Photosynthesis Chloroplasts in Elodea 1250 x Lessons From

Photosynthesis Chloroplasts in Elodea, 1250 x.

Lessons From Thin Air • In 1995, filmmakers from the Harvard Smithsonian Center for Astrophysics approached some new graduates from Harvard and the Massachusetts Institute of Technology. • One of the questions they posed: “Here’s a seed. Imagine I planted that seed in the ground, and a tree grew. …Where did all that weight come from? ” “The President, ” a giant 3, 200 year-old sequoia in California with a height of 247 feet. 2

Autotrophs • Plants, algae, and some bacteria have the ability to use sunlight to generate their own storage molecules of energy. • They are photoautotrophs. – From the Greek photo- meaning “light, ” autos- meaning “self” and -troph meaning “nutrition. ”

• The basic knowledge that plants need water, sunlight, and soil has been around since ancient times. – The working hypothesis was that plants grew by “eating” soil through their roots. • Jean Baptiste von Helmont wanted to verify this – to isolate the exact source of the increased mass of trees as they grew. 4

The Willow Tree Experiment • Von Helmont planted a 5 pound willow tree in 200 pounds of dry, potted soil. • He massed the tree, grew it for 5 years, then massed it and the soil again. • The tree gained over 160 pounds of mass. – The soil only decreased in mass by about 2 ounces. 5

Matter • According to the law of conservation of mass, mass cannot be created nor destroyed. It only changes form. – Where then, does the dry mass of wood come from?

• Part of the mystery of how plants worked was due to a lack of understanding of air. • For many centuries, scientists believed that air was a pure, elementary substance. – This stemmed from the Greek philosopher Aristotle’s idea of all matter being composed of four elements: earth, fire, wind, and water.

Priestley’s Experiment • Joseph Priestley, a British chemist, believed that air was not a single “elementary substance”, but a “composition” of gases. • During one experiment, he discovered that a candle placed in a sealed jar would extinguish very quickly. – He called the air “injured, ” because it was unable to support fire. • Placing a mouse in the jar would have a similar effect, and it would die. – Priestley had discovered oxygen! 8

• Priestley’s hypothesis about the composition of air was correct. • Air is primarily made of: – – – 78% nitrogen 21% oxygen 0. 9% argon 0. 03% carbon dioxide 78% 21% 0. 02% water 9

Ingenhousz’s Experiment • A Dutch physician, Jan Ingenhousz, decided to repeat Priestley’s experiment, with a few changes. • Two sealed containers were studied – one exposed to sunlight, the other left in the shade. – The sunlit container showed an increase in oxygen compared to the shaded one. – Plants must be releasing oxygen! 10

Photosynthesis • The final piece of the puzzle was solved by a Swiss botanist named Nicolas de Saussure. – He enclosed the plants in a sealed container of carbon dioxide and compared the air and plant masses before and after growth. • Based on these measurements, he concluded that plants were primarily composed of: – Water from the soil. – Carbon from the air. • The process was called photosynthesis, because light (“photo”) was required to make (“synthesize”) the plant tissues. 11

• The concept of photosynthesis difficult to understand, because the idea of producing a heavy plant from air and water is not intuitive. – Converting the matter in air and water to plants requires a great deal of energy.

Energy • There are many different forms of energy in the universe. • Potential energy is stored energy. It exists as a result of position or chemical structure. Position. Chemical Structure. • Kinetic energy is in motion. 13

Electromagnetic Spectrum • Kinetic energy travels in the form of waves. Each type of energy has its own wavelength. 14

Energy and Life • Most of the energy that supports life on Earth originates from the sun. • Solar radiation primarily contains three ranges of wavelengths of energy: – Visible light, which we are able to detect with our eyes. – Ultraviolet, which has a shorter wavelength than visible light and is able to penetrate living tissue. – Infrared, or heat, which has a longer wavelength than visible light. 15

Photosynthesis • Plants have the ability to harness some wavelengths of visible light to convert matter in the air to that of their own tissues. • The primary location of photosynthesis is in leaves. – Cells within leaves have a high concentration of chloroplasts, each of which contains a green pigment called chlorophyll.

Endosymbiosis • The evolutionary origin of chloroplasts is believed to be similar to that of mitochondria. – Like mitochondria, chloroplasts have their own independently-replicating circle of DNA. • A eukaryotic cell (one that already had mitochondria) engulfed a prokaryote called cyanobacteria. – Cyanobacteria were the first photosynthetic cells, and still exist today.

• The complete chemical reaction of photosynthesis is summarized like this: • Carbon dioxide and water are used to synthesize glucose, which can then be used to produce starch or cellulose. – Oxygen is produced as a waste product.

Light Reactions • The first part of photosynthesis is called the lightdependent reactions, because they can only occur when sunlight is available.

• A photon of light strikes an arrangement of chlorophyll called photosystem II, causing the excitement of electrons and the splitting of water.

• The electron is passed to photosystem I, where it is re-energized by another photon and used to generate an electron carrier called NADPH. – The photosystems are unfortunately named based on the order they were discovered, not the actual order in photosynthesis.

• The chlorophyll found in photosystem II and I is referred to as P 680 and P 700, because it responds best to light at wavelengths of 680 and 700 nanometers (red).

• The H+ ions produced in the Photosystem II are used to generate ATP through an electron transport chain and chemiosmosis, similar to what happens during cell respiration.

Light Independent Reactions • The molecules of ATP and NADPH are then utilized by the light-independent reactions to generate glucose (C 6 H 12 O 6). • A source of carbon is required to generate this molecule, so the plant must breathe in carbon dioxide (CO 2) through tiny pores in its leaves called stomata.

• During the Calvin cycle, 6 molecules of carbon dioxide are used to synthesize 2 molecules of a 3 -carbon sugar called glyceraldehyde-3 phosphate or G 3 P. – The two molecules of G 3 P are joined to make glucose. • The energy within ATP and NADPH from the light reactions power the Calvin cycle.

• The enzyme that attaches the molecule of carbon dioxide to the 5 carbon molecule Ru. BP is called rubisco. – Rubisco is sometimes referred to as the “bridge” or “gateway” to life, because carbon dioxide is considered a lifeless molecule, but Ru. BP is organic and part of the plant’s cellular processes.

Color • Plants have multiple pigments in their leaves besides chlorophyll to help absorb the energy from visible light. • Collectively, these pigments are able to absorb most wavelengths of visible light.

• The only wavelengths that plants do not utilize are green. – Plants are green because that light is reflected.

Photorespiration • One of the biggest problems in plants is dehydration – water loss through the stomata of leaves. • On hot, dry days, plants close their stomata to conserve water. – This limits photosynthesis as CO 2 levels decline and O 2 levels build up. • This causes photorespiration, where O 2 is added to the Calvin cycle instead of CO 2. – Photorespiration is considered wasteful, because no ATP or sugar is produced by this process, while a lot of the carbon compounds in the plant are consumed.

Plant Adaptations • C 4 plants reduce this problem by moving the carbon fixation process to the mesophyll, a different layer of tissue closer to the surface of the leaf. – Since oxygen does not build up in the chloroplasts, photorespiration is avoided.

• CAM plants only open their stomata at night, when the air is cooler. – CO 2 is absorbed and stored, then released into the Calvin cycle gradually throughout the next day.

Products of Photosynthesis • The end result of the glucose produced by photosynthesis depends on the needs of the plant. • If a plant needs to grow, the glucose will be used to generate the polysaccharide cellulose. • Excess glucose can be stored as starch to be used at a later time. – Potatoes and other plants have specialized storage organelles called amyloplasts to produce and store starch.

• In addition to chloroplasts, plant cells also contain mitochondria and do perform cell respiration. – This allows them to process glucose into ATP when the plant cells are in need of chemical energy.