Plant Structures and Life Cycles Diagram of a typical plant cell
What are plants? Plants are eukaryotes, their cells have nuclei and membrane-surrounded organelles Nuclei contain the genetic material, DNA Organelles are subcellular structures that perform specific cell functions Plants include microscopic, unicellular algae to multicellular trees All plant cells are enclosed by cell walls Nutritionally plants are photoautotrophs Phototrophs obtain energy by conversion of sunlight to chemical energy Autotrophs use CO2 as a source of carbon for constructing cells Plants are the only eukaryotic organisms that are photoautotrophs Some bacteria (cyanobacteria) are prokaryotic photoautotrophs Plants perform photosynthesis Conversion of light energy to chemical energy Assimilation of CO2 to make sugars Animals and fungi are chemoheterotrophs obtain energy from chemical energy from food require a source of fixed carbon, cannot assimilate CO2
Components of Plant Cells Plant cells are composed of: Cell wall composed of cellulose, hemicellulose sometimes two layers, primary and secondary wall secondary walls in woody plants contain lignin Plasma membrane Just inside the cell wall Surrounds the cytoplasm, the living part of the cell A phospholipid bilayer Differentially permeable water moves freely across membrane, osmosis water moves from higher solute concentration to lower water potential is difference in solute concentration between inside and outside of cells solutes decrease water potential if concentration of solutes in cell > outside, water moves into cell if concentration of solutes in cell< outside, water leaves the cell Ions, solutes do not move freely across membrane
solute conc. inside = outside
solute conc. inside > outside
solute conc. inside < outside
Water can move in and out of plant cells freely across the plasma membrane, depending on difference in water potential inside and outside the cell Water moves into plant cells when the solute concentration inside the cell is greater than outside the cell The cell wall is rigid, so the pressure of water moving into the cell creates pressure, turgor pressure Low vapor pressure (the concentration of water in the air) can also cause water to leave plant cells
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Components of Plant Cells Plant cells are composed of: Organelles Chloroplasts Are the organelles that perform photosynthesis Light reactions sun energy converted to chemical energy Calvin cycle converts CO2 from air to sugars
Components of Plant Cells The light reactions of photosynthesis convert sunlight to chemical energy and occur in internal membranes, grana, inside the chloroplasts
Chloroplasts contain chlorophyll, the light absorbing pigment Other light absorbing pigments, carotenes, xanthophylls
The Calvin cycle reactions uses the chemical energy from the light reactions to convert CO2 from air into sugars and occurs in the stroma of the chloroplast
Components of Plant Cells Components of Plant Cells Plant cells are composed of:
Plant cells are composed of:
Organelles Other plastids chromoplasts and leucoplasts chloroplast-like organelles that do not perform photosynthesis leucoplasts are used for storing metablolites such as starch chromoplasts contain pigments that give color to flowers and fruits
Organelles A central vacuole occupies the central portion of most plant cells contains cell sap, various chemical substances, waste and storage sugars, proteins, amino acids etc vacuoles often large, >90 of cell volume cytoplasm containing chloroplasts, mitochondria, nucleus etc confined to thin outer layer next to the plasma membrane Microbodies specialized organelles that contain enzymes for specific cell functions Ribosomes sites of protein syntheis are made of RNA and protein Endoplasmic reticulum (ER) internal membrane system in cytoplasm functions in the synthesis and â&#x20AC;&#x153;packagingâ&#x20AC;? of proteins proteins enclosed in a membrane for transport within the cell Golgi apparatus like the ER an internal cytoplasmic membrane system functions in protein storage, modification and transport
Mitochondria Perform energy metabolism in eurkaryotic cells, site of cellular respiration Aerobic respiration generates ATP ATP is consumed in the energy-requiring cell functions: growth, cell division Mitochondria have internal membranes like chloroplasts, but do not perform photosynthesis Produce the majority of ATP consumed in cellular metabolism
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Components of Plant Cells
Plant Tissues
Plant cells are composed of: Plant cells are organized into plant tissues A nucleus Contains the genetic material, DNA Directs cell processes Nucleus is surrounded by a double membrane, with pores a dark-staining region called the nucleolus contains the DNA coding for ribosomes
nuclear pores
Meristem is the site of plant growth and cell division All plant cells are derived from meristem Apical meristems are at the tips of stems, shoots and roots â&#x20AC;&#x201C; primary growth Apical meristem cells divide to produce undifferentiated cells Cells become differentiated into the different tissue types Woody plants have secondary (diameter) growth, radial meristems Radial meristems are cork cambium and vascular cambium
nucleolus
Plant Tissues Meristems are the sites of cell division Cells are initially undifferentiated, become differentiated into specialized tissue types as growth occurs
Dermal tissue is the outermost layer of cells Epidermis is usually a single layer of flattened cells leaf and stem epidermal cells are covered by a wax layer, the cuticle specialized epidermal cells may be formed hairs, glands, trichomes have various functions hairs, trichomes often are found on leaves of plants in dry environments help retard water loss from evaporation stomata are pores in the epidermis that allow exchange of water vapor CO2 and O2 between the plant and the atmosphere stomata are formed by specialized cells called guard cells guard cells swell or shrink in response to turgor pressure, opening or closing the stoma help to regulate water loss Periderm is the outer layer of bark of woody hosts composed of cork cambium and cork cells cork cambium produces cork cells as the tree grows radially
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Plant Tissues Ground tissue is the major tissue type in non woody plants Three major types are recognized Parenchyma cells are thin walled, variously shaped parenchyma tissue is spongy, loosely arranged with air spaces the photosynthetic cells of leaves “spongy mesophyll” Collenchyma cells have thicker walls, elongate shape provide structural reinforcement, support Sclerenchyma tissue includes fiber cells and sclerids sclerenchyma cells are nonliving at maturity supporting tissue, structural reinforcement, fibers have elongate shape important fiber plants, hemp, jute, flax etc sclerids are variously shaped, not long like fibers, tough bundles of cells the stone cells of pears are sclerids, cherry and peach pits are aggregations of sclerids
Plant Tissues Xylem tissue consists of tracheid cells and vessel elements that conduct water upward in the plant. Vessel elements are larger diameter “pipes” Tracheids have both a conducting and structural, supporting function Both have perforated side and end cell walls
Plant Tissues Vascular tissue Is the water conducting tissue in plants Two major types are recognized: xylem conducts water and dissolved mineral nutrients from soil via the roots to the shoots and leaves phloem conducts sugar and other organic molecules synthesized by the leaves downward Both tissue types are composed of different specialized cell types Tracheids and vessel elements conduct water in the xylem both are dead at maturity, only cell walls, used for “plumbing” Xylem can be produced by apical meristem cells (primary xylem) in nonwoody plants or by radial, vascular meristem (secondary xylem) in woody plants Secondary xylem is called wood
Plant Tissues Phloem tissue is composed of sieve tube members and companion cells Both types of cells are living at maturity, unlike xylem cells Cell walls of sieve tube members have perforate ends called sieve plates Sieve plates allow cytoplasmic connections between adjacent cells Provide contiguous channels for conducting organic materials Nuclei of sieve cell members degenerate at maturity Each sieve tube member has an associated companion cell Companion cells have functional nuclei, connected to sieve tube member via very fine perforations, plasmodesmata Companion cells load the sieve cells with material to be transported
Vessel elements are only found in Angiosperm plants, only tracheids found in Gymnosperms, ferns etc
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Plant Tissues
Phloem tissue consists of sieve tube members and companion cells, and functions to translocate organic molecules synthesized by the plant to distribute it to other parts of the plant. Sugars synthesized by photosynthesis are translocated by pholem to developing fruit, roots etc Materials to be translocated are loaded into the sieve tube elements by companion cells through fine connections through the cell walls, plasmodesmata that allow cytoplasmic connections between adjacent cells
Plant Organs Plant tissues are organized into more complex structures that have specific functions Plant vegetative organs function in plant growth and asexual reproduction Plant reproductive organs function in sexual reproduction Vegetative organs: Stems Roots Leaves
Plant Organs Stems Differ in tissue organization in the two major groups of plants, dicots and monocots In monocot stems (grasses) vascular tissue is regularly distributed within the stem Vascular bundles include both xylem and phloem Surrounded by a bundle sheath layer of fibers In nonwoody dicot stems vascular bundles only occur in the outer ring of the stem center part of stem is pith composed of parenchyma cells vascular bundles are composed of outer layer of phloem and inner layer of xylem xylem and phloem are separated by vascular cambium
Plant Organs
Comparison of the distribution of vascular bundles in monocot and non woody dicot stems Upper figures are monocot stems, lower figures are dicot stems
In woody dicot stems, secondary xylem is the dominant vascular tissue Secondary xylem is formed by the vascular cambium toward the inside Secondary phloem is formed by the vascular cambium toward the outside Woody dicot stems may also have ray parenchyma cells ray parenchyma transports materials radially in the stem the wood of oaks has very well developed ray parenchyma
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Plant Organs
Plant Organs Roots
The vascular system of a woody dicot is dominated by secondary xylem and secondary pholem produced by the vascular cambium
Taproots consist of a single main root with small lateral branch roots example carrots, turnips, parsnips Fibrous roots are highly branched Roots grow by an apical meristem, located just behind a root cap the root cap is a layer of dead cells that protects the meristem new cells are formed behind the meristem (toward the main stem) after new cells are formed they elongate before differentiating the region just behind the meristem is the zone of elongation the region where the cells begin to differentiate is the zone of maturation
Plant Organs
Root growth As new cells are formed behind the apical meristem they are initially undifferentiated
Plant Organs As in stems, vascular tissue in roots differs between monocots and dicots In dicot roots, vascular tissue is in a central bundle, the stele Xylem occupies the center of the stele, with spoke-like arms Phloem occupies the spaces between the arms of the xylem â&#x20AC;&#x153;spokesâ&#x20AC;? Monocot roots have alternating bundles of xylem and phloem xylem
phloem
xylem
pith
phloem
Cells elongate before differentiating into root tissue types
Dicot root vascular tissue
Monocot root vascular tissue
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Plant Organs Leaf structures and arrangement
Leaves The dominant function of leaves is to conduct photosynthesis, although leaves may be modified in various ways to serve various other functions in some plant species Leaves consist of a usually thin blade, a petiole may be present connecting the blade to the stem The point of attachment of a leaf to a stem is called a node, the space between nodes is the internode
Plant Organs
Leaf tissue arrangement Leaves have an upper and lower epidermal cell layer The interior or mesophyll is divided into an upper palisade layer of closely packed cells and a lower spongy layer of loose cells with air spaces
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