Plant Cells and Organs

Plant Cells and Organs Plants that produce seeds are better suited for life on land. Seeds turn into seedlings and finally into adult trees. They are able to do this by undergoing mitosis and cell specialization Cell specialization = the process by which cells develop from similar cells into cells that have specific functions within a multicellular organism

Seeds and Cell Differentiation • Each seed contains the young plant, the embryo. • Each seed also contains a supply of stored food (proteins, fats and carbohydrates). • A seed also has a protective seed coat that enables it to resist harsh conditions until it can sprout.

Structure of a Seed • Epicotyl: cells at the tip of the embryo that form the leaves and upper stem of the plant • Hypocotyl: middle part of the embryo that becomes the lower part of the stem • Radicle: cells on the other end of the embryo that develop into the roots system of the young plant • Cotyledon: a source of energy and nutrients for the embryo • Endosperm: stored food in the form of starch • Seed coat: protects the seed and also controls germination by restricting water and oxygen to the embryo

Germination • Germination requirements: • Sufficient water  allows the chemical reactions of germination to occur • An adequate supply of oxygen  growth requires energy • The proper temperature  the enzymes that work in the chemical reactions require a specific temperature to function

The Plant Emerges • After the absorption of sufficient water starts the chemical reactions necessary for the growth of the seed, the radicle lengthens, breaks through the seed coat and emerges into the soil to form the root system. • The hypocotyl arches as it grows into the first tiny leaves and the stem of the plant • The endosperm and the cotyledon provide the food energy required for this growth. • Cell differentiation = a stage of development of a living organism during which specialized cells form

The Plant Body • The seed: provides for the development of the new plant and is able to endure harsh conditions • The root: Absorbs water and dissolved mineral salts for plant growth; also stores food and anchors the plant to the soil • The stem: transfers soil water and manufactured organic materials to other parts of the plant; also supports the plant and stores food • The leaf: carries on the process of photosynthesis and makes the organic compound necessary for the life of the plant • The flower: reproduces the plant by starting the formation of the seed and fruit • The fruit: contains the seeds and the means for scattering them

Specialized Cells and Tissues in Plants • As plants grow, they always make new specialized cells • Groups of specialized cells form tissues  groups of tissues work together as organs (roots, stems, leaves) • Tissue = a cluster of similar cells that share the same specialized structure and function • Organ = a combination of several types of tissue working together to perform a specific function • Meristematic cells are undifferentiated cells that can form specialized cells in plants. They constantly produce more cells

Major plant tissues • Dermal tissue/epidermis: forms the outermost covering of the plant’s organs. Creates a barrier from the external environment, protects inner tissues from damage and controls the exchange of water and gases • Ground tissue: various functions  some cells perform photosynthesis, some provide support for the body of the plant • Vascular tissue: provides physical support for the plant’s body, transports water, minerals and sugars throughout the plant. The two specific types of cells that make up vascular tissue are called xylem cells and phloem cells.

The growth of plant tissues • Rapidly dividing meristematic cells at the tips of roots and branches cause the upward, downward and outward growth of plants • A bud is a swelling of the stem that contains meristem for new tissues • Most active growth occurs near the terminal bud • The lateral buds are dormant but can give rise to new branches, leaves and flowers.

Grow Up or Grow Out? • Plants elongate due to the presence of a chemical hormone called auxin. • Auxin is produced predominantly at the tips of the stems and roots • Plants will grow outwards (become bushy) if this source of auxin (in the terminal bud) is removed.

Forming new tissue • Meristematic cells give rise to new cells • They give rise to different kinds of cells that will be used to create the different organs of the plant

The Leaf • New leaves arise as a result of stem growth.

Structure of a Leaf • Epidermis: covering tissue on the upper and lower surfaces made up of a single layer of cells whose main function is protection. They allow sunlight to pass through to other cells in the leaf. • Cuticle: a thin, waxy layer that protects the epidermis from injury and excessive loss of moisture • Mesophyll Cells: cells located between the upper and lower epidermal layers. These cells are composed of two layers.

Mesophyll Cells • Palisade tissue: layer of elongated cells below the epidermis that are specialized to perform photosynthesis. The palisade cells contain numerous chloroplasts. • Spongy mesophyll: smaller more loosely packed mesophyll cells between the palisade layer and lower epidermis where photosynthesis takes place. Between the cells are air spaces that contain gases needed or produced by photosynthesis (water vapour, oxygen, carbon dioxide) • Vascular bundle/Vein (xylem and phloem): transport water and food Leaf structure

Transport System • The centre of the leaf contains xylem and phloem tissue arranged into a system called a vascular bundle. These bundles form veins that travel from the roots, up the stems and through the leaf and end in the spaces between the spongy mesophyll cells. • The xylem delivers water and the phloem picks up sugars that have been produced and delivers them to cells throughout the rest of the plant.

Transpiration • Plants take in much more water than they use for growth. Most of the water absorbed is eliminated. • The process of eliminating water vapour from the plant is called transpiration. • This occurs through tiny openings on the lower epidermis called stomata. • Temperature, humidity, amount of light and wind affect the rate of transpiration. • If water loss occurs too quickly the cells in the plant will shrink from lack of water --> wilting.

Gas Exchange in Leaves • Stomata: allow movement of gases and water vapour into and out of the leaf • Guard cell: these cells change shape and by doing so, open and close the stomata. These cells also contain chloroplasts unlike other epidermal cells The guard cells are the only cell in the epidermis that contain chloroplasts. They must therefore perform photosynthesis. It is the occurrence of photosynthesis that causes a stoma to open in the morning and close at night. An Open Stoma • In daylight, the guard cells produce glucose via photosynthesis • The presence of sugar inside the guard cells causes water to enter by osmosis from neighbouring epidermal cells • The increase in turgor pressure causes the guard cells to swell opening the stoma between them • The open stoma allow O2 and CO2 to leave and enter the leaf

Gas Exchange in Leaves A Closed Stoma • When the sun sets, photosynthesis stops and the guard cells stop producing glucose. • Water no longer enters the cells by osmosis and the turgor pressure decreases • The stoma then close http://trc.ucdavis.edu/biosci10v/bis10v/week8/stomata.mov

A look at Photosynthesis • Using the sun to make useful forms of energy • Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems

How Does a Plant Capture Light? • Plants have chlorophyll PIGMENTS (molecules that can absorb specific wavelengths of light) Plant leaves appear green. Therefore, what colours must the chlorophyll pigments absorb? reflect? http://www.johnkyrk.com/photosynthesis.html

Chloroplasts • They are extremely small but perform thousands of reactions in each minute b/c of their highly folded thylakoid membrane inside the chloroplast.

Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. http://wps.prenhall.com/esm_krogh_biology_3/0,8750,1135943-,00.html

Photosynthesis in plants • Light energy is used to transform carbon dioxide and water to energy rich food molecules composed of glucose monomers • There are 2 stages in this process

Overview of the two steps in the photosynthesis process CO2 + H2O -----> C6H12O6 + O2 The above equation shows us that photosynthesis is a process by which light energy is used to transform carbon dioxide and water into energy rich food molecules.

The Stem • Food (sap) manufactured in the leaves are carried by the stem to all parts of the plant. • The stem links roots and leaves and serves as the plant’s main organ for distribution of water and food. • Stems sometimes store food and water; the potato is an underground stem that stores starch • Stems also provide physical support for the plant • Stems contain most of a plant’s xylem tissue which form long tubes or vessels that act as pipes through which allow the easy flow of water • Xylem cells are closely associated with phloem cells that strengthen the structure of the vascular cells

The Roots • All roots are responsible for: • anchoring the plant to the ground • extracting water and minerals from the soil • Some plant roots also store food energy (as starch). • There are three main types of roots: • tap root • fibrous root • adventitious

Root Structures • Epidermis: the outermost layer of roots that function as the interface between plants and the soil • Root hairs: on the epidermis are considered to help in direct mineral nutrient uptake by increasing the surface area of roots • The Cortex is a band of parenchyma cells that develops beneath the epidermis. It stores food • The pericycle is a cylinder of cells surrounding the vascular cylinder xylem and phloem • The growing tip of roots is protected by a root cap consisting of concentric layers of cells surrounding the apical meristem where new root cells are produced

The Reproductive Organ: The Flower • The function of the flower is to reproduce the plant by producing fruit that contain seeds. • Flowers are specialized leaves --> some produce pollen which manufactures sperm and eggs • Fertilization occurs when the sperm and egg are combined by animals/bees/wind • This is pollination which results in seeds being formed • Seeds are sometimes embedded in fruit • Fruit start to ripen as a result of a plant hormone named ethylene

Plant Organ Systems • The body of a plant is considered to have two main organ systems: the roots system and the shoot system • The root system is responsible for taking in water and minerals from the soil • The shoot system is responsible for supporting the plant, performing photosynthesis and transporting sap.

Systems Working Together • The root and shoot systems are connected by the flow of water, nutrients, and various hormones through vascular bundles that contain xylem and phloem • Xylem --> moves water and minerals from the roots • Phloem --> moves sugars produced by photosynthesis to other plant parts

Movement of Water • Plants can only absorb nutrients if they are dissolved in water • This water has to move against gravity • Water enters the plant through the roots, passing first into the root hairs and then into the centre of the root. • The pericycle prevents the water from flowing backwards • A force called cohesion holds the water molecules together like glue • Adhesion helps water fight the force of gravity by helping water molecules to stick to the walls of the xylem cells

Transpiration Cohesion Theory • Water enters the roots by osmosis. This causes root pressure which pushes the water into the xylem. • Water molecules climb the sides of the xylem vessels by clinging to the narrow vessel walls (molecular adhesion) • Each water molecule clings to the next one forming a continuous column from the roots to the leaves (molecular cohesion) • Transpiration causes water to move through the leaf by osmosis, pulling water out of the xylem cells.

The Cohesion Tension Theory http://academic.kellogg.cc.mi.us/herbrandsonc/bio111/animations/0031.swf

Moving Nutrients through the System • The process of photosynthesis produces a form of sugar called glucose. • This glucose will usually be converted into starch and stored in the stem or the roots

The Bulk Flow Theory Sources and Sinks Source: the places where sugars are made. Since photosynthesis occurs in the leaves, this will be the place where sugars are loaded into the phloem Sink: The plant parts that require carbohydrates. These are the places where the sugars are unloaded from the phloem

Plant Cells and Organs