1. 5.2 Energy Transformations in a Cell
Metabolism - all chemical reactions in a cell
- cells constantly make and use energy
a) make energy:
exergonic reaction
- releases energy stored in molecules
- oxidation, catabolic, ex. cell respiration
b) use energy: endergonic reaction
- use energy (ATP) to do cell work
- reduction, anabolic, ex. move, transport, synthesize
Coupled reactions - energy released in exergonic reactions is used to power endergonic

2. 5.5 ATP shuttles energy in a cell
ATP - adenosine triphosphate
- bonds between phosphates are unstable
- easily broken  “high energy” bond
- energy released when bond breaks is used by cell

3. Coupled reactions:
ATP ADP Pi
triphosphate diphosphate phosphate group
Pi is free energy - powers cell work (endergonic)
phosphorylation - phosphate released from ATP is transferred to another molecule
- transfers energy to new molecule

4. ATP is renewable energy
ATP breakdown products (ADP + P) remain in cell
are used again to regenerate more ATP when needed
Energy from exergonic reactions
Energy for endergonicreactions
ATP made in cell respiration
ATP used for cellular work
Very fast!! 10 million ATP/second in a cell; needs help of enzymes

5. Why ATP?
-food molecules store large amount of potential energy in chemical bonds
- too much for cell to use at a time
- ex. $100 bill too much, easier to use $5 or $10
- cell respiration breaks down food molecule in many steps
- packages small amounts of energy in bonds of ATP

6. 5.4 How ATP powers cellular work
Membrane Transport
P  membrane protein
Protein changes shape
Protein pushes solute through membrane
P released
Biosynthesis
P  reactant
Reactants bond
- Product forms
P released
Mechanical Motion
P onto motor protein  protein changes shape
Pulls on muscle fibers  muscle contracts
P released

7. 5.10: Membranes organize chemical activities in a cell
a) keep structural order, make most organelles
b) compartments have specific enzymes
Plasma membrane - boundary between cell and its environment
- selectively permeable - controls what moves in or out
What goes through? Depends on:
- size of molecule
- charge (+ or -)
- concentration gradient
- type of cell
- needs of cell (oxygen and food in, wastes out)
- chemical signals to and within cell

8. 5.11: Phospholipids make up cell membranes
phospholipid - glycerol + two fatty acids + phosphate
a) hydrophilic head
- phosphates face outward, in contact with water
b) hydrophobic tails
- face inward, shielded from water
bilayer - lipid interior makes membrane selectively permeable
a) allows nonpolar and fat-soluble to pass through
b) polar, ions, and water-soluble can pass only with help of transport protein

9. 5.12: Membrane is a fluid mosaic of molecules
1. Mosaic - proteins and other molecules scattered among phospholipids
2. Fluid - molecules float and drift within layer
Membrane proteins – many functions

10. Fluid-Mosaic Model
Protein
Microfilaments

11. 4. Fatty acid tails – some chains unsaturated – kink
- keep molecules loosely-packed, membrane flexible
5. Cholesterol - in animal cells - embedded within fatty acids
- keeps phospholipids stable at body temperatures but fluid at cooler temps (ex. arctic organisms)
6. Carbohydrate antennae ‘tag” – on outer surface only
- identification and receptors

12. 5.13 Membrane proteins - many functions
1. Identification - self or non-self (ex. immunity)
2. Cell junctions - some anchor, some allow communication
3. Enzymes - catalyze reactions within membrane or inside cell
4. Receptors - for chemical messages from other cells (ex. hormones)
- signal transduction: relays a message from outside cell to specific molecule inside cell
- messenger binds to receptor
- relays message inside
- causes a response inside cell

13. Example: insulin receptors
- cause a different membrane protein to allow glucose into cell

14. Transport - move substances across the membrane
- “channels” or “pores”; some “carriers”
- small, nonpolar, fat-soluble can move between lipid molecules
- ex. oxygen, CO2, fatty acids
- larger, polar, and charged - need help of a membrane protein
- ex. glucose, water, ions
Structure – attach to cytoskeleton and to extra-cellular matrix
- cells connect to form tissues

15. 5.14: Passive Transport through a membrane
1. Diffusion - tendency of molecules to spread out evenly
- random motion of molecules – uses no cell energy
- net movement from area where highly concentrated to area where less concentrated
- reach equal concentrations, but still move randomly
Concentration gradient - difference in concentration between adjacent areas
- “down the concentration gradient” - a form of potential energy

16. Two or more solutes – each moves down its own gradient
Ex. Gas exchange in lungs

17. What goes through the membrane?
Small and nonpolar
- ex. gases (oxygen, carbon dioxide)
- fatty acids
What needs help to get through?
Larger, polar and charged particles
- ex. glucose, amino acids
- ions: Na+, K+, Ca+, Cl-

18. 5.15 Facilitated Diffusion - uses transport proteins
1. Membrane proteins help some molecules cross cell membrane
2. Down the gradient - no cell energy needed
3. Proteins -- specific for one substance
4. How do they do it?
a) some proteins have a “pore’ or “channel” (ex. water, ions)
some bind molecule on one side of membrane, change shape, release on other side (ex. glucose, amino acids)
“carrier” protein

19. 5.16: Osmosis - diffusion of water across a membrane
1. Selectively permeable membrane
- Dissolved solutes form a concentration gradient
- if solutes are present, fewer water molecules are free to diffuse
 creates a water concentration gradient
- ex. Pure water = 100% water; 3% solutes = 97% water
2. Water diffuses easily across membranes; many solutes don’t
Less water potential when solute is present: water molecules surround solute particles, fewer water molecules are free to diffuse through membrane

20. Osmotic Pressure – tendency of water to move across a membrane
Which way does water go?
Water moves from area of lower solute (higher water) concentration to area of higher solute (lower water) concentration.
- type of solutes doesn’t matter, only total concentration
Equilibrium – when concentrations are equal, or when force of gravity equals osmotic pressure

21. 5.17: Water balance in cells needed for homeostasis
Osmoregulation – control of water balance (all cells ~ 1% solutes)
Tonicity – tendency of cells to take in or lose water a) isotonic – same solute concentrations inside and outside of cell
- no net volume change
- ex. animal cells and intercellular fluid
b) hypotonic – lower solute concentration outside cell
- water moves into cell  cell swells
- animal cells may lyse; plant cells get turgid
c) hypertonic – higher solute concentration outside cell
- water leaves cell  cytoplasm shrinks
- plants (rigid wall) - plasmolysis

23. 5.18 Active transport uses cell energy
-membrane proteins move substances against the gradient
Cells can maintain a concentration that is NOT at equilibrium
- ex. Kidneys concentrate wastes in urine. return nutrients to blood
- ex. sodium-potassium pumps: in nerve cells, K+ and Na+ ion concentrations send nerve impulse
- ex. proton pumps: in respiration and photosynthesis; pump H+ across membranes

24. 5.19: Exocytosis and endocytosis – bulk transport
-large substances – too big to pass through membrane proteins
Exocytosis – enclose substance in membrane vesicle fuse with plasma membrane release contents outside cell
ex. Glands such as pancreas – insulin  bloodstream stomach, intestine – digestive enzymes  food cavity

25. Endocytosis – plasma membrane folds around substance,. - encloses it
Endocytosis – plasma membrane folds around substance, encloses it brings vesicle inside cell
phagocytosis (“cell eating”)
ex. amoeba
pseudopods wrap around food
pinocytosis (“cell drinking”) cell takes in droplets of fluid

26. “coated pit” on membrane
Receptor-mediated endocytosis
specific molecule attaches to receptor on plasma membrane
Plasma membrane encloses molecule  forms a vesicle
Example: cells take cholesterol from the blood this way
“coated pit” on membrane
cholesterol: essential for normal cell functioning and for making other lipids
a) carried in the blood by low-density lipoproteins (LDL)
b) LDL has a surface protein that fits a receptor on cell membranes; cells take it from the blood by receptor-mediated endocytosis

27. 5.20: hypercholesterolemia
If receptors faulty or missing, cholesterol remains in the blood  collects inside blood vessels (high risk for heart disease) and in pockets under the skin
Cholesterol in pockets under skin
cholesterol inside blood vessel

28. 5.21: Chloroplasts and mitochondria – make energy available to cell
These organelles have membrane assembly lines
Chloroplasts harvest energy from the sun
convert it into chemical energy in glucose & other foods
- endergonic
Mitochondria release chemical energy from food; exergonic
- convert it to chemical energy in ATP
- ATP is the only form of energy cells can use