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Chapter 8 Photosynthesis

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1 Chapter 8 Photosynthesis

2 Obtaining Energy The sun is the direct or indirect source of energy for most living things. Autotrophs —organisms that can make their own food Heterotrophs —organisms that can not make food. They obtain energy from eating food.

3 Photosynthesis Photosynthesis is the process used by autotrophs to convert light energy from sunlight into chemical energy in the form of organic compounds. Involves a complex series of chemical reactions known as a biochemical pathway. Product of one reaction is consumed in the next reaction

4 Overview Photosynthesis is often summarized in the following equation:
6CO H2O C6H12O O2 The Reactants are carbon dioxide and water The Products are glucose and oxygen Light energy

5 The Stages of Photosynthesis
There are two stages to the process Light Reactions —light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH Dark Reactions (Calvin Cycle)—organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH

6 The Light Reactions Require light to happen
Take place in the chloroplasts Chloroplasts contain pigments that absorb sunlight. Pigment —a compound that absorbs light

7 The Structure of a Chloroplast
Surrounded by an outer and inner membrane Thylakoids —membrane system arranged as flattened sacs. (from the Greek meaning “pocket”) Grana (pl.) Granum (singular)—stacks of thylakoid membrane sacs Stroma —solution that surrounds the grana

8 Thylakoids contain the pigments known as Chlorophylls .
Chlorophylls —absorb colors other than green. Therefore, green is reflected and is visible. Two types: Chlorophyll a and Chlorophyll b Chlorophyll a —directly involved in the light reactions Chlorophyll b —accessory pigment that assists in photosynthesis Carotenoids —accessory pigments responsible for fall colors and also assist in photosynthesis

9 Converting Light Energy to Chemical Energy
Chlorophylls and carotenoids are grouped in clusters embedded in proteins in the thylakoid membrane. These clusters are called photosystems Two photosystems exist, each with its own job to do: Photosystem I and Photosystem II Plants have both photosystems. Prokaryotic autotrophs only have photosystem II. It is only numbered as II because it was the second one discovered. However, it probably evolved 1st.

10 I II

11 The Calvin Cycle Named for Melvin Calvin
Most common pathway for carbon fixation Carbon fixation —changing CO2 into organic compounds (carbohydrates) It is the second set of reactions in photosynthesis and does not require light. It uses the energy that was stored in ATP and NADPH during the light reactions to produce organic compounds in the form of sugars. The Calvin Cycle occurs in the stroma of the chloroplasts and requires CO2

12 The Calvin Cycle 3 ADP 3 CO2 3 ATP 3 RuBP 6 PGA 6 ATP 6 ADP 5 G3P
glucose 6 G3P 1 G3P 6 NADPH starch 6NADP+ 6 P

13 Plant species that fix carbon using the Calvin Cycle only are known as C3 plants because of the three-carbon compound that is initially formed in the process. They include most plants.

14 Alternative Pathways Plants living in hot, dry climates have trouble using the Calvin Cycle to fix carbon. This is because they must partially close their stomata to conserve water. This allows less CO₂ to enter and an excess of O₂ to build up, both of which inhibit the Calvin Cycle Two alternate pathways have evolved for these plants—both allow the plants to conserve water. They are the C4 pathway and the CAM pathway

15 The C4 Pathway C4 plants include: corn, sugar cane and crab grass
Cells called mesophyll cells in C4 plants use an enzyme to fix CO2 into a four carbon compound This compound travels to other cells where CO2 can be released and enter the Calvin Cycle These plants lose about ½ as much water as C3 plants when producing the same amount of carbohydrates.

16 The CAM Pathway CAM plants include: cactuses, pineapples, and jade plants. These plants open their stomata at night and close them during the day (opposite of most plants). CO2 absorbed at night can enter the Calvin Cycle during the day, allowing the stomata to stay closed and conserve water. These plants lose less water than any other plants

17 Chapter 7 Cellular Respiration Mighty Mitochondria

18 Cellular Respiration Cellular Respiration —the process by which cells get energy from carbohydrates; oxygen combines with glucose to form water and carbon dioxide C6H12O O CO H2O energy (ATP)

19 The equation is a simple summary of a very complex process.
The overall purpose is to convert food into energy by breaking down organic fuel molecules. When oxygen is present during this process it is called aerobic respiration ( which is the most efficient). If no oxygen is present it is called anaerobic respiration (which is much less efficient). Both types (aerobic and anaerobic) start with a process called glycolysis.

20 Glycolysis Glycolysis —first stage of cellular respiration.
Glycolysis means “glucose splitting” Occurs in the cytosol No oxygen is needed Glucose molecules are broken down into two carbon molecules of pyruvic acid Pyruvic acid is then used in the Krebs Cycle (which is the second stage of aerobic respiration) Specific enzymes are needed 2 molecules of ATP are produced 2 molecules of NADH (an electron carrier molecule) are produced

21 G3P G3P

22 Summary of Glycolysis Basically:
One glucose (6C) is broken into two molecules of pyruvic acid (3C) If oxygen is available, the pyruvic acid will move into the mitochondria and aerobic respiration will begin. 4 ATP molecules are produced. Two are used to break apart the next glucose molecule and keep glycolysis going. This leaves a net yield of 2 ATP molecules for use by the cell. Two NAD+ are converted into 2 NADH and 2H+. These go to Electron Transport.

23 Efficiency of Glycolysis
Measured in kilocalories (kcal) One kilocalorie equals 1,000 calories (cal) Complete oxidation of glucose releases 686 kcal Production of ATP absorbs 7 kcal 2ATP are produced from every glucose molecule broken down by glycolysis The efficiency is therefore calculated by the following formula: Efficiency of Energy required to make ATP glycolysis = Energy released by oxidation of glucose = x 7 kcal x % = 2% 686 kcal

24 Aerobic Respiration In most cells, the pyruvic acid produced in glycolysis enters the pathway of aerobic respiration. This pathway produces nearly 20 times as much ATP as is produced by glycolysis alone and is therefore the most efficient. Oxygen must be available for this to happen. There are two major stages: The Krebs Cycle and the Electron Transport Chain

25 Intermediate Step Aerobic Respiration takes place in the mitochondria of the cell. Before the Krebs Cycle can begin, each of the two pyruvic acid molecules must be converted. The pyruvic acid enters the mitochondrial matrix (space inside the inner membrane of the mitochondria) It reacts with a molecule called coenzyme A to form Acetyl Coenzyme A (acetyl CoA)

26 The Krebs Cycle The Krebs Cycle (named for Hans Krebs) is a biochemical pathway that breaks down acetyl CoA. Two turns of the Krebs Cycle produce: 2 ATP molecules 4 CO2 molecules 6 NADH molecule 2 FADH2 molecules


28 Review of the Gylcolysis and the Krebs Cycle
In Glycolysis, one glucose molecule produces two pyruvic acid molecules, which can then form two molecules of Acetyl CoA. Both of the Acetyl CoA molecules enter the Krebs Cycle creating two turns of the cycle. This produces 6 NADH, 2 FADH2, 2 ATP and 4 CO2 molecules (waste product that diffuses out of the cell). The 6 NADH and 2 FADH2 molecules drive the next stage of aerobic respiration—the Electron Transport Chain.

29 Electron Transport Chain
The Electron Transport Chain, linked with chemiosmosis makes up the second stage of aerobic respiration. Electrons are transferred from one molecule to another by several electron carrying molecules located in the membrane of the mitochondria. All steps occur in the cristae (inner membrane)


31 Efficiency of Cellular Respiration
Through Aerobic Cellular Respiration, a maximum of 38 ATP molecules can be produced from one glucose molecule. 2 from Glycolysis 2 from Krebs cycle from the Electron Transport Chain

32 To see how we get 38, follow along…. 2 ATPs directly from glycolysis
2ATPs directly from Krebs cycle Each NADH can generate 3ATPs from electron transport (30 total) Each FADH2 can generate 2ATPs from electron transport (4 total)

33 The actual number of ATP molecules generated through Aerobic Respiration varies from cell to cell. (36-38) Most eukaryotic cells produce only 36 molecules per glucose molecule because the active transport of NADH through a cell membrane uses up some ATP. When 38 ATP molecules are generated the efficiency is calculated as follows: Efficiency of Energy required to make ATP Cellular Respiration = Energy released by oxidation of glucose = x kcal x 100% = 39% 686 kcal This is 20 times more efficient than glycolysis alone !!

34 Anaerobic Respiration
If no oxygen is present, the Krebs Cycle and Electron Transport Chain are not utilized. The cell must have a way to keep glycolysis going. Glycolysis would stop without a cellular process that recycles NAD+ and NADH. Without such a process, glycolysis would quickly use up all the NAD+ in the cell. Glycolysis and ATP production would stop and the cell would die. Fermentation to the rescue 

35 Fermentation Fermentation is the chemical pathway that recycles NAD+ in the absence of oxygen. It keeps glycolysis going. No additional ATP is made. Therefore, you still have the 2% efficiency rate for energy release. Two types of fermentation: Lactic Acid Fermentation Alcoholic Fermentation

36 Lactic Acid Fermentation
Pyruvic acid is converted by a specific enzyme into lactic acid. Two hydrogen atoms from NADH and H+ are transferred to pyruvic acid to form the lactic acid molecule. NADH is oxidized to NAD+ and reused to keep glycolysis going.

37 Occurs mostly in muscle cells during hard exercise.
Lactic acid fermentation occurs in foods such as yogurt and cheese as well as certain animal cells. Occurs mostly in muscle cells during hard exercise. Muscle cells use up oxygen too fast and switch from aerobic to anaerobic respiration. Lactic acid builds up reducing the cells ability to contract. This causes fatigue, pain and cramps. Slow down!!! Allow the lactic acid time to diffuse back into the blood stream and to the liver where it is converted back into pyruvic acid.

38 Alcoholic Fermentation
Converts pyruvic acid to carbon dioxide and ethyl alcohol. NAD+ is recycled in the same manner as before.

39 Bakers use the alcoholic fermentation of yeast to make bread.
CO2 is produced and trapped in the dough, causing it to rise. When the dough is baked, yeast cells die and the alcohol evaporates. You can’t get drunk from eating bread !!!

Click to reveal COMPARING PHOTOSYNTHESIS AND CELLULAR RESPIRATION PHOTOSYNTHESIS RESPIRATION FUNCTION   Production of Glucose   Oxidation of Glucose LOCATION   chloroplasts   mitochondria REACTANTS   6CO2 + 6H2O   C6H12O O2 PRODUCTS   C6H12O O2   6CO H2O EQUATION    light 6CO2 + 6H2O C6H12O O2  C6H12O O CO2 + 6H2O +ATP

41 Photosynthesis and Cellular Respiration Cycle

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