Shrinithi Mahadevan

Mechanisms of Transport Study Guide

Membrane transport refers to a set of transport mechanisms that control the movement of solutes such as ions and small molecules through biological membranes.


The ability of the cell to transport molecules in and out of itself is critical.

Small molecules, virtually always simple ions like hydrogen, potassium, or sodium, can pass through the plasma membrane. Passive diffusion may allow the ions to pass through the pore formed by membrane proteins. In this case, the protein that allows this transport is known as an ion channel. An ion pump is a transmembrane protein that uses energy, usually obtained from ATP, to actively drive ions from one side of the plasma membrane to the other.

Because membrane transport is so important, cells use various transport methods. Simple diffusion, enhanced diffusion, and active transport are the three types of processes.

Types of transport mechanism

There are four types of transport mechanisms in a cell. These are simple diffusion, facilitated diffusion, primary active transport and secondary active transport.

Passive Transport

  • To understand how substances pass passively through cell membranes, we need to understand concentration gradients and diffusion.
  • The concentration gradient is the difference in the concentration of a substance in a room.
  • Molecules (or ions) diffuse from where they are more concentrated to where they are less concentrated until they are evenly distributed in space (when a molecule moves in this way, it moves along a concentration gradient).
  • Three common types of passive transport include simple diffusion, osmosis, and facilitated diffusion.

● Diffusion

The molecules can travel directly through the membrane in simple diffusion. Diffusion occurs near the bottom of a concentration gradient, restricting the molecule's maximum concentration inside the cell (or outside the cell if it is a waste product). The molecule’s diffusion rate also limits diffusion’s effectiveness. As a result, while diffusion is an adequate transport mechanism for some substances (such as water), the cell must rely on other mechanisms for most of its transport requirements.

● Facilitated diffusion

Facilitated diffusion makes use of membrane protein channels to allow charged molecules to readily move in and out of the cell that would otherwise be unable to do so. Small ions like K, Na, and Cl- activate these channels the most. The number of protein channels accessible limits the speed of assisted transport, whereas the concentration gradient solely determines diffusion speed.

Types of Membrane Transport Source

Active transport

  • Although active transport necessitates energy expenditure to transport a molecule from one side of the membrane to the other, it is the only mode of transport that can transport molecules up and down a concentration gradient.

  • Active transport, like facilitated transport, is constrained by the number of protein transporters present. Primary and secondary active transportation are the two types of active transportation we are interested in.

    Primary active transport involves employing energy (typically from ATP hydrolysis) to create a conformational change in the membrane protein, resulting in molecular transport across the protein. The Na -K pump is the most well-known example of this. The Na -K pump is an antiport that transfers both K and Na into and out of the cell at the same time while using ATP.

    Secondary active transport includes energy to create a gradient across the cell membrane, which is subsequently used to transport a molecule of interest up to its concentration gradient.

  • Na - glucose secondary transport mechanism: The Na -K pump is the first step in another secondary active transport system, generating a high Na gradient across the cell membrane. The glucose-Na symport protein then transports glucose through the Na gradient into the cell.

Na-glucose secondary transport Source

  • In human gut epithelial cells, this mechanism is exploited differently. These cells use the coordinated actions of Na -glucose symporter, glucose permeases (glucose aided diffusion protein), and Na -K pumps to accept glucose and Na from the intestines and transfer them through to the bloodstream. Tight connections connect the epithelial cells, preventing seeping from the intestines into the bloodstream without being filtered by the epithelial cells.

● Endocytosis

The cell membrane wraps around a portion of the external medium, forming near-perfect spheres around it and drawing membrane-bound vesicles called endosomes into the cell. Different types of endocytosis are distinguished. In pinocytosis, the vesicles are small and contain fluid. In phagocytosis, vesicles are larger and contain solids.

In receptor-mediated endocytosis, substances bind to specific receptors on the outside of the cell membrane, which trigger the process of forming an envelope. Cholesterol enters cells in the last way.

Phagocytosis Source

● Exocytosis

During exocytosis, the intracellularly synthesized material packaged in membrane-bound vesicles is exported from the cell after the vesicles fuse with the outer membrane. The materials exported in this way are cell-specific protein products, neurotransmitters, and various other molecules.


  • Transfer proteins transport materials into and out of cells.
  • The cell does not have to expend any energy to use passive transport pathways.
  • Passive transport can be divided into simple diffusion and assisted diffusion.
  • Active transport pathways necessitate the cell's usage of ATP-stored energy.


1. What are the different types of transport mechanisms?

The different types of transport mechanisms are:

  • Diffusion
  • Facilitated Diffusion
  • Active Transport
  • Na - glucose secondary transport mechanism

2. What are transport mechanisms in biology?

The phrase "transport" refers to moving anything from one location to another. It can be used as an action term to describe transporting, moving, or conveying something from one place to another. The act or means by which molecules, ions, or substrates are transferred across a biological membrane, such as the plasma membrane, is referred to as transport in biology.

3. What are the 6 types of transport?

The 6 types of transport are

  • Simple Diffusion
  • Endocytosis
  • Exocytosis
  • Facilitated Diffusion
  • Osmosis
  • Active Transport

4. What are the three types of cell transport?

The three types of cell Transport are:

  • Simple Diffusion
  • Osmosis
  • Facilitated Diffusion

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  1. Cell transport https://www.ck12.org/biology/cell-transport/lesson/Cell-Transport-Advanced-BIO-ADV/ Accessed on 21 Dec, 2021
  2. Cell transport and homeostasis https://www.ck12.org/book/ck-12-biology/section/3.3/ Accessed on 21 Dec, 2021
  3. https://ncert.nic.in/textbook.php