Plant Cell Structure and Function Lecture 2 PDF

The Plant cell Plant cell is the basic unit of plants. Plant cells are eukaryotic cells, which have a true nucleus with specialized structures called organelles that carry out different functions. Structure of plant cell A plant cell is more or less a polyhedral or spherical structure su...

The Plant cell Plant cell is the basic unit of plants. Plant cells are eukaryotic cells, which have a true nucleus with specialized structures called organelles that carry out different functions. Structure of plant cell A plant cell is more or less a polyhedral or spherical structure surrounded by a cell wall. The content of the living cell is known as protoplast. The protoplast contains the protoplasmic and non- protoplasmic components. The protoplasmic or living components are—nucleus, mitochondria, plastids, endoplasmic reticulum, ribosomes, lysosomes, and Golgi bodies. The non-living components (non-protoplasmic ) are— vacuoles, food products, and secretory products. 1 Plant cell 2  1- Living components (protoplasmic components) (a) Cell membrane Composed of a phospholipid bilayer and proteins. Phospholipids have hydrophilic heads (phosphate-linked group) and hydrophobic tails (fatty acids). Proteins are integral proteins and peripheral proteins. Integral proteins are embedded in lipid bilayer and peripheral proteins are bound to membrane surface. 3 Cell membrane 4 Functions of cell membrane: It maintains the cell contents. It mediates the transport of substances into and out of the protoplasm. It has sites of enzymatic activities. 5  (b) Endoplasmic reticulum Endoplasmic reticulum consists of a network of membrane lamellae and tubules called cisternae; the internal space of the endoplasmic reticulum is called the cisternal space or the lumen. There are two types of endoplasmic reticulum, rough and smooth. Rough endoplasmic reticulum has ribosomes bound to the outside of the membranes, while smooth endoplasmic reticulum has no ribosomes bound to it. 6  The function of rough endoplasmic reticulum is protein synthesis. The ribosomes bound to the endoplasmic reticulum synthesize proteins. The functions of smooth endoplasmic reticulum and its associated enzymes are: i) synthesis of membrane components such as phospholipids, and ii) metabolism of carbohydrates. 7 Endoplasmic reticulum (ER) 8  (c) Plastids Plastids are found in the cells of plants and algae. They are responsible for manufacturing and storing of food. They contain pigments that are used in photosynthesis. Types of plastids i) Chloroplasts Chloroplasts are found in the mesophyll cells of plant. They are the sites for synthesis of food by the process of photosynthesis. They may be oval or spherical, spiral, star-shaped or cup-shaped. 9  The chloroplast is bounded by the outer and inner membranes. The inner membrane surrounds the stroma and the grana (stacks of thylakoids). One thylakoid stack is called a granum. The stacks of thylakoid sacs are connected by inter-grana lamellae. The lamellae keep all the thylakoid sacs a distance from each other to maximize the efficiency of the organelle in capturing the sun‘s energy. 10 Chloroplast 11 ii) Chromoplasts They produce and store pigments (Carotenoids). Chromoplasts are responsible for different colors found in flowers, fruits, and vegetables. Chromoplasts may develop from green chloroplasts. The colors given by chromoplasts attract insects and animals. So, they help in the pollination and dispersal of seeds. 12 iii) Leucoplasts These are the non-pigmented (colorless) organelles. Leucoplasts are usually found in the non- photosynthetic parts of the plant like roots. They act as storage for starch, lipids, and proteins. 13 Types of Leucoplasts i. Amyloplasts –are charged with storing starch. ii. Proteinoplasts –help in storing proteins and can be typically found in seeds. iii. Elaioplasts - help in storing fats and oils. 14  (d) Mitochondria Mitochondria are known as the powerhouses of the cell. Many of the reactions involved in cellular respiration happen in the mitochondria. Mitochondria are made of two membranes. The outer membrane covers the organelle. The inner membrane folds over many times and creates layered structures called cristae. The fluid contained in the mitochondria is called the matrix. The folding of the inner membrane increases the surface area inside the organelle. 15 Mitochondria 16  (e) Golgi bodies Golgi apparatus is made up of a series of compartments known as cisternae. Functions of Golgi apparatus are: modifying, packaging and transporting of proteins and lipids into vesicles for delivery to targeted destinations. 17 18  (f) The nucleus The cell nucleus acts like the brain of the cell. The nucleus is surrounded by the nuclear envelope. There are pores and spaces in the nuclear envelope for mRNA to pass through. The nucleus contains chromatin which is threads like bodies forming a reticulum and made of DNA, RNA, and nuclear proteins. There is a spherical round body, the nucleolus, inside of the nucleus. The functions of the nucleus are: 1) controlling the ongoing activities of the cell by determining which protein molecules are produced, and 2) storing the genetic information and passing it onto daughter cells during cell division. 19 Nucleus 20  (g) Ribosomes Ribosomes are small particles scattered in the cytoplasm (free ones) or associated with the outer endoplasmic reticulum membranes (bound ones). Ribosomes are the protein builders of the cell. Ribosomes are found in both prokaryotes and eukaryotes. There are two subunits in every ribosome. The small ribosomal subunits read the codons on mRNA, and the large subunits join the amino acids to form a polypeptide chain. 21 Protein synthesis Protein synthesis is the process in which cells make proteins. It occurs in two stages: transcription and translation. Transcription is the transfer of genetic instructions in DNA to mRNA in the nucleus. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm. Translation occurs at the ribosome. In translation, the instructions in mRNA are read, and tRNA brings the correct sequence of amino acids to the ribosome. The large ribosomal subunit catalyzes the peptide bond formation, producing a polypeptide chain. 22 Protein synthesis 23  2- The non-living components (non- protoplasmic components) (a) The cell wall The cell wall distinguishes plant cell from animal cell. The functions of cell wall are: (1) mechanical protection of cell, (2) porous medium for the circulation of water, minerals, and other small nutrient molecules, and (3) site of regulatory molecules that sense the presence of pathogenic microbes. 24  All cell walls contain two layers, the middle lamella and the primary cell wall. The middle lamella serves as a cementing layer between the primary walls of adjacent cells. It is composed of pectin. A fully grown plant cell may deposit an additional layer, which is the secondary cell wall. Secondary cell walls may be lignin as in xylem, suberin as in cork cells or cutin as the epidermal cells. 25 26  (b) Vacuoles The vacuoles contain the cell sap. The vacuoles contains other contents like starch grains, minerals, proteins, fats, phenolics, flavonoids, and alkaloids. (i) Crystals There are two main types of crystals: A) Calcium Oxalate Crystals The cell transforms oxalic acid to insoluble compounds (crystal form) to reduce its toxic effects. 27 Types of calcium oxalate crystals: 1- Solitary crystals: They are in the form of prism. 2- Druses crystals: They are star-like in shape. 3- Raphides crystals: They are long, thin and tapered ends crystals and accumulate in packages form. 28 B) Calcium carbonate crystals (cystolith): it is a crystalline deposit of calcium carbonate occurring as a knob on the end of a stalk within a plant cell. 29 (ii) Starch Grains The starch grain is characterized by hilum that surrounded by starch layers, it may contain one or more hilums. 30 (iii) Proteins (aleurone grains) Aleurone grains are protein materials. The grain composed from crystalline protein body (crystalloid), and spherical protein body (globoid). The grain is surrounded by a membrane. 31