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MEPS 316Plant and Cell Architecture

Plant Cells• Term derived from the Latin cella; meaning

storeroom or chamber

• 1665 – English botanist Robert Hooke

• Used term to describe individual units of the honeycomb-like structure observed in cork

• The “cells” Hooke observed were empty lumens of dead cells surrounded by cell walls.

Plant Life: Unifying PrinciplesSummarization of design elements of plants:

• Green plants are ultimate solar collectors.

– Harvest energy of sunlightHarvest energy of sunlight

– Convert light energy into chemical energy

– Store energy in bonds when they synthesize carbohydrates from CO2 + H2O.

• Other than certain reproductive cells, are nonmobile

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Plant Life: Unifying Principals

• Terrestrial plants are structurally reinforced to support their mass

– Grow toward sunlight and against gravity

• Terrestrial plants lose water continuously by• Terrestrial plants lose water continuously by evaporation.

– Evolved mechanism for avoiding desiccation

• Terrestrial plants have mechanisms for moving water and minerals from the soil to the sites of photosynthesis and growth.

Overview of Plant Structure• All seed plants have same

body plan

– Vegetative body composed of three organs

• Leaf → photosynthesis

• Stem → support• Stem → support

• Root → anchorage and absorption of water and minerals

– Leaves attached to stems at nodes

– Internode is region of stem between two nodes

– Shoot → stems and leaves

Overview of Plant StructureTwo categories of seed plants:

1. Gymnosperms → from Greek for “naked seed”

– Less advanced type

– About 700 species known

– Largest group is conifers → pine, spruce, fir, redwood

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Overview of Plant Structure2. Angiosperms → from Greek for “vessel

seed” or seeds contained in a vessel

• More advanced

• About 250,000 species known

• Major innovation is the flower; hence known as flowering plants.

Overview of Plant Structure• Plant cells are

surrounded by rigid cell walls

– Cell migration prevented because each walled cell and its neighbor are cemented together by a middle lamella

Overview of Plant Structure• Plant cells have two

types of walls:

– Primary cell walls → ytypically thin (<1µm) and characteristic of young, growing cells

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Overview of Plant Structure• Secondary cell walls →

thicker and stronger than primary walls; deposited when most cell enlargement ends.g

– Strength and toughness due to lignin, a brittle, glue-like material

Overview of Plant Structure

• Plant cells are surrounded by rigid cell walls

N ll d d b di idi• New cells are produced by dividing tissues called meristems

• Three major tissue systems make up the plant body

Overview of Plant Structure• Meristems → area

where plant growth is concentrated in localized regions of cell division– Site of nearly all

nuclear division (mitosis) and cell divisions (cytokinesis).

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Overview of Plant Structure• Apical meristem →

most active meristem in young plants; located at tips of stems and roots.

• Axillary nodes →Axillary nodes → located at nodes; contain apical meristem for branch shoots.

• Pericycle → internal meristematic tissue from which lateral roots arise.

Overview of Plant Structure

• Primary Growth

– Phase that gives rise to new organs and to the basic plant formthe basic plant form.

• Results from activity of apical meristems.

– Cell division followed by cell enlargement, typically elongation

Overview of Plant Structure• Secondary Growth

– May occur after elongation complete

I l t l t l– Involves two lateral meristems

• Vasular cambium → gives rise to secondary xylem (wood) and secondary phloem

• Cork cambium → produces the periderm, consisting mainly of cork cells

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Overview of Plant StructureThree Major Tissue Systems Make Up

Plant Body

• Dermal tissueDermal tissue

• Ground tissue

• Vascular tissue

Overview of Plant Structure• Dermal tissue

– Epidermis is the dermal tissue of young plants

Sh t f ll– Shoot surfaces usually covered by a waxy cuticle to prevent H2O loss.

– May also be covered with hairs, or trichomes, which are epidermal cell extensions.

Overview of Plant Structure

• Guard cells are pairs of specialized epidermal cells, surrounding pores in leaves (guard cells + pores = stomata).

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Overview of Plant Structure• Ground tissue

– Makes up bulk of plant cells.

– Three typesThree types

• Parenchyma

• Collenchyma

• Sclerenchyma

Parenchyma• Most abundant

ground tissue.

• Thin-walled,Thin walled, metabolically active; carries out variety of functions, including photosynthesis and storage.

Collenchyma• Composed of narrow,

elongated cells with thick primary walls.

• Provide structural• Provide structural support to the plant body, particularly the shoots.

• Typically arranged in bundles or layers.

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Sclerenchyma• Composed of two types

of cells → both types have thick secondary walls, frequently dead at maturity.

– Sclerids

– Fibers

• Main function is for mechanical support.

Vascular Tissue• Xylem → transports

water and mineral ions from the root to rest of plant.

• Phloem → distributes products of photosynthesis and a variety of other solutes throughout plant

Plant Membranes and OrganellesOrganelles

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Objectives• Be able to identify each of the organelles and

components of a plant cell from a drawing or electron micrograph.

H b i d t di f l t ll• Have a basic understanding of a plant cell and its components.

• Be able to locate, state a function for, and describe the composition of the primary and secondary cell wall.

Objectives cont.

• To be able to draw and explain the organization of a biological membrane.

T b bl t id tif ll l• To be able to identify cellular organelles and relate their structure and function to the cellular, tissue, and organ level of the whole plant.

Terms to Understand• Unit membrane (fluid-mosaic

model)

• Phospholipid bi-layer

• Integral vs. peripheral proteins

• Glyoxysome

• Spherosome

• Microtubule (tubulin)

• Microfilament (actin)• Tonoplast

• Cisternae

• Polysome

• Cristae vs. matrix

• Thylakoids vs. stroma

• Peroxisome

Microfilament (actin)

• Chromatin

• Nucleoplasm

• Nucleolus

• pH buffer

• Prokaryotic versus eukaryotic

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Eukaryotic versus Prokaryotic• Eukaryotic Cell

– Cell that contains a nucleus and membrane bound organelles

• Prokaryotic Cells

– Cell that lacks a nucleus and lacks membrane bound organellesorganelles.

– Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types.

– Think eu = “true” nucleus.

organelles.

– Only bacteria have prokaryotic cell types.

– Think pro = “no” nucleus or organelles

The Plant Cell• Plants are multicellular organisms composed

of millions of cells with specialized functions.

• All plant cells have same basic eukaryotic organization:– Nucleus

– Cytoplasm

– Subcellular organelles

– Enclosed in a membrane that defines their boundaries

– Surrounded by a cellulosic cell wall

The Plant Cell• In its simplest form, a cell is an aqueous

solution of chemicals called protoplasmsurrounded by a plasma membrane (also called plasmalemma).

• Collectively, the membrane and the protoplasm are referred to as a protoplast.

• The plant protoplast, in turn, is surrounded by a cell wall.

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Biological Membrane

Biological Membranes• Fluid-mosaic model →

all biological membranes have same basic organization.

– Double layer (bilayer) of phospholipids in which proteins are embedded.

– Or, in the case of chloroplast, glycosyl glycerides, in which proteins are embedded.

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Phospholipids• A class of lipids in which

two fatty acids are covalently linked to glycerol.

• Glycerol is covalently linked to a phosphate group.

• A variable component, called the head group, is attached to this phosphate group.

Examples: serine, choline, glycerol, or inositol

Plasma Membrane• Selectively permeable

– Allows some materials to pass through but not others

• Physically limits the cell• Also controls the exchange of materials and

serves to maintain essential differences between the cell and its environment

• Various transport proteins embedded in the plasma membrane are responsible for this selective traffic of solutes across the membrane.

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Additional Selectively Permeable Membranes

• Found throughout the protoplast• Form a variety of subcellular structures

called organelles (“little organs”)– Serve to compartmentalize major metabolic p j

activities– One of the organelles is the nucleus, which

contains the genetic information and is the control center of the cell

• The balance of the protoplasm, excluding the nucleus but including other organelles, is called cytoplasm.

Biological Membranes• Composed primarily of lipids and proteins,

with smaller amounts of carbohydrates

– Phospholipids → most abundant lipids in cell p p pmembranes

• Amphipathic molecules, meaning they have two hydrophobic (“water-fearing”) nonpolar hydrocarbon tails and a hydrophilic (“water-loving”) polar head group

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Biological Membranes• Spontaneously form a bilayer in an aqueous

environment

– Hydrophobic tails are buried in the interiory p

– Hydrophilic heads are exposed to the water

– Bilayers form because the nonpolar hydrophobic groups tend to avoid contact with water; associate only with other groups → a tendency known as hydrophobic bonding

Biological Membranes• Bilayer structure gives membrane two properties

that are particularly important to understanding how membranes perform their diverse functions

• They are highly fluid

• They are impermeable to most polar moleculesThey are impermeable to most polar molecules

• Phospholipid bilayer contains a high proportion of unsaturated fatty acids

• Giving the bilayer consistency of light machine oil

• Diffusion of a phospholipid from one side of the bilayer to the other, called “flip / flop”, is rare

Biological Membranes• Membrane bilayer is also impermeable to most polar

or charged solutes

• Includes virtually all ions and molecules normally encountered by cells

• A molecule must first dissolve in the fatty acid matrix, and polar or charged molecules do not readily dissolve in a strongly hydrophobic environment

• Notable exceptions include water, carbon dioxide, and oxygen

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Saturated versus Unsaturated Fatty Acids

• The fatty acids of phopholipids and glycosylglycerides are variable in length → usually 14 – 24 carbons.

• Saturated Fatty Acids• Fatty acids that have no double bonds• Fatty acids that have no double bonds

• Unsaturated Fatty Acids• Fatty acids that contain double bonds

• Examples:• Lard – is a saturated fatty acid

• Canola oil – is an unsaturated fatty acid

Fatty Acids• Number before the colon indicates number of

carbon atoms in chain

• Number following colon indicates number ofNumber following colon indicates number of double bonds

Saturated Fatty AcidsSaturated Fatty AcidsLauricLauric 12:012:0

MyristicMyristic 14:014:0

PalmiticPalmitic 16:016:0

StearicStearic 18:018:0

Unsaturated Fatty AcidsUnsaturated Fatty AcidsOleicOleic 18:118:1

LinoleicLinoleic 18:218:2

LinolenicLinolenic 18:318:3

Fats versus Oils

• Difference between fats and oils is a matter of melting point

• Melting point of fatty acids increases with chain length and extent of saturationg

• Saturated fatty acids tend to be solid at room temperature

• Unsaturated fatty acids tend to be liquid at room temperature

• A high proportion of unsaturated fatty acidscontribute to fluidity of membrane

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Fatty Acids and Melting PointStructure MP (°C)

Saturated Fatty Acids

CH3(CH2)10COOH 44

CH3(CH2)12COOH 52

CH3(CH2)14COOH 63

CH3(CH2)16COOH 69

Unsaturated Fatty Acids

CH3(CH2)5CH=CH(CH2)7COOH -0.5

CH3(CH2)7CH=CH(CH2)7COOH 13

CH3(CH2)4(CH=CHCH2)2(CH2)6COOH -9

CH3CH2(CH=CHCH2)3(CH2)6COOH -17

PhospholipidsSUMMATION:

• Often described as having a charged, polar phosphate “head” and a long hydrocarbon “tail” represented by two fatty acids

• Molecule thus had a dual character

• Phosphate head – hydrophilic (“water-loving”)

• Fatty acid tail – hydrophobic (“water-fearing”)

• Amphipathic – a molecule with both hydrophilic and hydrophobic properties

Membrane ProteinsMost membranes contain as much as 50% protein by

weight

• Membrane proteins may be categorized as either:• Integral – embedded in the lipid bilayer (e.g. proteins that

serve as ion channels)

– Also serve as specific receptors that serve in signal transduction pathways

• Peripheral – bound to membrane surface by noncovalent bonds, such as ionic or hydrogen bonds

– Some involved in interactions between the plasma membrane and components of the cytoskeleton, such as microtubules and actin microfilaments

• Anchored – bound to the membrane surface via lipid molecules

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Cellular Organelles

• Cytoplasmic interior of a cell is organized into a number of discrete, membrane-limited compartments called organelles.compartments called organelles.

• The liquid portion not included as one of the organelles is called the cytosol.

Vacuole• Conspicuous features of most mature higher

plant cells

• Occupies as much as 80 to 90% of cell volume

• Surrounded by the vacuolar membrane or tonoplast

• Contains water, a variety of inorganic ions, organic acids, sugars, enzymes, and secondary metabolic products including pigments

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Vacuoles• Meristematic cells have small

provacuoles (produced by Golgi).

• With cell maturity provacuoles fuse toWith cell maturity, provacuoles fuse to yield large vacuoles in mature cell and occupies about 90% cell volume.

• Cytoplasm is then just a thin strand/layer surrounding the vacuole.

Vacuoles cont.Functions:

• Role in maintaining turgor

• Turgor pressure provides structural rigidity

• Contains hydrolytic enzymes, including proteases, ribonucleases, and glycosidases

• Specialized protein-storing vacuoles, called protein bodies, are abundant in seeds.

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Vacuoles cont.− Vacuole is more acidic than cytosol and

maintains stable cytosolic pH by buffering pH.

− Pumps excess protons (H+) into vacuole ( f d l f it i(sour of oranges and lemons from citric acid in vacuole), pH = 3-5 in vacuole whereas cytosolic pH = 7.

− Vacuolar pH changes according to conditions, but not the cytosol or death of cell may occur (anaerobic, flooding stress).

Vacuoles cont.− Calcium, nitrate, and phosphate ions

accumulate in vacuole to 1000 X greater than in cytosol. "Warehouse" for ions and move to cytosol as needed.

− Provides water reservoir yielding turgor pressure of cell and allowing for enlargement of cell.

Tonoplast• Single unit membrane surrounding

vacuole

• Proteins within tonoplast participate inProteins within tonoplast participate in transport of solutes in/out of vacuole, therefore controlling water potential.

• Derived from ER via Golgi. ER may also swell directly to yield vacuole.

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Nucleus• Control center for all activities of the cell.

• Surrounded by double-membrane (nuclear envelope) which fuses at nuclear pores. Membrane continuous with ER. Macromolecules (ribosomal subunits, RNA, etc ) move out of nucleus via pores toetc.) move out of nucleus via pores to cytosol.

• Space between the two membranes of the nuclear envelope → perinuclear space.

• Nucleoplasm – watery, enzyme-filled solution containing chromatin (DNA + protein), chromosomes and nucleoli.

Nucleus cont.• Site of storage and replication of chromosomes,

composed of DNA and its associated proteins.

• Collectively, this DNA-protein complex is known as chromatin.

B b i th t h f th DNA th l• By being the storehouse of the DNA, the nucleus ultimately controls almost all protein synthesis and therefore the enzymes which drive the chemical reactions determining the structure and function of the cells.

• Collectively, the genes in the nucleus and their intervening sequence are referred to as the nucleargenome.

Nucleoli• Nucleus contains at

least 1 nucleolus

• Dense, granularDense, granular region, membraneless

• Site of ribosome synthesis, rRNA synthesis

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NucleusNuclear envelope

Nucleolus

Chromatin

Electron Micrograph of a Plant Cell

Nuclear Pores

Nuclear pores from a cell of an onion root.

Nuclear Pore Complex• Nuclear pores are

actually elaborate structures composed of more than 100 different proteins arranged to form nuclear pore

lcomplexes.

• Nuclear pores interrupt the integrity of the envelope and provide a channel for export of RNA from the nucleus into the cytoplasm.

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Endoplasmic Reticulum (ER)• Definition - Network of internal membranes, which

form flattened or tubular sacs, called cisternae. Continuous with outer membrane of the nuclear envelope.

• Types: Rough and Smoothyp g

– Rough - many ribosomes attached, parallel layers (lamellar); site for synthesis of membrane proteins and proteins to be secreted outside the cell or into the vacuoles

– Smooth - no ribosomes , tubular; site of lipid synthesis and membrane assembly

ER cont.• ER provides large surface area for chemical

reactions to occur; ER functions in secretion; gives rise to small vesicles which pinch off from membrane carrying proteins and other metabolites used in cell

all s nthesis ER transports proteinswall synthesis, ER transports proteins across membranes.

• The ER and the Golgi complex form an elaborate system of membranes involved in lipid and protein biosynthesis and secretion

ER Complex• Glycoproteins

synthesized in the ER are transported to the Golgi complex in transition vesicles.

• Transition vesicles fuse with the Golgi and the glycoproteins are released to the lumen of the Golgi cisterna.

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ER Complex• As the glycoproteins

are passed from the cis cisternae to the trans cisternae they are modified and additional sugar groups are dd dadded.

• Processed glycoproteins are then packaged in secretory vesicles, which bud off the Golgi and move to the plasma membrane.

Ribosomes• Site of protein synthesis (translation)

• Polysomes - chain or cluster of ribosomes held together by strand of mRNA. Therefore,held together by strand of mRNA. Therefore, translates message producing proteins.

NOTE: DNA → mRNA → ProteinTranscription Translation

(nucleus) (ribosomes)

Golgi Bodies• Stack of flattened cisternae and vesicles, separate

from ER.

• Function to process proteins transferred to it in membrane-bound vesicles from ER lumen.

• Vesicles form on ER, move through cytosol and fuse with Golgi. Within Golgi lumen, proteins are enzymatically modified (adds and removes sugars to form glycoproteins). After processing, glycoproteins leave in other vesicles from Golgi. Proteins are given "tags" or markers directing them to their destination within or outside of the cell.

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Golgi Bodies cont.• Function in cell wall formation. Site of

synthesis of hemi-cellulose, pectins and addition of carbohydrate to extensin protein. Does not function in cellulose synthesis.

• Secrete slime on outside of root cap when vesicles fuse with plasma membrane. Mucilage in insectivorous plants to trap insects comes from Golgi and fusion vesicles.

Mitochondria

• Principal site of cellular respiration

• Double membrane; smooth outer membrane and convoluted innermembrane and convoluted inner membrane (= cristae)

cristae = site of electron transport

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Mitochondria cont.• Soluble phase = matrix. Enzymes of tricarboxylic

acid (TCA) cycle.

• Site of oxidative respiration (sugar metabolized to yield ATP = high energy compound of cell)

• Both mitochondria and chloroplasts areBoth mitochondria and chloroplasts are semiautonomous → they contain their own DNA and protein-synthesizing machinery.

• Believed to have evolved from endosymbiotic bacteria.

• Endosymbionts. (A symbiotic relationship between a cyanobacterium and a simple nonphotosynthetic eukaryotic cell.)

Plastids

• A family of double membrane-bound, semi-autonomous organelles common to plant cells.

• Develop with light from proplastids.

Plastids cont.• Chloroplasts

– Most prominent plastids.

– Possess inner and outer membranes; 3rd inner membrane system = thylakoidsinner membrane system thylakoids

– Thylakoids contain pigments & protein, photochemical events of PS

– Fluid compartment surrounding thylakoids is called stroma.

– CO2 fixation in stroma

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Plastids cont.• Contain chlorophyll and its associated

proteins.

• Sites of photosynthesis.

• ATP synthases located on thylakoid membrane.

• A stack of thylakoids forms a granum.

Plastids cont.• Chromoplasts – plastids that contain high

concentration of carotenoid pigments → partly responsible for yellow, orange, and red colors of many fruits, flowers, as well as autumn leavesautumn leaves

• Leucoplasts – nonpigmented plastids; most important type of leucoplast is the amyloplast, a starch-storing plastid; abundant in storage tissue; specialized in root cap for gravity sensing

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Microbodies Play Specialized Roles in Leaves and Seeds

• Two main types: peroxisomes and glyoxysomes

– Single membrane (except spherosomes - see below) bound organelles specialized for one ofbelow) bound organelles specialized for one of several metabolic functions.

1. Peroxisome - site of photorespiration (discussed later with PS); catalase enzyme-breaks down hydrogen peroxide to water and oxygen

Microbodies cont.2. Glyoxysome - oil-storing seeds,

glyoxylate cycle enzymes convert fatty acids to sugars for transport in growing seedlings to produce energy, e.g, castor bean, cotton, peanut

• Oleosome - "lipid bodies” or spherosomes; store large quantities of triacylglycerol in the form of oil during seed development; possess a “half -unit membrane” with polar head groups toward the aqueous phase and fatty acid tails facing the lumen.

Microbodies cont.

– Contain oleosinswhich may function to keep each oleosome as a discrete organelle by preventing fusion.

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Cytoskeleton• 3-D network of filamentous proteins in cytosol.

• Provides spatial organization of organelles and assists in movement of organelles and other cytoskeletal components.

• Plays fundamental roles in mitosis, meiosis, t ki i ll d iti th i t f llcytokinesis, wall deposition, the maintenance of cell

shape, and cell differentiation.

• Three types of elements (assemblages of globular proteins): – Microtubules

– Microfilaments

– Intermediate filament-like structures.

Cytoskeleton cont.•• Microtubules function in mitosis and Microtubules function in mitosis and

cytokinesis:cytokinesis:

–– MitosisMitosis——process by which previously replicated process by which previously replicated p y p y pp y p y pchromosomes are aligned, separated, and chromosomes are aligned, separated, and distributed in an orderly fashion to daughter cells.distributed in an orderly fashion to daughter cells.

–– CytokinesisCytokinesis——process whereby a cell is process whereby a cell is partitioned into two progeny cells; usually begins partitioned into two progeny cells; usually begins late in mitosis.late in mitosis.

Cytoskeleton cont.• Microfilaments are involved in cytoplasmic

streaming and tip growth.

– Cytoplasmic streaming—the coordinated movement of particles and organelles through the cytosol.

– Actin microfilaments participate in the growth of the pollen tube.

• Intermediate filaments occur in the cytosol and nucleus of plant cells.

– Although identified, little is known about their function.

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Borrowed from Plant Physiology, Taiz, 2nd edition.

Plasmodesmata• Tubular extensions of the plasma membrane

• Transverse the cell wall and connect the cytoplasm of adjacent cells

• Symplast – continuum of cytoplasm• Symplast – continuum of cytoplasm

• Two types of plasmodesmata

1. Primary – formed during cytokinesis

2. Secondary – formed between cells after cell walls have been deposited

Plasmodesmata cont.

• Contain a complex internal structure that functions in regulating

l l t ffi f ll t llmacromolecular traffic from cell to cell.

• Each plasmodesmata contains a narrow tubule called a desmotubule.

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