Explain how the structure of the cell membrane controls the movement of materials by passive and active transport (North Carolina Standard Course of Study, Biology, LS.Bio.1).

What this topic is asking

North Carolina LS.Bio.1 asks how the cell membrane controls the movement of materials. For the Biology EOC you need the fluid mosaic structure, the idea of selective permeability, and the difference between passive transport (diffusion, osmosis, facilitated diffusion, no energy) and active transport (against the gradient, using ATP). Tonicity (hypotonic, isotonic, hypertonic) and its effect on cells is a favorite, often as a diagram or a data item.

The structure of the membrane

Each phospholipid has a hydrophilic ("water-loving") phosphate head and two hydrophobic ("water-fearing") fatty-acid tails. In water, the molecules arrange into a bilayer with the heads facing the watery inside and outside and the tails tucked together in the middle. This arrangement is what makes the membrane selectively permeable: small, non-polar molecules pass easily, but charged ions and large molecules need help from transport proteins. The membrane's job is to control what enters and leaves, keeping useful materials in and wastes moving out.

Passive transport: no energy needed

Passive transport moves substances down their concentration gradient (from high to low concentration), so it does not require the cell to spend energy.

• Diffusion. The net movement of particles from a region of high concentration to a region of low concentration until evenly spread. Oxygen diffuses into a cell because it is more concentrated outside.
• Osmosis. The diffusion of water across a selectively permeable membrane, from where water is more concentrated (fewer solutes) to where it is less concentrated (more solutes). Osmosis is just diffusion applied to water, so it is still passive.
• Facilitated diffusion. Movement down the gradient but through a protein channel, used by substances that cannot cross the lipid directly (such as glucose or ions). It still needs no energy, because it follows the gradient; the protein just provides a path.

Active transport: energy required

Larger movements also cost energy. Endocytosis brings large materials into the cell by wrapping them in membrane (forming a vesicle), and exocytosis releases materials by fusing a vesicle with the membrane. Both move bulk material and require energy, so they are forms of active transport.

Tonicity and its effect on cells

The same solution affects a cell depending on how its solute concentration compares to the cell's. Water always moves toward the higher solute concentration.

Plant cells respond differently because of their cell wall: in a hypotonic solution they become firm (turgid) rather than bursting, which is the healthy state for a plant; in a hypertonic solution the contents pull away from the wall (plasmolysis) and the plant wilts.

Try this

Q1. State two differences between passive and active transport. [2]

• Cue. Passive moves down the gradient and needs no energy; active moves up the gradient (low to high) and needs ATP (and a protein pump).

Q2. Explain why osmosis is classed as passive transport. [2]

• Cue. Osmosis is the diffusion of water down its concentration gradient (toward higher solute), so it needs no energy, which is the definition of passive transport.