structure of the cell membrane

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.

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

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.

Osmosis• The diffusion of water through a

selectively permeable membrane.

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.

Effect of Tonicity on Cells

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])

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)

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

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)

Life depends on the fact that energy can be converted from one form to another.

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

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

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?

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?

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?

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?

Potential Energy Diagrams

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

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?

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.

Energy fromexergonicreactions

Energy forendergonicreactions

Key Point: ATP is a renewable resource the cell can regenerate.

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

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

a.

b.c.

d.

e.

f.

Enzyme (sucrase)

Active siteSubstrate(sucrose)

Induced Fit

Products released

Substrate hydrolyzed

Catalytic cycle of an enzyme:

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

Competitive vs. Non-Competitive Inhibition

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