Cell Transport

Learning Objectives

• Understand the differences between passive and active transport.

• Explain the differences between simple and facilitated diffusion, including features of each.

• Describe the differences between channel and carrier proteins, including features of each type.

• Understand the difference between primary and secondary active transport.

• Describe the three forms of endocytosis and exocytosis, knowing the features of each and how they differ.

Passive VS Active Transport

• Passive Transport:

  • No energy input required.

  • Molecules move from high to low concentration.

  • Types: Simple diffusion and facilitated diffusion.

• Active Transport:

  • Energy input required for molecules to cross membranes.

  • Molecules move from low to high concentration.

Passive Transport

Simple Diffusion

• Direct diffusion of small and nonpolar (hydrophobic) molecules through the membrane.

• Examples: CO2, O2, ethanol.

• Water (H2O) can also pass, but not as efficiently due to its polar nature.

Facilitated Diffusion

• Requires transport proteins (transmembrane proteins) for large, polar (hydrophilic) or charged molecules to go through

• Examples: Ions, small carbohydrates, amino acids, nucleotides, water, sugars

• Types of proteins involved:

  1. Channel Proteins

  2. Carrier Proteins

Channel Proteins

• Continuous transport

• Transmembrane proteins that allow specific ions or molecules to pass through due to it’s shape

• Continuous transport

• Do not change shape

• Example: Aquaporins specialized for water transport. Enables a flow of water at 3 billion H20 molecules per second

• Channels can regulate traffic by being open or closed.

Carrier Proteins

• Change shape to transport molecules

• Transports molecules one at a time (discontinuous)

• Transport larger molecules by changing shape during the transport process.

• Can facilitate both passive and active transport.

Active Transport

• Moving substances against their concentration gradient requires energy (e.g., ATP).

• Types:

  1. Primary Active Transport: Direct use of ATP.

  2. Secondary Active Transport: Uses the concentration gradient of another molecule as energy.

Primary vs. Secondary Active Transport

• Primary Active Transport: Direct ATP usage for moving ions/molecules.

• Secondary Active Transport: Uses the gradient created by primary active transport to move other substances against their gradient.

Endocytosis and Exocytosis

• Endocytosis: Process of bringing substances into the cell via vesicles (import process). Bulk transport (one very large thing or many small things at once)

  • Types:

  1. Phagocytosis: Cell engulfs large solid particle/thing ("cellular eating"). (food)

  2. Pinocytosis: Cell engulfs small liquid droplets("cellular drinking"). (drinks/liquids)

  3. Receptor-mediated endocytosis: Cell engulfs specific molecules that bind to receptor proteins on membrane. (looking for one thing)

• Exocytosis: Bulk transport out of cell (releases things)

  • Example processes can include secretion of proteins.

Example Queries

• Which substances can move via simple diffusion?

  • Correct answer: Nitrogen molecule (N2).

• How does SARS-CoV-2 likely enter a host cell?

  • Correct answer: Receptor-mediated endocytosis through binding to ACE2 proteins.

Key Vocabulary Terms

• Passive transport

• Active transport (Primary vs. Secondary)

• Facilitated diffusion

• Channel protein

• Carrier protein

• Aquaporin

• Proton pump

• Endocytosis

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis

• Exocytosis