We all know that cells are the basic units of organisms. Containing organelles and being responsible for performing many of our vital functions, how do cells move particles in and out of them?
A process called cell transport is important for the functioning of cells as well as maintaining homeostasis. Read this article to learn more about cell transport!
The two basic methods of transport are passive transport and active transport. The basic difference between the two methods is that passive transport doesn’t use energy, while active transport requires the use of energy because it is the move of particles against the concentration gradient.
Passive transport
There are three types of passive transport: simple diffusion, osmosis, and facilitated diffusion.
Simple diffusion is known for the movement of small, non-polar, and lipid-soluble molecules such as oxygen and carbon dioxide. This is an unassisted process, meaning that solutes can move freely across the cell membrane with the concentration gradient.
Osmosis is the movement of water and can be further divided into hypotonic, hypertonic, and isotonic. Highly polar water molecules easily cross the plasma membrane through aquaporins, and it moves down its concentration gradient. Hypertonic solutions contain more solutes than the cells do, meaning there is more water inside the cell than outside the cell. Water will move out of the cell, causing the cell to shrink. In opposition, hypotonic solutions, which contain fewer solutes than the cells do, have more water outside of the cell. In this situation, water will move into the cell, causing the cell to plump. And lastly, in isotonic solutions, solutes inside and outside of the cell are balanced, which means that everything will maintain stability.
Facilitated diffusion is the transport of larger, polar, and lipid-insoluble particles such as glucose, sodium, and potassium. In this situation, protein membrane channels or protein molecules that act as carriers are used to facilitate the moving process.
Active transport
Active transport, sometimes called solute pumping, requires protein carriers to transport substances that may be too large, lipid-insoluble, and have to move against a concentration gradient. Adenosine triphosphate (ATP) is used for this transport.
A specific example of active transport is the sodium potassium pump, where ATP binds to the transport protein, breaking down the bond. The phosphate will cause the protein to change shape, letting out three sodium ions. After that, two potassium ions will be moved into the cell once the protein changes its shape again.
One type of active transport is vesicular transport, where substances are moved without actually crossing the plasma membrane. This usually refers to big and large amounts of particles. There are two types of vesicular transport: exocytosis and endocytosis.
Exocytosis: the movement of materials out of the cell. The materials are carried in a membranous sac called a vesicle. The vesicle will then migrate to the plasma membrane and combine with it. After that, material is emptied to the outside.
Endocytosis: a process where extracellular substances are engulfed by being enclosed in a membranous vesicle. The contents coming into the cell are digested by lysosomal enzymes. There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Phagocytosis can be considered as “cell eating”, where a cell engulfs large particles such as bacteria or dead body cells. It is a protective mechanism, but not for nutrients intake.
Pinocytosis, the “cell drinking”, where cells gulp droplets of extracellular fluid containing dissolved proteins or fats. This is a routine activity for most cells, such as those involved in absorption.
Receptor-mediated endocytosis is a method for taking up specific molecules, and the receptor proteins on the membrane surface bind only to certain substances. This is a highly selective process of taking in substances such as enzymes, hormones, cholesterol, and iron.
That is all for cell transport, an activity our cells are doing right now, without you even noticing.
We talk about cells a lot, from structures, functions, to transportation, but there are still many aspects of cells that we can go deeper on and understand more. By learning about the tiny things that make us alive, maybe we can exclaim how interesting and amazing life is!
Written by: Benetta Wang
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