8 and 9: REPRODUCTION; RESPIRATION IN HUMAN BEINGS THE PATHWAY UPSC Notes | EduRev

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8: REPRODUCTION

It is defined as a biological process in which an organism gives rise to young ones similar to Itself. There are two types of reproduction;

1) Sexual Reproduction

2) Asexual Reproduction

 

ASEXUAL REPRODUCTION  

When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual. It is common among single-celled organisms and in plants and animals with relatively simple organizations. In Protests’ and Monerans, the organism or the parent cell divides into two to give rise to new individuals.

 

SEXUAL REPRODUCTION  

When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction. All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. The period of growth is called the juvenile phase. It is known as vegetative phase in plants.

The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase. In non-primate mammals like cows, sheep, dogs such cyclical changes during reproduction are called oestrus cycle where as in primates (Monkeys, apes and humans) it is called menstrual cycle.

Many mammals exhibit such cycles only during favourable seasons in their reproductive phase and are therefore called seasonal breeders. Many other mammals are reproductively active throughout their reproductive phase and therefore termed as continuous breeders.

Sexual Reproduction in other words is the fusion of gametes. This process is called syngamy or fertilization which results in the formation of a diploid zygote. It is universal in all sexually reproducing organisms. Fertilisation is either external or internal.

 

EXTERNAL FERTI LIZATION 

External fertilization: In most aquatic organisms, such as a majority of fishes and algae as well as amphibians, fertilization occurs in the water, outside the body of organism.

 

INTERNAL FERTILIZATION 

Internal fertilization: Fertilisation occurs inside the body of the organism, hence the process is called internal fertilization.

 

EMBRYOGENESIS  

Embryogenesis is the process which traces the development of embryo from the zygote. During this process, Zygote undergoes cell division (mitosis) and cell differentiation. Cell differentiation helps groups of cells to undergo certain modifications to form specialized tissues and organs to form an organism.

Animals are further categorized into oviparous and viviparous. In oviparous animals like reptiles and birds the development of zygote takes place outside the body of the female parent; where they lay unfertilised/fertilised eggs.

In viviparous animals (majority of mammals including humans) female parent gives birth to young ones.

The term clone is used to describe such morphologically and genetically similar individuals.

 

 

SEXUAL REPRODUCTION IN FLOWERING PLANTS  

The pollen grains represent the male gametophytes while the gynoecium represents the female reproductive part of the flower. It may have one single pistil or may have more than one pistil. Each pistil has three parts; - The Stigma, style and ovary. The stigma serves as a landing platform for pollen grains. The style is the elongated slender part beneath the stigma. The basal bulged part of the pistil is the ovary.

Inside the ovary is the ovarian cavity. The placenta is located inside the ovarian cavity. Arising from the placenta are the mega sporangia, commonly called ovules. Each ovule has one or two protective envelopes called integuments. Enclosed within the integuments is a mass of cells called the nucleus. Located in the nucleus is the embryo sac or female gametophyte.

The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.

In a majority of flowering plants, one of the megaspores is functional while the other three degenerate. Only the functional megaspore develops into the female gametophyte (embryo sac).

 

POLLINATION  

Pollination is the mechanism under which pollen grains is transferred to the stigma of a pistil.

 

TYPES OF POLLINATION  

Depending upon the source of pollen, pollination can be divided into three types.

Autogamy- in Autogamy, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower.

Geitonogamy-Transfer of pollen grains from the anther to the stigma of another flower of the same plant.

Endogamy- Transfer of pollen grains from anther to the stigma of a different plant. This is the only typed of pollination which brings genetically different types of pollen grains to the stigma during pollination.

 

Agents of Pollination: plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination. Majority of plants use biotic agents for pollination. Only a small proportion of plants use abiotic agents.

Majority of flowering plants use a range of animals as pollinating agents. Bees, butterflies, ants, wasps and birds are the common pollinating agents.

 

HUMAN REPRODUCTION  

The male reproductive system is composed of a pair of testes, the male reproductive system is composed of a pair of testes, the male sex accessory ducts and the accessory glands and external genitalia. Each testis has about 250 compartments called testicular lobules, and each lobule contains one to three highly coiled seminiferous tubules. Each seminiferous tubule is lined inside by spermatogonia and sertoli cells.                                        

The female reproductive system consists of a pair of ovaries, a pair of oviducts, a uterus, a vagina, external genitalia and a pair of mammary glands. The ovaries produce the female gamete (ovum) and some steroid hormones (ovarian hormones). Ovarian follicles in different stages of development are embedded in the stroma. The oviducts, uterus and vagina are female accessory ducts. The uterus has three layers namely perimetrium, myometrium and endometrium. The female external genitalia include Mons pubis, labia major a, labia menorah, hymen and clitoris.

Spermatogenesis results in the formation of sperms that are transported by the male sex accessory ducts. A normal human sperm is composed of a head, neck, a middle piece and tail. The process of formation of mature female gametes is called oogenesis.

During copulation (coitus) semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic) of the fallopian tube. The ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilization takes place. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary-isthmic junction.

The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zone pellucid layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zone pellucid and the plasma membrane.

The reproductive cycle of female primates is called menstrual cycle which starts only after attaining sexual maturation termed as puberty. During ovulation only one ovum is released per menstrual cycle. After coitus, sperm fertilizes the ovum leading to formation of a diploid zygote. The presence of X or Y chromosome in the sperm determines the sex of the embryo. The zygote undergoes repeated mitotic division to form a blastocyst, which is implanted in the uterus resulting in pregnancy. The average duration of human pregnancy is about 9 months which is called the gestation period.

The structural and functional unit between developing embryo (foetus) and maternal body is called placenta. The placenta facilitates the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/ waste materials produced by the embryo. Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin(hCG), human placental lactogenic(hPL), oestrogens, progestogens, etc. In the later phase of pregnancy, a hormone called relaxin is also secreted by the ovary. It should be noted that hCG, hPL and relaxin are produced in women during pregnancy. In addition, during pregnancy the levels of other hormones like oestrogens, progestogens, cortisol, prolactin, thyroxine are increased many-folds in the maternal blood. Increased production of these hormones is essential for supporting the fatal growth, metabolic changes in the mother and maintenance of pregnancy.

The process of childbirth is called parturition which is induced by a complex neuroendocrine mechanism involving cortisol, oestrogens and oxytocin.

The major female and male hormones can be classified as oestrogens or androgens. Both classes of male and female hormones are present in both males and females alike, but in vastly different amounts. Most men produce 6-8 mg of the male hormone testosterone (an androgen) per day, compared to most women who produce 0.5 mg daily. Female hormones, oestrogens, are also present in both sexes, but in larger amounts for women.

Estrogens are the sex hormones produced primarily by a female's ovaries that stimulate the growth of a girl's sex organs, as well as her breasts and pubic hair, known as secondary sex characteristics. Estrogens also regulate the functioning of the menstrual cycle.

Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testicles of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid.

In men, testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle, bone mass, and the growth of body hair. In addition, testosterone is essential for health and well-being as well as the prevention of osteoporosis.

On average, in adult human males, the plasma concentration of testosterone is about 7–8 times as great as the concentration in adult human females' plasma, but as the metabolic consumption of testosterone in males is greater, the daily production is about 20 times greater in men.

 

ARTIFICIAL METHODS O F VEGETATIVE REPRODUCTION  

Besides natural methods of vegetative propagation, artificial modes of propagation are also being used. Farmers, gardeners and horticulturists have adopted several such methods like grafting, layering, cutting and tissue culture for propagating plants in gardens and nurseries.

 

GRAFTING 

In horticultural practices this method is commonly used. In this method the cutting of a plant (scion) is attached to the stem of another rooted plant (stock). After some time the attached cutting becomes an integrated part of the rooted plant. The scion and stock are placed in such a way that no gap remains between them. Finally they become joined in such a way that their vascular systems are united. Usually the scion is of a plant having desirable characters like large sized fruits and the stock has good absorbing capacity. Because of the arrangement of their vascular bundles, grafting experiments are successful only in divots and not in monocots. This method is commonly applied to improve the variety of fruits like mango. Wax is used to cover the place where grafting is being done. This is to avoid infection.

 

BUD GRAFTING  

A bud is taken along with portion of bark from a plant and is used as scion in this process. A T - shaped cut is made and the bud is fixed tightly on the stock with a tape. The bud gets attached to the stock after some time and new branches are formed. Pears, peaches, plum, citrus, roses, etc., are propagated by this method. This method is usually employed during spring.

 

CUTTING  

In rose, sugar-cane, Coleus, Bougainvillea, etc., this method is used to produce new plants. In this process stem cuttings with some nodes and internodes are placed in moist soil which gives rise to adventitious roots and a new plant subsequently. It is a very common method of vegetative propagation. Farmers divide up the rhizomes, tubers or roots stocks at the end of flowering or growing season. Each part grows into a separate plant in the following year. Some plants like dahlia are propagated by root cuttings.

 

LAYERING  

In some plants one or more branches are bent close to the ground and covered with moist soil. After some time, the underground portion of those branches produce new roots and develop into a separate plant as in jasmine, Rhododendron, Magnolia, etc. The stem or branch that develops adventitious roots while still attached to the parent plant is called a layer. In many plants, layering can also be induced artificially.

In mound layering the stem is pruned and the base of the plant is covered with soil. From the base, new shoots develop, which are separated from the parent plant, ad grown into a new plant. Many types of apples and gooseberries are grown using this method.

Air layering is another type of layering in which branches of the plants cannot be bent to the ground. A piece of the branch is scraped (girdled) in this method and polythene or plastic sheet is used as cover to preserve moisture. Roots arise from the scraped part after a few weeks. This branch is then detached from the parent plant which grows into a new plant after plantation.

Layering differs from cutting in that the developments of adventitious buds are induced before the stem is cut to form the new plant.

 

TISSUE CULTURE  

In this technique a small piece of tissue of a desired plant is cut. This is placed with a suitable nutrient medium under proper conditions. The tissue grows into an unorganized mass, known as callus. Small part of this tissue is put in another medium, which induces the formation of plantlets. The plantlets can be transplanted in soil or pots foe developing to maturity. This technique is also called micro propagation. This method is used in propagating plants like Asparagus, orchids, Chrysanthemum. This method allows us to grow whole plant from cells taken from various parts of the plant body.

 

9. RESPIRATION IN HUMAN BEINGS THE PATHWAY

  •  Air enters the nostrils 
  •  passes through the nasopharynx, 
  •  the oral pharynx 
  •  through the glottis 
  •  into the trachea 
  •  into the right and left bronchi, which branches and rebranches into 
  •  bronchioles, each of which terminates in a cluster of 
  •  alveoli

Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).

 

BREATHING  

In mammals, the diaphragm divides the body cavity into the 

  • Abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the 
  • Thoracic cavity, which contains the heart and lungs. 

The inner surface of the thoracic cavity and the outer surface of the lungs are lined with pleural membranes which adhere to each other. If air is introduced between them, the adhesion is broken and the natural elasticity of the lung causes it to collapse. This can occur from trauma. And it is sometimes induced deliberately to allow the lung to rest. In either case, reinflation occurs as the air is gradually absorbed by the tissues.

Because of this adhesion, any action that increases the volume of the thoracic cavity causes the lungs to expand, drawing air into them.

  • During inspiration (inhaling), 
    •  The external intercostal muscles contract, lifting the ribs up and out. 
    • The diaphragm contracts, drawing it down.
  • During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breathe 15–18 times a minute exchanging about 500 ml of air.
    • In more vigorous expiration, The internal intercostal muscles draw the ribs down and inward 
    • The wall of the abdomen contracts pushing the stomach and liver upward.

Under these conditions, an average adult male can flush his lungs with about 4 litres of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain.

The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the haemoglobin therein.

At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled.

The ease with which oxygen and carbon dioxide can pass between air and blood is clear from this electron micrograph of two alveoli (Air) and an adjacent capillary from the lung of a laboratory mouse. Note the thinness of the epithelial cells (EP) that line the alveoli and capillary (except where the nucleus is located). At the closest point, the surface of the red blood cell is only 0.7 µm away from the air in the alveolus.

 

CENTRAL CONTROL OF BREATHING  

The rate of cellular respiration (and hence oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20–25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing.

It is a rising concentration of carbon dioxide — not a declining concentration of oxygen — that plays the major role in regulating the ventilation of the lungs. Certain cells in the medulla oblongata are very sensitive to a drop in ph. As the CO2 content of the blood rises above normal levels, the pH drops and the medulla oblongata responds by increasing the number and rate of nerve impulses that control the action of the intercostal muscles and diaphragm. This produces an increase in the rate of lung ventilation, which quickly brings the CO2 concentration of the alveolar air, and then of the blood, back to normal levels.

However, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate but there has been no increase in the production of CO2.

The smooth muscle in the walls of the bronchioles is very sensitive to the concentration of carbon dioxide. A rising level of CO2 causes the bronchioles to dilate. This lowers the resistance in the airways and thus increases the flow of air in and out.

 

VITAL CAPACITY OF LUNG 

Vital capacity is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume.

A person's vital capacity can be measured by a wet or regular spirometer. In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease.

A normal adult has a vital capacity between 3 and 5 litres. A human's vital capacity depends on age, sex, height, weight, and ethnicity.

 

LUNG VOLUMES  

Lung volumes and lung capacities refer to the volume of air associated with different phases of the respiratory cycle. Lung volumes are directly measured, whereas lung capacities are inferred from volumes.

 

LUNG DISEASES  

 Asthma: The airways are persistently inflamed, and may occasionally spasm, causing wheezing and shortness of breath. Allergies, infections, or pollution can trigger asthma's symptoms.

 Chronic obstructive pulmonary disease (COPD): Lung conditions defined by an inability to exhale normally, which causes difficulty breathing.

 Chronic bronchitis: A form of COPD characterized by a chronic productive cough.

 Emphysema: Lung damage allows air to be trapped in the lungs in this form of COPD. Difficulty blowing air out is its hallmark.

 Acute bronchitis: A sudden infection of the airways, usually by a virus.

 Cystic fibrosis: A genetic condition causing poor clearance of mucus from the bronchi. The accumulated mucus results in repeated lung infections.

 Pneumonia: An infection of the alveoli, usually by bacteria.

 Tuberculosis: A slowly progressive pneumonia caused by the bacteria Mycobacterium tuberculosis.

 Emphysema results from damage to the fragile connections between alveoli. Smoking is the usual cause. (Emphysema also limits airflow, affecting the airways as well.)

 Pulmonary edema: Fluid leaks out of the small blood vessels of the lung into the air sacs and the surrounding area. One form is caused by heart failure and back pressure in the lungs' blood vessels; in another form, direct injury to the lung causes the leak of fluid.

 Lung cancer has many forms, and may develop in any part of the lungs. Most often this is in the main part of the lung, in or near the air sacs. The type, location, and spread of lung cancer determines the treatment options.

 Acute respiratory distress syndrome (ARDS): Severe, sudden injury to the lungs caused by a serious illness. Life support with mechanical ventilation is usually needed to survive until the lungs recover.

 Pneumoconiosis: A category of conditions caused by the inhalation of a substance that injures the lungs. Examples include black lung disease from inhaled coal dust and asbestosis from inhaled asbestos dust.

 Interstitial lung disease (ILD): A broad collection of lung conditions affecting the interstitium. Sarcoidosis, idiopathic pulmonary fibrosis, and autoimmune disease are among the many types of ILD.

 Pneumonias and pulmonary edams can also affect the interstitium.

 Pulmonary embolism (PE): A blood clot (usually in a deep leg vein, deep vein thrombosis) breaks off, travels to the heart, and is pumped into the lungs. The clot lodges in a pulmonary artery, often causing shortness of breath and low blood oxygen levels.

 Pulmonary hypertension: Various conditions can lead to high blood pressure in the pulmonary arteries. This can cause shortness of breath and chest pain. When no cause is identified, the condition is called idiopathic pulmonary arterial hypertension.

 Pleural effusion: Fluid collects in the normally tiny pleura space between the lung and the chest wall. Pneumonia or heart failure is usually responsible. If large, pleural effusions can impair breathing, and should be drained.

 Pneumothorax: Air may enter the space between the chest wall and the lung, collapsing the lung. To remove the air, a tube is typically inserted through the chest wall.

 Mesothelioma: A rare form of cancer that forms on the pleura. Mesothelioma tends to emerge several decades after asbestos exposure.

 Obesity hypoventilation syndrome: Extra weight on the chest and abdomen makes it difficult for the chest to expand. Serious breathing problems can result.

 Neuromuscular disorders: Poor function in the nerves controlling the respiratory muscles causes’ difficulty breathing. Amyotrophic lateral sclerosis and myasthenia gravis are examples of neuromuscular lung disease.

 

PHOTOSYNTHESIS  

Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants and some algae (Kingdom Protista). Plants need only light energy, CO2, and H2O to make sugar. The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis.

Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, etc. The parts of a typical leaf include the upper and lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the stomata’s. The upper and lower epidermal cells do not have chloroplasts, thus photosynthesis does not occur there. They serve primarily as protection for the rest of the leaf. The stomata’s are holes which occur primarily in the lower epidermis and are for air exchange: they let CO2 in and O2 out. The vascular bundles or veins in a leaf are part of the plant's transportation system, moving water and nutrients around the plant as needed. The mesophyll cells have chloroplasts and this is where photosynthesis occurs.

As you hopefully recall, the parts of a chloroplast include the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked in grana. The chlorophyll is built into the membranes of the thylakoids.

Chlorophyll looks green because it absorbs red and blue light, making these colours unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, it is the energy from the red and blue light that are absorbed that is, thereby, able to be used to do photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis. The overall chemical reaction involved in photosynthesis is: 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2. This is the source of the O2 we breathe, and thus, a significant factor in the concerns about deforestation.

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