Transport of Substances through Cell Membrane
| Type of molecule | Examples | Through which part of the membrane | Active or passive | With or against the concentration gradient | |
| Diffusion | Small, uncharged molecules | Carbon dioxide and oxygen | Phospholipid bilayer | Passive | With (high to low) |
| Facilitated Diffusion | Hydrophilic molecules | glucose | Through channel proteins/via carrier proteins | passive | With (high to low) |
| Osmosis | Water molecules | · | Phospholipid bilayer | passive | With (high to low) |
| Active Transport | Substances | Ions into root hairs, muscles and nerve cells | Through carrier proteins that change shape | active | Against (low to high) |
| Exocytosis | Very large molecules/particles | Proteins (insulin/
polysaccharides) |
Not through membrane | active | either |
| Endocytosis | Very large molecules/particles | cholesterol | Not through membrane | active | either |
Diffusion:
- Net movement of molecules/ions from an area of high concentration to an area of lower concentration
- Continues until equilibrium
- Small uncharged molecules
- Pass between the lipid molecules
- No energy needed
Facilitated Diffusion:
- Hydrophilic molecules
- Insoluble in lipids
- Cross the membrane by either diffusing through water-filled pores within channel proteins that than span the membrane
- Different channel proteins for transporting different molecules (each has a specific shape for a specific ion/molecule)
- Some may be opened or closed depending on a signal (hormone or change in voltage across the membrane) these are GATED CHANNELS
- No energy needed
- Carrier proteins; ion/molecule binds onto a specific site on the protein which then changes shape
- Movement can occur in either direction; net movement dependent on the concentration across the membrane
- Molecules move from high to low
- No energy needed
Osmosis:
- Net movement of water molecules from a solution with lower concentration to a solution with a higher concentration of solute.
- Partially permeable membrane
Active Transport:
- Transports substances against a concentration gradient (low to high)
- Energy required; comes from respiration and the energy transfer molecule ATP
- Specific carrier proteins are also needed; substance binds to carrier proteins
- Energy from ATP changes the shape of the carrier protein, causing the substance to be released on the other side of the membrane
- Occurs in every cell
- Transports ions across epithelial cells
Exocytosis:
- Very large molecules or particles transported across cell surface membranes
- Relies on fluid nature of the membrane
- Release of substances from the cell as vesicles fuse with the cell membrane (usually proteins or polysaccharides)
Endocytosis:
- Substances taken into the cell by the formation of vesicles
- Part of cell surface membrane engulfs the material to be transported
- Substance can also be absorbed by attaching to a receptor in the membrane then endocytosis