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Fluid mosaic’s are a “mosaic” in having diverse protein molecules embedded in a
framework of phospholipids. The membrane is “fluid” in that most of these molecules can
drift about in the membrane. Double bonds in the unsaturated fatty acid tails of many
phospholipids produce kinks that prevent phospholipids from packing tightly together,
keeping the membrane fluid. In animal cells, the steroid cholesterol, wedged into the
bilayer, helps stabilize the membrane at warm temperatures but also helps keep the
membrane fluid at lower temperatures. Some proteins give the membrane a stronger
framework. These proteins called integrins, span the membrane and attach to the
cytoskeleton on the inside and the extracellular matrix (ECM) on the outside. Glycoproteins
are involved in cell-cell recognition, a second function of plasma membrane proteins.
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Enzymes may work as a team to carry out sequential steps in pathways.
Other proteins function as receptors for chemical messengers from other cells. A receptor
protein has a shape that fits a specific messenger, such as a hormone. Often the binding of
the messenger to the receptor triggers a chain reaction involving other proteins, which
relay the messages to molecules that perform specific functions inside the cell. This
message-transfer process called SIGNAL TRANSDUCTION. A final important function of
membrane proteins is in transport. Membranes exhibit SELECTIVE PERMEABILITY; that is
they allow some substances to cross more easily than others. Non polar molecules can
easily pass through the membrane, in contrast polar molecules have a difficult time
crossing the membrane.
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Phospholipids could spontaneously self-assemble into simple membranes. When a mixture
of phospholipids and water is shaken, the phospholipids organize into bilayers surrounding
water filled bubbles. A membrane can enclose a solution that is different in composition
from its surroundings. A plasma membrane that allows cells to regulate their chemical
exchanges with the environment is a basic requirement for life.
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DIFFUSION is the tendency for particles of any kind to spread out evenly in an available
space, moving from where they are more concentrated to regions where they are less
concentrated. Molecules vibrate and move randomly as a result of a type of energy called
thermal motion (heat). Diffusion requires no work; it results from the thermal motion of
atoms and molecules. Because a cell does not perform work when molecules diffuse across
its membrane, the diffusion of a substance across a biological membrane is called PASSIVE
TRANSPORT.
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OSMOSIS- The diffusion of water molecules across a selectively permeable membrane. The
picture shows what happens if a membrane permeable to water but not to a solute
separates two solutions with different concentrations of solute. The solution on the right
side initially has a higher concentration of solute than that on the left. As you can see,
water crosses the membrane until the solute concentrations are equal on both sides. There
is a net movement of water down its own concentration gradient, from the solution with
the lower solute concentration and more free water molecules to that with the higher
solute concentration and fewer free water molecules.
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The term TONICITY describes the ability of a solution to cause a cell to gain or lose water.
The tonicity of a solution mainly depends on its concentration of solutes that cannot cross
the plasma membrane relative to the concentrations of solutes in the cell. The solute
concentration of a cell and its ISOTONIC environment are essentially equal, and the cell
gains water at the same rate that it loses it. In a HYPOTONIC solution, a solution with a
solute concentration lower than that of the cell. The cell gains water, swells, and may burst
(Lyse) like an overfilled balloon. An animal cell in a HYPERTONIC solution, a solution with a
higher solute concentration. The cell shrivels and can die from water loss. For an animal to
survive in a hypotonic or hypertonic environment, it must have a way to prevent excessive
uptake or excessive loss of water. The control of water balance is called
OSMOREGULATION.
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Recall that non-polar, hydrophobic molecules can dissolve in the lipid bilayer of a
membrane and cross it with ease. Numerous substances that do not diffuse freely across
membranes because of their polarity or charge can move across a membrane with the help
of specific transport proteins. When one of these proteins makes it possible for a substance
to move down its concentration gradient, the process is called FACILITATED DIFFUSION.
Facilitated diffusion is a type of passive transport because it does not require energy. A
common type of transport protein, which provides a hydrophilic channel that some
molecules or ions use as a tunnel through the membrane. Another type of transport
protein binds its passenger, changes shape, and releases its passenger on the other side.
The very rapid diffusion of water into and out of certain cells, such as plant cells, kidney
cells, and red blood cells, is made possible by transport proteins called AQUAPORINS.
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In ACTIVE TRANSPORT a cell must expend energy to move a solute against its concentration
gradient- that is, across a membrane toward the side where the solute is more
concentrated. The cell’s energy molecule ATP supplies the energy for most active transport.
1.) The process begins when solute on the cytoplasmic side of the plasma membrane
attaches to a specific binding site on the transport protein.
2.) ATP then transfers one of its phosphate groups to the transport protein.
3.) Causing it to change shape in such a way that the solute is released on the other side of
the membrane.
4.) Then the phosphate group detaches, and the transport protein returns to its original
shape, ready for a new round of active transport.
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A cell uses the process of EXOCYTOSIS to export bulky materials such as proteins or
polysaccharides. A transport vesicle filled with macromolecules buds from the Golgi
apparatus and moves to the plasma membrane. Once there the vesicle fuses with the
plasma membrane, and the vesicle’s contents spill out of the cell while the vesicle
membrane becomes part of the plasma membrane. ENDOCYTOSIS is a transport process
that is the opposite of exocytosis. In endocytosis, a cell takes in substances. A depression in
the plasma membrane pinches in and forms a vesicle enclosing material that had been
outside the cell. PHAGOCYTOSIS or “Cellular eating”. A cell engulfs a particle by wrapping
extensions called PSEUDOPODIA around it and packaging it within a membrane-enclosed
sac large enough to be called a vacuole.
PINOCYTOSIS or “Cellular drinking” The cell “gulps” droplets of fluid into tiny vesicles.
Pinocytosis is not specific; it takes in any and all solutes dissolved in the droplets.
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RECEPTOR_MEDIATED ENDOCYTOSIS is highly specific, in contrast to pinocytosis. Receptor
proteins for specific molecules are embedded in regions of the membrane that are lined by
a layer of coat proteins.
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ENERGY is defined as the capacity to perform work. Work is performed when an object is
moved against an opposing force, such as gravity or friction. Energy is the capacity to
rearrange matter. There are two basic forms of energy; Kinetic energy and potential energy
KINETIC ENERGY is the energy of motion. Moving objects can perform work by transferring
motion to other matter. HEAT or THERMAL ENERGY is a form of kinetic energy associated
with the random movement of atoms or molecules. Light is another kind of kinetic energy
that can be harnessed to perform work, such as powering photosynthesis in green plants.
POTENTIAL ENERGY, the second form of energy, is stored energy that an object possesses
as a result of its location or structure. Likewise, molecules possess potential energy
because of the arrangement of their atoms. CHEMICAL ENERGY is a term that refers to the
potential energy available for release in a chemical reaction.
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THERMODYNAMICS is the study of energy transformations that occur in a collection of
matter.
The first law of thermodynamics:
FIRST LAW OF THERMODYNAMICS also known as the law of energy conservation, the
energy in the universe is constant. Energy can be transferred and transformed, but it
cannot be created or destroyed.
The second law of thermodynamics:
It turns out that in every energy transfer or transformation, some energy becomes
unusable-unavailable to do work. In most energy transformations, some energy is
converted to heat, the energy associated with random molecular motion. Scientists use a
quantity called ENTROPY as a measure of disorder, or randomness. SECOND LAW OF
THERMODYNAMICS, energy conversions increase the entropy (disorder) of the universe.
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Chemical reactions are two types: They either release energy, or they require an input of
energy and store energy.
An EXERGONIC REACTION is a chemical reaction that releases energy. The reaction releases
to the surroundings an amount of energy equal to the difference in potential energy
between the reactants and the products.
CELLULAR RESPIRATION is a chemical process that uses oxygen to convert the chemical
energy stored in fuel molecules to a form of chemical energy that the cell can use to
perform work.
The other type of chemical reaction requires a net input of energy. ENDERGONIC
REACTIONS yield products that are rich in potential energy. Energy is absorbed from the
surroundings as the reaction occurs, so the products of an endergonic store more energy
than did the reactants.
The total of an organisms chemical reactions is called METABOLISM. A METABOLIC
PATHWAY is a series of chemical reactions that either builds a complex molecule or breaks
down a complex molecule into simpler compounds.
ENERGY COUPLING is the use of energy released from exergonic reactions to drive essential
en endergonic reaction.
ATP molecules is the key to energy coupling.
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ATP powers nearly all forms of cellular work. The structure of ATP, or adenosine
triphosphate. The adenosine part of ATP consists of adenine, a nitrogenous base and
ribose, a five carbon sugar. The triphosphate part is a chain of three phosphate groups.
When the bond of the third group breaks, a phosphate group leaves ATP- which becomes
ADP and energy is released.
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The hydrolysis of ATP is an exergonic reaction- it releases energy. By transferring the third
phosphate group from ATP to some other molecule. This transfer is called
PHOSPHORYLATION, and most cellular work depends on ATP energizing molecules by
Phosphorylating them. There are three main types of cellular work: Chemical, Mechanical,
and Transport.
In chemical work the phosphorylation of reactants provides energy to drive the endergonic
synthesis of products.
In mechanical work the transfer of of phosphate groups to special motor proteins in muscle
cells causes the protein to change shape and pull on actin filaments, in turn causing the
cells to contract.
In transport work ATP drives the active transport of solutes across a membrane against
their concentration gradient by phosphorylating certain membrane proteins.
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There is an energy barrier that must be overcome before a chemical reaction can begin.
Energy must be absorbed to contort or weaken bonds in reactant molecules so that they
can break and new bonds form.
ENERGY OF ACTIVATION (Ea) is the amount of energy needed to push the reactants over an
energy barrier, or “hill”, so that the “downhill” part of the reaction can begin.
One way to speed up the reactions is to add heat. But heating a cell would speed up all
reactions, not just the necessary ones, and too much heat would kill the cell.
ENZYMES proteins that function as biological catalysts, increasing the rate of a reaction
without being consumed by the reaction. An enzymes speeds up a reaction by lowering the
Ea barrier.
The black curve represents the course of the reaction without an enzyme; the Ea barrier is
higher than in the reaction with an enzyme (red curve).
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As a protein, an enzyme has a unique 3D shape, and that shape determines which chemical
reaction the enzyme catalyzes. A specific reactant that an enzyme acts on is called the
enzyme’s SUBSTRATE. A substrate fits into a region of the enzyme called an ACTIVE SITE. An
active site is typically a pocket of groove on the surface of the enzyme formed by only a few
of the enzyme’s amino acids. Enzyme’s are specific because their active sites fit only
specific substrate molecule.
1.) Sucrase starts with an empty active site.
2.) Sucrose enters the active site, attaching by weak bonds. The active site changes shape
slightly so that it embraces the substrate more snugly, like a firm handshake. This INDUCED
FIT may strain substrate bonds or place chemical groups of the amino acids making up the
active site in position to catalyze the reaction.
3.) The strained bond reacts with water, and the substrate is converted (hydrolyzed) too the
products glucose and fructose.
4.) The enzyme releases the products and emerges unchanged from the reaction.
OPTIMAL CONDITIONS FOR ENYZMES:
For every enzyme, there are conditions under which it is most effective. Temperature for
instance, affects molecular motion, and an enzyme’s optimal temperature produces the
highest rate of contact between reactant molecules and the enzyme’s active site. Higher
temperatures denature the enzyme, altering its specific shape and destroying its function
COFACTORS:
Many enzyme’s require non protein helpers called COFACTORS. The cofactors of some
enzymes are inorganic, such as the ions of zinc, iron, or copper. If the cofactor is an organic
molecule, it is called a COENZYME.
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A chemical that interferes with an enzyme’s activity is called an INHIBITOR. If an inhibitor
attaches to the enzyme by covalent bonds, the inhibition is usually irreversible. Inhibition is
reversible when weak interactions bind inhibitor and enzyme.
A COMPETITIVE INHIBITOR reduces an enzyme’s productivity by blocking substrates from
entering the active site. Competitive inhibition can be overcome by increasing the
concentration of substrate, making it more likely that a substrate molecule will be nearby
when an active site becomes vacant.
A NONCOMPETITIVE INHIBITOR does not enter the active site. Instead it binds to the
enzyme somewhere else, and its binding changes the shape of the enzyme so that the
active site no longer fits the substrate.
If a cell is producing more of that product than it needs, the product may act as an inhibitor
of one of the enzymes early in the pathway. This sort of inhibition, whereby a metabolic
reaction is blocked by its products, I called FEEDBACK INHIBITION and is one of the most
important mechanisms that regulates metabolism.
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