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DP IB Biology: SL

Revision Notes

Home / IB / Biology: SL / DP / Revision Notes / 1. Cell Biology / 1.3 Cells: Membrane Structure & Transport / 1.3.1 Phospholipid Bilayer Properties


1.3.1 Phospholipid Bilayer Properties


Amphipathic Properties

Phospholipids

  • Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
  • They are formed by a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails
  • As phospholipids have a hydrophobic and hydrophilic part they are known as amphipathic
  • The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water
  • The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water

The generalised molecular structure of a phospholipid, downloadable AS & A Level Biology revision notes

The generalised molecular structure of a phospholipid

  • Due to their amphipathic properties, phospholipids display an emergent property when placed into water
  • The hydrophilic phosphate heads orientate towards the water and the hydrophobic hydrocarbon tails orientate inwards (away from the water)
    • They form a phospholipid monolayer

_Phospholipid monolayer, downloadable AS & A Level Biology revision notes

A phospholipid monolayer

  • If phospholipids are mixed/shaken with water they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing inwards
    • This is called a micelle

Micelle, downloadable AS & A Level Biology revision notes

A micelle

  • Alternatively, when there is a sufficient concentration of phospholipids present then two-layered structures may form
  • These sheets are called phospholipid bilayers – this is the basic structure of the cell membrane

Phospholipid bilayer, downloadable AS & A Level Biology revision notes

A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads outwards

  • The two layers of phospholipids are loosely held together by weak hydrophobic interactions between the hydrocarbon tails allowing some membrane fluidity
  • The amphipathic properties result in the phospholipid bilayer acting as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)
  • This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in

Animal Cell Membranes: Cholesterol

  • Phospholipids and cholesterol are the two main components of animal cell plasma membranes (cholesterol is absent in plant membranes)

Cholesterol

  • Cholesterol is a lipid that belongs to the steroid group
  • It is amphipathic, with the majority of the cholesterol molecule being hydrophobic and therefore attracted to the hydrophobic hydrocarbon tails of the phospholipid
  • The hydroxyl group of the cholesterol molecule is hydrophilic. It is attracted to the phosphate heads of the phospholipid
  • Therefore in the plasma membrane cholesterol is positioned between phospholipids

Cholesterol structure_1, downloadable AS & A Level Biology revision notes

The molecular structure of cholesterol

Mammalian Membranes: Role of Cholesterol

  • The plasma membrane is fluid, meaning the components are free to move
  • The fluidity of the membrane needs to be controlled:
    • If it was too fluid the cell could not regulate what moved in and out
    • If it was not fluid enough then the cell would not be able to move and substances would be unable to move into or out of the cell

  • Cholesterol helps with the regulation of the membrane fluidity and permeability
    • Interaction between cholesterol and phospholipid tails stabilises the plasma membrane at higher temperatures by stopping the membrane from becoming too fluid
      • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
  • At colder temperatures cholesterol increases the fluidity of the membrane, stopping it crystallizing and becoming too rigid
    • This occurs because cholesterol stops the phospholipid tails packing too closely together

  • The impermeability of the membrane to hydrophilic ions (e.g. sodium and hydrogen) is also reduced by cholesterol
  • Cholesterol increases the mechanical strength and stability of membranes (without it membranes would break down causing cells to burst)
  • Exam Tip

    It is important to remember that cholesterol affects membrane fluidity and the permeability of hydrophilic ions (e.g. sodium and hydrogen) in mammal membranes.



    • 1. Cell Biology
      • 1.1 Cells: Theory
        • 1.1.1 Cell Theory
          • 1.1.2 Functions of Life
            • 1.1.3 Surface Area to Volume Ratio
              • 1.1.4 Cell Specialisation
                • 1.1.5 Stem Cells
                  • 1.1.6 Skills: Cell Theory
                  • 1.2 Cells: Origin & Ultrastructure
                    • 1.2.1 Origin of Cells
                      • 1.2.2 Endosymbiotic Theory
                        • 1.2.3 Prokaryotic Cell Structure
                          • 1.2.4 Eukaryotic Cell Structure
                            • 1.2.5 Exocrine Pancreatic & Palisade Mesophyll Cells
                              • 1.2.6 Comparison of Prokaryotic & Eukaryotic Cells
                                • 1.2.7 Microscopes
                                  • 1.2.8 Skills: Drawing Cells
                                    • 1.2.9 Skills: Cell Origin & Ultrastructure
                                    • 1.3 Cells: Membrane Structure & Transport
                                      • 1.3.1 Phospholipid Bilayer Properties
                                        • 1.3.2 Membrane Proteins
                                          • 1.3.3 History of Fluid Mosaic Model
                                            • 1.3.4 Membrane Transport
                                              • 1.3.5 Active Transport & Bulk Transport
                                                • 1.3.6 Skills: Membrane Structure & Transport
                                                  • 1.3.7 Skills: Estimation of Osmolarity
                                                  • 1.4 Cells: Division
                                                    • 1.4.1 Cell Cycle
                                                      • 1.4.2 Phases of Mitosis
                                                        • 1.4.3 Cancer
                                                          • 1.4.4 Skills: Cell Division
                                                        • 2. Molecular Biology
                                                          • 2.1 Metabolism & Water
                                                            • 2.1.1 Molecules
                                                              • 2.1.2 Metabolism
                                                                • 2.1.3 Hydrogen Bonds
                                                                  • 2.1.4 Water
                                                                  • 2.2 Carbohydrates & Lipids
                                                                    • 2.2.1 Carbohydrates
                                                                      • 2.2.2 Fatty Acids
                                                                        • 2.2.3 Lipids
                                                                          • 2.2.4 Skills: Visualising Carbohydrates
                                                                            • 2.2.5 Skills: Calculating BMI
                                                                            • 2.3 Proteins
                                                                              • 2.3.1 Proteins
                                                                                • 2.3.2 Protein Structure & Function
                                                                                  • 2.3.3 The Variety of Proteins
                                                                                    • 2.3.4 Skills: Molecules
                                                                                    • 2.4 Enzymes
                                                                                      • 2.4.1 Enzymes
                                                                                        • 2.4.2 Immobilised Enzymes
                                                                                          • 2.4.3 Skills: Enzyme Experiments
                                                                                          • 2.5 Nucleic Acids: Structure & DNA Replication
                                                                                            • 2.5.1 DNA & RNA Structure
                                                                                              • 2.5.2 DNA Replication
                                                                                                • 2.5.3 Skills: DNA Replication
                                                                                                • 2.6 Transcription & Translation
                                                                                                  • 2.6.1 Transcription
                                                                                                    • 2.6.2 Translation
                                                                                                      • 2.6.3 Biotechnology
                                                                                                        • 2.6.4 Skills: DNA, RNA & Protein Synthesis
                                                                                                          • 2.6.5 Skills: Interpreting Sequences
                                                                                                          • 2.7 Cellular Respiration
                                                                                                            • 2.7.1 Cellular Respiration
                                                                                                              • 2.7.2 Anaerobic Respiration
                                                                                                                • 2.7.3 Aerobic Respiration
                                                                                                                  • 2.7.4 Skills: Respiration
                                                                                                                  • 2.8 Photosynthesis
                                                                                                                    • 2.8.1 Photosynthesis
                                                                                                                      • 2.8.2 Photosynthesis Continued
                                                                                                                        • 2.8.3 Skills: Photosynthesis
                                                                                                                          • 2.8.4 Skills: Separating Photosynthetic Pigments
                                                                                                                        • 3. Genetics
                                                                                                                          • 3.1 Genes & Chromosomes
                                                                                                                            • 3.1.1 Genes
                                                                                                                              • 3.1.2 Alleles
                                                                                                                                • 3.1.3 Mutation
                                                                                                                                  • 3.1.4 Genome
                                                                                                                                    • 3.1.5 Prokaryotic Chromosomes
                                                                                                                                      • 3.1.6 Eukaryotic Chromosomes
                                                                                                                                        • 3.1.7 Chromosome Number
                                                                                                                                          • 3.1.8 Sex Determination
                                                                                                                                            • 3.1.9 Karyograms
                                                                                                                                              • 3.1.10 Skills: Using Databases
                                                                                                                                              • 3.2 Meiosis
                                                                                                                                                • 3.2.1 Meiosis
                                                                                                                                                  • 3.2.2 Stages of Meiosis
                                                                                                                                                    • 3.2.3 Genetic Variation
                                                                                                                                                      • 3.2.4 Non-disjunction
                                                                                                                                                        • 3.2.5 Skills: Meiosis
                                                                                                                                                        • 3.3 Inheritance
                                                                                                                                                          • 3.3.1 Inheritance
                                                                                                                                                            • 3.3.2 Inheriting Alleles
                                                                                                                                                              • 3.3.3 Skills: Inheritance
                                                                                                                                                                • 3.3.4 Inheritance of Genetic Diseases
                                                                                                                                                                  • 3.3.5 Mutations & Disease
                                                                                                                                                                  • 3.4 Genetic Modification & Biotechnology
                                                                                                                                                                    • 3.4.1 Electrophoresis & PCR
                                                                                                                                                                      • 3.4.2 DNA Profiling
                                                                                                                                                                        • 3.4.3 Genetic Modification
                                                                                                                                                                          • 3.4.4 Cloning
                                                                                                                                                                            • 3.4.5 Skills: Genetic Modification & Biotechnology
                                                                                                                                                                          • 4. Ecology
                                                                                                                                                                            • 4.1 Species, Communities, Ecosystems & Energy Flow
                                                                                                                                                                              • 4.1.1 Species & Population
                                                                                                                                                                                • 4.1.2 Methods of Nutrition
                                                                                                                                                                                  • 4.1.3 Community
                                                                                                                                                                                    • 4.1.4 Nutrient Cycling
                                                                                                                                                                                      • 4.1.5 Energy Flow
                                                                                                                                                                                        • 4.1.6 Energy Losses
                                                                                                                                                                                          • 4.1.7 Skills: Species, Communities, Ecosystems & Energy Flow
                                                                                                                                                                                            • 4.1.8 Skills: Chi-squared test & Statistical Significance
                                                                                                                                                                                              • 4.1.9 Skills: Pyramids of Energy
                                                                                                                                                                                              • 4.2 Carbon Cycling & Climate Change
                                                                                                                                                                                                • 4.2.1 Carbon Cycle: Carbon Dioxide
                                                                                                                                                                                                  • 4.2.2 Carbon Cycle: Carbon Dioxide in the Atmosphere
                                                                                                                                                                                                    • 4.2.3 Carbon Cycle: Methane
                                                                                                                                                                                                      • 4.2.4 Carbon Cycle: Organic Matter
                                                                                                                                                                                                        • 4.2.5 Climate Change: Greenhouse Effect
                                                                                                                                                                                                          • 4.2.6 Climate Change: Impact
                                                                                                                                                                                                            • 4.2.7 Climate Change: Causes
                                                                                                                                                                                                              • 4.2.8 Skills: Carbon Cycling & Climate Change
                                                                                                                                                                                                            • 5. Evolution and biodiversity
                                                                                                                                                                                                              • 5.1 Evolution & Natural Selection
                                                                                                                                                                                                                • 5.1.1 Evolution
                                                                                                                                                                                                                  • 5.1.2 Evolution: Evidence
                                                                                                                                                                                                                    • 5.1.3 Variation
                                                                                                                                                                                                                      • 5.1.4 Natural Selection
                                                                                                                                                                                                                        • 5.1.5 Natural Selection Examples
                                                                                                                                                                                                                        • 5.2 Classification & Cladistics
                                                                                                                                                                                                                          • 5.2.1 Classification System
                                                                                                                                                                                                                            • 5.2.2 Classifying Organisms
                                                                                                                                                                                                                              • 5.2.3 Plant Phyla Features
                                                                                                                                                                                                                                • 5.2.4 Animal Phyla Features
                                                                                                                                                                                                                                  • 5.2.5 Cladistics
                                                                                                                                                                                                                                    • 5.2.6 Reclassification
                                                                                                                                                                                                                                      • 5.2.7 Skills: Classification & Cladistics
                                                                                                                                                                                                                                    • 6. Human Physiology
                                                                                                                                                                                                                                      • 6.1 Digestion & Absorption
                                                                                                                                                                                                                                        • 6.1.1 Digestion
                                                                                                                                                                                                                                          • 6.1.2 Villi & Absorption
                                                                                                                                                                                                                                            • 6.1.3 Absorption
                                                                                                                                                                                                                                              • 6.1.4 Skills: Digestion & Absorption
                                                                                                                                                                                                                                              • 6.2 The Blood System
                                                                                                                                                                                                                                                • 6.2.1 The Blood System: History
                                                                                                                                                                                                                                                  • 6.2.2 The Blood System: Vessels
                                                                                                                                                                                                                                                    • 6.2.3 The Blood System: Double Circulation
                                                                                                                                                                                                                                                      • 6.2.4 The Blood System: Cardiac Cycle
                                                                                                                                                                                                                                                        • 6.2.5 The Heart Rate
                                                                                                                                                                                                                                                          • 6.2.6 Skills: The Blood System
                                                                                                                                                                                                                                                          • 6.3 Defence Against Infectious Disease
                                                                                                                                                                                                                                                            • 6.3.1 Skin
                                                                                                                                                                                                                                                              • 6.3.2 Blood Clotting
                                                                                                                                                                                                                                                                • 6.3.3 White Blood Cells
                                                                                                                                                                                                                                                                  • 6.3.4 Antibiotics
                                                                                                                                                                                                                                                                    • 6.3.5 Antibiotic Resistance
                                                                                                                                                                                                                                                                    • 6.4 Gas Exchange
                                                                                                                                                                                                                                                                      • 6.4.1 Ventilation: Function & Structures
                                                                                                                                                                                                                                                                        • 6.4.2 Ventilation: Mechanism
                                                                                                                                                                                                                                                                          • 6.4.3 Lung Diseases
                                                                                                                                                                                                                                                                            • 6.4.4 Skills: Monitoring Ventilation
                                                                                                                                                                                                                                                                            • 6.5 Neurones & Synapses
                                                                                                                                                                                                                                                                              • 6.5.1 Neurones: Function & Structure
                                                                                                                                                                                                                                                                                • 6.5.2 Nerve Impulses
                                                                                                                                                                                                                                                                                  • 6.5.3 Synapses
                                                                                                                                                                                                                                                                                    • 6.5.4 Skill: Neurones & Synapses
                                                                                                                                                                                                                                                                                    • 6.6 Hormones, Homeostasis & Reproduction
                                                                                                                                                                                                                                                                                      • 6.6.1 Hormones
                                                                                                                                                                                                                                                                                        • 6.6.2 Hormones Continued
                                                                                                                                                                                                                                                                                          • 6.6.3 Reproduction: Background
                                                                                                                                                                                                                                                                                            • 6.6.4 Reproduction: Sex Determination in Males
                                                                                                                                                                                                                                                                                              • 6.6.5 Reproduction: Sex Determination in Females
                                                                                                                                                                                                                                                                                                • 6.6.6 Skills: Reproduction Diagrams

                                                                                                                                                                                                                                                                                              Author: Catherine

                                                                                                                                                                                                                                                                                              Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.


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