They are round in shape. Fatty cells act as insulating layers. They also act as energy reserve and protect the skin from damage. Sense receptors: There are five sense receptors namely: touch, pressure, pain, heat and cold. Each receptor is sensitive to a particular type of stimulus.   Functions of the skin 1) Protection: The skin protects the body in the following ways: The skin covers and holds the body together. It forms barrier between external environment and the tissues below. Prevents harmful bacteria from invading the tissues. The sweat and sebaceous glands have certain constituents e.g (lysozyme) in them that are antagonistic to invading bacteria. Protects the body to some extent from physical injuries. It prevents the tissues from drying up. Melanin pigments prevent the ultra-violet rays from damaging the under-lying tissues. In some mammals such as porcupine, the hairs are modified into thick sharp spines that protect the animal from attack by predators. The hairs of many mammals have different colours and patterns, that camouflage the animals in their surroundings, making them difficult to be seen by their predators.   2) Sensitivity The skin contains different sense receptors which are sensitive to different environmental stimuli. The body can therefore react to changes in the environment. For example, on touching a hot object you quickly withdraw your hand to avoid burning injury.   3) Excretion and osmoregulation The sweat glands excrete surplus water containing salts and some little amount of waste nitrogenous substance (mainly urea).   4) Maintenance of a suitable constant body temperature Maintenance of a suitable constant body temperature is very important to mammals and birds, since their metabolic activities are tied to a constant temperature. These animals are known as warm blooded animals or homoiothermic animals. No matter the external changes in temperature, the body temperatures of mammals and birds remain the same. The constant body temperature in man is 98.4 degrees F (37 degrees C). The heat gained must be equal to heat lost. Other animals such as fish, amphibians and reptiles are cold blooded or poikilothermic since their body temperature changes with external temperatures.   5) The skin manufactures and stores vitamin D.   6) Storage of reserved food: Fats are stored under the dermis. The fats form an insulating layer.   7) Nails, claws, hoofs, horns and scales are outgrowths of skin. Nails give extra support to the fingers so that they can hold or grasp things well, Nails are also used in scratching the skin. Claws are mostly used by carnivores such as cats and lions as weapons of defence, offence and for catching their prey. Horns are used as weapons of offence and defence in mammals such as cows, deer, goats and rams. Scales protect the body of some mammals (e.g pangolin) from losing water. They also protect the inner organs.   8) The mammary glands in mammals are modification of the skin. They help to produce milk for feeding the young.   The care of the human’s skin Bathe regularly with soap to remove dirts, sweat and natural oils from the skin. This prevents bacteria, fungi, mites and dust from sticking to the skin and blocking the sweat pores. Dirty skin causes diseases such as, yaws, tinea and scabies. Do not bathe with harsh soap. Dry the washed body with clean towel. The feet must be washed frequently and cleaned with towel. Wearing clean stockings helps to absorb sweat from the clefts of the toes. Sweat staying in between toes inhabits fungus which causes tinea. Wearing of shoes prevents hookworm and jigger from infecting the skin and its underlying tissues. The skin must be kept moist always by using good creams especially during the harmattan period to keep the skin soft and flexible and protect it from cracking. On no account should bleaching creams be used on the skin. Bleached skin is proned to skin infection and cancer Wearing clean light and loose clothes allows free circulation of air round the body. Dirty and heavy clothes cause discomfort; rashes and fungal infections of the skin. Report any skin disease to a medical doctor for treatment. The skin should not be exposed to unnecessary direct radiation of the sun more especially the albinos. Direct hot sunlight may cause sun burns.   How mammals maintain their constant body temperature 1) In hot weather In a hot weather, the body temperature goes up. The following things happen to maintain a constant body temperature. More blood flows to the surface of the skin: The blood vessels in the skin dilate. More blood flows to the surface of the skin. The flow of blood stimulates the sweat glands to produce more sweat which is poured to the surface of the skin. The sweat evaporates thus producing cooling effect since the latent heat of vaporisation required to evaporate the sweat comes from the body. Failure of the skin to keep the body at constant normal temperature during hot weather leads to hyperthermia, heat stroke and death occurs. The hairs are lowered: The erector muscles of the hairs relax thus lowering the hairs and therefore the layer of air between the hairs and the skin is thin. This thinness of air allows more heat to be lost from the body (Note, air is a poor conductor of heat and if a thick layer of air is trapped more heat will be retained in the body.   2) In cold weather In a cold weather, the body temperature is lowered and the following things happen to maintain a constant body temperature of 98.4 degrees F (37 degrees C). Less blood flows to the surface: In a cold weather, the blood vessels constrict, therefore, less blood flows to the surface of the skin; less sweat is produced by sweat glands. Little latent heat is lost from the body and therefore less heat is lost from the body. That is why people are pale in cold weather. The hairs stand erect: The erector muscles of the hairs contract making the hairs to stand erect. A layer of thick air is trapped between them. Air being a poor conductor of heat insulates the body and less heat is lost by conduction, convection and radiation. Failure of the skin to keep the body at constant normal temperature during cold lead to hypothermia. Death occurs by frost bite. Shivering: When the body temperature is low, some involuntary muscles contract and relax continuously, thus producing heat. This is circulated to all parts of the body by the blood stream and helps to raise the body temperature.

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A cartilage is a tough elastic and semitransparent tissue devoid of blood vessels and nerves.   Functions of cartilage It prevents friction between two bones and allows smooth and free movement. It absorbs shock between bones. For example the shock resulting from the vertebral bones are absorbed by cartilaginous intervertebral discs. It keeps open the trachea (windpipe) so that air flows into the lungs always. The cartilage of the nose also keeps the nostrils open for in-take of fresh air and output of waste air. The cartilage of the ribs allows contraction and expansion of the thorax thus enhancing breathing mechanism. It forms cushion in the sockets of bones such as the glenoid cavity of the shoulder and hip sockets. The ear cartilage keeps the pinnae (external ear) erect so as to collect sound vibrations from the air. Cartilage and bone provide the firm areas for attachment of the tendons of the muscles. Fibro-cartilage is found in the pubis symphysis where it allows for considerable expansion of the pubis during parturition (child birth) without complete breakage.

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Carbohydrates are organic compounds containing carbon, hydrogen and oxygen atoms. The ratio of hydrogen to oxygen is 2:1 in all carbohydrates. Starch, sugar, glycogen and cellulose are carbohydrates. They are classified into three major groups namely:- Monosaccharides Disaccharides and Polysaccharides.   1) Monosaccharides (one molecule sugars) These are simple sugars which consist of one molecule each. These sugars may contain six or less carbon atoms per molecule. Examples are glucose, fructose both with chemical formula (C6H12O6) and ribose (C5H10O5).   2) Disaccharides (Double molecule sugars) When two molecules of simple sugar combine and one molecule of water is removed, a disaccharide is formed. When one molecule of glucose combines with one molecule of fructose, one molecule of sucrose (cane sugar) is formed. 1 molecule of glucose +1 molecule of fructose = 1 molecule of sucrose. Combination of two molecules of glucose will form one molecule of maltose or malt sugar. 1 molecule of glucose + 1 molecule of glucose = 1 molecule of maltose. 1 molecule of glucose combining with one molecule of galactose forms lactose (milk sugar). Examples of disaccharides are sucrose (cane sugar), maltose (malt sugar) and lactose (milk sugar).   3) Polysaccharides Polysaccharides are complex carbohydrates consisting of glucose molecules in chain with general formula of (C6H10O5)n, where letter n shows a large number. Starch, glycogen and cellulose are polysaccharides. Sources of starch are potato, yam, tapioca, rice, millet, wheat, bread and cassava. These are plant starches. Glycogen is an animal starch produced and stored in the liver and muscles. It is converted to glucose when the need arises. Cellulose is derived from vegetables and fruits. It forms the cell walls in most plant cells.   Importance of carbohydrates When oxidized, they give energy and heat. The energy is used for work while the heat is used to maintain body temperature in homoiothermic animals. Excess carbohydrates are converted into glycogen thus acting as food reserve only to be used when the need arises. Carbohydrates provide starting materials for the synthesis of proteins, fats, oil and vitamins.

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Nastic/sleeping movements The pinnate and bipinnate leaves of many leguminous plants open and close as a result of certain changes in temperature, light intensity or humidity of the atmosphere. The process which is however gradual is known as “sleeping movement” and occurs in such leaves as St. Thomas plant, and cassia. Flowers of certain plants are known to open only at certain time of the day. A good example is the “Four O’clock” plant which opens at about 4 pm daily. Sunflower changes its direction to face the sun. The flowers of”Snake tomato” remain closed during hot day and open at night or under conditions of high humidity. These flowers are pollinated at night by nocturnal insects (moth). These various responses of plant parts to external stimulus of the weather are known as nastic responses or simply nastic movements.   Taxis (tactic movement) Taxis: This is a locomotory movement of an entire organism or cell (e.g. gamete) in response to a directional stimulus. If the locomotory movement is towards the stimulus, it is positive taxis, if it is away it is negative taxis. Animals respond to stimulus of: Light (phototaxis). Water (hydrotaxis). Chemicals (chemotaxis). Current (rheotaxis). The response of animals to stimulus of light is called phototaxis, the response to stimulus of water is called hydrotaxis while the response to stimulus of chemicals is called chemotaxis. For example Euglena moves towards source of light (positive phototaxis). Earthworm move away from light (negative phototaxis). Sperm move towards chemical substances produced by eggs (positive chemotaxis). Mosquitoes avoid repellent (negative chemotaxis). Many freshwater fish (e.g. Tilapia) move against current.   Kinesis (kinetic movement) Kinesis is the rate of change of a living organism in response to the intensity of stimulus and not the direction of the stimulus, that is, the stimulus does not control the direction of movement. For example, when food substances are added to water, the tentacles of Hydra increase their waving movement.

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Enzymes are organic catalysts of protein origin produced by living cells which help to speed up or slow down the rate of chemical reactions but remain chemically unchanged at the end of the reaction.   Classes of enzymes Enzymes are usually classified according to the type of food they act upon. Generally three classes are known. They are amylases, proteases and lipases.   1) Amylases Amylases act on carbohydrates (starches and complex sugars) changing them to glucose which the body can absorb. Some amylase enzymes are: Maltase, sucrase, lactase and ptyalin. Maltase changes maltose to glucose. Sucrase changes sucrose to glucose and fructose. Lactase changes lactose to galactose and glucose. Ptyalin changes starch to maltose sugar.   2) Proteases Proteases act on proteins changing them into amino acids. Some proteases are pepsin, renin, trypsin and erepsin.   Pepsin changes protein to peptones. Renin coagulates milk proteins. Trypsin changes proteins to peptones while erepsin changes peptones to amino acids.   3) Lipases act on fats and oils changing them into fatty acids and glycerol.   Characteristics of enzymes Enzymes act as catalysts which help to speed up or slow down the rate of most chemical reactions within the body of the organism. They remain chemically unchanged at the end of the reaction. They are specific in action, that is, one or a group of enzymes will act on specific substrate. For example, the ptyalin in the saliva can only act on cooked starch changing the cooked starch to maltose sugar. Proteases (protein enzymes) will only act on proteins and nothing else. Enzymes are required in small quantity. A single molecule of one particular enzyme can change 40,000 molecules of its food in a second. Enzymes act best over a small range of temperature. They are denatured or destroyed at high temperature and inactive at a very cool tempera ture. Their reactions increase with a rise in temperature of 37 degrees Celsius. They are pH specific (acidity or alkalinity) and act best at certain level of pH. For example, the stomach enzymes act best in acidic medium while the enzymes of the small intestine act best in alkaline medium. Most enzyme actions are reversible. For example, starch is changed to sugar and from sugar to starch by enzyme diastase. Enzymes are retarded by poison or inhibitors. They are protein in nature. Some enzymes are inactive and therefore require co-enzymes or agents to activate them. For examples, the enzyme trypsinogen requires enterokinase to activate it into trypsin while pepsinogen requires dilute hydrochloric acid to activate it into pepsin. The activators of inactive enzymes are called zymogens. Thus the zymogens of trypsinogen and pepsinogen are enterokinase and dilute hydrochloric acid respectively. Enzymes can function outside the body of the organism that produces them.

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