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Plant Structures and Life Cycles

What are plants?

Plants are eukaryotes, their cells have nuclei and membrane-surrounded organellesNuclei contain the genetic material, DNAOrganelles are subcellular structures that perform specific cell functions

Plants include microscopic, unicellular algae to multicellular treesAll plant cells are enclosed by cell wallsNutritionally plants are photoautotrophsPhototrophs obtain energy by conversion of sunlight to chemical energyAutotrophs use CO2 as a source of carbon for constructing cellsPlants are the only eukaryotic organisms that are photoautotrophs

Some bacteria (cyanobacteria) are prokaryotic photoautotrophsPlants perform photosynthesis

Conversion of light energy to chemical energyAssimilation of CO2 to make sugars

Animals and fungi are chemoheterotrophsobtain energy from chemical energy from foodrequire a source of fixed carbon, cannot assimilate CO2

Diagram of a typical plant cell

Components of Plant Cells

Plant cells are composed of:

Cell wallcomposed of cellulose, hemicellulose

sometimes two layers, primary and secondary wallsecondary walls in woody plants contain lignin

Plasma membraneJust inside the cell wallSurrounds the cytoplasm, the living part of the cellA phospholipid bilayerDifferentially permeable

water moves freely across membrane, osmosiswater 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 cellif concentration of solutes in cell< outside, water leaves the cell

Ions, solutes do not move freely across membrane

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

solute conc. inside = outside

solute conc. inside > outside solute conc. inside < outside

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Plant cells are composed of:

OrganellesChloroplasts

Are the organelles that perform photosynthesis Light reactions sun energy converted to chemical energyCalvin cycle converts CO2 from air to sugars

Chloroplasts contain chlorophyll, the light absorbing pigmentOther light absorbing pigments, carotenes, xanthophylls

Components of Plant Cells

The light reactions of photosynthesis convert sunlight to chemical energy and occur in internal membranes, grana, inside the chloroplasts

Components of Plant Cells

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

Plant cells are composed of:

OrganellesOther plastids chromoplasts and leucoplasts

chloroplast-like organelles that do not perform photosynthesisleucoplasts are used for storing metablolites such as starchchromoplasts contain pigments that give color to flowers and fruits

MitochondriaPerform energy metabolism in eurkaryotic cells, site of cellular respirationAerobic respiration generates ATPATP 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

Components of Plant Cells

Plant cells are composed of:

OrganellesA central vacuole occupies the central portion of most plant cells

contains cell sap, various chemical substances, waste and storagesugars, proteins, amino acids etcvacuoles often large, >90 of cell volumecytoplasm containing chloroplasts, mitochondria, nucleus etc

confined to thin outer layer next to the plasma membraneMicrobodies

specialized organelles that contain enzymes for specific cell functionsRibosomes

sites of protein syntheis are made of RNA and protein

Endoplasmic reticulum (ER) internal membrane system in cytoplasmfunctions in the synthesis and “packaging” of proteinsproteins enclosed in a membrane for transport within the cell

Golgi apparatus like the ER an internal cytoplasmic membrane systemfunctions in protein storage, modification and transport

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Components of Plant Cells

Plant cells are composed of:

A nucleusContains the genetic material, DNADirects cell processesNucleus is surrounded by a double membrane, with poresa dark-staining region called the nucleolus contains the DNA coding for ribosomes

nucleolus

nuclear pores

Plant Tissues

Plant cells are organized into plant tissues

Meristem is the site of plant growth and cell divisionAll plant cells are derived from meristemApical meristems are at the tips of stems, shoots and roots – primary growthApical meristem cells divide to produce undifferentiated cellsCells become differentiated into the different tissue typesWoody plants have secondary (diameter) growth, radial meristemsRadial meristems are cork cambium and vascular cambium

Meristems are the sites of cell division

Cells are initially undifferentiated, become differentiated into specialized tissue types as growth occurs

Plant Tissues

Dermal tissueis the outermost layer of cellsEpidermis is usually a single layer of flattened cellsleaf and stem epidermal cells are covered by a wax layer, the cuticlespecialized epidermal cells may be formed

hairs, glands, trichomes have various functionshairs, trichomes often are found on leaves of plants in dry environments

help retard water loss from evaporationstomata are pores in the epidermis that allow exchange of water vapor CO2

and O2 between the plant and the atmospherestomata are formed by specialized cells called guard cellsguard cells swell or shrink in response to turgor pressure, opening or closing

the stomahelp to regulate water loss

Periderm is the outer layer of bark of woody hostscomposed of cork cambium and cork cellscork 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 recognizedParenchyma cells are thin walled, variously shaped

parenchyma tissue is spongy, loosely arranged with air spacesthe photosynthetic cells of leaves “spongy mesophyll”

Collenchyma cells have thicker walls, elongate shape provide structural reinforcement, support

Sclerenchyma tissue includes fiber cells and scleridssclerenchyma cells are nonliving at maturitysupporting tissue, structural reinforcement, fibers have elongate shapeimportant fiber plants, hemp, jute, flax etcsclerids 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

Vascular tissueIs 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 leavesphloem conducts sugar and other organic molecules synthesized by the

leaves downwardBoth tissue types are composed of different specialized cell types

Tracheids and vessel elements conduct water in the xylemboth 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

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

Vessel elements are only found in Angiosperm plants, only tracheids found in Gymnosperms, ferns etc

Plant Tissues Plant Tissues

Phloem tissue is composed of sieve tube members and companion cellsBoth types of cells are living at maturity, unlike xylem cells

Cell walls of sieve tube members have perforate ends called sieve platesSieve plates allow cytoplasmic connections between adjacent cellsProvide contiguous channels for conducting organic materialsNuclei of sieve cell members degenerate at maturity

Each sieve tube member has an associated companion cellCompanion cells have functional nuclei, connected to sieve tube member via

very fine perforations, plasmodesmataCompanion cells load the sieve cells with material to be transported

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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 reproductionPlant reproductive organs function in sexual reproduction

Vegetative organs:

StemsRootsLeaves

Plant Organs

Stems Differ in tissue organization in the two major groups of plants, dicots and

monocotsIn monocot stems (grasses) vascular tissue is regularly distributed within the stem

Vascular bundles include both xylem and phloemSurrounded by a bundle sheath layer of fibers

In nonwoody dicot stems vascular bundles only occur in the outer ring of the stemcenter part of stem is pith composed of parenchyma cellsvascular bundles are composed of outer layer of phloem and inner layer of xylemxylem and phloem are separated by vascular cambium

In woody dicot stems, secondary xylem is the dominant vascular tissueSecondary xylem is formed by the vascular cambium toward the insideSecondary phloem is formed by the vascular cambium toward the outsideWoody dicot stems may also have ray parenchyma cells

ray parenchyma transports materials radially in the stemthe wood of oaks has very well developed ray parenchyma

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

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Plant Organs

The vascular system of a woody dicot is dominated by secondary xylem and secondary pholem produced by the vascular cambium

Plant Organs

Roots

Taproots consist of a single main root with small lateral branch rootsexample carrots, turnips, parsnips

Fibrous roots are highly branched

Roots grow by an apical meristem, located just behind a root capthe root cap is a layer of dead cells that protects the meristemnew 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 elongationthe 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

Cells elongate before differentiating into root tissue types

As in stems, vascular tissue in roots differs between monocots and dicotsIn dicot roots, vascular tissue is in a central bundle, the steleXylem occupies the center of the stele, with spoke-like armsPhloem occupies the spaces between the arms of the xylem “spokes”

Monocot roots have alternating bundles of xylem and phloem

Plant Organs

Dicot root vascular tissue Monocot root vascular tissue

xylem phloem xylem phloempith

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Plant Organs

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

Leaf structures and arrangement

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