Photosynthesis

• Plants use water and carbon dioxide from the air to synthesize glucose (葡萄糖 ) and other carbohydrates (醣類 ). The synthesis of organic compounds begins in the chloroplast (葉綠體 ) with photosynthesis.

• The chloroplasts capture the energy from light and fabricate (製造 ) glucose molecules from carbon dioxide from the air and water from the soil.

• The implementation of the electron transport system (電子傳遞鏈 ) provides energy for organic synthesis in the form of ATP (腺嘌令核苷三磷酸 ) and the reduced coenzyme (輔脢 ) NADPH (菸鹼醯胺腺嘌呤二核苷酸磷酸 ).

• Along with the carbon dioxide and water, this energy can be used in the Calvin Cycle (卡爾文循環 ) to produce glucose.

glucose (葡萄糖 )

• Glucose is a carbohydrate, and is the most important simple sugar (單糖 ) in human metabolism (代謝作用 ).

• Glucose is also sometimes called dextrose (右旋葡萄糖 ). Corn syrup is primarily glucose.

• Glucose is one of the primary molecules which serve as energy sources (能源 ) for plants and animals. It is found in the sap (汁液 ) of plants, and is found in the human bloodstream where it is referred to as “blood sugar“ (血糖 ).

• The normal concentration of glucose in the blood is about 0.1%, but it becomes much higher in persons suffering from diabetes.

• Energy from glucose is obtained from the oxidation (氧化 ) reaction:

C6H12O6 + 6O2 --> 6CO2 + 6H2O

• When oxidized in the body, glucose produces carbon dioxide, water, and some nitrogen (氮 ) compounds and in the process provides energy which can be used by the cells.

• As a primary energy source in the body, it requires no digestion and is often provided intravenously (靜脈注射 ) to persons in hospitals as a nutrient.

• In living organisms, the oxidation of glucose contributes to a series of complex biochemical reactions which provides the energy needed by cells. The first step in the breakdown of glucose in all cells is glycolysis (糖解 ), producing pyruvate (丙酮酸 ) which is the starting point for all other processes in cellular respiration (呼吸作用 ).

• In cells where oxygen is present (aerobic respiration，有氧呼吸 ) these processes have been modeled in the TCA or Krebs cycle (檸檬酸或克列伯循環 ). A major part of the use of the energy from glucose oxidation is the conversion of ADP to ATP, with the energy-rich molecule ATP being subsequently used as the energy currency of the cell.

Fructose (果糖 )

• Fructose is a carbohydrate, and is a simple sugar found in fruits. It is often called just "fruit sugar".

• Fructose is also found in honey. It is classified as the sweetest of all the sugars.

Comparison of Glucose and Fructose

• Glucose and fructose are the two most important simple sugars for human consumption. They have the same molecular formula, C6H12O6, but have different structures，hence, an isomer (同分異構物 ).

• Glucose being classified as an aldehyde (醛糖 ) and fructose as a ketone (酮糖 ).

Cellulose (纖維素 )

• Cellulose is a form of carbohydrate in which some 1500 glucose rings chain together. It is the chief constituent of cell walls (細胞壁 ) in living organisms. Wood (木材 ) is mostly cellulose, making cellulose the most abundant type of organic compound on the Earth.

• Even though human digestion cannot break down cellulose for use as a food, animals such as cattle and termites (白蟻 ) rely on the energy content of cellulose. They have protozoa (單細胞原生動物 ) and bacteria (細菌 ) with the necessary enzymes (酵素 ) in their digestive (消化 ) systems. Cellulose in the human diet is needed for fiber (纖維 ).

Comparison of Starch and Cellulose

• Both starches and cellulose are carbohydrates which are classified as polysaccharides (多醣類 ) since they are composed of chains of glucose molecules. While they are similar, starches can be used as energy sources by the human body while cellulose cannot.

• Enzymes are important in the metabolism of foods, and these enzymes are very specific. They are somewhat like keys which will fit the geometry (幾何結構 ) of the starch bonds, but not those of the cellulose bonds.

Calvin Cycle (卡爾文循環 )

• Carbon dioxide is captured in a cycle of reactions known as the Calvin cycle or the Calvin-Benson cycle after its discoverers. It is also known as just the C3 cycle. Those plants that utilize just the Calvin cycle for carbon fixation are known as C3 plants.

• The fact that this 3-carbon molecule is the first stable (穩定的 ) product of photosynthesis leads to the practice of calling this cycle the C3 cycle.

C3 plants

• In C3 plants the photosynthesis, carbon fixation and Calvin cycle all occur in a single chloroplast.

• About 85% of plant species are C3 plants. They include the cereal grains: wheat, rice, barley, oats. peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C3 plants. Most lawn grasses such as rye and fescue are C3 plants.

C4 plants

• In C4 plants the photosynthesis takes place in a chloroplast of a thin-walled mesophyll (葉肉 ) cell and a 4-carbon acid (malic acid，蘋果酸 ) is handed off to a thick-walled bundle sheath (維管束鞘 ) cell where the Calvin cycle occurs in a chloroplast of that second cell. This protects the Calvin cycle from the effects of photorespiration (光呼吸 ).

• Only about 0.4% of the 260,000 known species of plants are C4 plants. But that small percentage includes the important food crops corn, sorghum (高粱 ), sugarcane and millet (小米 ).

CAM (景天酸 ) plants

• In CAM (Crassulacean Acid Metabolism) plants the photosynthesis and initial carbon fixation occur at night and a 4-carbon acid is stored in the cell‘s vacuole (液泡 ). During the day, the Calvin cycle operates in the same chloroplasts.

• They represent about 10% of the plant species and include cacti (仙人掌 ), orchids (蘭花 ), maternity plant (落地生根 ), wax plant (球蘭 ), pineapple (鳳梨 ), Spanish moss (松蘿菠蘿 ), and some ferns (蕨類 ). The only agriculturally significant CAM plants are the pineapple and an Agave (龍舌蘭 ) species used to make tequila (龍舌蘭酒 ) and as a source of fiber.