API  Note Home Page

Cell Membranes & Movement Through a Membrane

Cell membrane

Both plants and animal cells and prokaryotes have cell membranes. In the case of animals and man of the prokaryotes this membrane is also incased in a rigid cell wall. Animals do not have cell walls.

I). Plasma (cell) Membrane

        A). Structure

            1). Lipid Bilayer

Consists of a double layer of phospholipids :

phopholipid line of phospohlids double membrane of phospohlipids


               Permeable to molecules that are soluble in lipids:

Not permeable to molecules that are soluble in H2O:


            2). Proteins

                                Provide specialized functions for the cell

  i).  Receptor Proteins

                         Bind with external molecules (hormones) and

                        then carry messages into the inside of the cell

ii).  Integral proteins

    Act as channels for ions.

       iii). Peripheral proteins:

Associated with part of the membrane and act as enzymes and in signal transduction.

      iv).  Glycoproteins contain a branching sugar

                            Function in cell adhesion and cell recognition. 


        B). Function of cell membrane:


II.  Movement across a membrane

A.  Passive Transport--no energy needed

1). Diffusion:



The random motion of molecules that results in a net movement of molecules across a concentration gradient

from a region of high solute concentration to a region of low solute concentration.

When dispersion is complete

dynamic equilibrium is reached.


Energy cost = zero

i). Random motion:


ii). Net Movement:

{The movement in one direction}

- {The movement in the opposite direction}

= {The sum total or net movement}

  iii). Concentration Gradient:

The high solute area (high concentration) is said to be up

The low solute area (low concentration) is said to be "down".

 iv). Dynamic Equilibrium

v). Diffusion only occurs when:

i). There are some barriers to free movement of material.

ii). Membrane does not influence the direction of movement.


2). Diffusion: Facilitated Diffusion

Passive Movement

Energy cost = zero

A molecule that is too big or too polar to diffuse across the membrane combines with a specific a transport carrier protein and is released in the cytoplasm.




Glucose cannot pass through the membrane without using a special glucose channel.

The ability to cross the membrane is going to depend on the number of carrier proteins.

3). Osmosis

        i). The passive diffusion of water across the membrane.

When solute concentration (the number of solute particles not the type) differs water concentration differs.

Water diffusion occurs whenever water concentration differs on both sides and the solute is impermeable to the membrane.

Water rushes across the membrane until the solute concentration is even

Osmosis is passive.

ii). Net Movement

a). Isotonic:

The solute concentration of the outside of the cell is equal to the solute concentration inside of the cells so there is no that results in no net movement of water into and out of the cell.

Volume of water going in = Volume of water going out

b). Hypotonic:

The solute concentration of the outside of the cell is lower then the solute concentration inside of the cells

The water rushes into the cell and it lyses or brea

c). Hypertonic:

The solute concentration of the outside of the cell is higher then the solute concentration inside of the cells

That causes the net movement of water out of the cell.

                    The cell shrinks and shrivels up.


."Particles suck."

Tonicity (hypertonic, hypotonic) refers to concentration of particles, and water always goes toward the area of more particles. Thus particles suck water

B). Active Transport --Energy required

Active transport


1).  Active Transport

A cell uses energy from ATP to move solutes against their concentration gradient.

(From low concentration to high concentration)

These systems are called pumps

                                ie. Na+ pump, K+ pump, Ca++ pump

Active transport

                        Because they pump against the natural solute flow

                            ie. Sodium- Potassium pump in muscle and nerve cells

                                Na+ is higher outside the cell than inside.

                                K+ is higher inside the cell than outside the cell.

Both ions leak across their gradient from high to low.

                            The Na+-K+ pump works to drive:

                                        Na+ back out of the cell.

                                        It pushes Na+ against the gradient &

                                        brings K+ into the cell against its gradient.


2). Exocytosis & Endocytosis

i). Exocytosis

inside the cell to the outside of the cell.


ii). Endocytosis

outside the cell to inside the cell.