Phospholipids have:

• Two fatty acids attached to a glycerol molecule

• Phosphate molecule attached to the glycerol molecule.

Another important thing to notice, is the double bond between the Carbon molecules. One of the fatty acid chains is saturated (has no carbon-carbon double bonds), giving it a linear structure, while the other is an unsaturated fatty chain with a carbon-carbon double bond. This double bond is the reason our cells can diffuse nutrients and survive. Without this, we would not be alive.

This kink in the phospholipid spaces out the phospholipids, creating a fluid like structure. If there was no link and these fatty acid tails where straight, then the cell membrane would be much thicker, preventing molecules from easily diffusing in and out.

Therefore cells have to have a way of separating their internal environment from their

external environments.

The Plasma membrane allows the cell to control its internal environment. As discussed

before, it is semi-permeable, meaning the cell can choose that molecules enter the cell

and what molecules exit the cell.

The plasma membrane also protects the cell from foreign molecules entering the cell and damaging or destroy it.

The arrangement of all the molecules within the plasma membrane, give it a fluid like structure. This is referred to as, the Fluid Mosaic model.

Plasma

Membrane

(This is not an

Organelle, this is a structure)

Structure

Scroll over each part of the membrane

• Protein channels/Transporter proteins
• Receptor Proteins
• Cholesterol
• Phospholipid
• Recognition proteins / Glyco-proteins

The same type of membrane which protects the cell, is also present in many organelles. The term ‘membrane bound organelles’, means organelles such as, lysosomes, chloroplast, mitochondria, ER, vacuoles and the nucleus are enclosed within their own membrane.

As seen in the animation above, the phospholipids are arranged in tail to tail so there is a gap, where very little water exists. This arrangement creates a phospholipid Bilayer. Due to this Bilayer non-polar molecules diffuse quicker through the membrane compared to polar molecules.

Receptor Proteins

On the plasma membrane there are proteins which act as antennas, and gather

information about the external environment. The cell can then act accordingly

with this information.

Cholesterol

Cholesterol is embedded within the membrane, making it less fluid and more stable.

Without cholesterol membranes breakdown rapidly and release their content. It also

decreases the permeability of the membrane to small water-soluble molecules.

Transporter proteins (click here to learn more)

These proteins have openings on both sides of the membrane layer. Molecules which

are too large to diffuse through the membrane use these channels.

These channels can also be used to push molecules against the concentration gradient,

into or out of the cell, and ionised participles through the membrane.

However pushing against the gradient requires energy (ATP)

Recognition proteins / Glycoproteins

These proteins have carbohydrates attached to them and act as markers (antigens).

These markers are specific to each person and extremely important in immune system

recognition of self and invader cells.  This is why transplants are often rejected.

Phospholipids

The phosphate molecule is polar (dissolves easily in water) and makes the head hydrophilic. This means it is attracted to water.

As with all fats, they are non-polar and thus hydrophobic. This means the tails are water fearing.

If we where to align these phospholipids in a jar, they would create a barrier between the hydrophilic and hydrophobic parts of the liquid.

As seen in the image to the left.

Click here for more information on movement through the membrane.

Or

Click here for questions on the plasma membrane

Basic movement through membranes

(Scroll over each button to see how the molecule moves through)