Biology Week 8 PDF

Biology Topic D: Leaf structure, transpiration and photosynthesis By: Assoc. Prof. Rasha M. Abu El Khair Associate professor of Pharmacognosy and Vice Dean for Education affairs, College of Pharmacy, AASTMT Structure of the flowering plant  The bodies of flowering plants are composed of two maj...

Biology Topic D: Leaf structure, transpiration and photosynthesis By: Assoc. Prof. Rasha M. Abu El Khair Associate professor of Pharmacognosy and Vice Dean for Education affairs, College of Pharmacy, AASTMT Structure of the flowering plant  The bodies of flowering plants are composed of two major parts: the root system and the shoot system Root system  Roots are branched portions of a plant body, usually embedded in soil, that typically carry out six major functions:  anchor the plant in the ground  absorb water and minerals from the soil  transport water, minerals, sugars, and hormones to and from the shoot  store sugars and starches  produce hormones  interact with soil fungi and bacteria that help the plant to acquire nutrients. Shoot System  The shoot system consists of stems, leaves, buds, flowers, and fruits and usually grows aboveground.  The shoot system performs five major functions:  capture sunlight energy and photosynthesis transport materials synthesize sugars  transport materials to and from various parts of the plant  store surplus sugars and starches  reproduce  produce hormones. during  The principal support structures of a shoot are stems, which bear buds, leaves, and (in season) flowers and fruits  A bud is an embryonic shoot.  Different types of buds may produce branches, flowers, or additional growth at the top of an existing stem.  Leaves are the principal sites of photosynthesis in most plants.  Flowers are the plant’s reproductive organs, producing seeds enclosed in fruits which protect the developing seeds and often help disperse them Monocots and dicots  All flowering plants, however, can be placed into one of two large groups, called monocots and dicots  Monocots include lilies, daffodils, tulips, palm trees, and a wide variety of grasses, not only the familiar lawn grasses but also wheat, rice, corn, oats, and bamboo.  Dicots include virtually all “broad-leafed” plants, such as deciduous trees and bushes, most vegetables, cacti, and many of the plants whose flowers grace our fields and gardens.  There are several differences between monocots and dicots in both root and shoot systems. For example:  Monocots typically have long, thin leaves and bushy fibrous roots (think of grasses), while dicots have broader leaves and thick taproots (as in dandelions or carrots).  However, the characteristic that gives the groups their names is the number of cotyledons.  Monocots have a single cotyledon (Gk. mono, one), and dicots have two (Gk. di, two).  A cotyledon is the part of a plant embryo that absorbs and often stores food reserves in the seed and then transfers the food to the rest of the embryo when the seed sprouts.  When you eat a peanut, each half of the “nut” consists almost entirely of a cotyledon, which has stored starches and fats that power the growth of a peanut seedling.  Cotyledons are often called “seed leaves,” because they sometimes become the first green, photosynthetic leaves during seed sprouting Differentiated Tissues and Cell Types of Plants Parenchyma tissue  consists of living parenchyma cells with thin cell walls and diverse functions  Parenchyma cells carry out most of the plant’s metabolic activities, including photosynthesis, secretion of hormones, and food storage.  Storage structures including potatoes, seeds, fruits (such as apples and pears), and roots such as carrots are packed with parenchyma cells that contain various types of sugars and starches.  Thin-walled parenchyma cells, inflated with water, also help support the bodies of many non-woody plants.  Houseplants wilt if you forget to water them because their parenchyma cells deflate.  Parenchyma cells are also found in both the dermal and the vascular tissue systems. Collenchyma tissue  Collenchyma tissue is composed of living collenchyma cells that that are typically elongated, with irregularly thickened, but still flexible, cell walls  Collenchyma cells store nutrients.  They also form a kind of flexible skeleton that helps to support the bodies of young and nonwoody plants, providing them with supple strength against bending forces, such as wind.  Collenchyma also stiffens thick leaf veins and the leaf stalks, or petioles, of all plants.  For example, celery stalks (which are actually extremely thick petioles) are supported by ribs composed of bundles of collenchyma cells located just beneath the outer dermal cells Sclerenchyma tissue  Sclerenchyma tissue is composed of sclerenchyma cells with thickened cell walls  Like collenchyma, sclerenchyma cells support and strengthen the plant body.  However, unlike collenchyma and parenchyma cells, sclerenchyma cells die after they differentiate, leaving behind their cell walls as a source of support.  Sclerenchyma tissue forms nut shells and the outer covering of peach pits.  Scattered throughout the parenchyma cells in the flesh of a pear, sclerenchyma cells give pears their gritty texture.  Sclerenchyma cells also contribute to vascular tissue and form an important component of wood. The Ground Tissue System Makes Up Most of the Young Plant Body Most of the body of a young plant consists of cells of the ground tissue system. The Dermal Tissue System Covers the Plant Body  The dermal tissue system (L. derma, skin) covers and protects the entire plant body.  Cells of the epidermal tissue, collectively called the epidermis, form the outermost cell layer covering the leaves, stems, and roots of young plants and new growth in older plants  The epidermis is generally composed of flattened, tightly packed, thin-walled epidermal cells that lack chloroplasts. The Dermal Tissue System Covers the Plant Body  Some epidermal cells produce trichomes, projections that can be either outgrowths of a single cell or multicellular.  Trichomes have a variety of shapes and functions.  Some trichomes (such as root hairs) absorb water, others form a hairy coating on leaves, and some (such as cotton fibers) emerge from the epidermis of seeds and aid in dispersal. The Dermal Tissue System Covers the Plant Body  The aboveground parts of a plant are covered with a waterproof, waxy cuticle secreted by the epidermal cells.  The cuticle reduces the evaporation of water from the plant and helps protect it from pathogens.  As described later, specialized guard cells regulate the movement of water vapor, O2, and CO2 across the epidermis in stems and leaves. The Dermal Tissue System Covers the Plant Body  In woody plants, a type of protective dermal tissue called periderm replaces epidermal tissue on roots and stems as they undergo secondary growth. The Vascular Tissue System Transports Water and Nutrients  The vascular tissue system of plants conducts water and dissolved substances throughout the plant body.  It consists of two conducting tissues: xylem and phloem.  These tissues contain specialized conducting cells and often include sclerenchyma cells for added support and parenchyma cells that store various materials.  Here we focus on the conducting cells that are unique to the vascular tissue. Xylem Transports Water and Dissolved Minerals from the Roots to the Rest of the Plant  Xylem transports water and dissolved minerals from the roots to all parts of the shoot system.  Xylem contains two types of conducting sclerenchyma cells: tracheids and vessel elements  Both tracheids and vessel elements develop thick cell walls and then die, leaving behind hollow tubes of nonliving cell wall.  These tubes are perforated by pits, which are thin, porous dimples in the cell walls.  Pits allow water and minerals to pass among the conducting cells of the xylem, and also into adjacent cells of the plant body. Xylem Transports Water and Dissolved Minerals from the Roots to the Rest of the Plant  Tracheids are thin, elongated cells with tapered, overlapping ends connected by pits.  Vessel elements, which are larger in diameter than tracheids, form pipelines called vessels.  Vessel elements are stacked end to end, and their adjoining end walls may be perforated with large holes, or the end walls may disintegrate entirely, leaving an open tube Phloem Transports Dissolved Sugars and Other Organic Molecules Throughout the Plant Body  Phloem transports solutions containing organic molecules, including sugars, amino acids, and hormones, from the structures that synthesize them to the structures that use them.  Unlike xylem, in which fluids take a one-way trip from root to shoot, phloem transports fluids up or down the plant, to or from the leaves and roots, depending on the metabolic state of various parts of the plant at any given time. Phloem Transports Dissolved Sugars and Other Organic Molecules Throughout the Plant Body  Phloem contains two cell types that cooperate in conducting solutions rich in sugar and other organic nutrients:  sieve-tube elements and companion cells  Sieve-tube elements are joined end to end to form pipes called sieve tubes.  As sieve-tube elements mature, they lose their nuclei and most other organelles, leaving behind only a thin layer of cytoplasm lining the plasma membrane.  The junction between two sieve-tube elements is called a sieve plate.  Here, membrane-lined pores connect the insides of the sieve-tube elements, allowing fluid to move from one cell to the next. Phloem Transports Dissolved Sugars and Other Organic Molecules Throughout the Plant Body  How can sieve-tube elements maintain and repair their plasma membranes when they lack nuclei and most other organelles?  Life support for sieve-tube elements is provided by adjacent companion cells, which are connected to sieve tube elements by pores called plasmodesmata  Companion cells supply sieve tube elements with proteins and highenergy compounds such as ATP. What are the Structures and Functions of Leaves?  A typical leaf consists of a large, flat portion, the blade, connected to the stem by a stalk called the petiole.  Inside the blade and petiole, xylem and phloem connect the leaf to the vascular system of the rest of the plant  Leaves are the principal photosynthetic structures of most plants.  Photosynthesis uses the energy of sunlight (captured by chlorophyll) to convert water (H2O) and carbon dioxide (CO2) into sugar, releasing oxygen (O2) as a by-product  Water is absorbed from the soil by a plant’s roots and transported to the leaves, and CO2 enters leaves from the air. The Epidermis Regulates the Movement of Gases into and out of a Leaf  The leaf epidermis consists of a layer of nonphotosynthetic, transparent cells that secrete a waxy cuticle on their outer surfaces.  The cuticle is nearly waterproof and reduces the evaporation of water from the leaf.  Epidermal cells of many types of leaves bear trichomes, which serve a variety of functions;  for example, trichomes may reduce the evaporation of water, reflect harmful ultraviolet radiation, or deter insect predators.  The epidermis and cuticle are pierced by openings called stomata (singular, stoma) that regulate the diffusion of CO2, O2, and water vapor into and out of the leaf.  Most are in the shaded lower epidermis to minimize evaporation.  A stoma consists of two sausage-shaped guard cells that enclose, and adjust the size of, the pore between them.  Unlike other epidermal cells, guard cells contain chloroplasts and carry out photosynthesis. Photosynthesis Occurs in Mesophyll Cells  Transparent epidermal cells allow sunlight to reach the mesophyll (“middle of the leaf”), which consists of loosely packed parenchyma cells containing chloroplasts.  Many leaves contain two types of mesophyll cells:  an upper layer of columnar palisade cells  and a lower layer of irregularly shaped spongy cells.  Mesophyll cells carry out most of the photosynthesis in a leaf.  Air spaces between mesophyll cells allow CO2 from the atmosphere to diffuse to each cell and O2 produced during photosynthesis to diffuse away. Veins Transport Water and Nutrients Throughout the Leaf  Vascular bundles (in leaves, these are also called veins) containing xylem and phloem conduct materials between the leaf and the rest of the plant body.  Xylem delivers water and minerals to the mesophyll cells of the leaf.  Phloem transports soluble organic molecules.  For example, it carries sugar produced by mature leaves to other parts of the plant to provide energy or to store it for later use.  As leaves die, the phloem transports recaptured nutrients, particularly nitrogen-containing amino acids, into seeds, stems, and roots for storage. Differences between animal and plant cells Differences between animal and plant cells  One of the structures commonly found in animal cells which is absent from plant cells is the centriole. Plant  cells also differ from animal cells in possessing cell walls,  large permanent vacuoles and chloroplasts. Cell walls and plasmodesmata  With a light microscope, individual plant cells are more easily seen than animal cells. This is because they are usually larger and, unlike animal cells, are surrounded by a cell wall.  Note that the cell wall is an extra structure which is outside the cell surface membrane.  The wall is relatively rigid because it contains fibres of cellulose, a polysaccharide which strengthens the wall.  The cell wall gives the cell a definite shape.  It prevents the cell from bursting when water enters by osmosis, allowing large pressures to develop inside the cell  Movement of substances across membranes Cell walls and plasmodesmata  Cell walls may be reinforced with extra cellulose or with a hard material called lignin for extra strength  Cell walls are freely permeable, allowing free movement of molecules and ions through to the cell surface membrane.  Plant cells are linked to neighbouring cells by means of pores containing fine strands of cytoplasm.  These structures are called plasmodesmata (singular: plasmodesma).  They are lined with the cell surface membrane.  Movement through the pores is thought to be controlled by the structure of the pores. Vacuoles  Vacuoles are sac-like structures which are surrounded by a single membrane.  animal cells may possess small vacuoles such as phagocytic vacuoles  mature plant cells often possess a large, permanent, central vacuole.  The plant vacuole is surrounded by a membrane, the tonoplast, which controls exchange between the vacuole and the cytoplasm.  The fluid in the vacuole is a solution of pigments, enzymes, sugars and other organic compounds (including some waste products), mineral salts, oxygen and carbon dioxide.  In plants, vacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards)  as well as having a wide range of other functions.  For example, the pigments which colour the petals of certain flowers and the parts of some vegetables, such as the red pigment of beetroots, may be found in vacuoles. Chloroplasts  Chloroplasts are organelles specialised for the process of photosynthesis.  They are found in the green parts of the plant, mainly in the leaves.  They are relatively large organelles and so are easily seen with a light microscope. It is even possible to see tiny ‘grains’ or grana (singular: granum) inside the chloroplasts using a light microscope  These are the parts of the chloroplast that contain chlorophyll, the green pigment which absorbs light during the process of photosynthesis. Practical: Factors affecting osmosis in plant tissues  Useful videos:  https://youtu.be/M4LugywewAU?si=0fmTixqwex_dgU5A  https://youtu.be/k1O9jBHgsxs?si=FpjpVXelX8A8xz6S