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 self-

sustaining chemical reactions• So we want to know which chemical compounds and

reactions are involved in the plant products we study2. 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_reports/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.