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What 3 different mechanisms are involved in the movement of solutes across membranes?
1. simple diffusion 2. facilitated diffusion 3. active transport
What two different kinds of proteins are involved in facilitated diffusion?
channel protein and carrier proteins
What is simple diffusion?
the unassisted net movement of a solute from a region where its concetration is higher to a region where its concentration is lower
What kinds of molecules can undergo simple diffusion?
oxygen, ethanol, water, carbon dioxide
Why can only small and relatively nonpolar molecules undergo simple diffusion?
because membranes have a hydrophobic interior
What size of molecules are small enough to diffuse across membranes by simple diffusion?
up to 180 daltons
Are lipid bilayers relatively permeable to gases?
yes because they are nonpolar molecules
Is glycerol a polar molecule that is not permeable across membranes?
yes
Can ions simply diffuse across membranes?
no because 40kcal/mol of energy is required to move ions from an aqueous environment into a nonpolar environment
How can larger molecules move in and out of cells and organelles?
with the assistance of transport proteins like integral membrane proteins
Is an input of energy (ATP) required for facilitated diffusion?
no because the solute diffuses down the concentration gradient
What types of membrane transport are called passive transport?
simple and facilitated diffusion
What is the role of transport proteins in facilitated diffusion?
to provide a path through the lipid bilayer
How do carrier proteins facilitate diffusion?
bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane
How many solutes can carrier proteins transport?
one or two
What direction can carrier proteins transport solutes?
the directions can vary
What is it called when carrier proteins transport two solutes?
cotransport
What is it called when carrier proteins transport one solute?
uniport
What is it called when carrier proteins transport two solutes in the same direction?
symport
What is it called when carrier proteins transport two solutes in opposite directions?
antiport
What are examples of carrier proteins?
transporters or permeases
What is the glucose transporter?
a carrier protein that is a uniport carrier for glucose
In what direction is glucose transported by a glucose transporter?
inward
What is the glucose transporter called in erythrocytes?
GLUT1
How does GLUT1 work in erythrocytes?
it transports glucose through the membrane by the alternating conformation mechanism
What is the alternating conformation mechanism?
one conformation state, T1, has the binding site for glucose open on the outside of the cell and the other conformation state, T2, has the binding site open to the inside of the cell
What are the 4 steps of GLUT1 acting as a carrier protein?
1. D-glucose collides with and binds to GLUT1 in the T1 conformation
2. GLUT1 shifts to the T2 conformation
3. the conformational change causes the release of glucose
4. GLUT1 returns to the T1 conformation
How do channel proteins facilitate diffusion?
form hydrophilic transmembrane channels to provide a passage route for solutes
What are the 3 kinds of transmembrane protein channels that channel proteins form?
ion channels, porins, aquaporins
What are ion channels?
most of the smaller channels involved in ion transport
Is transport by carrier proteins faster than movement of solutes through ion channels?
no
Why are movements of solutes through ion channels faster than transport by carrier proteins?
because a conformation change is not required
What are porin channels?
large and nonspecific pores formed by transmembrane proteins called porins
What are examples of porin channels?
the pores on the outer membranes of bacteria, mitochondria, and chloroplasts
What molecular weight of solutes can pass through porin channels?
600 daltons
What are the characteristics of other channels besides porin channels?
small and highly selective
What are aquaporin channels?
transmembrane channels that allow rapid passage of water
What types of cell membranes do aquaporin allow rapid passage of water through in plants and animals?
animals: erythrocytes and kidney cell membranes
plants: root cells and vacuolar membranes in plants
What is active transport?
the movement of solutes against the concentration gradient using an input of energy (ATP)
What are the 3 major roles of active transport in cells and organelles?
1. uptake of essential nutrients from outside when their concentration is lower than inside the cell
2. export of various molecules from cell or organelle, even when their concentration outside is greater than inside the cell
3. maintenance of constant intracellular concentration of H+,K+,Na+,Ca++
How are active transport mechanisms differentiated?
by the source of energy and whether or not two solutes are transported concomitantly
What is direct active transport?
the accumulation of solute molecules or ions on one side of the membrane is coupled directly to an exergonic chemical reaction (hydrolysis of ATP)
What is indirect active transport?
depends on the cotransport of two solutes, with the movement of one solute down the concentration gradient driving the movement of the other solute up its gradient (can be symport or antiport)
What does direct active transport depend on?
four types of transport ATPases
What are the four types of transport ATPases required for direct active transport?
P-type, V-type, F-type, ABC-type
How do the different types of transport ATPases differ?
by structure, mechanism, location, and roles
How is P-type ATPase (phosphorylation-type) made?
transporter reversibly phosphorylated by ATP as part of the transport mechanism
Where is P-type ATPase located?
plasma membrane
What do P-type ATPase do?
pump H+ and cations (some are flippases)
What are V-type ATPase (vacuole-type)?
integral membrane protein and a peripheral ATPase
What do V-type ATPase do?
pump H+ into organelles such as vacuoles, vsicles, lysosomes, endosomes, and golgi complex
Where are F-type ATPase (factor type) found?
in bacteria, mitochondria, and chloroplasts
What is the purpose of F-type ATPase?
conserving solar energy (photosynthesis) or substrate oxidation (respiration) as ATP
What makes up the ABC-type ATPases (ATP-Binding Cassette)?
a large superfamily of transport proteins that are related to each other in amino acid sequence and molecular mechanism
What do ABC-type ATPases do?
transport a variety of solutes, the solute transported is specific to the particular ABC transporter of the superfamily
How are ABC-type ATPases used in the medical world?
used as antibiotic pumps or other drugs - pump drugs out of the cells making cells resistant to certain drugs
What are multi-drug resistance transport proteins?
involved in resistance of cancer cells to certain chemotherapeutic agents
Where can ABC-type ATPases be found?
in the cytosol