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Membrane Dynamics
Dr. Gary Mumaugh – Bethel University
Homeostasis Does Not Mean Equilibrium
 Two fluid compartments, the cell and the extracellular fluid (ECF)
 Osmotic equilibrium
 Chemical disequilibrium
 Electrical disequilibrium
 Important Point: Pressure, chemicals and electrical charge is always
dynamically moving between the two compartments and across the cell
membranes.
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Review of Cell Membranes
 The membrane is semi-permeable and selectively permeable
o It only allows certain substances to into and out of the cell
o The boundary separates substances from inside the cell (ICF) and outside the
cell (ECF).
o Is selectively permeable, “not everyone is on the A list!”.
 Is made up a double layer of phospholipid molecules embedded with proteins
o
The phospholipid has a split personality – the schizophrenic of chemistry
 The phosphate groups hate water – hydrophobic (inside of the two layers)
 The two fatty acid groups love water – hydrophilic (facing the water in the
intracellular and extracellular fluid)
 It is like an Oreo cookie! Two
layers (the cookie) love water and
are facing the I.CF and ECF. The
“creamy center” of the cookie is all
fat and is hates water.

The membrane has embedded proteins, which all have different functions
o This is a major part of cell physiology
o Some of the proteins are sticking outside into the ECF and some are
sticking out into the ICF.
o Some of the proteins go all the way through the membrane from the ICF to
the ECF.
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Review of Cell Membranes – continued

Functions of membrane embedded proteins
o Ion Channels
Na+, K+, Cl-, Ca++
 Ions are electrically charged atoms
 These channel proteins are like a hollow pipe on a tube, which
allow ions to flow in and out of the cell
 These channels are very selective and specific, so that only Na+
can flow through a Na+ channel and not K+, Cl-, or Ca++.
 These ion channels can open and close, but they are usually
closed. (See pictures on prior page)
o Transporter Proteins – Carrier Proteins
 They transport sugar and amino acids across the membrane
 They are also very specific.
 The ones that transport sugar cannot transport amino acids.
 The ones that transport amino acids cannot transport sugar.
 Uses Passive Transport
 Does not require ATP for energy and moves with facilitated
diffusion.
 Common for transporting sugar.
 Uses Active Transport
 Requires ATP and uses energy.
 Common for transporting amino acids.
o Enzymes
 Proteins that catalyze biochemical reactions
o Linker Proteins
 These proteins effect the cytoskeleton and the shape of the cell.
 They are on the cytoplasm side of the membrane and they attach to
the cytoskeleton proteins.
 They cause the cytoskeleton proteins to move, which changes the
shape of the cell.
 They use a lot of energy.
o Receptor Site Proteins
 They are activated by hormones, neurotransmitters, and other
chemicals. Any chemical that activates a receptor site is called a
signal molecule or ligand.
Remember Dr. M’s toy illustration in class
(Class Drawing - Receptors and Targets)
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Review of Cell Membranes - continued

Functions of membrane embedded proteins – continued
o Receptor Site Proteins - continued
 Activation of the Receptor Site Proteins can:
 Open and close ion channels
 Transport chemicals
 Enzyme activity
 Cause a shape of the cell
 Receptor sites can also act as a “blocking agent”
(Class Drawing - Membrane Blockers)

Steroid Hormones
 They are a receptor site protein
 They are made of cholesterol, and can easily diffuse across
the membrane because they are fat.
 The receptor sites for the steroid hormones are inside the
cytoplasm or on the nucleus membrane.
 When the steroid hormones activates the receptor of the
nucleus, it starts transcription of the genes.
(Class Drawing - Steroid Receptor)
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Review of Cell Membranes - continued

Functions of membrane embedded proteins – continued
o Receptor Site Proteins - continued
 Proteins Hormones
 They are a receptor site protein
 They are to big to get across the cell membrane and they
must attach on the outside of the cell
 Neurotransmitters
 They are a receptor site protein
 They are not fat soluble and cannot get across the
membrane.
 MHC – Major Histocompatibility Proteins
 Every person has a unique set of glycoproteins on the
surface of all cells.
 This allows WBCs to ”recognize” your cells from foreign
cells.
 Self vs. Non-Self concept
 Clinical Application
o Auto Immune Diseases
o Immunosuppressant Drugs
 Corticosteroids (prednisone)
o Organ Transplant Rejections
o Immunocompromised vs. Immunocompetent
Diffusion

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Diffusion is the spontaneous movement of substances (solutes) from an area of
higher concentration to an area of lower concentration.
This movement is called going down the concentration gradient.
All the molecules are in constant motion.
This kinetic energy moving the molecules is called Brownian movement or
motion.
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Diffusion – continued

The rate of diffusion is affected by several factors:
o The difference in the concentration gradient inside and outside the cell
(Class Drawing - Diffusion)
o The size of the chemical substances
(Class Drawing – Chemical Flow)
o The temperature – increased temperature >>> increased rate of diffusion
o Whether the substance moving is fat soluble or water soluble
 Because cells are two layers of fat, fat soluble substances move
faster.

Clinical applications:
 Why is it hard to wash off gasoline from your skin when you
are filling your gas tanks?
 Why are cortisone and SPF sun blocks in a gooey cream?
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Osmosis
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Osmosis is the diffusion of water through a cell membrane (through protein
channels).
o H2O is the only chemical that is in a liquid form and is flowing in and out of
the cells.
o All other substances are either solutes (solids) or gases.
Osmosis always flows from high to low concentration.
“Water always follows salt”
Isotonic environment = same concentration on both sides of the membrane
o Clinical application: Anytime anything is injected into the body, it is usually
isotonic (NS = Normal Saline)
Hypotonic Environment causes:
o swelling and lysis of the cell
o the cell bursts
Hypertonic Environment causes:
o shrinking of the cell or crenation
o the cell “wrinkles”
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(Class Drawing – Osmosis)
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Facilitated Transport and Active Transport “Pumps”
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Proteins that are embedded in the membranes that transport specific chemicals
through the membrane using energy supplied by ATP.
o If it uses ATP it is called Active Transport
o If it doesn’t use ATP it is called Facilitated Diffusion
Examples in which ions are being transported against the concentration gradient.
o Sodium pumps, Potassium pumps, Iodine pumps, Calcium pumps, Hydrogen
pumps.
Ion Channels
o They are tube shaped proteins that are usually closed.
o When they are open, they allow Ions to flow in or out of the cell on the
concentration gradient.
 Always flowing in >>>>> Sodium (Na+), Carbon (C+), Calcium (C++)
 Always flowing out >>>>> Potassium (K+)
Facilitated Diffusion
o Also called active transport
o Example: Glucose is to large to
enter the cell. In the presence of
insulin, it helps to open the
channels for it to enter.
 There is a connection
between the insulin
receptor on the
membrane to the glucose
channel that is like a
“slinky”. The presence of
insulin releases a GProtein (glucosetransportase-4) which
helps to open the
channel more. This is
called the Glut 4 transporter.
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(Class Drawing – Glut 4)
Facilitated Transport and Active Transport “Pumps” – continued

Primary Active Transport
o Active Transport processes account for about 40% of the energy used in the
body.
o Active Transport Proteins are called “Pumps”.
o ATP is used to force Hydrogen (H+) out of the cell.
o ATP is used to force Calcium (H++) out of the cell.
o ATP is used to force Sodium (Na++) out of the cell and, at the same time, it
forces Potassium (K+) into the cell.
 This is called the Sodium Potassium Pump. (Na+ / K+ Pump)
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Facilitated Transport and Active Transport “Pumps” – continued

Secondary Active Transport
o Sometimes the cells use ATP to force one chemical in and another chemical
hitches a ride and follows it.
o It is like a department store turn style door. You are pushing to circular door in
and someone else just follows you in, even though you used your energy and
they did nothing.
 This is called Symport – energy brining in 2 chemicals.
o The opposite also happens. You are pushing the circular door in and
someone goes out, even though you used your energy and they did nothing.
 This is called Antiport – energy brings in 1 chemical and another
chemical leaves.
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(Membrane Movements – Class Drawing)
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Vesicular Transport
 Phagocytosis

Vesicles created by the cytoskeleton

Cell engulfs bacterium or other particle into phagosome
 Endocytosis

Membrane surface indents and forms vesicles

Active process that can be nonselective (pinocytosis) or highly selective

Receptor-mediated endocytosis uses coated pits
Epithelial Transport
 Apical membrane faces the lumen or mucous
 Basal membrane faces the ECF
 Paracellular transport - Through junctions between
adjacent cells
 Transcellular transport - Through cells themselves
 Absorption from lumen to extracellular fluid (ECF)
 Secretion from ECF to lumen
 Transcellular transport of glucose uses membrane
proteins
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The Resting Membrane Potential
 Body is electrically neutral
 It is the potential difference across a cell membrane at
rest
 Negative on the outside and positive on the inside
 Chemical disequilibrium between ICF and ECF
 Potential values of RMP in various excitable tissues:
o Nerve fiber and skeletal muscle
-90
mV
o Cardiac muscle
-85
mV
o Nerve cell body
-70
mV
o Smooth muscle
-55 to -60 mV
The Cell Membrane Enables Separation of Electrical Charge in the Body
 Artificial cell explains the
distribution of charges across a
phospholipid bilayer.
 Movement of K+ ions out of the
cell according to a chemical
gradient
 Negative ions cannot follow
because the membrane is
impermeable to anions
 Electrical gradient is established
 Combination of electrical and
concentration gradient is an
electrochemical gradient
All Living Cells Have a Membrane Potential
 Resting membrane potential difference (membrane potential)
 Resting is the steady state
 Potential energy stored in the electrochemical gradient
 Difference in electric charges inside and outside of the cell
 Measuring cell’s membrane potential is done with a micropipette and voltmeter
 Resting membrane potential in actual cell is due mostly to potassium
Changes in Ion Permeability Change the Membrane Potential
 Change depends on concentration of ions across membrane
 Change depends on permeability membrane to ions
 Depolarization
 Repolarization
 Hyperpolarization
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