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Diffusion Moves Particles Down a Concentration Gradient

Definition

Diffusion

The net movement of particles of a gas, or of a dissolved substance, from a region of higher concentration to a region of lower concentration; it is passive and needs no energy from respiration.

  1. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
  2. The difference in concentration between the two regions is called a concentration gradient.

Diagram of a concentration gradient across a cell membrane, showing a high concentration of particles on one side and a low concentration on the other.

  1. Particles have kinetic energy and move randomly, colliding and spreading out until they are evenly spread.
  2. Diffusion is passive, so it uses the particles' own kinetic energy and no energy from respiration.
  3. Both gases and dissolved substances, called solutes, can diffuse.
Key Idea

Diffusion always moves a substance down its concentration gradient, from where it is more concentrated to where it is less concentrated.

Factors That Affect the Rate of Diffusion

Definition

Concentration gradient

The difference in concentration of a substance between two regions; particles diffuse down a concentration gradient, from a higher to a lower concentration.

  1. Three factors change how fast a substance diffuses.
  2. Concentration gradient:
    1. The bigger the difference in concentration, the steeper the gradient and the faster the rate of diffusion.
  3. Temperature:
    1. The higher the temperature, the more kinetic energy the particles have, so they move and diffuse faster.
  4. Surface area of the membrane:
    1. The larger the surface area, the more particles can cross at once, so the faster the rate of diffusion.
Example

Oxygen diffuses faster into a cell that is respiring quickly, because using it up keeps a steep concentration gradient.

Diffusion Across the Cell Membrane

Definition

Cell membrane

The partially permeable layer around a cell that controls which substances move into and out of it.

  1. Cells are surrounded by a partially permeable cell membrane.
  2. It lets small molecules pass through but holds back larger ones, such as starch.

A diagram of a cell membrane showing selective permeability. Small nonpolar molecules like nitrogen and small polar molecules like water can pass through the phospholipid bilayer, while ions and large molecules like sucrose are blocked.

A diagram showing passive diffusion across a cell membrane. Small particles move from an area of high concentration to an area of low concentration by passing directly through the phospholipid bilayer.

  1. Small molecules such as oxygen, carbon dioxide, glucose and urea can diffuse across it.
  2. The concentration gradient sets the direction, so a substance diffuses from wherever it is more concentrated to wherever it is less concentrated.
  3. Oxygen diffuses into a respiring cell while carbon dioxide diffuses out of it at the same time.
Example

Urea made in the liver diffuses into the blood plasma and is carried to the kidneys to be excreted.

Substances Moved by Diffusion

  1. Oxygen diffuses into cells for aerobic respiration.
  2. Carbon dioxide diffuses into photosynthesising plant cells and out of respiring cells.
  3. The soluble products of digestion, such as glucose, diffuse from the gut into the blood.
  4. Urea, a waste product, diffuses out of cells into the blood for excretion by the kidney.
Note

The direction can reverse: a leaf cell releases carbon dioxide at night, but takes it in during bright daylight when photosynthesis is faster than respiration.

Surface Area to Volume Ratio

Definition

Surface area to volume ratio

The surface area of an organism compared with its volume; it decreases as an organism gets larger, so bigger organisms need exchange surfaces and transport systems.

  1. Whether diffusion alone can supply a cell depends on its surface area to volume ratio.
  2. A small organism, such as a single-celled bacterium, has a large surface area compared with its volume.
  3. Substances have only a short distance to travel, so diffusion, osmosis and active transport across its surface are enough to meet its needs.
  4. As an organism gets larger, its surface area to volume ratio gets smaller.
  5. You compare ratios by working out the surface area and the volume, then dividing the surface area by the volume.
Example
  • A 1 cm cube has a surface area of 6 cm² and a volume of 1 cm³, giving a ratio of 6:1.
  • A 2 cm cube has a surface area of 24 cm² and a volume of 8 cm³, giving a ratio of 3:1, so the larger cube has the smaller ratio.

A table and diagrams comparing the surface area to volume ratio of a small cube, a large cube, and a collection of small cubes. It shows that as size increases, the ratio decreases, but dividing a large volume into many small units restores a high surface area to volume ratio.

Exchange Surfaces in Larger Organisms

Definition

Exchange surface

A surface in a multicellular organism where substances are exchanged with the environment, adapted with a large surface area and a thin barrier for a short diffusion path.

  1. A large, multicellular organism has a small surface area to volume ratio and a long distance from its surface to its centre.
  2. Diffusion across its outer surface alone is too slow to supply every cell.
  3. So larger organisms have specialised exchange surfaces and transport systems.
  4. An effective exchange surface has a large surface area and a thin membrane for a short diffusion path.
  5. In animals it also has a good blood supply and, for gases, is ventilated, which keeps steep concentration gradients.
Note

The blood supply and ventilation keep a steep gradient by bringing fresh supplies to the surface and carrying exchanged substances away.

Adaptations of Exchange Surfaces

  1. Small intestine (animals):
    1. Villi and microvilli give a very large surface area for absorbing digested food.

A diagram of a single epithelial cell from the small intestine, showing microvilli on its surface which increase the surface area for absorption. The nucleus and mitochondria are also labeled.

  1. The lining is one cell thick, giving a short diffusion path, and a rich blood supply keeps a steep gradient.
  2. Lungs (mammals):
    1. Millions of alveoli give a huge surface area for gas exchange.
    2. Each alveolus wall is one cell thick and moist, and a good blood supply and ventilation maintain the gradient.

Illustration of alveoli in the lungs, showing their adaptation for gas exchange. A detailed view shows the thin, one-cell-thick walls of the alveolus and capillary, facilitating the diffusion of oxygen and carbon dioxide.

  1. Gills (fish):
    1. Many filaments covered in lamellae give a large surface area.
    2. A dense blood supply flowing opposite to the water maintains a steep gradient.
  2. Roots (plants):
    1. Root hair cells give a large surface area for absorbing water and mineral ions.
  3. Leaves (plants):
    1. Being flat and thin, with internal air spaces and stomata, gives a large surface area and a short path for gas exchange.

A 3D cross-section of a leaf illustrating its adaptations for gas exchange, such as internal air spaces and stomata that provide a large surface area and short diffusion path for oxygen and carbon dioxide.

Self review
  • Define diffusion.
  • Give the three factors that affect the rate of diffusion.
  • Why can a single-celled organism rely on diffusion alone?
  • Give three features of an effective exchange surface.
  • How is the small intestine adapted for absorbing digested food?
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1.3.1 Diffusion Revision Guide

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