Transcript The Working Cell Ch. 5

The Working Cell
Ch. 5
A. Forms of Energy
• 1. Energy is capacity to do work; cells
continually use energy to develop, grow, repair,
reproduce, etc.
• 2. Kinetic energy is energy of motion; all moving
objects have kinetic energy.
• 3. Potential energy is stored energy.
• 4. Food is chemical energy; it contains potential
energy.
• 5. Chemical energy can be converted into
mechanical energy, e.g., muscle movement.
Two Laws of
Thermodynamics
First law of thermodynamics
called the law of conservation of energy)
(also
• a. Energy cannot be created or destroyed, but it
can be changed from one form to another.
• b. In an ecosystem, solar energy is converted to
chemical energy by the process of
photosynthesis; some of the chemical energy in
the plant is converted to chemical energy in an
animal, which in turn can become mechanical
energy or heat loss.
continued
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c. Neither the plant nor the animal
create energy, they convert it from
one form to another.
d. Likewise, energy is not
destroyed; some becomes heat that
dissipates into the environment.
Second law of
thermodynamics
• a. Energy cannot be changed from
one form into another without a loss
of usable energy.
• b. Heat is a form of energy that
dissipates into the environment; heat
can never be converted back to
another form of energy.
Cells and Entropy
• 1. Every energy transformation makes the
universe less organized and more disordered;
entropy is the term used to indicate the relative
amount of disorganization.
• 2. When ions distribute randomly across a
membrane, entropy has increased.
• 3. Organized/usable forms of energy (as in the
glucose molecule) have relatively low entropy;
unorganized/less stable forms have relatively high
entropy.
continued
• 4. Energy conversions result in heat;
therefore, the entropy of the universe is
always increasing.
• 5. Living things depend on a constant
supply of energy from the sun, because
the ultimate fate of all solar energy in the
biosphere is to become randomized in the
universe as heat; the living cell is a
temporary repository of order purchased
at the cost of a constant flow of energy.
Metabolic Reactions and
Energy Transformations
• 1. Metabolism is the sum of all the
biochemical reactions in a cell.
• 2. In the reaction A + B = C + D, A and B
are reactants and C and D are products.
• 3. Free energy ( G) is the amount of
energy that is free to do work after a
chemical reaction.
continued
• 4. Change in free energy is noted as G; a
negative G means that products have less free
energy than reactants; the reaction occurs
spontaneously.
• 5. Exergonic reactions have a negative G and
energy is released.
• 6. Endergonic reactions have a positive G;
products have more energy than reactants; such
reactions can only occur with an input of energy.
ATP: Energy for Cells
• 1. Adenosine triphosphate (ATP) is the
energy currency of cells; when cells need
energy, they “spend” ATP.
• 2. ATP is an energy carrier for many
different types of reactions.
• 3. When ATP is converted into ADP + P,
the energy released is sufficient for
biological reactions with little wasted.
continued
• 4. ATP breakdown is coupled to
endergonic reactions in a way that
minimizes energy loss.
• 5. ATP is a nucleotide composed of the
base adenine and the 5-carbon sugar
ribose and three phosphate groups.
• 6. When one phosphate group is removed,
about 7.3 kcal of energy is released per
mole.
Coupled Reactions
• 1. Coupled reactions are reactions that
occur in the same place, at the same time,
and in a way that an exergonic reaction is
used to drive an endergonic reaction.
• 2. ATP breakdown is often coupled to
cellular reactions that require energy.
• 3. ATP supply is maintained by breakdown
of glucose during cellular respiration.
Metabolic Pathways and
Enzymes
• 1. Enzymes are catalysts that speed
chemical reactions without the enzyme
being affected by the reaction.
• 2. Every enzyme is specific in its action
and catalyzes only one reaction or one type
of reaction.
• 3. Ribozymes are made of RNA rather
than proteins and also serve as catalysts.
• 4. A metabolic pathway is an orderly sequence
of linked reactions; each step is catalyzed by a
specific enzyme.
• 5. Metabolic pathways begin with a particular
reactant, end with a particular end product(s),
and may have many intermediate steps.
• 6. In many instances, one pathway leads to the
next; since pathways often have one or more
molecules in common, one pathway can lead to
several others.
• 7. Metabolic energy is captured more
easily if it is released in small
increments.
• 8. A reactant is the substance that
is converted into a product by the
reaction; often many intermediate
steps occur.
Energy of Activation
• 1.
A substrate is a reactant for an enzymatic reaction.
• 2. Enzymes speed chemical reactions by lowering the
energy of activation (Ea) by forming a complex with their
substrate(s) at the active site.
• a. An active site is a small region on the surface of the
enzyme where the substrate(s) bind.
• b. When a substrate binds to an enzyme, the active site
undergoes a slight change in shape that facilitates the
reaction. This is called the induced fit model of enzyme
catalysis.
• 3. Only a small amount of enzyme is needed in a cell
because enzymes are not consumed during catalysis.
• 4. Some enzymes (e.g., trypsin) actually
participate in the reaction.
• 5. A particular reactant(s) may produce
more than one type of product(s).
• a. Presence or absence of enzyme
determines which reaction takes place.
• b. If reactants can form more than one
product, the enzymes present determine
which product is formed.
Factors Affecting
Enzymatic Speed
• 1. Substrate concentration.
• Because molecules must collide to
react, enzyme activity increases as
substrate concentration increases; as
more substrate molecules fill active
sites, more product is produced per
unit time.
2. Optimal pH
a. Every enzyme has optimal pH at
which its rate of reaction is optimal.
b. A change in pH can alter the
ionization of the R groups of the
amino acids in the enzyme, thereby
disrupting the enzyme’s activity.
3.
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Temperature
As temperature rises, enzyme activity
increases because there are more
enzyme-substrate collisions.
Enzyme activity declines rapidly when
enzyme is denatured at a certain
temperature, due to a change in shape of
the enzyme.
4. Enzyme cofactors
• a. Many enzymes require an inorganic ion or non-protein
cofactor to function.
• b. Inorganic cofactors are ions of metals.
• c. A coenzyme is an organic cofactor, which assists the
enzyme (i.e., it may actually contribute atoms to the
reaction).
• d. Vitamins are small organic molecules required in trace
amounts for synthesis of coenzymes; they become part of a
coenzyme’s molecular structure; vitamin deficiency causes a
lack of a specific coenzyme and therefore a lack of its
enzymatic action.
5.
Enzyme inhibition
• a. Enzyme inhibition occurs when a substance (called an
inhibitor) binds to an enzyme and decreases its activity;
normally, enzyme inhibition is reversible.
•
b. In noncompetitive inhibition, the inhibitor
binds to the enzyme at a location other than the active site
(the allosteric site), changing the shape of the enzyme and
rendering it unable to bind to its substrate.
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c. In competitive inhibition, the substrate and
the inhibitor are both able to bind to the enzyme’s active
site.
Organelles and the Flow
of Energy
Photosynthesis
• 1. Photosynthesis uses energy to combine carbon
dioxide and water to produce glucose in the
formula:
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6 CO2 + 6 H2O + energy = C6H12O6 + 6 O2
• 2. Oxidation is the loss of electrons.
• 3. Reduction is the gain of electrons.
• 4. When hydrogen atoms are transferred to
carbon dioxide from water, water has been
oxidized and carbon dioxide has been reduced.
• 5. Input of energy is needed to produce the high-energy
glucose molecule.
• 6. Chloroplasts capture solar energy and convert it by way
of an electron transport system into the chemical energy of
ATP.
• 7. ATP is used along with hydrogen atoms to reduce
glucose; when NADP+ (nicotinamide adenine dinucleotide
phosphate) donates hydrogen atoms (H+ + e-) to a substrate
during photosynthesis, the substrate has accepted
electrons and is therefore reduced.
• 8. The reaction that reduces NADP+ is:
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NADP+ + 2e- + H+ = NADPH
Cellular Respiration
• 1.
The overall equation for cellular respiration is opposite
that of photosynthesis:
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C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy
• 2. When NAD removes hydrogen atoms (H+ + e-) during
cellular respiration, the substrate has lost electrons and is
therefore oxidized.
• 3. At the end of cellular respiration, glucose has been
oxidized to carbon dioxide and water and ATP molecules
have been produced.
• In metabolic pathways, most oxidations involve the
coenzyme NAD+ (nicotinamide adenine dinucleotide); the
molecule accepts two electrons but only one hydrogen ion:
NAD+ + 2e- + H+ = NADH
Electron Transport Chain
• 1. Both photosynthesis and respiration use an
electron transport chain consisting of
membrane-bound carriers that pass electrons
from one carrier to another.
• High-energy electrons are delivered to the
system and low-energy electrons leave it.
• The overall effect is a series of redox reactions;
every time electrons transfer to a new carrier,
energy is released for the production of ATP.
ATP Production
• 1. ATP synthesis is coupled to the electron
transport system.
• 2. Peter Mitchell received the 1978 Nobel Prize
for his chemiosmotic theory of ATP production.
• 3. In both mitochondria and chloroplasts,
carriers of electron transport systems are
located within a membrane.
• 4. H+ ions (protons) collect on one side of the
membrane because they are pumped there by
specific proteins.
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5.
The electrochemical gradient thus established across the
membrane is used to provide energy for ATP production.
6.
Enzymes and their carrier proteins, called ATP synthase
complexes, span the membrane; each complex contains a channel
that allows H+ ions to flow down their electrochemical gradient.
7.
In photosynthesis, energized electrons lead to the pumping
of hydrogen ions across the thylakoid membrane; as hydrogen ions
flow through the ATP synthase complex, ATP is formed.
8.
During cellular respiration, glucose breakdown provides
energy for a hydrogen ion gradient on the inner membrane of the
mitochondria that also couples hydrogen ion flow with ATP
formation.
Fluid-Mosaic Model
• 1.
The fluid-mosaic model describes the plasma
membrane.
• 2. The fluid component refers to the phospholipids
bilayer of the plasma membrane.
• 3. Fluidity of the plasma membrane allows cells to be
pliable.
• 4. Fluidity is affected by cholesterol molecules in the
plasma membrane.
• 5. The mosaic component refers to the protein content in
the plasma membrane.
• 6. Protins bond to the ECM and/or cytoskeleton to
prevent movement in the fluid phospholipid bilayer
. Permeability of the
Plasma Membrane
The plasma membrane is
differentially (selectively)
permeable; only certain molecules
can pass through.
• a. Small non-charged lipid molecules (alcohol,
oxygen) pass through the membrane freely.
• b. Small polar molecules (carbon dioxide, water)
move “down” a concentration gradient, i.e., from
high to low concentration.
• c. Ions and charged molecules cannot readily
pass through the hydrophobic component of the
bilayer and usually combine with carrier proteins.
Both passive and active
mechanisms move molecules
across membrane.
• a. Passive transport moves
molecules across membrane without
expenditure of energy; includes
diffusion and facilitated transport.
• b. Active transport requires a
carrier protein and uses energy
(ATP) to move molecules across a
plasma membrane; includes active
transport, exocytosis, endocytosis,
and pinocytosis.
• 3. The presence of a membrane
channel protein called an aquaporin
allows water to cross membranes
quickly.
• 4. Substances enter or exit a cell
through bulk transport.
Passive Transport
Across a Membrane
• 1. Diffusion is the movement of
molecules from higher to lower
concentration (i.e., “down” the
concentration gradient).
Diffusion continued
• a. A solution contains a solute, usually a
solid, and a solvent, usually a liquid.
• b. In the case of a dye diffusing in water,
the dye is a solute and water is the
solvent.
• c. Once a solute is evenly distributed,
random movement continues but with no
net change.
Diffusion continued
• d. Membrane chemical and physical properties
allow only a few types of molecules to cross by
diffusion.
• e. Gases readily diffuse through the lipid
bilayer; e.g., the movement of oxygen from air
sacs (alveoli) to the blood in lung capillaries
depends on the concentration of oxygen in alveoli.
• f. Temperature, pressure, electrical currents,
and molecular size influence the rate of diffusion.
water across a differentially
(selectively) permeable
membrane
• a. Osmosis is illustrated by the thistle tube example:
• 1) A differentially permeable membrane separates two
solutions.
• 2) The beaker has more water (lower percentage of
solute) and the thistle tube has less water (higher
percentage of solute).
• 3) The membrane does not permit passage of the solute;
water enters but the solute does not exit.
• 4) The membrane permits passage of water with a net
movement of water from the beaker to the inside of the
thistle tube.
• b. Osmotic pressure is the pressure
that develops in such a system due to
osmosis.
• c. Osmotic pressure results in water
being absorbed by the kidneys and
water being taken up from tissue
fluid.
2. Tonicity is strength of a
solution with respect to osmotic
pressure.
• a. Isotonic solutions occur where
the relative solute concentrations of
two solutions are equal; a 0.9% salt
solution is used in injections because
it is isotonic to red blood cells
(RBCs).
• b. A hypotonic solution has a solute
concentration that is less than another
solution; when a cell is placed in a
hypotonic solution, water enters the cell
and it may undergo cytolysis (“cell
bursting”).
• c. Swelling of a plant cell in a hypotonic
solution creates turgor pressure; this is
how plants maintain an erect position.
• When a plant cell is placed in a
hypotonic solution, it is turgid.
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A hypertonic solution has a solute
concentration that is higher than
another solution; when a cell is placed in
a hypertonic solution, it shrivels (a
condition called crenation).
Plasmolysis is shrinking of the cytoplasm
due to osmosis in a hypertonic solution;
as the central vacuole loses water, the
plasma membrane pulls away from the
cell wall.
• In a hypotonic solution, an animal cell
will lyse.
3. Facilitated Transport
• a. Facilitated transport is the
transport of a specific solute “down”
or “with” its concentration gradient
(from high to low), facilitated by a
carrier protein; glucose and amino
acids move across the membrane in
this way.
Active Transport Across
a Membrane
• A. Active transport is transport of a
specific solute across plasma
membranes “up” or “against” (from
low to high) its concentration
gradient through use of cellular
energy (ATP).
• 1. Iodine is concentrated in cells of
thyroid gland, glucose is completely
absorbed into lining of digestive tract, and
sodium is mostly reabsorbed by kidney
tubule lining.
• 2. Active transport requires both carrier
proteins and ATP; therefore cells must
have high number of mitochondria near
membranes where active transport occurs.
• 3. Proteins involved in active transport
are often called “pumps”; the
sodium-potassium pump is an important
carrier system in nerve and muscle cells.
• 4. Salt (NaCl) crosses a plasma membrane
because sodium ions are pumped across,
and the chloride ion is attracted to the
sodium ion and simply diffuses across
specific channels in the membrane.
Bulk Transport
• 1. In exocytosis, a vesicle formed by the
Golgi apparatus fuses with the plasma
membrane as secretion occurs; insulin
leaves insulin-secreting cells by this
method.
• 2. During endocytosis, cells take in
substances by vesicle formation as plasma
membrane pinches off by either
phagocytosis, pinocytosis, or receptormediated endocytosis.
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In phagocytosis, cells engulf large
particles (e.g., bacteria), forming an
endocytic vesicle.
a.Phagocytosis is commonly performed by
ameboid-type cells (e.g., amoebas and
macrophages).
b.When the endocytic vesicle fuses with
a lysosome, digestion of the internalized
substance occurs.
• 4. Pinocytosis occurs when vesicles
form around a liquid or very small
particles; this is only visible with
electron microscopy.
endocytosis, a form of
pinocytosis, occurs when specific
macromolecules bind to plasma
membrane receptors.
• a. The receptor proteins are shaped
to fit with specific substances
(vitamin, hormone, lipoprotein
molecule, etc.), and are found at one
location in the plasma membrane.
• b. This location is a coated pit with a layer of
fibrous protein on the cytoplasmic side; when the
vesicle is uncoated, it may fuse with a lysosome.
• c. Pits are associated with exchange of
substances between cells (e.g., maternal and fetal
blood).
• d. This system is selective and more efficient
than pinocytosis; it is important in moving
substances from maternal to fetal blood.
• e. Cholesterol (transported in a molecule
called a low-density lipoprotein, LDL)
enters a cell from the bloodstream via
receptors in coated pits; in familial
hypocholesterolemia, the LDL receptor
cannot bind to the coated pit and the
excess cholesterol accumulates in the
circulatory system.
Modification of Cell
Surfaces
• A. Cell Surfaces in Animals
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The extracellular
matrix is a meshwork of
polysaccharides and proteins
produced by animal cells.
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Collagen gives the matrix strength and elastin gives it
resilience.
Fibronectins and laminins bind to membrane receptors
and permit communication between matrix and cytoplasm;
these proteins also form “highways” that direct the
migration of cells during development.
Proteoglycans are glycoproteins that provide a packing gel
that joins the various proteins in matrix and most likely
regulate signaling proteins that bind to receptors in the
plasma protein.
points of contact between
cells that allow them to
behave in a coordinated
manner.
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Anchoring junctions mechanically attach
adjacent cells.
In adhesion junctions, internal cytoplasmic
plaques, firmly attached to cytoskeleton within
each cell are joined by intercellular filaments;
they hold cells together where tissues stretch
(e.g., in heart, stomach, bladder).
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In desmosomes, a single point of
attachment between adjacent cells
connects the cytoskeletons of adjacent
cells.
In tight junctions, plasma membrane
proteins attach in zipper-like fastenings;
they hold cells together so tightly that
the tissues are barriers (e.g., epithelial
lining of stomach, kidney tubules, bloodbrain barrier).
A gap junction allows cells to
communicate; formed when two
identical plasma membrane
channels join
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They provide strength to the cells involved
and allow the movement of small molecules
and ions from the cytoplasm of one cell to
the cytoplasm of the other cell.
Gap junctions permit flow of ions for heart
muscle and smooth muscle cells to contract.