The+Plant+Cell

=**The Plant Cell**= Plants are multicellular organisms composed of conglomerations of cells made up into specialized tissues and organs. The first plants are thought to of evolved from algae over 500 million years ago. Cells from the bark of an oak tree were the first cells of any kind examined by a British scientist Robert Hooke using an improved microscope in the late 1600s. Some plant cells are specialized due to metabolic, functional, or structural needs depending on spatial and temporal factors. Plants are eukaryotic and contain many of the same organelles and structures as other eukaryotes, yet there exist some distinguishing structures that are unique to plants. Many plant organelles and structures such as the endoplasmic reticulum, Golgi apparatus, cytoskeletal components mitochondria and a nucleus retain the same form and function as with other eukaryotes. The structures and organelles within plants that are the most distinguishing and unique would be the cell wall, plastids, plasmodesmata, and the vacuole. The plant cell is also lacking organelles found in other eukaryotes, such as the centrioles, lysosomes, intermediate filament, cilia, or flagella. An exception to the lack of flagella would be the flagellated gametes of some ancient plant species like // Ginko biloba // (Rost et al., 2005). Within the cell wall, plant cell protoplast is surrounded by a hydrophobic phospholipid bilayer, proteins, and a small amount of sterols that make up the plasma membrane. The phospholipid bilayer is a selectively permeable barrier that takes part in metabolic function. Proteins in the plasma membrane act as transport pumps and channels. Water on the other hand is transported into the cell through osmosis. The rigid structure of a plant is maintained by the balance of osmotic water uptake forces and cell wall pressure forces creating an internal pressure in the cell known as turgor pressure. Plant cells are interconnected through plasmodesmata which are tube like extensions of plasma membrane. Within the plasmodesmata, endoplasma reticulum and proteins are thought to aid in the transport between cells. A representative plant cell displaying the unique qualities is given in Figure 1.



Figure 1. A typical plant cell. Retrieved From: [|https://www.google.com/search?q=plant+cell&source=lnms&tbm=isch&sa=X&ei=FBeiUojhE5TLsQT384G4BQ&sqi=2&ved=0CAcQ_AUoAQ&biw=878&bih=746#facrc=_&imgdii=_&imgrc=q4rT3sGsQ4n7cM%3A%3BEom7j6fVOGQ3tM%3Bhttp%253A%252F%252Fmicro.magnet.fsu.edu%252Fcells%252Fplants%252Fimages%252Fplantcell450.jpg%3Bhttp%253A%252F%252Fmicro.magnet.fsu.edu%252Fcells%252Fplantcell.html%3B520%3B450]

The cell wall is a metabolically active supportive structure that defines the cell size and shape, provides protection, and is involved in life cycle regulation. The main structural component of the cell wall is cellulose arranged in fibrils and hemicellulose. Other components of the cell wall would be ligins, cutins, and suberin, as well as pectin. Depending on the growth habit of a given plant species, plant cells may form a secondary cell wall between the the primary cell wall and the plasma membrane after cell growth ceases. The network created by the cross linking of amino acids that make up lignins provide much of the rigid structural strength qualities of wood. The secondary cell wall deposition after growth ceases is represented in Figure 2. Pectins provide the glue like structure that holds adjacent cells together in the middle lamella through calcium cross linking of the polysaccharides.

Figure 2. A plant cell exhibiting extensive cell wall characteristics. Retrieved from: []

The nucleus is a large ovoid double membrane bound organelle that stores DNA in chromosomes, and regulates most of the functions in the cell through signals. Proteins production is regulated through nucleic signaling.

The cytoskeleton is a network of microfilaments and microtubules that aid in the transport of molecules and organelles, and provide cellular support.

Ribosomes are composed of ribosomal RNA and specific proteins. Ribosomes serve as a site of protein synthesis. Active ribosomes are found in clusters known as polyribosomes.

The endoplasmic reticulum acts as a site of protein packaging and transport across the lumen. In appearance ER looks like a series of sacs called cisternae.. The ER may also be found in the plasmodesmata connecting cells. The ER is a highly motile and dynamic organelle. It may be found in smooth (SER) or rough (RER) conformations. Rough ER appears bumpy from ribosomes attached to the surface. Smooth ER is more smooth in appearance, and is important in lipid, and membrane protein synthesis and digestion.

The Golgi apparatus is a series of flattened stacked membranes known as cisternae. Cisternae along with vesicles direct protein trafficking from the ER to cellular destinations. The trafficking through the ER to the Golgi and then to other membranous destinations is known as the endomembrane system and makes up a complex transport network within the cell.

DNA in plants can be found in the nucleus, mitochondria, and chlorplasts. Mitochondrial DNA and DNA within the chloroplast are bacterially derived from an ancient symbiotic relationship of bacterial species that were incorporated into the host cell at the time. This theory is greatly supported and is known as the endosymbiotic theory of the origin of eukaryotic cells.

Lysosomes contain digestive enzymes and function to break down proteins, carbohydrates, waste products, and nucleaic acids. Lysosomes accomplish cell dissolution when a cell is cued for destruction.

The mitochondria is a double membrane structure with the inner membrane heavily folded into cristae which maximize surface area for chemical reactions. Mitochondria contain DNA of circular chromosomal nature like bacteria. Within the mitochondria is a thick solution known as the matrix which contains enzymes. Mitochondria also contain ribosomes that are capable of synthesizing some proteins. The primary function of the mitochondria is to perform oxidative respiration. Carbon dioxide is converted to water in a process that releases energy that is used to synthesize ATP.

Chloroplasts are elaborate plastid organelles containing multiple membranous subunits. Stacks of grana contain thylakoid disks. Proteins within the thylakoid disks bind to chlorophyll. The enzyme filled liquid within the chloroplast is the stroma. These enzymes work in conjunction with the membranous proteins in the thylakoid disk to perform photosynthesis where light energy is used to convert carbon dioxide, ADP, NADP+ and water into ATP, NADPH, sugar, and oxygen in photosystem 1 and photosystem 2. Chloroplasts may also act as a carbohydrate reserve for starch grains. Analogous to the mitochondria, the DNA found in chloroplasts is circular in nature like a bacteria. Figure 3. Chloroplast representation displaying thylakoid stacks of grana. Retrieved from: []

Chromoplasts are developed from chloroplasts that lose their chlorophyll and replace it with specialized lipids like xanthophylls and carotenoids. This causes these plastids to exhibit orange or red coloration giving these organelles their descriptive name chrooplasts since chromo means color.

Leukoplasts act as starch storage sites and do not contain thylakoids.

Peroxisomses metabolize fatty acids, and proteins. Some peroxisomes contain peroxidases which catabolize hydrogen peroxide into less harmful byproducts.

The vacuole is a storage organelle bound by a single membrane called the tonoplast. There is a series of protein pumps and channels that regulate the transport of matter in and out of the vacuole. The vacuole may serve as a nutrient storage, and hazardous waste storage compartment simultaneously. The cell gets structural support from the turgor pressure associated with liquid uptake into the vacuole. The liquid within the vacuole is notably different than cytosol and is deemed cell sap. A mature plant cell contains a central vacuole that occupies 80-90% of the cell volume.

Plants continually undergo mitosis in dense aggregations of cells known as meristems in the tips of shoots, roots and branches. Meristematic regions are the site of cellular differentiation in plant organization. An interesting mitotic quality of plants is that during cell division plant cells lack centrioles. Representative meristems are depicted below in Figure 4.

Figure 4. Representations of meristems. Retrieved from: []

Literature Cited

Rost, T. L., Barbour, M. G., Murphy, T. M., Stocking, C. R. (2005) Plant Biology. Thomas/Brooks/Cole Publishing Company.