How do substances cross the cell surface membrane, and what distinguishes passive from active transport?
Explain the mechanisms of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis across membranes
A focused answer to the H2 Biology Cell Biology outcome on membrane transport. Simple and facilitated diffusion, osmosis and water potential, active transport with carrier proteins and ATP, and bulk transport by endocytosis and exocytosis.
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What this dot point is asking
SEAB wants you to explain how substances move across the cell surface membrane by simple diffusion, facilitated diffusion, osmosis, active transport, and bulk transport (endocytosis and exocytosis), to identify which are passive and which require energy, and to use the concept of water potential for osmosis. This builds directly on the fluid mosaic model.
The answer
Passive transport (no energy required)
Simple diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient, until equilibrium. Small non-polar molecules such as oxygen and carbon dioxide cross the bilayer directly.
Facilitated diffusion moves larger or charged molecules (such as glucose and ions) down their gradient through specific channel or carrier proteins, because these molecules cannot cross the non-polar core. It is still passive: no ATP is used.
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to one of lower water potential. Pure water has a water potential of zero; adding solute lowers (makes more negative) the water potential.
Active transport (energy required)
Active transport moves a substance against its concentration gradient using a carrier protein that acts as a pump. The carrier hydrolyses ATP, changes shape, and carries the substance from low to high concentration. This is how cells maintain steep gradients, such as the sodium-potassium pump.
Bulk transport
Endocytosis brings large materials into the cell by enclosing them in a vesicle formed from the membrane (phagocytosis for solids, pinocytosis for liquids). Exocytosis releases materials by fusing a vesicle with the membrane. Both require ATP and both rely on the fluidity of the membrane.
Examples in context
Example 1. The sodium-potassium pump. This carrier protein actively pumps three sodium ions out and two potassium ions in per ATP hydrolysed, against their gradients. The gradients it maintains are essential for nerve impulse conduction and for the secondary active transport of glucose in the gut.
Example 2. Glucose uptake in the small intestine. Glucose is first co-transported into the intestinal cell with sodium ions, then leaves into the blood by facilitated diffusion. The example shows passive and active mechanisms working together to absorb a nutrient efficiently.
Try this
Q1. State whether osmosis is an active or a passive process and give a reason. [1 mark]
- Cue. Passive, because water moves down a water potential gradient without the cell expending ATP.
Q2. Explain why oxygen can cross the phospholipid bilayer by simple diffusion but glucose cannot. [2 marks]
- Cue. Oxygen is a small non-polar molecule that dissolves in and crosses the hydrophobic core; glucose is larger and polar, so it requires a transport protein (facilitated diffusion).
Q3. Describe how a white blood cell uses endocytosis to engulf a bacterium. [2 marks]
- Cue. The cell surface membrane folds around the bacterium and pinches off to form a vesicle (phagosome) inside the cell, a process requiring ATP and membrane fluidity.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksCompare facilitated diffusion and active transport, referring to the direction of movement, the proteins involved and the energy requirement.Show worked answer →
The answer should contrast the two processes point by point.
Both use membrane proteins to move specific substances across the membrane. However, facilitated diffusion moves substances down their concentration gradient (from high to low concentration), uses channel or carrier proteins, and requires no metabolic energy because the movement is passive and driven by the gradient.
Active transport moves substances against their concentration gradient (from low to high concentration), uses carrier proteins that act as pumps, and requires metabolic energy in the form of ATP. The carrier protein changes shape using energy released by ATP hydrolysis to carry the substance across.
A full comparison notes that facilitated diffusion stops at equilibrium whereas active transport can maintain a steep gradient. Markers reward a clear contrast on each of the three named aspects rather than two separate descriptions, and correct use of the term against the gradient for active transport.
Original4 marksPlant tissue is placed in a concentrated sucrose solution. Using the idea of water potential, explain what happens to the cells and the term used to describe the result.Show worked answer →
The answer should reason from water potential to the cellular outcome.
A concentrated sucrose solution has a lower (more negative) water potential than the cell contents. Water therefore moves out of the cells by osmosis, down the water potential gradient, across the partially permeable cell surface membrane.
As water leaves, the cytoplasm and vacuole shrink and the cell surface membrane pulls away from the cell wall. This condition is called plasmolysis, and the cell is described as plasmolysed. The point at which the membrane just begins to pull away is incipient plasmolysis.
Markers reward the comparison of water potentials, the direction of osmosis (out of the cell), the resulting shrinkage and membrane detachment, and the correct term plasmolysis.
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