Biology Notes Chapter 6

1
Biology Notes Chapter 6

• Photosynthesis and Respiration

2
Photosynthesis

• In the process of photosynthesis, plants convert
  the energy of sunlight into the energy in the
  chemical bonds of carbohydrates, sugars and
  starches.
• Scientists have concluded that in the presence of
  light, plants transform carbon dioxide and water
  into carbohydrates and release oxygen.
• This gives us the basic outline of the process of
  photosynthesis.
• light
• CO2 H2O ? (CH2O)n O2
• Carbon Dioxide water yields in the presence
  of light carbohydrate and oxygen

3
Photosynthesis

• Because photosynthesis usually produces a
  particular carbohydrate which is the sugar
  glucose (C6H12O6) we can rewrite and balance the
  equation as follows.
• 6CO2 6H2O ? C6H12O6 6O2
• Carbon dioxide water yields in the presence of
  light glucose oxygen.
• Do not let the formula fool you, a great deal
  happens between the beginning and end of the
  equation.
• We do not get glucose and oxygen by simply
  putting water and carbon dioxide together.

4
Other Requirements of Photosynthesis

• Nearly all organisms on Earth depend on the sun
  for energy.
• Some organisms such as green plants use the suns
  energy directly.
• As a result plants are called autotrophs.
• Autotrophs are organisms that are able to produce
  food directly from simple inorganic molecules in
  the environment.
• Some organisms, such as animals cannot use the
  suns energy directly.
• These organisms are known as heterotrophs and
  must obtain their energy from the foods that they
  eat.
• Heterotrophs may eat autotrophs or heterotrophs
  or even both.

5
Pigments

• The process of photosynthesis begins when light
  is absorbed by pigments in the plant cell.
• Pigments are colored substances that absorb or
  reflect light.
• The principle pigment in green plants is
  chlorophyll.

6
Energy Storing Compounds

• The most important energy storing compound,
  meaning that it is one used by every living cell,
  is ATP, or adenosine triphosphate.
• An ATP molecule consists of a nucleotide called
  adenine, a 5 carbon sugar called ribose, and
  three phosphate groups.
• Energy is stored in the bond between adenosine
  and the phosphate groups.
• This energy is released when the bond is broken.

7
ATP continued

• Enzymes control the synthesis and breakdown of
  ATP within the cell. These enzymes enable ATP to
  be used as a source of energy for reactions that
  require energy.
• ATP supplies energy for many cellular activities
  such as muscle contraction, protein synthesis,
  and active transport.

8
The Light and Dark Reactions of Photosynthesis.

• Photosynthesis is broken down into two parts, the
  light and dark reactions.
• As their name implies the light reactions require
  light and the dark reactions do not require light
  to occur.

9
Light Reactions

• The light reactions absorb energy from the sun
  and store it in chemical bonds.
• Light reactions can be divided into four basic
  processes.
• Light absorption Clusters of pigment molecules
  called photosystems capture the energy from the
  sun.
• There are 2 photosystems in green plants,
  (photosystem 1 and 2)
• Each photosystem contains chlorophyll and other
  accessory pigments.

10
Light Reactions Continued

• The accessory pigments absorb light energy from
  areas of the spectrum that chlorophyll can not.
• Energy is passed on from pigment molecule to
  pigment molecule until it reaches a special pair
  of chlorophyll molecules that are surrounded by
  an apparatus that enables the energy to be
  captured in the chemical form.

11
Electron Transport

• The high energy electrons produced by light
  travel from the special pair of chlorophyll
  molecules to a molecule called an electron
  carrier.
• This is the beginning of electron transport.
• The high energy electrons are transferred along a
  series of electron carrier molecules in the
  photosynthetic membrane known as the electron
  transport chain.
• Electrons are passed form one carrier to another
  almost as pails of water are passed from one
  person to the next in a bucket brigade.

12
Electron Transport Continued

• At the end of the electron transport chain is an
  enzyme that passes the high energy to an electron
  carrier called NADP converting it to NADPH
• Adding a pair of electrons to this electron
  carrier requires energy.
• This is the first way in which part of the energy
  of sunlight is trapped in chemical bonds.

13
Oxygen Production

• As light shines on chlorophyll it looses
  electrons to form NADPH.
• The photosynthetic membrane is continuously
  replacing these electrons.
• These electrons are taken from water. (4
  electrons are removed from 2 water molecules,
  leaving 4 hydrogen ions and two oxygen atoms.)
• The two oxygen atoms form a single molecule of
  oxygen gas that leaves the chloroplast and is
  released into the air.

14
ATP Formation

• Hydrogen ions that are left behind when water is
  split are released inside the photosynthetic
  membrane.
• Electrons are passed from chlorophyll to NADP,
  and more hydrogen ions are pumped across the
  membrane.
• This makes the outside of the membrane negatively
  charged and the inside positively charged.

15
ATP Formation Continued

• The difference in charges across the membrane is
  a source of energy.
• An enzyme in the photosynthetic membrane makes
  use of this energy to attach a phosphate molecule
  to ADP, forming ATP
• This is the second way that the energy of
  sunlight is trapped in chemical form.

16
Summary of the Light Reactions

• The light reactions use water, ADP, and NADP
• They produce oxygen gas, which is expelled from
  the plant for us to breath.
• They also produce the energy storing molecules
  ATP and NADH.
• The dark reactions will convert these energy
  storing molecules to a more convenient form.

17
The Dark Reactions

• Do not let the name fool you, dark reactions
  generally take place in the sunlight , they just
  do not require light for them to occur.
• If ATP and NADPH are supplied, the dark reactions
  can be carried out in a test tube, even in total
  darkness without ever being exposed to light.

18
Calvin Cycle

• The dark reactions form a cycle, or circular
  series of reactions, known as the Calvin cycle.
• The Calvin Cycle has three major steps, which
  occur within the stoma of the chloroplasts

19
Step 1 of the Calvin Cycle

• An Enzyme combines a CO2 Molecule with a FIVE
  CARBON CARBOHYDRATE CALLED RuBP (ribulose
  bisphosphate).  The PRODUCT is a Six-Carbon
  Molecule that Splits into a Pair of Three-Carbon
  Molecules known as PGA (3-phosphoglycerate).

20
Step 2 of the Calvin Cycle

• PGA is Converted into another Three-Carbon
  Molecule, PGAL, in a Two Part Process
•     A. Each PGA Molecule Receives a Phosphate
  Group from a molecule of ATP - forming ADP
•     B. The resulting compound then Receives a
  Proton from NADPH (forming NADP) and Releases a
  Phosphate Group, Producing PGAL.
• In addition to PGAL, these Reactions produce ADP,
  NADP, and Phosphate.  These Three Products can
  be used again in the Light Reactions to Synthesis
  additional Molecules of ATP and NADPH.

21
Step 3 of the Calvin Cycle

• Most of the PGAL is Converted back into RuBP in a
  series of reactions to Return to Step 1 and allow
  the Calvin Cycle to Continue.  However, SOME PGAL
  Molecules LEAVE the Calvin Cycle and can be used
  by the Plant Cell to Make other Organic
  Compounds.

22
Glycolysis

• Glycolysis is the breaking down of glucose.
• Glucose (C6H12O6) is a simple 6-carbon sugar.
• If glucose is broken down completely in the
  presence of oxygen, carbon dioxide and water are
  produced
• C6H12O6 6 O2 ? 6 CO2 6 H2O
• This reaction gives us 3811 calories per gram of
  glucose.

23
Glycolysis

• A calorie is the amount of heat required to raise
  the temperature of 1 gram of water 1 degree
  Celsius.
• Cells take glucose apart slowly and capture
  energy a little at a time.
• The first stage of this is called glycolysis.
  Which is the breaking down of glucose in the
  cytoplasm of the cell.
• Enzymes break down glucose one step at a time
  into different types of molecules.

24
Glycolysis

• In the first step glucose is split from the 6
  carbon sugar into 2, 3 carbon PGAL molecules,
  which takes 2 ATP molecules to make this happen.
• The 2 PGAL molecules are then broken down into 2
  molecules of pyruvic acid.
• The energy from the PGAL molecules is used to
  make 4 molecules of ADP and 2 molecules of NADH,
  which are energy storing molecules.

25
Glycolysis

• In the process of glycolysis, 4 molecules of ATP
  are synthesized from 4 molecules of ADP.
• Since 2 molecule of ATP were used to start the
  process there is a net gain of 2 molecules of ATP
  during glycolysis.

26
Respiration

• Respiration is the process that involves oxygen
  and breaks down food molecules to release energy.
  
• Respiration uses pyruvic acid that was formed in
  glycolysis.
• By breaking down pyruvic acid in respiration, 34
  additional molecules of ATP are produced.
• Respiration occurs within the mitochondria of the
  cell.

27
The Krebs Cycle

• Summary of events
• 2 carbon atoms added from the breakdown of
  pyruvic acid.
• 2 carbon atoms removed in the form of 2 carbon
  dioxide molecules
• 3 molecules of NAD converted to NADH
• 1 molecule of FAD converted to FADH2
• 1 molecule of GDP converted to GTP

• Carbon dioxide is a waste product that is
  released from the cell at the end of the Krebs
  cycle.

28
Energy in the balance

• Photosynthesis and respiration can be thought of
  as opposite processes.
• Photosynthesis is the process of making glucose.
• Respiration is the process of breaking glucose
  down for the cell to use.
• The products of photosynthesis are the reactants
  of glucose breakdown and the products of glucose
  breakdown are the reactants of photosynthesis.

29
Fermentation

• Remember that glycolysis produces 2 ATP molecules
  per molecule of glucose and does not require
  oxygen.
• This enables cells to produce a limited amount of
  chemical energy in the form of ATP.
• Glycolysis also produces high energy electrons
  that convert NAD to NADH.
• In order for glycolysis to continue, NADH must be
  converted back to NAD.
• This is where fermentation comes in.

30
Fermentation

• Fermentation is anaerobic, which means that it
  does not require oxygen.
• In fermentation NADH is converted to NAD by
  adding the extra electrons in NADH to an organic
  molecule that acts as an electron acceptor.
• Fermentation allows cells to carry out energy
  production in the absence of oxygen.
• The combination of glycolysis and fermentation
  produces 2 molecules of ATP from a molecule of
  glucose.

31
Lactic Acid Fermentation

• In many cells, the pyruvic acid that accumulates
  as a result of glycolysis can be converted to
  lactic acid.
• Lactic acid fermentation regenerates NAD so that
  glycolysis can continue
• Pyruvic acid NADH ? lactic acid and NAD
• Lactic acid is produced in muscles during rapid
  exercise when the body cannot supply all the
  energy that is required.
• This is what causes the pain in your belly when
  running.

32
Alcohol Fermentation

• Alcohol fermentation occurs in yeasts and a few
  other microorganisms.
• In this process pyruvic acid ( a 3 carbon
  compound ) is broken down to produce a 2 carbon
  alcohol and carbon dioxide.
• Since alcohol is a byproduct this type of
  fermentation is called alcohol fermentation.
• As in lactic acid fermentation, the process that
  alters pyruvic acid also changes NADH back into
  NAD
• Pyruvic acid NADH ? alcohol CO2 NAD

33
Alcohol Fermentation

• Alcoholic fermentation is important to bakers and
  brewers.
• The carbon dioxide produced by yeast during
  fermentation causes dough to rise and forms the
  air spaces you see in a slice of bread.
• The carbon dioxide released by fermentation is
  the source of bubbles in beer and sparkling wine.