Gaseous Exchange

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Gaseous Exchange

By Mr. Nyamawi Waa Girls Secondary School

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Definition: It is t he pr oces s by which the respiratory gases (oxygen and carbon (iv) oxide are passed across the respiratory surface. In simple organism oxygen is absorbed by the exposed surface of the body by simple diffusion . In complex organisms, they have special organs e.g lungs or gills which absorb oxygen and remove carbon (iv) oxide from the body. Brought about by concentration gradient that exists between the body of living organism and the surrounding medium.

Definition: It is t he pr oces s by which the respiratory gases (oxygen and carbon (iv) oxide are passed across the respiratory surface. In simple organism oxygen is absorbed by the exposed surface of the body by simple diffusion . In complex organisms, they have special organs e.g lungs or gills which absorb oxygen and remove carbon (iv) oxide from the body. Brought about by concentration gradient that exists between the body of living organism and the surrounding medium.

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Importance of gaseous exchange Provide oxygen for respiration in all organisms. Provide carbon (iv) oxide for autotrophs for photosynthesis ; Facilitate carbon IV oxide removal from the body as a metabolic waste product .

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Gaseous exchange in plants.

- Green plants need oxygen for respiration and carbon (iv) oxide for photosynthesis. -During the day they absorb carbon (iv) oxide and give out oxygen while at night they give out carbon (iv) oxide and take in oxygen. -During photosynthesis some of the oxygen given out is used for respiration.

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Adaptations of respiratory surfaces in plants. They are moist to dissolve respiratory gases in order for them to diffuse in solution form. They have large surface areas to increase efficiency of gaseous exchange. They have increase ventilation due to the presence of stomata and lenticels and the epidermis, which are in contact with the surrounding medium. Mesophyll cells near the air spaces have thin walls. The thin membranes allow fast diffusion of gases.

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Examples of respiratory Surfaces in Plants Stomata in leaves pneumatophores e.g. Roots Lenticels in woody stems

Lenticel Cornplementary cell Epidermis Phellem(Cork) Phelloderm (Sec.ondary cortex)

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Structure and function of stomata

Stomata are tiny pores found on the surface of the leaf. A Stoma is an aperture on leaves surrounded by guard cells. A Guard cell is special Bean shaped epidermal cells surrounding the stoma that regulate opening and closing of the stomata . Functions of the guard cells Control the opeing and closing of the stomata hence regulating movement of gases e.g carbon (iv) oxide , oxygen and water vapour into and out of the leaf surface.

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Open —Nucleus Thid inn cell wall Chloreast Vnulc Tlin out cell wall Guard cell Epidermal Cf

Adaptations of the guard cells.

They contain chloroplasts and so can photosynthesize to produce glucose which facilitate closing and opening of the stomata. They are bean shaped/sausage shaped to facilitate the opening and closing of stomata. Inner walls are thicker while outer wall is thin to facilitate the opening and closing of stomata. They have uneven thickness of their cell walls ie thicker on the surrounding the pore .

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Answer the following questions What is gaseous exchange? What is the importance of gaseous exhache ? Name three structures through which gaseous exchange occurs in plants How is a guard cell adapted to its functions?

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The mechanisms of opening and closing of stomata.

There are three theories that try to explain how the stomata open and close. Photosynthetic theory Starch Sugar inter-conversion Theory. (effect of changes in pH of guard cells) Potassium Ion Theory. Photosynthetic theory. Chloroplast in guard cells carry out photosynthesis during the day. The glucose manufactured in the guard cells increases the osmotic pressure Of the guard cells . The cells then draw water from the surrounding epidermal cells by osmosis and become turgid. The outer thin wall stretches, puling the inner thick wall outwards hence opening the stoma. At night, no photosynthesis occurs. Manufactured glucose is changed into insoluble starch. This lowers osmotic pressure of the guard cells which lose water to the surrounding by osmosis and become flaccid. This closes the stoma.

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Summary of opening and closing of stomata .

During the day During the night -Presence of light -No light -Photosynthesis takes place -Photosynthesis does not take place. -The glucose formed increases the osmotic pressure in guard cells. -Any glucose present in the guard cell is converted to starch, causing low osmotic pressure in guard cells -Guard cells draw in water from epidermal cells and become turgid -Stoma opens -Guard cells lose water to epidermal cells and become flaccid - Stoma closes..

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Starch – sugar inter-conversion theory

During the day photosynthesis takes place in the guard cell using carbon (iv)oxide The pH in the guard cells tends to rise as the conditions becomes less acidic as carbon(iv)oxide is continuously used up. The increasing pH favours the conversion of starch into glucose . This increases the osmotic pressure of the guard cells. The guard cells draw water from epidermal cells and become turgid. This opens the stomata. At night, there is no sunlight so photosynthesis does not take place . Plants take in oxygen releasing carbon(iv)oxide. Carbon(iv)oxide from respiration accumulates in the guard cells lowering the pH which favours the conversion of glucose to starch. Starch is osmotically inactive. This reduce osmotic pressure in the guard cells which lose water to the surround cells and become flaccid. This close the stomata.

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Potassium Ion Theory.

In day time (light) adenosine triphosphate (ATP) is produced which causes potassium ions to move into guard cells by active transport. These ions cause an increase in solute concentration in guard cells that has been shown to cause movement of water into guard cells by osmosis. Guard cells become turgid and the stoma opens. At night potassium and chloride ions move out of the guard cells by diffusion and level of organic acid also decreases. This causes a drop in solute concentration that leads to movement of water out of guard cells by osmosis. Guard cells lose turgor and the stoma closes.

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Mechanism of Gaseous Exchange in Plants

Gaseous exchange takes place by diffusion. The structure of the leaf is adapted for gaseous exchange by having intercellular spaces that are filled. These are many and large in the spongy mesophyll. When stomata are open, during the day, carbon(IV)oxide from the atmosphere diffuses into the substomatal air chambers. From here, it moves into the intercellular space in the spongy mesophyll layer. The CO2 goes into solution when it comes into contact with the cell surface and diffuses into the cytoplasm. A concentration gradient is maintained between the cytoplasm of the cells and the intercellular spaces. CO2 therefore continues to diffuse into the cells. The oxygen produced during photosynthesis moves out of the cells and into the intercellular spaces. From here it moves to the substomatal air chambers and eventually diffuses out of the leaf through the stomata. At night Oxygen diffuses from the atmosphere where it is more concentrated into the plant through the respiratory surfaces. Similarly carbon (IV) oxide diffuses out as a metabolic waste product along a concentration gradient into the atmosphere.

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Stomata and habitat

Stomata are modified in number of ways depending on the habitat of the plant. Depending on the habitat where a plant leaves, there are three categories of plants i.e. Xerophytes Hydrophytes Mesophytes


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Distribution of stomata in aquatic(floating) plants Hydrophytes Aquatic plants such as water lily have numerous stomata on the upper leaf surface. The intercellular spaces in the leaf mesophyll are large. Gaseous exchange occurs by diffusion just as in terrestrial plants. Observation of internal structure of leaves of aquatic plants Transverse section of leaves of an aquatic plant such as Nymphaea differs from that of terrestrial plant. The following are some of the features that can be observed in the leave of an aquatic plant; Absence of cuticle Palisade mesophyll cells are very close to each other ie.compact . Air spaces ( aerenchyma ) in spongy mesophyll are very large to facilitate gaseous exchange and for bouyancy . Sclereids (stone cells) are scattered in leaf surface and project into air spaces for bouyancy . They strengthen the leaf making it firm and assist it to float.

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These are plants adapted to life in dry areas. They have less number of stomata that are small in size. Stomata may be sunken, hairy and in some they open during the night and close during the day.

upper Cuticle Vein Bundle-sheath Palisade multiple extension parenchyma epidermis Spongy multiple Guard cell bundle sheath epidermis

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They are plants growing in areas with adequate amounts of water. They have a fairly large number of stomata found on both leaf surfaces.

Leaf Cross Section •or cutK'e üxide •per sporq• mesovhyv ep_dez• oxmn

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Gaseous Exchange through the Lenticels

They are openings found on woody stems and they are made of loosely packed cells. They allow gaseous exchange between the inside of the plant and the outside by diffusion. Actual gaseous exchange occurs on some moist cells under the lenticels.


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Gaseous Exchange through the Roots

Plants like the mangroves growing in muddy salty waters have specialized aerial breathing roots called pneumatophores . These are roots that project above the ground level. Pneumatophores rise above the salty water to facilitate gaseous exchange.


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All animals take in oxygen for oxidation of organic compounds (respiration) to provide energy for cellular activities. The carbon (IV) oxide produced as a by-product is harmful to cells and has to be constantly removed from the body. Most animals have structures that are adapted for taking in oxygen and for removal of carbon (IV) oxide from the body. These are called " respiratory organs ". The process of taking in oxygen into the body and carbon (IV) oxide out of the body is called breathing or ventilation . Gaseous exchange involves passage of oxygen and carbon (IV) oxide through a respiratory surface by diffusion.

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Various types of respiratory surfaces have been developed by different animals to facilitate gaseous exchange depending on animals size, activity, environment/medium of operation. A continuous supply of oxygen is important for the survival of animals. Oxygen is necessary for respiration . Unicellular and simple organisms have a large surface area to volume ratio . The exchange of oxygen and carbon (iv) oxide is by simple diffusion through the cell membrane. Examples of unicellular organisms : . Amoeba . Paramecium .Earth worm In large multicelullar organism e.g humans, elephants etc. They have a small surface area to volume ratio . They have specialized structures for gaseous exchange e.g lungs, gills etc.

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Type of Respiratory surface Environment or medium of operation Example of organism. Cell membrane water Amoeba. Cell membrane water Paramecium. Tracheole Air Insects. Gill filaments water Fish. Gills water Tadpole. Buccal cavity/skin water/air Adult frog. Lungs Air Bird/reptiles. Lungs Air Human beings. Skin water Frog

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1.They are moist to dissolve respiratory gases during diffusion-gases diffuse faster in solution form. 2. They are thin to create a shorter distance for rapid diffusion of respiratory gases . 3. They have a large surface area in order to ensure maximum gaseous exchange. 4. They are highly vascularized ( well-supplied with blood vessels) to transport absorbed gases rapidly and to create a high diffusion gradient. 5. Have a high concentration gradient (High diffusion pressure deficit) to permit rapid diffusion of respiratory gases.

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Protozoa e.g amoeba carry out respiration inside the cell. It uses oxygen and releases carbon (iv) oxide. There is a high concentration of oxygen in the surrounding water than in the cell. Oxygen therefore diffuses across the cell membrane from the surrounding into the cell. The carbon (iv) oxide in the cell is at a higher concentration. Than in the surrounding and it diffuses out across the cell membrane.

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The respiratory system in insects is called Tracheal system. Components of Tracheal system Spiracles. Trachea. Tracheoles . Spiracles These are tiny pores on the abdomen and thorax. Are closed and opened by valves. Are surrounded by hair, which traps dust particles in the air.

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Trachea Forms two branches one on each side of the body. They are strengthened by chitinous ring. Tracheoles . They branch from the trachea. They are sites for gaseous exchange. They have thin walls and moist ends located in the tissues.

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Inhalation (Inspiration) Exhalation ( Epiration ) Abdominal muscles relax Abdominal muscle contract Pressure in the Abdomen and trachea reduces compared to atmospheric pressure Pressure in the trachea increase compared to the atmospheric pressure Thoracic spiracles open Abdominal spiracles opens; Air enters in them into the trachea , then into the tracheoles Carbon (iv) oxide is forced out through the spiracles; Gaseous exchange Oxygen being highly concentrated in the air in the tracheoles dissolve in the moisture in the tracheoles then diffuses into the tissues Carbon (iv) oxide being highly concentrated in the tissue cells diffuse out of the tissue into the tracheoles NB. Respiratory gases in insects are not transported by the blood.

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Fish uses gills for gaseous exchange; Fish live in water that contains oxygen and other gases are dissolved in it. Components of breathing system of a bony fish. Operculum, Opercular cavity/Gill chamber; Mouth cavity/buccal cavity) A gills comprising of; Gill bar. Gill filaments. Gill rakers .

Caudal Fin pectoral Fin Dorsal fins pelvic

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1.Operculum – These are gill covers on both sides of the head . They open into the opercular cavity by an operculum flap. 2.Opercular cavity – They are located on either side of the head, beneath the gills; 3.Gill bar – They are curved bony structures that holds the gill filaments on one Side and gill rakers on the other side. They are used for support of gill filaments and gill rakers . 4.Gill filaments – They are sites for gaseous exchange . Are membraneous projectionson the gill bar. They have a dense carpilary network. 5. Gill rakers – They are bony teeth-like structures /pointed facing the mouth cavity. - They filter solid material in incoming water preventing them from damaging the delicate gill filaments. They are attached to the gill bar.

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Adaptation of Gills for Gaseous Exchange

Gill filaments are thin walled to create a shorter distance for rapid diffusion of respiratory gases . Gill filaments are very many (about seventy pairs on each gill), to increase surface area for diffusion of respiratory gases . Each gill filament has very many gill lamellae that further increase surface area. The gill filaments are served by a dense network of blood vessels that ensure efficient transport of gases. It also ensures that a favorable diffusion gradient is maintained. The direction of flow of blood in the gill lamellae is in the opposite direction to that of the water (counter current flow) to ensure maximum diffusion of gases. The curved shape of the gill bar allows more fillaments to fit on it. Gill rakers prevent solid particles in water from reaching the delicate gill fillaments .

A Carp with the Operculum removed to show tty Gills Gill.a.mertts Gill-takers G" Atch

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Mechanisms of gaseous exchange in bony fish .

Inhalation Exhalation Mouth opens The floor of the mouth is lowered Volume in the mouth cavity increases Pressure reduces causing water to be drawn into the buccal cavity. Operculum cavity bulges out but opercular flap presses on the body remaining closed preventing water from entering or leaving through the opening. Volume in the gill chambers increases Pressure decreases allowing in the water from the mouth cavity to flow in to the gill chamber over the gill filaments. Mouth closes. Mouth floor is raised; Volume reduces in mouth cavity Pressure increases, forcing the water into the gill chambers. Operculum press inwards. Volume of the gill cavity reduce Water is forced out of the gill chamber Pressure increases Gaseous exchange in the gills As water passes over the gills Oxygen diffuses from water through the gill filaments into the capillaries alongside concentration gradient. Co 2 diffuses of the capillaries into the water alongside concentration gradient.

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NB – In order to have maximum gaseous exchange faster diffusion between the blood in the gill filaments and the flowing water, a steep concentration gradient must be maintained across the respiratory surface. This is achieved by the flow of water and blood in opposite directions. This system in which blood flows over the gills in the opposite direction with incoming water is called counter current flow . Importance – for maximum gaseous exchange /faster diffusion due to steep Concentration gradient.

Counter current flow .

Water flow over lamellae showing % 02 2 Blood flow through lamellae showing % 02

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When water flows over the gill filaments in one direction and blood in the opposite direction, the concentration of oxygen in blood and water may be 0% and 15% respectively at the beginning. As more oxygen diffuses from water into the blood, the concentration of oxygen in the blood becomes higher. In addition, as blood flows in vessels, it continues to be in contact with water that has a higher concentration of oxygen. At all the pts, the concentration of oxygen is higher in the water than in the blood. In parallel flow blood and water flow in the same direction. Oxygen in the water can diffuse into the blood rapidly at the beginning. This is because of the large difference in concentration of oxygen between blood (100% oxygen).

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Mechanisms of gaseous Exchange in amphibians (frog )

An adult frog lives on land but goes back into the water during the breeding season. A frog uses three different respiratory surfaces. These are the skin, buccal cavity and lungs.

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The skin

The skin of the frog has some important features to allow effective gaseous exchange with its environment. These are : Has a large network of capillaries;to transport respiratory gases to and from it. Its kept moist by secretions from mucus glands to dissolve respiratory gases. Have a thinner skin to permit rapid diffusion; Gaseous exchange through the skin takes place when the frog is on land and in Water ;

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When the frog is submerged in water, oxygen dissolved in water diffused through the skin into blood capillaries; This is because the concentration of oxygen is higher in the water than in the blood capillaries, carbon (iv) oxide from the blood capillaries below the When on land, oxygen from air dissolves and diffuses blood capillaries below it, carbon (iv) oxide from the blood diffuses in the opposite directions. Toads do not use the skin surface for gaseous exchange normally except when they are hibernating.

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The inner surface of the lungs is thin, moist and is richly supplied with blood capillaries; it has tiny folds which increase the surface area: -Used when the frog is very active i.e. when swimming /leaping -There valves in each nostril. - Lungs lie in the body cavity of the amphibian. Inspiration When the nostril are closed ,the air can be forced into the lungs by the pumping action of the flow of the mouth and the air reaches the alveoli which are moist and supplied with large network of capillaries, oxygen combines with haemoglobin and is transported to all parts of the body and Co 2 diffuses out. Expiration The nares and glottis open. The air in the mouth and the lungs passes out, helped by the natural elasticity of the lungs .

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The Mouth / Buccal Cavity :

Adaptation The lining of the buccal cavity is thin to reduce the distance travelled by the respiratory gases It is also richly supplied with blood capillaries to facilitate transport of the respiratory gases. It’s kept moist; by the mucous membrane to dissolve the respiratory gases. Inspiration and Expiration Air is taken or expelled from the mouth cavity by raising and lowering the floor of the mouth .

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Gaseous exchange. The concentration of oxygen in the air within the mouth cavity is higher than that of the blood inside the blood vessels. Oxygen, therefore dissolves in the moisture lining the mouth cavity and then diffuses into the blood through the thin epithelium. On the other hand, carbon (IV) oxide diffuses in the opposite direction along a concentration gradient.

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Mechanisms of Ventilation in Amphibians (frog ) in the buccal cavity

Inspiration During breathing hyoid muscles contract and are pulled down and the floor is lowered. The volume inside the mouth increases, the pressure decreases thus causes the atmospheric air to enter the mouth through nostril (nares ). Expiration When the transverse muscles contract, the floor of the mouth is raised and the volume in the mouth decreases so that air rushes out of the mouth through the nostrils

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Gaseous Exchange in Mammals

Part Brief Description and function   Nostrils -lined with ciliated epithelial cells; -Have goblet cells that secrete mucus; -Have hairs to trap dust particles; -Have capillaries that warm up incoming air; Glottis -Opening of trachea; Epiglottis -Open sand closes the glottis to prevent Solid particles ; from entering the trachea ;

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Larynx/voice box -uses vibrations of air flow to create sound; Trachea -C-shaped cartilaginous rings keep it open; -Passage of air; -Has mucus and cilia; to remove dust and harmful bodies -Strengthened by rings of cartilage to prevent it from Collapsing; Bronchi -two branches of trachea;-one for each lung -have rings of cartilage to strengthen it; Bronchioles -branches of bronchi; -many branches ending with sac like alveoli;

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Pleural membrane -two membranes surrounding the outer surface of lungs; -tightly attached to lungs; the other covers the inside the thoracic cavity; -secrete pleural fluids which lubricate the pleural membrane allowing easy movement of the lungs during breathing; Pleural cavity -found between the two pleural membranes; -filled with pleural fluid and reduces friction; therefore makes lungs move freely in chest cavity during breathing;

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Thoracic cavity/ Chest cavity -surrounded by the rib cage and sternum, thoracic vertebra -air tight; -protects lungs and heart; -any changes in its volume affect the lungs in the same way; Intercostal muscles -surround the ribs -cause movement of ribs that cause breathing; Diaphragm -muscular sheet which separates thoracic cavity from abdominal cavity; -floor of the thoracic cavity enhances breathing by changing the volume in the thoracic cavity;