AQA GCSE Biology (8461) — Cell Biology
A drop of food colouring in a glass of water slowly spreads out until the whole glass is tinted, even without you stirring it. This natural mixing happens because particles in liquids (solutions) and gases are constantly moving.
Diffusion continues until particles are evenly distributed, reaching an equilibrium. At this point, individual particles still move randomly in all directions, but there is no further net movement.
How do your cells get the ingredients they need to survive and get rid of their waste? Simple diffusion handles the transport of several vital molecules across the cell membrane.
Every time you make a cup of tea, you might notice the tea bag infuses the hot water much faster than if you used cold water. Several specific factors determine how quickly particles diffuse.
These relationships can be summarised conceptually:
Imagine trying to heat up a massive boulder versus a handful of small pebbles; the pebbles heat up almost instantly because they have much more exposed surface compared to their inside space. This principle dictates how organisms survive.
Exams often use cubes to represent organisms when comparing ratios. Let's compare a 1 cm cube to a 2 cm cube.
Formulas:
Step 1: Calculate for the 1 cm cube.
Step 2: Calculate for the 2 cm cube.
Conclusion: As the size of the object increases, the SA:V ratio decreases. In a practical experiment using indicator-soaked agar blocks, the smallest block will always change colour to the centre fastest because the diffusion distance is shortest relative to its surface area.
Students often think that diffusion stops completely when an even concentration is reached; in reality, individual particles continue to move randomly, but there is simply no further NET movement.
In 'Explain' questions asking about the effect of temperature on diffusion, you must mention 'kinetic energy' and 'faster movement' to get full marks — simply stating 'the rate increases' is a description, not an explanation.
When describing the transport of urea in exams, mark schemes specifically look for the exact phrase 'from cells into the blood plasma'.
Never confuse a 'large surface area' (which refers to the physical amount of membrane, like folds or microvilli) with a 'short diffusion path' (which refers to membrane thickness, like being one cell thick); these are always two separate marking points.
Diffusion
The spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Net movement
The overall direction in which most particles move; more particles move from the high-concentration area to the low-concentration area than in the opposite direction.
Passive process
A cellular process that happens naturally and does not require an input of metabolic energy (from respiration) to occur.
Kinetic energy
The energy that particles possess due to their movement, which increases at higher temperatures.
Cell membrane
A partially permeable boundary surrounding a cell that controls which substances can enter and exit.
Alveoli
Tiny air sacs in the lungs that provide a large surface area for efficient gas exchange.
Aerobic respiration
An exothermic cellular process that uses oxygen to break down glucose and release energy.
Urea
A waste product formed in the liver that diffuses from cells into the blood plasma to be transported to the kidneys.
Deamination
The process in the liver where excess amino acids are broken down and converted into urea for safe transport.
Blood plasma
The pale yellow liquid component of blood that transports dissolved substances, including urea and carbon dioxide.
Concentration gradient
The difference in the concentration of a substance between two areas; a steeper gradient leads to faster diffusion.
Ventilation
The mechanical process of breathing in and out, which helps maintain a steep concentration gradient for gas exchange in the lungs.
Surface area to volume ratio (SA:V)
A mathematical comparison of how much surface area an object has relative to its internal volume, critical for determining if simple diffusion is sufficient for an organism.
Villi
Microscopic, finger-like projections in the lining of the small intestine that significantly increase the surface area for absorption.
Put your knowledge into practice — try past paper questions for Biology
Diffusion
The spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Net movement
The overall direction in which most particles move; more particles move from the high-concentration area to the low-concentration area than in the opposite direction.
Passive process
A cellular process that happens naturally and does not require an input of metabolic energy (from respiration) to occur.
Kinetic energy
The energy that particles possess due to their movement, which increases at higher temperatures.
Cell membrane
A partially permeable boundary surrounding a cell that controls which substances can enter and exit.
Alveoli
Tiny air sacs in the lungs that provide a large surface area for efficient gas exchange.
Aerobic respiration
An exothermic cellular process that uses oxygen to break down glucose and release energy.
Urea
A waste product formed in the liver that diffuses from cells into the blood plasma to be transported to the kidneys.
Deamination
The process in the liver where excess amino acids are broken down and converted into urea for safe transport.
Blood plasma
The pale yellow liquid component of blood that transports dissolved substances, including urea and carbon dioxide.
Concentration gradient
The difference in the concentration of a substance between two areas; a steeper gradient leads to faster diffusion.
Ventilation
The mechanical process of breathing in and out, which helps maintain a steep concentration gradient for gas exchange in the lungs.
Surface area to volume ratio (SA:V)
A mathematical comparison of how much surface area an object has relative to its internal volume, critical for determining if simple diffusion is sufficient for an organism.
Villi
Microscopic, finger-like projections in the lining of the small intestine that significantly increase the surface area for absorption.
