CHAPTER 5: MEMBRANES

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)

Figure 5-1

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

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

Figure 5-2

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

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)

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

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

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)

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

Figure 5-3a

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)

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

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

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

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

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++

Figure 5-3b

Homeostasis and Equilibrium (p. 134-135) 4 phrases to learn: Dynamic disequilibrium Osmotic equilibrium Chemical disequilibrium Electrical disequilibrium

Homeostasis and Equilibrium (p. 134-135) 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

Homeostasis and Equilibrium (p. 134-135) 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)

Figure 5-3b

Homeostasis and Equilibrium (p. 134-135) “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

Membrane Transport (p. 136-158) “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)

Membrane Transport (p. 136-158) 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

Figure 5-4

Figure 5-5