Fluid Mosaic Model, Helped Diffusion, Co-Transport of Glucose & Osmosis Water Potential
The fluid mosaic model (Singer and Nicolson, 1972) describes the cell surface membrane as a dynamic phospholipid bilayer approximately 7-8 nm thick, interspersed with intrinsic and extrinsic proteins, cholesterol, glycolipids, and glycoproteins. Because the hydrophobic fatty acid core acts as a barrier to polar molecules and charged ions, transport across the membrane relies on specific channel and carrier proteins alongside ATP-driven pumps. Understanding how the sodium-potassium pump establishes an electrochemical gradient to drive secondary active co-transport of glucose in the ileum is a classic, high-tariff exam topic. You must also be fluent in water potential (ψ = ψs + ψp) terminology and beetroot membrane permeability experiments.
- Fluid Mosaic Model & Membrane Components
- 'Fluid' because phospholipids move laterally in the plane of the bilayer; 'mosaic' because intrinsic and extrinsic proteins are embedded in a scattered pattern. Cholesterol binds to hydrophobic fatty acid tails, restricting phospholipid movement at physiological temperatures to reduce fluidity and ion permeability while preventing solidification at low temperatures. Glycolipids and glycoproteins on the outer surface form the glycocalyx for cell-cell recognition, receptors, and tissue stability.
- Simple vs Helped Diffusion & Fick's Law
- Simple diffusion is the net passive movement of small, non-polar molecules (O₂, CO₂, steroid hormones) down a concentration gradient across the phospholipid bilayer, governed by Fick's Law: Rate ∝ (Surface Area × Concentration Gradient) / Diffusion Distance. Helped diffusion moves polar/charged species down their gradient via water-filled channel proteins (often gated for specific ions) or shape-changing carrier proteins, where Vmax is limited by the number of transport proteins.
- Osmosis & Water Potential (ψ = ψs + ψp)
- Osmosis is the net passive diffusion of water molecules across a partially permeable membrane from a region of higher (less negative) water potential (ψ) to a region of lower (more negative) water potential. Pure water at standard atmospheric pressure has ψ = 0 kPa; adding solutes lowers water potential (solute potential ψs is always negative), whereas turgor/pressure potential (ψp) in plant cells is positive.
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- Fluid Mosaic Model & Membrane Components
- 'Fluid' because phospholipids move laterally in the plane of the bilayer; 'mosaic' because intrinsic and extrinsic proteins are embedded in a scattered pattern. Cholesterol binds to hydrophobic fatty acid tails, restricting phospholipid movement at physiological temperatures to reduce fluidity and ion permeability while preventing solidification at low temperatures. Glycolipids and glycoproteins on the outer surface form the glycocalyx for cell-cell recognition, receptors, and tissue stability.
- Simple vs Helped Diffusion & Fick's Law
- Simple diffusion is the net passive movement of small, non-polar molecules (O₂, CO₂, steroid hormones) down a concentration gradient across the phospholipid bilayer, governed by Fick's Law: Rate ∝ (Surface Area × Concentration Gradient) / Diffusion Distance. Helped diffusion moves polar/charged species down their gradient via water-filled channel proteins (often gated for specific ions) or shape-changing carrier proteins, where Vmax is limited by the number of transport proteins.
- Osmosis & Water Potential (ψ = ψs + ψp)
- Osmosis is the net passive diffusion of water molecules across a partially permeable membrane from a region of higher (less negative) water potential (ψ) to a region of lower (more negative) water potential. Pure water at standard atmospheric pressure has ψ = 0 kPa; adding solutes lowers water potential (solute potential ψs is always negative), whereas turgor/pressure potential (ψp) in plant cells is positive.
- Primary Active Transport & Na⁺/K⁺-ATPase Pump
- Movement of ions or molecules against their concentration gradient using specific carrier proteins and ATP hydrolysis. ATP phosphorylates the carrier protein, inducing a conformational change. The sodium-potassium pump actively pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell per ATP hydrolysed.
- Co-Transport of Glucose in the Mammalian Ileum
- 1. Na⁺ ions are actively transported out of epithelial cells into the blood by the Na⁺/K⁺ pump using ATP, lowering intracellular [Na⁺]. 2. Na⁺ diffuses down its concentration gradient from the ileum lumen into the epithelial cell via a sodium-glucose co-transporter (symport) protein, carrying glucose into the cell against glucose's concentration gradient. 3. Glucose moves from the epithelial cell into the capillary blood down its concentration gradient via helped diffusion through a GLUT carrier protein.
- Beetroot Membrane Permeability & Temperature/Ethanol
- Investigated using beetroot (Beta vulgaris) discs containing red betalain pigment inside the vacuole (tonoplast) and cell surface membrane. Below 40°C, membranes remain intact; above ~45°C, kinetic energy increases phospholipid fluidity and denatures intrinsic membrane proteins, creating gaps through which betalain leaks (increasing absorbance in a colorimeter). Organic solvents like ethanol dissolve the non-polar phospholipid bilayer directly.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Why can polar molecules and charged ions not cross the phospholipid bilayer by simple diffusion?
What is the role of cholesterol in eukaryotic cell surface membranes?
What is the water potential (ψ) of pure water at atmospheric pressure, and how does adding solute affect ψ?
How do channel proteins and carrier proteins differ in helped diffusion?
Why is the co-transport of glucose in the ileum described as 'indirect' or 'secondary' active transport?
Why must beetroot discs be washed thoroughly in distilled water before starting a membrane permeability experiment?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Describe and explain how glucose is absorbed from the lumen of the ileum into the blood capillaries when the concentration of glucose in the lumen is lower than in the epithelial cells. (5 marks)
Click “Check Mark Scheme” to compare your answer, 1. Sodium ions (Na⁺) are actively transported out of the ileum epithelial cells into the blood by the sodium-potassium pump, using energy from the hydrolysis of ATP [1 mark]. 2. This maintains a lower concentration of Na⁺ inside the epithelial cell than in the lumen of the ileum [1 mark]. 3. Na⁺ ions diffuse down their concentration gradient from the lumen into the epithelial cell through a sodium-glucose co-transporter (carrier) protein in the microvilli membrane [1 mark]. 4. The co-transporter simultaneously carries glucose molecules into the epithelial cell against the glucose concentration gradient [1 mark]. 5. Glucose then moves out of the epithelial cell into the blood capillary down its concentration gradient by helped diffusion via a specific carrier protein [1 mark].
- Q2
A student placed plant tissue samples into a series of sucrose solutions of different concentrations (0.0 M to 1.0 M) and measured the percentage change in mass after 1 hour. Explain how they could use their results to find the water potential of the plant tissue, and explain the appearance of the plant cells in the 1.0 M solution. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Plot a calibration graph of percentage change in mass (y-axis) against sucrose concentration (x-axis) and find the x-intercept where percentage change in mass = 0% (no net movement of water by osmosis) [1 mark]. 2. At this isotonic sucrose concentration, the water potential of the plant tissue equals the water potential of the sucrose solution, which can be read from a standard calibration table [1 mark]. 3. In the 1.0 M sucrose solution, the external solution has a much lower (more negative) water potential than the plant cell cytoplasm/vacuole, so water leaves the cell by osmosis [1 mark]. 4. The vacuole shrinks and the protoplast pulls away from the rigid cellulose cell wall, so the cell becomes plasmolysed (flaccid) [1 mark].
- Q3
Explain the shape of a graph showing the rate of uptake of potassium ions into root hair cells against oxygen concentration, given that the curve starts above zero at 0% oxygen, rises steeply, and then levels off at high oxygen concentrations. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. At 0% oxygen, a small amount of ATP is still produced by anaerobic respiration (glycolysis), allowing a low rate of active transport (and some passive helped diffusion if down a gradient) [1 mark]. 2. As oxygen concentration increases, the rate of aerobic respiration (oxidative phosphorylation) increases, producing more ATP [1 mark]. 3. More ATP is available to hydrolyse and phosphorylate potassium carrier proteins, increasing the rate of active transport against the concentration gradient [1 mark]. 4. At high oxygen concentrations, the curve plateaus because all potassium carrier proteins in the membrane are saturated (working at Vmax), so carrier protein number becomes the limiting factor [1 mark].
- Q4
Explain why incubating beetroot discs at 65°C results in a dark red surrounding solution with a low percentage transmission in a colorimeter. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. At 65°C, phospholipids gain high kinetic energy and move much further apart, making the phospholipid bilayer excessively fluid and unstable [1 mark]. 2. The high temperature breaks hydrogen and ionic bonds in the tertiary structure of intrinsic channel and carrier proteins, causing them to denature [1 mark]. 3. Denatured proteins and disrupted phospholipid packing create gaps in both the tonoplast (vacuole membrane) and the cell surface membrane, greatly increasing membrane permeability [1 mark]. 4. Water-soluble betalain pigment diffuses out of the vacuole down its concentration gradient into the surrounding water, absorbing more light and reducing percentage transmission [1 mark].
Common Exam Questions (4)
Why does placing red blood cells in pure water cause them to burst (haemolysis), whereas plant cells do not burst?
Pure water has a water potential of 0 kPa, which is higher (less negative) than the cytoplasm of both cells, so water enters by osmosis. Red blood cells lack a cell wall, so their thin cell surface membrane stretches and bursts (osmotic lysis); plant cells have a rigid, inelastic cellulose cell wall that exerts an opposing inward pressure potential (turgor pressure), stopping further net water influx once the cell is fully turgid.
What colour filter should be used in a colorimeter when measuring red betalain pigment from beetroot?
A blue or green filter (around 470-530 nm) should be used because red pigment reflects/transmits red light and strongly absorbs complementary blue-green light, giving the greatest sensitivity and range of absorbance readings.
How do microvilli and mitochondria in epithelial cells of the ileum adapt them for absorption?
Microvilli (the brush border) fold the apical cell surface membrane to vastly increase surface area and house a high density of co-transporter proteins and membrane-bound disaccharidases. Numerous mitochondria provide the high supply of ATP required by the Na⁺/K⁺ pumps to maintain the sodium gradient.
What is the difference between endocytosis and exocytosis?
Both are bulk transport processes requiring ATP and membrane remodelling. Endocytosis (phagocytosis for solids, pinocytosis for liquids) involves the cell surface membrane invaginating and pinching off to form an intracellular vesicle; exocytosis involves intracellular secretory vesicles (from the Golgi apparatus) fusing with the plasma membrane to release contents (like neurotransmitters or digestive enzymes) outside the cell.
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