Membrane Structure, Synthesis, and Transport (Chapter 5) – Study Notes

Membrane Structure, Synthesis, and Transport

Introduction

The cell membrane is a critical structure that holds the contents of a cell together, enabling the chemistry of life to occur. The plasma membrane is a biomembrane that separates the internal contents of a cell from its external environment. It regulates the traffic of substances into and out of the cell and provides an interface for many vital cellular activities. (they are the same thing!)

Membrane Structure

Phospholipid Bilayer

• The fundamental framework of biological membranes (Khung cơ bản của màng sinh học) is the phospholipid bilayer, consisting of two layers of phospholipids.

• Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Membranes also contain:

  • Proteins

  • Carbohydrates attached to lipids (glycolipids) and proteins (glycoproteins)

  • Other lipids, such as cholesterol (in animal cells) or phytosterols (in plant cells)

Fluid-Mosaic Model

• The plasma membrane is described as a mosaic of lipid, protein, and carbohydrate.

• It is called a fluid-mosaic because lipids and proteins can move relative to each other within the membrane.

• A leaflet is half of a phospholipid bilayer; each faces a different region (cytoplasm or cell exterior).

• Phospholipid bilayer: The plasma membrane is made of two layers of phospholipids.

• Leaflet: Each leaflet is one of these layers. So, the bilayer has two leaflets:

  • Inner leaflet: Faces the cytoplasm (inside of the cell).

  • Outer leaflet: Faces the cell's exterior (outside environment).

• The two leaflets are asymmetrical (not identical); for example, glycolipids are found primarily in the extracellular leaflet. (Carbohydrates-lipids outside, Protein-lipids inside)

Membrane Proteins

Functions and Types

• Membrane proteins participate in transport, energy transduction, cell signaling, secretion, cell recognition, and cell-to-cell contact.

• Medically, about 70% of medications exert effects by binding to membrane proteins.

Types of Membrane Proteins

• Transmembrane proteins: Span from one side of the membrane to the other through the hydrophobic interior.

• Lipid-anchored proteins: Covalently attached to a lipid that anchors the protein into the bilayer.

• Peripheral membrane proteins: Non-covalently bound to other proteins or lipids on the membrane surface (do not interact with the membrane interior). Stick to the outside or inside surface of the membrane, but don’t go into the middle

Fluidity of Membranes

Semifluid Nature (not completely solid or completely liquid)

• Biomembranes are semifluid: lipid molecules move freely in two dimensions (rotationally and laterally within the leaflet), but not across leaflets.

• Fatty acyl tails remain within the hydrophobic interior, making these movements energetically favorable.

Bilayer Fluidity

• Optimal fluidity (just fluid enough) is essential for normal cell function, growth, and division.

• Membranes are less fluid at low temperatures and more fluid at high temperatures.

• Organisms can alter lipid composition to maintain optimal fluidity.

• Fluidity is influenced by:

  • Length of phospholipid tails (shorter tails = more fluid)

  • Double bonds in phospholipid tails (more double bonds = more fluid)

  • Presence of cholesterol: Cholesterol stabilizes fluidity, making membranes less fluid at high temps and more fluid at low temps

Length of Phospholipid Tails

• Lipid tails range from 14–24 carbon atoms; 16–18 carbons are most common.

• Shorter tails interact less, increasing membrane fluidity.

Double Bonds in Phospholipid Tails

• Double bonds create kinks in the lipid tails (unsaturated fatty acids), reducing interactions between adjacent tails and increasing fluidity.

Role of Cholesterol

• Cholesterol's polar head aligns with phospholipid heads; its nonpolar tail associates with phospholipid tails.

• The ring structure of cholesterol impairs dense packing of phospholipids, affecting membrane rigidity and fluidity.

• The ring structure of cholesterol prevents phospholipids from packing too closely together.

Cholesterol and Fluidity

• Cholesterol stabilizes membranes. (Cholesterol ổn định màng tế bào.)

• At high temperatures, cholesterol makes membranes less fluid; at low temperatures, it makes them more fluid.

Selective Permeability of Membranes

• Membranes composed only of phospholipids would have limited transport capabilities.

• Proteins embedded in the membrane create selective permeability.

• This ensures:

  • Essential molecules enter

  • Metabolic intermediates remain

  • Waste products exit

Transport Across Membranes

Passive Transport

• Does not require energy input; molecules move down their concentration gradient.

• Two types:

  • Simple diffusion: Direct movement through the phospholipid bilayer without a transport protein.

  • Facilitated diffusion: Movement with the aid of transport proteins.

Active Transport

• Moves molecules against their concentration gradient using energy from ATP.

Factors Affecting Simple Diffusion

• Hydrophobic interior is a barrier to ions and hydrophilic molecules.

• Permeability depends on:

  • Size

  • Polarity

  • Charge

  • Concentration

• Highest permeability: Gases and small, uncharged molecules.

Membrane Gradients

• Cells maintain a constant internal environment distinct from the external environment by establishing chemical and electrochemical gradients.

Osmosis

• Movement of water across membranes in response to solute concentration gradients.

• The bilayer is relatively impermeable to most solutes but somewhat permeable to water.

• If solute cannot cross, water moves to the environment with higher solute concentration.

Effects of Osmosis

• Hypertonic environment: Cells lose water and shrink.

  • In animal cells: Crenation

  • In plants/algae: Plasmolysis

• Hypotonic environment: Cells take up water.

  • Animal cells may swell and lyse.

  • Plant cell walls prevent major expansion, generating osmotic pressure that stops net water flow.

Transport Proteins

• Transmembrane proteins provide pathways for specific ions and hydrophilic molecules to cross membranes, bypassing the phospholipid bilayer.

• Allow selective permeability to small molecules and ions.

• Two main classes:

  • Channels

  • Transporters

Channels

• Form passageways for facilitated diffusion of ions or molecules.

• Most are gated (open/close in response to signals such as ligands).

• Allow rapid movement of solutes when open.

Transporters

• Bind a solute and undergo a conformational change to move it across the membrane.

• Slower than channels.

Categories of Transporters

Type	Description
Uniporters	Bind and transport a single molecule or ion across the membrane
Symporters (Co-transporters)	Bind two or more different types of ions/molecules and transport them in the same direction
Antiporters	Bind two or more different types of ions/molecules and transport them in opposite directions

Active Transport

• Moves solutes against their gradient (from low to high concentration).

• Primary active transport: Uses energy (usually ATP) directly to transport solutes (e.g., ATP-driven pumps).

• Secondary active transport: Uses a pre-existing gradient to drive the active transport of another solute (e.g., H+/sucrose symporter).

Na+/K+ Pump

• Exports sodium ions (Na+) and imports potassium ions (K+) against their gradients using ATP.

• Functions as an antiporter and an electrogenic pump (generates an electrical gradient by net export of one positive charge).

Mechanism of Pumping

1. Three Na+ ions bind from the cytosol; ATP is hydrolyzed, and phosphate (Pi) is covalently attached to the pump (E1 conformation).

2. Na+ ions are released outside the cell.

3. Two K+ ions bind from outside the cell.

4. Phosphate is released, and the pump switches to the E2 conformation, releasing K+ ions into the cytosol.

Example: The Na+/K+ pump is essential for nerve impulse transmission and maintaining cell volume.

Additional info: The selective permeability and active transport mechanisms of membranes are fundamental for maintaining homeostasis, cell signaling, and energy conversion in all living cells.