# Membrane Structure and Transport

> IB Biology HL · IB Biology HL (2025+ syllabus)
> Source: https://www.owlsprep.com/study/ib-biology-hl-u4-core-cells/

This sub-topic explores the structure of cell membranes and the mechanisms cells use to move substances across phospholipid bilayers, a core concept for understanding cell homeostasis and function in IB Biology HL.

**Prerequisites:** [Basic cell structure and organelles](https://www.owlsprep.com/study/ib-biology-hl-u4-core-cells-introduction/); Water polarity and hydrophobic interactions

## Learning objectives

- Describe the fluid mosaic model of cell membrane structure
- Distinguish between passive and active transport across membranes
- Explain osmosis in terms of solute concentration and water potential
- Apply membrane transport concepts to answer exam experimental questions

## Fluid Mosaic Model of Membrane Structure

**Fluid Mosaic Model** — Model describing cell membranes as a dynamic structure composed of a phospholipid bilayer with embedded and peripheral proteins, cholesterol, and carbohydrates. 'Fluid' refers to lateral movement of components, 'mosaic' refers to the mixed molecular composition.

*Example:* Human red blood cell membranes are ~52% protein and 40% lipid by weight, matching the model's mixed composition.

Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. This property spontaneously drives bilayer formation in aqueous environments, with hydrophobic fatty acid tails tucked into the membrane interior away from water.

- **Integral proteins**: Embedded through the entire bilayer, often act as channels or cell signalling receptors.
- **Peripheral proteins**: Attached to the bilayer surface, involved in signalling and structural support.
- **Cholesterol**: Modulates membrane fluidity across different temperature ranges.
- **Glycoproteins/Glycolipids**: Carbohydrate-modified molecules used for cell-cell recognition.

**Worked example:** Explain why the phospholipid bilayer is the most thermodynamically stable arrangement for cell membranes.

1. Step 1: Recall the amphipathic property of phospholipids:
2. Phospholipids have a charged, hydrophilic phosphate head that interacts favourably with polar water, and uncharged hydrophobic fatty acid tails that avoid interaction with water.
3. Step 2: Eliminate unstable arrangements: Any arrangement that exposes hydrophobic tails to water is energetically unfavourable and will not persist.
4. Step 3: Confirm bilayer stability: In the bilayer, all hydrophilic heads face the aqueous extracellular and intracellular environments, while all hydrophobic tails are sequestered in the membrane interior, making this the lowest energy, most stable arrangement.

> **Exam tip:** Always link the bilayer arrangement to the amphipathic nature of phospholipids when asked to explain its formation in exams.

## Passive Transport Across Membranes

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

*Example:* Oxygen diffusing from alveoli into blood capillaries is a type of passive transport.

Passive transport relies on the natural kinetic energy of molecules, which causes them to spread randomly from areas of higher concentration to lower concentration. There are two main types of passive transport:

- **Simple diffusion**: Small non-polar molecules (e.g. O₂, CO₂, steroid hormones) diffuse directly through the hydrophobic core of the bilayer.
- **Facilitated diffusion**: Large polar molecules or charged ions cannot pass through the hydrophobic core, so they diffuse through specific channel or carrier proteins down their concentration gradient.

**Worked example:** A plant cell is placed into a hypertonic solution (higher solute concentration than cell cytoplasm). Predict and explain the outcome.

1. Step 1: Define osmosis: net movement of free water across a selectively permeable membrane from lower to higher solute concentration.
2. Step 2: Compare concentrations: The external hypertonic solution has a higher solute concentration than the cell cytoplasm.
3. Step 3: Predict net movement: Net movement of water will be out of the plant cell, across the cell membrane into the surrounding solution.
4. Step 4: Explain the final state: As water leaves the vacuole and cytoplasm, cell volume decreases. The flexible cell membrane pulls away from the rigid cell wall, a process called plasmolysis.

## Active Transport and Bulk Transport

**Active Transport** — Movement of substances across a membrane against their concentration gradient (from lower to higher concentration), which requires ATP energy and specific carrier proteins called pumps.

*Example:* The sodium-potassium pump maintains resting potential in neurons by moving 3 Na⁺ out and 2 K⁺ in against their gradients.

Bulk transport is a type of active transport that moves large volumes of substances or whole particles across the membrane using membrane-bound vesicles. Endocytosis brings substances into the cell, while exocytosis removes substances from the cell.

**Worked example:** Explain why glucose cannot be transported against its concentration gradient via facilitated diffusion, and what mechanism is used instead.

1. Step 1: Recall the properties of facilitated diffusion: It is a passive process that only allows movement down a concentration gradient, relying on natural kinetic energy of molecules with no ATP input.
2. Step 2: Explain why it cannot work against the gradient: Moving against the gradient requires energy input to counteract the natural tendency of molecules to diffuse down the gradient. Facilitated diffusion does not supply this energy.
3. Step 3: State the correct mechanism: Glucose is transported against its gradient via active transport, using specific glucose pump proteins that hydrolyse ATP to provide the required energy.

> **tip**
>
> Always explicitly link the need for ATP to the movement against the concentration gradient when describing active transport in extended response questions.

## Common pitfalls

- **Wrong:** Claiming phospholipid fatty acid tails are hydrophilic
  - Why it fails: Common confusion between the properties of the head and tail of phospholipids
  - Correct: State that the phosphate head is hydrophilic, and the fatty acid tails are hydrophobic
- **Wrong:** Defining osmosis as movement of water from high to low water concentration
  - Why it fails: Ignores the requirement of a selectively permeable membrane and can be misleading for water potential calculations
  - Correct: Define osmosis as net movement of free water across a selectively permeable membrane from lower to higher solute concentration
- **Wrong:** Stating facilitated diffusion requires ATP energy
  - Why it fails: Confusion between facilitated diffusion (passive) and active transport; all diffusion is passive
  - Correct: Remember that channel/carrier proteins in facilitated diffusion only enable movement down the gradient, no ATP is required
- **Wrong:** Claiming cholesterol always increases membrane fluidity
  - Why it fails: Cholesterol has a temperature-dependent dual role that is often misremembered
  - Correct: State that cholesterol reduces fluidity at high temperatures (restricting phospholipid movement) and increases fluidity at low temperatures (preventing tight packing of tails)
- **Wrong:** Claiming all membrane proteins can move freely laterally
  - Why it fails: Overgeneralization of the 'fluid' property of the membrane model
  - Correct: Note that while most components move freely, some proteins are anchored to the cytoskeleton and are fixed in position

## Cheatsheet

| Membrane Component | Core Role | Transport Type | Key Features |
| --- | --- | --- | --- |
| Phospholipid bilayer | Basic membrane structure | Simple diffusion | Passive, down gradient, small non-polar |
| Integral channel protein | Facilitated transport | Facilitated diffusion | Passive, down gradient, polar/charged |
| Carrier pump | Active transport | Active transport | Requires ATP, against gradient |
| Cholesterol | Modulate fluidity | Endocytosis | Bulk transport into cell, ATP required |
| Glycoprotein | Cell-cell recognition | Exocytosis | Bulk transport out of cell, ATP required |

## What's next

Understanding membrane structure and transport is foundational for almost all subsequent topics in IB Biology HL, from cell signalling to osmoregulation and plant physiology. This topic appears frequently in both Paper 1 multiple choice and Paper 2 extended response, with common questions asking you to apply knowledge to novel experimental scenarios. Mastery of key definitions and mechanisms here will earn you easy marks and support your understanding of more complex topics later in the syllabus.

- [Core: Classification and Cladistics of Organisms](https://www.owlsprep.com/study/ib-biology-hl-u4-core-organisms/)
- [Core: Ecosystems](https://www.owlsprep.com/study/ib-biology-hl-u4-core-ecosystems/)
- [AHL: Nucleic acid structure](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-nucleic-acid-structure/)

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