Unit 4: Energy Generation

Energy production

Photosynthesis and cellular respiration work together to provide energy to all living organisms

Exergonic reaction

Reaction that produces or releases energy. For example, chemical energy stored in glucose is released as ATP and heat

How much ATP does your body use?

• Life sustaining activities: 75% of your energy

  • The brain uses 20% of your daily energy

• Life sustaining activities use 1300-1800 C per day

  • Basal metabolic rate (BMR)

• Voluntary activities: 25% of your energy

Redox reactions

The transfer of electrons form one reactant to another

Oxidation

The loss of electrons

Reduction

The gain of electrons

Redox Reaction in Cellular Respiration

Nicotinamide adenine dinucleotide (NAD+)

• A coenzyme made from the vitamin niacin

• Shuttles electrons in redox reactions

• Accepts electrons to become reduced to NADH

Dehydrogenase

• Enzyme that oxidizes the organic fuel molecule

• Strips two H atoms from the fuel molecule

• Transfers two electrons and one proton to the NAD+ molecule to make NADH, and releases the second proton

Electron Transport Chain (ETC)

• NADH delivers the electrons to a series of carrier molecules

• Final electron acceptor in cellular respiration is oxygen

• Reduces O2 to H2O

• The molecules are mostly embedded in the inner mitochondrial membrane

Stages of Cellular Respiration

Each stage is a series of chemical reactions and each stage takes place in a different region of the cell.

1. Glycolysis

2. Pyruvate oxidation and the citric acid cycle

3. Oxidative phosphorylation

Substrate Level Phosphorylation

The process by which ATP is produced in glycolysis and the citric acid cycle. A phosphate group is transferred from the substrate to ADP to form ATP

Stage 1: Glycolysis

• Takes place int he cytosol

• Breaks glucose into two molecules of pyruvate

• 9 chemical steps (product of one reaction is reactant for the next, different enzyme each time)

• Two molecules of NAD+ are reduced to two molecules of NADH

• Net gain of 2 molecules of ATP (2 ATP are used, 4 ATP are produced)

Energy Investment Phase

• First part of glycolsis

• Consumes energy to energize the glucose molecule in order to split it into two G3P molecules

Energy Payoff Phase

• Second part of glycolsis

• Energy is produced (4 ATP molecules, 2 NADH molecules)

• Final product is two molecules of pyruvate

Energy Production of Glycolisis

• No O2 used (anaerobic reaction)

• 2 net ATP and 2 NADH, about 6% of energy a cell can harvest from a glucose molecule

Glycolysis is an Ancient Process

• The universal energy harvesting process of life

  • Because its is so widespread in life, glycolysis likely evolved very early in the history of life

• Location of glycolysis in the cell also suggests antiquity

  • Occurs in the cytosol and does not require membrane-bound organelles

• Does not require oxygen

  • Used long before there was oxygen in the atmosphere

Stage 2: Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate is transported into the mitochondria. Part of this stage uses oxygen so it is aerobic.

Pyruvate Oxidation

Pyruvate is oxidized to a 2-carbon compound

1. Carboxyl group is removed from pyruvate and released as C02

2. The 2 carbon molecule is oxidized to reduce NAD+ to NADH

3. Coenzyme A (CoA) is added to the two carbon molecule (CoA is derived from vitamin B)

4. Final products: Acetyle CoA and 2 NADH

Citric Acid Cycle/Krebs cycle/TCA cycle

• Each step is catalyzed by a different molecule in the mitochondrial matrix or the inner mitochondrial membrane

• Acetyl CoA is combined with oxaloacetate to make citrate

• Redox reactions remove two carbon atoms and releases them as CO2

• Oxaloacetate is regenerated

• Output per acetyle CoA (double per glucose):

  • 2 CO2

  • 3 NADH

  • 1 FADH

  • 1 ATP

Energy Storage Molecules

• A small amount of energy is produced through glycolysis and citric acid cycle

• Most of the energy from glucose is stored as NADH and FADH2

• High energy electrons must be shuttled through an electron chain

Stage 3: Oxidative Phosphorylation

• Energy captured by the NADH and FADH2 molecules is used to phosphorylation ADP to ATP

• Takes place in the mitochondria

• Two parts: electron transport chain and chemiosmosis

• 90% of the ATP of cellular respiration is produced

Electron Transport Chain

• NADH and FADH2 shuttle electrons to this

• H+ ions are pumped across the inner mitochondrial membrane into the intermembrane space

  • Active transport that results in an H+ concentration gradient that holds lots of potential energy

• Terminal electron accepter is O2 which is reduced to H2O

Chemiosmosis

• The potential energy of the H+ concentration gradient is used to make ATP

• H+ atoms are driven back down to their concentration gradient through the enzyme complex ATP synthase

• Phosphorylates ADP to ATP

Fermentation

• The process of harvesting energy from organic matter without using oxygen as a terminal electron acceptor.

• Does not use oxygen so it is anaerobic respiration

• Harvests energy for the cell using glycolysis

  • Enough energy for your muscles to contract for a short amount of time when starved of oxygen

  • Many microorganisms use this to supply all their needs

NAD+ Regeneration

Aerobic cellular respiration:

• NAD+ is regenerated when NADH passes its electrons through the electron transport chain during oxidative phosphorylation

Anaerobic fermentation:

• Oxidative phosphorylation does not take place because there is no oxygen

• A different anaerobic path is needed to regenerate NAD+

Latic Acid Fermentation

• Done by animal muscle cells and some bacteria

• Pyruvate is reduced to lactate, allowing 2 NADH to be oxidized to 2 NAD+

• Examples:

  • In muscle cells this occurs when the need for ATP outpaces the delivery of oxygen

  • Liver cells oxidize lactate back to pyruvate

  • Bacterial lactic acid fermentation makes cheese and yogurt

  • Lactic acid bacteria turn soybeans into soy sauce and cabbage into sauerkraut

Alcohol Fermentation

• Yeasts and some bacteria reduce pyruvate to ethanol and CO2 while oxidizing NADH to NAD+

• Examples:

  • Ethanol used in alcoholic beverages (brewing, winemaking) since CO2 causes the the bubbles

  • CO2 causes bread dough to rise

Yeasts and respiration

• Single celled fungi

• Normally use aerobic cellular respiration to process their food

• Also able to survive in anaerobic conditions by using alcohol fermentation

Anaerobes

Organisms that can live in anaerobic conditions

Obligate anaerobes

Require anaerobic conditions, are poisoned by oxygen

Facultative anaerobes

• Can live in anaerobic or aerobic conditions

• Will make ATP by either fermentation or oxidative phosphorylation depending on whether or not O2 is available

• The organism always takes the available route that produces the most energy

Photosynthesis

• Solar energy is used to convert CO2 and H2O into sugars and O2

• Used by plants and some microorganisms

• In some prokaryotes photosynthesis is not linked to carbon fixation

Carbon Fixation

Process by which living organisms convert inorganic, atmospheric carbon dioxide CO2 into organic compounds which are reduced to form sugars that can be used for energy and biological growth

Autotrophs

“Self-feeders”. Organisms that make their own food. The ultimate source of organic molecules for almost all life on earth (take in inorganic material and turn into organic)

Photoautotrophs

Autotrophs that use energy from light. Primary producers of the biosphere

Heterotrophs

• The consumers of the biosphere

• Cannot make their own food

• Consume plants or animals, or decompose organic material

• Dependant on photoautotrophs for oxygen and for organic fuel to maintain life

Chlorophyll

A light absorbing pigment in the chloroplasts. Causes the green colouring in plants.

Chloroplasts

Specialized organelles found in plant cells and eukaryotic algae that convert sunlight into chemical energy through photosynthesis. Most concentrated in the leaves.

Leaf Structure

• Chloroplasts are concentrated in mesophyll cells. Each mesophyll cell has 30-40 chloroplast

• Also contains stomata and veins

Stomata

Pores that allow CO2 to enter the leaf and O2 to exit the leaf

Veins

Deliver water, minerals, sugars to all parts of the plant

Chloroplasts Structure

• Photosynthetic enzymes and chlorophyll are embedded in the thylakoid membranes

• Sugars are manufactured in the stroma

Chlorophyll a

A pigment that absorbs blue-violent and red light. Participates directly in the light reactions of photosynthesis

Chlorophyll b

Absorbs blue and orange light. Transfers energy to chlorophyll a

Carotenoids

• Pigments that reflect shades of yellow and orange

• More stable than chlorophyll

• Broaden the spectrum of colours that can drive photosynthesis

• Pass their energy to chlorophyll

Photoprotection

• Protection from excessive light energy

• Some carotenoids capture and dissipate excessive light energy that would otherwise damage the cells

• Light energy can damage chlorophyll or react badly with it to damage other cellular components

Photosynthesis is a Redox Reaction

• The opposite of cellular respiration

• Potential energy of the electrons increases as they move from H2O to CO2

• Energy from sunlight gives the electrons this energy boost

Stages of Photosynthesis

• Light reactions:

  • In the thylakoids

  • The capture of light energy

• Calvin Cycle

  • In the stroma

  • Carbon fixation

Stage 1: Light Reactions

• Light energy is absorbed by chlorophyll

• H2O is split to release O2

• Electrons are transferred from H2O to NADP+ to reduce it to NADPH

• ATP is produced

Capturing Solar Energy

• When a pigment molecule absorbs a photon, an electron jumps to an energy level farther from the nucleus

  • Raised from a stable ground state to an unstable excited state

• The excited electron drops back down to its ground state and releases energy in the form of heat and sometimes light (fluorescence)

Photosystems

• Two complexes:

  • Reaction centre surrounded by light harvesting complexes

• When a pigment molecule in the light harvesting complex absorbs a photon the energy is passed from molecule to molecule

• The energy will finally be passed to the reaction centre complex

Light-harvesting complex

• Pigment molecules bound to proteins

• Harvests light from a wide range of different wavelengths

• Large numbers of pigment molecules allows for efficient light harvesting

Reaction Centre

• A pair of specialized chlorophyll a molecules bound to proteins

• Contains the primary electron acceptor

• When an electron in the reaction centre chlorophyll molecule is boosted to an excited state it is captured by the primary electron acceptor

Two Photosystems

• Photosystem II and photosystem I are coupled together by an electron transport chain (ETC)

• A photon excites an electron in the photosystem II pigment, the energy is passed to the primary electron acceptor, and moves down an ETC to produce ATP

• Another photon excites an electron in photosystem I pigments, the energy is passed to a primary electron acceptor and NADPH is formed

Light Reactions Step 1

• Water is split into one oxygen atom (O), two hydrogen ions (H+) and two electrons

• O joins with another O to form O2 which is released from the cell

• Electrons are passed to the reaction centre chlorophyll a

  • Replaces the electrons that was passed from chlorophyll to the primary electron accepter

• H+ stays in the thylakoid space to build up a concentration gradient

Light Reactions Step 2

• Electrons pass through an ETC

• Terminal electron acceptor is the photosystem I reaction centre chlorophyll a to replace the electrons that were passed to the primary electron acceptor

Light Reactions Step 3

• After photosystem I there is another short electron transport chain

• Electrons are passed to NADP+ to reduce it to NADPH

Light Reaction Step 4

• Chemiosmosis produces ATP

• H+ gradient formed by the ETC between photosystems and the splitting of water

• H+ is pumped through ATP synthase to phosphorylate ADP to ATP

Redox reaction of the Calvin Cycle

• Reactants:

  • CO2 from the air

  • ATP and NADPH from the light reactions

• Products:

  • Glyceraldehyde-3-phosphate (G3P), used by the cell to make glucose, sucrose, and other organic molecules

Stage 2: Calvin Cycle/Dark reactions/Light-independent reactions

• 4 main steps

• Starting material: 5-carbon sugar called ribulose bisphosphate (RuBP)

• The cycle turns three times to generate one molecule of glyceraldhyde-3-phosphate (G3P)

• Each turn incorporates one CO2 molecule. Three turns incorporates three C atoms

Calvin Cycle Step 1 - Carbon Fixation

• CO2 is attached to RuBP

• Done using enzyme rubisco

• The resulting 6-carbon molecule is unstable so it immediately splits into two 3-carbon molecules

Calvin Cycle Step 2 - Reduction

• NADPH and ATP are used to reduce the 3-carbon molecule to G3P

• NADPH provides the electrons for reduction

• ATP provides energy for the process to take place

Calvin Cycle Step 3 - Release of G3P

For every 3 CO2 molecules fixed, one G3P molecule is released as a product

Calvin Cycle Step 4 - Regeneration of RuBP

Five GSP molecules are rearranged to make RuBP using energy from ATP

Energy Input of Calvin Cycle

• The cycle turns three times

  • 3 CO2 molecules enter the cycle combining with 3 RuBP molecules

  • Makes 6 G3P molecules (one is released)

  • 5 G3P molecules (15 carbon atoms total) rearrange to make 3 RuBP molecules (15 carbon atoms total)

• Synthesis of one G3P molecule

  • Uses 9 ATP

  • Uses 6 NADPH

  • ATP and NADPH were provided by the light reactions

• G3P can be used to construct glucose

  • Energy storage

  • Cellular respiration releases the energy stored in glucose

C3 Plants

• Standard plants that use CO2 directly from the air and use rubisco to add CO2 to RuBP. Includes many important agricultural crops

• Problem: Hot, dry weather decreases crop yield.

  • Plants close their stomata to reduce water loss through evaporation

  • CO2 cannot enter the leaf, so concentrations decrease

  • O2 cannot exit the leaf so concentrations increase, resulting in photorespiration

Photorespiration

• Reaction that occurs in the light and consumes O2 to release CO2

• O2 levels build up in the plant

• Rubisco adds O2 to RuBP instead of adding CO2

  • Rubisco is “sloppy” enzyme

• Results in a 2-carbon product instead of G3P

• Uses ATP and NADPH without yielding any sugar

C4 Plants

• Avoid photorespiration by splitting the mesophyll cell into two cells

• New mesophyll cells have an enzyme with a very high affinity for CO2

• Fixes CO2 into a 4-carbon intermediate compound, even when its concentration is very low

• The 4-carbon molecule shuttles the CO2 to the bundle-sheath cells that perform the Calvin Cycle

• Includes sugarcane and corn

CAM Plants

• These plants are adaptive to very dry climates (pineapples, cacti, succulents, etc)

• Converse water by only opening its stomata at night

• CO2 is fixed into a 4-carbon molecule in the mesophyll cell which stores the carbon at night to be used during the day

• During the day that CO2 is released into the cell so that the Calvin Cycle can proceed