Simple and Facilitated Diffusion
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of membrane structure and the concept of concentration gradients. You can test your knowledge on these below.
Describe the fluid mosaic model?
- Phospholipids move laterally in the membrane.
- Transport proteins and extrinsic proteins are embeded within the membrane.
Define the term ‘concentration gradient’?
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A difference in concentration of a substance between two areas.
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Molecules move from a region of high concentration to low concentration (down the gradient) by diffusion.
Describe why phospholipids form a bilayer.
Hydrophilic phosphate heads face towards water (they are attracted to water)
Hydrophobic fatty acid tails face away from water (they repel water)
Topic Explainer Video
Check out this @LauraDoesBiology video that explains simple and facilitated diffusion or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Simple Diffusion
Definition: The passive movement of small, non-polar molecules across the phospholipid bilayer without the need for energy from ATP or transport proteins.
Key Features:
Occurs down the concentration gradient (from higher to lower concentration).
Does not require energy from ATP (passive process).
The rate of diffusion depends on:
- Concentration gradient: Steeper concentration gradients increase diffusion rate.
- Surface area: Larger surface area increases diffusion rate.
- Thickness of membrane: Membranes that are 1 cell thick/ thinner allow faster diffusion.
- Temperature: Higher temperature increases kinetic energy of particles, speeding up diffusion.
Examples:
- Movement of oxygen and carbon dioxide across alveolar membranes in the lungs.
- Absorption of glycerol and fatty acids in the intestines.
Facilitated Diffusion
Facilitated Diffusion
The passive transport of larger, polar, or charged molecules across the membrane via channel or carrier proteins.
Key Features:
Still passive (does not require energy from ATP).
Relies on specific transport proteins embedded in the membrane.
Types of transport proteins:
- Channel Proteins: Form hydrophilic pores for charged ions (e.g., Na⁺, K⁺).
- Carrier Proteins: Change shape to transport larger molecules across the membrane (e.g., glucose or amino acids).
Transport proteins can be opened and closed to control the movement of substances in and out of the cell.
Factors Affecting Rate:
- Number of transport proteins: More transport proteins increase diffusion rate.
- Concentration gradient: Steeper concentration gradients increase diffusion rate.
- Surface area: Larger surface area increases diffusion rate. There is more surface area for an increased number of transport proteins.
- Thickness of membrane: Membranes that are 1 cell thick/ thinner allow for faster diffusion.
- Temperature: Higher temperature increases kinetic energy of particles, speeding up diffusion.
Examples:
- Glucose uptake in cells via carrier proteins called GLUT transporters.
- Ion transport in nerve cells via voltage-gated channel proteins.
Key Terms
- Diffusion: Passive movement of particles from high to low concentration.
- Simple Diffusion: Direct passage of small, non-polar molecules cross the membrane.
- Facilitated Diffusion: Transport of larger and polar molecules via membrane transport proteins.
- Channel Protein: Protein pores allowing specific ions to pass.
- Carrier Protein: Proteins that change shape to move molecules.
- Concentration Gradient: Difference in concentration between two areas.
No answer provided.
Exam Tips
Lots of questions link into the factors that affect diffusion. Always have diffusion on your mind if you see any of the factors in the question.
Always state the direction and type of diffusion for particles. For example, glucose moves into the cells by facilitated diffusion.
Thickness of the membrane refers to the tissue membrane, how many cells thick it is, not the cell membrane itself. Don't make the mistake of saying that the cell membrane or cell wall is thinner.
No answer provided.
Glucose cannot pass through a cell membrane by simple diffusion. Explain how glucose is transported into a cell. (3 marks)
- Glucose is transported by facilitated diffusion.
- It moves down its concentration gradient.
- It passes through a specific carrier protein in the membrane.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!