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Plant Anatomy and Nutrient Transport
Chapter 43
In order to survive, plants have to…The best ways to appreciate plants is to consider how they
overcome the challenges encountered by life on Earth
Obtain energy Obtain water and other nutrients Distribute water and nutrients through the body Exchange gases Support the body Grow and develop Reproduce
Evolution has produced a variety of different types of plants
Plant body Organization
• Two major parts – The root system of a plant – The shoot system
Root Systems
• Branched portions of the plant body • Embedded in the soil• Six functions -
– Anchor the plant – Absorb water and minerals from soil – Store surplus food, carbohydrates manufactured in the shoot– Transport water, minerals, sugars, hormones to and from shoot – Produce hormones – Interact with soil fungi and bacteria that help provide nutrients to
the plant
The Shoot
The shoot system is buds, leaves, flowers, fruits - all on parts of stems • Buds give rise to leaves or flowers • Leaves - sites of photosynthesis • Flowers - reproductive organs, producing male and female gametes,
then help them to reach one another • Flowers produce seeds enclosed within fruits (protect and aid in
dispersal)
Stems - branched, elevate the leaves, flowers, fruit• Elevating the fruit helps disperse the seeds • Some parts are specialized to transport water, minerals,
food molecules, others produce hormones
Two groups of flowering plants
Monocots - lilies, daffodils, tulips, palm trees, grasses— lawn grasses, and wheat, rice, corn, oats, bamboo
Dicots - “broad-leafed” plants, including deciduous trees, bushes, vegetables, and flowers in fields and gardens
There are differences between monocots and dicots, but the characteristic that gives these groups their name is the number of cotyledons
The part of a plant embryo that absorbs and stores food reserves in the seed, then transfers the food to the rest to the embryo when the seed sprouts
Monocots have a single cotyledon Dicots have two cotyledons
structures
leaf primordiaapical meristem
leaf
terminal bud
branch roots
lateral bud
flower
stem
branch
fruit
blade
petiole
node
root hairsroot cap
energy acquisitionby photosynthesis;gas exchange
reproduction
body support;transport of waterand nutrients
Acquisitionof water andminerals
reproduction
growth anddevelopment ofplant structures
functions
root
shoo
t sys
tem
root
sys
tem
The Structures and Functions of a Flowering Plant
Characteristics of Monocots and Dicots Flowers Leaves Roots SeedsStems
cotyledon
embryo
cotyledons
embryo
Flower parts are inthrees or multiplesof three
Flower parts are infours or fives or multiplesof four or five
Leaves have smooth edges,often narrow, with parallel veins
Leaves are palmate (handlike)or oval with netlike veins
Vascular bundlesare scatteredthroughout the stem
Monocots have afibrous root system
The seed has onecotyledon (seed leaf)
The seed hastwo cotyledons(seed leaves)
Dicots have ataproot system
Vascular bundlesare arranged in aring around the stem
Dic
ots
Mon
oco
ts
Plant Development
Dramatically different from animalsOne difference - timing and distribution of growth
In animals, the proportions of a newborn differs from an adult, parts of a newborn’s body grow until they reach adult size and structure, then growth stops
Flowering plants grow throughout their lives, never reaching a stable adult body form
Most plants grow longer or taller only at the tips of their branches and roots
A swing tied to a tree branch or initials carved in tree bark do not move farther up from the ground as the tree grows
Plants are composed of two types of cells
• During plant growth, meristem cells give rise to differentiated cells
– Meristem cells, like animal stem cells, are unspecialized and capable of mitotic cell division
– Some daughter cells lose the ability to divide and become differentiated cells, with specialized structures and functions
• Continued division of meristem cells keep the plant growing throughout its life• Differentiated daughter cells form the non-growing parts of the plant, as leaves
Where Growth Occurs• Plants grow as a result of cell division and differentiation of
meristem cells found in two general locations –
– Apical meristems - located at the tips of roots and shoots • Growth produced by apical meristem cells is primary
growth• Increase in the height or length of a shoot or root,
development of specialized parts of the plant - leaves and buds
– Lateral meristems (side meristems, cambium) - concentric cylinders of meristem cells
Animation: Primary Growth
Secondary Growth Division of lateral meristem cells and differentiation of their daughter cells
produce further concentric cylinders of secondary growth, an increase in the diameter and strength of roots and shoots – Occurs in woody plants - deciduous trees, shrubs, conifers – Some woody plants become very tall and thick and may live hundreds to thousands of
years
Many plants do not undergo secondary growth
– Plants that lack secondary growth are soft bodied, with flexible, fairly short stems
– These herbaceous, typically short-lived plants include lettuce, beans, lilies, and grasses
Tissues and Cell Types? As meristem cells differentiate, they produce a variety of
cell types – One or more specialized types of cells work
together to perform a specific function, as conducting water and minerals = tissue
– Functional groups of more than one tissue = tissue systems • Dermal tissue system covers the outer surface of the
plant • Ground tissue system makes up the body of young
plants; its functions include photosynthesis, storage, and support • Vascular tissue system transports fluids throughout the
plant body
Tissues and Cell Types
Dermal Tissues• Dermal tissue system covers the plant body – Two types of dermal tissues: • epidermal tissue • periderm
Epidermal Tissues Epidermal tissue forms the epidermis - outermost cell layer covering the
leaves, stems, and roots of all young plants, also covers flowers, seeds, and fruit – In herbaceous plants, it forms the outer covering of the entire
plant body throughout its life – Above ground - generally composed of tightly packed, thin-
walled cells, covered with waterproof, waxy cuticle secreted by the epidermal cells • The cuticle reduces the evaporation of water from the plant and helps protect
it from the invasion of disease microorganisms
– Adjustable pores regulate the movement of water vapor, O2, and CO2 across the epidermis of leaves and young stems
– In contrast, the epidermal cells of roots are not covered with cuticle that would prevent them from absorbing water and minerals
PeridermReplaces epidermal tissue on the roots and stems of
woody plants as they age
Composed of multiple layers of cork cells on the outside of the root or stem and a layer of lateral meristem tissue - cork cambium - that generates the cells • Cork cells produce thick, waterproof cell walls as they grow,
then die as they reach maturity • Because of multiple layers of waterproof cork cells on their
surface, root segments that are covered with periderm help anchor the plant in the soil, but can not absorb water and minerals
Ground Tissue
Compromises most of the young plant body
– All of the tissue of the plant body except dermal and vascular tissues
– Three types of ground tissues are parenchyma, collenchyma, and sclerenchyma
Parenchyma Parenchyma - most abundant - makes up most of young plant body – The cells - called parenchyma cells - have thin cell walls
and are alive at maturity – They carry out the plant’s metabolic activities,
photosynthesis , secretion of hormones, food storage • Potatoes, seeds, fruits, storage roots are packed with
parenchyma cells that store sugars and starches
– Help to support the bodies of many plants, especially herbaceous plants
– Some cells can divide – In addition to making up much of the ground tissue
system, parenchyma cells are found in periderm and vascular tissues
Parenchyma
(a) Parenchyma cells in a white potato
thin cell wall
stored starch
Collenchyma Cells that are elongated, with thickened but flexible cell walls
– Alive at maturity, generally cannot divide – Collenchyma tissue provides support for entire
body of young and non-woody plants, the leaf stalks, or petioles, of all plants • Celery stalks are thick petioles, are supported by “strings”
composed of collenchyma cells
(b) Collenchyma cells in a celery stalk
thick cell wall
Collenenchyma
Sclerenchyma Composed of cells with thick, hardened cell walls
– Sclerenchyma cells support and strengthen the plant body; they die after they differentiate
– Their thick cell walls then remain as a source of support • Sclerenchyma cells form nut shells and the outer
covering of peach pits • Scattered throughout the parenchyma cells in a pear,
sclerenchyma cells give pears their gritty texture • Sclerenchyma cells support vascular tissues and form an
important component of wood
(c) Sclerenchyma cells in a pear
thick cell wall
Sclerenenchyma
• Transports water and nutrients – Conducts water and dissolved substances throughout the body – Consists of two conducting tissues: xylem and phloem
The Vascular Tissue System
Xylem Transports water and dissolved minerals from the roots to the rest of the
plant, only in one direction.
– In angiosperms, xylem contains supporting sclerenchyma fibers and two specialized conducting cell types: tracheids and vessel elements
– Both tracheids and vessel elements develop thick cell walls, then die as their final step of differentiation, leaving hollow tubes of nonliving cells wall
Xylem Tissues
• Tracheids - thin, elongated cells stacked atop one another – Tapered, overlapping cells resemble the tips of hypodermic needles – The ends and sides of tracheids contain pits - porous dimples in the
walls that separate adjacent cells – Because the cell wall in a pit is both thin and porous, water and
minerals pass freely from one tracheid to another an adjacent vessel element
• Vessel elements - larger in diameter than tracheids, form pipelines called vessels – Vessel elements are stacked end to end – Their adjoining end walls may be connected by fairly large holes or
the walls may disintegrate, leaving an open tube
Animation: Xylem Adaptations
tracheids
pits
end wall
vesselelement
Xylem
Phloem Transports sugars and other organic molecules throughout the
plant body – Transports sugars, amino acids, and hormones—from structures that
synthesize them to structures that need them – Transports fluids up or down the plant, depending on the metabolic
state of the parts of the plant at any given time
• Two cell types: sieve-tube elements and companion cells – Sieve-tube elements - joined end to end to form pipes
• As sieve-tube elements mature, they lose their nuclei and other organelles, only a thin layer of cytoplasm lining the plasma membrane
– The junction between two sieve-tube elements is a sieve plate • Membrane-lined pores connect the insides of two sieve-tube
elements, allowing fluid to move from one cell to the next
Sieve-tube Cells• Sieve-tube function requires an intact plasma membrane – How, then, can sieve-tube elements maintain and
repair their plasma membranes when they lack nuclei and most other organelles? • Life support of sieve-tube elements is provided by smaller,
adjacent companion cells, which are connected to sieve-tube elements by pores called plasmodesmata
• Companion cells help maintain the integrity of the sieve-tube elements by providing them with proteins and high-energy compounds such as ATP
– Like xylem, phloem also contains supporting sclerenchyma fibers
sieve plate
companioncell
companion cell
sieve-tubeelement
Phloem
Leaves Major photosynthetic structures of most plants
– Their green color arises from chlorophyll molecules – Shape and structure of leaves has evolved in response to
environmental challenges that plants face in obtaining the essentials for photosynthesis: sunlight, carbon dioxide (CO2), and water
Water is absorbed from the soil by the roots and transported to leaves by the xylem – Assuming adequate water supply, maximum photosynthesis
would occur in a porous leaf (allows CO2 to diffuse easily from air to the leaf) with a large surface area
Land plants cannot always get enough water from soil – On a hot, sunny day - large, porous leaf loses more
water through evaporation than the plant could replace
– The leaves of flowering plants are an compromise between conflicting demands • They have a large, waterproof surfaces with adjustable
pores that can open and close to admit CO2 or restrict water evaporation
Leaves are a compromise…
Angiosperm Leaves A broad, flat portion - the blade is connected to the
stem by a stalk, or petiole – The petiole positions the blade – Inside, vascular tissues provide a conducting system
between the leaf and the rest of the plant
The epidermis regulates movement of gases in and out – Leaf epidermis is a layer of nonphotosynthetic, transparent
cells that secrete a waxy cuticle on the outer surfaces – The cuticle is waterproof and reduces evaporation
Stomata (stoma) Adjustable pores in the cuticle and epidermis,
they regulate the diffusion of CO2, O2, water vapor in and out – Two sausage-shaped guard cells that enclose and
adjust the size of the opening – Unlike the other epidermal cells, guard cells contain
chloroplasts and carry out photosynthesis
Functions of Leaves• Photosynthesis occurs in mesophyll cells – The transparent epidermal cells allow sunlight to reach
the mesophyll (“middle of the leaf”), which consists of loosely packed cells containing chloroplasts • Mesophyll cells carry out most of the photosynthesis of a
leaf • Air spaces between mesophyll cells allow CO2 from the
atmosphere to diffuse to each cell and O2 produced during photosynthesis to diffuse away
– Many leaves possess two types of mesophyll cells—an upper layer of columnar palisade cells and a lower layer of irregularly shaped spongy cells
Vascular Bundles• Veins transport water and nutrients throughout the
leaf – Vascular bundles (veins) contain xylem and phloem – Conduct materials between leaf and the rest of the plant
• Veins send thin branches to each photosynthetic cell • Xylem delivers water and minerals to the mesophyll cells of the
leaf, and phloem carries away the sugar they produce during photosynthesis
upper epidermis
petioleblade
mesophyll
palisadelayer
spongylayer
lowerepidermis stoma guard cell chloroplasts
xylem phloem
vascular bundle
cuticle
cuticle bundle-sheathcell
A Typical Dicot Leaf
Structures and Functions of Leaves Temperature, availability of water and light have exerted
selection pressure on leaves – Dim light - the floor of a tropical rain forest - very large leaves,
low light level and abundant water – Desert-dwelling cacti have spines , no surface area for
evaporation– Plump leaves of succulents store water in the central vacuoles of
their cells and are covered with a thick cuticle to reduces water evaporation
– Some plants have surprising structures and functions, including storing nutrients, capturing prey, or climbing
– Onions, Venus Flytraps, Pea plant tendrils
Specialized Leaves
Stems Support and separate the leaves, lifting them to the sunlight
and air – Stems transport water and dissolved minerals from the roots up
to the leaves – They also transport sugars produced in the photosynthetic parts
of the shoot to the roots and other parts of the shoot, such as buds, flowers, and fruits
Adaptations of stems
• Potato eyes• Strawberry runners• Grapes and ivies with grasping tendrils• thorns
Cork
Functions of Roots
• Anchor plant• Absorb water and mineral• Store water and food
• Dicots generally have taproots• Monocots have fibrous root systems
Taproots and Fibrous Roots
Structures of Roots?
• 4 distinct regions– Root cap– Epidermis– Cortex– Vascular cylinder
root hairepidermis
cortex
endodermisof cortex
pericycle
xylem
phloem
apicalmeristem
vascularcylinder
rootcap
Primary Growth in Roots
Root Cap
• Primary growth in a root• Protects apical meristem• Thick cell walls, lubricant• Continuously replaced
Epidermis
• Permeable to water and minerals
• No cuticle• Epidermal cells grow root
hairs to increase surface area
Root Hairs
Cortex
• Located between the epidermis and vascular cylinder
• Large, loose packed parenchyma with porous cell walls
• Sugar is transported to these cells and stored as starch
• Innermost layer, endodermis circles vascular cylinder– Caspian Strip (more later) – waxy
on top bottom, sides but not inner/outer faces
Vascular Bundle - Pericycle
• Pericycle – outermost layer– Regulate movement of
minerals and water into xylem– Source of branching in roots– Hormone release causes
formation of branch root– Punches out through
epidermis, cortex by crushing and releasing enzymes
• Xylem and Phloem
Branch Roots
Specialized Roots
Secondary Growth
• Similar but not identical to secondary growth in stems– Vascular cambium produces secondary xylem and
phloem in the root interior– Cork cambium produces cork cells on the outside
of the root
Essential Nutrients
• Plants only need inorganic nutrients because they can make their own organic molecules
• Macronutrients – needed in large quantity• Micronutrients - <1% total nutrients needed
CO2 - O2 - H
Minerals (K, Ca)Ionic compounds – nitrate, phosphateWater
Water is crucial
• Transport minerals, sugars, hormones and other organic molecules
• Plants require a large amount of water• Source is soil
Roots Transport Minerals
• Absorb mineral from soil and transport to xylem
• Minerals must be dissolved in soil water• Transported from root to shoot in tracheids
and vessel elements of xylem
Young Root Structure
• Living cells• Extracellular space• Tracheids and vessel elements (dead)
• Cell walls are all very porous• Caspian strip divides extracellular space into
two compartments – inside (vascular bundle) and outside
vascular cylinder
xylem
pericycle endodermis
endodermalcells
Casparian strip
cell walls
soil particles
water
roothair
air
plasmodesmata
cortex epidermis
12
3
45
(a) Pathways of mineral and water uptake
(b) Endodermal cells, showing the Casparian strip
Mineral and Water Uptake by Roots
Mineral and water uptake
1. Minerals dissolved in water fill space between cells (blue)2. Minerals are actively transported across plasma
membrane of root hairs, epidermal, cortex, and outside face of endodermal cells; water follows by osmosis (black)
3. Minerals diffuse from cell to cell through plasmodesmata (red)
4. Minerals diffuse or are actively transported across plasma membanes of pericycle cells; water follows by osmosis
5. Minerals and water enter tracheids and vessel elements of xylem (blue)
Importance of the caspian strip
• Soil water in the outer root compartment has low concentration of mineral
• Inner compartment has higher concentration• = diffusion gradient• The caspian strip prevents movement of
minerals from inner outer compartments
Author Animation: Xylem Transport
Root Pressure
• Water moves by osmosis from the soil, across plasma membranes and into the tracheids and vessel elements of the roots
• Sometimes this pressure is so strong it causes root pressure – water entering pushes minerals up the root into the shoot and the water droplets can be visible on the leaves
• This is influenced by transpiration –evaporation of water from the leaves
Root Pressure
Symbiotic Relationships
• Mutually beneficial relationships• Fungal mycorrhizae – fungi in a symbiotic relationship
with plant roots.• Fungal strands twine between roots, increasing area of
root in contact with soil• Some fungi can extract elements that plants cannot, ie.
Phosphates• Fungus receives sugars, amino acids, vitamins from
plant– Desert and high altitude locations– In some forested areas the mycorrhizae interlink trees of
different species, allowing nutrient exchange between them
Mycorrhizae: A Root–Fungus Symbiosis
Nitrogen-fixing bacteria
• Plants can only use N in form of ammonia or nitrate
• Legumes – peas, alfalfa, soy beans• Symbiotic relationship with bacteria that are
able to fix nitrogen (N2 NH3)• Bacteria enter cell and move to cortex where
they form a nodule• Use plant nutrients
Nodules House Nitrogen-Fixing Bacteria
nitrogen-fixing bacteria withincortex cells of nodules
epidermis
nodule
Transpiration
• 90% of water absorbed is lost through the stomata of the leaves, minerals are carried along with the water
• Cohesion – tension theory = water is pulled up the tree by transpiration– Cohesion – attraction between water molecules, forms a
chain-like column within the xylem– Tension – chain of water is pulled up xylem by tension
produced by evaporation from leaves
– Redwood trees that are 350 ft. tall
2
1
3
water molecules
Waterevaporatesthrough thestomata of leaves
Water entersthe vascularcylinder of theroot
Cohesion ofwater moleculesto one another byhydrogen bondscreates a “waterchain”
flow
of w
ater
The Cohesion–Tension Theory
Author Animation: Cohesion and Adhesion
Amazon Rain Forest
• Warm weather and abundant rainfall
• Supports hundreds of trees per acre
• Humidity partially due to transpiration
• When clearcut, local climate is much drier and hotter
• Read page 858
Stomata Regulation
• Mechanically – how is the size changed?• Physiologically – how do they respond to stimuli?
• Two guard cells, slightly curved. • Cellulose fibers encircle the cells
• K+ enters cells in response to light and CO2 conc., water follows by osmosis
• Cellulose belts prevent cells from getting fatter so they get longer, curve outward – open central pore
• Closes when it loses water
Stomata
How Guard Cells Open a Stoma K+ entersthe guard cells(red arrows)
1
Water followsby osmosis(blue arrows)
2
The guardcells lengthenand bendoutward
3
The poreopens4
cellulose“belts”
K+ ions
pore
guardcells
(a) Closed stoma (b) Opening a stoma
How is sugar transported?
• Synthesized in leaves, carried by phloem• Botanists use aphids to learn how phloem
works
• Chemical analysis of phloem fluid = 12-20% sugar, +amino acids, protein and hormones
Aphids Feed on the Sugary Fluid in Phloem Sieve Tubes
Pressure-flow theory
• Differences in water pressure drive flow of fluid through sieve tubes
• Pressure differences are created by the production and use of sugars– Sugar source – synthesizes more than uses– Sugar sink – uses more than it synthesizes– May be source or sink depending on season• Roots, sink – summer (conv. sugar to starch)• Roots, source – following spring (conv. starch to sugar)
How it works
1. Sugar produce in source cell, transported to phloem
2. Water from xylem follows sugar into phloem, increasing pressure
3. Water pressure drives fluid to regions of lower pressure
4. Cells of sugar sink transport sugar out of phloem, water follows by osmosis = lower pressure
The Pressure-Flow Theory of Sugar Transport in Phloem
xylem vessel phloem sieve tube
sugarsink
sugar source
sugar sourcecell
sugar sinkcell
sunlight
1
2
3
4
Fig. 43-25
Author Animation: Phloem Transport
Why do leaves turn color in the fall?
• Fall - temperatures cool, days are long and sun is bright
• Photosynthetic pathways are less efficient, leaf cannot use all light available
• Excess light harms chloroplasts• Red pigmented leaves – anthocyanin – are better
protected against intense light• Why protect? Complex molecules in the leaves are
broken down and stored in cells in the root and stem. Photosynthesis must continue to provide energy for this process