Cell Walls


There are three strata or layers that may be found in plant cell walls.

The outmost layer or middle lamella forms the crossing point linking neighboring plant cells and bonds them as one. This is a layer rich in pectin. Theprimary cell wall is commonly a slender, stretchy and expandable layer that forms while the cell is maturing. Thesecondary cell wall forms within the primary cell wall once the cell is done maturing. It isn't common in all cell types. In some cells the secondary wall consists oflignin that adds support to the cell wall and creates a watertight seal. The middlelamellais developed first, and formed from thecell plateduringcytokinesis. The primary cell wall, composed of cellulose microfibrils aligned at different angles, is then placed inside the middle lamella. Microfibrils are bonded collectively by hydrogen bonds to supply a high tensile strength. The cell wallis the flexible but occasionally reasonably stiff layer that encompasses cells. It is located on the exterior thecell membraneand provides structural support and security. One of the major functions of the cell wall is to prevent over-expansion when water enters the cell. The cell walls in a few plants also acts as storage device for carbohydrates to later be gradual breakdown into its component materials and dispersed to meet the development requirements of the plant. These cells interact thoughplasmodesmata, which are adjoining channels of cytoplasm that tie to the protoplasts of adjoining cells across the cell wall.

Question 2

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The chloroplast and mitochondrion are as alike as they are different.

The similarities are:

  • They are both specialized subunit within a cell involved in energy conversion within a cell. These organelles are responsible for utilizing and converting the energy that is used by the cell in every process that doesn't occur at the same time, such as diffusion.
  • They are both involved in ATP, as listed before, such as production via a proton gradient, have ATP-synthesis appearing as stalked particles, and have electron transport chains.
  • Both are enclosed by a double phospholipid bi-layers or double membrane.
  • Both share in a circular or ovoid shape.
  • They both have 70S ribosome

The differences are:

  • Chloroplasts are found in plant cells only, and mitochondria are found in both plant and animal cells.
  • Mitochondria are 1-10 micrometers in diameter, where as chloroplasts are a 2-10 micrometers in diameter.
  • Mitochondria generate ATP from glucose duringcellular respiration, where as chloroplasts generate ATP from light or photosynthesis to produce glucose for storage.
  • The chloroplast contains thylakoids which is a separate entity from the inner membrane. Mitochondria contain cristae which are continuous with the inner membrane.
  • The same space within both organelles has different names. The space within the mitochondria, equivalent to the cytosol of the cell, is called the mitochondrial matrix where as in the chloroplasts the same space is called the stroma.
  • The mitochondria does not contain starch grains, where as the chloroplasts has them in the stroma.

Question 3

Primary growth is the lengthening of the stem and roots. Apical meristems at shoot and root tips produce primary tissues. All plant growth occurs by cell division and cell elongation. Cell division occurs primarily in regions of undifferentiated cells known as meristems. Cell division in the apical meristems and subsequent elongation and maturation of the new cells produces primary growth. The tissues parenchyma, collenchymas, and sclerenchma are composed of a single type of cell and are called simple tissues. The xylem, phloem and epidermis are composed of two or more cell types and are called complex tissues. A perennial plant is a plant that lives for two or more years, they can live from a few years or as many as several thousand years, such as trees. Perennials are usually woody plants, shrubs and trees. Primary growth occurs in all vascular plants, it is the lengthening in roots and stems. The growth happens by cell elongation and division. All primary growth results from the activity of apical meristems. After embryogenesis, most plant development occurs through the activity of meristems, which consist of initials and their immediate derivatives. The apical meristems are involved primarily with extension of roots and stems. This growth results in the formation of primary tissues, which constitute the primary body of the plant. Some tissues parenchyma, collenchymas and scherenchyma are composed of a single type of cell and are simple tissues. Phloem, xylem, and epidermis are made up of more than one cell types and are considered complex tissues. Growth, is an permanent boost in size and is created mainly through cell amplification. Morphogenesis is the attainment of a specific figure or appearance, and differentiation is the procedure by which cells with the same genetic materials develop into differentiated forms from one to another due to differential gene appearance. The destiny of a plant cell is defined by its ultimate location in the growing organ, even though cellular differentiation is based on the control of gene appearance. Perennials grow each season from theirroot-stockrather than seeding themselves as anannual plantdoes. Perennials just flower during the best conditions during some seasons. Perennials mature and blossom over the spring and summer and then die off every autumn and winter, only to grow back the very next spring.

Question 4

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Beginning in the second year of life secondary tissues are produced from two lateral meristems and continue on growing throughout the life of the plant. The two lateral meristems tissues are the vascular cambium and the cork cambium. These two very different tissues, like the apical meristems present in primary growth, are continually emergent, and will continue to produce new tissues or secondary tissues each year over the life span of the perennial plant. Where the apical meristems creates primary tissues that add to the height and depth of the plant body each year, the lateral meristems manufactures secondary tissues, adding to the girth of the plant body. Periclinal divisions of the fusiform, ray initials, and their immediate derivatives form the secondary xylem and secondary phloem. The division producessecondary xylemcellson the outside of the meristem andsecondary phloemcells on the inside. Throughout secondary growth, the primary phloem can get pressed to the exterior causing the thin walled cells to be destroyed. In such a case the thick walled primary phloem fibers are the only part that remains intact, if they are present. Due to this growth, theepidermisof the stem or roots usually ruptures, causing plants with secondary growth to begin to develop acork cambium. The cork cambium creates thickened cork cellsto defend the exterior of the plant and reduce water loss. This process may produce a layer of cork meters thick if this process continues over the years. This very common in trees who grow a ring each year in secondary growth.

Question 5

The force that drags water through a plant originates at the top, where the water evaporates to form water vapor. Water once in leaves of the plant evaporates through the walls of the mesophyll, creating a vacuum. Water vapor diffuses through the stomata, the area that controls the swap of vapor in and out of the plant, to the drier air outside. But before the water gets to the mesophyll it must be absorbed through the soil and travel the length of the plant. So long as the water concentration is less in one area of the plant than in the other, water will travel to the area of lower concentration. Low water concentration in the root relative to the damp soil draws water into the roots through clusters of hair-like epidermal tissues. The apoplectic flow of water through cell walls and spaces is easy and rapid outside the plasma membrane. There is regulation of entering molecules to avoid toxic amounts of materials from traveling through the plant and into the xylem, which is the next stop for water traveling through the plant. The thin skin forces the water to move through the cell membrane and past the protoplasm which slows down the process. If osmotic forces are not satisfactory active transport may happen if necessary. Water molecules are held together by cohesion to form continuous columns. Water moleculesadhereto the walls, resistance is created by the bound water wall, while flowing water must evade the bound layer. The principal dynamic force here istranspiration, wherethe lower water potential of air to that of the leaf moves the water outward. This is produced due to the difference in vapor pressure between the evaporating surface in the leaf and that in the air above the leaf.

Question 6

Most plants collect the water and minerals or food they need from their roots. Minerals like K+and Ca2+ travel broken down in water through the soil and into the plant through the roots which are sometimes joined by a variety of organic molecules furnished by root cells. The majority of the water is lost during transpiration. Water and minerals enter the roots through different routs which eventually come together in thestele. Soil water gets absorbed through the root through its epidermis. Water then travels in both the cytoplasm of root cells called thesymplastand in the nonliving parts of the root, or the apoplast. Throughthe symplast, water crosses the plasma membrane and then passes from cell to cell throughplasmodesmata. In the apoplast water travels in the spaces among the cells as well as in the cells walls, but water has not traveled through a plasma membrane. Minerals, however, enter the root throughactive transportand enter the symplast of epidermal cells and travel into the stele through the plasmodesmata, which connects the cells. The minerals enter the water in the xylem from the cells of the pericycle and from parenchyma cells neighboring the xylem through particular transmembrane channels. When water enters the stele, it is free to travel between cells and through them. In young roots, water enters directly into thexylemvessels and or tracheids. These are nonliving conduits so are part of the apoplast. When water enters the xylem, accompanied by the minerals that have been deposited in it and organic molecules furnished by the root tissue somewhere down the line, water then subsequently makes its way upward through the vessels and tracheids. Water now exits the xylem and moves every which way to deliver to the requirements of additional tissues. Once water gets to the leaves, the water enters into the petiole and then spreads out through the various veins of the plant leaf. The cells of thespongy and palisade layers are just about the last place to receive water.

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