Biological membranes

Biological membranes

• Partially permeable barriers:

  • Control movement of substances in and out of cells and organelles

  • Separate cell from environment and organelles from cytoplasm

  • Help maintain internal conditions

• Sites of chemical reactions:

  • Provide surfaces for enzymes and metabolic reactions

  • Increase efficiency by organising reactions (e.g. in mitochondria, chloroplasts)

• Cell communication:

  • Contain receptor proteins for signalling molecules (e.g. hormones)

  • Allow cells to detect and respond to sign

Components of fluid mosaic model

• Phospholipids:

  • Form a bilayer with hydrophilic heads and hydrophobic tails

  • Create a partially permeable barrier controlling movement of substances

• Cholesterol:

  • Fits between phospholipids

  • Regulates fluidity and stability of the membrane

  • Prevents membrane from becoming too rigid or too fluid

• Proteins:

  • Include channels, carriers, enzymes, and receptors

  • Control transport and enable chemical reactions and signalling

• Glycoproteins:

  • Proteins with carbohydrate chains

  • Act as cell recognition markers and receptors for signalling

• Glycolipids:

  • Lipids with carbohydrate chains

  • Also involved in cell recognition and membrane stability

Fluid mosaic model

Temperature on membrane structure and permeabilty

• Increasing temperature increases kinetic energy of phospholipids and proteins

• Membrane becomes more fluid and more permeable

• At high temperatures, proteins denature and the bilayer is disrupted, causing leakage of cell contents

• Low temperatures make the membrane more rigid and less permeable

Solvents on membrane structure and permeabilty

• Dissolve membrane lipids (phospholipids)

• Disrupt the bilayer structure

• Increase membrane permeability or completely destroy membrane integrity

• Can also denature membrane proteins

Permeabiltiy:
Membrane permeability can be investigated using beetroot, where pigment leaks out if the cell membrane is damaged and is measured by colour intensity in solution. Increasing temperature or solvent concentration increases permeability by disrupting the phospholipid bilayer and denaturing membrane proteins.

Molecules movement:
A common practical is using agar blocks with indicator dye (e.g. phenolphthalein or universal indicator). The blocks are placed in acid or alkali, and the time taken for the colour change to spread is measured to show diffusion rate under different conditions (e.g. different sizes of blocks for surface area or different temperatures).

Passive movements across plasma membrane

• Diffusion: movement of molecules from high to low concentration (no ATP required)

• Facilitated diffusion: diffusion through channel/carrier proteins (no ATP required)

• Osmosis: movement of water across a partially permeable membrane from a region of higher water potential to lower water potential (no ATP required)

Active movements to move across membranes

• Active transport:

  • Moves substances against a concentration gradient (low → high)

  • Uses ATP and carrier proteins

• Endocytosis:

  • Cell membrane engulfs substances to form vesicles

  • Requires ATP

• Exocytosis:

  • Vesicles fuse with membrane to release substances outside the cell

  • Requires ATP

Osmosis on cells

• Animal cells:

  • In hypotonic solution (higher water potential outside): water enters → cell may burst (lysis)

  • In hypertonic solution (lower water potential outside): water leaves → cell shrinks (crenation)

• Plant cells:

  • In hypotonic solution: water enters → cell becomes turgid (healthy, firm)

  • In hypertonic solution: water leaves → cell becomes flaccid, and may undergo plasmolysis

Practical investigrations plant and animal cells

• Plant cells (e.g. potato cylinders):

  • Place in solutions with different concentrations

  • Measure change in mass or length to show water gain/loss

• Animal cells (e.g. red blood cells):

  • Observe under microscope in different solutions

  • Look for swelling, shrinking, or bursting