Cell Transport

Cell Membrane

The Cell membrane is the most important organelle in an organism’s cell. It controls how substances move in and out of the cell.

5 methods for movement in and out of cell:

• 1) lipid diffusion
• 2) osmosis
• 3) passive transport
• 4) active transport
• 5) vesicles

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Lipid Diffusion

• Carried out by very small molecules – H₂O, O₂, CO₂
• No energy is involved
• Substances move down concentration gradient
• Cannot be controlled by cell

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Osmosis

• Diffusion of water (abundant in cells) across a membrane
• Higher concentration of solution = less water molecules
• Net flow of water diffuses down concentration gradient
• Water moves from dilute to concentrated solution

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Solution Types

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Passive Transport

• Transport of substances across a membrane by a trans-membrane protein molecule
• Transport proteins are often specific to one molecule
• No energy is involved
• Substances move down concentration gradient

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Channel proteins

• form channel in membrane which is filled with water
• ions diffuse across the membrane
• many channels can be opened or closed

Carrier proteins

• flip around so that only one side of membrane is open
• high concentration: substance binds
• low concentration: substance is released

Active Transport

• Protein pump molecule “pumps” substances across membrane
• Molecule of substance binds to protein and is then released on opposite face
• Each molecule fits a particular protein pump
• Energy is required; substances move up concentration gradient

Vesicles

Move large molecules in and out of cells e.g. proteins, polysaccharides, nucleotides

• In to cell: endocytosis
• Out of cell: exocytosis

Cell Transport Summary

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