3.Movement in and out of cells

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration.
• In diffusion molecules move down a concentration gradient, as a result of their random movement
• Diffusion across the cell membrane

  
•  cell membrane can restrict the free movement of molecules
• The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all
• Diffusion helps living organisms to:

  • obtain many of their requirements
  • get rid of many of their waste products
  • carry out gas exchange for respiration
•  plants require oxygen for respiration at all times, as well as carbon dioxide for photosynthesis when conditions for photosynthesis are right
• SITE: small intestine

  MOLECULES MOVING: glucose, amino acids, fatty acids and glycerol etc.

  FROM: lumen of small intestine

  TO: blood/lymph in villi
• SITE: leaf

  MOLECULES MOVING: Oxygen

  FROM: air spaces between mesophyll cells

  TO: mitochondria in all cells
• SITE: leaf

  MOLECULES MOVING: carbon dioxide

  FROM: air spaces between mesophyll cells

  TO: chloroplasts in mesophyll cells
• SITE: leaf

  MOLECULES MOVING: water vapour

  FROM: stomatal pores

  TO: air outside stomata
• SITE: lungs

  MOLECULES MOVING: carbon dioxide

  FROM: blood capillaries around alveoli

  TO: alveolar airspace
• SITE: lungs

  MOLECULES MOVING: oxygen

  FROM: alveolar airspace

  TO: blood capillaries around alveoli
• Where does the energy for diffusion come from?

  • All particles move randomly at all times
  • This is known as Brownian motion
  • The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions
• Brownian motion

  
• Surface area to volume ratio influences diffusion
• the bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
• Many cells which are adapted for diffusion have increased surface area in some way - eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)
• Distance influences diffusion rate

  • The smaller the distance molecules have to travel the faster transport will occur
  • This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible
• Temperature influences diffusion rate

  • The higher the temperature, the faster molecules move as they have more energy
  • This results in more collisions against the cell membrane and therefore a faster rate of movement across them
• Concentration Gradient influences diffusion rate

  • The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
  • This is because on the side with the higher concentration, more random collisions against the membrane will occur
• Water is important for all living organisms as many substances are able to dissolve in it 
• water is a solvent
• water is important as a solvent as dissolved substances can be easily transported around organisms - eg xylem and phloem of plants and dissolved food molecules in the blood
• water is important as a solvent as digested food molecules are in the alimentary canal but need to be moved to cells all over the body - without water as a solvent this would not be able to happen
• water is important as a solvent as toxic substances such as urea and substances in excess of requirements such as salts can dissolve in water which makes them easy to remove from the body in urine
• water is important as a solvent as water is also an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen as necessary in cells 
• All cells are surrounded by a cell membrane which is partially permeaable
• Water can move in and out of cells by osmosis
• Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
• in osmosis water is moving down its concentration gradient
• the cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)
• osmosis experiment

  
• dialysis tubing experiment

  
• When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall
• Water entering the cell by osmosis makes the cell rigid and firm
• osmosis is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight
• The pressure created by the cell wall stops too much water entering through osmosis and prevents the cell from bursting
• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts
• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane