Plasma Membranes

Phospholipid Bilayer

• Hydrophilic phosphate heads form outer + inner surfaces.

• Hydrophobic fatty acid tails form the core.

• Amphipathic nature → forms a selective barrier.

• Cell membranes exist in aqueous environments (cytosol and extracellular fluid).

Model for Membranes

• Fluid mosaic model:

  • Phospholipids free to move → membrane flexibility.

  • Proteins embedded vary in shape, size, position → like a “mosaic”.

Phospholipid

• Core structure of bilayer.

• Movement of phospholipids → drives movement of other components.

• Hydrophobic barrier prevents passage of water-soluble substances.

Intrinsic Protein

Two main types:

• Intrinsic proteins (integral):

  • Transmembrane, embedded through bilayer.

  • Contain hydrophobic R groups that interact with core.

Channel Protein

• → provide hydrophilic channel for passive diffusion of polar molecules/ions (down conc. gradient).

  • Held together by the interactions of the hydrophobic core & R groups.

Carrier Protein

• → used for passive AND active transport (against gradient, shape change).

  • Also helps with facilitated diffusion

Extrinsic Protein

• Present on one side of bilayer. (peripheral)

• Hydrophilic R groups interact with phospholipid heads or intrinsic proteins.

• Can be present in either layer or move between.

• Usually involved in reactions and have enzymes attached to them.

• Used in cell signalling or recognition.

Glycoproteins

• Intrinsic proteins with carbohydrate chains attached.

• Roles:

  • Cell adhesion.

  • Receptors for chemical signals (hormones, neurotransmitters, antibodies).

  • Example responses:

    • Neurotransmitter binding → triggers impulse.

    • Insulin/glucagon binding → regulates blood glucose.

    • Drugs (β-blockers) can bind to receptors.

• Function: cell signalling/ communication/ recognition.

Glycolipids

• Lipids with carbohydrate chains attached.

• Roles:

  • Cell markers / antigens.

  • Identified by immune system as self / non-self.

Cholesterol

• Lipid with hydrophilic end + hydrophobic end.

• Sits between phospholipids:

  • Hydrophilic end interacts with heads.

  • Hydrophobic end interacts with tails.

• Roles:

  • Adds stability, prevents membranes becoming too rigid or bursting.

  • Reduces fluidity at high temperatures.

  • Prevent leakage of water & dissolved ions from cell.

  • Prevents phospholipids crystallising → maintains fluidity.

Functions of Plasma Membranes

1. At cell surface:

   • Separate cell contents from environment.

   • Selectively permeable barrier.

   • Allow cell communication/signalling.

   • Allow cell recognition.

   • Site of some chemical reactions.

2. Within cells:

   • Compartmentalisation of organelles (separate different areas within the cells from each other)

   • Form vesicles.

   • Control entry/exit of substances into organelles.

   • Provide reaction surfaces (e.g. inner mitochondrial membrane, thylakoid membranes).

Sites of Chemical Reactions

• Many enzymes are bound to membranes.

• Correct positioning is vital for reactions:

  • Mitochondria → cristae contain electron carriers + ATP synthase (respiration).

  • Chloroplasts → thylakoid membranes contain photosynthetic enzymes.

Factors affecting structure:

• Temperature

• Solvents

How does temperature affect?

• Phospholipids: constantly moving.

• Increased temperature → more kinetic energy → phospholipids move more → membrane becomes:

  • More fluid.

  • More permeable.

• If temperature continues rising → phospholipid bilayer structure breaks down completely.

• Result:

  • Easier for particles to cross membrane.

  • Carrier & channel proteins denature at high temps → membrane transport disrupted → increased permeability.

⚠ Study tip: Proteins are denatured by heat, but phospholipid membranes are described as disrupted or destroyed, not denatured.

How does solvent affect?

• Water (polar solvent): essential for phospholipid bilayer formation.

  • Hydrophilic phosphate heads interact with water.

  • Hydrophobic tails cluster away from water → bilayer integrity maintained.

• Organic solvents (less polar or non-polar, e.g. alcohol, benzene):

  • Dissolve membranes → disrupt cell membranes.

  • Alcohols used in antiseptic wipes: dissolve membranes of bacteria → kill them → reduced infection risk.

• Low conc. alcohols (e.g. alcoholic drinks):

  • Don’t fully dissolve membranes but still disrupt.

  • Non-polar molecules enter bilayer, creating gaps → membrane becomes more permeable.

• Physiological effect: Neurone membranes disrupted → nerve impulse transmission affected → altered behaviour after alcohol.

Practical investigation – beetroot membrane permeability

• Why beetroot? Contains betalain pigment (red). Released if cell membranes disrupted → solution colour changes.

• Method:

  • Equal beetroot discs prepared, washed in running water.

  • Placed in 100 ml distilled water at different temps (water bath, 10°C intervals).

  • At each temp, 5 samples of surrounding solution taken.

  • Absorbance of pigment measured by colorimeter with blue filter.

  • Experiment repeated ×3, mean absorbance calculated.

• Results:

  • Graph shows increasing absorbance with rising temperature.

  • Point where curve rises steeply = membrane disruption threshold.