What is the primary difference between simple and facilitated diffusion?

Simple diffusion occurs directly through the phospholipid bilayer without help, while facilitated diffusion requires specific transport proteins (channels or carriers) to move substances across the membrane.

How do carrier proteins differ from channel proteins in facilitated diffusion?

Channel proteins act as fixed pores or gates for ions, whereas carrier proteins must bind to a specific molecule and undergo a conformational change (shape change) to transport the substance.

Why is facilitated diffusion considered a passive process despite using proteins?

It is passive because substances move down their concentration gradient (from high to low) and the process does not require metabolic energy in the form of ATP.

What is a common misconception regarding 'equilibrium' in diffusion?

A common error is thinking that molecular movement stops at equilibrium. In reality, molecules continue to move randomly, but there is no further 'net' change in concentration.

Why can't glucose cross the membrane via simple diffusion?

Glucose is a large, polar molecule. It is too big to fit between phospholipids and its polarity makes it insoluble in the hydrophobic fatty acid tails of the bilayer.

What happens to the rate of diffusion if the membrane thickness is doubled?

According to Fick's Law, the rate of diffusion is inversely proportional to thickness; therefore, doubling the thickness would halve the rate of diffusion.

Define 'Net Movement' in the context of diffusion.

Net movement is the overall direction of particle flow, calculated as the difference between particles moving in one direction and those moving in the opposite direction.

What does it mean for a transport protein to be 'specific'?

Specificity means the protein has a unique binding site or pore shape that only allows one particular type of molecule or ion to pass through.

State the proportional relationship described by Fick's Law.

The rate of diffusion is proportional to $(\text{Surface Area} \times \text{Concentration Difference}) \div \text{Thickness of Membrane}$.

How does temperature affect the rate of facilitated diffusion?

Higher temperatures increase the kinetic energy of molecules, leading to faster movement and a higher frequency of successful bindings with carrier proteins, thus increasing the rate.

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Diffusion & Facilitated Diffusion

Summary

Diffusion is the passive movement of substances down a concentration gradient, occurring either directly through the lipid bilayer or via specialized transport proteins. This process is fundamental to cellular homeostasis, allowing for the exchange of gases, nutrients, and ions without the expenditure of metabolic energy.

1. Definition & Core Concepts

Diffusion is defined as the net movement of a substance from a region of higher concentration to a region of lower concentration. This movement is driven by the intrinsic kinetic energy of molecules or ions, which causes them to move randomly in all directions.

The term net movement refers to the overall direction of the particles; while individual molecules move randomly, the statistical trend is toward the area of lower concentration until equilibrium is reached.

As a passive process, diffusion does not require energy in the form of ATP from the cell. It relies entirely on the concentration gradient existing across a space or a membrane.

Diagram showing simple diffusion of small molecules through the lipid bilayer and facilitated diffusion of larger molecules through a transport protein.

2. Simple Diffusion

Simple diffusion occurs when molecules pass directly through the phospholipid bilayer of the cell membrane. This is only possible for substances that are compatible with the hydrophobic core of the membrane.

Molecules that utilize this pathway are typically small and non-polar, such as oxygen ( $O_2$ ) and carbon dioxide ( $CO_2$ ). Their lack of charge allows them to interact with and pass through the non-polar fatty acid tails of the phospholipids.

Small polar molecules, like water, can sometimes pass through the bilayer due to their size, but the rate is significantly slower compared to non-polar substances.

3. Facilitated Diffusion

4. Transport Protein Types

5. Factors Affecting Diffusion Rate

6. Key Distinctions

7. Exam Strategy & Tips

Diffusion & Facilitated Diffusion

Summary

1. Definition & Core Concepts

As a passive process, diffusion does not require energy in the form of ATP from the cell. It relies entirely on the concentration gradient existing across a space or a membrane.

Diagram showing simple diffusion of small molecules through the lipid bilayer and facilitated diffusion of larger molecules through a transport protein.

2. Simple Diffusion

Small polar molecules, like water, can sometimes pass through the bilayer due to their size, but the rate is significantly slower compared to non-polar substances.

3. Facilitated Diffusion

Facilitated diffusion is the movement of substances across a membrane via specific transport proteins. This mechanism is required for molecules that are too large or too highly charged (polar/ions) to pass through the lipid bilayer directly.

This process remains passive because the substances still move down their concentration gradient. The proteins merely provide a shielded pathway through the hydrophobic membrane interior.

Commonly transported substances include glucose, amino acids, and ions like sodium ( $Na^+$ ) or chloride ( $Cl^-$ ).

4. Transport Protein Types

Channel Proteins: These act as water-filled pores that extend through the membrane. They are often 'gated,' meaning they can open or close to regulate the flow of specific ions based on cellular signals.
Carrier Proteins: These proteins have a specific binding site for a molecule. When the molecule binds, the protein undergoes a conformational change (changes shape) to flip the molecule to the other side of the membrane.

Both types of proteins are highly specific, meaning a glucose carrier will not transport amino acids, and a sodium channel will not allow potassium to pass.

5. Factors Affecting Diffusion Rate

The rate of diffusion is governed by several physical variables, often summarized by Fick's Law. The rate is directly proportional to the surface area and the concentration gradient, but inversely proportional to the thickness of the membrane.

Temperature increases the rate of diffusion by providing more kinetic energy to the particles, causing them to move and collide more frequently.

The properties of the substance also matter; smaller molecules diffuse faster than larger ones, and non-polar molecules diffuse faster through the bilayer than polar ones.

6. Key Distinctions

Feature	Simple Diffusion	Facilitated Diffusion
Protein Required	No	Yes (Channel or Carrier)
Substance Type	Small, Non-polar	Large, Polar, or Ions
Energy (ATP)	No	No
Direction	Down Gradient	Down Gradient

It is critical to distinguish facilitated diffusion from active transport. While both use proteins, facilitated diffusion is passive (down gradient), whereas active transport requires ATP to move substances against a gradient.

7. Exam Strategy & Tips

Identify the Molecule: If an exam question mentions an ion (like $Na^+$ ) or a large sugar (like glucose), always look for 'facilitated diffusion' or 'active transport' rather than simple diffusion.
Check the Gradient: Always determine if the movement is from high-to-low (passive) or low-to-high (active). This is the most common point of confusion.
Fick's Law Application: If a cell is adapted for fast diffusion (like an alveolus), explain it using Fick's Law: mention the large surface area, the short diffusion pathway (thin walls), and the steep concentration gradient (maintained by blood flow).

This process remains passive because the substances still move down their concentration gradient. The proteins merely provide a shielded pathway through the hydrophobic membrane interior.

Commonly transported substances include glucose, amino acids, and ions like sodium ( $Na^+$ ) or chloride ( $Cl^-$ ).

Identify the Molecule: If an exam question mentions an ion (like $Na^+$ ) or a large sugar (like glucose), always look for 'facilitated diffusion' or 'active transport' rather than simple diffusion.
Check the Gradient: Always determine if the movement is from high-to-low (passive) or low-to-high (active). This is the most common point of confusion.
Fick's Law Application: If a cell is adapted for fast diffusion (like an alveolus), explain it using Fick's Law: mention the large surface area, the short diffusion pathway (thin walls), and the steep concentration gradient (maintained by blood flow).