Textbook Notes (270,000)
CA (160,000)
UTSC (20,000)
A.Elia (20)
Chapter 5

BIOD27H3 Chapter Notes - Chapter 5: Dynamic Equilibrium, Extracellular Fluid, Passive Transport


Department
Biological Sciences
Course Code
BIOD27H3
Professor
A.Elia
Chapter
5

This preview shows pages 1-2. to view the full 8 pages of the document.
Human Physiology (Fifth Edition)
Silverthorn, D. U.
Chapter 5 Membrane Dynamics
Mass Balance and Homeostasis
To maintain homeostasis, the body uses mass balance
The law of mass balance: if the amount of a substance in the body has to stay
constant, any gain must be offset by an equal loss;
oTotal amount of x in the body = intake + production excretion
metabolism
To maintain mass balance, the body can use excretion (elimination of
material from the body through urine, feces, lungs, skin) or metabolize the
substance into a different substance (which will create a new mass balance
disturbance through addition of metabolite)
Excretion Clears Substances from the Body
oClearance: rate at which a molecule leaves the body by excretion,
metabolism, or both; usually expressed as a volume of blood plasma
cleared of x per unit of time;
oKidney and liver major organs involved in clearing materials from
the body
oSaliva, sweat, breast milk, and hair contain solutes cleared from the
body
oMass flow = concentration x volume flow; used to determine the rate of
uptake, output, or production of x, as well as its movement from one
compartment to another
Homeostasis Does Not Mean Equilibrium
oHomeostasis often refers to the stability of the extracellular fluid
compartment (consisting of the plasma and interstitial fluid)
oDynamic disequilibrium (chemical disequilibrium): state of the two
fluid compartments in which the different concentrations of solutes in the
ECF and ICF create a concentration gradient; continual input of energy is
needed to keep the body in this state
oFree movement of water allows the ECF and ICF to reach a state
called osmotic equilibrium where the total amount of solute per volume of
fluid is equal on either side of the cell membrane
oThe body as a whole is electrically neutral, but a few anions are found
in the ICF while their matching cations are in the ECF, making the inside
of cells slightly negative (electrical disequilibrium)
oICF, ECF = osmotic equilibrium, chemical and electrical
disequilibrium, which are dynamic steady states (meaning materials are
constantly moving, but there is no net movement)
oHomeostasis works to maintain the dynamic steady states
Transport Occurs Within and Between Compartments
www.notesolution.com

Only pages 1-2 are available for preview. Some parts have been intentionally blurred.

oSome transport mechanisms need energy from ATP, while others are able
to use kinetic/potential energy
oMovement between compartments requires a molecule to cross one or
more cell membranes, whereas movement within the compartment is less
restricted
oBulk flow: a pressure gradient causes fluid from regions of higher
pressure to regions of lower pressure
Diffusion
Cells membranes are selectively permeable
Membrane permeability is variable and can be changed by altering the
proteins or lipids of the membrane
The size of the molecule and its lipid solubility influence its movement across
membranes (large, less lipid-soluble molecules usually cannot pass the
membrane unless the cell has specific membrane proteins to transport them or
vesicles to move them)
Passive transport: movement across the membrane not requiring energy input
Active transport: movement across the membrane needing energy input
Diffusion Uses Only the Energy of Molecular Movement
oDiffusion: passive movement of uncharged molecules from an area of
higher concentration to lower concentration due to random molecular
movement
o7 properties:
1. A passive process: does not require outside energy; uses only
kinetic energy from molecules
2. Moves down concentration gradient: high to low; rate of diffusion
depends on concentration gradient (larger difference, faster
diffusion)
3. Net movement of molecules occurs until equilibrium: molecules
are now evenly distributed; dynamic equilibrium state
4. Fast over short distances, slow over long distances: time needed
for a molecule to diffuse = distance2
5. Directly related to temperature: rate of diffusion increases as
temperatures increase
6. Inversely related to molecular size: the larger the molecule, the
slower its diffusion through a given medium
7. Takes place in an open system (where there are no boundaries)
or across a partition that separates two systems (but only if the
partition allows the molecules to cross)
oIons do not move by diffusion; diffusion is random molecular motion down
a concentration gradient
Ion movement depends on electrical gradient because of the
attraction/repulsion of charges
Lipophilic Molecules Can Diffuse Through the Phospholipid Bilayer
oSimple diffusion: diffusion directly across the bilayer, only moves lipids,
steroids, and small lipophilic molecules
www.notesolution.com
You're Reading a Preview

Unlock to view full version

Only pages 1-2 are available for preview. Some parts have been intentionally blurred.

rate of diffusing depends on ability of diffusion molecules to
dissolve in lipid layer
rate of diffusion is directly proportional to the surface area of the
membrane; the larger the surface area, the more molecules that
can diffuse
rate of diffusion is inversely proportional to membrane thickness;
thicker membrane, slower rate
Protein-Mediated Transport
mediated transport: transport across membranes with the help of membrane
proteins
facilitated diffusion: movement of molecules across the membrane according to
concentration gradient with the help of membrane proteins
active transport: movement across membrane against concentration gradient,
needing energy from ATP
Membrane Proteins Function as Structural Proteins, Enzymes,
Receptors, and Transporters
oStructural Proteins: connect the membrane to the cytoskeleton to
maintain shape, create cell junctions that hold tissue together, attach
cells to the extracellular matrix
oEnzymes: catalyze chemical reactions; some attach to intracellular
surface for an important role in transferring signals
oReceptors: part of the bodys chemical signaling system; when bound to
its ligand another event at the membrane is often triggered
oTransporters: move molecules across membranes; divided into channel
and carrier proteins
Channel proteins allow more rapid transport across the
membrane, but limited to transporting small ions and water
Carrier proteins are slower, but can move larger molecules
Channel Proteins Form Open, Water-Filled Passageways
oWhen water-filled channels are open, tens of millions of ions per second
can get through unobstructed
oMore than 100 ion channels have been identified which may be specific for
one or a few ions
oSelectivity of a channel is determined by the diameter of its central pore
and the charge of the amino acids lining the channel (opposites attract)
oGated channels are most often closed, which allows them to regulate the
movement of ions through them
Gated channels may be chemically gated (messenger molecules),
voltage-gated (electric state), or mechanically gated (physical
forces)
Carrier Proteins Change Conformation to Move Molecules
oCarrier proteins bind to specific substrates and carry them across the
membrane by changing conformation; large, complex proteins with
multiple subunits
www.notesolution.com
You're Reading a Preview

Unlock to view full version