Revise Movement into and out of cells for Cambridge 9700 (4.2)

Before diving into transport, recall that the cell surface membrane is a dynamic, partially permeable structure primarily composed of a phospholipid bilayer with embedded proteins (the fluid mosaic model). Its selective nature dictates what enters and exits the cell, maintaining homeostasis. The hydrophobic tails of the phospholipids create a barrier to most polar and charged substances, while allowing small, non-polar molecules to pass through.

Passive transport processes occur down a concentration gradient (or water potential gradient for osmosis) and do not require metabolic energy (ATP) from the cell.

1. Diffusion

This is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random kinetic energy. It continues until the particles are evenly distributed (dynamic equilibrium). Small, non-polar molecules like oxygen, carbon dioxide, and fat-soluble vitamins can diffuse directly across the phospholipid bilayer. Factors affecting the rate of diffusion include:

• Concentration gradient: Steeper gradient = faster rate.
• Surface area: Larger surface area = faster rate.
• Diffusion distance: Shorter distance = faster rate.
• Temperature: Higher temperature (more kinetic energy) = faster rate.
• Size and nature of diffusing molecule: Smaller, non-polar molecules diffuse faster.

1. Facilitated Diffusion

Larger polar molecules (e.g., glucose, amino acids) or charged ions (e.g., Na⁺, Cl⁻) cannot pass directly through the lipid bilayer. They require the assistance of specific transmembrane transport proteins. This process is still passive as it occurs down a concentration gradient and does not require ATP.

• Channel proteins: Form water-filled pores or channels that allow specific ions to pass through. They provide a hydrophilic pathway. Many are 'gated', meaning they can open or close in response to a signal (e.g., voltage change or binding of a ligand), thus controlling ion flow.
• Carrier proteins: Bind to specific molecules (like glucose), which causes the protein to undergo a conformational change (change in shape). This change moves the molecule to the other side of the membrane, where it is released. This process is slower than channel-mediated diffusion and can become saturated if all carrier proteins are occupied.

1. Osmosis

Osmosis is a special case of diffusion. It is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

• Water potential (Ψ): A measure of the potential energy of water, or its tendency to move. It is measured in kilopascals (kPa). Pure water has the highest possible water potential, defined as 0 kPa at standard temperature and pressure. Adding solutes lowers the water potential, making it negative. Water always moves from a less negative Ψ to a more negative Ψ.
• Components of Water Potential: Water potential is determined by two factors: solute potential (Ψs) and pressure potential (Ψp). The formula is Ψ = Ψs + Ψp.
  • Solute potential (Ψs): The effect of dissolved solutes on water potential. It is always negative or zero (for pure water).
  • Pressure potential (Ψp): The effect of physical pressure on water potential. In animal cells, it is usually zero. In plant cells, the inward pressure of the cell wall creates a positive pressure potential (turgor pressure).
• Effects on animal cells (no cell wall):
  • Isotonic solution: Same water potential as the cell cytoplasm; no net water movement, cell remains normal.
  • Hypotonic solution: Higher water potential than the cell; water enters, causing it to swell and potentially burst (lysis).
  • Hypertonic solution: Lower water potential than the cell; water leaves, causing it to shrink and crenate.
• Effects on plant cells (with cell wall):
  • Isotonic solution: No net water movement; the cell is flaccid (lacking turgor).
  • Hypotonic solution: Water enters, the vacuole swells, pushing the cell membrane against the rigid cell wall. The cell becomes turgid. The cell wall prevents lysis and the resulting turgor pressure is vital for plant support.
  • Hypertonic solution: Water leaves, causing the protoplast (cell membrane and contents) to pull away from the cell wall. This is plasmolysis, and the cell becomes flaccid.

Active transport is the movement of substances against their concentration gradient (from a region of lower concentration to a region of higher concentration). This process requires metabolic energy in the form of ATP and is carried out by specific carrier proteins (often called 'pumps').

The process involves:

1. The target molecule or ion binds to a specific site on the carrier protein.
2. ATP is hydrolysed to ADP + Pi, releasing energy. The phosphate group often binds to the protein.
3. This energy causes the carrier protein to change its three-dimensional shape (conformational change).
4. The molecule/ion is carried across the membrane and released on the other side.
5. The phosphate group is released, and the protein returns to its original shape.

A key example is the sodium-potassium (Na⁺/K⁺) pump in animal cells, which actively pumps three Na⁺ ions out of the cell for every two K⁺ ions pumped in, maintaining electrochemical gradients crucial for nerve impulses and other processes.

For very large molecules (like proteins) or bulk quantities of substances, cells use processes that involve the formation of vesicles. These processes are active and require ATP for moving the cytoskeleton and changing the membrane shape.

1. Endocytosis

The process by which cells take in substances by engulfing them. A section of the cell surface membrane folds inwards to form a vesicle or vacuole that then pinches off into the cytoplasm.

• Phagocytosis: 'Cell eating'; the ingestion of large solid particles, such as bacteria by phagocytic white blood cells.
• Pinocytosis: 'Cell drinking'; the ingestion of external fluid, including dissolved substances.

1. Exocytosis

The process by which cells release substances. Vesicles, often formed by the Golgi apparatus, move to the cell surface membrane, fuse with it, and release their contents to the outside. This is how cells secrete substances like hormones (e.g., insulin from pancreatic cells), neurotransmitters, or digestive enzymes.