Movement in and out of cells (2.1-2.2)

• Materials pass in and out of cells by:

1. Diffusion
2. Osmosis
3. Active transport
4. Phagocytosis

Diffusion

• Molecules and ions in liquids or a gas move continuously and randomly and so they change direction as they bump into eachother

• The particles collide more often when they are concentrated

• They tend to spread out until they are spaced out through diffusion

• Diffusion: the net movement of molecules within a gas or liquid from a region of high concentration to a region of low concentration (down a concentration gradient) across a semi permeable membrane, until equilibrium is reached

Importance of Diffusion

• Gas exchange in the aleveolu of lungs takes place by diffusion

• Gas exchange through the stomata of the leaves occurs by diffusion

Factors affecting the rate of diffusion

1. Concentration gradient: The higher the difference in concentrations the faster the rate of diffusion
2. Temprature: An increase in temperature will increase the rate of diffusion; Molecules gain kinetic energy and so will move faster (faster diffusion)
3. Surface area of the membrane: Larger surface area of the membrane the higher the rate of diffusion
4. Distance over which diffusing molecules travel: Shorter the distances faster the rate of diffusion
5. Size of diffusing molecules: Smaller/lighter molecules will move faster and so will have a higher rate of diffusion

Osmosis

• Water concentration is referred to as water potential

  • A solution with many water molecules is one with high water potential
  • A solution with fewer water molecules is one with low water potential

• A concentrated salt solution contains more salt molecules than water molecules so is said to have low water potential

• Osmosis: The net movement of water molecules from a solution of high water potential to a solution of low water potential (down a water potential gradient) across a partially permeable

Osmosis in plant cells

• When a plant cell is placed in a solution of low water potential: water leaves the the cell
  • Cytoplasm pulls away from the cell wall and the cell becomes flaccid
• When it is placed in a solution of high water potential: water enters the cell
  • Cytoplasm pushes against the cell wall and the cell becomes turgid

Osmosis in animal cells

• When it is immersed in a solution of low water potential: the cell loses water
  • Shrinks and becomes crenated
• When it is immersed in a solution of high water potential: the cell takes in water
  • Swells and bursts (haemolysis) leaving just the membrane

Active transport

• Active transport: the movement of molecules from a region of LOWER concentration to a region of HIGHER concentration (against a concentration gradient)
• It is an active process and requires energy from the cell (respiration) and takes place through a carrier protein in the cell membrane

1. Particle is recognised by the carrier protein in the membrane
2. Respiration supplies energy to drive carrier protein across membrane
3. Particles released into the cell, even against a concentration gradient
4. Carrier protein returns to collect more of the same particles