Nature and Composition of Plants PDF

Topic: Nature and Composition of Plants Jedi Joy Mahilum Faculty, Dept of Plant Breeding and Genetics Visayas State University Structure of Plant Cells Most plants h...

Topic: Nature and Composition of Plants Jedi Joy Mahilum Faculty, Dept of Plant Breeding and Genetics Visayas State University Structure of Plant Cells Most plants have green leaves, stems, roots, and flowers. Some plants like conifers do not have flowers but cones. Cacti and many succulents also do not have leaves but are considered plants because they resemble the plants and capable of photosynthesis. Similarly, ferns and mosses are also plants. Characteristics of plants: 1. Autotrophs, plants use energy from sunlight to make food from carbon dioxide and water. 2. Plants are eukaryotic. They are made up of many cells with true nucleus. 3. It has a waxy cuticle layer that coats most of the surfaces of plants that are exposed to air. 4. Plant cells are surrounded by a rigid cell wall that contributes to its rectangular shape. 5. Plants have two stages in their life cycle— the sporophyte stage and gametophyte stage. 6. Plant adapt to the environment they live. 7. Plants are highly integrated organisms. The Structure of Plant Cells A cell is the basic unit of life. All living organisms are made up of one or more cells. Unicellular organisms or prokaryote such as bacteria is composed of only one cell containing organelles inside but without true nucleus. Eukaryotes on the other hand are living organisms that are made up of many cells like many plants and animals. They are characterized with the presence of a true nucleus and membrane-bound organelles. Eukaryotes are also more complex compared to prokaryotic organisms. Organelles of plant cells: 1. Plasma membrane. The plasma membrane or plasmalemma is the membrane that completely covers the surface of the protoplasm. It is the outermost surface of the protoplast. The plasma membrane is impermeable to harmful materials and permeable to beneficial ones, hence it is considered as a selectively permeable membrane. 2. Nucleus (plural nuclei). An organelle that serves as an archive or permanent storage place for the organism’s genetic information. The nucleus of eukaryotic cells is surrounded by a nuclear envelope composed of outer and inner membranes. The nuclear envelope separates the nuclear material from the rest of the cell and contains numerous holes called nuclear pores. Nuclear pores on the other hand, is involved in the transport of material between the nucleus and the rest of the protoplasm. While the substance inside a nucleus is termed, Nucleoplasm. 3. Central Vacuole. It is an organelle that serves as storage of both nutrient reserves and waste products of the cell. It is a digestive organelle that digests wastes as other organelle age and become impaired (animal cells have small vacuoles called lysosomes). Vacuoles also functions for cell growth. It primarily contains water and salts which can expand rapidly forcing the plant cell to grow rapidly. Vacuoles are surrounded by a single membrane called Tonoplast. In seed cells, vacuoles may be filled with protein that will be used when the seed germinates 10-50 years after the material was deposited in the vacuole 4. Cytoplasm. It is the fluid of the cell plus organelles (protoplasm) found between the cell membrane and the nucleus. 5. Mitochondria (singular, mitochondrion). It is an organelle that carries out cell respiration. It is also known as the ‘powerhouse’ of the cell because it supplies energy needed to fuel cellular processes. The inner membrane is folded into plate-like cristae where respiratory enzymes are embedded. 6. Ribosome. Ribosomes form clusters of polysomes or polyribosomes that are bound in the protoplasm responsible for protein synthesis. They are complex aggregates of three molecules of RNA (ribosomal RNA) and approximately 50 types of protein that associate and from two subunits. 7. Plastids. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids are found in all plants and algae but not in animals, fungi and prokaryotes. Table 1. Types of Plastids and their functions. Types of Plastids Functions a. Amyloplasts Store starch b. Chloroplasts Carry out photosynthesis c. Chromoplasts Contain abundant colored lipids; in flowers and fruits e.g. carotenoid pigments d. Etioplasts A specific stage in the transformation of proplastids to chloroplasts; occur when tissues are grown without light e. Leucoplasts Colorless plastids; synthesize lipids and other materials; e.g. amyloplast f. Proplastids Small, undifferentiated plastids 8. Endoplasmic reticulum. It is a system of narrow tubes and sheets that form a network throughout the cytoplasm and carry large molecules. E.g. proteins. There are three forms of endoplasmic reticulum (ER): a. Rough endoplasmic reticulum (RER) - RER aids in the movement of proteins in the cell. They are rough because of the ribosomes are attached to them giving a rough appearance. b. Smooth endoplasmic reticulum (SER) – SER is involved in lipid synthesis and membrane assembly. It lacks ribosomes. c. Cortical endosplasmic reticulum – regulates the level of Calcium ions in the cytosol; interconnected by cytoplasmic strands called plasmodesmata. 9. Golgi apparatus. It is also known as dictyosomes, golgi complex or collectively called golgi bodies. It is composed of 5-8 stacks of flattened cisternae or disk-shaped sacs. Golgi apparatus functions for the secretion of polysaccharides incorporated in the cell wall. It also processes and secrete glycoproteins. 10. Microbodies. They are small, spherical bodies inside the cells approximately 0.5 to 1.5 µm in diameter. There are two classes of microbodies: a. Peroxisomes – are involved in detoxifying certain byproducts of photosynthesis and are found closely associated with chloroplasts. In animals, they are abundant in liver and kidney cells, where they break down foreign compounds that contaminate our food. b. Glyoxysomes – occur only in plants. They are involved in converting stored fats into sugars; important during germination of fat-rich, oily seeds such as peanut, sunflower and coconut. 11. Cytosol. It is also called Hyaloplasm. It is a clear substance, mostly water, enzymes and numerous chemical precursors, intermediates, and products of enzymatic reactions. Plant Cells vs. Animal Cells Table 2. Comparison between plant cells and animal cells. Chacteristic Plant Cell Animal Cell a. Cell size Plant cells are larger than Generally small in size animal cells b. Cell shape Fixed (rectangular) Irregular or spherical c. Presence or absence of Plasma membrane is Cell wall is absent cell wall surrounded by cell wall (cellulose) d. Presence or absence of Plastids are present Plastids are absent plastids e. Presence or absence Large central vacuole Vacuoles are many and small and/or size of vacuole f. Presence or absence of No Lysosomes (except in With Lysosomes lysosomes lower forms) g. Presence or absence of Simpler units of Golgi Highly complex Golgi body Golgi body (dictyosomes) h. Presence or absence of Lacks centrosome/centrioles Have centrosomes/centrioles plastids centrioles/centrosome i. Food storage Starch Glycogen and Lipids 12. Microtubules. This organelle acts as ‘cytoskeleton’ holding certain regions of the cell surface back while other parts expand. It provides the framework that moves chromosomes during cell division of the nucleus, move the whole nuclei, mitochondria ns other organelles. Microtubules guide vesicles to specific sites and aids in the motility for both organelles and whole cells. 13. Microfilaments. It is made up of globular proteins called actin. Microfilaments are also implicated in the different types of structure and movement inside the cell. For example, each part of a centriole contains a circle of nine sets of three microtubules attached to each other by fine filaments along their sided. 14. Cell wall. It serves as constrains of expansion of the protoplast and prevents rupture of the plasma membrane. The presence of cell wall is the distinguishing characteristics of plant cells that differ from animal. Cell wall largely determines the size and shape of the cell, texture of the tissues, therefore contributes to the final form of plant organs. Tissues of Plant Body What is a tissue? A tissue is an aggregate of cells in an organism that have similar structure and function. Tissues can be grouped into meristematic, such as those found in the root and shoot apical meristems, permanent tissues and reproductive tissues. Tissue Systems of the Primary Plant Body Tissues undergo primary and secondary growths. The primary meristems are initiated during embryogenesis. They become partly differentiated tissues that remain meristematic for a certain period of time and later on differentiate into specific cell types of the primary tissues. This type of growth is called primary growth. Primary growth is focused on the increase in cell number and length of the primary plant body. While secondary growth is a further division of the primary tissues that lead to the increase in diameters such as thickening of the stem and roots. Secondary growth generally occurs in dicotyledonous, perennial plants.  Figure 2. Tissues of the primary plant body. Three (3) broad types of tissue systems: A. Ground tissue system. They are composed of tissues found between or surrounding dermal and vascular tissues. Examples of ground tissues are parenchyma, collenchyma and sclerenchyma. B. Dermal tissue system. They are the outside layer(s) of tissues. Example: epidermis and periderm C. Vascular tissue system. They are made up of tissues for conduction in plants. Example: phloem and xylem Tissues can also be classified into a.) Simple and b.) Complex Tissue. Simple tissues are composed of single cell type while complex tissues have two or more cell types. Ground tissues; parenchyma, collenchyma are classified as simple tissues. While complex tissues are sclerenchyma, vascular tissues (xylem and phloem) and dermal tissues. Ground Tissue System Ground tissues are the most abundant tissues of the plant body. While dermal tissues are mostly the outermost layer of tissues that serve as protective structures especially the soft tissues of plants. Three types of ground tissues: a. Parenchyma tissue b. Collenchyma tissue c. Sclerenchyma tissue A. Parenchyma Tissue Parenchyma tissue is a group of parenchyma cells. It is the most common of the ground tissues comprising the soft parts of the plant body. It is capable of cell division and located throughout the plant body, as parenchyma tissue in cortex, pith and pith rays in xylem and phloem. Parenchyma cells with numerous chloroplasts involved in photosynthesis is called chlorenchyma. They have thin walls that allow light and carbon dioxide to pass through. Characteristics of parenchyma cells: a. Living when functional (living at maturity) b. Shape: commonly polyhedral (many-sided); variable c. Functions: for wound healing and regeneration; metabolic processes such as respiration, secretion, and photosynthesis; storage and conduction In many succulents, such as the Cactaceae and Peperomia, parenchyma is specialized as a water-storage tissue. B. Collenchyma Tissue Collenchyma tissue is made up of collenchyma cells with either thick or thin walls. It is usually produced in the shoot tips and young petioles. Aerial roots of epiphytes like orchids have a thick layer of collenchyma. Characteristics of Collenchyma Cells: 1. Shape of cells is elongated; unevenly thickened nonlignified primary walls, which are soft and pliable 2. The cell wall is unevenly thickened primary only and lacks lignin 3. They are living when functional. 4. They are found on the periphery (beneath the epidermis) in young elongating stems and in the ribs along veins of some leaves. 5. Functions for support in the primary plant body C. Sclerenchyma Tissue Sclerenchyma tissue is a simple (correction) type of tissue. It is made up of two types of cells; a.) fibers and b.) sclereids. Sclerenchyma cells have both primary and thick, lignified secondary walls. They develop from parenchyma cells in mature organs after maximum growth. Economically important fibers such as in Manila Hemp are extracted from the stem and leaves. a.) Characteristics of Fiber cells: Shape of cell is long and slender that occurs as strands. They have primary and thick secondary cell walls that are often lignified Often (not always), the functional cells are dead Location: Sometimes in cortex of stems, most often associated with xylem and phloem; in leaves of monocots Functions: Support and storage b.) Characteristics of Sclereids: ▪ Shape is variable and generally shorter than fibers ▪ They contain primary and thick secondary cell walls that are typically lignified ▪ May be living or dead when functional ▪ Location: found throughout the plant body ▪ Function: For mechanical support and protection Sclereids also make up the seed coats of many seeds, the shells of nuts, and the endocarp of stone fruits such as olive, peach, and cherry, and they give pears their characteristic gritty texture. Dermal Tissue System (Complex Tissues) Dermal tissue is the outermost layer of tissue which functions for protection of the soft tissues of plants. Among the dermal tissues are: epidermis and periderm. Roots hairs and trichomes are extensions to the epidermis. Root hairs are trichomes that facilitate the absorption of water and minerals from the soil. Guard cells of the stomata are also found in epidermal tissues. a. Epidermis It is the outermost layer of parenchyma cells of the primary plant body constituting the dermal tissues of leaves, floral parts, fruits and seeds, stems and roots until they undergo considerable secondary growth. It serves as a barrier against the invasion of pathogens and shield the soft internal cells. Functions: Mechanical protection Minimizes water loss (cuticle) Aeration of internal tissues via stomata Trichomes, also called hairs, are extensions of epidermal cells that elongate outwards. Trichomes protects plants from insect pests. They exist in many sizes and shapes. Some are unicellular and narrow that die shortly after maturity. Other trichomes remain alive at maturity and serve as secretory glands that secrete salt, antiherbivore compounds and digestive enzymes especially in carnivorous plants. The epidermis also contains pairs of guard cells and a stomatal pore between them. Both the guard cells and stomatal pore constitute the stoma (stomata or stomates, plural). The stomata serve as the passage way of carbon dioxide, oxygen and vapor. The guard cells regulate the movement of gases, including water vapor, into and out of the plant. b.) Periderm The periderm are dermal cells that results from the secondary increase in thickness. It replaces the epidermis as protective layer in roots and stem that undergo secondary growth. Periderm initials are located beneath the epidermis and subsequently formed periderms occur deeper in the bark of perennials like trees. They replace the epidermis as protective tissues in roots and stem when the plant undergoes secondary growth. Lenticels are also found in the periderm which functions for aeration of internal tissues.  The periderm has three layers: a.) phellem or cork, b.) phellogen or cork cambium and c.) phelloderm. Cork or phellem is consist of nonliving and heavily suberized cells at maturity. The phelloderm is a living parenchyma-like tissue, while the cork cambium is the layer between the cork tissue on its outer surface and phelloderm on its inner surface. Vascular Tissue System The vascular tissue system consists of two conducting tissues: a.) xylem and b.) phloem. Xylem and phloem form a continuous system of vascular tissue extending throughout the plant body. A. Xylem Xylem is the water-conducting tissue of plants. It is made up of two types of treachery elements: tracheid and vessel element. Both are elongated cells that have secondary walls, lack protoplasts at maturity and may contain pits in their walls. Vessel elements have perforation plates that lack both primary and secondary walls. Perforation plate occur on both the end walls that joins the vessel elements from end to end forming long continuous columns or tubes called vessels. In the primary plant body, the xylem is derived from the procambium. While in secondary growth, the xylem is derived from the vascular cambium. Two types of Xylem Tracheary Elements: a.) Tracheid Characteristics of tracheid cells: 1. Shape is elongated and tapering. 2. They have primary and secondary cell walls, lignified and contain pits but not perforations. 3. They are dead when functional or at maturity. 4. Function: The chief water-conducting element in gymnosperms and seedless vascular plants; also found in some angiosperms b.) Vessel element Characteristics of vessel element: 1. Shape is elongated but shorter than tracheids 2. They have primary and secondary cell walls; lignified with pits and perforations 3. They are dead when functional or at maturity 4. Function: The chief water-conducting element in angiosperms. B. Phloem Phloem is also known as “superinformation highway” of vascular plants. It is characterized as the principal food-conducting tissue that transport sugars and many other substances including amino acids, lipids, micronutrients, hormones, the floral stimulus (florigen), and numerous proteins and RNAs, some of which serve as signaling molecules. But, phloem is also the route for movement of a range of plant viruses.  Phloem is composed of four cell types: sieve cell, albuminous cell, sieve-tube element and companion cell. Cell Types of Phloem: a. Sieve Cell Characteristics of Sieve Cell: 1. Shape is elongated and tapering 2. Primary cell walls are present in most species with sieve areas; callose (polysaccharide) often associated with wall and pores 3. Living at maturity which lack or with remnants of a nucleus at maturity; lacks distinction between vacuole and cytoplasm; contains large amounts of tubular endoplasmic reticulum; lacks P-protein (slime). 4. Function: The food-conducting element in gymnosperms. b. Albuminous cell 1. They are specialized parenchyma cells also known as Strasburger cells that contain nucleus. 2. Their shape is elongated 3. They only have primary cell walls. 4. They are living at maturity, associated with sieve cell, but generally not derived from same mother cell as sieve cell; they have numerous plasmodesmatal connections with sieve cell. 5. Function: For the delivery of substances to the sieve cell, including informational molecules and ATP (the same role as companion cells. c. Sieve-tube element Several sieve-tube elements in a vertical series constitute a sieve tube. 1. Shape is elongated. 2. They have primary cell wall, with sieve areas on end walls (sieve plates) with larger pores than those on side walls. They have callose that is often associated with walls and pores. 3. They are living at maturity that either lack a nucleus at maturity or contains only remnants of a nucleus. 4. They contain proteinaceous substance known as P-protein. 5. Function: The food-conducting element in angiosperms. Table 3. Summary of cell types of vascular tissues and their functions. Cell types Principal function Xylem 1. Tracheary elements Conduction of water and minerals Tracheids Vessel elements 2. Fibers Support; sometimes storage 3. Parenchyma Storage Phloem 1. Sieve elements Long distance transport of food materials Sieve cells with albuminous cells and signaling molecules. Sieve tube elements with companion cells 2. Sclerenchyma Support; sometimes storage Fibers Sclereids 3. Parenchyma Storage d. Companion cell 1. Shape is variable but often times elongated. 2. They have primary cell wall that is living at maturity. 3. They are closely associated with sieve-tube element with numerous pore-plasmodesmata connections. 4. Function: For the delivery of substances to the sieve-tube element, including informational molecules and ATP. Secondary Growth Secondary growth is the increase in girth in regions that are no longer elongating. It involves the activities of two lateral meristems: (a.) vascular cambium and (b.) cork cambium or phellogen. This occurs in all gymnosperms and in most angiosperms except monocots. Herbaceous dicots undergo little or no secondary growth while woody plants such as trees and shrubs may continue to increase in thickness for several years. Angiosperms are flowering plants consisting of 297,000 species and the largest division in the plant kingdom. The early angiosperms are grouped into three known as basal angiosperms, eudicots or the true dicots and monocots. Examples: Basal eudicots (early dicots) –waterlily and laurel Eudicots (true dicots) – rose, aster and many others Monocots –grasses, palm, cattail, philodendron and bromeliads In woody plants, the protective layer called the periderm replaces the epidermis which serves as the protective layer during secondary growth of the plants. It is formed by cell divisions that occur in the cork cambium. Formation of the periderm is preceded by the initiation of secondary vascular xylem and phloem in the vascular cambium. Divisions of the pericyle also form radial increase in layers of the pericycle. Table 4. Comparison between primary and secondary plant bodies. Primary plant body Secondary plant body Derived from shoot and root apical meristems Derived from primary tissues (cork cambium and vascular cambium) Made up of primary tissues Made up of secondary tissues: wood and bark Herbaceous parts of a plant Constitutes the woody, bark-covered parts of a plant An herb consists of only primary plant body A woody plant has primary tissues at its root and shoot tips but after secondary growth, wood and bark arise in the primary tissues of stems and roots