Lesson 3: Photosynthesis


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Only green plants possess the amazing ability of trapping solar energy to produce food.
Photosynthesis is the process in which green plants absorb solar energy to make food from carbon dioxide and water.

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Lesson 3: Photosynthesis

  1. 1. Plants•Plants are everywhere. However how well do we knowthem? Firstly, how do they different from animals and us?Yes, they make food themselves through photosynthesis.What do they need in order to carry out photosynthesis?•Do plants have any circulatory system like us to carry foodand water to other parts of the plants? How do theyreproduce? Do plants response to stimuli?
  2. 2. Photosynthesisa.Green plants are producers.b.Only green plants possess the amazing ability of trappingsolar energy to produce food.c.Photosynthesis is the process in which green plants absorbsolar energy to make food from carbon dioxide and water.
  3. 3. d. Oxygen is released as a by- product.e. Chlorophyll is the green pigment present in the leaves that can absorb sunlight.f. The glucose produced during photosynthesis is then converted into starch. Starch is stored in the stems, fruits or roots of plants.Photosynthesis can be represented by the following equation:Water + carbon dioxide sunlight glucose + oxygen chlorophyll
  4. 4. Requirements of photosynthesis1. The necessary factors for photosynthesis:a.Sunlightb.Chlorophyllc.Waterd.Carbon dioxide2. Photosynthesis will not take place if any one of thesefactors is absent.3. We should test for the presence of starch in leaves todetermine whether photosynthesis has taken place in aplant.
  5. 5. Chlorophyll Sunlight Carbon dioxideWater
  6. 6. •Testing for the presence of starch in leaves1. Below are the steps to test for the presence ofstarch in a leaf. immerse a leaf in boiling water for a minute to soften and break the cells.
  7. 7. transfer the boiled leaf into aboiling tube containingalcohol . Place the boilingtube in a water bath for afew minutes to decolourisethe leaf. remove the leafand put it back into the hotwater for a few seconds tosoften the leaf.
  8. 8. 1. Spread the leaf into a white tile.Add a few drops of iodine solutionto test for the presence of starch.2. The presence of starch can betested using iodine solution after thechlorophyll of the leaf is removed.3. The areas containing starch willbe stained blue-black when iodinesolution is added.
  9. 9. The importance of photosynthesis-Photosynthesis is an important process. Thisprocess manufactures food for all organisms onEarth.-All organisms need food but only green plants arecapable of trapping solar energy to manufacturefood from simple substances.-All other organisms, animals or non- green plants,are directly or indirectly dependent on green plantsfor food.
  10. 10. - Green plants start the food chain and they are the producers in any ecosystem.- Therefore, photosynthesis is an important process that provides food for all organisms.- Photosynthesis replaces the oxygen that is used up in the following process.a. Respiration of living things.b. Combustion of fuelc. Decomposition of dead plants and animal material.
  11. 11. - On the other hand, photosynthesis absorb carbon dioxide in the atmosphere that is given out during respiration, combustion and decomposition.- The concentration of oxygen and carbon dioxide in the atmosphere are maintained by the oxygen and carbon cycles.- Therefore, photosynthesis and respiration are important in maintaining a balanced level of oxygen and carbon dioxide in the atmosphere. These processes will lead to a balanced ecosystem.
  12. 12. The transport system in plants•Wilting-Non-woody plants depend on thewater stored in the cells of the stemfor support.-Wilting occurs in non-woody plantswhen water loss through the aerialparts of the plants exceeds waterabsorption by the roots.-Cells in the plants lose their turgiditystiffness) and the plant droops.
  13. 13. -Wilting can be important to the plants because theleaf surfaces are removed from the direct rays ofthe Sun and the stomata close. Water lose from theplants will be reduced.-Normally, a plant which has wilted will remain inthis condition until evening.-In the evening, water absorption exceeds waterloss and the cells become turgid again.
  14. 14. •Transpiration-Transpiration is the evaporation of water from theaerial parts of plants.-90% of the water absorbed by the roots is lost byevaporation from the surfaces of cells in the leavesand subsequent diffusion of water vapour throughthe stomata. 9% is lost through the cuticle. Theremaining 1% is used for photosynthesis.
  15. 15. •Stomata-Stomata are pores in theepidermis of the leavesand stems.-A pair of bean-shapedguard cells bound eachstomata. Surfaces view of a stoma
  16. 16. - In most dicotyledons, stomata appear only in thelower epidermis of the leaf.-In monocotyledons, stomata are found on bothsides of the leaf.-Most stomata open during the dday and close atnight.-The main function of the stomata is to allow gasesto diffuse in and out the leaf. During photosynthesis,carbon dioxide diffuses from the atmosphere intothe leaf and oxygen diffuses out from the leaf intothe atmosphere.
  17. 17. - When the stomata open, water vapour is releasedto the surroundings through the stomata bytranspiration. However, the stomata will close whenthe transpiration exceeds water absorption.
  18. 18. Factors that affect the rate of transpiration•Light – light stimulate the opening of stomata andconsequently increases transpiration.•Temperature- water molecules move faster inwarm air, hence the rate of transpiration isincreased.•Relative humidity – the rate of water loss dependson the difference in the concentration of watermolecules in the leaf and in the atmosphere. Highrelative humidity reduces water loss.
  19. 19. •Wind speed – an increase in wind speed increasesthe rate of transpiration because the movement ofair carries away the water vapour from the stomata.•Water availability – short supply of water causesthe plant to wilt and the stomata to close. Thisreduces the rate of transpiration.•Altitude – in the mountains, the atmosphericpressure decreases sufficiently to cause anincrease in the rate of transpiration.
  20. 20. The roles of transpiration•The pulling force developed by transpirationprovides the pathway through which water andminerals are transported in the plant.•Evaporation from the leaf surface has a coolingeffect which helps to prevent the heat of directsunlight from damaging the delicate cells.
  21. 21. Vascular tissues of plant•The vascular tissues are concerned with transportand are functionally equivalent to the circulatorsystem of mammals•The two types of vascular tissues are called xylemand ploem.•Xylema. The main function of the xylem is to transportwater and mineral salts from the roots to the stemand leaves.
  22. 22. b. Xylem forms wood in shrubs andtrees. Therefore, xylem providessupport to the plantsc. Xylem consists mainly of vessels,which are elongated tubes with thickwalls. The walls are strengthenedwith a substance called lignin. Xylemvessels are dead structures withoutprotoplasmic contents andtransverse walls. Xylem vessels
  23. 23. •Phloema.Phloem transports synthesised food substancesfrom one part of the plant to another.b.Phloem consists mainly of sieve tubes. Sievetubes are living cells without nucleus. The walls ofsieve tubes are composed largely of cellulose. Thetransverse walls of sieve tubes are perforated bypores and are called sieve plates.c.Each sieve tube cell has a companion cell besideit.
  24. 24. Phloem Cells
  25. 25. The sexual reproductive system of flowering plantsStructure of flower•Flowers are the sexual reproductive organs ofplants.•The flower sits on the expanded end of a flowerstalk called the receptacle.•The outermost parts of the flower are the sepals.The sepals are usually small, green and leafy-like.•The petals maybe coloured, scented andsometimes fused into a tube.•The female part of the flower is called the stamen.Each stamen consists of an anther at the end of astalk-like filament. The anther consistspollen grains.
  26. 26. •The female part of the flower is called the pistil orcarpel. The pistil consists of stigma style ovary andovules.
  27. 27. There are two types of flowersa. Unisexual flowers contain either the stamens(male flowers) or the pistils (female flowers) Example of unisexual flower
  28. 28. b. Bisexual flowers contain both the stamens andthe pistils Example of bisexual flowers
  29. 29. •Pollen grains come in a diversity of shape andsizes. The outer wall of each pollen grain has apattern characteristics of each species. Pollen grains from (a) grass, (b) rose an (c ) hibiscus plant
  30. 30. •Pollen grains are carries of the male gametes ofthe plants.•The ovules are formed in the ovary and theycontain the female gametesa.The cross section b. the longitudinal section Ovary
  31. 31. Parts of flowers FunctionReceptacle Supports the flower Supports the young flower when itSepals is in the budding stage Attract animal pollinators throughPetals their colour and scentStigma Receive pollen grainsStyle Connects the stigma to the ovary Protects the ovules. After fertilization, the ovary developsOvary into a fruit to protect the seed Contains the female gamete (egg cell). After fertilization, the ovuleOvule developes into a seed. Produce male gametes (in pollenAnther grains)Filament Supports the anther
  32. 32. Pollination•Pollination is the transfer of pollen grains fromanthers to stigmas.•Pollen grains cannot move independently and theirmain means of transport is by agents of pollination.•The chief agents of pollination are wind andinsects but birds, bats, snails and even water maycarry pollen grains from one flower to another. Thechief insect pollinators are bees and butterflies.
  33. 33. Structure of an insect- Structure of a wind- pollinated flower pollinated flower
  34. 34. Differences between insect-pollinated and wind- pollinated flowers Insect- pollinated Wind- pollinated•Large and conspicuous Size •Small and inconspicuous•Brightly couloured Petals •Greenish and dull couloured•Present Nectaries •Absent Scent •Absent•Scented
  35. 35. Insect- pollinated Wind- pollinated•Inside flower, where •Dangled outside Anthersinsect have to brush past flower, where theythem to reach the catch the wind.nectaries.•Small sticky and remain •Large, feathery and Stigmasinside the flower. hang outside the flower. •Very large quantities,•Quite large quantities,larger, heavier, and Pollen small, light andrough – surfaced. grains smooth
  36. 36. •Self pollination is the transfer of pollen grains fromthe anther to the stigma of the same flower, or fromthe anther to the stigma of a different flower in thesame plant.•Cross pollination is the transfer of pollen grainsfrom the anther of one plant to the stigma ofanother plant of the same species.(a)Self- pollination (b) Cross- pollination
  37. 37. Similarity and differences between self-pollination and cross- pollination Self- pollination and cross- pollination are both involve the transfer of pollen grains from the anther to the stigma. Differences Self- pollination Cross- pollination•Involves pollen grains and •Involves pollen grains andstigmas from the same stigmas from flowers offlowers, or from different different plants of the sameflowers of the same plant. species.
  38. 38. Self- pollination Cross- pollination•Offspring have the genetic •Offspring have the geneticmaterials of only one parent. materials of two parents. ThisIt is form of inbreeding and results in increased variabilityresults in less varied in the offspring and greateroffspring. adaptability to new environments•Can occur even before theflower opens fully. •Depends on agents of pollination•Less common from More common form ofpollination. pollination.
  39. 39. The advantages of cross- pollination•Cross- pollination has the distinct disadvantage ofproviding more genetic variations of the offspring.•More variation will help the offspring to survive in achanging environment.Use of cross- pollination in agriculture•For centuries, farmers have been trying to cultivatenew varieties of crops through cross breeding ofclosely related plants.•The main aim of cross breeding is to produceoffspring with desirable characteristics of each parent.
  40. 40. •Cross breeding can be achieved through cross-pollination. For example, the pollen grains of a highyielding variety of rice are artificially placed on thestigmas of another variety of rice that hasresistance to diseases. Cross pollination mightproduce offspring that are both disease resistantand high yielding.•The tenera variety of oil palm is produced bycross- pollination the dura variety with the pisiferavariety.
  41. 41. Cross- pollinating the dura variety with the pisifera variety yields the tenera variety
  42. 42. The development of fruits and seeds in plantsFertilization in plants•After pollination, the male gamete inside the pollengrain on the stigma still has not reached the femalegamete because the female gamete is inside theovule and the ovule is inside the ovary.•Sugary secretion of the stigma causes the pollengrain to germinite and produce a pollen tube.•The pollen tube grows down through the style andenters the ovule through a tiny hole called themicropyle.•The male gamete in the pollen tube fuses with thefemale gamete (egg) to form a zygote. this processis called fertilization.
  43. 43. Fertilization in plants
  44. 44. •One pollen grain can only fertilizes one ovule.•If there are many ovules in the ovary, then manypollen grains will be needed to fertilize them all. Formation of fruits and seeds•Once the ovule have been fertilized, the sepals,petals and stamens wither and fall off.•The fertilized ovule develops into a seed and theovary as a whole develops into the fruit.•The number of seeds in a fruit depends in howmany ovules there were in the ovary and how manywere fertilized.
  45. 45. •The wall of the fruit called the pericarp developsfrom the wall of the ovary.•As the fruit matures, the pericarp may becomehard and dry (e.g. sweet pea, castor oil) or juicy andfleshy (e.g. mango and tomato). Juicy fruits arecalled succulent fruits.(a)Fruit of sweet pea plant (b)fruit of mango plant
  46. 46. The germination of seedsStructure of a seed•Seeds have protective covering called the testa.•The zygote develops into an embryo whichconsists of:a.A young root called the radicle.b.A young shoot called plumule,c.One or two seed leaves called cotyledons•Seeds with two cotyledons are calleddicotyledonous seeds ( e.g. soya bean and broadbean) and seed with one cotyledon are calledmonocotyledonous seeds (e.g. paddy and wheat).
  47. 47. •Food for the embryo is deposited either inside thecotyledons or in the endosperm.•On the testa is a scar called the hilum, which is the pointwhere the seed is attached to the ovary wall.•The micropyle lies above the hilum.•The fruit wall called the pericarp protects the seeds. The maize grain