10/5/2015
Energy
Biology 102
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Required to drive all chemical reactions that
sustain life
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Cannot be created or destroyed, so living things
must obtain it from the environment
Lecture 7: Photosynthesis
And Cellular Respiration
Trapping Sunlight
Photosynthesis
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Ultimately, all living things on Earth derive energy
from the sun
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Process by which the sun’s energy is trapped as
chemical energy in molecular bonds of sugar
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Some directly by photosynthesis
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Overall chemical reaction:
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Plants, some protists and bacteria
Others indirectly through the food chain
6 CO2 + 6 H2O
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6 CO2 + 6 H2O
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Compilation of dozens of reaction steps
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Utilizes dozens of enzymes
Takes place mostly in the leaves
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Large, flat, maximum surface area
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Specialized structures that allow all required
components to come together
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H2O
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CO2
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Sunlight
Photosynthesis in Plants
C6H12O6 + 6 O2
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C6H12O6 + 6 O2
Extremely simplified
Photosynthesis in Plants
Energy from
sunlight
Energy from
sunlight
6 CO2 + 6 H2O
Energy from
sunlight
C6H12O6 + 6 O2
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Source of H2O: taken in through roots,
transported to leaves by vascular bundles
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Problem: large surface area means potential
Problem:
water loss
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Solution:: cuticle
Solution
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Waxy protective coating reduces water loss
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10/5/2015
Photosynthesis in Plants
6 CO2 + 6 H2O
cuticle
upper
epidermis
(a) Leaves
Energy from
sunlight
C6H12O6 + 6 O2
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Source of CO2: air
mesophyll
cells
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Problem:: cuticle keeps gases out
Problem
lower
epidermis
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Solution:: stomata
Solution
vascular
bundle
(b) Internal leaf structure
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Adjustable pores allow gases in (and out)
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Where the plant “breathes”
Photosynthesis in Plants
6 CO2 + 6 H2O
Energy from
sunlight
C6H12O6 + 6 O2
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Sunlight captured by chloroplasts
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Primarily in mesophyll layer of leaf
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One cell may contain 4040-50 chloroplasts
Light
cuticle
upper
epidermis
mesophyll
cells
(a) Leaves
lower
epidermis
outer membrane
inner membrane
thylakoid
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Composed of tiny packets of energy called
photons
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Energy of photons correspond to wavelength
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Long wavelength = low energy
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Short wavelength = high energy
(b) Internal leaf structure
stroma
(d) Chloroplast
(c) Mesophyll cell containing chloroplasts
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Light
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Infinite number of wavelengths contained in
sunlight
Light
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3 possible outcomes when photons strike an
object
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Correspond to different colors
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Absorbed (captured)
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Some wavelengths are visible to humans;
most are not
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Reflected (bounce back)
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Transmitted (pass through)
Reflected
Light
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Absorbed wavelengths generate heat, drive
biological processes
Why is white so “bright?”
Transmitted
Light
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Reflected or transmitted wavelengths reach the
eye of observers
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Perceived as color
Thought Question
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Absorbed
Thought Question
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Why are our pupils black?
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Photosynthesis
Photosynthesis
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Sunlight is captured by pigments in chloroplasts
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Sunlight is captured by pigments in chloroplasts
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Primarily chlorophyll
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Primarily chlorophyll
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Others (example: catotenoids)
catotenoids)
What colors does chlorophyll absorb? Reflect?
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Others (example: catotenoids)
catotenoids)
What colors does chlorophyll absorb? Reflect?
Photosynthesis
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Photosynthesis can be split into 2 sets of
reactions
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Take place in different parts of the chloroplast
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Light reactions (thylakoids
(thylakoids))
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Calvin cycle (stroma
(stroma))
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Light--dependent
Light
outer membrane
inner membrane
Light--independent
Light
thylakoid
stroma
Just Because It’s Interesting
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Beta--carotene
Beta
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Plant pigment that gives orange vegetables
their color
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Converted to vitamin A in animals
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Forms lightlight-absorbing pigments in eye
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Same compounds capture light in plants and
animals
Light Reactions of Photosynthesis
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Light can strike an object and eject electrons
from its surface
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Photoelectric effect
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Can be useful, damaging
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Light Reactions of Photosynthesis
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Light strikes chlorophyll and ejects an electron
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High energy electrons release energy to make…
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ATP
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NADPH
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Both of these go on to fuel the Calvin cycle
(more on that in a minute)
Light Reactions
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Light Reactions
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Light Reactions
Water split apart in the process
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Hydrogen fuels more ATP generation
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Oxygen goes to mitochondria to fuel cellular
respiration (more on that in a minute)
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Some oxygen is also released
Calvin Cycle
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Photosystem I
ATP and NADPH go on to fuel the Calvin cycle
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Light--independent reactions of
Light
photosynthesis
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Still in the chloroplast, just a different part
(Light reactions)
(Calvin cycle)
Question
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How does an acorn become an oak?
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Needs energy
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Stored in seed initially
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Later from photosynthesis
Needs carbon to form biomolecules
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Where does the carbon come from?
?
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Answer
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Calvin Cycle
Carbon in biomolecules of plants (and ultimately
all living things) comes from CO2 in the air
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Carbon is “fixed” into larger organic molecules
(sugars) through the Calvin cycle
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Comes up through the food chain to higher
organisms
Input
CO2
ATP
NADPH
air
Light
reactions
CALVIN
CYCLE
Energy
CO2
Output: G3P + G3P
Photosynthesis
Light
H2O
Chloroplast
Photosynthesis
CO2
Light
reactions
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Products of photosynthesis: sugars, oxygen
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What happens to these products?
Stroma
NADP+
ADP
P
Thylakoid
membranes
glucose
Calvin
cycle
ATP
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Some sugar turned into tissues, stored for
later use
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Oxygen and most sugars used to fuel cellular
respiration in mitochondria
NADPH
O2
Sugar
Cellular Respiration
Cellular Respiration
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Animals do this too
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We just bypass photosynthesis by eating,
breathing
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Cellular Respiration
Cellular Respiration
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Process by which organisms liberate energy
stored in glucose
C6H12O6 + 6 O2
(exhaled)
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Extremely simplified
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Dozens of steps involving dozens of enzymes
Cellular Respiration
C6H12O6 + 6 O2
6 CO2 + 6 H2O + 38 ATP + heat
6 CO2 + 6 H2O + 38 ATP + heat
Cellular Respiration
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Three steps
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Glycolysis – generates 2 ATP
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Process is EXTREMELY important
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Kreb’s Cycle – generates 2 ATP
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Organisms absolutely depend on ATP generated
this way
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Electron Transport Chain – generates 34 ATP
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38 ATP total (in theory)
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Blocking the process causes death in a few
minutes
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Oxygen depravation
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Metabolic poisons (eg
(eg cyanide, CO, Rotenone)
Cellular Respiration
Fermentation
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What happens when oxygen is unavailable?
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Cells must generate ATP without oxygen
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Fermentation
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Less efficient – 2 ATP per glucose
Waste products
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Lactic acid (animals, bacteria)
C6H12O6
2 Lactic acid + 2 ATP + heat
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Fermentation
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Cellular Respiration
Waste products in yeast: ethanol and CO2
C6H12O6
2 CO2 + 2 Ethanol + 2 ATP + heat
Summary
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Photosynthesis
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Where: in chloroplast
Where:
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Uses:: light, water, CO2
Uses
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Produces:: sugar, O2, ATP, NADPH
Produces
Summary
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Plants, animals are interdependent in the energy
cycle
Cellular respiration
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Where: in mitochondria
Where:
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Uses: products of photosynthesis
Uses:
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Produces: ATP, CO2
Produces:
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Alternate pathway:
pathway: fermentation
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