Cell Structure and Function · Grade 10

How is the cell membrane built and what does it decide?

The membrane is a double layer of phospholipids: the water-loving heads face outwards and inwards, the water-repelling tails hide in the middle. Proteins float within that layer and let through substances the fat alone would block. The result is a selective boundary — neither a wall nor an opening.

Learning objectives

Two ends, two behaviours

A phospholipid molecule has a head that is drawn to water and two tails that repel it. In a watery environment they arrange themselves: heads facing the water on both sides, tails pushed inwards towards each other.

That produces a double layer without anybody building it — it is simply the most stable arrangement. It is also why a torn membrane closes itself again.

The result is a barrier to anything that is drawn to water. Ions and sugars do not pass through the fatty tail region, while small uncharged molecules such as oxygen cross without difficulty.

The fluid mosaic model

Mosaic, because the layer is studded with many proteins and also with cholesterol and sugars — it is not one uniform substance.

Fluid, because none of it is fixed in place. Phospholipids and proteins drift within the plane like boats on a lake, and that is what lets the membrane change shape, fold, and engulf vesicles.

What the proteins do

Channel proteins form a watery passage through which ions can move. Carrier proteins bind a molecule, change shape and deliver it to the other side. Others act as receptors or as anchors for structure.

Without proteins the membrane would be an almost total barrier. They are what turns it from a wall into something selective — the cell chooses what enters, rather than merely blocking whatever is too large.

For a cell living in fresh water this is critical. There is almost nothing dissolved outside it, and without control over what comes and goes it would fill with water and burst.

Worked examples

  1. Why does oxygen cross the membrane easily and a sodium ion not?

    1. The centre of the membrane is fatty and repels water
    2. Oxygen is a small uncharged molecule, so it passes
    3. A sodium ion is charged and surrounded by water, so it is repelled

    Answer: Oxygen by free diffusion; sodium only through a protein

  2. What does fluid mean in the model, if the membrane holds the cell's shape?

    1. It is not fluid in the sense of pouring away
    2. Its components move within its plane
    3. Stability comes from the arrangement, not from being fixed

    Answer: The components move within it; it remains a layer

  3. A membrane is pierced by a fine needle. What happens?

    1. The fatty tails are exposed to water at the edges of the hole
    2. That is an unstable state
    3. The molecules rearrange and close it

    Answer: The hole seals itself — by the same force that built the layer

Common mistakes

Drawing the membrane as a solid wall
It is selectively permeable: some substances cross freely, others only through a protein, others not at all. A cell inside a solid wall would be dead.
Reversing the heads and tails
The heads are drawn to water, so they face out and in, towards the watery surroundings on both sides. The tails hide in the middle.
Treating membrane proteins as decoration
They are what lets the cell choose. Without them there is no sugar uptake, no ion balance, and no receiving of signals.

What to remember

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