Cell Transport Diffusion

This section explains diffusion. Covering, Ficks law of diffusion, surface area to volume ratios and adaptions to maximise diffusion. 

Cell transport is a fundamental topic in biology, focusing on how substances move into and out of cells. This movement occurs via several processes, including diffusion, osmosis, and active transport. In this guide, we will focus on diffusion and its relationship with key concepts such as Fick's law, surface area to volume ratios, and adaptations in organisms.

Diffusion

Definition:
Diffusion is the movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient, until equilibrium is reached. It is a passive process, meaning it does not require energy.

Examples of diffusion:

• Oxygen moving into cells from the bloodstream.
• Carbon dioxide moving out of cells into the bloodstream.
• Nutrients like glucose entering cells from the surrounding fluid.

Key Points:

• Concentration gradient: The difference in concentration of a substance between two areas. The steeper the gradient, the faster the rate of diffusion.
• Temperature: Higher temperatures increase the rate of diffusion as particles move more quickly.
• Particle size: Smaller particles diffuse faster than larger ones.

A Protist called Amoeba can absorb oxygen through diffusion.

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Fick’s Law of Diffusion

Fick's Law is used to describe the rate of diffusion and how it is affected by different factors.

Fick’s Law Formula

$$\text{Rate of Diffusion} = \frac{\text{Surface Area} \times \text{Concentration Gradient}}{\text{Thickness of the Membrane}} $$

Explanation of each factor:

• Surface Area: A larger surface area allows for more particles to move across the membrane at once, thus increasing the rate of diffusion.
• Concentration Gradient: The greater the difference in concentration between two areas, the faster the rate of diffusion.
• Thickness of the Membrane: The thinner the membrane, the faster diffusion can occur, as the particles don’t have to travel as far.

Surface Area to Volume Ratios

The surface area to volume ratio (SA:V) is an important concept when discussing diffusion in cells and organisms. The ratio influences the rate at which substances can diffuse into or out of the cell.

Why is it important?

• Small cells or organisms have a high SA:V ratio, meaning they can exchange materials more efficiently.
• Larger organisms or cells have a low SA:V ratio, meaning diffusion is less efficient.

Example:
A small cube with side length 1 cm has a surface area of 6 cm² and a volume of 1 cm³, giving it a surface area to volume ratio of 6:1. If the cube's size increases to 10 cm, its surface area increases to 600 cm², but its volume increases to 1000 cm³, giving it a ratio of just 0.6:1.

Adaptations to Maximise Diffusion

Many organisms have evolved adaptations that maximise diffusion by increasing surface area or improving concentration gradients. Some of these adaptations include:

• Alveoli in the lungs: These tiny air sacs have a large surface area and a thin wall to maximise the diffusion of gases (oxygen and carbon dioxide) between the lungs and the bloodstream.

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• Root hair cells in plants: These cells have long, thin extensions (root hairs) that increase the surface area for the absorption of water and minerals from the soil.
• Intestinal villi: In the small intestine, finger-like projections (villi) increase the surface area for the absorption of nutrients from digested food.
• Fish gills: Fish gills have a large surface area and a very thin membrane, allowing for the efficient exchange of gases between the water and the bloodstream.

Recap: Factors Affecting Diffusion

• Concentration Gradient: A higher concentration gradient leads to faster diffusion.
• Surface Area: Larger surface areas lead to more particles being able to diffuse across the membrane.
• Temperature: Higher temperatures increase particle movement, speeding up diffusion.
• Membrane Thickness: Thinner membranes allow for faster diffusion.

Key Points to Remember

• Diffusion is a passive process that happens down a concentration gradient.
• Fick’s Law describes the factors affecting the rate of diffusion.
• Surface area to volume ratio is crucial for effective diffusion in cells and organisms.
• Adaptations in cells and organisms maximise diffusion efficiency to meet their metabolic needs.

By understanding these key concepts, you'll be able to explain the principles behind how substances move into and out of cells, and how organisms have evolved to make these processes as efficient as possible.