Diffusion and Facilitated Diffusion

Diffusion

Diffusion is a fundamental process in cellular transport, involving the movement of molecules from areas of high concentration to areas of low concentration.

Definition: Diffusion is the movement of molecules down a concentration gradient until there is a uniform distribution.

Factors affecting the rate of diffusion include:

• Thickness of the exchange surface's membrane
• Surface area of the membrane
• Size and nature of the diffusing molecule
• Temperature

Facilitated Diffusion

Facilitated diffusion is a specialized form of diffusion that allows molecules unable to pass through the membrane naturally to be transported across.

Highlight: Facilitated diffusion is a passive process that occurs along the concentration gradient, requiring no energy input.

Two types of proteins facilitate this process:

1. Channel proteins: Molecules with pores lined with polar groups, allowing small polar molecules to pass through. These channels can open and close based on cellular needs.

2. Carrier proteins: Enable diffusion of large polar molecules like sugars and amino acids. They work by binding to a molecule, changing shape, and releasing the molecule on the other side of the membrane.

Example: Glucose transport into cells often occurs through facilitated diffusion using specific carrier proteins.

Active Transport

Active transport is an energy-requiring process that moves ions and molecules across the membrane against a concentration gradient.

Key features of active transport include:

• Movement from lower to higher concentration
• Utilization of intrinsic carrier proteins
• Energy requirement (usually in the form of ATP)
• Rate limitation by the number and availability of carrier proteins

Highlight: The difference between active transport and diffusion is that active transport requires energy and moves substances against the concentration gradient, while diffusion is passive and follows the concentration gradient.

The process of active transport involves:

1. Molecule or ion binding to a specific carrier protein
2. ATP transferring a phosphate group to the carrier protein
3. Carrier protein changing shape and transporting the molecule across the membrane
4. Release of the molecule or ion into the cytoplasm
5. Regeneration of ATP and return of the carrier protein to its original state

Vocabulary: Co-transport is a type of facilitated diffusion that brings molecules and ions into cells together on the same transport protein.

Cell Membranes and Transport

The cell membrane is a complex structure composed primarily of phospholipids and proteins. Its main function is to regulate the passage of substances in and out of the cell.

Phospholipids and Membrane Structure

Phospholipids in cell membranes are crucial components that form the basis of membrane structure. They have unique properties that allow them to create a bilayer:

• Their hydrophilic heads are polar, attracted to water both inside and outside the cell.
• The hydrophobic tails are non-polar, repelled by water, and point towards each other inside the membrane.

Definition: A phospholipid bilayer is a double layer of phospholipid molecules that forms the cell membrane.

This structure allows lipid-soluble molecules to pass through but prevents water-soluble molecules from crossing freely.

Proteins in the Membrane

Proteins are scattered throughout the phospholipid bilayer in two main forms:

1. Extrinsic proteins: Located on either surface of the bilayer, providing structural support and forming recognition and receptor sites.
2. Intrinsic proteins: Extend across both layers, acting as carriers for water-soluble substances or forming channels for active transport of ions.

The Fluid Mosaic Model

Definition: The fluid mosaic model describes the structure of the cell membrane as a mosaic of components, including phospholipids, cholesterol, proteins, and carbohydrates, that move fluidly within the membrane.

Key features of this model include:

• Fluidity of phospholipid molecules within the layer
• Varied shapes, sizes, and patterns of embedded proteins
• Presence of cholesterol for membrane rigidity and stability
• Glycoproteins and glycolipids serving roles such as hormone receptors or cell-to-cell recognition

Membrane Permeability

The cell membrane is selectively permeable, allowing:

• Lipid-soluble substances to dissolve in the phospholipid and diffuse across
• Water-soluble substances to pass through intrinsic protein channels

Highlight: The selective permeability of the cell membrane is crucial for maintaining cellular homeostasis and controlling the movement of substances in and out of the cell.