Botany Final Exam Notes - PDF

Plant Structure and Function Notes Outline – Teacher Answer Key (guide) © Lisa Michalek Tissues  A plant’s body is made of tissues that form organs.  In vascular plants, there are three types of tissue systems.  Dermal Tissue System  Forms the protective outer...

Plant Structure and Function Notes Outline – Teacher Answer Key (guide) © Lisa Michalek Tissues  A plant’s body is made of tissues that form organs.  In vascular plants, there are three types of tissue systems.  Dermal Tissue System  Forms the protective outer layer of a plant.  Ground Tissue System  Makes up much of the inside of the nonwoody parts of a plant, including roots, stems, and leaves.  Vascular Tissue System  Forms strands that conduct water, minerals and organic compounds throughout a vascular plant. Dermal Tissue System  Dermal tissue covers the outside of a plant’s body.  In the nonwoody parts of a plant, dermal tissue forms a “skin” called the epidermis.  The epidermis of most plants is made up of a single layer of flat cells.  A waxy cuticle, which prevents water loss, coats the epidermis of the stems and leaves.  Often the cells of the epidermis have hair-like extensions or other structures.  Extensions of the epidermal cells on leaves and stems often help to slow water loss.  Extensions of the epidermal cells on root tips help increase water absorption.  The dermal tissue on woody stems and roots consists of several layers of dead cells that are referred to as cork.  Cork cells contain a waterproof chemical and are not covered by a waxy cuticle.  In addition to protection, dermal tissue also functions in gas exchange and in the absorption of mineral nutrients. Ground Tissue System  Ground tissue makes up much of the inside of most plants.  Most ground tissue consists of thin-walled cells that remain alive and keep their nucleus after they mature.  Some ground tissue contains some thick-walled cells.  Ground tissue has different function, depending on where it is located in a plant.  The ground tissue in leaves, which is packed with chloroplasts, is specialized for photosynthesis.  The ground tissue in stems and roots functions mainly in the storage of water, sugar, and starch.  Throughout the body of a plant, ground tissue also surrounds and supports the third kind of plant tissue – vascular tissue. Vascular Tissue System  Plants have two kinds of vascular tissue.  xylem and phloem.  Both xylem and phloem contain strands of cells that are stacked end to end and act like tiny pipes.  These strands of cells act as a plumbing system, carrying fluids and dissolved substances throughout a plant’s body. Xylem  Xylem has thick-walled cells that conduct water and mineral nutrients from a plant’s roots through its stems to its leaves.  The conducting cells in xylem must lose their cell membrane, nucleus, and cytoplasm before they can conduct water.  At maturity, all that is left of these cells is their cell walls.  One type of xylem cell found in all vascular plants is called a tracheid.  Tracheids are narrow, elongated, and tapered at each end.  Water flows from one tracheid to the next through pits, which are thin areas in the cell walls.  Gnetophytes and flowering plants also have a second type of xylem cell, which makes up conducting strands called vessels.  The vessel cells are wider than tracheids and have large perforations in their ends.  The perforations allow water to flow more quickly between vessel cells. Phloem  Phloem contains cells that conduct sugars and other nutrients throughout a plant’s body.  The conducting cells of phloem have a cell wall, a cell membrane, and cytoplasm.  These cells either lack organelles or have modified organelles.  The conducting strands in phloem are called sieve tubes.  Pores in the walls between neighboring sieve-tube cells connect the cytoplasms and allow substances to pass freely from cell to cell.  Beside the sieve tubes are rows of companion cells, which contain organelles.  Companion cells carry out cellular respiration, protein synthesis, and other metabolic functions for the sieve-tube cells. Plant Cells and Tissues Plant Tissue Cell Types Dermal Tissue Epidermal cells, Guard cells, Cork cells Ground Tissue Mesophyll cells, Cortex cells, pith cells Vascular Tissue Vessel cells, Tracheids, Sieve-tube cells, Companion cells Roots  Most plants are anchored to the spot where they grow by roots, which also absorb water and mineral nutrients.  In many plants, roots also function in the storage of organic nutrients, such as sugar and starch.  Many dicots, such as carrots and radishes, have a large central root from which much smaller roots branch.  This type of root system is called a taproot system.  Most monocots such as grasses, have a highly branched, fibrous root system.  Some plants have roots that grow from aboveground stems or leaves.  These roots are called adventitious roots.  The prop roots of corn and the aerial roots of orchids are examples of adventitious roots.  A root has a central core of vascular tissue that is surrounded by ground tissue.  The ground tissue surrounding the vascular tissue is called the cortex.  Roots are covered by dermal tissue.   An epidermis covers all of a root except for the root tip.  The epidermal cells just behind a root tip often produce root hairs, which are slender projections of the cell membrane.  Root hairs greatly increase the surface area of a root and its ability to absorb water and mineral nutrients.  A mass of cells called the root cap covers and protects the actively growing root tip.  A layer of cork replaces the epidermis in the older sections of a root.  Many plants have roots that become woody as they get older.  Layers of xylem replace the ground tissue in woody roots. Stems  The shoots of most plants consist of stems and leaves.  Stems support the leaves and house the vascular tissue, which transports substances between the roots and the leaves.  Many plants have stems that are specialized for other functions.  Stems of cactuses store water.  Potatoes are stems that are specialized for nutrient storage and for asexual reproduction.  Leaves are attached to a stem at points called nodes.  The space between two nodes is called an internode.  Buds that can grow into new branches are also located at the nodes on a stem.  Other features of a stem depend on whether the stem is woody or nonwoody. Nonwoody Stems  A plant with stems that are flexible and usually green is called an herbaceous plant.  Herbaceous plants include violets, clovers, and grasses.  The stems of herbaceous plants contain bundles of xylem and phloem called vascular bundles.  Herbaceous stems are covered by an epidermis.  Stomata in the epidermis enable the stems to exchange gases with the outside air.  The vascular bundles are surrounded by ground tissue.  In monocot stems, such as that of corn, the vascular bundles are scattered in the ground tissue.  In dicot stems, the vascular bundles are arranged in a ring.  The ground tissue outside the ring of vascular bundles is called the cortex.  The ground tissue inside the ring is called the pith. Woody Stems  Trees and shrubs, such as pines, oaks, roses, and hollies have woody stems.  Woody stems are stiff and nongreen.  Buds, which produce new growth, are found at the tips and at the nodes.  They exchange gasses through pores in their bark.  A young woody stem has a central core of pith and a ring of vascular bundles, which fuse into solid cylinders as the stem matures.  Layers of xylem form the innermost cylinder and are the major component of wood.  A cylinder of phloem lies outside the cylinder of xylem.  Woody stems are covered by cork, which protects them from physical damage and helps prevent water loss.  Together, the layers of cork and phloem make up the bark of a woody stem.  A mature woody stem contains many layers of wood and is covered by a thick layer of heartwood.   The xylem in heartwood, which can no longer conduct water, provides support.  Sapwood, which lies outside the heartwood, contains vessel cells that can conduct water. Leaves  Leaves are the primary photosynthetic organs of plants.  Most leaves have a flattened portion, called the blade, that is often attached to a stem by a stalk called the petiole.  A leaf blade may be divided into two or more sections called leaflets.  Leaves with an undivided blade are called simple leaves.  Leaves with two or more leaflets are called compound leaves.  Leaflets reduce the surface area of a leaf blade.  Many plants have highly modified leaves that are specialized for particular purposes.  The spines of a cactus and the tendrils of a garden pea are modified leaves.  Cactus spines are specialized for protection and water conservation, while garden-pea tendrils are specialized for climbing.  A leaf is a mass of ground tissue and vascular tissue covered by epidermis.  A cuticle coats the upper and lower epidermis.  Both xylem and phloem are found in the veins of a leaf.  Veins are extensions of vascular bundles that run from the tips of roots to the edges of leaves.  In leaves, the ground tissue is called mesophyll.  Mesophyll cells are packed with chloroplasts, where photosynthesis occurs.  The chlorophyll in chloroplasts makes leaves look green. Leaves  Most plants have leaves with two layers of mesophyll.  One or more rows of closely packed columnar cells make up the palisade layer, which lies just beneath the upper epidermis.  A layer of loosely packed, spherical cells, called the spongy layer, lies between the palisade layer and the lower epidermis.  The spongy layer has many air spaces through which gases can travel.  Stomata, the tiny holes in the epidermis, connect the air spaces to the outside air. Movement of Water in Plants  Water and mineral nutrients move from a plant’s roots to its leaves through xylem.  Water is pulled up through a plant as it evaporates from the plants leaves.  The surface of leaves are coved with many tiny pores, the stomata.  When the stomata are open, water vapor diffuses out of a leaf.  This loss of water vapor from a plant is called transpiration.  In most plants, more than 90% of the water taken in by the roots is ultimately lost through transpiration.  The xylem contains a column of water that extends from the leaves to the roots.  The cohesion of water molecules causes water molecules that are being lost by a plant to pull on the water molecules still in the xylem.  This pull extends through the water in the xylem.  Water is drawn upward in the same way liquid is drawn through a siphon.  As long as the column of water in the xylem does not break, water will keep moving upward as transpiration occurs.  Roots take in water from the soil by osmosis.  This water enters the xylem and replaces the water lost through transpiration. Guard Cells and Transpiration  A stoma is surrounded by a pair of guard cells that are shaped like two cupped hands.  Changes in water pressure within the guard cells cause the stoma to open or close.  When the guard cells take in water, they swell.  Extra cellulose strands in their cell walls permit the cells to increase in length but not in diameter.  As a result, guard cells that take in water bend away from each other, opening the stoma and allowing transpiration to proceed.  When water leaves the guard cells, they shorten and move closer to each other, closing the stoma and stopping transpiration.  The loss of water from guard cells (for any reason) causes stomata to close, stopping further water loss.  This is an example of homeostasis. Movement of Organic Compounds in Plants  Organic compounds move throughout a plant within the phloem.  Botanists use the term source to refer to a part of a plant that provides organic compounds for other parts of the plant.  A leaf is a source because it makes starch during photosynthesis.  A root that stores sugar is also a source.  Botanists use the term sink to refer to a part of a plant that organic compounds are delivered to.  Actively growing parts, such as root tips and developing fruits, are examples of sinks.  The movement of organic compounds within a plant from a source to a sink is called translocation. Translocation  The movement of organic compounds in a plant is more complex than the movement of water for three reasons.  Water flows freely through empty xylem cells, but organic compounds must pass through the cytoplasm of living phloem cells.  Water only moves up in xylem, while organic compounds move in all directions in phloem.  Water can diffuse through cell membranes but organic compounds cannot.  The German botanist Ernst Munch proposed a model of translocation in 1924. In this pressure –flow model:  Sugar from a source enters phloem cells by active transport.  When the sugar concentration in the phloem increases, water enters the sieve tubes in phloem from xylem by osmosis.  Pressure builds up inside the sieve-tube cells and pushes sugar through the sieve tubes.  Sugar moves from phloem cells into a sink by active transport.