Photorespiration and C4 Plants All plants carry on photosynthesis by

1. 55 • Photorespiration and C4 Plants • All plants carry on photosynthesis by • adding carbon dioxide (CO2) to a phosphorylated 5-carbon sugar called ribulose bisphosphate. • This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RUBISCO). • The resulting 6-carbon compound breaks down into two molecules of 3-phosphoglyceric acid (PGA). • These 3-carbon molecules serve as the starting material for the synthesis of glucose and other food molecules. • The process is called the Calvin cycle and the pathway is called the C3 pathway.

2. C4 Plants : the Calvin cylcle is confined to a bundle of sheathe cells. • Over 8000 species of angiosperms, scattered among 18 different families, have developed adaptations which minimize the losses to photorespiration. • They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C4 plants. (Plants that have only the Calvin cycle are thus C3 plants.) • Some C4 plants — called CAM plants — separate their C3 and C4 cycles by time. CAM plants are discussed below. • Other C4 plants have structural changes in their leaf anatomy so that • their C4 and C3 pathways are separated in different parts of the leaf with • RUBISCO sequestered where the CO2 level is high; the O2 level low. • These adaptations are described now. • The details of the C4 cycle • After entering through stomata, CO2 diffuses into a mesophyll cell. • Being close to the leaf surface, these cells are exposed to high levels of O2, but • have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle). • Instead the CO2 is inserted into a 3-carbon compound (C3) called phosphoenolpyruvic acid (PEP) forming • the 4-carbon compound oxaloacetic acid (C4). • Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is • transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells • are deep in the leaf so atmospheric oxygen cannot diffuse easily to them; • often have thylakoids with reduced photosystem II complexes (the one that produces O2). • Both of these features keep oxygen levels low. • Here the 4-carbon compound is broken down into • carbon dioxide, which enters the Calvin cycle to form sugars and starch. • pyruvic acid (C3), which is transported back to a mesophyll cell where it is converted back into PEP.

3. area of detail • It is generated by a stimulus. • Na+ enters, and cell becomes positively charged. • K+ leaves, and area of positive charge moves. • 56 An action potential is a moving electrical impulse.

4. 57 gallbladder • The adult human gallbladder stores about 50 millilitres (1.8 imp fl oz; 1.7 US fl oz) of bile, which is released when food containing fat enters the digestive tract, stimulating the secretion of cholecystokinin (CCK). The bile, produced in the liver, emulsifies fats in partly digested food. • After being stored in the gallbladder, the bile becomes more concentrated than when it left the liver, increasing its potency and intensifying its effect on fats.

5. 58 stomach • The stomach is a muscularorgan of the digestive tract. It is located between the esophagus and the small intestine. The stomach is hollow and sac-shaped. It is involved in the second phase of digestion, following mastication (chewing).The stomach produces protease enzymes and hydrochloric acid which kills bacteria and gives the right pH for the protease enzyme to work. The word stomach is derived from the Latinstomachus which is derived from the Greek word stomachos, ultimately from stoma (στόμα), "mouth". The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster (γαστήρ). The stomach churns food before it moves on to the rest of the digestive system.

6. 59 Pancreas • The pancreas is a glandorgan in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestiveenzymes that pass to the small intestine. These enzymes help in the further breakdown of the carbohydrates, protein, and fat in the chyme.

7. 60Liver • The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function. • This organ plays a major role in metabolism and has a number of functions in the body, including glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification. It lies below the diaphragm in the thoracic region of the abdomen. It produces bile, an alkaline compound which aids in digestion, via the emulsification of lipids. It also performs and regulates a wide variety of high-volume biochemical reactions requiring highly specialized tissues, including the synthesis and breakdown of small and complex molecules, many of which are necessary for normal vital functions.[2]

8. Not choice: Kidneys • The kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are an essential part of the urinary system, but have several secondary functions concerned with homeostatic functions. These include the regulation of electrolytes, acid-base balance, and blood pressure. In producing urine, the kidneys excrete wastes such as urea and ammonium; the kidneys also are responsible for the reabsorption of glucose and amino acids. Finally, the kidneys are important in the production of hormones including calcitriol, renin and erythropoietin. • Located behind the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes urine into a ureter, itself a paired structure that empties into the urinary bladder.

9. 61 Chlorophyll • The "head" of a chlorophyll molecule is a ring called a porphyrin. The porphyrin ring of chlorophyll, which has a magnesium atom at its center, is the part of a chlorophyll molecule that absorbs light energy. Examine the illustration to view the chlorophyll molecule and to learn which wavelengths of light are absorbed by this pigment. Compare the absorption spectrum of chlorophyll with the action spectrum of photosynthesis that Engelmann found.

10. 62 Auxin • Auxins are a class of plant growth substance and morphogens (often called phytohormone or plant hormone). Auxins have an essential role in coordination of many growth and behavioral processes in the plant life cycle. Auxins and their role in plant growth were first revealed by the Dutch scientist Frits Went.[1]

11. 63 phytochrome • Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms. It also regulates other responses including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.

12. 64 chlorophyll • Chlorophyll molecules embedded in the thylakoid membrane absorb light energy. These molecules are the most important pigments for absorbing the light energy used in photosynthesis. A chlorophyll molecule has a hydrophobic "tail" that embeds the molecule into the thylakoid membrane.

13. Not used ethylene • Ethylene (IUPAC name: ethene) is the chemical compound with the formula C2H4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin. It is extremely important in industry and also has a role in biology as a hormone.[2] Ethylene is the most produced organic compound in the world; global production of ethylene exceeded 107 million metric tonnes in 2005.[3] To meet the ever increasing demand for ethylene, sharp increases in production facilities have been added globally, particularly in the Persian Gulf countries.

14. Not used abscisic acid • Abscisic acid (ABA), also known as abscisin II and dormin, is a plant hormone. ABA functions in many plant developmental processes, including bud dormancy; it is degraded by the enzyme, (+)-abscisic acid 8'-hydroxylase.

15. 65 nonsense codon • In genetics, a nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a nonsense codon in the transcribedmRNA, and in a truncated, incomplete, and usually nonfunctional protein product. It differs from a missense mutation, which is a point mutation where a single nucleotide is changed to cause substitution of a different amino acid. Some genetic disorders, such as thalassemia and DMD, result from nonsense mutations.

16. 66 ribosome • Ribosomes are the components of cells that make proteins from amino acids. One of the central tenets of biology is that DNA makes RNA, which then makes protein. The DNA sequence in genes is copied into a messenger RNA (mRNA). Ribosomes then read the information in this RNA and use it to produce proteins. Ribosomes do this by binding to a messenger RNA and using it as a template for the correct sequence of amino acids in a particular protein. The amino acids are attached to transfer RNA (tRNA) molecules, which enter one part of the ribosome and bind to the messenger RNA sequence. The attached amino acids are then joined together by another part of the ribosome. The ribosome moves along the mRNA, "reading" its sequence and producing a chain of amino acids.

17. 67 poy-a tail • A long stretch of (about ten to 200 or more) adeninenucleotides added to the "tail" or 3' end of the pre-mRNA via the process called polyadenylation catalyzed by the enzyme, polyadenylate polymerase (PAP).

18. 68 anticodon • The anticodon region of a transfer RNA is a sequence of three bases that are complementary to a codon in the messenger RNA. During translation , the bases of the anticodon form complementary base pairs witht the bases of the codon by forming the appropriate hydrogen bonds.

19. Not chosen exon • An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule after either portions of a precursor RNA (introns) have been removed by cis-splicing or by two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA or a functional form of a non-coding RNA such as rRNA or tRNA. Depending on the context, exon can refer to the sequence in the DNA or its RNA transcript.