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Section 2

Cell Structure

All plant cells are surrounded and confined by a rigid cell wall. Each cell has usually one nucleus which is surrounded by cytoplasm. In higher plants the nucleus in enclosed by nuclear membrane.

Figure 2.2.1: Eukaryotic cell - Structure of a plant cell

Figure 2.2.2: Parts of a Cell

Cell wall - the cell walls of plants are generally thick, strong, porous, rigid and none living structures, which enclose the cells including the plasma membranes. Cell wall is secreted by the living matter of cell. Cell wall is highly permeable to water, solutes and gases. This structure making the outermost boundary of the cell, is known as extra cellular matrix (ECM). The function of extra cellular matrix is - Transport of a variety of materials on either side Recognition of a variety of stimuli with the help of receptor proteins and Cell adhesions (including cell to cell adhesions and attachment to the substratum).

Structure of cell wall - The cell wall of a mature plant cell consists of:

  1. Primary wall is the first formed wall. It is thin,permeable, elastic (only semi rigid) and capable of growth. Primary walls surround growing and dividing plant cells such as meristematic cells, parenchymatous cells and root hairs.

Chemical composition of primary wall - It is made up of cellulose micro fibrils running through a matrix of complex polysaccharides (hemicellulose) and pectic material.

Figure 2.2.3: Parts and Layers of cell wall

  1. Secondary wall - After maturity, more layers of cells are added on the inner side of the primary wall formed secondary cell wall. At certain areas, secondary wall is not formed, these plaes are called pits. Secondary wall is rigid and thicker. The cells of sclerenchyma collenchya xylem fibers, tracheids, and sclereids have secondary deposits of lignified cellulose which provide mechanicl strength to the tissue. In tracheids of conifers, secondary wall divided into three layers termed as S1, S2, S3. S2 is relatively thick.

Chemical composition of Secondary wall is same as primary wall. Secondary cell walls that are comprised mostly of cellulose and cross-linking glycans and can also be lignified.

  1. Tertiary wall - It is rarely present and is found on inner side of secondary wall. It is made up of cellulose and xylan.
  2. Middle lamella - It is the first structure that is formed from cell plate between the newly formed daughter cells at the time of cytokinesis. Adjacent cells in plant tissues are held together and separated from one another by cementing material called middle lamella and also known as intercellular substance. It is primarily composed of pectic substance.

The cell wall is primarily made up of cellulose and pectin, but some additional materials are deposited for specific functions Lignin(lignification) in secondary walls of the cells of woody plant Cutin(cutinisation) on exposed surface of cell walls as in leaf surface Suberin(suberisation) in the secondary wall of cork cells Silica in cell wall of grasses.

Plasmodesmata(single plasmodesma) are living cytoplasm strands which establish connections between two adjacent cells through very fine pores in adjacent cell walls.

Figure 2.2.4.

 

Functions of Cell Wall :- Cell wall provides shape and rigidity to cell It protects the protoplasm from external injury.

Cell Membrane Cell Membrane(C. Nageli and C. Cramer in1855) or Plasma Membrane:- All living cells are enclosed by a selectively permeable, thin molecular layer termed as plasma membrane or plasma lemma(J. Q. Plowe in 1931). The plasma membrane separates the cell from its surroundings, protects it from changes in the chemical and physical environment, and regulates movements of molecules into and out of the cell. The study of plasma membrane is based on the Fluid Mosaic Model .To perform the function of the organelle, plasma membrane is composed primarily of two types of molecules—lipids, which are fatty or oily molecules, and proteins.All membranes have a common general structure in which two layered sheets or bilayer of lipid molecules have proteins embedded in them The most common lipids in the plasma membrane are the phospholipid, which has a polar (hydrophilic it can mix with water) head and two nonpolar (hydrophobic they do not mix well with water) tails. In the plasma membrane’s bilayer construction, phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each other in the middle of the membrane. By bridging both the hydrophilic and hydrophobic regions of the membrane, these proteins are "bound" to the membrane. Most interestingly, however, is that these bridging proteins can form channels through the membrane for a particular kind of chemical to pass. In most cells, the plasma membrane is about 40 percent lipid and 60 percent protein, but these proportions vary greatly, from as little as 20 percent to as much as 75 percent protein depending on the type of cell.

The carbohydrates occur only at the outer surface of the membrane. Their molecules are covalently linked to

  1. the polar heads of some lipid molecules (forming glycolipids) and
  2. most of the proteins exposed at outer surface (forming glycoproteins).

The carbohydrates so bound to membrane components constitute the glycocalyx of cell surface.

 

Figure 2.2.5: Fluid-mosaic model of cell membrane

Function of plasma membrane - Cell membraneof the cell is the important import/export control area . It can "pump" other substance into the cell against the concentration gradient or pump other "wastes" etc. out of the cell is known Membrane transport . The difference between the concentration of a substance in two different areas is known as a concentration gradient. Membrane transport is of two types:

  1. Passive transport Passive transport does not require an expenditure of metabolic energy, and materials flow down the concentration gradient. Examples of passive transport are(a) diffusion-movement of molecules down a concentration gradient (b)osmosis is the diffusion of water through a semipermeable membrane and(c) facilitated diffusionis the diffusion of a substance across a membrane.
  2. Active transport Active transport uses energy (in the form of ATP), and materials flow against the concentration gradient.

Endocytosis, the plasma membrane folds inward, forming a pouch that traps molecules. The pouch continues to press inward until it forms a closed sac that breaks loose from the plasma membrane and sinks into the cell. Exocytosis, is a reversal of endocytosis. A sac inside the cell containing proteins and other molecules moves toward the outer edge of the cell until it touches the plasma membrane. The membrane of the sac then joins with the plasma membrane, and the contents of the sac are released from the cell.

In multicellular organisms, the plasma membrane also plays a critical role in communication between cells.

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