What is the primary difference between integral and peripheral proteins?

Integral proteins are permanently embedded within the phospholipid bilayer, often spanning its entire width (transmembrane), whereas peripheral proteins are temporarily attached to the inner or outer surface of the membrane. Integral proteins usually function in transport or signaling, while peripheral proteins often serve as enzymes or structural supports.

How does facilitated diffusion differ from active transport?

Facilitated diffusion is a passive process that moves substances down their concentration gradient through protein channels or carriers without using energy. Active transport moves substances against their concentration gradient and requires metabolic energy in the form of ATP to power carrier proteins.

What is the difference between a glycolipid and a glycoprotein?

A glycolipid consists of a carbohydrate chain covalently bonded to a lipid molecule, while a glycoprotein consists of a carbohydrate chain bonded to a protein. Both are found on the outer surface of the membrane and contribute to the glycocalyx for cell recognition.

Why is it incorrect to say the cell membrane is 'semi-permeable'?

The term 'semi-permeable' implies the membrane only filters substances based on size (like a sieve). 'Partially permeable' or 'selectively permeable' is more accurate because the membrane selects substances based on size, charge, and lipid solubility, often using specific transport proteins.

What is a common misconception about the role of cholesterol in the membrane?

A common error is thinking cholesterol only makes the membrane rigid. In reality, it acts as a fluidity buffer; it prevents the membrane from becoming too fluid at high temperatures and prevents it from freezing/solidifying at low temperatures by disrupting tight phospholipid packing.

What error occurs when students describe the movement of water across a membrane?

Students often forget that water moves from a region of higher water potential to lower water potential. They may incorrectly describe it as moving from 'high concentration' without specifying 'water concentration' or 'water potential', which can lead to confusion with solute movement.

Define the 'Fluid Mosaic Model'.

It is a model describing the cell membrane as a fluid phospholipid bilayer in which various proteins are embedded (mosaic). The 'fluid' part indicates that both lipids and proteins can move laterally within the plane of the membrane.

What is the 'Glycocalyx'?

The glycocalyx is the outer layer of the cell membrane formed by the carbohydrate side chains of glycoproteins and glycolipids. It plays a vital role in cell recognition, adhesion, and acting as a receptor site for chemical signals.

What does 'Amphipathic' mean in the context of the cell membrane?

Amphipathic refers to a molecule that possesses both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region. Phospholipids are amphipathic, which allows them to form the bilayer structure essential for membrane integrity.

Why can't ions like $Na^+$ pass directly through the phospholipid bilayer?

Ions are highly charged and hydrophilic, making them insoluble in the hydrophobic interior of the phospholipid bilayer. They are repelled by the fatty acid tails and therefore require specific channel or carrier proteins to cross the membrane.

The Cell Surface Membrane | AQA AS-Level Biology

1. Definition & Core Concepts

The cell surface membrane is an ultra-thin boundary (approximately 7-10 nm thick) that encloses the cytoplasm of all living cells, acting as a selective barrier.
It is described by the Fluid Mosaic Model, where 'fluid' refers to the lateral movement of phospholipids and proteins, and 'mosaic' refers to the random arrangement of various protein molecules embedded within the lipid bilayer.
The membrane is partially permeable, meaning it allows certain small or non-polar molecules to pass through freely while restricting the movement of large or highly charged ions.

Diagram of the Fluid Mosaic Model showing the phospholipid bilayer, an integral protein, and cholesterol molecules.

2. The Phospholipid Bilayer

Phospholipids are amphipathic molecules, containing a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails.
In an aqueous environment, these molecules spontaneously arrange into a bilayer, with heads facing the outward watery environments and tails shielded in the interior.
This arrangement creates a hydrophobic core that prevents the free passage of polar substances, such as glucose and ions, while allowing lipid-soluble molecules like oxygen and carbon dioxide to diffuse through.

3. Membrane Proteins and Carbohydrates

Integral proteins span the entire bilayer and often act as transport channels or carriers for specific molecules that cannot cross the lipid core.
Peripheral proteins are attached to the surface of the membrane and often serve as enzymes or structural anchors for the cytoskeleton.
Glycoproteins and glycolipids (carbohydrates attached to proteins or lipids) form the glycocalyx, which is essential for cell-to-cell recognition and acting as receptor sites for hormones and neurotransmitters.

4. Role of Cholesterol

Cholesterol molecules are positioned between the fatty acid tails of phospholipids, playing a critical role in regulating membrane fluidity.
At high temperatures, cholesterol restricts the movement of phospholipids, preventing the membrane from becoming too fluid and losing its structural integrity.
At low temperatures, it prevents the tails from packing too tightly together, ensuring the membrane does not solidify and remains functional.

5. Key Distinctions: Transport Mechanisms

Feature	Simple Diffusion	Facilitated Diffusion	Active Transport
Energy Required	No (Passive)	No (Passive)	Yes (ATP)
Direction	Down gradient	Down gradient	Against gradient
Protein Needed	No	Yes (Channel/Carrier)	Yes (Carrier/Pump)
Substances	Small, non-polar	Large, polar, ions	Ions, specific sugars

Simple diffusion occurs directly through the phospholipid bilayer for molecules like $O_2$ and $CO_2$ .
Facilitated diffusion utilizes specific transmembrane proteins to move polar molecules like glucose or $Na^+$ ions without expending energy.

6. Exam Strategy & Tips

Terminology Precision: Always use the term 'partially permeable' rather than 'semi-permeable' when describing biological membranes, as they are selective based on more than just size.
Fluidity Factors: When asked about membrane permeability changes, consider both temperature (which increases kinetic energy) and the presence of organic solvents (which dissolve lipids).
Component Identification: In diagrams, identify cholesterol by its position between tails, and glycoproteins by the branched carbohydrate chains extending into the extracellular space.
Check the Gradient: Before deciding if a process is active or passive, always identify the concentration on both sides of the membrane; movement 'against' the gradient always requires ATP.

The Cell Surface Membrane

Summary

1. Definition & Core Concepts

2. The Phospholipid Bilayer

3. Membrane Proteins and Carbohydrates

4. Role of Cholesterol

5. Key Distinctions: Transport Mechanisms

6. Exam Strategy & Tips

The Cell Surface Membrane

Summary

1. Definition & Core Concepts

2. The Phospholipid Bilayer

3. Membrane Proteins and Carbohydrates

4. Role of Cholesterol

5. Key Distinctions: Transport Mechanisms

6. Exam Strategy & Tips