This document discusses cellular transport mechanisms. It describes passive transport mechanisms like diffusion, facilitated transport, and osmosis which move molecules across membranes down their concentration gradients and require no energy. Active transport mechanisms like active transport, endocytosis, and exocytosis move molecules against their gradients or larger particles/cells and require energy. Diffusion allows small noncharged molecules, water, and gases to passively enter and leave cells. Osmosis involves the passive movement of water across membranes. Facilitated transport uses carrier proteins to transport molecules too large for diffusion.

1. Cellular Transport
An introduction
HL Biology
Cells & Biomolecules
Mr Goodwin

2. Passive transport
 The movement of molecules across the plasma
membrane from an area of high concentration
to an area of low concentration.
 A process requiring no energy
 Made up of three processes:
– Diffusion
– Facilitated Transport
– Osmosis

3. Diffusion
 The movement of molecules from an area of high
concentration to an area of low concentration
 A natural process requiring no energy
 Some molecules enter and leave cells through the
plasma membrane by diffusion.
 Such molecules are:
1. Small noncharged molecules like urea
2. Water
3. Gases like oxygen and carbon dioxide.

4. Osmosis
 The movement of water molecules across a semi-
permeable membrane, from an area of high water
concentration to an area of low water concentration
 A natural process requiring no energy
 An area of high water concentration has low solute
concentration, while an area of low water concentration
has high solute concentration
 Solution tonicity (how relatively concentrated)
1. Isotonic: No water movement
2. Hypertonic: Water moves in.
3. Hypotonic: Water moves out

5. Facilitated transport
 The movement of molecules from an area of high
concentration to an area of low concentration
 A natural process requiring no energy
 Unlike diffusion, a carrier or channel protein is required.
 Such molecules are:
1. Some sugars
2. Water
3. Some amino acids
Note that these molecules are generally too big to simply diffuse on their own

6. Active Transport
 The movement of molecules across the plasma
membrane from an area of low concentration
to an area of high concentration.
 A process that requires energy
 Made up of three processes:
– Active transport
– Endocytosis
– Exocytosis

7. Active Transport
 The movement of molecules from an area of low
concentration to an area of high concentration. This is
also known as going against the concentration
gradient.
 A process that requires energy
 A protein carrier is also required.
 Such molecules are:
1. Ions like Na+, K+ and Cl-
2. Some sugars
3. Some amino acids

8. Exocytosis
A process where very large molecules
(macromolecules) leave the cell by the formation
of vesicles from the plasma membrane. Molecules
like:
 Digestive enzymes: Ex, Produced in the pancreas
and released in the small intestine.
 Hormones: Ex, Insulin, produed in the pancreas,
released into the blood to regulate sugar levels.

9. Endocytosis
A process where very large molecules
(macromolecules) or even entire cells enter
the cell by the invagination of the plasma
membrane. Things like:
 Cholesterol: Travels in the blood in protein-
lipid “bubbles” which enter cells by
endocytosis.
 Bacteria: Are “eaten” by white blood cells
(the body’s “soldiers”) by endocytosis.

10. Endocytosis
A process where very large molecules
(macromolecules) or even entire cells enter
the cell by the invagination of the plasma
membrane. Things like:
 Cholesterol: Travels in the blood in protein-
lipid “bubbles” which enter cells by
endocytosis.
 Bacteria: Are “eaten” by white blood cells
(the body’s “soldiers”) by endocytosis.