Membrane Transport
When we discuss membrane transport, we are referring to the specific properties of the cell membrane and how different particles can move across that membrane. Thus, understanding membrane transport, requires an understanding of the cell membrane itself. So, this is where we will begin; at the cell membrane! |
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The Cell Membrane (Plasma Membrane)
- The Main Function of the Cell Membrane is PROTECTION -
The cell membrane is also called the plasma membrane. You can think of the membrane as the "skin" of the cell. Anything outside of the cell is considered "extracellular" and the contents inside the cell are considered "intracellular". The cell membrane protects the cell by creating a barrier between what is inside the cell and what is outside the cell.
Cross Section of the Cell Membrane
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The cell membrane surrounds the entire cell and act as protection. You can think of the cell membrane as acting like our skin! |
In addition to this, the cell membrane does something our skin can’t do... It regulates what comes into the cell and what goes out of the cell. For this reason, we consider the cell membrane to be “SELECTIVELY PERMEABLE” which means that it allows some substances to enter or exit the cell, but not others. This is a very important function.
The Cell Membrane is selectively permeable due to its structure. The cell membrane is made up of a phospholipid bilayer.
The Cell Membrane is selectively permeable due to its structure. The cell membrane is made up of a phospholipid bilayer.
Phospholipid
The phospholipid bilayer of the cell membrane has a unique structure. It is made up of an inner layer and an outer layer of phospholipids that are oriented with their 'tails' facing each other.
Phospholipids are considered amphiphilic, because they contain a polar, hydrophillic head that consists of a phosphate group and two nonpolar, hydrophobic fatty acid chains as 'tails'.
When the phospholipids form the cell membrane, the polar, hydrophillic (water-loving) heads are oriented towards the liquid outside the cells (extracellular fluid) and the liquid inside the cell (extracellular fluid). The tails of the phospholipids are oriented towards each other, away from the liquid, since they are made up of hydrophobic (water-fearing) fatty acid chains. This formation creates a barrier between the extracellular matrix and the intracellular fluid (cytology).
Phospholipids are considered amphiphilic, because they contain a polar, hydrophillic head that consists of a phosphate group and two nonpolar, hydrophobic fatty acid chains as 'tails'.
When the phospholipids form the cell membrane, the polar, hydrophillic (water-loving) heads are oriented towards the liquid outside the cells (extracellular fluid) and the liquid inside the cell (extracellular fluid). The tails of the phospholipids are oriented towards each other, away from the liquid, since they are made up of hydrophobic (water-fearing) fatty acid chains. This formation creates a barrier between the extracellular matrix and the intracellular fluid (cytology).
Click Below to View a Video on the Mighty Membrane
Types of Membrane Transport
ANIMATED GIF Courtesy of AmoebaSisters see more www.amoebasisters.com/gifs.html
There are a lot of ways that substances are permitted to pass through the cell membrane. There are 2 main categories of membrane transport; active and passive.
ACTIVE TRANSPORT mechanisms require energy.
PASSIVE TRANSPORT mechanisms do not require energy.
There are a lot of ways that substances are permitted to pass through the cell membrane. There are 2 main categories of membrane transport; active and passive.
ACTIVE TRANSPORT mechanisms require energy.
PASSIVE TRANSPORT mechanisms do not require energy.
Passive Membrane Transport Mechanisms
The 3 types of passive membrane transport diffusion, facilitated diffusion and osmosis. In all of these transport mechanisms, substances move from an area of higher concentration to an area of lower concentration. When we have a difference in the concentration of a substance, we call this a concentration gradient. In passive transport mechanisms, molecules are traveling "down" their concentration gradient, from an area of high concentration to an area of low concentration.
Diffusion
Diffusion is a natural process that does not require energy. Small, charged molecules freely diffuse across the cell membrane.
Facilitated Diffusion
For molecules that do not freely diffuse across the membrane, specialized membrane proteins can allow for diffusion of these molecules across the membrane.This mechanism of transport is called facilitated diffusion.
Osmosis
Diffusion of water is so important to biological processes and to osmoregularity, that it is given its own name; osmosis!
A hypotonic solution, is a solution that has a lower concentration of solutes (dissolved particles) than the liquid inside the cell. Since the cell membrane is only semi-permeable, the solutes cannot move across the membrane, but the water can! So, when a cell is placed into a hypotonic solution, water will rush into the cell causing it to bloat and swell. If the solution is very hypotonic, the cell can burst open! OUCH!
An isotonic solution is a solution that has the same concentration of solutes as the liquid inside of the cell. In this situation, since the concentration is the same inside the cell as it is outside the cell, water is happy! There will be no net movement of water flowing into or out of the cell. In this situation, the cell maintains its shape and is healthy and happy!
A hypertonic solution is a solution that contains a higher concentration of dissolved substance than the liquid inside of the cell. When a cell is placed in a hypertonic solution, water will move out of the cell, causing the cell to shrink and shrivel up!
Diffusion of water is so important to biological processes and to osmoregularity, that it is given its own name; osmosis!
A hypotonic solution, is a solution that has a lower concentration of solutes (dissolved particles) than the liquid inside the cell. Since the cell membrane is only semi-permeable, the solutes cannot move across the membrane, but the water can! So, when a cell is placed into a hypotonic solution, water will rush into the cell causing it to bloat and swell. If the solution is very hypotonic, the cell can burst open! OUCH!
An isotonic solution is a solution that has the same concentration of solutes as the liquid inside of the cell. In this situation, since the concentration is the same inside the cell as it is outside the cell, water is happy! There will be no net movement of water flowing into or out of the cell. In this situation, the cell maintains its shape and is healthy and happy!
A hypertonic solution is a solution that contains a higher concentration of dissolved substance than the liquid inside of the cell. When a cell is placed in a hypertonic solution, water will move out of the cell, causing the cell to shrink and shrivel up!
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Active Membrane Transport Mechanisms
Active transport mechanisms require energy and act to move the molecule(s) against their concentration gradient, from an area of lower concentration to an area of higher concentration. Active transport requires an energy source and a transmembrane protein that allows passage of the molecule(s) against its concentration gradient.
Another type of active membrane transport is vesicular transport. This type of transport is explained in more detail below!
Active transport mechanisms require energy and act to move the molecule(s) against their concentration gradient, from an area of lower concentration to an area of higher concentration. Active transport requires an energy source and a transmembrane protein that allows passage of the molecule(s) against its concentration gradient.
Another type of active membrane transport is vesicular transport. This type of transport is explained in more detail below!
Vesicular Transport
exocytosis = secretion and endocytosis = uptake
EndocytosisEndocytosis is the process by which contents from the extracellular matrix are taken up into the cell through a mechanism that involves the cell membrane essentially "pinching off" part of itself to form a vesicle that surrounds the particles being transported into the cell. |
ExocytosisExocytosis is when a vesicle from inside the cell fuses with the membrane and the contents are released into the extracellular fluid.
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http://academic.brooklyn.cuny.edu/biology/bio4fv/page/endocytb.htm
Vesicular transport is a vital part of the cell's function. Vesicles perform a variety of functions, including transport, metabolism, and temporary storage .
There are 3 different types of endocytosis.
- Phagocytosis – “cell eating” - In phagocytosis, one cell will engulf the target and then proceed to digest or destroy the contents of the vesicle. Phagocytosis is demonstrated by white blood cells that are appropriately named, "phagocytes".
- Pinocytosis – “cell drinking” - Cells tend to sample small amounts of the extracellular fluid via pinocytosis. Pinocytosis is when the endocytosis results in a vesicle having only extracellular fluid as its contents.
- Receptor-mediated endocytosis - This occurs when the plasma proteins bind only to certain molecules via a "lock and key" mechanism. The binding triggers the section of the membrane to "invaginate" and form a vesicle around the molecule that transports it into the cell from the extracellular fluid.
Receptor-Mediated EndocytosisGIF Courtesy of Source s10.lite.msu.edu
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Endocytosis and Exocytosis |
Vesicle
The Main Function of the Vesicle is Transport
By SuperManu - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2918850
The vesicle consists of a small amount of fluid (and sometimes particles) surrounded by a phospholipid bilayer. This bilayer is made up of the same phospholipids that are found in the cell membrane of the plasma membrane. this is due to the fact that vesicles are actually made from the cell membrane itself!
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