Unit 2: Cell Energetics

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Unit 2 Cell Energetics

• Biology
• 2001-2002

2
Homeostasis

• Definition the maintenance of a stable internal
  environment in the presence of a dynamic external
  environment
• Consequences of failing to maintain homeostasis
  include
• Accumulation of wastes
• Decrease in food supply
• Increased susceptibility to foreign invaders
• Note Any of the above may result in cell death
  (ouch!)

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Homeostasis (contd)

• Which organelle is responsible for the
  maintenance of homeostasis?

The cell membrane
Which special property of the membrane
facilitates this function?
Selective permeability of the phospholipid bilayer
4
Homeostasis (contd)

• Membrane structure

Peripheral Protein
Hydrophilic Head
Bilayer
Hydrophobic Tail
H2O
H2O
Integral Protein
Note Different membranes will be permeable to
different substances, depending upon the
functions and needs of the cell.
5
Passive Transport - Diffusion

• Diffusion is the movement of molecules from an
  area of higher concentration to lower
  concentration, until equilibrium is reached.
• This is also referred to as movement down the
  concentration gradient.
• This movement is passive, requiring no ATP
  (cells energy currency).

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Passive Transport Diffusion 2
When there is a membrane between solutions of
two different concentrations, the ability of a
molecule to diffuse through the membrane depends
on
1. The size and nature of molecule
2. chemical composition of the membrane
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Passive Transport - Osmosis

• Osmosis is the diffusion of water molecules
  (e.g. when crossing a membrane) from an area of
  higher concentration to lower concentration
• Osmosis, as a specialized type of diffusion,
  requires no ATP

O
H
H
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Passive Transport - Osmosis

• A solution is hypotonic if it has a lower solute
  concentration relative to another solution
• A solution is hypertonic if it has a higher
  solute concentration relative to another solution
• What might isotonic mean?

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Passive Transport - Osmosis

• Since water is typically the solvent, not the
  solute, then water molecules will move via
  osmosis from a hypotonic solution to a hypertonic
  solution.
• This movement will occur until equilibrium is
  reached

Solution A
Solution B
20 g NaCl L H2O
10 g NaCl L H2O
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Passive Transport - Osmosis
Direction of osmosis Condition Net Movement of
Water External solution is Into the
Cell Hypotonic to Cytosol External solution
is Out of the Cell Hypertonic to
Cytosol External solution is None Isotonic to
Cytosol
H2O
H2O
H2O
What condition results from each direction of
movement?
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Passive Transport - Osmosis

• Plasmolysis results from the continual movement
  of water molecules out of a cell, leading to
  dehydration of the cytosol and eventual wilting
• Cytolysis results from the continual movement of
  water molecules into a cell, which leads to
  flooding of the cytosol and eventual bursting

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Passive Transport - Facilitated Diffusion

• If molecules are too large or too insoluable to
  diffuse across the membrane, then they rely on
  carrier molecules to transport them across in a
  process called facilitated diffusion.

Help!
Glucose
Carrier Protein
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Passive Transport - Diffusion Thru Ion Channels

• Ions, too charged to cross the hydrophobic lipid
  layer, travel through ion channels.
• These channels are specific to the ion traveling
  through (e.g. Na, Ca2, Cl-).
• Some channels are gated others are open all the
  time.

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Active Transport - Cell Membrane Pumps

• Active transport requires ATP, since molecules
  travel against the concentration gradient.
• Carrier proteins in the membrane also serve as
  membrane pumps during active transport.

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Active Transport - Cell Membrane Pumps

• The Sodium-Potassium Pump is one of the most
  common. It moves
• Na outside the cell
• K inside the cell

• This works to maintain a concentration gradient
  across the membrane, facilitating normal
  functioning of the cell.

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Active Transport - Endocytosis

• Endocytosis is the process by which cells ingest
  external fluids, macromolecules, and even other
  cells. Endocytosis occurs in two forms

1. Pinocytosis is the transport of liquids or
solutes
2. Phagocytosis is the transport of larger solid
particles or whole cells
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Active Transport - Endocytosis

• To move particles in, the cell membrane folds in
  and forms a small pouch around the molecules
• This pouch eventually pinches off, becoming a
  vesicle and moving the particles to the cells
  interior

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Active Transport - Exocytosis

• Exocytosis is the reverse process of endocytosis
  (i.e., molecules move from the interior to the
  exterior of the cell).
• A vesicle containing wastes fuses with the cell
  membrane, dumping its contents to the environment
  external to the cell.

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20
Photosynthesis Intro
Photosynthesis is a biochemical pathway that
converts the suns radiant energy into chemical
energy in the form of glucose.
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
The net reaction for photosynthesis
sunlight 6CO2 6H2O C6H12O6 6O2
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Photosynthesis Intro 2
Where do the reactants for photosynthesis come
from?
The net reaction for photosynthesis
sunlight 6CO2 6H2O C6H12O6 6O2
Diffuses into leaf from atmosphere thru stomata
Absorbed by roots from soil via osmosis
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Photosynthesis Intro 3
A SEM micrograph of a stoma of a pea plant
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
23
Photosynthesis Intro 4
Which organisms are responsible for
photosynthesis?

• Only autotrophs can perform photosynthesis
• plants (responsible for 10 organic
  products)
• algae (90)
• cyanobacteria (lt1)

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Photosynthesis Intro 5
Interdependence in Nature via Energy Transfer
Photosynthesis
Autotrophs
Autotrophs Heterotrophs
Cell Respiration
25
Photosynthesis Light Absorption
How do plants absorb sunlight?
The chloroplasts on the surface of leaves contain
special pigments, called chlorophylls, that
absorb the red and blue wavelengths of sunlight.
They eventually transfer this energy into
chemical energy in the form of glucose.
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
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Photosynthesis Chloroplast
The chloroplast is the photosynthetic organelle
found in autotrophs.
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
27
Photosynthesis Chloroplast 2
A TEM micrograph of a chloroplast
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
28
Photosynthesis The Reactions

• The process of photosynthesis occurs in two
  phases
• The light reactions (a.k.a. the light-dependent
  reactions) take place in the thylakoid membrane
• The dark reactions (a.k.a. the light-independent
  reactions) take place in the stroma

During the light reactions, water is used to
generate a transfer of energy via electrons as it
picks up energy from the sun. During the dark
reactions, carbon dioxide uses that energy to
become fixed into glucose.
Note
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Photosynthesis Light Reactions
Step 1 Radiant energy strikes the leaf much is
absorbed by the chlorophyll a within Photosystem
II (PS II) and electrons there are excited
remainder is reflected back giving leaves their
green coloring.
2H2O O2 (4H) 4e-
Step 2 Inside the grana, the excited electrons
are picked up by the primary electron acceptor.
Step 3 The electrons are then passed thru a
series of molecules known as the electron
transport chain (ETC).
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Photosynthesis Light Reactions 2
Step 3
Step 2
Step 1
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
31
Photosynthesis Light Reactions 3
Step 4 Light excites electrons in Photosystem I
(PS I). As they move to their primary electron
acceptor, they are replaced by the electrons from
PS II.
Step 5 The electrons from PS I then move down
an ETC. At the end of the chain, the electrons
combine with NADP and H to form NADPH
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Photosynthesis Light Reactions 4
Step 5
Step 3
Step 2
Step 4
Step 1
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
33
Photosynthesis Chemiosmosis
Chemiosmosis is the process by which ATP is
generated due to a concentration of protons (H)
across the thylakoid membrane. These protons are
continually generated by the splitting of water
in the light reactions.
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
34
Photosynthesis Dark Reactions
The NADPH and ATP formed during the light
reactions, along with CO2, combine during the
dark reactions in a process called the Calvin
cycle.
The Calvin cycle uses a 5-C recycled molecule
called RuBP to fix inorganic CO2 into organic
6-C glucose.
From http//www.botany.uwc.ac.za/ecotree/leaves.h
tmtop
35
Photosynthesis Alternate Pathways
The Calvin cycle is the most common pathway for
carbon fixation by plants using a 3-C
intermediate however, some plants use other
pathways.
The C4 pathway fixes CO2 into a 4-C intermediate
in a water-conserving mechanism. Examples of C4
plants include corn sugarcane.
The CAM pathway allows for extreme water
efficiency in hot, dry climates by keeping
stomata closed during the day. Succulents and
cacti are examples of CAM plants.
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Photosynthesis Factors Influencing Photosynthesis

• The rate that a plant is capable of carrying out
  its photosynthetic function depends upon several
  environmental factors
• Light intensity
• CO2 availability
• Temperature

CO2
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Cell Respiration Intro
Cellular respiration is the biochemical pathway
responsible for the conversion of chemical energy
in the form of glucose into cellular energy in
the form of ATP.
ATP
Glucose
The net equation for cell respiration C6H12O6
6O2 6CO2 6H2O ATP
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Cell Respiration Intro 2
Which types of organisms undergo cell respiration?
All aerobic eukaryotic organisms, since the
possession of mitochondria is a necessity.
How might prokaryotes generate ATP?
Bacteria undergo fermentation in the cytoplasm,
an alternate, less-efficient pathway.
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Cell Respiration Intro 3
Where do organisms get the reactants for cell
respiration from?
The net equation for cell respiration C6H12O6
6O2 6CO2 6H2O ATP
and consumed by heterotrophs as they eat!
Oxygen simply diffuses across a membrane, as in
the lung of mammals, gill of fishes, or trachea
of insects
Glucose is produced by autotrophs during
photosynthesis
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Cell Respiration The Mitochondria
The mitochondria is the organelle of cell
respiration. Its double membrane allows for
extreme specialization, and its inner folds
callled cristae allow for maximum surface area.
From http//www.people.virginia.edu/rjh9u/eltran
s.html
From http//www.ultranet.com/7Ejkimball/BiologyP
ages/C/CellularRespiration.html
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Cell Respiration The Mitochondria 2
TEM micrographs of mitochondria
From http//biology.about.com/library/weekly/aa04
0600a.htm
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Cell Respiration The Mitochondria 3
Depictions of the Internal Structure
From http//projects.edtech.sandi.net/miramesa/Or
ganelles/mito.html
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Cell Respiration The Phases

• Cell Respiration occurs in three steps
• Glycolysis
• Citric Acid Cycle (CAC) aka Krebs
• Electron Transport Chain (ETC)

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Cell Respiration Glycolysis
Glycolysis (breaking sugar) is the process of
breaking down 1 6-C glucose molecule into two
(3-C) pyruvate molecules.

• Glycolysis occurs in 2 phases
• Energy-investment phase
• Energy-yielding phase

2 ATP molecules must be invested to begin the
breakdown of glucose before 4 can be generated
during oxidation-reduction reactions, resulting
in 2 net ATP.
WHAT?!?!
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Cell Respiration Glycolysis 2
-1
The breakdown of glucose into 2 pyruvate
molecules, with net gain of 2 ATP.
-1
From http//gened.emc.maricopa.edu/bio/bio181/BIO
BK/BioBookGlyc.html
Along the pathway, NAD is reduced to NADH.
2
Pyruvic acid is produced.
2
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Cell Respiration Citric Acid Cycle
The pyruvic acid produced during glycolysis is
first modified into Acetyl CoA
From http//gened.emc.maricopa.edu/bio/bio181/BIO
BK/BioBookGlyc.html
Notice that NAD is again reduced to NADH during
the process, and CO2 is given off.
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Cell Respiration Citric Acid Cycle 2
The 2-C Acetyl CoA then joins a 4-C recycled
molecule to produce 6-C citrate. This molecule
in transformed during the Citric Acid Cycle.
From http//gened.emc.maricopa.edu/bio/bio181/BIO
BK/BioBookGlyc.html
2 ATPs are produced per glucose molecule in the
CAC!
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Cell Respiration Citric Acid Cycle 3
Another view, this time with intermediates shown
in more detail
From http//www.ultranet.com/7Ejkimball/BiologyP
ages/C/CellularRespiration.html
49
Cell Respiration Electron Transport Chain
The ETC is a series of molecules embedded in the
membrane of the mitochondria that have a high
electron affinity.
From http//gened.emc.maricopa.edu/bio/bio181/BIO
BK/BioBookGlyc.html
NADH is the primary electron acceptor, but each
neighbor in the ETC is more electronegative than
the previous, so the electron moves down the
chain. Oxygen is the final electron acceptor and
produces water.
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Cell Respiration Electron Transport Chain 2

• The ETC is helpful in 2 ways
• It allows an electron to be passed from Acetyl
  CoA to oxygen indirectly thru a series of
  intermediates, conserving energy.
• It involves a series of oxidation-reduction
  reactions, generating a gradient of H ions in
  the intermembrane space of the mitochondria. SO
  WHAT?!?!?

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Cell Respiration Chemiosmosis
The H molecules naturally try to establish
equilibrium across the mitochondrial membrane.
They are able to diffuse across with the help of
ATP synthase, a carrier protein complex. This
movement thru the membrane causes a
conformational change in the protein, which
results in the linking up of ADP to Pi to form
ATP!
From http//www.ultranet.com/7Ejkimball/BiologyP
ages/C/CellularRespiration.html
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Cell Respiration Chemiosmosis 2
Chemiosmosis due to the ETC produces the most ATP
than glycolysis or the Citric Acid Cycle. (30-32
per glucose)
From http//www.ultranet.com/7Ejkimball/BiologyP
ages/C/CellularRespiration.html
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Cell Respiration CAC ETC
The ETC close up and in action
From http//www.people.virginia.edu/rjh9u/eltran
s.html
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Cell Respiration ATP Production
From http//www.ultranet.com/7Ejkimball/BiologyP
ages/C/CellularRespiration.html
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Cell Respiration Fermentation
What happens if oxygen is not available but
energy is still needed?
Anaerobic fermentation allows for a less
efficient method of producing ATP. Glucose is
still broken down into pyruvate during glycolysis
(2 ATP), but the remaining fermentation only
regenerates NAD to further encourage glycolysis.
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Cell Respiration Fermentation 2
Fermentation occurs in one of two types,
depending upon the organism.
Lactic acid fermentation changes pyruvate into
lactic acid
From http//gened.emc.maricopa.edu/bio/bio181/BIO
BK/BioBookGlyc.html
and alcohol fermentation changes pyruvate into
ethyl alcohol.
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Cell Respiration Review
From http//gened.emc.maricopa.edu/bio/bio181/BIO
BK/BioBookGlyc.html