Biology Lab C_FALL

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Xylem and Phloem Structure

23-1

23.1 Transport begins in both the roots

and leaves of a plant

 Xylem

 Carries water and minerals from roots to leaves

 In addition to other cell types, has two types of

nonliving, conductive cells:

1. Tracheids – tapered at both ends, ends overlap, and pits

allow water to pass between tracheids

2. Vessel elements – long and tubular with perforation plates

at each end, form hollow pipeline from roots to leaves

23-2

23.1 Transport begins in both the roots

and leaves of a plant

 Phloem

 Transports sugar to all parts of plant

 Several cell types, including:

1. Sieve-tube members – living conducting cells, contain

cytoplasm but have no nucleus

2. Companion cells – provide proteins to sieve-tube

members

 Water is large part of xylem sap and phloem

sap

23-3

Figure 23.1A 23-4

xylem phloem stoma

CO2

Phloem is transporting sugar

from the leaf to the root. H2O

water sugar

H2O sugar

blue = phloem

pink =sugar

red = xylem

light blue = water

23-5

H2O sugar

Xylem transports water and

minerals from the root to the leaf.

H2O

Root phloem xylem Figure 23.1A

(continued)

blue = phloem

pink =sugar

red = xylem

light blue = water

23-6 Figure 23.1B

23-7 Figure 23.1C

Xylem and Phloem Structure

23-8

23.2 Water is pulled up in xylem by

evaporation from leaves

 Water rises to top of plant due in part to root

pressure

 Water entering root cells creates positive (internal)

pressure compared to water in surrounding soil

 Primarily occurs at night

 Result is guttation – drops of water are forced out of

vein endings along edges of leaves

 Root pressure probably not mechanism by

which water reaches tops of very tall trees

23-9

Figure 23.2A 23-10

HOW SCIENCE PROGRESSES

23A Competition for Resources Is One

Aspect of Biodiversity

 Minnesota grasslands often harbor more than

100 plant species within only few hectares

 Held in check due to competition

 Mathematical model showed stable coexistence

of range of plant species that differ according to

their abilities to compete for nitrogen and to

disperse to new areas

23-11

Figure 23A.1 Figure 23A.2 23-12

Poor disperser

Good competitor

for nitrogen

Good disperser

Poor competitor

for nitrogen

23.2 Water is pulled up in xylem by

evaporation from leaves

 Cohesion-tension model

 Requires no expenditure of energy by plant and is

dependent on properties of water

 Cohesion – tendency of water molecules to cling

together

 Water molecules form continuous water column in xylem,

from leaves to roots

 Adhesion – ability of water to interact with molecules

making up vessel walls in xylem

 Gives water column extra strength and prevents it from

slipping back

23-13

23.2 Water is pulled up in xylem by

evaporation from leaves

 Leaf:

 Evaporation of water through leaf stomata is called

transpiration

 At least 90% of water taken up by roots is lost by

transpiration

 Transpiration exerts pulling force, or tension, that

draws water column through xylem to replace water

lost by leaf cells

 Root:

 Due to active transport of minerals into root, water

enters root hairs passively by osmosis, and from

there it enters xylem

23-14

Figure 23.2B 23-15

xylem

mesophyll

cells

Leaves

•Transpiration creates

tension.

•Tension pulls the water

column upward from the

roots to the leaves.

H2O

intercellular

space

stoma

cohesion due to hydrogen bonding

between water molecules

adhesion due to

polarity of

water molecules

cell wall

water molecule

(tree): © Paul Davies/Alamy

23-16

xylem

root hair

Roots

•Water enters xylem

at root.

•Water column

extends from

leaves to root

Stem

•Cohesion makes

water column

continuous.

•Adhesion keeps

water column in

place.

(tree): © Paul Davies/Alamy

Figure 23.2B

(continued)

23.3 Guard cells regulate water

loss at leaves

 Stoma (pl., stomata) – small pore in leaf

epidermis bordered by modified epidermal cells

called guard cells

 Water enters guard cells – stoma opens

 Water exits guard cells – stoma closes

 Entrance of K+ into guard cells creates osmotic

pressure causing water to follow and stoma

opens

 Stoma closes when turgor pressure decreases due to

exit of K+ followed by exit of water

23-17

Figure 23.3A 23-18

K+

343X

K+ enters guard cells and water follows.

H2O H2O

vacuole

guard cell

stoma

(Right): © Jeremy Burgess/SPL/Photo Researchers, Inc.

23-19

(Right): © Jeremy Burgess/SPL/Photo Researchers, Inc

K+

H2O H2O

370X K+ enters guard cells and water follows.

Figure 23.3B

23.3 Guard cells regulate water

loss at leaves

 Three additional factors regulate whether

stomata open or close:

1. Light causes stomata to open

2. High concentration of CO2 causes stomata to close

3. Abscisic acid (ABA) produced by cells in wilting

leaves causes stomata to close

23-20

HOW BIOLOGY IMPACTS OUR LIVES

23B Plants Can Clean Up Toxic Messes

 Phytoremediation – use of plants to clean up

pollutants

 Plants can be used to clean up organic or

inorganic pollutants

 Inorganics absorbed and trapped inside plant, plant

then harvested to be disposed of or reclaimed

 Limitations of phytoremediation include slow

pace

 May take several growing seasons to clean site

23-21

Figure 23B 23-22

Organic Nutrient Transport in

Phloem

23-23

23.4 Pressure flow explains

phloem transport

 Plants transport organic molecules resulting

from photosynthesis to parts that need them

 Aphids and radioactive 14C tracers are used to

follow sugar from source to sink

 Aphids are phloem-feeding insects and are used to

extract sap from phloem

 Sap movement through phloem can be as fast as 60–

100 cm per hour and possibly up to 300 cm per hour

23-24

Figure 23.4A 23-25

Under microscope

An aphid feeding on a plant stem

waste due to feeding

on phloem sap

(top): © Bruce Iverson/SPL/Photo Researchers, Inc.; (bottom):From M.H. Zimmerman "Movement of Organic Substances in

Trees" in SCIENCE 133 (13) January 1961

23.4 Pressure flow explains phloem

transport

 Pressure-flow model

 Explanation for movement of organic

materials throughout plant in phloem

 Flow is from source to sink

 During growing season, leaves photosynthesize

and are source of sugar

 Roots, which are growing, are sink for sugar

 Sugar transport

 Transport can account for any direction of flow in

sieve tubes if we consider that direction of flow is

always from source to sink

23-26

Figure 23.4B 23-27

H2O

H2O H2O

Bulb2:

dilute

sucrose

solution

Bulb1:

concentrated

sucrose

solution

differentially permeable membranes

pressure flow of solution

3 1

Figure 23.4C 23-28

palisade mesophyll

cell of leaf

xylem phloem

Leaf

sugar water

xylem phloem

6 3

cortex cell

of root

xylem phloem Root

water sugar

Plant Nutrition and Soil

23-29

23.5 Certain nutrients are essential

to plants

 Approximately 95% of a typical plant’s dry

weight is carbon, hydrogen, and oxygen

 Minerals as nutrients:

 Mineral – inorganic substance usually containing two

or more elements needed to help build molecules

 Essential nutrients

 Have identifiable roles

 No other nutrients can substitute and fulfill same roles

 Deficiency causes death or failure to reproduce

 Divided into:

1. Macronutrients – needed in large quantities

2. Micronutrients – needed in trace amounts

23-30

23.5 Certain nutrients are essential

to plants

 Beneficial nutrients either are required for or

enhance growth of plant

 Hydroponics allows plants to grow in water,

instead of soil, if supplied with all necessary

nutrients

23-31

23-32 Figure 23.5A

H20

CO2

H2O

Water evaporates

from leaves.

Oxygen escapes from

photosynthesizing leaves.

Carbondioxide

enters photo-

synthesizing

leaves.

Water enters

roots.

Minerals enter

roots.

minerals Oxygen enters and

carbondioxide exits

respiring roots.

23-33

23-34

Figure 23.5B 23-35

solution lacks nitrogen complete nutrition solution

solution lacks phosphorus complete nutrition solution (both): Courtesy Mary E. Doohan

23-36

Courtesy Mary E. Doohan

solution lacks calcium complete nutrition solution

Figure 23.5B (continued)

23.6 Roots are specialized for the uptake

of water and minerals

 Soil – mixture of mineral particles (sand, silt,

and clay), decaying organic material, living

organisms, air, and water, which together

support plant growth

 Humus – soil that contains high percentage of

decomposing organic material

 Soil profile – vertical section of soil, from

surface to rock below

 Usually has parallel layers known as horizons

23-37

23-38 Figure 23.6A

S o

il h o

riz o

n s

root hair

negatively charged soil particle cortex

epidermis of root

film of water

b. Minerals in soil

air space

a.Simplified soil profile

Topsoil: humus

plus living

organisms

Zone of leaching:

removal of nutrients

Subsoil:

accumulation

of minerals and

organic materials

Parent material:

weathered rock

Ca2+

K+

K+ K+

K+

23.6 Roots are specialized for the uptake

of water and minerals

 For water to reach root xylem, it must pass

through cortex:

1. Apoplast route – between cells

2. Symplast route – through cells using

plasmodesmata

 Regardless of pathway, water enters root cells

by aquaporins

 Minerals are actively taken up by plant cells

 Astonishing ability to concentrate minerals

 Ions need to be actively transported into plant cells

23-39

Figure 23.6B 23-40

50 µm

symplast route

of water and

minerals root hair

epidermis apoplast route

of water and

minerals

cortex

endodermis

andCasparian

strip

pericycle

vascular cylinder

endodermis

pericycle

phloem

xylem

cortex

23-41

1 2

P +

K+

K+ K+

K+

H+

ATP

H+

An ATP-driven

pump transports

H+ outofcell.

The electrochemical

gradient causes K+

to enter by way of a

channel protein.

Endodermal Cell

Water Outside Endodermal Cell

H+

I–

I– I–

I–

ADP

H+ H+ Negatively charged ions

(I–) are transported

along with H+ into cell

I– H+

Figure 23.6B (continued)

23.7 Adaptations of plants help them

acquire nutrients

 Root nodules

 Some plants, such as legumes, soybeans, and alfalfa,

have roots colonized by Rhizobium bacteria

 Rhizobium can reduce atmospheric nitrogen (N2) to

NH4 + for incorporation into organic compounds

 Mycorrhizae

 Involves fungi and almost any type of plant root

 Fungus increases the surface area available for

mineral and water uptake and breaks down organic

matter in soil

23-42

Figure 23.7A Root nodules 23-43

nodule root

bacteria

23-44 Figure 23.7B

 Parasitic plants, such as dodders, broomrapes,

and pinedrops, send out rootlike projections

called haustoria that tap into xylem and phloem

of host stems

23-45

23-46 Figure 23.7C

 Carnivorous

plants, such

as Venus

flytrap and

sundew digest

insects as

source of

nitrogen

 Bypass need

for nitrates

from soil,

which may be

lacking

Connecting the Concepts:

Chapter 23

 Land offers advantages for plants: more light

and CO2 for photosynthesis

 Transport system was critical for plants to make

full use of land environment

 Brings water and minerals from roots to leaves

 Takes products of photosynthesis down to roots

 Allows materials to be distributed to rapidly-growing

parts of plant

 Distributes hormones that regulate plant responses to

environment

23-47