BIOL 1030
Introduction to
Biology: Organismal
Biology. Spring 2011
Section A
Steve Thompson: [email protected]
http://www.bioinfo4u.net
1
Sunday, February 27, 2011
Thank you, Dr.
Richard Clark . . .
I hope that Dr. Clark’s
introduction to the world of
plants will get you ‘rarin’’ to
go, so that you’ll want to learn
all about how they work . . .
At least that’s my hope.
2
Sunday, February 27, 2011
So now, time for a major change
of emphasis in the course.
We’ll be leaving the realm of molecular and
cellular biology for the tissues and organ
systems of plant and animal physiology. The
molecules and cells are still there, as is the
evolution that put it all in place; we’ll just be
moving our focus out a bit to look at how cells
come together to form specialized tissues, and
how tissues come together to form the organs
that make up multicellular organisms.
3
Sunday, February 27, 2011
Here’s a very brief intro’ into
just what physiology is.
“Physiology (from Greek physis, "nature, origin"; and
logia, "study of") is the study of the mechanical,
physical, and biochemical functions of living
organisms. Physiology has traditionally been
divided bet ween plant . . . animal and all living
things . . . but the principles . . . are universal, no
matter what particular organism is being studied.
For example, what is learned about the physiology
of yeast cells may also apply to human cells that
one may be studying.” Wikipedia
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Sunday, February 27, 2011
Wikipedia also does a decent job
of describing the history . . .
Of the science of physiology, which goes way, way back:
http://en.wikipedia.org/wiki/Physiology
Merriam-Webster’s breaks it down . . .
Physiology is “a branch of biology that deals with
the functions and activities of life or of living
matter (as organs, tissues, or cells) and of the
physical and chemical phenomena involved;” and
“the organic processes and phenomena of an
organism or any of its parts or of a particular
bodily process.”
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Sunday, February 27, 2011
Some relevant sources of
further information:
BMC Physiology: An open source refereed
journal — http://
www.biomedcentral.com/bmcphysiol/.
The National Library of Medicine: With a
human emphasis — http://
www.nlm.nih.gov/ser vices/anatomy.html.
physiologyINFO.org: A public ser vice of the
American Physiological Society — http://
www.physiologyinfo.org/.
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Sunday, February 27, 2011
The “Levels of Biological
Organization”
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Sunday, February 27, 2011
LEVEL 1 - Cells . . .
Are the basic unit of structure and function in living things.
They may serve a specific function within an organism.
Examples — blood cells, nerve cells, bone cells, etc.
LEVEL 2 - Tissues . . .
Are made up of cells that are similar in structure and function, and that work together to
perform a specific activity.
Examples — blood, nervous, bone, etc.
Humans have four basic tissue types: connective, epithelial, muscle, and ner ve.
LEVEL 3 - Organs . . .
Are made up of tissues that work together to perform a specific activity.
Examples — heart, brain, skin, kidney, etc.
LEVEL 4 - Organ Systems . . .
Are groups of t wo or more organs working together to perform a specific overall function.
Examples - circulatory system, nervous system, skeletal system, etc.
The Human body has about a dozen — circulatory, digestive, endocrine, excretory, immune,
integumentary, muscular, nervous, reproductive, respiratory, and skeletal.
LEVEL 5 - Organisms . . .
Are entire living things that carry out all basic life processes, i.e. taking in nutrients,
harnessing energy, releasing waste, growing, responding, reproducing, and evolving.
Multicellular organisms are made up of organ systems, but an organism may be made up of only
one cell, such as bacteria, archeaons, and protists.
Examples — E. coli, amoeba, mushroom, sunflower, human . . . . . . . . . .
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Sunday, February 27, 2011
Maybe this will
help?
http://www.youtube.com/watch?
v=Nmo5OcivpaI&feature=related
And, for fun,
check out the
levels of
organization
at . . .
http://micro.magnet.fsu.edu/primer/java/
scienceopticsu/powersof10/
Zooming in from “far, far away” all the way into
the atoms of an oak tree!
9
1
Sunday, February 27, 2011
Now, plant physiology.
Your text launches right into ‘higher’ plant
“Form and Function;” however, it is important
to remember that around half of the
atmospheric oxygen in the world comes from
phytoplankton! Therefore, it’s important to
understand a bit how that works, and that’s
the realm of ‘lower’ plant physiology. So,
we’ll very briefly cover a bit of that and then
launch into the chapter material.
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Sunday, February 27, 2011
Phytoplankton?
“Phytoplankton comprise 80% of all plant life on the
planet and are responsible for 40% – 80% of the world's
oxygen [depending on which author you read]. Perhaps you
thought that the rain forest or old growth forests were
the main contributors of the world's oxygen. Not so. In
fact old growth forests consume more oxygen than they
produce, and while trees do contribute to the world's
oxygen supply, their contribution is less than the
phytoplankton and other plants on the planet.”
From: http://www.kidscruz.com/NAT_PP.HTM and see
http://news.nationalgeographic.com/news/
2004/06/0607_040607_phytoplankton.html and
http://earthobservatory.nasa.gov/IOTD/view.php?id=6956.
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Sunday, February 27, 2011
These guys need light to live, and they float (or kinda,
sorta, swim) about in the top layers of aquatic systems.
“There are three main types of phytoplankton: Diatoms,
Dinoflagellates, and Cocolithophores. The diatoms are single cell,
yellow-green algae. They exist in single units or in long chains. They
have cell walls made of silica (glass plants). The Greek meaning for
their name is "cut in t wo." That is because under a microscope, you can
see that half of their cell wall fits over the other half. They are the
most abundant form of phytoplankton.”
“Dinoflagellates resemble both plants and animals. They have
cellulose cell walls that act like armor, but use flagella to swim.
There are t wo species of dinoflagellates that are responsible for red
tide (a deadly bloom of these organisms results in a huge release of
their toxins into the water, which kill fish and sicken humans).
Cocolithophores are the smallest of the phytoplankton and are made
of calcium carbonate.” [Add Cyanobacteria and micro and tiny green
algae to the list! SMT]
Also from http://www.kidscruz.com/NAT_PP.HTM
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Sunday, February 27, 2011
OK, how do they work?
Much of the phytoplankton are unicellular, some
are colonial, and a few are very small yet
multicellular. There are at least 5,000 species of
marine phytoplankton, and more are discovered
all the time (especially Cyanobacterial).
They serve as the base of most aquatic food
chains; and are absolutely essential to most life
on earth (hydrothermal vents are the exception).
They almost all use oxygenic photosynthesis, with
chlorophyll, pretty much the same as land plants
(except Cyanobacteria don’t have chloroplasts).
See http://planktonnet.awi.de/ for great pic’s.
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Sunday, February 27, 2011
So, let’s now move onto those
‘higher’ plants . . .
A vascular, flowering (angiosperm – 260,000 living
species classified in 453 families, http://tolweb.org/
Angiosperms/20646) plant’s body is divided into:
Vegetative – are the non reproductive parts;
Roots – are usually below ground to anchor and
absorb water and minerals, they depend on
“shoots” for their cellular energy requirements;
Stems and leaves – “shoots” – are the
aboveground portion of a plant that produces
carbohydrates through oxygenic photosynthesis;
Reproductive parts – are flowers, which become
fruits.
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Sunday, February 27, 2011
There are
t wo major
subgroups of
angiosperms.
The more
‘primitive’ is the
Monocotyledons
(monocots).
(http://
tolweb.org/
Monocotyledons
/20668)
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Sunday, February 27, 2011
And the
‘higher’ is
the . . .
Dicotyledons,
which is a
“polyphyletic”
group, of which
the Eudicots
(http://
tolweb.org/
eudicots/
20666), a true
clade, comprise
75% of all
angiosperm
species.
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Sunday, February 27, 2011
Angiosperms
have . . .
Stems:
Node – point at which
leaves attach;
Axillary buds at nodes;
Internode – stem area
bet ween nodes.
Leaves:
Blade – flattened portion;
Petiole – supporting
stalklike structure.
And . . .
Roots:
absorb water and
nutrients.
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Sunday, February 27, 2011
Specialized
plant stems:
Tendrils – support.
Stolon – grow
along soil surface
asexually forming
new plants at
nodes.
Rhizomes –
underground
stems that
produce new roots
and shoots.
Tubers – swollen
underground
stems for storage,
e.g. potatoes.
Tendril
Rhizome
Cactus bodies – photosynthesis and water storage (but
not cactus needles — they’re modified leaves).
Other thorns – often modified branches (stems) for
protection.
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Sunday, February 27, 2011
Types of leaves:
Simple leaves have undivided blades.
Compound leaves are divided into leaflets.
Only leaves have axillary buds at their bases.
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Sunday, February 27, 2011
Specialized plant leaves . . .
For example, onion bulbs – store
nutrients;
Cotyledons – embryonic leaves that
store carbohydrates;
Cactus spines – defense;
Flower sepal and petals;
Carnivorous plant’s leaves attract,
capture, and digest prey.
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Sunday, February 27, 2011
Types of
roots:
Taproot – primary
root enlarges to
form a major root
persisting
throughout the life
of the plant.
Fibrous – shortlived primary root
replaced by
branching roots at
the base of the
stem.
Sunday, February 27, 2011
21
Specialized roots:
Beet and carrot roots store starch;
Desert plants use roots to store water;
Roots in swamp plants may grow up into the air for
oxygen diffusion; plus roots . . .
Buttress and prop up plants to provide support.
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Sunday, February 27, 2011
And, as
‘they’ say,
“many
parts are
edible . . .”
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Sunday, February 27, 2011
Growth patterns
Shoots become larger by adding repeated nodes
and internodes.
Modular growth allows for extreme flexibility.
1) Determinate growth – plant stops growing
after reaching its mature size;
This is more common in herbaceous plants
(little or no woody tissue).
2) Indeterminate growth – plant grows
indefinitely persisting as long as environmental
conditions allow;
This is more common in woody plants.
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Sunday, February 27, 2011
Meristem is the . . .
Source of new plant cells. And are . . .
Regions of a plant undergoing active mitosis.
They are like stem cells in animals.
There are three types:
1) Apical – actively growing, near the tip of the root
and the shoot.
There are three primary meristems in the apical
region – protoderm, procambium, ground meristem.
2) Lateral – produce cells that thicken the stem or root
– they make secondary growth in woody plants.
3) Intercalary – bet ween the nodes of a mature stem,
and usually at the base of the internodes; it regrows
leaves from its base.
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Sunday, February 27, 2011
Parts of the meristem:
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Sunday, February 27, 2011
One more time: types of meristem:
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Sunday, February 27, 2011
Plant cells
build tissues.
Parenchyma – most
abundant cells in primary
plant body, functions include
respiration, photosynthesis,
and storage (a);
Collenchyma – elongated
living cells that can stretch
as the cell grows, provide
support (b);
Sclerenchyma – dead at
maturity, provide support
Fibers – strands (c);
and . . .
Sclerids – many shapes
(d).
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Sunday, February 27, 2011
Waterconducting
cells: the xylem
Transport water and
dissolved minerals.
Water-conducting
cells are elongated
and dead at
maturity.
Lignin-rich cell walls
provide support.
Tracheids – water
moves slowly from
one cell to the next.
Vessel element –
greater diameter lets
water pass faster.
Sunday, February 27, 2011
Parts of
the
xylem
29
Sucroseconducting cells:
the phloem
Transport dissolved
organic compounds.
The cells are alive at
maturity.
Strands of cytoplasm
pass though pores so
sugars can pass from cell
to cell.
Sieve tube element –
make up the sieve tube
along with sieve plates.
These cells carry on
metabolism but lack
nuclei!
Companion cells retain
their nucleus and help to
transfer carbohydrates
in and out of sieve tube
elements.
Sunday, February 27, 2011
Phloem
parts
30
The three
main
mature
tissue
types:
1) dermal,
2) ground,
and
3) vascular.
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Sunday, February 27, 2011
1) Dermal tissue . . .
Covers the plant. It consists of . . .
1) Epidermis derived from the protoderm.
In plants with secondary growth, lateral
meristems produce tissue that replaces the
epidermis in the stems and roots.
Epidermal cells are flat, transparent, and tightly
packed.
2) Cuticle – waxy layer that helps to conser ve water.
3) Stomata – pores for O2 and CO2 gas exchange.
4) Guard cells – control the opening and closing of
the stomata – also evolved for water
conser vation.
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Sunday, February 27, 2011
Here’s a look . . .
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Sunday, February 27, 2011
And even closer . . .
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Sunday, February 27, 2011
2) Ground tissue is the . . .
Majority of the primary plant
body.
It is mostly parenchyma cells
derived from ground meristem.
And is structurally unspecialized,
but has important sites for
photosynthesis, respiration, and
storage.
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Sunday, February 27, 2011
3) Vascular tissue is . . .
Specialized conducting tissue.
It transports water, minerals, carbohydrates,
and other dissolved compounds.
The xylem and phloem are derived from the
procambium. And the . . .
The vascular bundle is found in stems and
leaves, and consists of . . .
Xylem and phloem together with
parenchyma and sclerenchyma.
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Sunday, February 27, 2011
One more time . . .
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Sunday, February 27, 2011
And a great overview – check this out . . .
http://www.accessexcellence.org/RC/VL/GG/ecb/ecb_images/
Panel_21_01HigherPlants.pdf
All the parts laid out, with all those connections that are so important.
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Sunday, February 27, 2011
Tissues build stems, leaves, and roots.
Stems:
Grow and differentiate at their tips.
New cells originate at the apical meristem in
a terminal bud.
Daughter cells give rise to ground tissue,
epidermis, and primary xylem and phloem.
If a shoot loses its terminal bud, axillary
buds begin to divide and grow.
Vascular tissue is arranged in bundles.
Monocot stem has scattered bundles;
Eudicot stem has ring of bundles around pith.
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Sunday, February 27, 2011
The Monocot stem . . .
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Sunday, February 27, 2011
The Eudicot stem . . .
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Sunday, February 27, 2011
Leaves . . .
Originate as leaf primordia on flanks of the
apical meristem.
Leaf epidermis contains stomata.
Ground tissue called mesophyll is composed
mainly of parenchyma cells.
There are abundant chloroplasts.
Veins – vascular bundles:
Monocots have parallel veins, and . . .
Eudicots have netted veins.
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Sunday, February 27, 2011
Vein differences
Eudicot
Monocot
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Sunday, February 27, 2011
Let’s zoom in again . . .
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Sunday, February 27, 2011
Roots . . .
Apical meristem located just behind root tip.
The root cap protects the apical meristem.
The apical meristem produces cells that
differentiate into ground meristem,
protoderm, and procambium.
Root epidermis has root hairs.
The Casparian strip ensures that all
materials entering the vascular cylinder
pass through the cytoplasm of the
endodermal cells first.
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Sunday, February 27, 2011
And
zoom in
on a
root
tip . . .
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Sunday, February 27, 2011
Even closer into a root hair . . .
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Sunday, February 27, 2011
And
closer
still into
the
primary
root of a
Monocot.
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Sunday, February 27, 2011
And a of Eudicot . . .
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Sunday, February 27, 2011
Lateral meristems
There are powerful selective pressure for
tall plants. Why? Reach for sunlight!
Secondary growth allows for increased
girth of stems and roots – support tallness.
Wood and bark come from this secondary
growth.
Vascular cambium:
Internal cylinder of meristem tissue;
Produces most of the diameter of a woody
root or stem.
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Sunday, February 27, 2011
Here’s
the
picture.
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Sunday, February 27, 2011
Vascular cambium cells divide
producing t wo daughters . . .
One remains a meristem cell. The other, if . . .
It’s inside the cambium, becomes secondary
xylem - most growth - wood. The other, if . . .
It’s outside the cambium becomes secondary
phloem.
The vascular cambium also produces rays for
lateral water and nutrient transport; and . . .
Bark – all tissue outside the vascular
cambium.
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Sunday, February 27, 2011
Let’s
look at
wood a
little
more
closely.
Varies in hardness due to sclerenchyma fibers.
Heart wood is oldest secondary xylem, unable to conduct water.
Sapwood is for transport.
And it has growth rings in seasonal climates.
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Sunday, February 27, 2011
And to zoom in a bit . . .
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Sunday, February 27, 2011
So, how do plants get
the stuff they need
Feed me . . . feed me . . .
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Sunday, February 27, 2011
On to – soil and air provide plants
water and inorganic nutrients.
Autotrophic, but still require essential
nutrients, . . .
Which are chemicals required for metabolism,
growth, and reproduction.
At least 16 essential to all plants. Nine of the 16
are . . .
Macronutrients – needed in fairly large amounts –
Carbon (C), hydrogen (H), oxygen (O), phosphorus
(P), potassium (K), nitrogen (N), sulfur (S),
calcium (Ca), and magnesium (Mg).
Micronutrients are required in smaller amounts.
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Sunday, February 27, 2011
The macronutrients:
C, H, and O are the most abundant:
96% of dry weight of plant. But, N, P, & K are limiting.
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Sunday, February 27, 2011
Fertilizer numbers reflect the N:P:K ratio
N, P, and K are
often limiting
in the
environment
and,
therefore, are
common
ingredients in
fertilizer to
prevent or
treat
nutrient
deficiencies.
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Sunday, February 27, 2011
The micronutrients:
They’re less than half a percent of a plant’s dry weight.
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Sunday, February 27, 2011
All these nutrients come from soils, which develop
distinct layers. The layers are composed of . . .
Litter (layer of dead, decomposing leaves and t wigs, etc.)
lies on the soil surface.
Microorganisms release carbon from decaying litter as
CO2. However, . . .
Some carbon remains as humus, which is a . . .
Chemically complex, hard-to-digest, spongy organic
material.
Topsoil is the upper layer of soil. It’s also known as the . . .
“A” horizon. Most humus is located there.
Less organic matter is found in the “B” horizon, but roots
are still present.
The “C” horizon is just above bedrock, and is quite
inorganic.
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Sunday, February 27, 2011
Layers
of soil
Like most
things in
biology –
it’s a
gradation
with no
clear
start and
stop.
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Sunday, February 27, 2011
What comes from where?
Plants obtain C, H, and O from water and
the atmosphere.
Water enters through the roots.
Carbon and oxygen atoms come from the
atmosphere as CO2 gas (but plants need O2
as well, which they get from the
atmosphere).
Plants use water and CO2 to produce glucose
through photosynthesis.
Roots take up all other the other required
elements from the soil.
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Sunday, February 27, 2011
However, . . .
Nitrogen availability often
limits plant growth.
N2 is 78% of the atmosphere, but it’s
chemically unavailable to plants.
Several types of bacteria use nitrogenfixation to convert N2 into usable forms.
Rhizobium lives in legumes’ nodules.
Carnivorous plants obtain nitrogen from
the insects they consume.
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Sunday, February 27, 2011
Check out the carnivorous plants video.
http://www.youtube.com/watch?v=KYGwgzehf6c
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Sunday, February 27, 2011
Who cares about nitrogen? All of life! It’s a part of
almost all biological molecules. So, the assimilation
of nitrogen into organic compounds matters.
like Rhizobium
Ammonia is incorporated into organic compounds by all organisms.
Some bacteria are capable of converting atmospheric nitrogen to ammonia,
and most bacteria, fungi, and plants can utilize nitrate from soil.
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Sunday, February 27, 2011
Nitrogen fixation?
Many species would die without this.
It converts atmospheric nitrogen gas (N2) to
ammonia (NH3) “fixing” nitrogen for the ecosphere.
Rhizobium live in legume (e.g. peas, soybeans, beans,
alfalfa) root nodules in a symbiotic relationship.
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Sunday, February 27, 2011
Nitrogen fixation in legumes
A good
infection –
Rhizobium in
Leguminosae.
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Sunday, February 27, 2011
Nitrogen-fixing bacteria-legume symbiosis
Rhizobium are able to
fix N2 alone only under
microaerophilic
conditions (too much O2
inhibits nitrogenases).
In the nodule, O2 levels
are kept low by the O2
binding protein
leghemoglobin.
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Sunday, February 27, 2011
90% of leguminous
plants can undergo
nodulation, but the
legume-rhizobium
symbiosis is speciesspecific.
Nitrogen-fixing Rhizobium, cont.
A number of nod
genes are required
for nodulation.
The nod genes
control speciesspecific nodulation
in rhizobia.
The nif genes are
required for
nitrogen fixation.
They are often
found on plasmids.
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Sunday, February 27, 2011
Getting stuff around – transport
A plant may use 200-1000 liters of water to produce one
kg of tissue in one growing season; a large temperate,
hardwood tree could use 70 gallons a day during the
growing season!
Plant cells are mostly water. It is needed for metabolism –
hydrolysis and photosynthesis directly use it. Plus . . .
Turgor pressure helps to keep plants upright. And . . .
Leaf mesophyll cells must remain moist for CO2 diffusion.
But this accounts for only a tiny bit of the water a plant
goes through – the rest simply evaporates.
Xylem and phloem from a continuous plumbing system
throughout plant:
Phloem distributes the products of photosynthesis. And . . .
Xylem transport xylem sap – water and dissolved minerals.
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Sunday, February 27, 2011
Check out the flows.
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Sunday, February 27, 2011
Part of the BBC series “The
Private Life of Plants.”
http://www.youtube.com/watch?v=J1PqUB7Tu3Y
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Sunday, February 27, 2011
Transpiration – evaporation of
water from a leaf (relates to xylem)
Heat causes water to evaporate. Which . . .
Helps to cool the leaf. But this establishes a . . .
Concentration gradient and water vapor diffuses out of
the open stomata, to try to equalize the gradient.
Any environmental factor that increases evaporation
increases transpiration, e.g. . . .
Low humidity, high wind, or temperatures; although
stomata do close down when conditions get too
extreme.
The water that evaporates is replaced by water drawn
up through the stem – if there is enough available.
Plants wilt and eventually die if water cannot be
replaced.
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Sunday, February 27, 2011
Cohesion-tension theory
Since all the cells of the xylem are dead at maturity,
they can’t ‘power’ up the water from the roots to the
leaves – so how’s it get up the stem?
Remember water’s physical properties:
Cohesion — water molecules cling to each other.
Water molecules are pulled toward the leaf,
because . . .
The system is under tension (negative pressure).
Water in xylem forms a continuous hydraulic system
throughout the plant body.
Adhesion helps to counter gravity — water sticks to
the walls of xylem tubes with hydrogen bonds.
This is called — capillary action.
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Sunday, February 27, 2011
Water is
‘pulled’ up
the tree.
Transpiration
creates the
tension.
The physical
properties of
water allow it
to ‘climb’ the
xylem.
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Sunday, February 27, 2011
Here’s a nice transpiration animation.
http://www.youtube.com/watch?v=At1BJJDcXhk
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Sunday, February 27, 2011
Additional water enters from the roots
as evaporation pulls water up the stem.
The root epidermis is fringed with root hairs that
dramatically increase available surface area for
water and mineral absorption.
Mycorhizzal fungi increase surface area even more.
There’s usually less solute concentration outside the
plant than inside, so water enters roots by osmosis.
It can take t wo different pathways:
Extracellular – moves in spaces bet ween and along
cell walls; versus . . .
Intracellular – moves from cell to cell via
plasmodesmata (remember them).
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Sunday, February 27, 2011
Eventually water and minerals
contact the endodermis, the
innermost layer of the cortex.
And the impermeable Casparian strip
forces water to enter cells at this point.
Ion-channel transport proteins in the
endodermal cells only admit certain ions.
Water and dissolved minerals enter
xylem after this for transport to all the
tissues of the plant.
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Sunday, February 27, 2011
Two routes for water in the roots
The Casparian strip ensures that it all goes through the
living endodermis to get to the xylem.
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Sunday, February 27, 2011
Water conser vation!
The waxy cuticle is an important watersaving adaptation in land plants. It is . . .
Impermeable to water and gases.
Stomata permit the leaf to exchange gases.
Guard cells border and control stomata.
Plentiful water — K+ enters guard cells,
water follows, guard cells swell and open.
Drought — abscisic acid triggers loss of K+,
guard cells collapse and stomata close.
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Sunday, February 27, 2011
They’re
pretty
cool little
organs.
Guard cells
actively
pump K+ ions
into
themselves
when water
is abundant.
Hormonal
signals
stimulate K+
to leave
when water
is scarce.
Sunday, February 27, 2011
81
However, whenever
stomata are closed . . .
Plants can’t get any CO2 for
photosynthesis (or O2 for respiration), nor
get rid of 02 waste products.
Most plants close their stomata at night
when photosynthesis can’t occur anyway,
to conserve water.
But CAM desert plants open theirs at night,
then store the CO2 until the next day when
they can photosynthesize behind closed
stomata during the day. Neat trick!
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Sunday, February 27, 2011
YouTube has a decent lesson . . .
http://www.youtube.com/watch?v=clw_OcDX5lI
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Sunday, February 27, 2011
But what about the sugar and
other organics? Phloem transport.
Remember – phloem is made of live cells –
sieve tube elements and perforated sieve
tube plates.
Phloem sap – dissolved organic compounds
are carried in the phloem. This includes . . .
Carbohydrates, amino acids, hormones,
enzymes, mRNAs . . . .
An aside: aphids can har vest phloem sap
without triggering wound response –
scientists collect “honeydew” to study
phloem.
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Sunday, February 27, 2011
A
direct
hit.
Aphids can
collect
phloem sap
without
the plants
‘knowing’
it. We can
use that
trick.
Sunday, February 27, 2011
85
Pressure flow theory
Phloem sap moves under positive pressure from “sources”
to “sinks.” This is the opposite of xylem flow! But it’s still
driven by an osmotic gradient.
Source – produces or releases sugars (makes an increased
concentration of sugar there);
Sink – any plant part that does not photosynthesize (as
sugar gets used up by respiration, its concentration falls).
Companion cells load sucrose into sieve tube elements by
active transport (this requires energy, i.e. ATP).
Water moves by osmosis out of the xylem and into the
phloem sap, because of this sugar concentration gradient.
Resulting increased turgor pressure drives phloem sap
through sieve tube elements (pumps it throughout the
plant).
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Sunday, February 27, 2011
Plant organs may be a sink or
a source, depending on needs.
A sink takes up compounds through facilitated
diffusion or active transport.
Water moves by osmosis out of the phloem and
into the xylem, depending on the gradient.
This relieves pressure.
Any particular organ may be sink or source,
e.g. . . .
A potato tuber is a . . .
Sink when storing starch; but a . . .
Source when releasing starch for growth.
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Here’s an illustration.
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Sunday, February 27, 2011
So that’s how plants work and
can keep living, once they get
going . . .
But what about their reproduction and
development?
That’s what we’ll cover in the last lecture of
this section — the way plants have
succeeded as well as they have on the earth
all these eons — through reproductive
success! And then after that we’ll take a
break and watch some of The Botany of
Desire by Michael Pollan as presented by PBS.
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Sunday, February 27, 2011