In simple terms
A friendly intro before the formal notes — no formulas yet.
How cells move stuff around
Every living cell needs to control what goes in and out. This lesson breaks down the different ways cells transport substances, from tiny molecules diffusing passively to large particles being actively hauled using energy. Understanding these processes is key to how organisms function.
Imagine a busy border crossing. Some people (small molecules) can just walk across freely (diffusion). Others need a special pass or specific gate (facilitated diffusion). Some need a dedicated vehicle and energy to go against the flow (active transport). Large groups or cargo need specialised vehicles and processes to enter or exit (bulk transport).
- 1
Review the structure of the cell surface membrane – it's crucial for understanding transport.
- 2
Categorise transport into passive (no ATP) and active (requires ATP) processes.
- 3
Memorise the key characteristics for each mechanism: gradient, proteins, energy requirement.
- 4
Practise applying concepts to different cell types and scenarios, especially osmosis calculations.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 7.2.1
State that some mineral ions and organic compounds can be transported within plants dissolved in water
- 7.2.2
Describe the transport of water from the soil to the xylem through the: • apoplast pathway, including reference to lignin and cellulose • symplast pathway, including reference to the endodermis, Casparian strip and suberin
- 7.2.3
Explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
- 7.2.4
Explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
- 7.2.5
Make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
- 7.2.6
State that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes
- 7.2.7
Explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
- 7.2.8
Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink
Explore the concept
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
1. Passive Transport: No Energy Required
Passive transport processes rely on the inherent kinetic energy of molecules and do not require the cell to expend metabolic energy (ATP). Substances move down their respective gradients.
1.1. Simple Diffusion
Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, directly across the cell surface membrane, until equilibrium is reached. It’s a passive process, meaning no ATP is involved.
Characteristics:
- Down a concentration gradient: Molecules move from where there are more of them to where there are fewer.
- Directly through the lipid bilayer: Small, non-polar molecules (e.g., oxygen, carbon dioxide, ethanol, urea) can pass through without assistance.
- No carrier proteins involved.
- Passive: Does not require metabolic energy (ATP).
Factors Affecting Rate of Diffusion:
water moving thorough cells walls – apoplastic pathway.
suberin deposits increase with age of endodermal cells except for certain passage cells.
water can pass freely through these passage cells Image: By Dylan W. Schwilk - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55396563.
most water travels via the apoplastic pathway (when transpiration rates are high).
these are the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem.
the water moves by diffusion as it isn’t crossing a partially permeable membrane.
the water can move from cell wall to cell wall directly or through the intracellular spaces.
movement of water via this pathway occurs more quickly than in the symplastic pathway.
this movement is however stopped at the endodermis due to the presence of the Casparian strip Symplastic pathway.
movement through the cytoplasm, plasmodesmata, or vacuole of cells (crossing membranes).
the sinks (where the assimilates are required) could be: 1) meristems (apical or lateral) that are actively dividing 2) roots that are growing and / or actively absorbing mineral ions 3) any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs).
xerophytes (from the greek xero for ‘dry’) are plants that are adapted to dry and arid conditions.
xerophytes have physiological and structural (xeromorphic) adaptations to maximise water conservation.
water molecules cling together by hydrogen bonds between molecules known as cohesive forces.
water molecules experience attraction towards the cellulose in the cell walls of the xylem (adhesion) Cohesion-adhesion theory.
water molecules tend to cling to one another via hydrogen bonds (cohesion).
1.2. Facilitated Diffusion
Facilitated diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, across a cell surface membrane, via specific protein channels or carrier proteins. It is also a passive process.
Characteristics:
- Down a concentration gradient.
- Uses transport proteins:
- Channel proteins: Form hydrophilic pores through the membrane, allowing specific ions or small polar molecules (e.g., water via aquaporins) to pass. Gated channels can open or close.
- Carrier proteins: Bind to specific molecules, undergo a conformational change, and then release the molecule on the other side. They act like a 'shuttle'.
- Specificity: Each protein is specific to certain molecules or ions.
- Saturation: The rate of transport reaches a maximum when all available channel or carrier proteins are occupied (saturated).
- Passive: Does not require metabolic energy (ATP).
1.3. Osmosis
Osmosis is a special type of facilitated diffusion involving water. It's the net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane, until equilibrium is reached.
Key terms:
- Partially permeable membrane: A membrane that allows small molecules (like water) to pass through, but not larger solute molecules.
- Water potential (Ψ): The potential energy of water molecules. Pure water at atmospheric pressure has a water potential of 0 kPa (the highest possible). Adding solutes lowers the water potential (makes it more negative).
- Solute potential (Ψs): The reduction in water potential due to the presence of solutes. It's always negative or zero. More solute means more negative Ψs.
- Pressure potential (Ψp): The pressure exerted by water against a cell wall (in plant cells) or by hydrostatic pressure. It is usually positive (turgor pressure) or zero. For animal cells, Ψp is generally considered zero.
- Equation: Water potential (Ψ) = Solute potential (Ψs) + Pressure potential (Ψp)
Effects on cells:
- Animal cells (no cell wall):
- Hypotonic solution (higher water potential outside): Water enters the cell, causing it to swell and eventually burst (lysis).
- Isotonic solution (same water potential): No net movement, cell maintains normal shape.
- Hypertonic solution (lower water potential outside): Water leaves the cell, causing it to shrink and become crenated.
- Plant cells (with cell wall):
- Hypotonic solution: Water enters the cell by osmosis, causing the protoplast to swell and push against the rigid cell wall, leading to turgidity. The cell wall prevents lysis.
- Isotonic solution: Little net movement, cell is flaccid (limp).
- Hypertonic solution: Water leaves the cell, the protoplast shrinks and pulls away from the cell wall – this is called plasmolysis.
Ψ = Ψs + Ψp
2. Active Transport: Energy Required
Active transport is the movement of molecules or ions against their concentration gradient (from a region of lower concentration to a region of higher concentration), across a cell surface membrane, using specific carrier proteins and requiring metabolic energy (ATP).
Characteristics:
- Against a concentration gradient: Moves substances from low to high concentration.
- Requires carrier proteins: These proteins bind to specific molecules/ions and use ATP to change shape, 'pumping' the substance across the membrane.
- Requires metabolic energy (ATP): ATP provides the energy to change the conformation of the carrier protein.
- Specificity: Carrier proteins are specific to the molecules or ions they transport.
- Saturation: The rate of transport reaches a maximum when all available carrier proteins are occupied and working at their fastest.
3. Bulk Transport: For Large Quantities
When cells need to transport very large molecules or large quantities of substances (e.g., whole cells, cell debris, proteins, hormones), they use bulk transport mechanisms which involve the formation and fusion of vesicles. These processes always require ATP.
3.1. Endocytosis (entering the cell): The cell membrane engulfs a substance, forming a vesicle that pinches off and moves into the cytoplasm.
- Phagocytosis: 'Cell eating' – uptake of solid particles (e.g., bacteria by white blood cells).
- Pinocytosis: 'Cell drinking' – uptake of liquids or small dissolved molecules (e.g., absorption of digested fats).
3.2. Exocytosis (exiting the cell): Vesicles containing substances fuse with the cell membrane, releasing their contents outside the cell.
- Secretion: Releasing useful substances (e.g., hormones, enzymes).
- Waste removal: Expelling unwanted substances.
Clearly distinguish between passive (diffusion, facilitated diffusion, osmosis) and active (active transport, bulk transport) processes based on ATP requirement and gradient.
For osmosis, always refer to 'water potential' and 'partially permeable membrane' in your definitions. Avoid 'concentration of water'.
Remember that facilitated diffusion and active transport both use specific membrane proteins and can exhibit saturation.
Practice interpreting diagrams of cells in different solutions (hypotonic, isotonic, hypertonic) for both plant and animal cells.
Relate transport mechanisms to specific biological examples, e.g., glucose absorption in the small intestine, gas exchange in the lungs, water uptake by roots.
Worked examples
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The diagram shows a cell surface membrane. Explain how the movement of glucose into a cell differs when transported via facilitated diffusion compared to active transport.
- 1
Fac facilitated Diffusion: Glucose moves down its concentration gradient (from higher to lower concentration). It uses a specific carrier protein but does not require metabolic energy (ATP). The carrier protein binds to glucose, changes shape, and releases it inside the cell.
A plant cell has and . Calculate the water potential . [2 marks]
- 1
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How it all connects
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Glossary
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Quick check
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Revision flashcards
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Define simple diffusion.
Net movement of molecules down a concentration gradient across the membrane without proteins or ATP.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
water moving thorough cells walls – apoplastic pathway.
- ✓
suberin deposits increase with age of endodermal cells except for certain passage cells.
- ✓
water can pass freely through these passage cells Image: By Dylan W. Schwilk - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55396563.
- ✓
most water travels via the apoplastic pathway (when transpiration rates are high).
- ✓
these are the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem.
- ✓
the water moves by diffusion as it isn’t crossing a partially permeable membrane.
- ✓
the water can move from cell wall to cell wall directly or through the intracellular spaces.
- ✓
movement of water via this pathway occurs more quickly than in the symplastic pathway.
- ✓
this movement is however stopped at the endodermis due to the presence of the Casparian strip Symplastic pathway.
- ✓
movement through the cytoplasm, plasmodesmata, or vacuole of cells (crossing membranes).
- ✓
the sinks (where the assimilates are required) could be: 1) meristems (apical or lateral) that are actively dividing 2) roots that are growing and / or actively absorbing mineral ions 3) any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs).
- ✓
xerophytes (from the greek xero for ‘dry’) are plants that are adapted to dry and arid conditions.
- ✓
xerophytes have physiological and structural (xeromorphic) adaptations to maximise water conservation.
- ✓
water molecules cling together by hydrogen bonds between molecules known as cohesive forces.
- ✓
water molecules experience attraction towards the cellulose in the cell walls of the xylem (adhesion) Cohesion-adhesion theory.
- ✓
water molecules tend to cling to one another via hydrogen bonds (cohesion).
Practice — then mark it
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