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From Wikipedia, the free encyclopedia

Plant cell structure
Plant cell structure

Plant cells are eukaryotic cells of the types present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagellae or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.

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Transcription

Plants are freaking great because they have this magical wizard power that allows them to take carbon dioxide out of the air and convert it into wonderful, fresh, pure, oxygen for us to breathe. They're also way cooler than us because, unlike us and every other animal on the planet, they don't require all kinds of Hot Pockets and fancy coffee drinks to keep them going The only thing plants need to make themselves a delicious feast is sunlight and water. Just sunlight and water! Paula Deen can't do that and she makes fried-egg bacon donut burgers. I'm telling you this is surprisingly good. This is a different kind of magic. But you know, part of this is plants! And everything in it, in fact, everything that is in this McDonalds in fact, everything that you have ever eaten in your life is either made from plants, or from something that ate plants. So, let's talk about plants! Plants probably evolved more than 500 million years ago. The earliest land-plant fossils date back more than 400 million years ago. These plants were lycophytes which are still around today and which reproduce through making a bunch of spores, shedding them, saying a couple of Hail Marys and hoping for the best. Some of these lycophytes went on to evolve into "scale trees," which are now extinct, but huge, swampy forests of them used to cover the Earth. Some people call these scale tree forests "coal forests" because there were so many of them and they were so dense and they covered the whole Earth and they eventually fossilized into giant seams of coal, which are very important to our lifestyles today. So this is now called the Carboniferous Period. See what they did there? Because Coal is made out of carbon, so they named the epoch of geological history over how face-meltingly intense and productive these forests were. I would give my left eyeball, three fingers on my left hand -- the middle ones, so that I could hang loose -- and my pinky toe if I were able to go back and see these scale forests because they were freaking awesome. Anyway, Angiosperms, or plants that use flowers to reproduce, didn't develop until the end of the Cretaceous Period, about 65 million years ago, just as the dinosaurs were dying out. Which makes you wonder if in fact the first angiosperms assassinated all the dinosaurs. I'm not saying that's definitely what happened, I'm just saying it's a little bit suspicious. Anyway, on the cellular level, plant and animal cells are actually pretty similar. They're called eukaryotic cells, which means they have a "good kernel." And that "kernel" is the nucleus. Not "new-cue-lus." And the nucleus can be found in all sorts of cells. Animal cells, plant cells, algae cells. You know, basically all of the popular kids. Eukaryotic cells are way more advanced than prokaryotic cells. We have the eukaryotic cell and we have the prokaryotic cell. Prokaryotic basically means "before the kernel." Pro-kernel. And then we have eukaryotic, which means "good kernel!" The prokaryotes include your bacteria and your archaea, which you've probably met before in your lifetime, every time you've had strep throat, for example, or if you've ever been in a hot spring or an oil well or something. They're everywhere. They covered the planet. They cover you! But like I said, eukaryotes have that separately enclosed nucleus. That all important nucleus that contains its DNA and is enclosed by a separate membrane Because the eukaryotic cell is a busy place -- there's chemical reactions going on in all different parts of the cell -- it's important to keep those places divided up. Eukaryotic cells also have these little stuff-doing factories called organelles. I guess we decided we would name everything something weird... But, organelles. And they're suspended in cytoplasm, continuing with the really esoteric terminology that you're going to have to know. Cytoplasm is mostly just water, but it's some other stuff too. Well basically if you want to know about the structure of the eukaryotic cell you should watch my video on animal cells. Let's just link to it right here. Plant and animal cells are very similar environments. They control themselves in very similar ways, but obviously, plants and animals are very different things. What are the differences in a plant cell that makes it so different from an animal? Well that's what we're going to go over now. First, plants are thought to have evolved from green algae, which evolved from some more primitive prokaryotes, and something plants inherited from their ancestors was a rigid wall surrounding the plasma membrane of each cell. So, this cell wall of plants is mainly made of cellulose and lignin, which are two really tough compounds. Cellulose is by far the most common and easy to find complex carbohydrate in nature, although if you were to include simple carbohydrates as well, glucose would win that one. And this is because, fascinating fact: cellulose is just a chain of glucose molecules! You're welcome. If you want to jog your memory about carbohydrates and other organic molecules, you can watch this episode right here. Anyway, as it happens, you know who needs carbohydrates to live? Animals. But you know what's a real pain in the ass to digest? Cellulose. Plants weren't born yesterday. Cellulose is a far more complex structure than you will generally find in a prokaryotic cell, but it's also one of the main things that differentiates a plant cell from an animal cell. Animals cells don't have this rigid cell wall--they have just a flexible membrane that frees them up to move around and eat plants and stuff. However, the cell wall gives structure to a plant's leaves, roots and stems, and it also protects it to a degree. Which is why trees aren't squishy and don't giggle when you poke them. The combination of lignin and cellulose is what makes trees, for example, able to grow really, really freaking tall. Both of these compounds are extremely strong and resistant to deterioration. When we eat food, lignin and cellulose is what we call "roughage" because we can't digest it. It's still useful for us in certain aspects of our digestive system, but it's not nutritious. Which is why eating a stick is really unappetizing. And like, your shirt. This is a 100% plant shirt, but it doesn't taste good. We can't go around eating wood like a beaver or grass like a cow because our digestive systems just aren't set up for that. However, other animals that don't have access to delicious donut burgers have either developed gigantic stomachs like sloths or multiple stomachs like goats in order to make a living eating cellulose. These animals have a kind of bacteria in their stomach that actually does the digestion of the cellulose for it. It breaks the cellulose into individual glucose molecules, which can then be used for food. But other animals, like humans -- mostly carnivores -- don't have any of that kind of bacteria, which is why it's so difficult for us to digest sticks. Ah! But there is another reason why cellulose and lignin are very very useful to us as humans: It burns, my friends! This is basically what would happen in our stomachs. It's oxidizing. It's producing the energy that we would get out of it if we were able to, except it's doing it very very quickly. And this is the kind of energy, like, this energy that's coming out of it right now, is the energy that would be useful to us if we were cows. But we're not. So instead, we just use it to keep ourselves warm on the cold winter nights. Ow! It's on me! Ow! Ahh! Anyway, while we animals are walking around, spending our lives searching for ever more digestible plant materials, plants don't have to do any of that. They just sit there and they make their own food. And you know how they do that? They do it with photosynthesis! Another thing that plant cells have that animal cells just don't have are plastids, organelles that plants use to make and store compounds that they need. And you wanna know something super interesting about plastids? They and their fellow organelles, the mitochondria that generate energy for the cell, actually started as bacteria that were absorbed into plant cells very early in their evolution like maybe some protist-like cell absorbed a bacteria, and it found that instead of digesting that bacteria for the energy that it has, it could use that bacteria. That bacteria could create energy for the cell or convert light into lovely glucose compounds, which is crazy! Nobody's really, precisely sure how this happened, but they know that it did happen because plastids and mitochondria both have double membranes. One from the original bacteria, and one from the cell as it wrapped around it. Cool, huh? Anyway, the most important of the plastids are chloroplasts, which convert light energy from the sun into the sugar and into oxygen, which the plant doesn't need, so it just gets rid of it. All the green parts of a plant that you see -- the leaves, the non-woody stems, the unripened oranges -- are all filled with cells which are filled with chloroplasts, which are making food and oxygen for you. You're very welcome, I'm sure. Another big difference between a plant cell and an animal cell, is the large, central vacuole. Plant cells can push water into vacuoles which provides turgor pressure from inside the cell, which reinforces the already stiff cellulose wall and makes the plant rigid like a crunchy piece of celery or something. Usually when soil dries out or a celery stalk sits in your refrigerator for too long, the cells lose some water, turgor pressure drops, and the plant wilts or gets all floppy. So, the vacuoles are also kind of a storage container for the cell. It can contain water, which plants need to save up, just in case. And also other compounds that the cell might need. It can also contain and export stuff the cell doesn't need anymore, like wastes. Some animal cells also have vacuoles, but they aren't as large and they don't have this very important job of giving the animal shape. So now, let's do this. Let's just go over the basics of plant cell anatomy: 1. They've got a cell wall that's made out of cellulose and so it's really rigid and not messing around. 2. They've got a nucleus in its own little baggie that's separate from all the other organelles. This is basically the headquarters of any eukaryotic cell: it stores all the genetic information for the plant and also acts as the cell's activities director, telling it how to grow, when to split, when to jump and how high...that sort of thing. Animal cells have this kind of nucleus too, but prokaryotes don't. Which is why they're stuck hanging around in oil wells and stuff. 3. They've got plastids, including chloroplasts, which are awesome green food-making machines. 4. They've got a central vacuole that stores water and other stuff and helps give the cell structural support. And so, stack these cells on top of one another like apartments in an apartment building and you've got a plant! And all of these unique features are what make it possible for plants to put food on our table and air in our lungs. So next time you see a plant, just go ahead and shake its hand and thank it for its hard work and its service. Now, we went over that stuff pretty fast, so if you want to go back and listen to any of it, we have a review section over here for stuff that you may not have totally picked up on or just want to watch again. It's not a huge piece of your life to re-watch some stuff so go ahead and click on these things. If you have questions to do with plant cell anatomy, please leave them for us in the comments and we will hopefully get to those. You can also hook up with us on Facebook and Twitter of course and we will see you on episode 7 of Biology Crash Course.

Contents

Characteristics of plant cells

Types of plant cells and tissues

Plant cells differentiate from undifferentiated meristematic cells (analogous to the stem cells of animals) to form the major classes of cells and tissues of roots, stems, leaves, flowers, and reproductive structures, each of which may be composed of several cell types.

Parenchyma

 Parenchyma cells are living cells that have functions ranging from storage and support to photosynthesis (mesophyll cells) and phloem loading (transfer cells). Apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells. Some parenchyma cells, as in the epidermis, are specialized for light penetration and focusing or regulation of gas exchange, but others are among the least specialized cells in plant tissue, and may remain totipotent, capable of dividing to produce new populations of undifferentiated cells, throughout their lives.[15] Parenchyma cells have thin, permeable primary walls enabling the transport of small molecules between them, and their cytoplasm is responsible for a wide range of biochemical functions such as nectar secretion, or the manufacture of secondary products that discourage herbivory. Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are called chlorenchyma cells. Others, such as the majority of the parenchyma cells in potato tubers and the seed cotyledons of legumes, have a storage function.

Collenchyma

Collenchyma cells – collenchyma cells are alive at maturity and have thickened cellulosic cell walls.[16] These cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become apparent. Plastids do not develop, and the secretory apparatus (ER and Golgi) proliferates to secrete additional primary wall. The wall is most commonly thickest at the corners, where three or more cells come in contact, and thinnest where only two cells come in contact, though other arrangements of the wall thickening are possible.[16] Pectin and hemicellulose are the dominant constituents of collenchyma cell walls of dicotyledon angiosperms, which may contain as little as 20% of cellulose in Petasites.[17] Collenchyma cells are typically quite elongated, and may divide transversely to give a septate appearance. The role of this cell type is to support the plant in axes still growing in length, and to confer flexibility and tensile strength on tissues. The primary wall lacks lignin that would make it tough and rigid, so this cell type provides what could be called plastic support – support that can hold a young stem or petiole into the air, but in cells that can be stretched as the cells around them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma does. Parts of the strings in celery are collenchyma.

Cross section of a leaf showing various plant cell types

Sclerenchyma

Sclerenchyma is a tissue composed of two types of cells, sclereids and fibres that have thickened, lignified secondary walls[16]:78 laid down inside of the primary cell wall. The secondary walls harden the cells and make them impermeable to water. Consequently, scereids and fibres are typically dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity. Sclereids or stone cells, (from the Greek skleros, hard) are hard, tough cells that give leaves or fruits a gritty texture. They may discourage herbivory by damaging digestive passages in small insect larval stages. Sclereids form the hard pit wall of peaches and many other fruits, providing physical protection to the developing kernel. Fibres are elongated cells with lignified secondary walls that provide load-bearing support and tensile strength to the leaves and stems of herbaceous plants. Sclerenchyma fibres are not involved in conduction, either of water and nutrients (as in the xylem) or of carbon compounds (as in the phloem), but it is likely that they evolved as modifications of xylem and phloem initials in early land plants.

Xylem

Xylem is a complex vascular tissue composed of water-conducting tracheids or vessel elements, together with fibres and parenchyma cells. Tracheids [18] are elongated cells with lignified secondary thickening of the cell walls, specialised for conduction of water, and first appeared in plants during their transition to land in the Silurian period more than 425 million years ago (see Cooksonia). The possession of xylem tracheids defines the vascular plants or Tracheophytes. Tracheids are pointed, elongated xylem cells, the simplest of which have continuous primary cell walls and lignified secondary wall thickenings in the form of rings, hoops, or reticulate networks. More complex tracheids with valve-like perforations called bordered pits characterise the gymnosperms. The ferns and other pteridophytes and the gymnosperms have only xylem tracheids, while the flowering plants also have xylem vessels. Vessel elements are hollow xylem cells without end walls that are aligned end-to-end so as to form long continuous tubes. The bryophytes lack true xylem tissue, but their sporophytes have a water-conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.

Phloem

Phloem is a specialised tissue for food transport in higher plants, mainly transporting sucrose along pressure gradients generated by osmosis, a phenomenon called translocation. Phloem is a complex tissue, consisting of two main cell types, the sieve tubes and the intimately associated companion cells, together with parenchyma cells, phloem fibres and sclereids.[16]:171 Sieve tubes are joined end-to-end with perforate end-plates between known as sieve plates, which allow transport of photosynthate between the sieve elements. The sieve tube elements lack nuclei and ribosomes, and their metabolism and functions are regulated by the adjacent nucleate companion cells. The companion cells, connected to the sieve tubes via plasmodesmata, are responsible for loading the phloem with sugars. The bryophytes lack phloem, but moss sporophytes have a simpler tissue with analogous function known as the leptome.

This is an electron micrograph of the epidermal cells of a Brassica chinensis leaf.  The stomates are also visible.
This is an electron micrograph of the epidermal cells of a Brassica chinensis leaf. The stomates are also visible.

Epidermis

The plant epidermis is specialised tissue, composed of parenchyma cells, that covers the external surfaces of leaves, stems and roots. Several cell types may be present in the epidermis. Notable among these are the stomatal guard cells that control the rate of gas exchange between the plant and the atmosphere, glandular and clothing hairs or trichomes, and the root hairs of primary roots. In the shoot epidermis of most plants, only the guard cells have chloroplasts. Chloroplasts contain the green pigment chlorophyll which is needed for photosynthesis. The epidermal cells of aerial organs arise from the superficial layer of cells known as the tunica (L1 and L2 layers) that covers the plant shoot apex,[16] whereas the cortex and vascular tissues arise from innermost layer of the shoot apex known as the corpus (L3 layer). The epidermis of roots originates from the layer of cells immediately beneath the root cap. The epidermis of all aerial organs, but not roots, is covered with a cuticle made of polyester cutin or polymer cutan (or both), with a superficial layer of epicuticular waxes. The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin.[19]

See also

References

  1. ^ Keegstra, K (2010). "Plant cell walls". Plant Physiology. 154 (2): 483–486. doi:10.1104/pp.110.161240. PMC 2949028.
  2. ^ Raven, JA (1997). "The vacuole: a cost-benefit analysis". Advances in Botanical Research. 25: 59–86. doi:10.1016/S0065-2296(08)60148-2.
  3. ^ Oparka, KJ (1993). "Signalling via plasmodesmata-the neglected pathway". Seminars in Cell Biology. 4: 131–138. doi:10.1006/scel.1993.1016.
  4. ^ Hepler, PK (1982). "Endoplasmic reticulum in the formation of the cell plate and plasmodesmata". Protoplasma. 111: 121–133. doi:10.1007/BF01282070.
  5. ^ Bassham, James Alan; Lambers, Hans, eds. (2018). "Photosynthesis: importance, process, & reactions". Encyclopedia Britannica. Retrieved 2018-04-15.
  6. ^ Anderson, S; Bankier, AT; Barrell, BG; de Bruijn, MH; Coulson, AR; Drouin, J; Eperon, IC; Nierlich, DP; Roe, BA; Sanger, F; Schreier, PH; Smith, AJ; Staden, R; Young, IG (1981). "Sequence and organization of the human mitochondrial genome". Nature. 290: 4–65. doi:10.1038/290457a0. PMID 7219534.
  7. ^ Cui, L; Veeraraghavan, N; Richter, A; Wall, K; Jansen, RK; Leebens-Mack, J; Makalowska, I; dePamphilis, CW (2006). "ChloroplastDB: the chloroplast genome database". Nucleic Acids Research. 34: D692–696. doi:10.1093/nar/gkj055. PMC 1347418. PMID 16381961.
  8. ^ Margulis, L (1970). Origin of eukaryotic cells. New Haven: Yale University Press. ISBN 978-0300013535.
  9. ^ Lewis, LA; McCourt, RM (2004). "Green algae and the origin of land plants" (PDF). American Journal of Botany. 91: 1535–1556. doi:10.3732/ajb.91.10.1535. PMID 21652308.
  10. ^ López-Bautista, JM; Waters, DA; Chapman, RL (2003). "Phragmoplastin, green algae and the evolution of cytokinesis". International Journal of Systematic and Evolutionary Microbiology. 53: 1715–1718. doi:10.1099/ijs.0.02561-0. PMID 14657098.
  11. ^ Silflow, CD; Lefebvre, PA (2001). "Assembly and motility of eukaryotic cilia and flagella. Lessons from Chlamydomonas reinhardtii". Plant Physiology. 127: 1500–1507. doi:10.1104/pp.010807. PMC 1540183. PMID 11743094.
  12. ^ Manton, I; Clarke, B (1952). "An electron microscope study of the spermatozoid of Sphagnum". Journal of Experimental Botany. 3: 265–275. doi:10.1093/jxb/3.3.265.
  13. ^ Paolillo, Jr., DJ (1967). "axoneme in flagella of Polytrichum juniperinum". Transactions of the American Microscopical Society. 86: 428–433. doi:10.2307/3224266.
  14. ^ Raven, PH; Evert, RF; Eichhorm, SE (1999). Biology of Plants (6th ed.). New York: W.H. Freeman. ISBN 9780716762843.
  15. ^ G., Haberlandt (1902). "Kulturversuche mit isolierten Pflanzenzellen". Mathematisch-naturwissenschaftliche. Akademie der Wissenschaften in Wien Sitzungsberichte. 111 (1): 69–92.
  16. ^ a b c d e Cutter, EG (1977). Plant Anatomy Part 1. Cells and Tissues. London: Edward Arnold. ISBN 0713126388.
  17. ^ Roelofsen, PA (1959). The plant cell wall. Berlin: Gebrüder Borntraeger. ASIN B0007J57W0.
  18. ^ MT Tyree; MH Zimmermann (2003) Xylem structure and the ascent of sap, 2nd edition, Springer-Verlag, New York USA
  19. ^ Kolattukudy, PE (1996) Biosynthetic pathways of cutin and waxes, and their sensitivity to environmental stresses. In: Plant Cuticles. Ed. by G. Kerstiens, BIOS Scientific publishers Ltd., Oxford, pp 83–108
This page was last edited on 12 December 2018, at 18:23
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