Download Introduction to Plant Products and Human Affairs

Introduction to Plant Products
and Human Affairs
How Do We Use Plants?
•
•
•
•
Food and fuel
Shelter
Medicine
Pleasure
Food
• Plants convert carbon dioxide and
water into sugar, using energy from
sunlight.
• CO2 + H2O  C6H12O6 (sugar)
– The chemical bonds in the sugar
molecules store energy
• Sugar is our primary food source—
everything else is made from it. To
get energy, we reverse the reaction
above:
– C6H12O6 (sugar)  CO2 + H2O
– This is the same process as burning
fuel, but our bodies do it in a slower
and more controlled way.
– Plants also use the sugar they make
as food.
More Food
• Plants store sugar as starch, to help
survive the winter and to help new
seedlings get started. We take
advantage of this process to get our
food.
– About 80% of the human diet is
starch
– To use starch, our bodies convert it
back into sugar
• Starch mostly comes from:
– the seeds of cereal grains like maize
(corn), wheat, and rice
– The underground storage organs of
potatoes, yams, and cassavas
• Starch molecules are composed of
many sugars joined together.
Protein
• Proteins are an essential part of all living
things.
• A fundamental difference between plants and
animals: our bodies are mostly protein, and
plants are mostly carbohydrate.
• Carbohydrates are made of carbon, oxygen,
and hydrogen, but proteins are 1/3 nitrogen
(as well as C, H, and O).
• Nitrogen gas in the air must be “fixed” before
it can be used. This process is difficult and
energy-intensive.
– Bacteria in the root nodules of legumes
perform nitrogen fixation.
• We eat legumes (things like beans and peas)
because they produce lots more protein than
other plants.
Root nodules containing
nitrogen-fixing bacteria
Plant-derived Materials
• Wood, fiber, paper, rubber
• Plants live on land. They need
sunlight and carbon dioxide (above
ground), plus water and minerals
(below ground) to photosynthesize.
• The way to satisfy these needs: a
vascular system to transport water
up from the roots and sugar from
the leaves to everywhere else.
– With mechanical support to hold the
plant upright
• Wood and fibers are derived from
the vascular tissue.
• Cellulose is also composed of sugar
molecules (similar to starch)
Medicine
• Plants have to survive whatever
predators attack them—they can’t
hide or run away.
• A major form of defense is
chemical: many plants produce
chemical compounds that are
poisonous.
• We have found that in small
amounts, many of these poisons
can be used as medicinal drugs.
– Also, recreational drugs and spices.
– Things that are poisonous to
organisms like bacteria or fungi or
insects are often useful for humans.
Estrogen-like
molecules produced
by plants
Pleasure
• Most plants rely on animals
for reproduction: the
animals spread the pollen
from one plant to another,
and also disperse seeds to
new locations.
• Plants need to attract
pollinators, and we often
benefit: perfumes,
ornamental flowers, fruits,
dyes.
Principles of Biology
Principles of Biology
• Biology is based on two fundamental
principles. Throughout this class we will
constantly be referring to these principals.
1. Life is applied chemistry; life is a set of selfsustaining chemical reactions
•
So we want to know which chemical compounds and
reactions are involved in the plant products we study
2. Life evolves by natural selection
•
So we often ask what advantage some useful trait
confers on the plant.
1. Chemistry
• This is the physical reality of life:
what we are made of, the
mechanisms used in all living
processes.
• All living things are composed of
atoms
– Mostly carbon, hydrogen, oxygen, and
nitrogen.
– About 20 others used in lesser
amounts
• Atoms consist of a nucleus
containing protons and neutrons,
and a cloud of electrons surrounding
the nucleus
– The number of protons determines
which element an atom is
Molecules
• Atoms usually combine together
into molecules, such as carbon
dioxide, water, and glucose.
• Atoms are connected together by
chemical bonds, which are shared
electrons.
– All chemical reactions involve the
movement of electrons
– Each type of atom forms a
characteristic number of bonds:
carbon has 4, nitrogen has 3,
oxygen has 2, hydrogen has 1
• Chemical reactions are used to
rearrange the atoms in molecules
to create different molecules.
Macromolecules
• Atoms can be combined into very large
molecules, called macromolecules.
• Macromolecules are built up from
smaller subunits:
– Carbohydrates are long chains
(sometimes branched) of sugars.
– DNA molecules are millions of
nucleotides strung together.
– Proteins are chains of 20 different types
of amino acid. Several hundred amino
acids in a chain for a typical protein.
Energy
• Energy is needed to do any form of
work.
• Thermodynamics: energy is neither
created nor destroyed, it just changes
form. But, disorder (entropy)
increases.
– In practice, useful forms of energy get
converted into waste heat, which can’t
be used for anything.
• Chemical bonds contain energy:
different amounts in different
molecules. Most chemical reactions
either need energy added or they
release energy.
– Thus, converting carbon dioxide and
water into sugar needs energy from
sunlight added, and metabolizing
sugar back into carbon dioxide and
water releases energy.
Enzymes
• Chemical reactions in living cells happen because of
catalysts called enzymes.
– Technically, enzymes just greatly speed up the reactions,
which otherwise would occur at an impossibly slow rate.
• Each different reaction in the cell uses a different
enzyme. Thousands of different enzymes in a typical
cell.
Enzymes and Proteins
• Enzymes are made of proteins
– Enzymes catalyze all chemical reactions in the cell
– Proteins are macromolecules made from chains of
amino acids
• Enzymes are used to create the amino acid
subunits and then assemble them into the
proteins which make up the enzymes. This is
the basic self-sustaining set of chemical
reactions that is life.
DNA and Proteins
• The sequence of amino acids in
each protein determines its
function.
• Information about the sequence is
stored in DNA.
– DNA is a very stable molecule that
can be easily copied so its
information is passed to future
generations.
• Each gene is a short region of DNA
that codes for a particular protein.
2. Evolution
• Evolution by natural selection is
one of the cornerstones of
biology: nothing makes much
sense without it.
– Unfortunately, the concept has been
caught up in a religious and political
fight.
– This is a science class, so we are only
going to teach the scientific view.
You are welcome to believe
whatever you like outside of class.
Natural Selection
• The basic concept is very simple: if
one organism is better able to
survive and produce than another
organism, there will be more
descendants of the first organism
than the second after a few
generations.
– The ability to survive and reproduce
is called evolutionary fitness.
– Having greater fitness needs to be
inherited, so the descendants also
have greater fitness.
– It’s like compound interest: a little
bit better to start leads to a much
greater result after a long time has
passed.
• We can say that a given trait confers
a selective advantage if it increases
the fitness of an individual.
Artificial Selection
• Artificial selection is when
humans decide which
organisms will survive and
reproduce.
– As opposed to natural
selection, when only natural
forces affect reproduction
– The point is, artificial and
natural selection work the
same way, by allowing some
individuals to reproduce more
than others.
• Most of plant (and animal)
breeding uses artificial
selection: we choose the best
individuals to be parents for
the next generation.
Genetic Variation
• For natural selection to work, there
needs to be genetic variation within a
species, so that some variants will be
more fit than others.
• Most species have a lot of naturally
occurring variation: differences in their
DNA due to random mutations.
– Some mutations are quite small: changing
a single DNA base, for example.
– Others are quite large: genes fused
together or split in two, for example.
Entirely new traits can arise from these
changes.
Simple base
change mutation
Effects of Genetic Variation
• Most mutations have little or no effect on
the organism.
– More than 95% of the DNA in a typical
organism is not involved in making proteins
or anything else needed for life (as far as we
know).
– Even mutations within genes often have little
effect: proteins work quite well with minor
variations in their amino acid sequence
• Many mutations that cause minor changes
have no immediate effect on fitness, but
when external conditions change, they
become important.
– Called pre-adaptive characteristics
– For example, a mutation that makes an
enzyme work better at low temperatures
might become very important if the climate
cools off during an Ice Age.
• Important point: fitness depends on the
environment you find yourself in.
Randomness
• A basic principle of evolution is that
genetic change is random but
natural selection causes mutations
with greater fitness to increase.
– Lots of new mutations occur, but
only a few persist due selective
advantage.
• This is opposed to the false idea that
changes that occur within the body
of an individual affect future
generations.
– Reading a lot won’t make your
children smarter, and exercising a lot
won’t make them stronger
– This false idea is called “inheritance
of acquired characteristics”, and is
often associated with Jean-Baptiste
Lamarck, an otherwise excellent
scientist from the 1700’s.
Exchanging DNA
•
•
•
Evolution would be a very slow process if it had to
depend on mutations alone. It is much faster to
pick up a new useful trait by getting whole genes or
groups of genes from another organism.
Bacteria and other lower organisms trade random
pieces of DNA frequently, often between very
different species. This is called horizontal gene
transfer, and it is very common.
Higher organisms (eukaryotes, including plants and
animals) have a regular sexual process that mixes
the DNA from two different individuals to produce
offspring that are genetically different from either
parent.
– This allows several good traits to be combined in a single
individual
– Also, some unlucky individuals get several bad traits:
they then fail to reproduce, which removes the bad traits
from the population.
Animation!
http://www.nsf.gov/news/special_r
eports/fibr/gt_horizon.htm
Diploidy
• Most lower organisms have only 1 copy of
each gene: this is called haploid.
• The sexual process in eukaryotes means that
at least for a short time, two copies of each
gene are present: one from each parent. This
is called diploid.
– Simple eukaryotes (like yeast) quickly go back
to the haploid state.
• But, being diploid has a real advantage: you
have a backup copy of every gene, so if one
copy gets inactivated (by random mutation),
there is a second copy to take over.
• This seems to work very well: almost all large
organisms (anything you can see) are diploid,
including all plants more complicated than
mosses as well as most animals.