Edited by simon
2025-12-04 02:29 · Updated content
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Molecular Transport Across Membranes
Diffusion • Osmosis • Passive Transport • Active Transport • Pumps • Channels • Vesicle Transport
Cell membranes control what enters and leaves the cell.
This regulation is essential for life — without it, cells would swell, burst, dry out, or lose all chemical gradients needed for metabolism.
This page introduces the main mechanisms by which molecules cross cell membranes, with examples from onion cells and yeast.
1. Structure of the Plasma Membrane
The plasma membrane is a phospholipid bilayer containing:
- lipids
- proteins (channels, pumps, receptors)
- carbohydrates (on the outer surface of animal cells)
Properties:
- selectively permeable
- allows some molecules through easily
- blocks others
- uses proteins for controlled transport
The membrane is ~5–10 nm thick — far too small to see with a light microscope.
2. Passive Transport (No Energy Required)
Passive transport relies on natural motion of molecules.
2.1. Simple Diffusion
Molecules move from high concentration to low concentration.
Can cross the membrane if:
- small
- nonpolar
- uncharged
Examples:
- oxygen (O₂)
- carbon dioxide (CO₂)
- small lipids
- steroid hormones
Yeast and onion cells use diffusion constantly for gases.
2.2. Facilitated Diffusion
Molecules that cannot cross the membrane directly use proteins:
- channels
- carrier proteins
Still passive (no ATP required).
Examples:
- glucose transporters (GLUT)
- ion channels (K⁺, Na⁺, Ca²⁺)
- aquaporins (water channels)
Yeast rely heavily on glucose transporters to detect and import sugar.
3. Osmosis (Water Transport)
Osmosis = diffusion of water across a membrane.
Water moves from:
- low solute concentration → high solute concentration
This process shapes cells dramatically.
3.1. In Plant Cells (e.g., onion)
Because they have rigid cell walls, plant cells behave differently:
- In hypotonic solution → water enters → turgid (healthy)
- In hypertonic solution → water leaves → plasma membrane shrinks from wall (plasmolysis)
Plasmolysis is visible under a microscope — great beginner experiment.
3.2. In Yeast
Yeast have a strong cell wall, so they resist lysis.
But osmotic shock activates the HOG pathway (High Osmolarity Glycerol), a key stress signaling response.
Osmosis is never directly visible — only consequences (shrinking, swelling).
4. Active Transport (Requires Energy, Usually ATP)
Cells often need to move molecules against their concentration gradient.
This requires:
- pumps
- transporters
- ATP or chemical gradients
4.1. Sodium-Potassium Pump (Animals)
3 Na⁺ out, 2 K⁺ in.
Maintains membrane potential in neurons and muscle cells.
No equivalent in yeast or plants (they use different ion pumps).
4.2. Proton Pumps (H⁺ pumps)
Used by:
- yeast
- plants
- fungi
Proton pumps create electrochemical gradients powering:
- nutrient uptake
- ATP production in mitochondria
- pH regulation
Yeast use proton pumps extensively for nutrient transport.
4.3. Calcium Pumps
Cells maintain extremely low Ca²⁺ levels in the cytoplasm.
Small openings of Ca²⁺ channels trigger powerful signals.
5. Co-Transport (Coupled Transport)
Transporters can move two molecules at once.
5.1. Symport
Both molecules move in the same direction.
Example:
- glucose + H⁺ symporter in yeast
- amino acid symporters
5.2. Antiport
Molecules move in opposite directions.
Example:
- Na⁺/Ca²⁺ antiporter in muscles
- H⁺/K⁺ antiporter in yeast vacuoles
6. Bulk Transport (Vesicle Transport)
Large molecules cannot cross the membrane through channels or pumps.
Cells use vesicles instead.
6.1. Endocytosis
Cell membrane folds inward to bring material into the cell.
Types:
- phagocytosis (“cell eating” — not in yeast)
- pinocytosis (“cell drinking”)
- receptor-mediated endocytosis
6.2. Exocytosis
Vesicles fuse with the membrane to release substances.
Used for:
- secretion
- neurotransmitters
- enzymes
- hormones
Yeast secrete enzymes and mating pheromones via exocytosis.
These processes are not visible with a light microscope.
7. Transport in Yeast
Yeast transport is central to:
- nutrient sensing (glucose, nitrogen)
- stress response (osmotic shock)
- proton gradients for metabolism
- transport of aging-related molecules
Key yeast-specific transport examples:
- HXT glucose transporters
- Pma1 proton pump
- Fps1 aquaglyceroporin (important for osmotic stress)
- Mating pheromone receptors (GPCRs)
8. Transport in Plant Cells (e.g., Onion)
Plant cells have:
- rigid cellulose walls
- large vacuoles
- proton pumps instead of Na⁺/K⁺ pumps
- plasmodesmata (channels between cells)
Unique plant features:
8.1. Plasmodesmata
Tiny channels connecting cells → allow ions and small molecules to move directly between cytoplasms.
Too small to see with optical microscopy.
8.2. Vacuolar Transport
Vacuoles store:
- ions
- water
- pigments
- toxins
Transport proteins in vacuolar membranes regulate cell pressure.
9. What Can Be Seen Under a Light Microscope?
Transport proteins and membranes themselves are too small (5–10 nm).
But effects of transport can be visible.
Visible:
- plasmolysis in onion cells
- swelling of cells in hypotonic solutions
- budding behavior changes in stressed yeast
- vacuole size changes
Not visible:
- channels
- pumps
- carriers
- vesicles
- ion gradients
- membrane potentials
- diffusion itself
Only large-scale outcomes can be observed, not the molecular processes.
10. Summary Table of Transport Types
| Type | Energy Required? | Uses Proteins? | Direction | Example |
|---|---|---|---|---|
| Simple diffusion | No | No | High → Low | O₂, CO₂ |
| Facilitated diffusion | No | Yes | High → Low | Glucose channels |
| Osmosis | No | Yes (aquaporins) | Water | Plant turgor |
| Active transport | Yes (ATP) | Yes | Low → High | Proton pump |
| Symport | Yes (gradient) | Yes | Same direction | Glucose + H⁺ |
| Antiport | Yes (gradient) | Yes | Opposite directions | Na⁺/Ca²⁺ |
| Endocytosis | Yes | Yes | Bulk import | Receptor uptake |
| Exocytosis | Yes | Yes | Bulk export | Yeast pheromones |
11. Quick Beginner Summary
- Membranes are selectively permeable.
- Small uncharged molecules diffuse easily.
- Ions and polar molecules need transport proteins.
- Water moves by osmosis — great to observe in onion cells.
- Yeast use proton pumps and nutrient transporters heavily.
- Transport machinery is invisible, but its effects can be seen.