Plant Cells

PLANT CELLS Plant cells have certain organelles that animal cells do not have, such as cell walls and plastids. a. Cell Wall Apart from the plasma membrane, plant cells have a cell wall which is located outside the plasma membrane. Cell walls are divided into primary cell walls and secondary cell walls. Primary cell walls are composed of cellulose, hemicellulose, pectin and several other compounds. Cellulose molecules form microfibrils, which are long fibers and have very strong tensile strength. Young, growing cells only have a primary wall. The secondary cell wall is only present in mature cells, which is located between the primary wall and the plasma membrane. This wall is produced by protoplasm after the cell no longer grows in size. It has more cellulose content and also contains hemicellulose and lignin. These materials cause the secondary cell walls to be thicker and stiffer than the primary cell walls. Between the adjacent cell walls, there is a central lamella consisting of magnesium and calcium in the form of gelatin. Between two adjacent walls (neighbors) there is a connection of plasma threads through very small holes (points) called plasmodesmata. b. Plastids Plastids are organelles that only plant cells have. Consisting of two layers of membrane, it contains DNA, ribosomes, several types of proteins, a number of enzymes and pigments found in certain plastids. There are various types of plastids, namely: 1) leukoplasts are colorless plastids 2) amyloplasts, plastids containing starch; 3) aleuroplasts, plastids containing protein 4) elaioplasts, plastids containing fat 5) chromoplasts, plastids that contain pigments other than chlorophyll, such as red, orange and yellow pigments 6) chloroplasts are plastids that contain chlorophyll. Chloroplasts are a type of plastid, namely organelles covered by two layers of membrane. Contains the pigment chlorophyll or green leaf substance, carotene which gives the orange color, and xanthophyll for the yellow color. Chloroplasts are discshaped with a diameter and thickness of 2 – 4 ÿm. This organelle is found in photosynthetic cells of plants and several types algae. Inside the membrane there is a granum, which is a pile of sacs, each of which contains chlorophyll pigments, carotenoids, proteins and fats. Each sac is called a thylakoid and includes a flat sac, which connects one grana to another grana. All grana are immersed in the stroma, which is a basic matrix that is transparent and contains many enzymes that make up carbohydrates, DNA, RNA and ribosomes. So that you better understand the structure of chloroplasts c. Vacuoles Vacuoles in plant cells are limited by a layer of membrane called the tonoplast, which originates from the endoplasmic reticulum and plays a role in regulating the transport of substances in and out of the vacuole. Mature cells generally have one large vacuole located in the center (central vacuole), while young cells have several small vacuoles. Plant vacuoles function to store synthetic materials or metabolic waste, including mineral salts, carbohydrates, amino acids, alkaloids and anthocyanins which determine flower color. Vacuoles in animal cells are smaller and are not equipped with tonoplasts. In some protozoa there are food vacuoles which contain intracellular digestive enzymes, there are also contractile vacuoles which function as osmoregulators, namely regulating the concentration of cell fluid.