# Cell membranes and transport

> IB Biology SL · Theme A: Unity and Diversity
> Source: https://www.owlsprep.com/study/ib-biology-sl-u1-cell-membranes-and-transport/

This module covers the fluid mosaic model of cell membrane structure, and all major mechanisms of substance transport across membranes, including both passive and active processes aligned with IB Biology SL requirements.

**Prerequisites:** [Basic cell structure](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-theory-introduction/); [Biological molecule properties](https://www.owlsprep.com/study/ib-biology-sl-intro-biological-molecules/)

## Learning objectives

- Describe the fluid mosaic model of cell membrane structure
- Explain the role of each membrane component
- Distinguish between passive and active transport mechanisms
- Predict osmotic outcomes for animal and plant cells in different solute concentrations

## 1. The Fluid Mosaic Model of Membrane Structure

**Fluid Mosaic Model** — Describes the cell membrane as a dynamic phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol. Most components can move laterally within the bilayer, giving the membrane fluidity.

*Example:* Amphipathic phospholipids arrange with hydrophilic phosphate heads facing aqueous cytoplasm/extracellular fluid, and hydrophobic fatty acid tails facing inward.

- Cholesterol modulates fluidity: restricts movement at high temperatures, prevents packing at low temperatures
- Integral proteins span the full bilayer; peripheral proteins bind to the membrane surface
- Glycoproteins/glycolipids function in cell recognition and signaling

**Worked example:** Predict how a lack of cholesterol would affect an animal cell membrane at high temperatures.

1. Recall the core function of cholesterol in animal cell membranes
2. At high temperatures, cholesterol normally restricts movement of fatty acid tails in the bilayer
3. If cholesterol is absent, there is no restriction on molecular movement within the bilayer
4. Conclusion: The membrane becomes overly fluid, loses structural integrity, and has uncontrolled permeability

> **tip**
>
> IB exam questions frequently ask you to label and explain the role of each membrane component, so memorize each component's function.

*Calculator:* forbidden

## 2. Passive Transport: Simple Diffusion and Osmosis

**Passive Transport** — Net movement of substances across a membrane down their concentration or water potential gradient, that does not require ATP energy input from the cell.

Simple diffusion occurs for small, non-polar molecules that can pass directly through the hydrophobic phospholipid bilayer. Common examples include oxygen, carbon dioxide, and steroid hormones.

**Osmosis** — Net passive movement of free water molecules across a semipermeable membrane, from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).

**Worked example:** A human red blood cell with 0.9% solute concentration is placed into a 10% saline solution. Predict and explain the outcome.

1. Classify the tonicity: the extracellular solution is hypertonic to the red blood cell cytoplasm
2. Water potential is higher inside the cell than outside the cell
3. By osmosis, free water moves out of the cell down the water potential gradient
4. Outcome: The red blood cell loses water and undergoes crenation (shriveling)

**Check your understanding**

1. Which substance crosses cell membranes by simple diffusion?

   - A) Glucose
   - B) Oxygen
   - C) Sodium ions
   - D) Insulin

   *Why:* Glucose is too large/polar, sodium ions are charged, and insulin is a large protein: all cannot cross via simple diffusion. Only small non-polar oxygen diffuses directly through the bilayer.

*Calculator:* forbidden

## 3. Facilitated Diffusion

**Facilitated Diffusion** — Passive movement of large, polar, or charged substances down their concentration gradient, via specific channel or carrier proteins embedded in the membrane.

The hydrophobic core of the phospholipid bilayer repels polar and charged molecules, so these substances cannot cross via simple diffusion, even when moving down their gradient. Channel proteins form hydrophilic pores for specific ions, while carrier proteins change shape to transport larger molecules like glucose.

**Worked example:** Explain why glucose cannot cross cell membranes via simple diffusion.

1. Glucose is a large polar molecule
2. The hydrophobic core of the phospholipid bilayer repels all polar molecules
3. Glucose cannot pass directly through the hydrophobic core
4. It must therefore use a specific carrier protein for facilitated diffusion, even when moving down its concentration gradient

> **info**
>
> Many ion channel proteins are gated: they open and close in response to chemical or electrical signals, which is critical for nerve cell function.

*Calculator:* forbidden

## 4. Active Transport and Bulk Transport

**Active Transport** — Movement of substances across a membrane against their concentration gradient, which requires ATP energy and specific transmembrane protein pumps.

The most well-studied example is the sodium-potassium pump in animal cells, which moves 3 sodium ions out of the cell and 2 potassium ions into the cell against their gradients, using one ATP per cycle. Bulk transport (endocytosis and exocytosis) is also an active process that moves large volumes of material via membrane vesicles.

**Worked example:** Iodine concentration in thyroid gland cells is 100x higher than in blood plasma. What transport mechanism moves iodine into thyroid cells, and why is this required?

1. Iodine is moving against its concentration gradient (higher inside the cell than outside)
2. Movement against a concentration gradient requires energy input in the form of ATP
3. This process is carried out by a specific membrane protein pump
4. Conclusion: The mechanism is active transport, required to accumulate iodine against its gradient

Endocytosis brings material into the cell (phagocytosis for large particles, pinocytosis for fluid), while exocytosis releases material from the cell, for example insulin secretion from pancreatic beta cells.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Stating osmosis is the movement of solute down a concentration gradient.
  - Why it fails: Osmosis specifically describes the movement of free water molecules, not solute.
  - Correct: Osmosis is the net movement of free water across a semipermeable membrane from higher to lower water potential.
- **Wrong:** Claiming facilitated diffusion requires ATP because it uses proteins.
  - Why it fails: All passive transport, including facilitated diffusion, moves down a gradient and does not need ATP.
  - Correct: Facilitated diffusion is passive, only requiring specific proteins to enable movement down the concentration gradient.
- **Wrong:** Predicting a plant cell placed in pure water will burst.
  - Why it fails: Plant cells have a rigid cell wall that resists osmotic pressure and swelling.
  - Correct: A plant cell placed in pure water will become turgid (firm), the cell wall prevents bursting.
- **Wrong:** Claiming all membrane proteins are free to move in the bilayer.
  - Why it fails: Some proteins are anchored to the cytoskeleton and are fixed in place.
  - Correct: Most membrane components move laterally, but not all are free to move in the fluid mosaic model.

## Cheatsheet

| Process | ATP Required? | Direction | Needs Protein? | Example |
| --- | --- | --- | --- | --- |
| Simple diffusion | No | Down gradient | No | O₂, CO₂ crossing membrane |
| Osmosis | No | Down water potential | No (aquaporins optional) | Water movement into plant roots |
| Facilitated diffusion | No | Down gradient | Yes | Glucose entering body cells |
| Active transport | Yes | Against gradient | Yes (pump) | Na⁺/K⁺ pump in nerve cells |
| Endo/Exocytosis | Yes | Any direction | No (uses vesicles) | Insulin secretion from pancreas |

## What's next

Understanding cell membranes and transport is foundational for almost all other topics in IB Biology, from cell signaling and division to physiological processes like gas exchange and nerve conduction. The principles of osmosis are key for understanding water movement in plants, while active transport underpins processes like glucose reabsorption in the kidney and electrical action potentials in neurons. This topic also includes a required core practical on osmosis that is frequently assessed in experimental questions on paper 2 and 3. Build on this knowledge with the related topics below.

- [Theme B: Form and Function](https://www.owlsprep.com/study/ib-biology-sl-u2-overview/)

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