Introduction to Plant Products and Human Affairs

1. Introduction to Plant Products and Human Affairs

2. How Do We Use Plants? • Food and fuel • Shelter • Medicine • Pleasure

3. 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.

4. 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.

5. 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

6. 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)

7. 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

8. 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.

9. Principles of Biology

10. Principles of Biology • Biology is based on two fundamental principles. Throughout this class we will constantly be referring to these principals. • 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 study • Life evolves by natural selection • So we often ask what advantage some useful trait confers on the plant.

11. 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

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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

22. 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.

23. 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.

24. 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

25. 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.