Gayathri Mani

Membrane Transport Study Guide

Plasma membranes allow certain substances to enter and leave a cell through various types of transport. Let's read about these different modes of membrane transport!

Introduction

Plasma membranes are selectively permeable—they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells; they must have a way of obtaining these materials from extracellular fluids. The most direct forms of membrane transport are passive.

Active transport:

In active transport, substances move against the concentration gradient, from an area of low concentration to an area of high concentration. This process is called "active" since it requires energy (generally in the form of ATP).

During active transport, ATP is essential to move a substance across a membrane, often with the help of protein carriers and generally against its concentration gradient. One of the most frequent types of active transport involves proteins that serve as pumps.

Primary Active Transport

  • The primary active transport which functions with the active transport of sodium and potassium ions allows secondary active transport to occur. The secondary transport method is also considered active because it depends on the use of energy, just like primary transport.

  • One of the most crucial pumps in animal cells is the sodium-potassium pump ( Na+-K+ ATPase ), which maintains the electrochemical gradient in living cells. The sodium-potassium pump moves two K+ ions into the cell while moving three Na+ ions out of the cell. Depending on the orientation to the interior or exterior of the cell and the pump's affinity for either sodium or potassium ions, the Na+-K+ ATPase exists in two forms. This process consists of the following steps:

    1. With the enzyme oriented towards the cell's interior, the carrier has a high affinity for sodium ions. Three sodium ions bind to the protein.
    2. The protein carrier hydrolyzes ATP, and a low-energy phosphate group attaches to it.
    3. Post this, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein's affinity for sodium decreases and the three sodium ions leave the carrier.
    4. The shape change increases the carrier's affinity for potassium ions, and two such ions attach to the protein. As a result, the low-energy phosphate group detaches from the carrier.
    5. The carrier protein repositions itself towards the cell's interior with the phosphate group removed and potassium ions attached.
    6. In its new configuration, the carrier protein has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process is repeated again.
  • Subsequently, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. Hence, the interior is slightly more negative relative to the exterior. This difference in charge is vital in creating the conditions necessary for the secondary process. Therefore, the sodium-potassium pump is an electrogenic pump (a pump that creates a charge imbalance), that creates an electrical imbalance across the membrane and contributes to the membrane potential.

Sodium-Potassium Pump Source

Secondary Active Transport

  • In secondary active transport, a particle is moved down its electrochemical gradient as another is moved up its concentration gradient. Secondary active transport does not depend on chemical energy molecules like ATP. Rather, secondary active transport relies on the potential energy stored in a concentration gradient. Although this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane. Therefore, it is known as secondary active transport.

Secondary Active Transport Source

Bulk Transport

Endocytosis and exocytosis are how the cell can import or export large amounts of material while using large plasma membrane folds. Endocytosis can be further subdivided into three categories: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

1. Endocytosis:

Phagocytosis("cell eating") is a type of endocytosis in which large particles, such as cells or cellular debris are transported into the cell. Single-celled eukaryotes like amoebas use phagocytosis for hunting and consuming their prey.

Phagocytosis Source

Pinocytosis ("cell drinking") is a type of endocytosis in which a cell takes in quite small amounts of extracellular fluid. Pinocytosis occurs in several cell types and takes place continuously, with the cell sampling and resampling the surrounding fluid to get whatever nutrients and other molecules happen to be present.

Pinocytosis Source

Receptor-mediated endocytosis is a type of endocytosis in which receptor proteins on the cell surface capture a specific target molecule. The receptors (transmembrane proteins) cluster in regions of the cell membrane known as coated pits. This name comes from a layer of proteins, called coat proteins, found on the cytoplasmic side of the pit. Clathrin is known to be the best-studied coat protein.

Receptor-mediated Endocytosis Source

2. Exocytosis:

Cells must take in specific molecules, such as nutrients, but they also need to release other molecules, like signaling proteins and waste products, to the external environment. Exocytosis is a type of bulk transport in which materials are transported from the inside to the outside of the cell in membrane-bound vesicles that fuse with the plasma membrane.

Exocytosis Source

Passive Transport

There are three types of passive transport: simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

  • Simple diffusion involves the movement of substances from an area of high concentration to an area of low concentration until the concentration becomes equal throughout a space. This is also true for some substances moving into and out of cells.
  • Only small, uncharged substances like carbon dioxide and oxygen can quickly diffuse across the cell membrane as it (the cell membrane) is semipermeable. Charged ions or large molecules require different kinds of transport.

Simple Diffusion Source

Facilitated Diffusion

  • Although several gases can diffuse easily between the phospholipids of the cell membrane, many polar or charged substances (like chloride, ions, sucrose, etc.) require help from membrane proteins as they are both large and polar.
  • Membrane proteins can be either channel proteins or carrier proteins.
  • Regardless of a concentration gradient that may exist for these substances, their charge (polarity) prevents them from crossing the hydrophobic center of the cell membrane.
  • Substances or particles transported through facilitated diffusion still move with the concentration gradient, but the transport proteins defend them from the hydrophobic region while they pass through.

Facilitated diffusion Source

Osmosis

  • Osmosis is the diffusion of water molecules through a semipermeable membrane. Water can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself. However, whether or not water will move into the cell, out of the cell, or both is determined by the concentration of solutes within the water.

The more the solute in a solution, the greater is the osmotic pressure of the solution. A solution that has a higher concentration of solutes than another solution is hypertonic. As the higher osmotic pressure pulls water, water molecules tend to diffuse into a hypertonic solution.

In contrast, a solution with a lower concentration of solutes than another solution is hypotonic. Cells in a hypotonic solution usually take on too much water and swell. Hence, they carry the risk of eventually bursting. This process is called lysis.

Osmosis Source

Conclusion

  • The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur.
  • In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient. Secondary active transport does not rely on chemical energy molecules like ATP.
  • The secondary transport method is still considered active because it depends on the use of energy as does primary transport.
  • In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient. Secondary active transport does not rely on chemical energy molecules like ATP.
  • Exocytosis (exo = external, cytosis = transport mechanism) is a form of bulk transport in which materials are transported from the inside to the outside of the cell in membrane-bound vesicles that fuse with the plasma membrane.
  • In passive transport, substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a single substance concentration range has a concentration gradient.
  • There are three types of passive transport: simple diffusion, facilitated diffusion and osmosis.

FAQs

1. What organelles are membrane sacs used to transport molecules?

Vesicles are membrane-bound sacs that are invloved in organizing metabolism, transport, and storage of molecules.

2. What is the difference between active and passive transport across the plasma membrane?

Active transport is the movement of molecules across the plasma membrane by using cellular energy, whereas, passive transport is the movement of molecules across the membrane without the use of ATP.

3. Under what circumstances does membrane transport require energy?

When a molecule moves down its concentration gradient is it participating in passive transport; moving up the concentration gradient requires energy making it active transport.

4. What transport process can create a concentration gradient for sodium across the plasma membrane?

Primary active transport moves ions across a membrane, thereby, creating an electrochemical gradient for sodium across the plasma membrane.

5. Within the fluid mosaic of a plasma membrane, what is the role of transport and channel proteins?

Transmembrane protein channels and transporters are invloved in the transport of small organic molecules such as sugars or amino acids.

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Sources

  1. Lumen. “Transport across Membranes | Boundless Anatomy and Physiology.” Lumenlearning.com, 2013, courses.lumenlearning.com/boundless-ap/chapter/transport-across-membranes/. Accessed 12 Feb. 2022.
  2. “5.2 Passive Transport - Biology 2e | OpenStax.” Openstax.org, openstax.org/books/biology-2e/pages/5-2-passive-transport. Accessed 12 Feb. 2022.
  3. “Passive Transport and Active Transport across a Cell Membrane Article (Article).” Khan Academy, www.khanacademy.org/test-prep/mcat/cells/transport-across-a-cell-membrane/a/passive-transport-and-active-transport-across-a-cell-membrane-article#:~:text=When%20a%20molecule%20moves%20down. Accessed 12 Feb. 2022.