In other words, the transport process does not involve chemical modification of any of the compounds bound to the carrier.
Rather, carriers catalyze the movement of inorganic ions and simple organic solutes across the membrane.
One defining feature of the carrier is that they display saturability when the kinetics of transport are expressed to the substrate concentration.
The array of ions and solutes translocated by the carrier is vast.
The principal inorganic nutrients including NH4+/ NO3-,Pi,K+ and SO4- are all transported into the cell by plasma membrane carrier. Carriers are also responsible for taking up ions that play less roles in metabolism like Cl- ions.
The organic solutes translocated into the cell by carrier are the fundamental building blocks of polymers: sugars, amino acids, purines and pyrimidines. .(Fig I).
Active transport involves membrane carrier that moves substance against an electrochemical gradient ie. an energy requiring process.
The energy releasing activity includes the hydrolysis of high energy molecules such as ATP, the absorption of photons of light, the transfer of electrons and the movement of other substances down their respective electrochemical gradients.
The active transport of sodium and potassium ions is the classic example of an energy requiring transport system powered by the hydrolysis of ATP.
In most cells the concentration of potassium is higher than the concentration of sodium.
Maintaining this unequal distribution of sodium and potassium is important because potassium is needed inside the cell for the activity of certain enzymes and the sodium is required for the active transport of sugar and amino acids.
MCQs on Carriers
Which of the following is true about carriers? a) They catalyze ATP hydrolysis b) They transport inorganic ions and organic solutes c) They chemically modify the bound compounds d) They cannot transport ions Ans: b
Carriers show which characteristic property in terms of transport kinetics? a) Non-saturability b) Saturability c) No kinetic dependence d) Random transport Ans: b
What type of molecules are translocated by carriers? a) Inorganic ions only b) Organic solutes only c) Both inorganic ions and organic solutes d) Large macromolecules Ans: c
Which of the following is an inorganic nutrient transported by plasma membrane carriers? a) Glucose b) NH₄⁺ / NO₃⁻ c) Fatty acids d) DNA Ans: b
Carrier-mediated transport does not involve: a) Active transport b) Passive diffusion c) Chemical modification of transported substances d) Specificity for substrates Ans: c
Active transport requires: a) ATP hydrolysis b) Passive movement c) Concentration equilibrium d) No energy Ans: a
Which of the following can provide energy for active transport? a) Hydrolysis of ATP b) Absorption of photons c) Electron transfer d) All of the above Ans: d
The sodium-potassium pump maintains which type of ion concentration gradient? a) Equal concentration of Na⁺ and K⁺ b) Higher Na⁺ inside the cell c) Higher K⁺ inside the cell d) No ion gradient Ans: c
Why is it important to maintain an unequal distribution of Na⁺ and K⁺ in cells? a) To create an electrochemical gradient b) To facilitate enzyme activity c) To transport sugars and amino acids d) All of the above Ans: d
Which of the following is an example of an energy-dependent transport system? a) Diffusion of oxygen b) Na⁺/K⁺ pump c) Facilitated diffusion of glucose d) Osmosis Ans: b
Which of the following is NOT a function of membrane carriers? a) Facilitated diffusion b) Active transport c) Catalyzing substrate breakdown d) Ion transport Ans: c
The transport of Cl⁻ ions is important for: a) DNA replication b) Maintaining osmotic balance c) Protein synthesis d) ATP production Ans: b
Which organic molecules are transported by carriers? a) Sugars b) Amino acids c) Purines and pyrimidines d) All of the above Ans: d
Which ion plays a crucial role in enzyme activation inside the cell? a) Na⁺ b) K⁺ c) Cl⁻ d) H⁺ Ans: b
The process where molecules move against their concentration gradient is called: a) Passive transport b) Active transport c) Facilitated diffusion d) Simple diffusion Ans: b
Which of the following statements about active transport is incorrect? a) It requires ATP b) It moves substances against the concentration gradient c) It is always independent of carrier proteins d) It can use electrochemical gradients for energy Ans: c PYQ: CSIR NET 2021
Which of the following is an example of secondary active transport? a) Na⁺/K⁺ ATPase b) Glucose-Na⁺ symporter c) Diffusion of gases d) Osmosis Ans: b PYQ: GATE 2020
The driving force for passive transport is: a) ATP hydrolysis b) Carrier proteins c) Concentration gradient d) None of the above Ans: c PYQ: NEET 2019
The sodium-potassium pump moves: a) 3 Na⁺ out, 2 K⁺ in b) 2 Na⁺ out, 3 K⁺ in c) Equal Na⁺ and K⁺ d) No ions Ans: a PYQ: IIT JAM 2022
Which transporter type allows ions to pass through membranes without ATP? a) Ion channels b) Carrier proteins c) ATPases d) ABC transporters Ans: a PYQ: CSIR NET 2020
Which of the following best describes carrier proteins? a) They facilitate passive diffusion only b) They bind to specific molecules and change conformation c) They hydrolyze ATP directly d) They transport molecules without specificity Ans: b
Which of the following is NOT transported by carriers? a) Amino acids b) Purines and pyrimidines c) Large polysaccharides d) Simple sugars Ans: c
Carrier-mediated transport differs from channel-mediated transport because carriers: a) Are always passive b) Bind solutes before transport c) Require ATP hydrolysis d) Only transport inorganic ions Ans: b
Which type of transport is facilitated by carrier proteins without using ATP? a) Simple diffusion b) Facilitated diffusion c) Primary active transport d) Endocytosis Ans: b
Which characteristic of carrier proteins leads to saturation at high substrate concentration? a) Carrier proteins have limited binding sites b) Transport is energy-independent c) Only inorganic ions are transported d) Transport rate decreases at higher substrate concentration Ans: a
The Na⁺/K⁺ pump is an example of: a) Passive transport b) Facilitated diffusion c) Primary active transport d) Secondary active transport Ans: c
In which type of active transport does the movement of one molecule drive the transport of another? a) Uniport b) Symport c) Antiport d) Passive diffusion Ans: b
The electrochemical gradient generated by the Na⁺/K⁺ pump is essential for: a) Protein synthesis b) Nerve impulse conduction c) ATP synthesis d) DNA replication Ans: b
Which of the following provides energy for secondary active transport? a) ATP hydrolysis b) Ion gradient c) Electron transfer d) Light absorption Ans: b
Active transport differs from passive transport because: a) It does not require membrane proteins b) It requires energy input c) It moves substances down the gradient d) It allows only water transport Ans: b
Which transport process involves the direct use of ATP? a) Passive diffusion b) Primary active transport c) Secondary active transport d) Facilitated diffusion Ans: b PYQ: CSIR NET 2020
Which statement about ion channels and carrier proteins is incorrect? a) Ion channels are always passive b) Carrier proteins can be active or passive c) Carrier proteins change conformation d) Ion channels require ATP Ans: d PYQ: GATE 2021
Which of the following is an example of facilitated diffusion? a) Na⁺/K⁺ pump b) Glucose transport by GLUT proteins c) Proton pump d) Phagocytosis Ans: b PYQ: NEET 2018
What is the ratio of Na⁺ to K⁺ transported by Na⁺/K⁺ ATPase? a) 3 Na⁺ out, 2 K⁺ in b) 2 Na⁺ out, 3 K⁺ in c) 1 Na⁺ out, 1 K⁺ in d) 4 Na⁺ out, 3 K⁺ in Ans: a PYQ: IIT JAM 2023
Which of the following does NOT require energy input? a) Active transport b) Endocytosis c) Facilitated diffusion d) Proton pumps Ans: c PYQ: CSIR NET 2019
The uptake of Cl⁻ ions by a cell is mainly for: a) Enzyme activation b) Osmotic balance c) Energy production d) Protein synthesis Ans: b
Which ion is required for nerve signal transmission? a) Na⁺ b) K⁺ c) Cl⁻ d) Both Na⁺ and K⁺ Ans: d
The Na⁺/K⁺ pump helps maintain: a) pH balance b) Membrane potential c) Enzyme function d) Protein synthesis Ans: b
Which of the following does NOT involve carrier proteins? a) Simple diffusion b) Active transport c) Facilitated diffusion d) Secondary active transport Ans: a
Why does facilitated diffusion require carrier proteins? a) To speed up diffusion b) To provide energy c) To transport molecules against a gradient d) To convert molecules into ATP Ans: a
Which of the following is a characteristic of passive transport? a) Requires ATP b) Moves molecules against their gradient c) Does not require energy d) Only occurs in animal cells Ans: c PYQ: GATE 2017
The function of an antiporter is to: a) Move two molecules in the same direction b) Move molecules down their gradient c) Exchange one molecule for another in opposite directions d) Facilitate diffusion Ans: c PYQ: CSIR NET 2018
Which of the following ion gradients drive secondary active transport? a) Na⁺ gradient b) K⁺ gradient c) H⁺ gradient d) All of the above Ans: d PYQ: NEET 2020
The proton gradient in mitochondria is used to: a) Transport glucose b) Synthesize ATP c) Regulate temperature d) Generate DNA Ans: b PYQ: IIT JAM 2022
Which of the following is a function of ion channels? a) ATP hydrolysis b) Selective permeability c) Chemical modification of solutes d) Endocytosis Ans: b PYQ: CSIR NET 2021
Which type of transport does NOT involve conformational changes in proteins? a) Carrier-mediated transport b) Ion channel-mediated transport c) Active transport d) Secondary transport Ans: b
Which of the following ions is actively pumped out of the cell? a) Na⁺ b) K⁺ c) Cl⁻ d) H⁺ Ans: a
The energy for primary active transport comes from: a) ATP hydrolysis b) Light absorption c) Ion gradients d) Carrier proteins Ans: a
The Na⁺/K⁺ pump is an example of: a) Uniport b) Symport c) Antiport d) Diffusion Ans: c
Carrier proteins are required for: a) Facilitated diffusion b) Active transport c) Secondary active transport d) All of the above Ans: d
