describe and explain the processes of simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis

Movement into and out of Cells – Cambridge A‑Level Biology (9700) – Topic 4.2

Understanding how substances cross the plasma membrane is essential for explaining nutrient uptake, waste removal, cell signalling, growth and defence. The six transport processes required by the syllabus are described below, each linked to the fluid‑mosaic model (proteins embedded in a phospholipid bilayer) and followed by a short “Why it matters” statement.

1. Simple Diffusion

Net movement of molecules from an area of higher concentration to an area of lower concentration driven solely by the concentration gradient.

• Occurs directly through the phospholipid bilayer – no protein required.
• Only small, non‑polar or uncharged molecules pass efficiently (e.g. O₂, CO₂, lipid‑soluble hormones).
• Passive: no ATP is required.
• Rate depends on temperature, surface area, membrane thickness and the diffusion coefficient (D).

Fick’s first law (exam‑relevant):

\$J = -D\frac{dC}{dx}\$

where J = flux (mol m⁻² s⁻¹), D = diffusion coefficient, dC/dx = concentration gradient.

Why it matters: Simple diffusion supplies cells with O₂ for respiration and removes CO₂ waste without any energy cost.

Practical ideas – Simple diffusion

• Starch‑iodine diffusion in agar: cut agar blocks of different thicknesses, place a drop of iodine on one face and record the time taken for the blue colour to appear on the opposite face.
• Dialysis‑tube experiment: fill tubing with glucose (small) or starch (large) and place in water. Only glucose diffuses out, illustrating size‑selectivity.

2. Facilitated Diffusion

Movement of substances down their concentration gradient, mediated by specific membrane proteins.

• Carrier proteins bind the solute, change conformation and release it on the opposite side.
• Channel proteins provide a permanent or gated hydrophilic pore (e.g. Na⁺, K⁺, Cl⁻ channels).
• Highly selective – only the correct substrate fits the carrier or channel.
• Passive – no ATP is consumed, although the protein itself may undergo conformational changes.

Why it matters: Facilitated diffusion enables rapid uptake of essential polar molecules (e.g. glucose) and ions that cannot cross the lipid bilayer unaided.

Typical examples

• GLUT carriers for glucose transport.
• Voltage‑gated Na⁺ and K⁺ channels in nerve and muscle cells.
• Urea reabsorption via facilitated diffusion in renal tubules.

3. Osmosis

Diffusion of water across a semi‑permeable membrane toward the region of higher solute concentration (lower water potential).

• Water moves through the lipid bilayer and/or specialised aquaporin channels.
• Direction is governed by water potential (Ψ) – water moves from higher Ψ to lower Ψ.
• Water‑potential equation (syllabus terminology):
  

  \$\Psi = \Psis + \Psip\$

• Ψ_s = solute (osmotic) potential (always negative).
• Ψ_p = pressure potential – in plant cells this is the turgor pressure that can oppose water influx.

Why it matters: Osmosis controls cell volume, turgor pressure in plants and the movement of water in animal capillaries and kidneys.

Practical ideas – Osmosis

• Potato core experiment: place equal‑sized potato cylinders in sucrose solutions of 0, 0.2, 0.4, 0.6 M. Measure mass change to calculate water potential of each solution.
• Dialysis‑tube osmosis: fill tubing with a known solute concentration, immerse in pure water and record volume change.

4. Active Transport

Movement of substances against their concentration (or electro‑chemical) gradient, requiring an input of energy.

Primary active transport

• ATP is hydrolysed directly by the pump.
• Typical example: Na⁺/K⁺‑ATPase – 3 Na⁺ out, 2 K⁺ in per ATP, creating an electro‑chemical gradient.

Secondary active transport

• Energy is stored in an ion gradient generated by a primary pump.
• Transport of a second solute is coupled to the downhill movement of the ion.

• Symport – both species move in the same direction (e.g. Na⁺‑glucose cotransporter in intestinal epithelium).
• Antiport – species move in opposite directions (e.g. Na⁺/Ca²⁺ exchanger in cardiac muscle).

Why it matters: Primary active transport establishes ion gradients that power secondary transport, nerve impulse propagation, muscle contraction and the uptake of nutrients against a concentration gradient.

Key exam points

• Active transport is the only way to move substances *against* a gradient.
• Electro‑chemical gradients consist of both concentration and electrical components; they are expressed as the sum of a chemical gradient (ΔC) and a membrane potential (ΔΨ).
• Each cycle of the Na⁺/K⁺‑ATPase consumes one ATP molecule.

5. Endocytosis

Process by which cells internalise extracellular material by engulfing it with the plasma membrane, forming a vesicle.

Forms and purpose

• Phagocytosis (“cell eating”) – large particles (bacteria, dead cells) are enclosed in a pseudopod‑derived vesicle. Purpose: immune defence and nutrient acquisition in unicellular organisms.
• Pinocytosis (“cell drinking”) – bulk fluid and dissolved solutes are taken up in numerous small vesicles. Purpose: continual sampling of the extracellular environment.
• Receptor‑mediated endocytosis – specific ligands bind surface receptors, causing clathrin‑coated pits to invaginate and pinch off as vesicles (e.g. LDL uptake, hormone‑receptor complexes). Purpose: highly efficient, selective uptake of low‑concentration substances.

All forms require ATP for membrane remodelling and vesicle trafficking.

Why it matters: Endocytosis allows cells to import large macromolecules, particles and signalling complexes that cannot cross the membrane by diffusion.

6. Exocytosis

Reverse of endocytosis: vesicles fuse with the plasma membrane to release their contents to the extracellular space.

• Essential for secretion of hormones (insulin), neurotransmitters, digestive enzymes and for insertion of membrane proteins.
• Vesicle docking, priming and fusion are mediated by SNARE protein complexes.
• ATP is required for vesicle transport along cytoskeletal tracks and for the fusion process.

Why it matters: Exocytosis provides a rapid, regulated way for cells to communicate (e.g. neurotransmission) and to export products that would otherwise accumulate intracellularly.

Mathematical Note – Surface‑Area : Volume (SA:V) Ratios

A high SA:V ratio favours diffusion and osmosis; many exam questions ask you to calculate or compare these ratios.

• Cube (side = a):
  

  Surface area = 6a², Volume = a³ → SA:V = 6/a

• Sphere (radius = r):
  

  Surface area = 4πr², Volume = (4/3)πr³ → SA:V = 3/r

Worked example (syllabus style):

1. Sphere, diameter = 20 µm (r = 10 µm).
   

   SA = 4π(10)² = 400π µm².
   

   V = (4/3)π(10)³ = (4000/3)π µm³.
   

   SA:V = 400π ÷ (4000/3)π = 3/10 = 0.3 µm⁻¹.

2. Sphere, diameter = 5 µm (r = 2.5 µm).
   

   SA = 4π(2.5)² = 25π µm².
   

   V = (4/3)π(2.5)³ ≈ (62.5/3)π µm³.
   

   SA:V ≈ 25π ÷ (62.5/3)π = 3/2.5 = 1.2 µm⁻¹.

3. Conclusion: the smaller cell has a four‑fold higher SA:V, so diffusion of gases and water will be relatively faster.

Comparison of the Six Transport Mechanisms

Key Points to Remember

1. Passive processes (simple diffusion, facilitated diffusion, osmosis) rely on existing gradients and require no cellular energy.
2. Active transport creates and maintains electro‑chemical gradients; these gradients power secondary active transport, nerve impulses and muscle contraction.
3. Vesicular transport (endocytosis & exocytosis) moves large or highly polar substances that cannot cross the lipid bilayer directly.
4. Membrane proteins (carriers, channels, pumps, receptors) are the functional units of the fluid‑mosaic model; their structure determines selectivity and transport rate.
5. In plant cells the rigid cell wall means that water influx generates turgor pressure (Ψp), a major driver of cell expansion and stomatal movements.
6. Surface‑area : volume ratios strongly influence the efficiency of diffusion and osmosis – smaller cells or cells with folded membranes have higher SA:V and therefore exchange substances more rapidly.