1. CHAPTER 5: MEMBRANES

2. Mass Balance and Homeostasis
Human body is an open system
Exchanges heat and materials with external environment
Principle of MASS BALANCE is used to maintain Homeostasis
Law of Mass Balance:
If the amount of a substance in the body is to remain constant,
any gain must be offset by an equal loss
Example:
Water loss (output) must be balanced by water intake (from external environment and from metabolic water production)

3. Figure 5-1

4. Mass Balance and Homeostasis
Law of Mass Balance:
If the amount of a substance in the body is to remain constant,
any gain must be offset by an equal loss
Total amount (load) of substance X in body =
Intake + production – excretion - metabolism

5. Mass Balance and Homeostasis
Applies to:
Water
Oxygen
Carbon Dioxide
Salts (electrolytes)
Hydrogen ions (pH)

6. Figure 5-2

7. Mass Balance and Homeostasis
Input:
Most substances enter the body from the outside
Water, nutrients, enter via digestive tract
Oxygen and other gases enter via lungs:
Some lipid-soluble molecules can enter through the skin

8. Mass Balance and Homeostasis
Output:
1. Excretion
Through urine, feces, lungs, or skin
2. Metabolize the substance to a different one
Metabolite: a product created in a metabolic pathway
Clearance:
The rate at which a molecule disappears from the body
(via excretion or metabolism)

9. Mass Balance and Homeostasis
Clearance example:
Volume of blood plasma cleared of the substance per unit of time
Mass Flow: a more direct way to measure this
Mass Flow = concentration x volume flow
Amount of substance/min = amount of substance/vol x vol/min

10. Mass Balance and Homeostasis
Mass Flow: example:
A person is given an intravenous infusion of glucose solution (50 g glucose per liter), given at a rate of 2 ml per min
Mass flow is:
50 g glucose/1000 ml x 2 ml solution /min = 0.1 g glucose/min
Mass flow applies to entry, production, and removal of substances
And also to movement of substances from one functional compartment to another

11. Homeostasis and Equilibrium
Homeostasis: usually refers to stability of the internal environment
Usually refers to stability of the ECF (plasma, interstitial fluid)
Cells maintain a state of cellular homeostasis
But, lots harder to measure stability of ICF (inside environment of cells)

12. Homeostasis and Equilibrium
In a state of homeostasis, the composition of both body compartments is relatively stable
BUT
The composition of the compartments are different

13. Figure 5-3a

14. Electrolytes These dissociate (come apart) in liquids
and form ions (charged particles)
Acids: HCl (hydrochloric acid)
Dissociates to form H+ and Cl-
Bases: NaOH (sodium hydroxide)
KOH (potassium hydroxide)
Salts: NaCl (sodium chloride or table salt)
KCl (potassium chloride)

15. Electrolytes are
Chemically reactive in metabolism
Major cations
Na+, K+, Ca2+, H+
Major anions
Cl-, HCO3-, PO43-
Cation: positive ion
Anion: negative ion

16. Electrolytes Main functions in body: 1. Many are essential minerals
2. As the most numerous solutes, they control osmosis of water between body compartments

17. Electrolytes Main functions in body:
3. They help maintain the acid-base balance required for normal cellular activities
4. They can carry an electric current:
This allows Action Potentials, graded potentials to happen; controls secretion of some hormones and neurotransmitters

18. Distribution of Electrolytes in Extracellular Fluid
1. Interstitial fluid contains:
Na+ (most abundant positive ion)
Cl- (most abundant negative ion)
H2CO3 (Bicarbonate ion; abundant) K+
Ca++
Mg++
2. Plasma contains:
Same ions as interstitial fluid and
Lots of protein anions

19. Distribution of Electrolytes 3. Intracellular fluid (ICF) contains:
K+ (most abundant positive ion)
Protein ions; phosphate ions (HPO42-)
(most abundant negative ions)
H2CO3
(Bicarbonate ion; not as abundant as in ECF's)
Cl-
Na+
Mg++

20. Figure 5-3b

21. Homeostasis and Equilibrium
(p )
4 phrases to learn:
Dynamic disequilibrium
Osmotic equilibrium
Chemical disequilibrium
Electrical disequilibrium

22. Homeostasis and Equilibrium
(p )
Dynamic disequilibrium
ECF and ICF contain different concentrations of various solutes, resulting in a state of dynamic disequilibrium
Osmotic equilibrium
Water moves freely between ECF and ICF, so these can reach a state of osmotic equilibrium

23. Homeostasis and Equilibrium
(p )
Chemical disequilibrium
Certain solutes are more concentrated in one compartment than in the other
Electrical disequilibrium
Body as a whole is electrically neutral
But due to ion concentrations, ICF is slightly negative relative to ECF
Changes to the ionic imbalance create electrical signals (nerve cells, muscle cells)

24. Figure 5-3b

25. Homeostasis and Equilibrium
(p )
“The goal of homeostasis is to maintain the dynamic steady states of the body's compartments
Dynamic:
Materials are constantly moving back and forth between compartments
Steady state:
No net movement of materials between the compartments

26. Membrane Transport
(p )
“Humans are large complex organisms and the movement of materials within and between compartments is necessary for communication”
Movement of materials across selectively permeable membranes requires a variety of transport mechanisms
(Cell membranes are selectively permeable)

27. Membrane Transport
(p )
Transport Mechanisms:
Some require energy in the form of ATP
(Active transport)
Some do not require ATP (Passive transport)
This kind uses energy of molecular motion

28. Figure 5-4

29. Figure 5-5