Describe the cellular circumstances of membrane proteins, concentration and charge differences that allow diffusion or require active transport. Solution Answer: Diffusion: it is going to takes place as per differences of solute concentrations on both sides of a cell For example: Normally glucose enters into the cell by passive facilitated diffusion via GLUT 4 membrane protein channels (muscle or adipose cells) or via GLUT-2 (brain, kidney, pancreatic beta cells) and will be converted to glucose -6-phosphate (by enzyme glucokinase) later by oxidation process The amphipathic nature of cell membrane allows the selective permeability through the cell membrane. Cell membrane is further embedded with many ion channels and receptor membrane proteins that participate in a variety of transport mechanisms used by cell. The transport is going to takes place as per charge differences across the cell either \"down the concentration gradient or against the concentration gradient\". Cells control the movement of substances across the cell membrane through, 1. Passive transport or gradient diffusion 2. Facilitated diffusion 3. Active transport Cell performs the above mechanisms to regulate the movement of substances through lipid bilayer, ion channels and membrane transporters (trnasmembrane proteins). Simple diffusion does not require energy but primarily relies upon the membrane solubility of the solute to promote free diffusion into the cells & rate of transport will be doubled if the concentration is increased. Ex. Aquaporins. Active transport: Active transport needs energy require ATPases, and it help to move ions against their concentration gradients. The concentration of some of the ions is more inside the cell and some is more outside the cell. For example, the concentration of sodium ions is more outside the cell due to charge differences across the cells. Still, the sodium ions move from the inside of cell to outside through ion channels, by means of active transport. Facilitated diffusion: Facilitated diffusion is a passive process and uses \"carrier proteins(integral membrane transport)\" to move specific solutes molecules down the concentration gradient. This type of transport does not require energy to transport a molecule across the membrane. Its rate is higher than that of simple diffusion of the molecule finally attain a plateau or saturable stage until the concentration attains equilibrium. Facilitated diffusion involves the transport of transport of molecules using the membrane bound proteins. For example, ion channels allow the transport of ions into and out of the cell, which are otherwise, cannot pass through cell membrane. Glucose binds to the carrier molecule present on the cell membrane, and enter into the cell; this process is facilitated by insulin. Passive transport: It does not need energy source but the driving force is concentration gradient. Both active transport and facilitated diffusion use carrier molecules for the transpo.

1. Describe the cellular circumstances of membrane proteins, concentration and charge differences
that allow diffusion or require active transport.
Solution
Answer:
Diffusion: it is going to takes place as per differences of solute concentrations on both sides of a
cell
For example: Normally glucose enters into the cell by passive facilitated diffusion via GLUT 4
membrane protein channels (muscle or adipose cells) or via GLUT-2 (brain, kidney, pancreatic
beta cells) and will be converted to glucose -6-phosphate (by enzyme glucokinase) later by
oxidation process
The amphipathic nature of cell membrane allows the selective permeability through the cell
membrane. Cell membrane is further embedded with many ion channels and receptor membrane
proteins that participate in a variety of transport mechanisms used by cell. The transport is going
to takes place as per charge differences across the cell either "down the concentration gradient
or against the concentration gradient". Cells control the movement of substances across the cell
membrane through,
1. Passive transport or gradient diffusion
2. Facilitated diffusion
3. Active transport
Cell performs the above mechanisms to regulate the movement of substances through lipid
bilayer, ion channels and membrane transporters (trnasmembrane proteins).
Simple diffusion does not require energy but primarily relies upon the membrane solubility of
the solute to promote free diffusion into the cells & rate of transport will be doubled if the
concentration is increased. Ex. Aquaporins.
Active transport: Active transport needs energy require ATPases, and it help to move ions
against their concentration gradients. The concentration of some of the ions is more inside the
cell and some is more outside the cell. For example, the concentration of sodium ions is more
outside the cell due to charge differences across the cells. Still, the sodium ions move from the
inside of cell to outside through ion channels, by means of active transport.
Facilitated diffusion:
Facilitated diffusion is a passive process and uses "carrier proteins(integral membrane
transport)" to move specific solutes molecules down the concentration gradient. This type of

2. transport does not require energy to transport a molecule across the membrane. Its rate is higher
than that of simple diffusion of the molecule finally attain a plateau or saturable stage until the
concentration attains equilibrium.
Facilitated diffusion involves the transport of transport of molecules using the membrane bound
proteins. For example, ion channels allow the transport of ions into and out of the cell, which are
otherwise, cannot pass through cell membrane. Glucose binds to the carrier molecule present on
the cell membrane, and enter into the cell; this process is facilitated by insulin.
Passive transport: It does not need energy source but the driving force is concentration gradient.
Both active transport and facilitated diffusion use carrier molecules for the transport of ions or
molecules, thus they are similar in this regard. But in active transport, ions move against their
concentration gradient (from lower to higher concentration) and in facilitated diffusion, ions
move according to their concentration gradient (from higher to lower concentration).
The “enzyme” model is based on carriers, channels, and pumps that work in synchrony to
mediate solute transport. Primary active transport using H+-ATPase or proton improves
movement sodium inside and potassium outside using energy followed by building up of
interstitial sodium concentration.
The Na/H+ antiporter: It is mediated by movement of solutes in the presence of "proton
ATPases" that is potentially expressed in plant parts such as roots, leaves during salt stress.
Symport & co-transport: the proton ATPase (H+
-ATPase) has mainly enzymatically mediate the sucrose/H+ cotransport using ATP hydrolysis
to load sucrose into the phloem.
Facilitated diffusion of water & H+ ions if mainly through secondary active transport based on
electrochemical gradient in the presence of H+-ATPase. For example, in plants, fungi, protists
etc plasma membrane "osmotic pressure is maintained in the presence of voltage sensitive
potassium channels result in "hyperpolarization" finally leading to influx of K+ in the cell.