The Cell Membrane

Structure, function and the movement of materials

Cell Membrane The cell membrane is a bi-layered

membrane composed mainly of phospholipids.

Phospholipids have 2 fatty acids bonded to a glycerol backbone.

The third carbon in the glycerol chain is bonded to a phosphorous group.

The phosphorous group makes up the head of the molecule and has an overall charge to the group.

The fatty acids that make up the tail of the molecule do not have any charge.

As a result of the phosphorous head being charged it has an attraction for water, and the non polar tail has an attraction for the other non polar tail of the inner layer.

The non polar layers do not dissolve in water and thus create a barrier that separates the water based outside of the cell away from the water based cytoplasm inside the cell.

There are other molecules located within the cell membrane; there are many kinds of protein molecules studded through the membrane.

They tend to move sideways with the phospholipids layers thus supporting the fluid-mosaic membrane model.

M e m b r a n e s A r e F l u i d

Glycerol

Fatty acids

Phosphate Head Group

Water

No water

Serine

Phosphate

Hydrophilic heads interact with water

Hydrophobic tails interact with each other

Hydrophilic heads interact with water

Lipid micelles

Lipid bilayers

The membrane contains other types of lipids such as cholesterol.  The cholesterol allows the cell membrane to function in a wide range of temperatures.

At high temperatures it helps the lipid layer maintain rigidity.

The Role of Cholesterol

At low temperatures it keeps the membrane flexible, and prevents it from freezing.

It also helps to make the membrane less permeable to most biological molecules.

There are many different types of lipids found in specialized cells.  Each has the ability to aid that cell function in its environment.

Homeostasis

The self-regulating and self-maintaining system of internal stability of an organism.

The conditions within the cell must remain relatively constant at all times in order for the cell to remain alive and functioning properly.  This condition of maintaining a constant internal environment within the cell is called Homeostasis.  It is mainly the role of the cell membrane to control the movement of materials into and out of the cell.

Homeostasis

There are a number of methods used by the cell membrane in maintaining homeostasis.  This is due to the fact that the cell membrane is a selectively permeable membrane, allowing certain substances to pass freely but prevent others.

Plasma membrane and Homeostasis

Permeable - if a substance can pass through the membrane

Selectively permeable - membrane lets some substances pass in/out but not others.

Non permeable – if a substance can’t pass through the membrane

Water as an Environment for life

Water is the solvent on both sides of the cell membrane that provides a medium for the movement of particles across the cell membrane. Many single celled organisms exist only in water, while cells of multi-cellular organisms are bathed in a thin layer of extracellular fluid – mainly water.

The fluid inside the cell that helps make up the cytoplasm is also composed mostly of water. It is called intercellular fluid.

Methods of Material Movement Diffusion Osmosis Facilitated Diffusion Active Transport Bulk Membrane Transport

Endocytosis Pinocytosis, Phagocytosis, Receptor Mediated

endocytosis Exocytosis

Diffusion Diffusion is a method of passive transport

where small molecules pass through the cell membrane.  It can be defined as the movement of molecules from a region of high concentration to a region of lower concentration.

Passive transport refers to the movement of material across the cell membrane without the cell expending any of its energy.

Small molecules such as oxygen and carbon dioxide will pass freely in this manner.

It works because molecules are in constant motion and this random motion causes molecules to collide with one another (Brownian motion).

Molecules of another substance when added to water will be bounced around because of the motion of the water. 

This will be noticeable as long as the new substance is not evenly distributed through the water. 

In any case the particles will follow the concentration gradient – that is move from a region of higher concentration to a region of lower concentration.

Diffusion works well where the distances are short and the molecules are small. 

The cell is always using oxygen so the concentration gradient always moves oxygen into the cell and carbon dioxide is constantly being produced in the cell so it is constantly being moved out of the cell where its concentration is lower.

Hydrophobic molecules

Small, uncharged polar molecules

Large, uncharged polar molecules

Ions

O2, CO2, N2

glycerol

Glucose, sucrose

H+,Na+,NCO3–,

Ca2+,CL-,Mg2+,K+

Phospholipid bilayerRelative permeability / diffusionthrough plasma membrane

Osmosis Osmosis is the diffusion of water across a

cell membrane.  While molecules move from one side of the cell membrane to the other so does water. 

It is important that the water balance within a cell be kept constant.  If there is either too much entering or too much leaving the cell problems can arise. 

Water, just like other substances, will move along the concentration gradient.

The three conditions that can exist for osmosis are:Isotonic Condition: This occurs when the concentration of water

inside the cell equals the concentration outside the cell.  There is an equal amount of water entering as there is leaving the cell.  In this condition there is no change to the cell.

Hypotonic Condition: When the concentration of water outside the cell

is greater than that inside the cell.  More water will move into the cell causing it to swell.  Animal cells may burst and plant cells will become rigid.

Three conditions of osmosis:Hypertonic Condition: When the concentration of water is greater inside

the cell than outside.  More water will migrate out of the cell than enter it.  In both animal and plant cells the cytoplasm shrinks.

Osmosis is also a passive transport process because the cell does not use any of its own energy to move water across the membrane also it can’t stop the process because the cell membrane is permeable to water.

Osmosis Water passes from a

dilute to a concentrated solution through a semi permeable membrane

Both plant and animal cells are semi permeable

Semi permeable membrane

Dilute solution Concentrate solution

Water

Osmosis in animal and plant cells

Facilitated Diffusion

Not all molecules that must enter the cell are small enough to pass easily through the cell membrane. 

Glucose is a good example.  The concentration of glucose outside the cell is greater than it is inside the cell because it is large it needs help getting in. 

Specialized and highly specific transport proteins carry out this function.  The transport protein will recognize the molecule and help it enter the cell.

In the case of glucose, a carrier protein recognizes the molecule and facilitates its move through the cell membrane. 

The process is still an example of passive transport because it is the concentration gradient for glucose (higher outside than inside) that drives the passage of glucose through the carrier protein.

The carrier molecule will only accept a non polar molecule with a specific shape.

Active Transport In many cases the internal

environment of a cell must be vastly different from its external environment.  The cell must: Concentrate nutrients for maintenance

and growth. Carry out any specialized functions that

the cell might have. Many toxic waste products have to be

completely removed.

Passive transport will handle some of these functions but not all. 

In order to complete all of these the cell must use energy to cause materials to move in the direction it needs. 

This process is called Active Transport and materials (solutes) must move against the concentration gradient. 

That is materials must move from a region of low concentration to a region of high concentration.

At rest your cells might use as much as 40% of its energy on active transport. 

Other specialized cells would use up to 90% on active transport.  Some examples of these specialized cells are:

Kidney cells pump glucose and amino acids out of urine and back into the blood.

Intestinal cells pump in nutrients from the gut.

Root tip cells pump in nutrients form the soil.

Gill cells in fish pump out sodium ions.

Active Transport Pump Usually against a concentration gradient. The active transport pump is a system that

creates unnatural concentrations of both Na+ and K+ on both sides of the cell membrane.  Na+ is pumped out of the cell and K+ is pumped into the cell. 

The process requires a specialized carrier protein and energy from ATP.  There are a number of stages involved in creating these unnatural concentrations.  They are:

The shape of the carrier protein allows 3 sodium ions to be taken up from inside the cell.  The sodium migrates to the outer edge of this protein and will be released when the protein’s shape is changed by the addition of a phosphate molecule.

Energy in the form of ATP is converted into ADP and a free P.  The phosphate binds to the carrier protein changing its shape this releases the sodium ions and allows 2 potassium ions to enter the cell.

As the potassium ions move into the cell they have to wait for the releasing of the phosphate molecule before they are let go.

The carrier protein is back to its original shape and can transport 3 more sodium ions outside the cell.  Throughout this process there is an increase in the concentration of sodium ions outside the cell.  This is the key to moving substances into the cell against the concentration gradient.

Bulk Membrane Transport Endocytosis

Pinocytosis, Phagocytosis, Receptor Mediated Endocytosis

Exocytosis

Endocytosis The process where the cell membrane

folds into itself and pinches off forming a vesicle.

Material too large to pass either through the cell membrane or carrier proteins can be brought into the cell in this manner.

Pinocytosis: (Cell Drinking)

This process involves the cell taking in of small droplets of extracellular fluid along with dissolved materials.

The cell membrane encircles the droplets and pinches off inside the cell once it is inside.

This activity is common to all cell types.

Phagocytosis: (Cell Eating) Same process of the cell membrane

encircling the particles and then pinching off inside the cell.

The particles are much bigger and more specific.   They can be either worn out cell fragments, bacteria, or other substances.

Specialized cells within the body or single celled organisms carry out this process.

Receptor-assisted Endocytosis Involves the intake of specific molecules that

would other wise not be able to enter the cell. Cholesterol is a fat and is not soluble in water.  All

cells require cholesterol and it cannot pass into the cell by Pinocytosis.

Cholesterol is brought to the cells by blood in small vesicles that are made of a single layer of phospholipids.

Each droplet has its own tag, a protein marker that will match a protein receptor on the cell membrane.

Receptor-assisted Endocytosis The tag binds to the receptor and this

stimulates endocytosis. The cholesterol is brought into the cell. The vesicle empties its contents into the

cell and returns to the cell membrane. It turns inside out so that the receptors are

pointing outward again.

Exocytosis This is the reverse process of endocytosis. Vesicles inside the cell move to the cell

membrane where they empty their contents outside the cell.

In this way vesicle material is returned to the cell membrane.