BIO151 UNIT 3 CH. 36 FROM HAZEL

4 regions of a growing root

root cap

cell division

elongation

maturation

sclereids

boxier than fibers & irregular in shape, has thick, lignified secondary walls

impact the hardness to nutshells & seed coats, & gritty texture to pear fruits

fibers

usually grouped in strands

long, slender, & tapered

some used commercially, such as hemp fibers for making rope

spongy mesophyll

located inward from lower epidermis

consists of irregularly shapes cells that have fewer chloroplasts

these cells form a labyrinth of air spaces through which CO2 and O2 circulate to & from the palisade layer

the air spaces are particularly large in the vicinity of the stomata, where CO2 is taken up from the outside air and O2 is released

3 main types of ground tissue

parenchyma cells

collenchyma cells

sclerenchyma cells

describe the ground tissue of a typical system

consists of mostly parenchyma cells

however, collenchyma cells just beneath the epidermis strengthen many stems during primary growth

sclerenchyma cells especially fiber cells, also provide support in those parts of the cell that are no longer elongating

how does the stele of angiosperms vary between roots & shoots?

root: a solid central vascular cylinder of xylem & phloem

stems & leaves: consists of vascular bundles, separate strands containing xylem & phloem

what organs make up the shoot system of a vascular plant?

reproductive shoot (flower), vegetable shoot

apical bud, axillary bud

node, internode

leaf, stem

ground tissue

includes cells specialized for functions such as storage, photosynthesis, support & short-distance transport

located between dermal & vascular tissue

when internal to the vascular tissue, is the “pith”

when external to the vascular tissue, is the “cortex”

responsible for most metabolic functions

apical meristem

located at the root & shoot tips

provide cells that enable primary growth

lateral meristems

2 lateral meristems extend along the length of a shoot/root where primary growth has ceased

enables secondary growth

functions of the xylem

conducts water & dissolved minerals upward from roots into shoots

2 types of water-conducting cells found in the xylem

tracheid & vessel elements

both are tubular, elongated cells that are dead & lignified at functional maturity

the secondary walls of both are hardened w. lignin, this hardening provides support & prevents collapse under the tension of water transport

during secondary growth of a woody stem, which stem generates vascular tissue?

the vascular cambium, a thin layer of actively dividing cells located between the xylem & phloem tissues in plants

epidermis

a layer of tightly packed cells (dermal tissue)

protects plant from water loss & disease

has specialized characteristics in each organ

in roots, water & minerals absorbed from the soil enter through this structure, especially in root hairs

what organs make up the root system of a vascular plant?

taproot, lateral branch roots

what 2 organ systems make up all parts of a vascular plant?

shoot system & root system

they are connected by vascular tissue, which is continuous throughout the plant

functions of roots

anchors a vascular plant in the soil

absorbs minerals & water

often stores carbohydrates & other reserves

what happens to the outermost layers of periderm on a tree (or other woody plant)

older layers of periderm are sloughed off

this occurs as a part of the process in which during the early stages of secondary growth, the epidermis is pushed outward, causing it to split, dry & fall off the stem or root

within the process of secondary growth which layer forms the bark?

all tissue exterior to the vascular cambium (secondary phloem, cork cambium, cork, layers of periderm)

give examples of special adaptations of roots

pneumatophore/air roots

“strangling” aerial roots

storage roots (plants such as beets will store food & water in their roots)

tendrils

after it has “lassoed” a support, a tendril forms a coil that brings the plant closer to the support, they are typically modified leaves but some are modified stems

spines

are actually leaves, carry out photosynthesis

storage leaves

bulbs have a short underground stem & modified leaves that store food

reproductive leaves

the leaves of some succulents produce adventitious plantlets, which fall off the leaf & take root in the soil

what is translocation?

refers to transport of the products of photosynthesis

carried out by the phloem

where is the pericycle found?

in the vascular cylinder, it is a cell layer that surrounds a solid core of xylem & phloem tissues

which tissues in the plant facilitate bulk flow?

specialized cells in vascular tissue:

tracheids

vessel elements of xylem

sieve-tube elements of phloem

symplast

the entire mass of cytosol of all the living cells in a plant, as well as the plasmodesmata, the cytoplasmic channels that interconnect them

apoplast

consists of everything external to the plasma membranes of living cells & includes cell walls, extracellular spaces and the interior of dead cells such as vessel elements & tracheids

2 major compartments of plant tissue

apoplast & symplast

bulk flow

the movement of liquid in response to a pressure gradient

always occurs from higher to lower pressure

unlike osmosis, is independent of solute concentration

occurs because diffusion is much too slow to function in long-distance transport within a plant

turgor pressure

the force within the cell that pushes the plasma membrane against the cell/wall

what is the main component of phloem sap?

sugar, typically sucrose in most species

sucrose content may be as high as 30% by weight, giving the sap a syrupy thickness

transpiration

the loss of water vapor from leaves & other aerial parts of the plant

what process moves water & minerals along the xylem?

bulk flow

substances are transported long distances to the veins that branch throughout each leaf

describe the anatomy of a typical leaf

leaf epidermis: covered by a waxy cuticle that reduces water loss except where it’s interrupted by stomata

ground tissue (mesophyll): is sandwiched between upper & lower epidermal layers, consists mainly of parenchyma cells specialized for photosynthesis

palisade mesophyll: located beneath the upper epidermis

spongy mesophyll: located inward from the lower epidermis

vascular tissue of each leaf is continuous w. the vascular tissue of the stem; veins subdivide repeatedly & branch throughout the mesophyll

functions of leaves

main photosynthetic organ in most vascular plants, intercepts light

exchanges gases w. atmosphere

dissipates heat

defends itself from herbivores/pathogens

**this can vary as some have conflicting anatomical/physiological requirements

vessel elements

possessed by most angiosperms, a few gymnosperms & a few vascular plants

generally wider, shorter, thinner walled & less tapered than the tracheids

are aligned end to end, forming long pipes known as vessels that may be visible

the end walls have perforation dates that enable water to flow freely through the vessels

vascular rays

radial files of mostly parenchyma cells that connect the secondary xylem & phloem

their cells move water & nutrients between the secondary xylem & phloem, store carbohydrates & other reserves, aids in wound repair

tracheids

is in the xylem of all vascular plants

long, thin cells w. tapered ends, water moves from cell to cell mainly through the pits, where it doesn’t have to cross thick secondary walls

casparian strip

barrier located in the transverse & radial walls of each endodermal cell

a belt made of suberin, a waxy material impervious to water & dissolved minerals

due to the barrier, water & minerals can’t cross the endodermis & enter vascular cylinder via the apoplast, instead they cross through the selectively permeable plasma membrane of an endodermal cell before entering the vascular cylinder

why is turgor pressure important?

it is critical for plant functions bc it helps maintain the stiffness of plant tissues & also serves as the driving force for cell elongation

describe which materials are transported within a tall, photosynthetic plant

sugars produced by photosynthesis in leaves

phloem sap, xylem sap

water & minerals

CO2 is taken up, O2 is produced

sugar-conducting cells of the phloem

these cells are alive at functional maturity

includes sieve cells & companion cells

how is the opening & closing of the guard cells controlled?

controlled via turgor pressure, when they take in water from neighboring cells by osmosis

changes in turgor pressure in these cells results primarily form the reversible absorption and loss of K+ from neighboring epidermal cells

3 routes by which solutes can move through or among the apoplast & symplast

apoplastic route

symplastic route

transmembrane route

apoplastic route

water & solutes move along the continuum of cell walls & extracellular spaces

transmembrane route

water & solutes move out of one cell, across the cell wall, and into the neighboring cell, which can then be passed to the next cell

this route requires repeated crossings of plasma membranes as substances exist & enter

symplastic route

water & solutes move along the continuum of cytosol

this route requires substances to cross a plasma membrane once, when the first enter the plant

after entering, substances can move from cell to cell via plasmodesmata

in which direction does phloem sap travel relative to sources & sinks?

for each sieve tube, the direction of transport depends on the locations of the sugar source & sugar sink that are connected by that tube

therefore, neighboring by that tube may carry sap in different directions if they originate & end in different locations

sugar sink

an organ that is a net consumer or depository of sugar

includes growing roots, buds, stems, & fruits

usually, receive sugar from the nearest sugar sources

ex: upper leaves will export sugar to growing shoot tip; lower leaves will export sugar to the roots

sugar source

a plant organ that is a net producer of sugar via photosynthesis or breakdown of starches

in the process of transportational pull, how is tension transferred through the xylem?

pulling forces are transferred to the xylem because each water molecule is cohesively bound to the next via hydrogen bonds

how can living cells in the interior tissues of woody organs absorb oxygen if they are surrounded by waxy periderm?

through small, raised areas called lenticels, which are dotting the periderm. there is more space between the cork cells, enabling cells within a woody stem or root to exchange gases w. the outside air

what happens when the living cellular contents of a tracheid or vessel element disintegrate?

the cells thickened walls remain behind, forming a nonliving conduit through water can flow

the secondary walls are often interrupted by pits, thinner regions where only primary walls are present

water can migrate laterally between neighboring cells through pits

axillary buds

found in shoots

located in the upper angle formed by each leaf & stem

can potentially form a lateral branch, thorn, or flower

region of cell division

includes the stem cells of the root apical meristem & their immediate products

new root cells produced in this region, including cells of the root cap

function of the phloem

transports sugars, the products of photosynthesis, from where they are made (usually the leaves) to where they are needed or stored (usually roots & sites of growth such as developing leaves & fruits)

sieve tubes

in the phloem of angiosperms, nutrients are transported through these tubes

they consist of chains of cells called sieve = tube element/sieve-tube members

sieve cells

long, narrow cells that transport sugars & other organic nutrients in seedless vascular plants & gymnosperms

sieve-tube elements/sieve-tube members

though alive, they lack a nucleus, ribosomes, a distinct vacuole, and cytoskeletal elements

this reduction in cell contents allows nutrients to pass more easily through the cell

alongside each is a companion cell

how do roots absorb water & minerals into the xylem of their stele?

water gets absorbed via osmosis, minerals via root hairs;

they move via the apoplast/symplast through the cortex

the Casparian strip forces selective entry

ions are pumped into the xylem and water follow via root pressure

goes upwards via transpiration pull

in plant tissues that facilitate bulk flow, how is their structure adapted to it?

mature tracheids & vessel elements are dead cells, therefore have no cytoplasm

meanwhile, the cytoplasm of sieve-tube elements is almost devoid of organelles

this allows water to more easily pass through the xylem & phloem

flow is also enhanced by the perforation plates at end of vessel elements & porous sieve plates connecting sieve-tube elements

how does water move across plant plasma membrane?

by osmosis (diffusion of free water across a membrane)

what gradient drives most cotransport in plants?

the H+ gradient

unlike animal cells, hydrogen ions (H+) rather than sodium ions (Na+) play primary roles in basic transport processes

how do water & minerals pass into the tracheids & vessel elements of the xylem? (last step of the soil-to-xylem pathway)

these water-conducting cells lack protoplasts when mature & are parts of apoplast

endodermal cells, as well as living cells within the vascular cylinder, discharge minerals from their protoplasts into their own cell walls

both diffusion & active transport are involved in this transfer of solutes from the symplast to the apoplast

water & minerals can now enter the tracheids & vessel elements

what drives bulk flow in phloem? (mechanism of translocation in angiosperms)

phloem sap flows from source to sink at rates that are as great as 1m/hr, much faster than diffusion as cytoplasmic streaming

researchers concluded that it moves through the sieve tubes of angiosperms by bulk flow driven by positive pressure, known as pressure flow

building of pressure at the source & reduction of that pressure at the sink cause cap to flow from source to sink

endodermis

innermost layer of cells in the root cortex

functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular cylinder

prevents solutes that have accumulated in the xylem from leaking back into the soil solution

how are solutes transported across plant plasma membranes?

the selective permeability of the plasma membrane controls the short-distance movement of substances into & out of cells

both active & passive transport mechanisms occur

cell membranes equipped w. the same general types of pumps & transport proteins that function in other cells (channel proteins, carrier proteins, cotransporters)

membranes of cells have ion channels that allow only certain ions to pass. most channels are gated, opening or closing in response to stimuli such as chemicals, pressure or voltage

cohesion-tension hypothesis

posits that transpiration provides the pull for the ascent of xylem sap & the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots. hence xylem sap is normally under negative pressure or tension

transportational pull considered main mechanism that generates a pressure gradient in xylem between leaves & roots

what prevents water loss from parts of the leaf that aren’t the stomata?

about 95% of the water a plant loses escapes through the stomata, although these pores account for only 1-2% of the external leaf surface

the waxy cuticle limits water loss through the remaining surface of the leaf

how does the reversible absorption & loss of K+ from neighboring epidermal cells work in guard cells?

stomata open when guard cells actively accumulate K+ from neighboring epidermal cells

flow of K+ across the plasma membrane of the guard cells is coupled to the generation of a membrane potential by proton pumps

stomatal opening correlates w. active transport of H+ out of the guard cell, the resulting voltage (membrane potential) drives K+ into cell via specific membrane channels

the absorption of K+ causes the water potential to become more negative as water enters via osmosis

collenchyma cells

help support young parts of the plant shoot

are generally elongated cells that have thicker primary walls than parenchyma cells, through the walls are unevenly thickened

young stems/petioles often have strands of these cells just below the epidermis

provide flexible support without restraining growth

at maturity, are living & flexible, elongating with the stems & leaves they support

sclerenchyma cells

also function as supporting elements in the plant but are more rigid than collenchyma cells, the secondary cell wall is thick & contains large amounts of lignin

mature cells can’t elongate & occur in regions of the plant that have stopped growing in length

are so specialized for support that many are dead at functional maturity but they produce secondary walls before the protoplast (living part of cell) dies

the rigid walls remain as a “skeleton” that supports the plant, in some cases for hundreds of years

parenchyma cells

mature cells have primary walls that are relatively thin/flexible, most lack secondary walls. generally have a large central vacuole

perform most metabolic functions of the plant, synthesizing & storing various organic products

compose the fleshy tissue of many fruits

photosynthesis occurs within their chloroplasts

some in stems/roots have colorless plastids called amyloplasts that store starch

most retain the ability to divide/differentiate into other types of cells under particular conditions, such as during wound repair