3.1 Movement of Substances across the Plasma Membrane



Biological membrane

All cells in nature are surrounded by Biological Membranes, which all have the same basic structure.

The cell membrane is a biological membrane that separates the interior of all cells from the outside environment .The cell membrane is selectively-permeable to ions and organic molecules and controls the movement of substances in and out of cells. It consists of the phospholipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for the extracellular glycocalyx and cell wall and intracellular cytoskeleton.

Membranes separate their contents from the environment. Cell membranes separate the cell contents from its environment, and organelle membranes separate the organelle contents from their environment. Membranes regulate the movement of materials through them. For example, cell membranes might not allow starch molecules to leave the cell.


According to the fluid mosaic model of S. J. Singer and Garth Nicolson 1972, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less easily. This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as: (a) protein-protein complexes; (b) lipid rafts, and (c) pickets and fences formed by the actin-based cytoskeleton.

The barrier is differentially permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.

Fluid mosaic model

According to the fluid mosaic model of S. J. Singer and Garth Nicolson 1972, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less easily.This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as: (a) protein-protein complexes; (b) lipid rafts, and (c) pickets and fences formed by the actin-based cytoskeleton.

1. Living Organisms can only survive if :

    @ All the metabolic activities in the cell are able to take place properly.

    @ A constant internal enviroment is well-maintained.

2. In order for the metabolic activities to take place properly, the cells require a certain substancesto eliminate the by-product or waste product of the reaction.

3. There are two metabolic activities

    @ Anabolism

~ involves the biulding up of complex molecules from simple molecules.

~nessasary for production of new enzymes, protein and tissues which are important for growth, maintenence and tissues repair.

   @ Catabolism

~refers to breaking down of complex  molecules to simple ones, mainly for energy production.

  • When placed in water, the ‘heads’ orientate themselves towards water molecules and the ‘tails’ away, meaning that phospholipids will form a layer above water if left. If Phospholipid Molecules are completely surrounded by water, they may form a Bilayer.

  • A Phospholipid Bilayer consists of two layers of Phospholipids where the ‘tails’ point inwards and the ‘heads’ point outwards, towards water. One layer is like a mirror image of the other.


  • The Phospholipid Molecules are not bonded together, however, their Amphipathic Nature gives the Bilayer a degree of stability, since the Hydrophilic ‘head’ cannot easily through the Hydrophobic region created by the ‘tails’. The molecules can however move freely as a fluid in the plane of the Bilayer.

  • Small, non-polar molecules can pass through the Phospholipid Bilayer since they can ‘squeeze’ between the Phospholipid Molecules and are not repelled by the hydrophobic region. Water molecules can also move through the Bilayer, despite being polar.



Passive transport


Molecules are in constant motion and tend to move from regions where they are in higher concentration to regions where they are less concentrated. Diffusion is the net movement of molecules down their concentration gradient. Diffusion can occur in gases, in liquids, or through solids. An example of diffusion in gases occurs when a bottle of perfume is opened at the front of a room. Within minutes people further and further from the source can smell the perfume.


 Osmosis is the movement of molecules down a concentration gradient and at the same time across a membrane. Cell membranes do not allow all molecules to cross them. They are said to be “selectively” or “differentially” permeable. Only certain molecules can cross the membrane into or out of a cell. For example, water can cross the membrane while sodium and chlorine ions (dissolved salt) cannot. If there is a concentration gradient across the membrane (if there is more salt and less water on one side than on the other), water will move across the membrane down the concentration gradient while the salt cannot. If there is more salt and less water inside a cell than outside, water will flow into the cell from the surrounding environment. This process is called osmosis.







Facilitated diffusion

Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is a process of passive transport, facilitated by integral proteins. Facilitated diffusion is the spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins. The facilitated diffusion may occur either across biological membranes or through aqueous compartments of an organism.

Polar molecules and charged ions are dissolved in water but they can not diffuse freely across the plasma membrane due to the hydrophobic nature of the fatty acid tails of phospholipid that make up the lipid bilayers. Only small nonpolar molecules, such as oxygen can diffuse easily across the membrane. All polar molecules are transported across membranes by proteins that form transmembrane channels. These channels are gated so they can open and close, thus regulating the flow of ions or small polar molecules. Larger molecules are transported by transmembrane carrier proteins, such as permeases that change their conformation as the molecules are carried through, for example glucose or amino acids.

Non-polar molecules, such as retinol or fatty acids are poorly soluble in water. They are transported through aqueous compartments of cells or through extracellular space by water-soluble carriers as ritinol binding protein.The metabolites are not changed because no energy is required for facilitated diffusion. Only permease changes its shape in order to transport the metabolites. The form of transport through cell membrane which modifies its metabolites is the group translocation Transportation.



Active transport

Active transport is the movement of a substance against its concentration gradient (from low to high concentration). In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids.  Active transport uses energy, unlike passive transport, which does not use any type of energy. Active transport is a good example of a process for which cells require energy.The energy for active transport usually comes from ATP (adenosine triphosphate) Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.



  • Water, ethanol, and Chloroform exemplify simple molecules that do not require active transport to cross a membrane.

  • Metal ions, such as Na+,K +,Mg 2+, or Ca2+, require ion pumps or ion channels to cross membranes and distribute through the body

  • The pump for sodium and potassium is called sodium- potassium pump or Na +/K+-ATPase

  • In the epithelial cells of the stomach, gastric acidis produced by hydrogen potassium ATPase, a proton pump




Why osmosis and diffusion are important?


  • All living things have certain requirements they must satisfy in order to remain alive – maintain homeostasis

  •  These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes.

  • These tasks happen at the cellular level.

  • Molecules move through the cell membrane by diffusion

  • All living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell.

  • This membrane is a complex structure that is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell, and for interacting with the environment surrounding the cell.


Types of Solutions Based on Solute Concentration

The terms hypotonic, hypertonic, and isotonic are used to compare solutions relative to their solute concentrations.

In the illustration, the solution in the bag contains less solute than the solution in the beaker. The solution in the bag is hypotonic (lower solute concentration) to the solution in the beaker. The solution in the beaker is hypertonic (higher solute concentration) to the one in the bag. Water will move from the hypotonic solution into the hypertonic solution.

In this illustration the two solutions are equal in their solute concentrations. We say that they are isotonic to each other.


Will there be a net movement of water between two isotonic solutions?


Water will move freely between the two solutions if they are separated by a selectively permeable membrane. However, there will be no net change in the concentration of water on either side of the membrane. Differences in solute concentration will allow you to predict net changes in water movement. 


When the environment outside a cell has a lower concentration of dissolved molecules than inside the cell, the solution is said to be hypotonic, and water will move from the solution into the cell. If the surrounding solution has a higher concentration of dissolved molecules than the cell, the solution is hypertonic. In that case, water will move from the cell out into the surrounding solution. An isotonic solution is one in which the concentration of dissolved molecules is the same inside and outside the cell, and there is no net movement of water across the membrane. When cells are placed in a hypertonic solution, water flows out of them and they shrink or shrivel up. When cells are placed in a hypotonic solution, water flows into them. If the cell does not have a cell wall or some other means of protecting the membrane, it will burst in a hypotonic solution.


Water Potential

Because you will be working with potato cells in the laboratory, you need to understand the concept of water potential. Biologists use this term to describe the tendency of water to leave one place in favor of another. Water always moves from an area of higher water potential to an area of lower water potential.

Water potential is affected by two factors: pressure and the amount of solute. For example, imagine a red blood cell dropped into distilled water. Water will move into the red blood cell and cause the cell to expand, stretching the flexible membrane. At some point, the pressure of the incoming water will cause the cell to pop, just like an over-filled balloon.



Why don’t red blood cells pop in the bloodstream?

  Answer: Red blood cells don’t pop because the blood provides an isotonic environment for the cells.

If a plant cell is placed in distilled water, water will enter the cell and the cell contents will expand. However, the elastic cell wall exerts a back pressure, which will limit the net gain of water.



The affect application of osmosis in everyday life

It is important for cells to maintain an osmotic balance with its surroundings. Obviously cells must not be allowed to burst or crenate in a living organism. What stop this from happening? The interstitial fluid has the same concentration as the cytoplasmic fluid in cells, so the rate at which water enters and leaves the cell by osmosis is the same. Can you really recall what you have learn in chapter 2 about how the body maintain the internal environment, so that it is in a stable condition for the cells to function optimally?

     How about animals living in an aquatic in which the surrounding are not isotonic to their body tissue? What are the problem they may face due to the process of osmosis? For example, unicellular organisms such as paramecium sp. that live in freshwater are surrounding by a hypotonic solution. At all times, water is flowing  into the cytoplasm by osmosis. The paramecium sp. is in constant danger because its membrane may burst due the rising osmotic pressure. Can you recall how unicellular organisms such as amoeba sp. overcome this problem? Refer to chapter 2 to learn how they expel the excess water.

     How about the plant? Under natural conditions, plasmosis is usually rare. However, if plants are given too much fertilizer, wilting occurs. Fertilisers such as potassium nitrate when dissolve in the soil water will make the soil water more concentrated and hypertonic to the cell roots. As a result, water diffuses from the cell sap into the soil by osmosis and the cells are plasmolysed. A wilting plant eventually dies if the plant is not watered immediately.


Biology ><

~ by bio520 on February 24, 2011.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: