Eukaryotic cells possess an outer membrane that protects a cell’s contents. However, the outer membrane is semi-permeable, and allows certain materials to enter it.
Inside eukaryotic cells, smaller sub-structures called organelles possess their own membranes. Organelles serve several different functions in cells, including moving molecules across the cellular membrane or through the organelle’s membranes.
TL;DR (Too Long; Didn't Read)
Molecules can diffuse across membranes via transport proteins, or they can be aided in active transport by other proteins. Organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria and peroxisomes all play a role in membrane transport.
Cell Membrane Characteristics
The membrane of a eukaryotic cell is often referred to as a plasma membrane. The plasma membrane is comprised of a phospholipid bilayer, and is permeable to some molecules, but not all.
Components of the phospholipid bilayer include a combination of glycerol and fatty acids with a phosphate group. These yield the glycerophospholipids that generally make up the bilayer of most cell membranes.
The phospholipid bilayer possesses water-loving (hydrophilic) qualities on its exterior, and water-repellant (hydrophobic) qualities on its interior. The hydrophilic portions face the outside of the cell as well as the inside of it, and are both interactive and attracted to the water in these environments.
Throughout the cell membrane, pores and proteins help determine what enters or exits the cell. Of the different kinds of proteins found in the cell membrane, some extend only into part of the phospholipid bilayer. These are called extrinsic proteins. The proteins that cross the entire bilayer are called intrinsic proteins, or transmembrane proteins.
Proteins make up about half of cellular membranes’ mass. While some proteins can move around easily in the bilayer, others are locked in place and need help if they must move.
Transport Biology Facts
Cells need a way to get necessary molecules into them. They also need a way to release certain materials back out again. Released materials can of course include wastes, but often certain functional proteins must be secreted outside of cells as well. The phospholipid bilayer membrane maintains a flux of molecules into the cell, by means of osmosis, passive transport or active transport.
The extrinsic and intrinsic proteins work to help with this transport biology. These proteins may possess pores to allow for diffusion, they may work as receptors or enzymes for biological processes, or they might work in immune responses and cellular signaling. There are different types of passive transport as well as active transport that play a role in the movement of molecules across membranes.
Types of Passive Transport
In transport biology, passive transport refers to the transport of molecules across the cell membrane that does not require any assistance or energy. These are typically small molecules that can simply flow into and out of the cell, relatively freely. They might include water, ions and the like.
One example of passive transport is diffusion. Diffusion occurs when certain materials enter the cell membrane via pores. Essential molecules such as oxygen and carbon dioxide are good examples. Typically diffusion requires a concentration gradient, meaning the concentration outside of the cell membrane has to be different from the inside.
Facilitated transport requires assistance via carrier proteins. Carrier proteins bind the materials needed for transport at binding sites. This joining makes the protein change shape. Once the items are helped through the membrane, the protein releases them.
Another type of passive transport is via simple osmosis. This is common with water. Water molecules strike a cell membrane, creating pressure and building up “water potential.” Water will move from high to low water potential to enter the cell.
Active Membrane Transport
Occasionally, certain substances cannot cross a cell membrane simply by diffusion or passive transport. Moving from low to high concentration, for example, requires energy. To make this happen, active transport occurs with the help of carrier proteins. Carrier proteins hold binding sites that the necessary substances attach to so they can be moved across the membrane.
Larger molecules such as sugars, some ions, other highly charged materials, amino acids and starches cannot drift across the membranes without aid. Transport or carrier proteins are built to specific needs depending on the type of molecule that needs to move across a membrane. Receptor proteins also work selectively to bind molecules and guide them across membranes.
Organelles Involved in Membrane Transport
Pores and proteins are not the only aids for membrane transport. Organelles also serve this function in a number of ways. Organelles are smaller sub-structures inside cells.
Organelles have diverse shapes and they perform different functions. These organelles make up what is called the endomembrane system, and they possess unique forms of protein transport.
In cytosis, large amounts of materials can cross a membrane via vesicles. These are bits of cell membrane that can move items into the cell or out (endocytosis or exocytosis, respectively). Proteins are packaged by the endoplasmic reticulum in vesicles to be released outside the cell. Two examples of vesicular proteins include insulin and erythropoietin.
The endoplasmic reticulum (ER) is an organelle responsible for making both membranes and their proteins. It also aids molecular transport through its own membrane. The ER is responsible for protein translocation, which is the movement of proteins throughout the cell. Some proteins can fully cross the ER membrane if they are soluble. Secretory proteins are one such example.
For membrane proteins, however, their nature of being part of the membrane’s bilayer requires a little help to move around. The ER membrane can use signals or transmembrane segments as a way to translocate these proteins. This is one of the types of passive transport that provides a direction for the proteins to travel to.
In the case of the protein complex known as Sec61, which functions mostly as a pore channel, it must partner with a ribosome for the purpose of translocation.
The Golgi apparatus is another crucial organelle. It gives proteins final, specific additions that give them complexity, such as added carbohydrates. It uses vesicles to transport molecules.
Vesicular transport can occur in part due to coating proteins, and these proteins aid in vesicle movement between the ER and the Golgi apparatus. One example of a coat protein is clathrin.
In the inner membrane of the organelles called mitochondria, numerous proteins must be used to help with energy generation for the cell. The outer membrane, in contrast, is porous for small molecules to pass through.
Peroxisomes are a kind of organelle that breaks down fatty acids. As their name implies, they also play a role in the removal of harmful hydrogen peroxide from cells. Peroxisomes can also transport large, folded proteins.
Researchers only recently discovered the immense pores that allow peroxisomes to do this. Ordinarily proteins are not transported in their full, large, three-dimensional states. Much of the time they simply are too large to pass through a pore. But peroxisomes are up to the task in the case of these giant pores. Proteins must carry a particular signal in order for a peroxisome to transport them.
The diverse methods of types of passive transport make transport biology a fascinating subject for study. Gaining knowledge about how materials can be moved across cell membranes can aid in understanding cellular processes.
Because many diseases involve malformed, poorly folded or otherwise dysfunctional proteins, it becomes clear how relevant membrane transport can be. Transport biology also provides limitless opportunities to discover ways to treat deficiencies and diseases, and perhaps to make novel medications for treatment.
- Royal Society of Chemistry: In and Out of Cells
- Scitable by Nature Education: Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes
- Phys.org: Researchers Discover Giant Pores in the Membrane of the Cell Organelles
- Howard Hughes Medical Institute: Protein Transport in Cells
- Scitable by Nature Education: Cell Membranes
About the Author
J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction & fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.