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The Key Differences Between Plant and Animal Cells
Figure 1 Diagram of a ChloroplastClick thumbnail to view full-size
Figure 2 Diagram of a Plant Cell
Figure 3 Diagram of a Animal Cell
The three main differences between plant and animal cells is that of plastids, the cell wall and a large central vacuole, which will now be explained in greater detail (Davidson, 2005).
The first difference is that plant cells have plastids such as chloroplasts when animal cells do not [Fig 1] (Vasil and Vasil, 1972). Plastids are semi-autonomous organelles that are generally interconvertable and can divide independently of the cell. Their functions include storage, photosynthesis and pigment formation. Their properties include: a double membrane, having many copies of circular chromosomes, having a complete protein synthesis machinery, being the only site in the plant cells where lipid is made, they have genes lost to the nuclear genome. Each cell of the plant can only contain one type, since there are six main types: eoplasts (which are the most simple and consist of only a double membrane and DNA fibrils), etioplasts (which are from dark-grown leaves and have a prolamellar body for lipid storage), chromoplasts (which accumulate pigments e.g. flowers and have granum and thylakoids), amyloplasts (which store starch e.g. potato tubers, elatioplasts which store lipids e.g. anthers) and, finally, chloroplasts (Davidson 2005).
Chloroplasts are the most common type of plasmid and are found in all green tissues. Their function is to do photosynthesis which is where energy is trapped in the pigment chlorophyll, which is found in the chloroplasts. It is also this chlorophyll which makes many plants appear green, since it usually reflects green and yellow wavelengths of light. This process of photosynthesis converts carbon dioxide and water into sugars, releasing oxygen as a bi-product (Ridge, 2002). Chloroplasts have an outer membrane which is permeable to small molecules and an inner membrane which allows phosphate and sucrose precursors to be transported. They have single sheets forming numerous small interconnected flattened stack or grana to increase the surface area for light absorption. However animal cells only have mitochondria (which animal cells also have) so they are limited to only respiration.
Plastids are believed to have been formed from cyanobacteria, in endosymbiosis theory, which were very successful because of their ability to do photosynthesis. This is when water is utilised as an electron donor and carbon dioxide is taken up and then converted, using energy from the sun, into oxygen (a bi-product) and sugars-the same way it is still done today by chloroplasts within plants Over time, a few prokaryotic cells became large enough to become predators of the smaller, by attacking, engulfing and ingesting them. However, as endosymbiosis theory suggests, some of these smaller prokaryotes survived, leaving them still functioning within the other organisms (Gault and Marler, 2009). Thus, they provided sugars to the larger organism and received nutrients and CO2 in return, which slowly evolved into the chloroplasts of today.
A second difference between plant and animal cells is that while animal cells have only a cell membrane, plant cells also have a cell wall [Fig 2, 3]. It is essentially strands of cellulose running horizontally, ‘glued’ to each other by branched polysaccharides e.g. pectin (Burgert 2006), combined with the cell membrane and microtubules to help brace the membrane from the inside. Cellulose is made by b1-4 linkage joining molecules of glucose end-to-end. It is the most abundant organic molecule in the biosphere and a major structural component of plants, to the extent where cotton and paper are almost pure cellulose. Microfibrils are these long thin molecules of cellulose combined, which then themselves wind together to produce macrofibrils. In addition, the cell wall consists of lignin which has a supportive function because it is a very rigid molecule. Also, there are suberin cutin waxes which are fatty substances that are found on the outside surfaces of plat to help with waterproofing, to prevent dehydration via evaporation. The functions of the cell wall are: to lead the cells stability, determine its shape, influences its development, counterbalances the osmotic pressure and protects the cell against pathogens. It is the major structural material which plants are made from. This means that plant cells are more rigid in structure when animal cells are able to form different shapes.
Although vacuoles can sometimes be present in animal cells, they are must smaller and far less significant than those in plant cells [Fig 5, 6]. Young plants, too have small vacuoles but this then grows to the large central vacuole of maturity (Staley et al 2007). Vacuoles are membrane-bound sacs with little or even no internal structure. This surrounding membrane is called the tonoplast and is very active. Many mature plant cells have a single large central vacuole that have several important functions: they store foods (for example the proteins in seeds), they store wastes, they store special metabolites (e.g. malic acid) and they maintain tugor in the cell by filling the vacuole with water.
There are also several smaller differences between animal and plant cells. Cilia, for instance, are generally present in animal cells but they are very rare in plant cells. Also, centrioles are present in most animal cells but are only present in lower life forms. Lysosomes are generally present in the cytoplasm of animal cells when they are usually not evident in plant cells. To communicate between cells, plant cells have linking pores however animal cells use an analogue system of gap-junctions. In addition, plant cells can be totipotent. They are also non-motile.
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