IB Biology - Curriculum Notes 

1.4 Membrane transport  

∑ - Understandings:

∑ - Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

Passive transport means there is no expenditure of energy (ATP).

Passive transport requires the substance to move from an area of high concentration to low concentration.
Osmosis is the passive movement of water molecules, across a semi-permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.
Simple diffusion is the passive movement of particles from an area of high concentration to an area of low concentration (follows its concentration gradient). 

Simple diffusion across membranes occurs when substances other than water move across the phospholipid bilayer (between the phospholipids) or through protein channels.
Substances that move across the membrane are usually small non-charged particles (i.e. Oxygen, Carbon Dioxide, and Nitrogen) or other lipids.
Facilitated Diffusion - Specific ions and other particles that cannot move through the phospholipid bilayer sometimes move across protein channels

During facilitated diffusion, the membrane protein changes its shape to allow a specific substance to move across the membrane.
Each protein channel structure allows only one specific molecule to pass through the channel. For example, magnesium ions pass through a channel protein specific to magnesium ions. Active transport is the movement of substances across membranes using energy from ATP.

Active transport generally moves substances against their concentration gradient (low to high concentration).
Many different protein pumps are used for active transport. Each pump only transports a particular substance; therefore cells can control what is absorbed and what is expelled.
Pumps work in a specific direction; substances enter only on one side and exit through the other side.
Substances enter the pump from the side with a lower concentration.
Energy from ATP is used to change the conformational shape of the pump.
The specific particle is released on the side with a higher concentration and the pump returns to its original shape. A good link for membrane transport


∑- The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

The phospholipids in the bilayer are loosely packed together creating fluidity and allowing movement along the horizontal plane.
The hydrophilic properties of the phosphate heads and the hydrophobic properties of the hydrocarbon tails prevent flipping of the molecules across the vertical plane, maintaining the stable bilayer.
Cholesterol embedded in the membrane will reduce the fluidity making the membrane more stable.


The plasma membrane is pinched as a result of the membrane changing shape.
External material (i.e. Fluid droplets) are engulfed and enclosed by the membrane.
A vesicle is formed that contains the enclosed particles or fluid droplets, now moves into the cytoplasm.
The plasma membrane easily reattaches at the ends that were pinched because of the fluidity of the membrane.
Vesicles that move through the cytoplasm are broken down and dissolve into the cytoplasm.
https://www.youtube.com/watch?v=iZYLeIJwe4w  (White blood cell consuming bacteria)


After a vesicle created by the rough ER enters the Golgi apparatus, it is again modified, and another vesicle is budded from the end of the Golgi apparatus, which moves towards the cell membrane.
This vesicle migrates to the plasma membrane and fuses with the membrane, releasing the protein outside the cell through a process called exocytosis.
The fluidity of the hydrophilic and hydrophobic properties of the phospholipids and the fluidity of the membrane allows the phospholipids from the vesicle to combine to the plasma membrane to form a new membrane that includes the phospholipids from the vesicle.  

 Video showing endocytosis and exocytosis

Diagram of Endocytosis and Exocytosis (to the right)

Applications and skills:

ß - Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.


Watch the animation on McGraw-Hill to see how the sodium/potassium pumps are used pump ions in and out of neurons to facilitate the transmission of nerve impulses.  Every cycle pumps three sodium ions out of the axon and two potassium ions into the axon.


***Using your textbook and the online resource, create a diagram explaining the steps of active transport of sodium and potassium and the use of the facilitated diffusion of potassium ions in axons during nerve impulse transmission.***

ß - Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

Hypertonic solution – Is a solution with higher osmolarity (higher solute concentration) than the other solution. If cells are placed into a hypertonic solution, water will leave the cell causing the cytoplasm’s volume to shrink and thereby forming indentations in the cell membrane.

Hypotonic solution – Is a solution with lower osmolarity (lower solute concentration) than the other solution. If cells are placed in a hypotonic solution, the water will rush into the cell causing them to swell and possibly burst.

Both of the above solutions would damage cells, therefore isotonic solutions are used (same osmolarity as inside the cell)

Isotonic solution: A solution that has the same salt concentration as cells and blood. 

In medical procedures, isotonic solutions are commonly used as

1) Intravenously infused fluids in hospitalized patients.
2) Used to rinse wounds and skin abrasions
3) Saline eye drops
4) Packing donor organs for transport (frozen to slush)
5) During skin grafts, used to keep the damaged area moist
(normal saline is approx. 300 mOsm)

ß - Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. (Practical 2)

Osmolarity of Potato or Yam Cores Lab

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