Lecture 4 - Transport across cell membranes

🧠 VERY DETAILED MULTI-PARAGRAPH SUMMARY

Cellular transport across membranes is a fundamental principle underpinning all physiology, particularly in excitable tissues like neurons and muscle. At its core lies the phospholipid bilayer, a dynamic “fluid mosaic” composed of hydrophilic heads and hydrophobic tails. This structural arrangement creates a selectively permeable barrier, meaning that not all substances can freely pass through. Small nonpolar molecules such as oxygen and carbon dioxide diffuse easily, whereas charged ions and large polar molecules require specialised transport systems.

Transport mechanisms are broadly divided into passive and active processes. Passive transport, including simple diffusion and facilitated diffusion, moves substances down their concentration gradient without energy expenditure. Facilitated diffusion relies on carrier proteins or channels, making it faster but saturable. In contrast, active transport requires ATP to move substances against their concentration gradient. A key example is the sodium-potassium pump, which maintains ionic gradients by exporting sodium and importing potassium, creating an essential electrochemical imbalance that enables electrical signalling.

Ion channels are the central players in rapid physiological signalling. These transmembrane proteins can open or close in response to stimuli such as voltage changes (ions), ligand (neurotransmitter, hormone, substrates and inhibitors) binding, or mechanical stretch. When open, they allow ions to flow rapidly across the membrane, generating electrical signals within milliseconds. This rapid ion movement is what enables neuronal communication and action potentials, distinguishing it from slower processes like hormone signalling or protein synthesis.

The electrochemical gradient, combining both concentration and electrical forces, governs ion movement. For example, sodium is highly concentrated outside the cell, while potassium is higher inside. This imbalance is critical for generating the resting membrane potential and subsequent action potentials. Without these gradients, excitable cells would lose their ability to transmit signals.

Water movement across membranes, or osmosis, is equally vital. Water travels across semipermeable membranes to balance solute concentrations, often via specialised channels called aquaporins. Maintaining isotonic conditions is crucial; otherwise, cells may shrink (hypertonic environment) or swell and burst (hypotonic environment). This is especially important in the brain, where extracellular fluid composition must remain tightly regulated.

The blood-brain barrier adds another layer of complexity. It restricts the entry of substances into the brain, requiring molecules to pass through endothelial cells and astrocytes. This barrier protects neural tissue but also complicates drug delivery.

Ion channel function is highly specific and tightly regulated. Channels selectively permit certain ions based on size and charge, often requiring ions to shed hydration shells before passing through. Malfunction of these channels leads to channelopathies, such as myotonia congenita (chloride channel defect causing muscle hyperexcitability- goat video) or certain forms of epilepsy and pain disorders linked to sodium channel mutations.

Overall, cellular transport mechanisms form the foundation of neuronal excitability, signaling speed, and physiological homeostasis. Without controlled ion movement and membrane permeability, processes such as cognition, muscle contraction, and synaptic transmission would not be possible.

📌 BULLET POINT SUMMARY

Membrane Structure

  • Phospholipid bilayer: hydrophilic heads + hydrophobic tails

  • Fluid mosaic model: dynamic with embedded proteins

  • Selectively permeable barrier

Types of Transport

  • Passive transport

    • Simple diffusion (no proteins)

    • Facilitated diffusion (via carriers/channels)

  • Active transport

    • Requires ATP

    • Moves substances against gradient

    • Example: Na⁺/K⁺ ATPase (3 Na⁺ out, 2 K⁺ in)

Ion Channels

  • Highly selective (Na⁺, K⁺, Ca²⁺, Cl⁻)

  • Can be:

    • Voltage-gated

    • Ligand-gated

    • Mechanically gated

  • Enable rapid signaling (milliseconds)

Electrochemical Gradient

  • Combines:

    • Concentration gradient

    • Electrical gradient

  • Essential for action potentials

Osmosis

  • Water moves from high → low water concentration

  • Controlled by aquaporins

  • Tonicity:

    • Hypertonic → cell shrinks

    • Hypotonic → cell swells

    • Isotonic → stable

Brain Environment

  • Extracellular space ≈ 20% of brain volume

  • Blood-brain barrier regulates entry of substances

Channelopathies

  • Ion channel defects cause disease:

    • Myotonia congenita (Cl⁻ channel)

    • Epilepsy (Na⁺ channels)

    • Pain disorders (NaV1.7 mutations)

✏ FILL-IN-THE-BLANK SUMMARY

Section A: Basics

  1. The cell membrane is primarily composed of a __phospho________ bilayer.

  2. Hydrophilic regions face ___aqua_______ environments.

  3. Hydrophobic regions prevent passage of ___charged______ molecules.

Section B: Transport

  1. Movement down a concentration gradient without energy is called ___passive______ transport.

  2. Movement against a gradient requires __energy___ATP_____.

  3. The sodium-potassium pump moves _3_ Na⁺ out and __2_ K⁺ in.

Section C: Ion Channels

  1. Ion channels allow rapid movement of ___ions_______ across membranes.

  2. Channels that open in response to neurotransmitters are called ___ligand_______ gated channels.

  3. Electrical changes open _voltage___ gated channels.

Section D: Osmosis

  1. Water movement across membranes is called __osmosis________.

  2. Channels for water are called ___auqaporin_______.

  3. Cells in a hypotonic solution will ___burst_______.

Section E: Physiology

  1. Sodium concentration is higher ___outside_______ the cell.

  2. Potassium concentration is higher __inside________ the cell.

  3. The electrochemical gradient combines electrical and ____chemical______ forces.

✅ ANSWERS

  1. Phospholipid

  2. Aqueous (intracellular/extracellular)

  3. Charged (or polar ions)

  4. Passive

  5. ATP (energy)

  6. 3, 2

  7. Ions

  8. Ligand

  9. Voltage

  10. Osmosis

  11. Aquaporins

  12. Swell (and possibly burst)

  13. Outside

  14. Inside

  15. Chemical

🧪 40 HARD EXAM-STYLE MCQs

Q1

Which component primarily forms the hydrophobic core of the membrane?
a) Cholesterol
b) Phospholipid heads
c) Fatty acid tails
d) Proteins
e) Carbohydrates

Q2

Which transport type requires ATP?
a) Diffusion
b) Facilitated diffusion
c) Osmosis
d) Active transport
e) Filtration

Q3

Which ion is highest inside cells?
a) Na⁺
b) Cl⁻
c) K⁺
d) Ca²⁺
e) H⁺

Q4

Which of the following is NOT passive transport?
a) Diffusion
b) Facilitated diffusion
c) Osmosis
d) Active transport
e) Channel-mediated transport

Q5

Aquaporins transport:
a) Sodium
b) Potassium
c) Water
d) Glucose
e) Calcium

Q6

Which process maintains resting membrane potential?
a) Diffusion
b) Na⁺/K⁺ ATPase
c) Osmosis
d) Filtration
e) Exocytosis

Q7

Ion channels open in response to:
a) Temperature only
b) Light only
c) Voltage or ligands
d) DNA replication
e) Protein synthesis

Q8

Which ion triggers neurotransmitter release?
a) Na⁺
b) K⁺
c) Ca²⁺
d) Cl⁻
e) Mg²⁺

Q9

Facilitated diffusion differs from simple diffusion because it:
a) Uses ATP
b) Requires transport proteins
c) Moves against gradient
d) Is slower
e) Uses vesicles

Q10

Which environment causes cell shrinkage?
a) Hypotonic
b) Isotonic
c) Hypertonic
d) Neutral
e) Acidic

Q11–40 (FULL MCQs)

Q11

Which structure forms the blood-brain barrier?
a) Neurons
b) Astrocytes and endothelial cells
c) Microglia
d) Oligodendrocytes
e) Synapses

Q12

Which ion cannot freely diffuse across the membrane?
a) O₂
b) CO₂
c) Na⁺
d) Benzene
e) Ethanol

Q13

The driving force for ion movement is:
a) Osmotic pressure only
b) ATP only
c) Electrochemical gradient
d) Temperature
e) pH only

Q14

Which transporter moves two substances in the same direction?
a) Antiporter
b) Uniporter
c) Symporter
d) Channel
e) Pump

Q15

Which is NOT a gating mechanism of ion channels?
a) Voltage
b) Ligand
c) Mechanical
d) DNA binding
e) Phosphorylation

Q16

Which best describes facilitated diffusion?
a) Energy dependent
b) Non-specific
c) Carrier mediated
d) Moves uphill
e) Uses vesicles

Q17

Na⁺/K⁺ ATPase is:
a) Passive
b) Secondary active
c) Primary active
d) Diffusion
e) Osmotic

Q18

What happens when Na⁺ enters a neuron?
a) Hyperpolarization
b) Depolarization
c) No change
d) Repolarization
e) Inhibition

Q19

Which ion typically causes inhibition?
a) Na⁺
b) K⁺
c) Ca²⁺
d) Cl⁻
e) Mg²⁺

Q20

Which structure allows rapid ion flow?
a) Carrier
b) Channel
c) Vesicle
d) Receptor
e) Enzyme

Q21

Which is a secondary active transport example?
a) Na⁺/K⁺ ATPase
b) Na⁺-glucose cotransport
c) Diffusion
d) Osmosis
e) Ion channel

Q22

Which is TRUE of cholesterol?
a) Makes membrane rigid only
b) Regulates fluidity
c) Forms channels
d) Pumps ions
e) Stores energy

Q23

Which molecule diffuses most easily?
a) Glucose
b) Na⁺
c) O₂
d) Amino acids
e) Proteins

Q24

Which condition causes cell swelling?
a) Hypertonic
b) Hypotonic
c) Isotonic
d) Neutral
e) Acidic

Q25

Which protein transports glucose in neurons?
a) GLUT1
b) GLUT2
c) GLUT3
d) GLUT4
e) SGLT

Q26

Which ion channel defect causes myotonia congenita?
a) Na⁺
b) K⁺
c) Ca²⁺
d) Cl⁻
e) Mg²⁺

Q27

Which is NOT part of membrane proteins?
a) Ion channels
b) Carriers
c) Receptors
d) DNA
e) Enzymes

Q28

Which ion is highest extracellularly?
a) K⁺
b) Na⁺
c) Ca²⁺
d) Mg²⁺
e) H⁺

Q29

Which process uses vesicles?
a) Diffusion
b) Active transport
c) Endocytosis
d) Osmosis
e) Filtration

Q30

Which defines osmolarity?
a) Ion charge
b) Number of particles
c) Size
d) Shape
e) Weight

Q31

Which is fastest signaling?
a) Hormones
b) Protein synthesis
c) Neural transmission
d) DNA replication
e) Cell division

Q32

Which channel opens during depolarization?
a) K⁺
b) Cl⁻
c) Na⁺
d) Ca²⁺
e) Mg²⁺

Q33

Which gradient moves ions?
a) Thermal
b) Electrical only
c) Chemical only
d) Electrochemical
e) Mechanical

Q34

Which ion exits during repolarization?
a) Na⁺
b) K⁺
c) Ca²⁺
d) Cl⁻
e) H⁺

Q35

Which is NOT a feature of ion channels?
a) Selectivity
b) Speed
c) Saturation
d) Gating
e) Specificity

Q36

Which structure clears debris in brain?
a) Astrocytes
b) Microglia
c) Neurons
d) Axons
e) Dendrites

Q37

Which allows synaptic transmission?
a) Osmosis
b) Ion channels
c) Diffusion only
d) DNA
e) ATP only

Q38

Which ion channel mutation causes pain loss?
a) NaV1.1
b) NaV1.5
c) NaV1.7
d) NaV1.2
e) NaV1.3

Q39

Which ion channel toxin blocks Na⁺ channels?
a) Insulin
b) Dopamine
c) Tetrodotoxin
d) Glucose
e) ATP

Q40

Which is NOT a function of membranes?
a) Barrier
b) Transport
c) Communication
d) DNA replication
e) Signal transduction

✅ ANSWERS (MCQs)

  1. c

  2. d

  3. c

  4. d

  5. c

  6. b

  7. c

  8. c

  9. b

  10. c

  11. b

  12. c

  13. c

  14. c

  15. d

  16. c

  17. c

  18. b

  19. d

  20. b

  21. b

  22. b

  23. c

  24. b

  25. c

  26. d

  27. d

  28. b

  29. c

  30. b

  31. c

  32. c

  33. d

  34. b

  35. c

  36. b

  37. b

  38. c

  39. c

  40. d

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