1. Cells Maintain Their Internal Environments
Chapter 7
Cells Maintain Their Internal Environments

2. 1
Transport proteins 2
Pumps, carriers, and nutrient distribution
Cells, salts, and water balance 3

3. Cell Membrane Components of cell membrane
Lipid: phospholipids, sterols
Proteins : Receptor, adhesion proteins, recognition protein, transport protein
Membrane-spanning domain
Hydrophobic surface and hydrophilic core
Transport of sugars, amino acids, ions
Attachment to cytosolic or exterior face of membrane

4. Transport Across Membrane
Diffusion
Free diffusion by concentration gradient
Hydrophobic substance, nonpolar molecules (O2, CO2), small polar molecules (water, ethanol)
Transport proteins
Channel proteins
Transport of ions (Na+, K+, Ca2+, Cl-) along their concentration gradients
Aquaporin: channel for water
Gated channel
Carrier proteins
Escort metabolic building block along the concentration gradients

5. Simple Diffusion vs. Facillitated diffusion
FIGURE Energy changes accompanying passage of a hydrophilic solute through the lipid bilayer of a biological membrane. (a) In simple diffusion, removal of the hydration shell is highly endergonic, and the energy of activation (ΔG‡) for diffusion through the bilayer is very high. (b) A transporter protein reduces the ΔG‡ for transmembrane diffusion of the solute. It does this by forming noncovalent interactions with the dehydrated solute to replace the hydrogen bonding with water and by providing a hydrophilic transmembrane pathway.

6. Classification of Transporters
FIGURE Classification of transporters

7. FIGURE 11-25 Summary of transport types.

8. FIGURE 11-33 Three general classes of transport systems
FIGURE Three general classes of transport systems. Transporters differ in the number of solutes (substrates) transported and the direction in which each solute moves. Examples of all three types of transporters are discussed in the text. Note that this classification tells us nothing about whether these are energy-requiring (active transport) or energy-independent (passive transport) processes.

9. FIGURE Chloride-bicarbonate exchanger of the erythrocyte membrane. This cotransport system allows the entry and exit of HCO3– without changing the membrane potential. Its role is to increase the CO2-carrying capacity of the blood.

10. FIGURE 11-34 Two types of active transport
FIGURE Two types of active transport. (a) In primary active transport, the energy released by ATP hydrolysis drives solute movement against an electrochemical gradient. (b) In secondary active transport, a gradient of ion X (often Na+) has been established by primary active transport. Movement of X down its electrochemical gradient now provides the energy to drive cotransport of a second solute (S) against its electrochemical gradient.

11. Transport Across Membrane
Active Transport
Pump
Transport against concentration gradient
high Na+ outside of cell, high K+ inside of cell
Energy source
ATP : e.g. Na+/K+ ATPase
Concentration gradient of ions

12. ATPase in Animal cell
FIGURE Role of the Na+K+ ATPase in animal cells. In animal cells, this active transport system is primarily responsible for setting and maintaining the intracellular concentrations of Na+ and K+ and for generating the membrane potential. It does this by moving three Na+ out of the cell for every two K+ it moves in. The electrical potential across the plasma membrane is central to electrical signaling in neurons, and the gradient of Na+ is used to drive the uphill cotransport of solutes in many cell types.

13. Transport Proteins in Animals
Nerve Impulses
Resting membrane potential of -70mV (~EK=RT/ZFln[Kout]/[Kin]
Opening of Na+ channel by stimulation
Generation of action potential of 50 mV
Opening of voltage-gated K+ channel
Repolarization of membrane potential
Restoration of membrane potential by Na+/K+ ATPase

14. ATP hydrolysis and synthesis are done by one enzyme
FIGURE Reversibility of F-type ATPases. An ATP-driven proton transporter also can catalyze ATP synthesis (red arrows) as protons flow down their electrochemical gradient. This is the central reaction in the processes of oxidative phosphorylation and photophosphorylation, both described in detail in Chapter 19.

16. Three states of the cystic fibrosis transmembrane conductance regulator, CFTR.
BOX 11-3 FIGURE 1 Three states of the cystic fibrosis transmembrane conductance regulator, CFTR. The protein has two segments, each with six transmembrane helices, and three functionally significant domains extend from the cytoplasmic surface: NBD1 and NBD2 (green) are nucleotide-binding domains that bind ATP, and a regulatory domain (blue) is the site of phosphorylation by cAMP-dependent protein kinase. When this R domain is phosphorylated but no ATP is bound to the NBDs (left), the channel is closed. The binding of ATP opens the channel (middle) until the bound ATP is hydrolyzed. When the regulatory domain is unphosphorylated (right), it binds the NBD domains and prevents ATP binding and channel opening. The most commonly occurring mutation leading to CF is the deletion of Phe508 in the NBD1 domain (left). CFTR is a typical ABC transporter in all but two respects: most ABC transporters lack the regulatory domain, and CFTR acts as an ion channel (for Cl–), not as a typical transporter.

17. Transport Proteins in Animals
Muscle contraction
Action potential at the neuromuscular junction
Opening of Ca2+ channel in the sarcoplasmic reticulum (SR: 근소포체)
Released Ca2+ binding to troponin
Movement of tropomyosin exposing myosin binding site

18. When Gradient Fail LongQT (LQT) syndrome Inherited heart failure
Long recovery periods before new heart contraction
Cell to cell variation of recovery periods
Can cause arrhythmia (lack of rhythm)
Defects in K+ or Na+ channels
Inherited heart failure
Mutation in the regulatory protein of Ca2+ channel in SR

19. Transport proteins 1
Pumps, carriers, and nutrient distribution 2
Cells, salts, and water balance 3

20. Pumps, Carriers, and Nutrient Distribution
Epithelial cells
Cells cover body surfaces and line internal organs
Intestinal epithelium
Microvilli facing the intestinal track
Structure
Tight junction between cell: prevent transport of large molecules
Extra cellular matrix support epithelial cells

21. Transport of Nutrients across Epithelial Cells
Intestinal side
active transport of glucose powered by Na+ gradient
Co-transport of Na+ and glucose
Membrane side: carrier proteins

22. Glucose transport in intestinal epithelial cells:
The Na+/K+ ATPase continues to pump Na+ outward to maintain the Na+ gradient that drives glucose uptake.
FIGURE Glucose transport in intestinal epithelial cells. Glucose is cotransported with Na+ across the apical plasma membrane into the epithelial cell. It moves through the cell to the basal surface, where it passes into the blood via GLUT2, a passive glucose uniporter. The Na+K+ ATPase continues to pump Na+ outward to maintain the Na+ gradient that drives glucose uptake.

23. 1
Transport proteins
Pumps, carriers, and nutrient distribution 2
Cells, salts, and water balance 3 3

24. Cells, Salts, and Water Balance
Movement of water across the cell
Water movement to equalize the total concentration of solutes
Osmosis: movement of water across membranes
Osmotic balance: A system with no net water movement
Osmotic balance of cells
Higher concentration of ions outside
Cells contain many proteins, amino acids, and other small molecules to keep the osmotic balance

25. Cells, Salts, and Water Balance
Water in human body (75 kg)
45 L total
30 L: intracellular
3.75 L: Blood plasma
11.25 L: extracellular fluid
Importance of water balance for proper function of a body
Lactose intolerance
Lack of lactase breaking milk sugar lactose into glucose and galactose
No digestion of lactose  movement of water into the intestine
Metabolism of lactose by intestinal bacteria  gas production
High-magnesium laxative : relieving constipation
Cystic fibrosis
Mutation in Cl- channel : reduced water secretion  thick mucus in epithelia of respiratory and gastrointestinal tracts
Same solute

26. Biotechnology Rehydration therapy
Diarrhea: kill 2 million children/year by dehydration
Solution of sugar and salt is effective to treat dehydration: e.g. sports drinks
Enzyme treatments for lactose intolerance
Add lactase enzyme in milk or dairy products