Study Guide
Cellular Physiology Facilitated Diffusion
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University:
Kentucky State University -
Course:
BIO 303 | Human Physiology Academic year:
2020
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Views:
300
Pages:
25
Author:
marthamanayay9qr12
Cellular Physiology
L3, L4, L5 Main Topics
• Fluid mosaic model of the cell membrane
• The cell membrane components
• Transport across the cell membrane (passive
transport and active transport)
• Passive transport (simple and facilitated
diffusion)
• Osmosis and osmotic pressure
• Tonicity
• Osmolarity and Tonicity
• Facilitated diffusion
• Active transport ( primary and secondary)
• Bulk transport (Vesicle transport) Facilitated Diffusion
Characteristics
• Occurs down an electrochemical gradient ("downhill"), similar
to simple diffusion
• Does not require metabolic energy and therefore is passive
• It is more rapid than simple diffusion
• It is carrier-mediated
1. Stereo-specificity (D-glucose is transported by facilitated
diffusion but L-glucose is not. Simple diffusion cannot tell the
difference)
2. Saturation (transport rate increases as the concentration of the
solute increases until the carriers are saturated (Vm)
3. Competition (structurally related solutes compete for transport
sites on carrier molecules. Galactose is competitive inhibitor of
glucose transport in the small intestine. Facilitated Diffusion…..
Two types of proteins are
involved in facilitated diffusion:
a. Carrier proteins: such as the
glucose transporter (GLUT1)
move solute molecules across
a membrane
b. Channels: facilitate diffusion
of ions down a concentration
gradient, a single channel
opening event may allow flux
of many thousands of ions.
Glucose
Carrier protein
(6.16) Human Physiology, S. Fox Facilitated Diffusion
Effect of
adding more
protein carriers
on rate of
diffusion
Rate of
diffusion
(pmol/min/mg
protein)
Vmax
Effect of adding
more protein
carriers on Vmax
Concentration gradient (Co-Ci) Simple vs. Facilitated Diffusion
Facilitated diffusion
is saturable because
the binding sites are
limited and has
maximum transport
What limits maximum rate of facilitated diffusion?
AnsNumber of
carrier proteins Sheet#1
- carrier mediated transport is present in Facilitated and Active diffusion.
- facilitated diffusion is considered a passive diffusion because it doesn’t require energy
but it requires specific proteins ( carrier proteins , channels)
- what distinguishes carrier proteins and some channels that a conformational change will
occur when it’s activated which will allow the solutes to enter the cell.
- in saturation the rate of the solute entering the cell will equal the rate of the binding solutes
with the carrier proteins .
- in facilitated diffusion Vmax will be effected when the number of carrier proteins .
- in simple diffusion there is positive relationship between the rate of diffusion and
concentration gradient because it’s only dependent on the concentration gradient. Main Topics
• Fluid mosaic model of the cell membrane
• The cell membrane components
• Transport across the cell membrane (passive
transport and active transport)
• Passive transport (simple and facilitated
diffusion)
• Osmosis and osmotic pressure
• Tonicity
• Osmolarity and Tonicity
• Facilitated diffusion
• Active transport ( primary and secondary)
• Bulk transport (Vesicle transport) Active Transport
Primary Active Transport
• Molecules are “pumped” against an electrochemical
gradient at the expense of energy (ATP). It is carrier mediated
– direct use of energy
Secondary Active Transport
• Transport is driven by the energy stored in the
electrochemical gradient of another molecule (usually Na+).
– indirect use of energy Primary Active Transport
1. Na+-K+ ATPase pump
• Located on the plasma membrane of all animal cells
• Pumps 3 sodium ions out of cells and pumps 2 potassium ions
into cells against electrochemical gradients (electrogenic)
• It helps regulate cell volume by controlling the concentration of
solutes inside the cell, regulating osmotic balance by maintaining
Na+ and K+ balance
• The energy used to run the pump also indirectly serves as the
energy source for the co-transport of glucose and amino acids
across the membrane (intestine and kidney cell)
• Inhibited by ouabain that causes cell to swell and burst Primary Active Transport ….
Na+-K+ ATPase
Extracellular fluid
Na+
Na+/K+ ATPase
gradient
Cytosol
K+
gradient
3 Na+ expelled
P
3 Na+
1
2K+
ATP
2
ADP
3
P
2 K+
4 imported Sheet#2
- The Na+-K+ ATPase pumps is a cell membrane protein which uses ATP to pump the ions.
- Na+-K+ ATPase pump is called electrogenic because it preserves the difference in the
concentration of the ions inside and outside the cell. Primary Active Transport…
2. Ca2+ ATPase
• Present on the cell membrane and the sarcoplasmic reticulum
in muscle fibers
• Maintains a low cytosolic Ca2+ concentration
3. H+ ATPase
• Found in parietal cells of gastric glands (HCl secretion)
and intercalated cells of renal tubules (controls blood pH)
• Concentrates H+ ions up to 1 million-fold Sheet#3
- there are subtypes of H+ ATPase such as :
-B H+ ATPase found in the cytoplasm
- V H+ ATPase found in vascular
- H+ ATPase Concentrates H+ ions up to 1 million-fold so it would be excreted with the
urine . Secondary Active Transport
• The transport of two or more solutes is coupled
• One of the solutes (usually Na+) is transported
"downhill" and provides energy for the "uphill" transport
of the other solute (s)
• Metabolic energy is not provided directly, but indirectly
from the Na+ gradient that is maintained across cell
membranes.
• Inhibition of Na+, K+-ATPase will decrease transport of
Na+ out of the cell, decrease the trans-membrane Na+
gradient, and eventually inhibit secondary active
transport. Secondary Active Transport
1. Symporters: transport substance in same direction as a “driver”
ion like Na+.
Na+
AA
Examples:
Extracellular
Na+
AA
Na+ glucose
Na+ 2 HCO3-
Intracellular
Figure 4-13 Secondary Active Transport…..
2. Antiporters: transport substance in opposite direction of a “driver” ion
like Na+
Na+
Ca2+
Figure 4-14
Examples:
Extracellular
Na+
Na+
Ca2+
Na+/HCO3-
H+
Cl-/H+
Intracellular Secondary Active Transport ….. Sheet#4
- symport is called cotranspoert .
- antiport is called exchange . Main Topics
• Fluid mosaic model of the cell membrane
• The cell membrane components
• Transport across the cell membrane (passive
transport and active transport)
• Passive transport (simple and facilitated
diffusion)
• Osmosis and osmotic pressure
• Tonicity
• Osmolarity and Tonicity
• Facilitated diffusion
• Active transport ( primary and secondary)
• Bulk transport (Vesicle transport) Cell membrane mediated transport
Bulk transport or Vesicular transport
Polypeptides and proteins, hormones, neurotransmitters, are too large to be transported
through a membrane by the carriers proteins
Types of bulk transport:
1. Endocytosis (uses energy)
a. Phagocytosis: process by which the cells take in large particles by in folding
the cell membrane to form endocytotic vesicles (ex. WBC capture and kill bacteria) Bulk transport …….
b. Pinocytosis: a process of taking in fluids into the cell by invagination of the cell
membrane
Ex, Engulf fat droplets in small intestine
Adopted from the internet Bulk transport …….
c. Receptor – mediated endocytosis (vesicle formation): very specific
1. The plasma membrane forms a pit.
2. The pits has a specific receptor proteins which binds to specific molecules that
surrounds the cell.
3. The pit will close and pinch off to form a vesicle that carries the molecule inside the cell
4. Ex. Iron, Cholesterol Sheet#5
- In bulk transport molecules will not cross the cell membrane .
- endocytosis is called cell eating .
- Pinocytosis is called cell drinking.
- what distinguishes Receptor – mediated endocytosis that the vesicles will be coated with proteins Bulk transport …….
2. Exocytosis:
• Is a process in which a substance is exited from the cell without passing through the
cell membrane. Ex. secretion of proteins like enzymes, hormones and antibodies.
• Needs energy
• The materials are forced to move outside the cell, the membrane that surrounds the
material fuses with the cell membrane and releases its content out (ex. waste removal,
release of neurotransmitters)
Cellular Physiology Facilitated Diffusion
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