Active Transport, Practice Problems and FAQs

Have you ever noticed that climbing down the stairs is always easy compared to climbing up the stairs? While climbing uphill we fight against the force of gravity and thus it requires more energy whereas climbing downhill can be achieved without using extra energy.

Let us take another scenario. While boarding a heavily crowded train compartment we have to put in a lot of effort and energy but when we are about to get down from a crowded train, all we have to do is align ourselves in a line and the crowd pushes us down, without us having to use much energy.

You will be amazed to know that both of these scenarios are applicable to molecules that move in and out of the cells. While movement of molecules from their region of high concentration to their region of low concentration, that is, downhill movement is easy and does not require energy, the uphill movement from low concentration to high concentration comes at a price. The price paid is in terms of cellular energy and such a mode of transport is known as active transport. In this article we will learn more about active transport.

Table of contents

Means of transport
Active transport
Types of Active Transport
Practice Problems
FAQs

Means of transport

Transport of substances into and out of the cell is very crucial for the survival of the cell. The cell takes up water, nutrients, enzymes, hormones, etc for carrying out different metabolic functions and gives out wastes, excess water and other substances which it does not need. The transport of substances across the cell membrane can either be active or passive. Active transport requires the expenditure of energy while passive transport does not require energy and is driven by concentration gradient. Let us try to understand the process of active transport in detail.

Active transport

It is the mode of transport which involves the expenditure of cellular energy and is often carried out against a concentration gradient (from lower to higher concentration) with the help of special membrane or carrier proteins.

The cellular energy used in active transport helps to move the molecules against any sort of resistance as is posed by a negative concentration gradient or the polar repulsion between the hydrophobic lipid bilayer of the cell membrane and hydrophilic substances to be transported across it. Active transport helps in the accumulation of high concentrations of ions and molecules that the cell needs.

Fig: Active transport

Let us take an example. We know that water is absorbed by the root hair cells through the process of osmosis. This is because solute concentration in the cell sap is higher than that in the soil which results in less water concentration in cell sap. Hence water from the soil crosses the semipermeable cell membrane and enters the root hair cell, from its region of high concentration to its region of low concentration by the process of osmosis. But, if the cell sap already has a high concentration of solutes, then how does it absorb more mineral ions which are also essential for plant growth? Here, the process of active transport comes into the picture. Mineral ions are absorbed from the soil by the root hair cells, against their concentration gradient, by the process of active transport and utilisation of energy.

Absorption of glucose from the intestinal lumen in humans, absorption of amino acids across the lining of intestines, outward movement of calcium ions from cardiac muscle cells are some of the other examples of active transport in living beings.

Types of Active Transport

Active transport can be of two types based on the type of cellular energy it utilises to transport the molecules -

primary active transport uses the energy currency of the cell that is adenosine triphosphate (ATP)
secondary active transport uses an electrochemical gradient to drive the transport of molecules against the concentration gradient

Primary active transport

The type of active transport that uses chemical energy in the form of ATP for transporting molecules across a membrane is known as primary or direct active transport. Active transport helps in moving metal ions, such as Na⁺, K⁺, Mg²⁺, and Ca²⁺ across the cell membrane with the help of ion pumps or ion channels. This process uses transmembrane ATPases for the hydrolysis of ATP to generate energy.

A very common ATPase driven ion pump found universally in all animal cells is the sodium-potassium pump, which helps in the maintenance of membrane potential of the cells. It helps to move three sodium ions out of the cell in exchange of every two potassium ions that is brought into the cell by directly utilising energy in the form of ATP.

Primary active transport can also use energy obtained from redox reactions and photons (light) for carrying out transport. The electron transport chain occurring in the mitochondria uses the energy obtained from the reduction of NADH and the subsequent reduction and oxidation of the electron carriers in the chain to pump protons from the mitochondrial matrix into the intermembrane space across the inner mitochondrial membrane from their lower to their higher concentration.

During the light reactions of photosynthesis, the energy of photons is used to generate a proton gradient between the two sides of the thylakoid membrane and synthesise NADPH.

The different types of primary active transporters are -

P-type ATPase: Na⁺-K⁺ pump, Ca²⁺ pump, proton pump
F-ATPase: ATP synthase present in inner mitochondrial membrane and thylakoid membrane
V-ATPase: vacuolar ATPase
ABC (ATP binding cassette) transporter: PhABCG1 transporter in Petunia that helps in active transport of volatile organic compounds, NtPDR1 in Nicotiana tabacum helps in exporting antimicrobial metabolites.

Mechanism of active transport

Let us understand the mechanism of active transport using the sodium-potassium pump as an example.

The protein acting as the pump is initially open towards the cell cytoplasm and allows three sodium ions to bind to itself. The pump has a high affinity towards sodium at this point.

Fig: Initial configuration of pump

This induces hydrolysis of ATP into ADP and inorganic phosphate (P_i) and this phosphate is used to phosphorylate the pump.
Phosphorylation of the pump induces a conformational change in the pump causing it to open towards the cell exterior. Change in conformation lowers the affinity of the pump towards sodium, thereby releasing the sodium ions into the extracellular space.

Fig: Change in conformation of pump due to binding of sodium ions and phosphorylation

Change in conformation also increases the affinity of the pump towards the potassium ions present in the extracellular fluid. Thus two potassium ions bind to the pump and cause the release of the attached phosphate group.

Fig: Release of sodium ions and binding of potassium ions

As the pump is dephosphorylated, it returns to its original conformation and faces the interior of the cell.
This, again reverses the pump's affinity from potassium ions to sodium ions and hence the potassium ions detach from the pump and are released into the cell cytoplasm. The pump is now free again to bind to sodium ions and repeat the process.

Fig: Release of potassium inside the cell

Secondary active transport

This is the type of active transport in which no direct coupling of ATP is required to transport molecules across a semipermeable membrane against their concentration gradient. Secondary active transport, also known as cotransport or coupled transport utilises the electrochemical gradient that is generated due to movement of ions into or outside the cell. The energy synthesised due to movement of an ion along its electrochemical gradient is utilised to drive the transport of another ion against its electrochemical gradient.

Allowing an ion to move down its electrochemical gradient can increase entropy which in turn can serve as a source of energy. For example, in the electron transport chain, the movement of protons along the electrochemical gradient across the inner mitochondrial membrane (from intermembrane space to mitochondrial matrix) through the proton channel of the F₀-F₁ particles activates the ATP synthase activity of the F₁ subunit and drives the synthesis of ATP from ADP. When protons are pumped across a biomembrane, the energy derive from it is often used as a source of energy in secondary active transport.

Fig: Mechanism of ATP synthesis in electron transport chain

In humans, the electrochemical gradient of sodium (Na⁺) ions is commonly utilised to power the cotransport of another ion or molecule across the plasma membrane, against its gradient.

Depending on the direction of the movement of the substances being co-transported, cotransporters can be symporters or antiporters.

Fig: Two types of secondary active transporters

Antiporter

An antiporter pumps two different types of solutes, ions or molecules, in opposite directions, that is one is pumped into the cell while the other is pumped out. One of the solutes moves from high to low concentration and the increase in entropic energy due to that is used to drive the movement of another solute in the opposite direction, that is from the lower to the higher concentration.

An example of an antiporter is the sodium-calcium exchanger which transports three Na⁺ ions into the cell in exchange of one Ca²⁺ ion moving out of the cell. This antiporter pump is of great significance in the cardiac muscle cells and helps to keep the cytoplasmic calcium concentration low. The cardiac muscle cells also possess calcium ATPases or primary active transporters which operate when the intracellular concentration of calcium is low and help to maintain the resting concentration of calcium in the muscle cells. But the ATPase transporters are much slower compared to the antiporters. Hence antiporters are used when there is a steep rise in the calcium ion concentration of the cytoplasm of cardiac muscle cells and rapid recovery is required.

Fig: Sodium calcium exchanger

Symporter

A symporter transports two different species of solutes in the same direction, that is either from outside into the cell or from inside the cell to the outside. But in either case, the movement of one solute from high to low concentration (downhill) drives the movement of another solute, against its concentration gradient, from low concentration to high concentration (uphill).

Glucose transporter SGLT1 is an example of a symporter which takes in two sodium ions into the cell and uses that to drive the entry of one glucose molecule into the cell. This symporter is found in the membrane of intestinal cells, brain cells, cardiac muscle cells and the cells lining the third segment of the proximal convoluted tubule of kidney nephrons. SGLT2 cotrasporter is also found in segments 1 and 2 of the PCT.

Fig: Glucose symporter

Practice Problems

Which of the following helps in secondary active transport?

F-ATPase
Sodium calcium exchanger
Na⁺-K⁺ pump
Both b and c.

Solution: Active transport is the mode of transport which involves the expenditure of cellular energy and is often carried out against a concentration gradient (from lower to higher concentration) with the help of special membrane or carrier proteins. Active transport can be of two types based on the type of cellular energy it utilises to transport the molecules - Primary active transport and secondary active transport.

The type of active transport that uses chemical energy in the form of ATP for transporting molecules across a membrane is known as primary or direct active transport. The P-type ATPase transporter such as Na⁺-K⁺ pump, Ca²⁺ pump, proton pumps and F-ATPase transporter such as ATP synthase present in inner mitochondrial membrane and thylakoid membrane are some examples of primary active transporters.

In secondary active transport, no direct coupling of ATP is required to transport molecules across a membrane against a gradient. The energy synthesised due to movement of an ion along its electrochemical gradient is utilised to drive the transport of another ion against its electrochemical gradient. The transporters used in such transport can be a symporter or an antiporter. Sodium calcium exchanger is an antiporter which transports three sodium ions into the cell in exchange of one calcium ion moving out of the cell. Thus, the correct option is b.

2. Which of the following statements is true?

Glucose transporter SGLT1 co-transports one glucose molecule and two sodium ions into the cell
Glucose transporter SGLT1 co-transports two glucose molecule into the cell for every two sodium ions it imports into the cell
Glucose transporter SGLT1 co-transports one glucose molecule into the cell for every two sodium ions it exports out of the cell
Glucose transporter SGLT1 co-transports two glucose molecule into the cell for every two sodium ions it exports out of the cell

Solution: A symporter transports two different species of solutes in the same direction, that is either from outside into the cell or from inside the cell to the outside. But in either case, the movement of one solute from high to low concentration (downhill) drives the movement of another solute, against its concentration gradient, from low concentration to high concentration (uphill). Glucose transporter SGLT1 is an example of a symporter which takes in two sodium ions into the cell and uses that to drive the entry of one glucose molecule into the cell. Thus, the correct option is a.

3. In a sodium potassium pump, the affinity of the pump towards sodium is more when

it is phosphorylated
it is not phosphorylated
it is facing the interior of the cell
it is facing the exterior of the cell

I and III
I and IV
II and III
II and IV

Solution: In its original conformation, the sodium potassium pump is initially open towards the interior of the cell, facing the cytoplasm, and allows three sodium ions to bind to itself. The pump has high affinity towards sodium at this point and is in a non-phosphorylated state. Binding of sodium induces hydrolysis of ATP and phosphorylation of the pump which in turn causes the pump to change conformation and face the exterior of the cell. This phosphorylated pump facing the cell exterior has lower sodium affinity and higher potassium affinity.

Thus, the correct option is c.

4. A sodium potassium pump

uses ATP to import three sodium ions into the cell and export two potassium ions out of the cell
uses ATP to export three sodium ions out of the cell and import two potassium ions into the cell
uses ATP to import two sodium ions into the cell and export three potassium ions out of the cell
uses ATP to export two sodium ions out of the cell and import three potassium ions into the cell

Solution: A very common ATPase driven ion pump found universally in all animal cells is the sodium-potassium pump, which helps to maintain the membrane potential of the cells. It helps to move three Na⁺ ions out of the cell in exchange of every two K⁺ ions that move into the cell by directly utilising energy in the form of ATP. Thus, the correct option is b.

FAQs

Who was the first person to discover that glucose absorption in the intestine occurs with the help of a symporter?

Solution: Robert K. Crane discovered that the sodium-glucose symporter helps in intestinal glucose absorption. He presented his discovery in August 1960, in Prague.

How can defects in the SGLT2 transporter affect the human body?

Solution: The SGLT2 transporter is a glucose transporter that is largely responsible for absorption of glucose in the PCT of the kidney nephrons. Defects in SGLT2 prevent efficient glucose reabsorption resulting in high levels of glucose in the urine which leads to a condition known as renal glycosuria.

3. Do defects in active transport systems of the human body lead to cystic fibrosis?

Answer: Cystic fibrosis (CF) is caused due to mutations in the CFTR (Cystic Fibrosis Conductance Regulator) gene which codes for an ATP-gated chloride channel. Thus the protein forming the channel is misfolded and is not sent to the cell membrane to help in membrane transport. Normally, the CFTR coded chloride channel would allow chloride ions to move out of cells which is followed by exit of sodium and water molecules. This outward movement of water helps to hydrate the mucosal surface and thins out the mucosal secretion making it easy for the cilia in the secretory ducts or respiratory passes to clear the mucus from these tubular structures. In cystic fibrosis, the mutation of the channel protein impairs the movement of chloride ions, and subsequently sodium ions and water molecules. Thus, the mucosal surface remains dehydrated, which leads to thick mucus in the bronchial passages, pancreatic ducts, etc. This promotes bacterial growth, pulmonary infections, etc.

4. How are root hair cells adapted to active transport?

Answer: Root hair cells have a high solute concentration in the cell sap compared to the soil which builds the negative concentration gradient needed for active transport. They also have a higher number of mitochondria to generate ATP for active transport. The cell membrane of these cells are also equipped with multiple carrier proteins or pumps to facilitate active transport.

YOUTUBE LINK: