CIE AS/A Level Biology -4.1 Fluid mosaic membranes- Study Notes- New Syllabus
CIE AS/A Level Biology -4.1 Fluid mosaic membranes- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -4.1 Fluid mosaic membranes- Study Notes- New Syllabus
Key Concepts:
- describe the fluid mosaic model of membrane structure with reference to the hydrophobic and hydrophilic interactions that account for the formation of the phospholipid bilayer and the arrangement of proteins
- describe the arrangement of cholesterol, glycolipids and glycoproteins in cell surface membranes
- describe the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in cell surface membranes, with reference to stability, fluidity, permeability, transport (carrier proteins and channel proteins), cell signalling (cell surface receptors) and cell recognition (cell surface antigens – see 11.1.2)
- outline the main stages in the process of cell signalling leading to specific responses:
• secretion of specific chemicals (ligands) from cells
• transport of ligands to target cells
• binding of ligands to cell surface receptors on target cells
Fluid Mosaic Model of Membrane Structure
🌱 Overview
- Proposed by Singer and Nicolson (1972).
- Describes the plasma membrane as a flexible, dynamic structure made up of a phospholipid bilayer with proteins embedded in it.
🧩 Phospholipid Bilayer
- Phospholipids have:
- Hydrophilic (polar) phosphate heads → attract water.
- Hydrophobic (non-polar) fatty acid tails → repel water.
- In water, phospholipids arrange themselves into a bilayer:
- Heads face outward towards aqueous environments (inside cytoplasm & outside cell).
- Tails face inward, shielded from water.
🌊 Hydrophobic & Hydrophilic Interactions
- Hydrophobic tails avoid contact with water → cause bilayer formation.
- Hydrophilic heads interact with water on both membrane surfaces.
- This arrangement creates a stable barrier that is selectively permeable.
🧬 Proteins in the Membrane
- Integral (intrinsic) proteins:
- Span across the bilayer or are embedded within it.
- Often function as transport channels, carriers, or receptors.
- Peripheral (extrinsic) proteins:
- Loosely attached to surface (inside or outside).
- Often involved in cell signalling or structural support.
🎯 “Fluid” Aspect
- Phospholipids and proteins can move laterally within the layer.
- This fluidity allows:
- Membrane repair
- Flexibility
- Dynamic changes in protein arrangement
🧩 “Mosaic” Aspect
- The membrane is a patchwork of:
- Proteins
- Phospholipids
- Cholesterol
- Glycolipids & Glycoproteins
- Cholesterol helps regulate fluidity and stability.
📌 Key Functions from the Structure
- Selective permeability
- Cell signalling
- Transport of molecules
- Cell recognition
✅ Summary:
The fluid mosaic model explains how hydrophobic and hydrophilic interactions drive phospholipids into a bilayer, with proteins embedded in a dynamic, flexible structure. This arrangement supports selective transport, cell communication, and structural integrity.
The fluid mosaic model explains how hydrophobic and hydrophilic interactions drive phospholipids into a bilayer, with proteins embedded in a dynamic, flexible structure. This arrangement supports selective transport, cell communication, and structural integrity.
Arrangement of Cholesterol, Glycolipids & Glycoproteins in Cell Surface Membranes
🌱 Cholesterol
- Location: Found between phospholipid molecules in both layers of the bilayer.
- Structure: Small, rigid, hydrophobic molecule with a small polar hydroxyl group.
- Role in arrangement:
- Hydroxyl group aligns near phosphate heads (hydrophilic region).
- Rigid steroid ring structure inserts into hydrophobic tails.
- Function:
- Reduces phospholipid movement → membrane stability.
- Prevents packing of tails at low temperatures → maintains fluidity.
🌿 Glycolipids
- Location: Lipids with a carbohydrate chain attached; always in outer layer of the bilayer.
- Arrangement:
- Lipid portion embedded in the hydrophobic region of bilayer.
- Carbohydrate chain extends into the extracellular fluid.
- Function:
- Cell recognition & signalling.
- Acts as a receptor for certain molecules.
- Contributes to glycocalyx (carbohydrate-rich cell surface).
🧩 Glycoproteins
- Location: Proteins with carbohydrate chains attached; carbohydrate part always projects into extracellular space.
- Arrangement:
- Protein portion can be integral (spanning membrane) or peripheral.
- Carbohydrate chain faces outward, forming part of glycocalyx.
- Function:
- Cell-cell recognition (e.g., immune responses).
- Acts as receptors for hormones & neurotransmitters.
- Helps with cell adhesion.
📌 Integrated Arrangement in the Fluid Mosaic Model
| Component | Location in Membrane | Orientation | Main Role |
|---|---|---|---|
| Cholesterol | Between phospholipid tails in both layers | Hydroxyl group near heads, rings in tails | Stability & fluidity |
| Glycolipids | Outer layer only | Carbohydrate chain outward | Cell recognition, signalling |
| Glycoproteins | Outer layer (protein in bilayer) | Carbohydrate chain outward | Receptors, recognition, adhesion |
✅ Summary:
In the cell surface membrane, cholesterol sits between phospholipids to regulate fluidity, glycolipids anchor into the outer layer with carbohydrates extending outward, and glycoproteins project carbohydrate chains from membrane proteins into the extracellular space for recognition, signalling, and adhesion.
In the cell surface membrane, cholesterol sits between phospholipids to regulate fluidity, glycolipids anchor into the outer layer with carbohydrates extending outward, and glycoproteins project carbohydrate chains from membrane proteins into the extracellular space for recognition, signalling, and adhesion.
Roles of Phospholipids, Cholesterol, Glycolipids, Proteins & Glycoproteins in Cell Surface Membranes
🧱 1. Phospholipids
- Structure: Amphipathic – hydrophilic phosphate heads + hydrophobic fatty acid tails.
- Roles:
- Form phospholipid bilayer → basic membrane structure.
- Hydrophobic core prevents passage of water-soluble/polar molecules, allowing selective permeability.
- Provides fluidity – phospholipids move laterally within the bilayer.
- Creates a barrier between internal and external environments.
🪨 2. Cholesterol
- Structure: Small, rigid lipid with a polar hydroxyl group and non-polar rings/tail.
- Roles:
- Stability – fits between phospholipids, reducing movement of fatty acid tails.
- Fluidity regulation:
- Prevents membrane becoming too rigid at low temps.
- Prevents membrane becoming too fluid at high temps.
- Permeability control – reduces permeability to small water-soluble molecules.
🌿 3. Glycolipids
- Structure: Lipid + carbohydrate chain; found in outer layer of bilayer.
- Roles:
- Cell recognition (act as cell surface antigens).
- Cell signalling – carbohydrate chains act as receptors for specific chemicals.
- Help form glycocalyx for protection.
🧩 4. Proteins
- Two main types:
- Intrinsic (integral) proteins – span the bilayer.
- Extrinsic (peripheral) proteins – on inner or outer surface.
- Roles:
- Transport:
- Carrier proteins – change shape to move substances across membrane (active transport or facilitated diffusion).
- Channel proteins – form hydrophilic pores for passive movement of ions/water-soluble molecules.
- Enzymes – catalyse reactions at membrane surfaces.
- Cell signalling – some act as receptors for hormones/neurotransmitters.
- Transport:
🧬 5. Glycoproteins
- Structure: Protein + carbohydrate chain; carbohydrate projects into extracellular fluid.
- Roles:
- Cell signalling – receptors for hormones, neurotransmitters, and other messenger molecules.
- Cell recognition – act as antigens.
- Cell adhesion – help cells stick together in tissues.
- Form part of the glycocalyx.
📊 Summary Table
| Component | Stability | Fluidity | Permeability | Transport | Cell Signalling | Cell Recognition |
|---|---|---|---|---|---|---|
| Phospholipids | ✔ (bilayer integrity) | ✔ (lateral movement) | Selective barrier | ✘ | ✘ | ✘ |
| Cholesterol | ✔ | ✔ (temp regulation) | ✔ (reduces) | ✘ | ✘ | ✘ |
| Glycolipids | ✘ | ✘ | ✘ | ✘ | ✔ (receptors) | ✔ (antigens) |
| Proteins | ✔ (structure support) | ✘ | ✔ (channels) | ✔ (carrier/channel) | ✔ (receptors) | Sometimes |
| Glycoproteins | ✘ | ✘ | ✘ | ✘ | ✔ (receptors) | ✔ (antigens) |
✅ Summary Box
Phospholipids form the barrier and control basic permeability.
Cholesterol stabilises and regulates fluidity.
Glycolipids and glycoproteins are key in recognition and signalling.
Proteins are essential for transport, signalling, and structural support.
Phospholipids form the barrier and control basic permeability.
Cholesterol stabilises and regulates fluidity.
Glycolipids and glycoproteins are key in recognition and signalling.
Proteins are essential for transport, signalling, and structural support.
Stages in the Process of Cell Signalling
🧪 1. Secretion of Ligands
- Ligand = a chemical messenger (e.g., hormone, neurotransmitter).
- Produced and secreted by specific cells in response to a stimulus.
- Examples: Hormones (insulin, adrenaline), Neurotransmitters (acetylcholine).
🚚 2. Transport of Ligands to Target Cells
- Ligands travel from secreting cell to target cell via:
- Bloodstream (endocrine signalling – hormones).
- Diffusion across synaptic cleft (paracrine signalling – neurotransmitters).
- Ligand remains intact and active until reaching its target.
🎯 3. Binding to Cell Surface Receptors
- Target cells have specific receptor proteins on their surface membranes.
- Receptors have complementary shapes to their ligand → ensures specificity.
- Binding causes a conformational change in the receptor, triggering a response inside the cell.
🌀 4. Triggering the Response
- Receptor activation leads to:
- Activation of enzymes inside cell.
- Opening/closing ion channels.
- Activation of second messengers (e.g., cAMP) → amplifies signal.
- Changes in gene expression.
📊 Overview Table
| Stage | Key Event | Example |
|---|---|---|
| 1. Secretion | Ligand released from signalling cell | Insulin secreted by pancreas |
| 2. Transport | Ligand moves to target | Blood carries insulin to muscles |
| 3. Binding | Ligand binds receptor | Insulin binds receptor on muscle cell membrane |
| 4. Response | Cellular change occurs | Glucose uptake increases |
✅ Summary Box
Cell signalling is a specific communication system where ligands secreted by one cell travel to target cells, bind to complementary receptors, and trigger specific cellular responses. This ensures coordination between cells in multicellular organisms.
Cell signalling is a specific communication system where ligands secreted by one cell travel to target cells, bind to complementary receptors, and trigger specific cellular responses. This ensures coordination between cells in multicellular organisms.