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Module 1: Plant Cell Technology

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Plant Tissues and Functions

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So, yesterday what did we do, westudied about the basic structure of plant cells, the differentorganelles in plant cells, the distinct features of plant cells, the in what ways plant cellsare different in terms of organelles and their functions.So, nowwe will briefly go through how plants evolved and and then what are the differenttypes of plant tissues, therefore, during evolutions how did the different types oftissues evolved.And then coming onto the leaf structure and then talking about the transports in rootsand stems which means the vascular bundles .So, now talking about the evolutions of plant cells, why it is important, you will cometo know how the structure of the plant cells changed during evolution.So, now, to begin with they were nonvascular plants.So, once they came from aquatic to the land, so the land plants who which initially camein they were non vascular in nature.So, when I say they were non vascular in nature, what does that mean, any idea?Vascular bundle.So,the vascular bundle was missing the vascular tissue which you as you said it is xylem andphloem.So, these plants therefore, in order to survive, so what would be the best way to survive?Now, because the vascular bundle is missing, the mechanism to transport the water and mineralsto long distances against a gravity is missing.So, then they will have to grow crawling at the level of the ground, and also even atthe level of the ground to long distances long lengths they cannot survive, becausethen again the transport of minerals and water has to be longer.So, therefore, you will find them very close to water bodies where they can easily find.So, like for example, mosses, algae, so liverworts.So, these kind of a plants which were nonvascular in nature used to grow near the food sourceor the water source.Then with evolution with in order to survive higher order functions of a survival, thenthey called vascular in nature, vascular in nature which means that getting the vascularbundles xylem and phloem.So, now conducting of water and minerals could happen.So, then the initial ones were non seed bearing plants.So, through pores they used to have their progenies.So, for example, ferns they are non seed bearing plants.Then further evolution happened, they came to seed bearing plants.Now, even in seed bearing plants they were angiosperms and gymnosperms.Angiosperms if you remember they are they do not have non-flowering plants, they havenaked seeds like for example, conifers pine trees.So, the the ovule does not have an ovary around it, there are no fruits.So, the seeds are open.But then further evolution means where the seeds could be the only way the seeds couldbe dispersed was the wind was the major mechanism.So, but now when the fruits came in or the flowers came in pollination, then fertilizationcould be long distances.So, they could get genetically improved varieties.So, then came the seed bearing plants or your flowering plants where the seeds could beencapsulated in fruits and all.So, then we will talk about mainly of about the vascular tissue.So, now talking about the different tissues in the plant.So, now what is a tissue?We have heard about tissues, we have heard about so cells, then organs.So, what is a tissue?It is somewhere between the cell and the organ.So, tissue is nothing but when similar cells from the same origin and come together toperform a specialized function, so then they form a tissue.When a number of tissues come together having or carrying out different functions in a functionalmanner then they become an organ.So, in plants, there are different kinds of tissues.What are the different kinds of tissues?Dermal tissue, then ground tissue, then meristematic tissue and vascular tissue .So, now let us talk about all these tissues.Dermal tissue, dermal tissue by the name itself the skin of the plant . So, they are composedof many different types of cells, but let us talk about the aerial parts and the groundparts . Now, one kind of the dermal tissue is called epidermis which you must have heardeven during your school days.Now, epidermis is mainly made up of parenchyma type of cells.Now, for example, in leaves they form the thin skin the transparent layer which alsosometimes these kind of cells at the epidermis is they also secrete a wascy a waxy layerwhich is called as cuticle.Now, it serves the purpose of protecting or preventing the loss of water from the surfaceof the leaf or the stem.Thenwhat else one is epidermis, now specially this you will find in non woody plants oryoung plants.Then dermal tissue when you talk about ground level, under ground level, then the modificationsinto root hairs.Now, roots they form the epidermis on the roots form root hairs.There the purpose of epidermis these long extensions or root, small, small extensionsof the epidermis is to increase the absorption capacity of the root to absorb water and mineralsto enhance the uptake of water and minerals through these extensions.So, there the epidermis is playing an extra role.Then on leaves and some and sometimes you will find wherever it is depending on thelocation at which it is present on the plant, the function and the structure varies.Like for example, on the surface I said you will find cuticle, it is called as cuticleor the waxy layer or waxy compound generation.Now, below the surface of the leaf, the entire leaf surface, you will find the epidermiswill have large amount of pores or spaces which as called as stomata.And you will find guard cells which make these openings.Now, these openings we have heard already they are used to for gas exchange and forthe exchange of or for the water.So, transpiration happens from there.So, then in woody plants, epidermis is replaced by the dead cork cells.So, when there are woody plants rather than having thin skin, this thin skin gets modifiedinto cork cells.Now, cork cells are nothing but mature cells which are no longer growing and they becomedead at maturity.The cytoplasm is lost, so they are only used for structural and protection purpose.So, then the epidermis is modified into cork cells region.Talking about ground tissue, ground tissue is the mass of the plant; it forms the majorflesh of the plant in the stem the thickness.So, this ground tissue again majority of the cells in the ground tissues are parenchymacells.And these cells they make up the ground tissue.Now, they carry out many different kinds of functions.Now, these parenchyma cells because they can differentiate into different types of cells,so they can take part in various metabolic activities, like in food storage, in synthesis,then inphotosynthesis which meanspreparing food.Then what else and in support which means making the flesh supporting the stem or theleaf.So, that is how these un in woody.Non-woody root stems leaves they are majorly made up of primarily made up of ground tissuenon-woody plants.So, vascular tissue system now this is something which through evolution came into the plantcells.Vascular tissue there are two kinds of vasculartissues, one is the xylem and the phloem . Now, xylemis what, xylem is responsible for?Water conduction Water conduction.Phloem is responsible for?. The food which is prepared in terms of carbohydratesor sugars which is prepared in the leaves or the stems is then transported through differentparts of the plant.Now, how does that happen?Now, in in roots when we say xylem now xylem contains different kinds of cells, and phloemcontains different kinds of cells which have specific structures to facilitate this process.Let us talk about the xylem.Now, when the water gets absorbed onto the roots, now how do you think the water is ableto get transported through the xylem against gravity till the leaves.It has to travel to the leaves where the photosynthesis is happening.Capillary action.So, from where is that one is the capillary action, but do you think capillary actionwould be enough to take it to the site of a photosynthesis they are such tall trees.So, that creates a negative pressure in this xylem.Once the water because of transpiration gets evaporated, there is a pull for more watertill the leaves.hm So, there is a negative pressure in the xylem,and then the water is sucked up from the xylem for the photosynthesis in the leaves.So, then again the roots to fill up, the adsorption takes place.Now, one is that.What other do you think when you say capillary action,what determines capillary action whatkind of forces?Cohesive forces.Cohesive, and what else?Cohesive, and?Adhesive forces.Adhesive forces.So, now, how do you think cohesive force in this, an additive adhesive force in this ishelping or facilitating the water movement in the xylem?This is called as cohesive tension theory.So, I will come onto that in the next slide.Let us talk about the different types of cells then I will be taking in detail.Now, the different types of cells in xylem, there are two different types one is calledthe vessel elements and the others are tracheids.So, now, vessel elements, these are nothing but columnar cells which have perforationsat the top and the bottom.So, they become like pipe lines with no cytoplasm empty chambers lignified cells thick cellswith secondary walls also the water can travel easily.So, these perforations they are also called as perforated plates.So, these are openings in the cells at the top and the bottom.Then this is vessel elements.Now, tracheids, tracheids are very long cells which become close to each other long in lengths,and these are simple cells, but because of the length they the water can diffuse throughthese cells and can pass.Again these are dead cells upon maturity.So, they become empty chambers as and therefore they have stronger secondary walls so similarto vessel elements.So, then talking about phloem, phloem it isthe purpose is to transport sugar to differentparts of the plants.So, the type of cells which are responsible, these are called as sieve tube elements . Now,the sugar to be transported through sieve tube element, these are again long cells.Now, the sugar to be transported through sieve through elements, how do you think, the sugarsap is so thick, once the sugar is prepared, how does the flow happen through these cells?It is not the site of synthesis of the sugar.So, first the sugar from some site of synthesis, the cells which are preparing sugar wherephotosynthesis is taking place.The sugar has to be dropped into sieve these sieve cells, and then from sieve tube elements.And then from there it has to be dropped back to the sink wherever sugar is needed.So, how do you think this is happening?Sugar in the sieve tube elements to begin with is not present.So, so it can be through active transport or passive.So, active when it is from higher from to the sink, it has to be transported to thesink.Passive, it can be from the source to the sieve tube elements.But then how in the sieve tube elements the flow is happening, it is lateral also down,also up also.So, how in the sieve tube elements the flow is happening, you will always find a hint,you will always find these vascular bundles together.hm The xylem and the phloem together.Yeah.Why do you think it is so?Nature does everything for a purpose, it beautifully designs the structure.The flow of the sugar in these sieve tube elements is through hydrostatic pressure.So, what happens when it is loaded onto the sieve tube elements the sugar, the water becauseof xylem is close to it.The water will flow where.From the xylem to the phloem.So, once the water flows, then there will be hydrostatic pressure, then the sugar startsflowing.Now, in order to pour it into a sink, even though active transport or again this is facilitatedby a removal of water at the site the water is again diffused back from those cells atthe site to the xylem, so that the hydrostatic pressure can be reduced.And then the active transport can take place to put the sugars into the source cells.And what does it depend on the loading and unloading it depends in the sieve tube elementsthereare lot of we have heard about plasmodesmata.So, it depends on the number of plasmodesmata, those connecting channels through which thistransport happens.There are two patterns of growth, one is primary growth and the second is secondary growth.Primary growth where elongation of the lengthening of the aerial parts or the underground partsis happening, so that is by the meristematic tissue only.So, there there are are and not all throughout like animals ones or mammals once the growthhas happened, we do not keep growing continuously.So, but in plants the growth can happen till the plant is alive.Now, but it does not happen throughout the plant, it only happens in certain regionswhich are called as meristems, these are meristematic tissue regions which are composed of cellswhich are continuously dividing.So, it is the function of these tissues to increase the thickness or the length, or evenbetween the phloem and the xylem there is another meristematic tissue which is calledas vascular meristem which further is rapidly dividing and then differentiates into eitherphloem or xylem and causes thickness there.So, now secondary growth, secondary growth is increasing the thickness.You will find it in specially woody plants.Those thick trunks which you see in the plants is because of secondary growth that happensbecause of your lateral meristems or vascular meristem.So, as I said that it happens through the meristematic regions that is what is thereon this slide.And how do these meristematic tissues or these meristems divide, it is through mitosis simplecell division.Now, because these meristematic cells, when we talk about totipotency in plant cells,it is because of these cells which have the ability to continuously divide and then differentiateinto different types of tissues or different types of cells.Now, growth in meristems as I was telling you, the apical meristem then the lateralmeristem is at the inter nodes or nodal.And there are rapid meristematic regions even between the nodes which will help in lengtheningthe inter node.So, there are different kinds of depending on wherever this meristem region is present,they support the either the elongation of the plant or the increase in the secondarygrowth.Talking about cambium, cambium is your vascular cambium.Now, this is a lateral meristem in the vascular plants which means how does the xylem andthe phloem of the cell keeps on increasing or gets rejuvenated, having more cells toit.So, now, this happens because of the tissue as I said vascular meristem which is presentbetween xylem and the phloem, and it increases the size of the vascular cambium.Now, as I discussed which process helps in the transport of water through the roots tillthe upper regions of the plant is diffusion.Now, for diffusion what is the driving force.Concentration, concentrating.So, how do you think from the root from the ground, it is being sent to the through diffusionto the upper region, what will guide it?It is the concentration of other solute molecules or other substances present around which willcause the water movement which we call as osmosis,So, yesterday I was talking about plasmolysis this is what you see e in wilted leaves.Because of transpiration water is lost, so it withdraws the plasma membrane withdrawsitself, and thus they shrink . And if you put it in a hypertonic solution, these leavesthen they will become turgid because of the movement of water molecules from outside tothe inside.And we know that cellular membranes, these are semi permeable membranes, and water caneasily move in through these semi permeable membranes.Now, once the water is moving through the roots and going till the xylem.So, it has to cross the root hairs, then the cortex region, then it has to cross the phloem,then the xylem.So, how do you think that the water after entering the xylem is not able to come comeout as it is going in, it can even come out?No, it is not active transport, but there are sealants you will find that a once ithas crossed the phloem, and then it goes to the xylem.So, now,before the phloem, there is another layer of cells which have waterproofing mechanism.The water then it is when we say semi permeable which means it is differentially permeable.So, there the mechanism is selectively going one way transport, it will not allow the waterto leak out once it has reached the xylem.In an isotonic solution nothing would happen, because there is no concentration gradient.Both inside and outside the concentration of the solutes is same.So, now, where do you think this active transport for water it is mainly diffusion.And then for minerals and salts and for flow a movement of sugars, it is active transport.This is how it takes place in roots.So, it has been absorbed.It can takes place through two ways, one is the water can pass through the cells throughthe cells and reach the xylem; another way is the gaps between the cells are utilizedto reach till the xylem.So, transport as we had already discussed, transpiration is one of the ways by whichthe water is able to flow through the xylem.It creates that negative pressure in the xylem for the water movement to happen.How much percentage of water do you think every day the plant is losing because of transpiration?So, but you know actually 98 percent of the water which is absorbed every day is lost.Now, this loss is one this causes the movement that is one.And what other advantage do you think this loss?Even though it is getting lost, but there is some advantage which is happening.Now, why is water needed by the plant?Photosynthesis, water has to be needed.So, it becomes the bottleneck or the rate limiting step in the photosynthesis, knowingthat 98 percent of the water is getting lost.What can be other additional advantages not additional, but other advantages which arebalancing out this loss?So, one is the mineral transport along with water not food, but the minerals which arecoming along with water.So, they will be pulled up because there is transpiration.So, they will be pulled up.This is how it gets transported in the so the hypothesis which is used to explain theflow of sugar molecules in the sieve tube elements is pressure flow hypothesis.So, as I said because the xylem is adjacent to phloem and the water molecules diffusein from the xylem to the phloem at the site of source.So, once it is the source where the sugar is being prepared.So, in order for the sugar to move now towards the sink in the hydrostatic pressure is needed.So, for that the water is driven in into the sieve tube elements.Now, through the sieve tube elements, then it will pass and as soon as the side whereit has to be downloaded it reaches now the hydrostatic pressure has to be reduced.So, the water is again diffused back into the xylem.So, now, tell me which all steps will include active transport in this picture if you cansee, what all steps will have active transport at the site of sink?. Ok.For what?For the sugar.So, that is also active transport hm.What else?So, do you think water what will be the best way because ultimately the sugar has to bedownloaded, why should the cell spend extra energy doing active transport for all molecules?. So, what will it do to automatically takecare of the movement of the other entity?. It actively transports the sugar moleculesinto the sink through membrane proteins.The cell membrane as I said is permeable to water.So, once the sugar is downloaded, then the water is able to diffuse.So, these things happen in parallel.So, there are membrane proteins, transport proteins which help in downloading of thesecells against the these molecules against the concentration gradient that is calledas pressure flow hypothesis.Now, let us talk about the leaves.What is the structure of the leaf, where does the photosynthesis takes place?As I said the epidermis is modified.The upper surface of the epidermis is very thin transparent you, you can yourself I donot know whether you did that experiment, I remember during my 12th standard, we usedto peel off the upper layer.So, that used to be transparent that was the upper epidermis.And thenbelow that it is your parenchyma cells, modified parenchyma cells which are the sitefor photosynthesis.So, the palisade mesophyll and your spongy mesophyll, spongyy mesophylls they are randomlyarranged.So, there are spaces; it is more flexible.The upper layer which are palisade mesophylls, these are again parenchyma cells only, whereyour light reactions take place, where it gets captured.Now, then in the this spongy mesophyll there is an area where you see in the veins.Now, these veins are made up of your vascular tissue your xylem and the phloem.Now, near these xylem and the phloem, adjacent regions of the cells is called bundle sheathcells, these are again modified parenchyma cells.Now, these bundle sheath cells are the site for dark reactions or the spongy mesophyll,where the carbohydrates gets formed.Now, see very nicely nature has made such, now the site for carbohydrates or sugars synthesishas been made close to the vascular bundle, so that it can be transported . So, that isa source, and then it has to be transported into the phloem for the movement to differentparts of the plant.And your lower epidermis will have openings called stomata.Now, what do you think the plants would do?Plants cannot move.There are seasonal variations, lot of sunlight, sometimes no sunlight.So, how do you think the plants are adapting themselves to cater to their needs of photosynthesis?Too much of sunlight which means lot of loss of water also, so at the cost of sunlightcaptured the there is damage also which is happening.So, then what would the plant do to survive?It will close the stomata.But if it closes the stomata, what will happen?. The damage for it the demerit, it is a meritnow, it has closed no water loss, but now what is the demerit for photosynthesis letus.Yeah.Stick to a brief photosynthesis gas exchange.Yeah.Is a demerit.SuddenlyCO 2 now concentration how will the it capture in the presence of sunlight, weall know it needs CO 2 also.Yeah.To synthesize in the dark reactions, convert them into sugars with ATP generated throughlight reactions light photons captured.Now, once the stomata is closed, what will happen, no CO 2 coming in.So, inside the cells, what will happen where these light reactions are happening, the oxygenconcentration will rise.So, now, once the oxygen concentration rises, the enzyme which is responsible for photosynthesislight dark reactions is for synthesizing carbohydrates.Do you remember the name of the enzyme?Rubisco.Rubisco.So, rubisco can get activated both ways by CO 2 as well as O 2.But we will see later that once the oxygen concentration increases rather than doingphotosynthesis, it becomes because of higher concentration of oxygen, rubisco starts actingtowards oxygen, starts preferring oxygen rather than CO 2, and some other kinds of undesirablethen reactions start happening which is not what the plant or the rubisco is meant todo.. So, there are different types of plants dependingon the regions where they are where they need to adapt to these situations, these are classedas generally whatever plants you see they are C 3 class of plants.And the plants in arid regions, these are called as C 4 plants.hm.So, now in the next class we will see we will talk about photosynthesis photorespirationwhich is the cause for all this, yes, and then we will come on to photosynthesis.And then we will talk about the adaptations into C 3 and c 4.And even there is a third type from C 4, and why does it happen how does the plant adaptitself to these situations.