CHAPTER 5

The Working Cell

Energy

•The capacity to do work•Comes in two forms:Kinetic energy – it does worke.g. heat or light produced by

molecular movement/breaking of chemical bonds

- gravity or friction

Sect 5.1

Potential energy – it is stored energy

e.g. chemical energy stored in molecular bonds

(most important type of energy for living organisms)

Governing Laws of Energy Conversion

•Thermodynamics – the study of energy conversions in matter

sect 5.2

System-matter under study

Surroundings-everything outside the system

Open system-exchanges both energy & matter w/surroundings (living organism)

2 Laws of ThermodynamicsFirst Law of Thermodynamics – (Law of Energy Conservation) energy can be transferred and transformed but not created or destroyed

1) solar (light) energy is stored in glucose’s chemical bonds during photosynthesis

2) chemical energy in glucose is converted into a useable form for cells in ATP

Second Law of Thermodynamics - energy conversions reduce the state of order, or increases entropy (amount of disorder in a system) in the universe1) energy conversions are not 100% efficient, energy lost to the universe (usually heat or light)

2) energy is lost in every cellular chemical reaction

Life’s Chemical Reactions

•Endergonic reactions - require energy input (energy stored in covalent bonds of product molecules); e.g. photosynthesis producing glucose

Sect 5.3

Exergonic reactions - releases energy e.g. burning wood (1 step means releasing all at once), cellular respiration (many steps - heat and chemical energy of ATP)

Chemical energy converts glucose’s energy to ATP

Cellular metabolism - the sum of endergonic and exergonic reactions in an organism

ATP Shuttle

•ATP (Adenosine triphosphate) is the power unit that shuttles energy used in cellular work

• it uses energy coupling - using energy from exergonic reactions to drive endergonic reactions

Sect 5.4

•ATP uses unstable bonds between its phosphate groups

•the breakdown of glucose:

3 Things Happen:

1) a phosphate is removed

2) ATP becomes ADP

3) energy is released (p. 75)

Phosphorylation - the energy from ATP is actually released in the 3rd phosphate and transferred to a molecule

• most cellular work depends on ATP energizing other molecules by phosphorylating them Fig. 5.4B p. 75

• ATP is a renewable source of energy (recycle 10 million ATP each second)

How Enzymes Work

Speeding up Chemical Reactions

• energy of activation (EA) is the amount of energy needed to start a chemical reaction (breaking the bond between the 2nd and 3rd phosphate)

Sect 5.5

• in many essential metabolic processes in our body

•EA is too high for processes to occur quickly enough otherwise things would denature

• EA acts as a barrier in sustaining lifeEnzyme - protein molecule that serves as a biological catalyst

• enzymes lower this barrier by lowering the EA and speeding up the rate of reactions w/out being changed itself

Enzyme Specificity

•Very selective on the reactions they catalyze because of their 3-D shape

•Substrate(s) is the reactant in a chemical reaction the enzyme acts upon

Sect 5.6

•Enzyme and substrate bind at the enzyme’s active site (groove or pocket); this changes the substrate in the product(s)

•Because of an enzyme’s specificity for a given substrate(s), many different enzymes are needed to catalyze all of a cell’s reactions p.77

Cellular Environment• Body’s pH (6-8)• Temperature (35-40o C)• Salt concentration

(around 4%)

•Many enzymes need non-protein helpers: cofactors – inorganic molecules (Zn, Fe, Cu)

coenzymes – organic molecules made from vitamins or vitamins themselves

Sect 5.7

Enzyme Inhibitors

• Interfere with an enzyme’s activity

• Competitive Inhibitor(p. 78) – substrate that resembles the enzyme’s normal substrate and competes for the active site

Sect 5.8-5.9

•Noncompetitive Inhibitor (p.78)

-substrate that does not compete for the normal substrate’s active site

- binds at another active site

- alters the normal substrate’s active site (no binding occurs for the normal substrate)

•Most inhibition is reversible because of weak H-bonds and is used by cells to regulate metabolism•Feedback Inhibition – substrate exceeds demand and inhibits its enzyme until substrate levels become lower•Many pesticides and antibiotics irreversibly inhibit enzymes of the target organisms (covalent bonds form instead of H-bonds)

Membrane Form & FunctionsSelective Permeability

•Items needed by the cell enter and waste products leave through the PM

•All other substances are blocked from entry

Sect 5.10-5.13

Membrane Structure Phospholipid Bilayer• Composed of 2 layers w/each

layer made of molecules that have:- polar,hydrophilic head w/a phosphate group- 2 nonpolar,hydrophobic hydrocarbon tails

p. 79 Fig. A (look at 3 lines)

•In water, these molecules will spontaneously form the 2-layer framework called the phospholipid bilayer

water

Hydrophilic heads

Hydrophobic tails water

Fluid Mosaic

•Mosaic refers to the membrane surface w/many different embedded proteins

•Fluid refers to lateral drifting of individual proteins and phospholipids w/in the membrane

•Also embedded in the surface are glycoproteins and glycolipids (sugars attached to proteins or lipids) that recognize self from nonself

Ex: white blood cells, embryo

• Other membrane proteins function as:

1)Enzymes

2)Receptors of chemical messages from other cells which can relay a signal to inside the cell – signal transduction

3)Transport molecules through cell membrane

Traffic Across Membranes

•Passive Transport-Osmosis-Osmoregulation-Facilitated Diffusion

•Active Transport-ATP-Exocytosis-Endocytosis

Passive Transport

- based on diffusion of molecules across a cell membrane• no energy used

• movement of high [ ] of molecules down its concentration gradient toward low [ ] to achieve equilibriumO2 in lungs vs blood/CO2 in blood vs lungs

Sect 5.15

Osmosis – special case of diffusion of water across a cell membrane

• Hypertonic – soln has a relatively higher amount of solute molecules than water molecules

• Hypotonic – soln has a relatively higher amount of water molecules than solute molecules

•Isotonic – the [ ] of water molecules is the same on each side of the membrane

Sect 5.16

• water will move across a membrane from hypotonic to hypertonic to reach an equilibrium

•Tonicity – tendency of a cell in a given solution to lose or gain water

•Osmoregulation – a process of cells to control water balance; to prevent excessive loss of water

Plasmolysis – plant cell loses water, shrivel & plasma membrane pulls away from cell wall

Facilitated Diffusion:

- involves diffusion of molecules that cannot diffuse easily across a membrane

- uses a transport protein embedded in the membrane that forms a channel (pore)

Sect 5.15

Active Transport- moves molecules against the [ ] gradient across a cell membrane

• requires energy (ATP)

•Many transport systems involve the movement of one substance out w/one substance into the cell

Ex: Na-K pump

Sect 5.18

Movement of Large Molecules/Substances

Exocytosis – export of substances from the cytoplasm through vesicles that fuse w/the cell membrane

Ex: tears, hormones

Sect 5.19

Endocytosis – import of substances into the cytoplasm through vesicles formed by the cell membrane

• Phagocytosis – “cell eating” – engulfing large particles (bacteria)

• Pinocytosis – “cell drinking” – nonspecific engulfing droplets of liquids w/any dissolved solutes

• Receptor-mediated endocytosis – highly specific on what is to be engulfed; has receptor proteins that pick up specific moleculesWhen does this go wrong?

Hypercholesterolemia – excessively high levels of cholesterol in the blood

LDL (low-density lipoprotein) vs HDL (high-density lipoprotein)

- caused by faulty receptor-mediated endocytosis in liver cell membranes that do not remove excess cholesterol from the blood