Symport Antiport Uniport: Overview, Types, Example

Membrane Transport Mechanisms

The mechanisms for transport across plasma membranes are the major methods by which substances move into or out of the cell. It can be broadly divided into passive transport and active transport.

Importance in Cellular Homeostasis

Membrane transport mechanisms have very important roles in maintaining cellular homeostasis, uptake of nutrients, waste product removal, and signalling. When they function properly, the cells will have enough energy to grow and respond to environmental changes.

Types of Membrane Transport

It can be broadly divided into passive transport, which is without energy expenditure, and active transport, where energy is utilised in the form of ATP. In contrast, passive transport occurs via simple diffusion and facilitated diffusion, and active transport involves pumps and cotransporters.

• Simple diffusion: Movement of molecules from an area of high concentration to an area of low concentration without the aid of transport proteins.

• Facilitated diffusion: A way of moving molecules across the cell membrane using transport proteins moving down their concentration gradient.

• Active Transport: The movement of molecules against their concentration gradient, requiring energy input, typically from ATP.

Passive vs Active Transport

The difference between passive and active transport is:

Role of Transport Proteins

• Integral membrane proteins which actively or passively assist in the translocation of substances across the cell membrane.

• Such proteins are essential for maintaining the interior of the cell and performing communication and transport functions between the cell and its environment.

What is Symport?

Symport is a co-transport mechanism in which two substances are transported across the membrane by one transport protein in the same direction.

Mechanism

• Symport coupling of the movement of one molecule with its concentration gradient to the movement of another molecule against its concentration gradient.

• In the sodium-glucose symporter, sodium ions diffuse down their concentration gradient into the cell, powering the cotransport of glucose against its concentration gradient.

Example – Sodium-Glucose Cotransport (SGLT)

• SGLT couples the movement of sodium ions with glucose uptake into the intestine or kidney.

• It allows glucose absorption against concentration gradients.

Significance in Nutrient Uptake (Gut & Kidney)

• Symport plays an important role in the uptake of nutrients from the gut. An example includes glucose and amino acids.

• Symport plays a significant role in the kidneys that deal with the reabsorption of valuable nutrients and ions into the blood from the filtrate.

What is Antiport?

Antiport is a transport system where one transport protein moves two substrates in opposite directions across the membrane.

Mechanism

• Antiport is the exchange of one molecule moving in one direction for another molecule moving in the other.

• The sodium-potassium pump is an example of an active transporter that pumps three sodium ions outside a cell and two potassium ions inside a cell, using ATP as the source of energy to maintain the electrochemical gradient.

Example – Sodium-Potassium Pump (Na⁺/K⁺ ATPase)

• The Na⁺/K⁺ ATPase pump actively transports 3 Na⁺ ions out and 2 K⁺ ions into the cell.

• This pumping of ions utilizes ATP.

Role in Nerve Impulse and Ion Balance

• Antiport mechanisms are crucial to maintaining proper intracellular and extracellular ion concentrations.

• The sodium-potassium pump itself is of critical importance in maintaining the resting membrane potential of the nerve cells and thereby helps in the propagation of the nerve impulse.

What is Uniport?

Uniport is transportation across the membrane for a single type of molecule, either facilitated diffusion or active transport.

Mechanism

• Uniport is a transport mechanism by which one substance is moved across the membrane by the transport protein without being coupled to the movement of some other molecule.

Example – Glucose Transport (GLUT1 in RBCs)

• GLUT1 is facilitated diffusion present in RBCs.

• It allows glucose to move down its concentration gradient without use of energy.

Role in Cellular Metabolism

• Uniport mechanisms are of central importance in taking onboard glucose and other molecules which are essen­tial for cellular metabolism.

• Glucose is taken into the cells by GLUT1 to make a continuous supply for energy production.

Difference Between Symport, Antiport, and Uniport (Table)

The difference between symport, antiport and uniport is given in the table below:

Significance in Physiology

The mechanisms of membrane transport are significant in normal physiology for the maintenance of homeostasis and various cellular functions that are crucial for survival.

Role in Homeostasis and Ion Regulation

The membrane transport system help to maintain homeostasis by controlling the movement of ions like Na⁺, K⁺, and Ca²⁺. This ensures proper functioning of the body like muscle contraction and nerve function.

Nutrient Absorption and Waste Removal

It facilitates uptake of nutrients like glucose and amino acids and removal of wastes such as urea and carbon dioxide

Regulation of pH and Electrochemical Gradient

The transport proteins are responsible for the internal environment of the cell. It maintains the proper pH, establishes electrochemical gradients, action potential and enzyme activity.

Medical Relevance

Defective transport proteins lead to a wide variety of diseases and disorders.

CFTR Mutation – Cystic Fibrosis (Chloride Transport)

The CFTR gene is responsible for the transportation of chlorine ions across membranes. Any mutation in this gene causes disruption in chloride ion transport. This leads to mucus build up in the lungs, causing breathing problems in individuals with cystic fibrosis.

Sodium Imbalance and Hypertension

The imbalance in the transport of sodium and potassium ions can cause an increased blood volume leading to high blood pressure (hypertension).

Potential Therapeutic Targets

Understanding membrane transport mechanisms has led to the development of targeted therapies. These drugs treat conditions like hypertension and cystic fibrosis by making ion transport normal.

Symport, Antiport, and Uniport NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Types of Membrane transport

• Active vs Passive Transport

• Uniport, Symport, Antiport (Definition & Examples)

Practice Questions for NEET

Q1. When there are two solutes moving across the membrane in the same direction, their transport is called

1. Uniport

2. Antiport

3. Symport

4. All of these

Correct answer: 3) Symport

Explanation:

It is a type of active transport in which two molecules or ions are simultaneously transported across a membrane in the same direction by a carrier protein. For example, the sodium-glucose symporter transports sodium ions and glucose together into the cell.

Hence, the correct answer is option 3) Symport.

Q2. When bicarbonate ions move across the membrane of RBCs, the chlorine ions are taken in. This type of transport is called

1. Symport

2. Antiport

3. Uniport

4. All of these

Correct answer: 2) Antiport

Explanation:

It transports the solute (in or out) and the cotransporter solute in the opposite direction. For example, HCO3- - Cl- transporter. It transports the solute (in or out) and the cotransporter solute in the opposite direction. For example, the HCO3⁻ - Cl⁻ transporter is an antiporter system where bicarbonate ions and chloride ions are exchanged across the membrane. This type of transport is crucial in maintaining pH balance and ion homeostasis in cells. Such systems are energy-efficient as they utilize the existing concentration gradients of ions. Antiporters like this play a vital role in processes such as respiration and acid-base balance in the blood.

Hence, the correct answer is option 2) Antiport.

Q3. Which one of the following structures between two adjacent cells is an effective transport pathway?.

1. Plasmodesmata

2. Plastoquinones

3. Endoplasmic reticulum

4. Plasmalemma

Correct answer: 1) Plasmodesmata

Explanation:

Plasmodesmata are fine cytoplasmic connections between adjacent cells which facilitate material passage.

- wherein

Found in plant cells.

Hence, the correct answer is option 1) Plasmodesmata.

Also Read:

• MCQs on Symport, Antiport and Uniport

• Passive Transport

• Stomata

Recommended Video On 'Symport Antiport Uniport'