How does the cell membrane control what enters and leaves the cell?
Explain the fluid mosaic model and the mechanisms of passive and active transport across the membrane
The fluid mosaic membrane controls movement of substances by passive transport (diffusion, osmosis, facilitated diffusion) and active transport, which requires energy.
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What this dot point is asking
You need to describe the structure of the membrane using the fluid mosaic model and explain each mechanism of transport, including whether it requires energy and the direction relative to the concentration gradient.
The fluid mosaic model
The cell membrane is described by the fluid mosaic model:
- It is built from a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) phosphate head facing outward and two hydrophobic (water-fearing) fatty acid tails facing inward.
- It is fluid because the phospholipids can move sideways within the layer.
- It is a mosaic because various proteins are scattered through the bilayer, including channel and carrier proteins, plus cholesterol and carbohydrate markers.
The membrane is selectively (partially) permeable: small non-polar molecules cross freely, but ions and large or polar molecules need protein channels or carriers.
Passive transport (no energy)
Passive transport moves substances down their concentration gradient (high to low), so it needs no cellular energy.
Diffusion
Diffusion is the net movement of particles from a region of high concentration to a region of low concentration until evenly spread. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the bilayer.
Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane, from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).
Facilitated diffusion
Facilitated diffusion is passive movement of substances that cannot cross the bilayer alone (such as glucose and ions) through channel or carrier proteins. It is still down the gradient, so it needs no energy.
Active transport (requires energy)
Active transport moves substances against their concentration gradient (low to high). This requires energy from ATP and uses specific carrier proteins (pumps). An example is the sodium-potassium pump in nerve cells.
Bulk transport also requires energy: endocytosis brings large materials into the cell in vesicles, and exocytosis releases them.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 SACE Stage 21 marksWhich one of the following correctly identifies a property of simple diffusion and a property of facilitated diffusion: simple does not require energy and facilitated requires energy; simple requires membrane proteins and facilitated does not; simple moves with the gradient and facilitated against it; or both do not require energy?Show worked answer →
The correct combination is that both do not require energy. Simple diffusion and facilitated diffusion are both passive transport, moving substances down (with) their concentration gradient without using ATP. The differences are that simple diffusion does not need membrane proteins while facilitated diffusion uses channel or carrier proteins, and both still move substances with the gradient (not against it).
2018 SACE Stage 23 marksInsulin is a protein packaged in the Golgi body. Describe how insulin is secreted from a cell.Show worked answer →
Three points describe exocytosis.
Insulin is packaged into a membrane-bound vesicle, which buds off the Golgi body.
The vesicle moves to and fuses with the cell (plasma) membrane.
The vesicle membrane joins the plasma membrane and the insulin is released to the outside of the cell (into the blood). This process is exocytosis and requires energy (ATP).