Transcript Chapter 4 How Cells Work Energy • Energy is central to life – Universal relationship between energy and work • Ultimate energy source = SUN –

Chapter 4
How Cells Work
Energy
• Energy is central to life
– Universal relationship between energy and
work
• Ultimate energy source = SUN
– Plants transform light energy into chemical
energy (C6H12O6)
– Photosynthesis
What is energy?
• Energy = capacity to do work
• Work = movement against an opposing
force
The Energy Currency Molecule
• Adenosine triphosphate, ATP
• Structure of ATP
– Adenosine
• Ribose + adenine
– 3 phosphate groups
• Negatively charged molecules that repel each
other
– 2 HIGH ENERGY phosphate bonds
How do cells use ATP?
• Breakage of the last bond (release of a
phosphate group) releases energy and allows
the cell to do work
• ATP = STORED ENERGY = Potential
• ADP = CELL PERFORMED WORK = Kinetic
How Cells Use ATP
ATP → ADP + Pi + energy
How do cells use ATP?
• From where does the energy to make ATP
from ADP come?
– Covalent bonds in macromolecules!
– Cells recycle the ADP and phosphates
– This process requires ENERGY!!
– Analogous to recharging a battery:
• The components are in the battery – energy needs
to be added to the battery to make it useable
Energy Reactions and Cycles
• Endergonic Reactions
– Require energy
– i.e. Synthesis of glucose from CO2 and water
during photosynthesis
• Exergonic Reactions
– Release energy
– i.e. Breakdown of glucose to CO2 and water by
aerobic respiration
Endergonic
Exergonic
Enzymes
• Reaction characteristics
– Exergonic reactions in living things may not
occur very quickly
• Energy of Activation (high temperature, light) is
needed to start the reaction
• Amount of energy needed to start a reaction
– Many different reactions are needed to
complete a task
• These reactions are linked together
Activation Energy
Ways to Lower the Energy of
Activation
• Enzymes
– Protein catalysts that lower the amount of energy
needed to get the chemical reaction going
– They maintain their original chemical composition
while causing a change in the substrate (reactant)
– The specific shape of the enzyme allows it to
catalyze only one reaction
– Active site = place on the enzyme that binds
substrate
– Since the enzyme does NOT change its shape, it is
REUSABLE
Figure 5.5
Figure 5.6
Figure 5.7
Altering the Rate of an
Enzymatic Reaction
• One can alter the rate by altering two key
factors:
1. Temperature
2. pH
3. Coenzymes and Cofactors
4. Allosteric Regulators
5. Salt Concentration
Altering Temperature
• Gradual ↑ in temperature will INCREASE the rate of
the reaction
– How? By an increase in the speed at which the molecules
are moving
– This results in increased collisions of the enzyme and
substrate
• Extremely low temperatures will SLOW DOWN or
STOP the reaction
– Why? The enzyme and substrate are moving too slow to
collide
• Extremely high temperatures will STOP the reaction
– Why? Because the enzyme will be denatured!
Altering pH
• Alterations in pH will STOP the reaction
because the enzyme will be denatured!
• Remember, a small pH change does NOT
correlate with a small change in the pH of
the environment!!
– Why? pH scale is logarithmic
Figure 5.8
Enzyme Questions
• The presence of an enzyme _____ the required
energy of activation of a chemical reaction.
• Generally, as the amount of substrate is
increased, the rate of the reaction _____.
• Raising the temperature to over 50C ___ the
rate of an enzymatic reaction.
• Lowering the pH for an enzyme that works best
in a highly acidic environment ___ the rate for
the reaction.
•
Ways that substances can move
across the PM
Passive
– Process that does NOT require energy
– Includes:
• Diffusion
• Osmosis
• Active
– Process that DOES REQUIRE energy!
– Includes:
• Endocytosis
– Phagocytosis
– Pinocytosis
– Receptor-Mediated Endocytosis
• Exocytosis
How exactly do things move back
and forth across a cell’s plasma
membrane??
Selective Permeability
• Protein channels located in the plasma
membrane act as channels
• Each channel passes only a certain kind of
molecule (some are specific, some nonspecific)
• Types of selective permeability
– Selective Diffusion
– Facilitated Diffusion
– Active Transport
Selective Diffusion
• Movement of molecules from high
concentration to low concentration
• Channels may act as ‘open doors’
• Example includes ion channels
– Ion channels allow passage of any ion that
can fit in the channel
– Essential roles in nervous system signaling
Diffusion
• Oxygen, Carbon dioxide (CO2) and lipids can pass across the
PM using diffusion
• One way in which water and other substances can move across
the PM
• RANDOM movement of molecules in a solution from regions of
HIGH concentration to regions of LOW concentration
– HIGH
low
• Random movement occurs until equilibration occurs
– Until there is NO NET MOVEMENT in any particular
direction
– NOTE: Individual molecules are still moving – but there is no
overall directionality!
Diffusion
Terms to know:
• Concentration gradient
– A system that is imposed on a solution by
molecules present in that solution.
– Ex. Sugar in water
• When sugar is dropped in water, the sugar
molecules break up and dissolve over time. The
individual sugar molecules moving into the water
move DOWN their concentration gradient – they
are moving from the cube of sugar to spread out in
the water where there is no sugar.
Osmosis
• Movement of WATER ONLY across the PM from
the side with more water (less solute) to the side
with less water (more solute)
• Water passes into and out of a cell down its
concentration gradient (DIFFUSION)
• DIFFERENT from diffusion in that water
movement depends upon the concentration of
other substances in solution
Osmosis
Terms to know:
• Osmotic concentration
– Concentration of ALL molecules dissolved in
a solution
• Hypertonic
– The solution with higher solute concentration
• Hypotonic
– The solution with lower solute concentration
• Isotonic
– Solutions are isotonic when the solute
concentrations of both are equal
Figure 4.27
Osmosis
Osmotic pressure
• Generated by movement of water into a
cell by osmosis
• Ex. Red blood cell, Figure 4.28
Osmosis
What is another way that cells
can take in food and liquids?
• Diffusion
• Osmosis
• Endocytosis
– Phagocytosis
– Pinocytosis
– Receptor-Mediated Endocytosis
• Exocytosis
Endocytosis
• Allows for BULK PASSAGE of food and
liquids INTO the cell
• Two types:
– Phagocytosis
• “Cell eating”
– Pinocytosis
• “Cell drinking”
• The PM engulfs the particle(s) forming a
vesicle thus allowing a means of entry into
the cell
Endocytosis
Phagocytosis:
• “Cell eating”
• Material that the cell takes in may include
particulate, digested particles or other
fragments of organic matter
Pinocytosis:
• “Cell drinking”
• Material that the cell takes in is liquid
Phagocytosis and Pinocytosis
Endocytosis, continued
• Rates of endocytosis vary among cells!
• Ex. Muscle cells during exercise
Exocytosis
• Process by which material is discharged
from the cell
• Material to be discharged is packaged into
vesicles inside the cell (by what
organelle?)
• Vesicles then make they way (along
what?) to the plasma membrane for
secretion into the cell exterior
Exocytosis
Problems with endocytosis
• Expensive for the cell
– Cell uses a lot of its membrane to form
vesicles
• Non selective
– Anything can enter the cell through
Everyday Science
Hypercholesterolemia
• Human genetic disease
• Receptors are normally embedded in the PM
• In patients with HC, the receptors are not help
in place by clathrin
• This results in a failure of cholesterol uptake
into the cell (failure of the mouse-trap triggering
mechanism) thus leaving the cholesterol to
travel though the bloodstream and bind to
arteries
Discussion Question
• Do you think muscle cells have a higher or
lower rate of endocytosis during exercise?