• Cells transport ions and small molecules across their membranes under different conditions.
• Molecules and ions move spontaneously down their concentration gradient (i.e., from a higher to a lower region of concentration) by diffusion.
• Molecules and ions can also be moved against their concentration gradient, through active transport, requiring the expenditure of energy (usually from ATP)

DIFFUSION

• Molecules move from high to low concentration down a concentration gradient as a result of random movement.
• For example, water and oxygen molecules move freely in and out of cells based on concentration gradient.
• Charged molecules cannot pass freely, e.g. glucose.
• The kinetic energy of molecules and ions results in their diffusion.

Why Diffusion is Important?

• Oxygen moves into organisms by diffusion down a concentration gradient during respiration.
  • Carbon dioxide, produced during aerobic respiration, is potentially toxic if it builds up, it is removed using the same mechanisms, again by diffusion.
  • Photosynthetic plants need carbon dioxide for making their food. This diffuses through the stomata in the leaves and the oxygen produced during photosynthesis, along with water vapor from the transpiration stream, diffuses out of the leaf through the stomata
• Mineral ions in solution, such as nitrates and magnesium, are thought to diffuse across the tissues of plant roots, but most are absorbed into the roots by active transport.
  • In the ileum, water-soluble vitamins such as vitamin B and vitamin C are absorbed into the bloodstream by diffusion.
  • In the kidneys, some solutes in the renal capsule, such as urea and salts, pass back into the bloodstream by diffusion.

Factors that Affect the Rate of Diffusion

1. Surface Area: The larger the surface area, the faster the rate of diffusion
2. Temperature: An increase in temperature causes an increase in the kinetic energy which molecules and ions possess, therefore, the rate of diffusion increases.
3. Concentration Gradient: The bigger the difference in the concentration of a substance on either side of a membrane, the faster it will tend to diffuse. The difference is called a concentration gradient or diffusion gradient
4. Distance: An increased distance slows down the diffusion rate.
5. Size of Molecule/Ion: The larger the molecules or ions, the slower they diffuse.

OSMOSIS

• Osmosis involves the diffusion of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Effects of Osmosis on Plant and Animal Cells

1. When Placed in H₂O: Concentration of H₂O outside the cell is higher than inside it. Cells will take in H₂O by osmosis:
   • plant cells become turgid (swollen) but do not burst (they have a tough cell wall which is fully permeable)
   • animal cells will burst (no cell wall)
2. When placed in concentrated sugar or salt solutions: Concentration of H₂O inside the cell is higher than outside it. H₂O gets out of the cell by osmosis:
   • plant cells become flaccid (soft and limp), cytoplasm is no longer pressed against the cell wall. The plant loses its firmness and begins to wilt.
   • animal cells shrink, become crenated

ACTIVE TRANSPORT

• Sometimes substances are required to be moved against their concentration gradient, of faster than they would by passive transport.
• In these cases, active transport is used, which requires energy.
• There are many occasions when cells need to take in substances which are only present in small quantities around them.
• For example, root hair cells in plants take in nitrate ions from the soil. Their concentrations are often higher inside the root hair cell than in the soil, so the diffusion gradient is from the root hair to the soil. Despite this, the root hair cells still can take nitrate ions in, by active transport.

The Importance of Active Transport

• Energy-consuming processes by which substances are transported against a concentration gradient, e.g., ion uptake by root hairs and glucose uptake by epithelial cells of villi.