Chapter 5

Biological Membranes and Transport

Cell Membranes: Surround cell organelles and compartments.
- Share common features and basic composition.
- Composed primarily of a phospholipid bilayer.

Characteristics of Bilayers:
- Fluid nature due to hydrogen bonding of water.
- Individual phospholipids and proteins can move within the membrane.
Types of Membrane Lipids:
- Glycerol phospholipids
- Sphingolipids
- Sterols

Composition by Weight:
- Varies across organisms (e.g., human myelin sheath, mouse liver, maize leaf).
- Includes proteins, phospholipids, sterols, and other lipids.

Phospholipid Structure:
- Composed of glycerol, fatty acids, and phosphate.
- Fatty acids range from 12 to 20 carbons.
Sphingolipids:
- Contain sphingosine and one fatty acid.
- Important for biological recognition (e.g., blood types).

Fluidity and Movement:
- Phospholipids held together by hydrophobic interactions.
- Membrane can enlarge or shrink by adding/removing phospholipids.
Components of Cellular Membranes:
1. Phospholipid bilayer (including sphingolipids).
2. Transmembrane proteins (integral proteins).
3. Interior protein network (peripheral proteins).
4. Cell-surface markers (glycoproteins and glycolipids).

Microscopy Techniques:
- Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM).
- Freeze-fracture microscopy visualizes membrane interiors.

Factors Influencing Fluidity:
- Saturated vs. unsaturated fatty acids.
- Temperature effects on membrane fluidity.
Distinct Lipid Composition:
- Varies between ER membrane, Golgi stack, and plasma membrane.

Roles of Membrane Proteins:
- Transporters, enzymes, receptors, identity markers, adhesion, and cytoskeleton attachment.
Transmembrane Domains:
- Hydrophobic amino acids in α helices; proteins can have multiple domains.

Types of Transport:
- Passive Transport: No energy required; molecules move down concentration gradient.
- Facilitated Transport: Accelerated by specific proteins (channels and carriers).
- Active Transport: Requires energy (ATP) to move substances against a gradient.

Ion Channels:
- Allow passage of ions; gated channels respond to stimuli.
Osmosis:
- Movement of water across a membrane toward higher solute concentration.
- Aquaporins facilitate rapid water transport.

Osmotic Concentration:
- Hypertonic, hypotonic, and isotonic solutions.
Maintaining Osmotic Balance:
- Cells use extrusion, isosmotic regulation, and turgor pressure.

Energy Requirements:
- Active transport moves substances from low to high concentration using ATP.
Types of Active Transport:
- Primary and secondary active transport (coupled transport).
  - Secondarry active transport
    - used ATP indirectly
    - symporter and antiporter can be used
    - the glucose-Na+ symporter captures the energy
  - intestinal tansport of glucose Na, NA/K, ATPase and a transporter

Endocytosis:
- Movement of substances into the cell (phagocytosis, pinocytosis, receptor-mediated).
  - Phagocytosis- cell taakes particulate matter
  - Pinocytosis- cell takes in only fluid
  - receptor mediated endocytosis- specific molecules are taken in after they bind to a receptor
Exocytosis:
- Discharge of materials out of the cell (used for hormone secretion, etc.).
Clinical Relevance:
- Example: Familial hypercholesterolemia, the LDL recptors lack tails in receptor mediated endocytosis