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 non-specific)

• 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!

DiffusionTerms 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

OsmosisTerms 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

OsmosisOsmotic 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

EndocytosisPhagocytosis:

• “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 ScienceHypercholesterolemia

• 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?