structure of the cell membrane the composition of nearly all cell membranes is a double- layered...
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Structure of the Cell Membrane• The composition of nearly all cell membranes is a
double-layered sheet called a lipid bilayer. • Because the various components of the bilayer (phospholipids,
proteins, carbohydrate chains & cholesterol) are able to move around, it is often referred to as a fluid-mosaic model.
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Functions of the Plasma Membrane
• Embedded enzyme proteins help carry out chemical reactions of the cell
• Receptor proteins allow to receive chemical messages• Surface carbohydrate molecules allow recognition &
communication between cells• Transport proteins serve as channels or pumps to
move materials in and out of cells• Proteins from adjacent cells allow intracellular joining• Embedded cholesterol molecules provide
reinforcement
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Diffusion Through Cell Boundaries
• Particles in a solution tend to move from an area where they are more concentrated to an area where they are less concentrated. (visualize this as a downhill movement, [high] to [low])
• This process is called diffusion• When the concentration of the solute is the
same throughout a system, the system has reached equilibrium.
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Osmosis• The diffusion of water through a
selectively permeable membrane.
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Osmotic Pressure & Tonicity
• If you compare two solutions, the more concentrated solution (less water) is hypertonic.
• The more dilute (more water) solution is hypotonic.
• When the concentration is equal in both solutions, the solution is described as isotonic.
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Effect of Tonicity on Cells
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Facilitated Diffusion• Cell membranes have protein channels that act
as channels (pores) or carrier molecules, making it easy for certain molecules to cross.
• The movement of specific molecules across cell membranes through protein channels is known as facilitated diffusion.
• This process usually involves solute molecules (rather than solvent molecules) and is a form of passive transport because it requires no energy, ([high] to [low])
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Aquaporins• Membrane proteins
channels; Critical role in water transport into and out of cells (facilitated diffusion)
• At least 11 variants in humans; understanding of their specific physiological roles is currently a major area of research
• Important role in kidneys (maintain body fluid balance)
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Active Transport• Active transport involves the use of energy (ATP) to
move substances across a cell membrane• Ex) Movement of materials in the opposite direction
from which the materials would normally move - against a concentration difference
• This is achieved by membrane proteins that act as pumps
• Visualize this as UPHILL, [low] to [high])• Large materials also move by active transport regardless of concentration gradient
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Active Transport of Large Materials(“Bulk Transport”)
• Endocytosis is the process of taking large materials into the cell using vesicles, or pockets, of the cell membrane. (endo=enter)
– Phagocytosis – cell “eating”; actively moving solids into the cell
– Pinocytosis – cell “drinking”; actively moving liquids into the cell
• Exocytosis, the membrane of the vesicle surrounding the material fuses with the cell membrane, forcing the contents out of the cell (exo=exit)
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Life depends on the fact that energy can be converted from one form to another.
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EnergyEnergy• Energy: the capacity to rearrange matter/ “do
work”• There are two basic forms of energy:
– Kinetic energy- energy of motion; moving objects do work by transferring motion to other matter
• Examples: think of an example where this is true• Includes: heat (random motion of molecules) & light
– Potential energy- stored energy as a result of location or structure
• Includes: Chemical energy- energy available to cell to do work
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Energy Transformations
• Thermodynamics- study of energy transformation that occur in a system (collection of matter)
What is the difference between an open and closed system?
• 1st Law: Law of Energy Conservation• 2nd Law: Energy conversions increase entropy of the
universe– Efficiency of car vs cell energy conversions
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Fuel
Gasoline
Energy conversion in a cell
Energy for cellular work
Cellular respiration
Waste productsEnergy conversion
Combustion
Energy conversion in a car
Oxygen
Heat
Glucose
Oxygen Water
Carbon dioxide
Water
Carbon dioxide
Kinetic energyof movement
Heatenergy
Which do you think is more efficient?
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Explain the following statement:
“In a thermodynamic sense, a cell or an organism is an island of low entropy in an increasingly random universe.”
Concept:Chemical reactions either release or store energy.
How could one detect if energy was released or stored?
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Reactants
Amount ofenergy
released
Pot
enti
al e
ner
gy o
f mol
ecu
les
Energy released
Products
•In an exergonic reaction, energy is released.
•Bonds of reactants contain more energy than those in the products.Examples of exergonic reactions?
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Reactants
Pot
enti
al e
ner
gy o
f mol
ecu
les
Energy required
Products
Amount ofenergy
required
•Endergonic reaction, energy is absorbed from surroundings.
• Products contain more energy in bonds than reactants.Examples of endergonic reactions?
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Potential Energy Diagrams
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Identify the following as exergonic or endergonic.
• Use of gasoline in a lawn mower• Photosynthesis• Synthesis of a protein in a cell• Synthesis of glycogen in a cell• Cell Respiration• Hydrolysis of ATP
All the chemical reactions that take place in an organism is referred to as metabolism.A Metabolic pathway is a series of rxns that builds or breaks down a complex molecule (controlled steps)Energy released from exergonic rxns drives endergonic rxnsin a cell – energy coupling
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Ribose
Adenine
Triphosphate (ATP)
Adenosine
Phosphategroup
Hydrolysis
Diphosphate (ADP)Adenosine
ATP:a. Riboseb. Adeninec. 3 negatively charged phosphate groups
ATP analogy: “compressed spring””
Is the hydrolysis of ATP exergonic or endergonic?
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ATP drives cellular work
Cellular work:
1.Transport
2.Chemical
3.Mechanical
* Phosphorylating certain molecules results in work to be completed. Explain this based on what you learned about energy.
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Energy fromexergonicreactions
Energy forendergonicreactions
Key Point: ATP is a renewable resource the cell can regenerate.
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Why don’t macromolecules such as DNA, carbohydrates, lipids spontaneously break down into simpler less energetic molecules?
• Energy barrier must be overcome, bonds between atoms must be weakened
• Referred to as energy of activation (EA)
• How can this be accomplished?
1. add heat- problem?
2. enzyme- proteins that function as biological catalyst, lowers EA and are not consumed in reaction
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Reactionwithoutenzyme
EA with enzyme
En
erg
y Reactants
Reaction withenzyme
EA withoutenzyme
Netchangein energy(the same)
Products
Progress of the reaction
Figure 5.14 The effect of an enzyme is to lower EA
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a.
b.c.
d.
e.
f.
Enzyme (sucrase)
Active siteSubstrate(sucrose)
Induced Fit
Products released
Substrate hydrolyzed
Catalytic cycle of an enzyme:
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Enzyme Functioning
• If enzyme shape changes (denatured) then it can no longer function.
• Denaturation can be caused by: temperature, pH, pressure.
• Enzyme non-protein helpers: cofactor(inorganic- Zn, Fe, Cu ions) or coenzyme (organic- vitamins)
• Enzyme function can be blocked by an inhibitor
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Competitive vs. Non-Competitive Inhibition
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Enzyme Inhibitors• Important in regulating cell metabolism
– Feedback inhibition- product of metabolic reaction blocks the reaction from occurring
• Important to medicine and agriculture:– Component to medicines and pesticides-blocks enzyme
functioning– Some antibiotics work by inhibiting enzymes of
pathogenic bacteria– Some HIV drugs (proteases) target viral enzymes• Many toxins and poisons interrupt metabolism in
this way- cause irreversible effects- Nerve gases bind in the active site of an enzyme vital to
the transmission of nerve impulses- Some pesticides work by targeting the corresponding
enzyme in insects