Botany for Pre-Medicine Students PDF

Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. DEFINITION,STRUCTURE & FUNCTION OF PLANT CELL Plant Cell Definition: Plants are multicellular organisms composed of millions of cells with specialize...

Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. DEFINITION,STRUCTURE & FUNCTION OF PLANT CELL Plant Cell Definition: Plants are multicellular organisms composed of millions of cells with specialized function. All plant cells have the same basic eukaryotic organization. Plant cells are the basic unit of life in organisms of the kingdom Plantae. They are eukaryotic cells, which have a true nucleus along with specialized structures called organelles that carry out different functions. Animals, fungi, and protests also have eukaryotic cells, while bacteria and archaea have simpler prokaryotic cells. Plant cells are differentiated from the cells of other organisms by their cell walls, chloroplasts, and central vacuole. Functions of Plant Cells: Plant cells are the basic building block of plant life, and they carry out all of the functions necessary for survival. Photosynthesis, the making of food from light energy, carbon dioxide, and water, occurs in the chloroplasts of the cell. The energy molecule adenosine triphosphate (ATP) is produced through cellular respiration in the mitochondria. There are five types of plant cells, each with different functions:  Parenchyma cells are the majority of cells in a plant. They are found in leaves and carry out photosynthesis and cellular respiration, along with other metabolic processes. They also store substances like starches and proteins and have a role in plant wound repair.  Collenchyma cells provide support to growing parts of a plant. They are elongated, have thick cell walls, and can grow and change shape as a plant grows.  Sclerenchyma cells are hard cells that are the main supporting cells in the areas of a plant that have ceased growing. Sclerenchyma cells are dead and have very thick cell walls.  Xylem cells transport mostly water and a few nutrients throughout a plant, from the roots to the stem and leaves.  Phloem cells transport nutrients made during photosynthesis to all parts of a plant. They transport sap, which is a watery solution high in sugars. Plant Cell Structure: The plant cell has many different parts. Each part of the cell has a specialized function. These structures are called organelles. 14 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. This diagram shows the various parts of a plant cell. Specialized structures in plant cells include chloroplasts, a large vacuole, and the cell wall. Chloroplasts: Chloroplasts are found only in plant and algae cells. These organelles carry out the process of photosynthesis, which turns water, carbon dioxide, and light energy into nutrients. They are oval-shaped and have two membranes: an outer membrane, which forms the external surface of the chloroplast, and an inner membrane that lies just beneath. Between the outer and inner membrane is a thin inter-membrane space about 10-20 nanometers wide. Within the other membrane, there is another space called the stroma, which is where chloroplasts are contained. Chloroplasts themselves contain many flattened disks called thylakoids, and these have a high concentration of chlorophyll and carotenoids, which capture light energy. The molecule chlorophyll also gives plants their green color. Thylakoids are stacked on top of one another in vascular plants in stacks called grana. 15 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. Chloroplasts – sites of photosynthesis. Vacuoles: (Regulates water balance, Stores wastes, Stores secondary metabolites, Stores water soluble pigments); Plant cells are unique in that they have a large central vacuole. A vacuole is a small sphere of membrane within the cell that can contain fluid, ions, and other molecules. Vacuoles are basically large vesicles. They can be found in the cells of many different organisms, but plant cells characteristically have a large vacuole that can take up anywhere from 30-80 percent of the cell. The central vacuole of a plant cell helps maintain its turgor pressure, which is the pressure of the contents of the cell pushing against the cell wall. A plant thrives best when its cells have high turgidity, and this occurs when the central vacuole is full of water. If turgor pressure in the plants decreases, the plants begin to wilt. Plant cells fare best in hypotonic solutions, where there is more water in the environment than in the cell; under these conditions, water rushes into the cell by osmosis, and turgidity is high. Animal cells, on the other hand, can lyse if too much water rushes in; they fare better in isotonic solutions, where the concentration of solutes in the cell and in the environment is equal and net movement of water in and out of the cell is the same. Cell Wall: The cell wall is a tough layer found on the outside of the plant cell that gives it strength and also maintains high turgidity. In plants, the cell wall contains mainly 16 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. cellulose, along with other molecules like hemicellulose, pectin, and liginins. The composition of the plant cell wall differentiates it from the cell walls of other organisms. For example, fungi cell walls contain chitin, and bacterial cell walls contain peptidoglycan, and these substances are not found in plants. A main difference between plant and animal cells is that plant cells have a cell wall while animal cells do not. Plant cells have a primary cell wall, which is a flexible layer formed on the outside of a growing plant cell, and a secondary cell wall, a tough, thick layer formed inside the primary plant cell wall when the cell is mature. Other Organelles: Plant cells have many other organelles that are essentially the same as organelles in other types of eukaryotic cells, such as animal cells. The nucleus contains a cell’s deoxyribonucleic acid (DNA), its genetic material. DNA contains instructions for making proteins, which controls all of the body’s activities. The nucleus also regulates the growth and division of the cell. Proteins are synthesized in ribosomes, modified in the endoplasmic reticulum, and folded, sorted, and packaged into vesicles in the Golgi apparatus. 17 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. Nucleus: _ The nucleus is surrounded by a double membrane called the nuclear envelope. The space between these two membranes is called the perinuclear space. The joining sites of the two nuclear membranes are called the nuclear pores. _ The material filled in the nucleus is called nucleoplasm (or nuclear sap). _ About 8% of the surface area of the nuclear membrane is occupied by pores. These pores allow the transport of substances between cytosol and nucleus. _Nucleus is the site of storage and replication of chromosomes, composed of DNA and its associated proteins (histones). The DNA-protein complex is known as chromatin. _ Nucleus contains a densely granular region called the nucleolus, which is the site of ribosome (ribosomal RNA) synthesis. _ Ribosomal proteins are synthesized in cytosol and transported into nucleus via nuclear pores, where they bind with rRNA to form 40S and 60S subunits. These subunits pass into cytosol and aggregate to form 80S ribosomes. _ The genes are transcribed in nucleus to form mRNA and rRNA, mRNA and tRNA pass from nucleus to cytosol where they are used for protein synthesis. _ The nucleotide sequence of mRNA is translated into amino acid sequence of proteins by ribosomes. tRNA assists by transferring amino acids to mRNA codons. 18 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. Endoplasmic Reticulum: _Cells have an elaborate network of internal membranes called endoplasmic reticulum (ER). _ER is continuous with the outer membrane of nuclear envelope (but not plasma- lemma). _ The ER lumen of one cell is connected to adjacent cell via plasmodesmata. _ There are 2 types of ER, smooth and rough, which are interconnected. _ Rough ER is covered with ribosomes which synthesize proteins to be delivered to lumen of ER. _ Smooth ER lacks ribosomes. _ Smooth ER is the site of lipid synthesis and membrane assembly. _ Both types of ER are involved in secretion. Golgi Apparatus: _Golgi apparatus (or Golgi complex) is made of one or more dictysomes (or Golgi bodies) which are stacks of 3-10 flattened sacs (cisternae) and vesicles. _Plant cells contain up to several hundred Golgi bodies dispersed in cytoplasm. _The cisternae close to plasma lemma are called transface, and the cisternae close to center of cell are called cis face. The medial cisternae are between trans and cis cisternae. _Golgi body is a dynamic structure; new cisternae are continuously produced from endoplasmic reticulum at cisface while old cisternae are lost in the form of vesicle at transface. _Golgi apparatus has intermediary position between ER and extracellular space. _It is involved in the transport and processing of many substances that are produced in ER and eventually discharged outside the cell via Golgi. It plays a key role in synthesis and secretion of complex polysaccharides and in processing of glycoproteins. _Various proteins (including enzymes) are first synthesized in rough ER then they reach to Golgi via vesicles that bud off from ER and fuse with Golgi. 19 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. _Vesicles formed from membrane of outer face of Golgi apparatus move to different locations in cell. At plasma membrane they discharge their contents as secretions. Mitochondria: are also found in plant cells. They produce ATP through cellular respiration. Photosynthesis in the chloroplasts provides the nutrients that mitochondria break down for use in cellular respiration. Interestingly, both chloroplasts and mitochondria are thought to have formed from bacteria being engulfed by other cells in an endosymbiotic (mutually beneficial) relationship, and they did so independently of each other. Mitochondria: _ Mitochondria are cytoplasmic organelles. _ Mitochondria are the sites of oxidative phosphorylation (ATP synthesis). _Mitochondria are surrounded by two membranes. The outer membrane is smooth and the inner membrane is highly convoluted. The folds of inner membrane are called ‘cristae’. _The components of respiratory electron transport chain are found in inner membrane. _The inner membrane is also characterized by the presence of stalked particles with spherical heads containing ATPase. ATPase catalyses the synthesis of ATP. _The inner membrane is highly impermeable to the passage of protons (H+), which allows the formation of electrochemical gradient necessary for ATP synthesis. _The compartment enclosed by inner membrane is called ‘matrix’. Matrix contains the enzymes of Krebs cycle (TCA cycle or citric acid cycle). _Mitochondria contain their own protein synthesizing machinery (ribosomes, tRNA etc.). _Mitochondrial ribosomes are smaller (70 S) than those found in cytosol (80 S). _Mitochondria contain circular, histone-free DNA molecule, similar to those of bacteria. Mitochondrial genome of plants consists of about 200 kb (200,000 base pairs), which is much larger than animal mitochondria. 20 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. Cytosol is the liquid contained within cells. It is mostly made of water, and also contains ions like potassium, proteins, and small molecules. Cytosol and all the organelles within it, except for the nucleus, are called the cytoplasm. The cytoskeleton is a network of filaments and tubules found throughout the cytoplasm of the cell. It has many functions; it gives the cell shape, provides strength, stabilizes tissues, anchors organelles within the cell, and has a role in cell signaling. The cell membrane, a double phospholipid layer, surrounds the entire cell. Cytoskeleton: _The cytosol in organized into a 3-dimensional network of filamentous proteins called 'cytoskeleton'. _Cytoskeleton serves as scaffolding for the movement of organelles and other components. _Cytoskeleton plays an important role in maintenance of cell shape as well as in cell division. _Basically 2 types of cytoskeletal elements are found in plant cells; microtubules and microfilaments. _Microtubules are hollow cylinders with an outer diameter of 25 nm. _Microtubules are composed of polymers of the globular protein ‘tubulin’. _A single microtubule consists of thousands of tubulin monomers arranged in 13 columns called proto-filaments. _Microfilaments are solid with 7 nm diameter. _Microfilaments are composed of protein globular actin (or G-actin). _A microfilament consists of two chains of polymerized actin subunits that inter-wind in a helical fashion. Plasma Membrane (Plasmalemma): _All cells are enclosed in a membrane that serves as their outer boundary, separating the cytoplasm from the external environment. _This plasma membrane allows the cells to take up and retain certain substances while excluding others. Thus, plasmalemma accounts for selective traffic of solutes across membrane. _All biological membranes consist of a double layer (bi layer) of phospholipids in which proteins are embedded. _The membrane is not a static structure, but it is a dynamic structure. Both lipid and protein molecules are free to move and are usually in a constant motion. However, these molecules readily move in the plane of membrane, a process known as lateral diffusion. 21 Botany for Pre- Medicine students. Prof: M. A. Al-Ryani. 22