Diffusion, Osmosis and Active Transport

diffusion is the movement of particles from a higher concentration to a lower concentration. it is a passive process, meaning it does not require any energy.

increasing the rate of diffusion:

• increasing temperature 
  • at lower temperatures, particles have less energy, causing them to move slower and diffuse slower
  • at higher temperatures, particles have more energy, causing them to move faster and diffuse faster
• increasing the concentration gradient
  • if the difference in concentration is low, the particles will diffuse slower
  • if the difference in concentration is high, the particles will diffuse faster
• increasing the surface area
  • a smaller surface area results in a slower rate of diffusion because there is a longer diffusion distance
  • a larger surface area results in a faster rate of diffusion because there is a shorter diffusion distance. 

cells carrying out respiration

• oxygen moves in by diffusion 
• carbon dioxide moves out by diffusion

plant cells carrying out photosynthesis

• carbon dioxide moves in by diffusion
• oxygen diffuses out of the cell

transport of urea in the blood in humans

• proteins are digested to produce amino acids, which are used to repair and grow muscle
• excess is made into urea in the liver
• urea diffuses out of the liver into the blood
• urea diffuses out of the blood into the kidney

active transport is the movement of particles from a lower concentration to a higher concentration and requires energy

it is an active process and requires energy, so cells that are specialised to carry out active transport have many mitochondria

mitochondria releases energy by respiration, which means that cells carrying out active transport need oxygen

absorption of glucose in the small intestine

• at first the concentration of glucose is higher in the intestine than in the blood, so diffusion occurs
• gradually, the concentration of glucose in the small intestine decreases until it is lower than in the blood. now active transport occurs.

absorption of mineral ions in plant roots 

• mineral ions are essential for the plant but there is a lower concentration in the soil than in the root
• therefore, active transport occurs

osmosis is the diffusion of water from a higher concentration of water to a lower concentration of water across a semi permeable membrane.

dilute - high concentration of water, and a low concentration of something else (eg sugar, salt etc)

concentrated - high concentration of something else, low concentration of water

cell/tissue is placed in a more dilute solution:

• water moves in 
• mass increases 
• length increases

cell/tissue is placed in a solution with the same concentration:

• same amount of water has moved in and out 
• mass stays the same
• length stays the same

cell/tissue is placed in a more concentrated solution:

• water moves out
• mass decreases 
• length decreases

animal cells:

dilute - cell gets bigger and could burst

concentrated - cell shrinks and no longer works effectively 

plant cells:

dilute - becomes turgid (bigger) but does not burst. cell walls prevent bursting

concentrated - cell becomes flaccid and leads to a shrunken vacuole. the cell shrinks which leads to less cytoplasm and the cell membrane moves away from the cell wall. the plant wilts.

percentage change = (change/original) * 100

if the change is positive, the mass has increased

if the change is negative, the mass has decreased

calculating the percentage change allows cells/tissues with different starting masses to be compared

single celled organisms are able to effectively transport substances in and out of the cell. multi cellular organisms require specialised exchange surfaces to provide an efficient supply of materials.

as the size of an object increases, the surface area to volume ratio decreases. a smaller object has a larger ratio than a larger object

having a large surface area to volume ratio:

• increases the rate of diffusion
• increases the rate of energy transfer

exchange surfaces are systems that allow multi-cellular organisms to exchange materials with the environment.

they have adaptations that enable substances to move every quickly in and out of the body

they are needed because larger organisms have a smaller surface area to volume ratio, so diffusion is slower

specialised for diffusion, active transport and osmosis 

key features of any exchange surface:

• large surface area
• thin walls 
• good blood supply in animals

alveoli in the lungs: 

• purpose - site of gas exchange in mammals 
• oxygen diffuses into the blood and carbon dioxide diffuses out of the blood
• each alveolus has:
  • large surface area
  • thin walls to give a short diffusion distance
  • good blood supply
  • well ventilated so maintains a high concentration of oxygen in the alveoli

villi in the small intestine:

• purpose - the site of nutrient absorption in the small intestine
• glucose and other nutrients move by diffusion or active transport into the blood
• each villus has:
  • large surface area
  • thin walls to give a short diffusion distance
  • good blood supply

root hair cells in plants:

• purpose - site of water and nutrient absorption in plants 
• mineral ions move in by diffusion or active transport, water moves in by osmosis
• each root hair cell has:
  • large surface area
  • thin walls to give a short diffusion distance
  • many mitochondria to release energy for active transport

gills in fish:

• purpose - the site of gas exchange in fish
• oxygen diffuses into the blood from the water, carbon dioxide diffuses out 
• gills have:
  • large surface area
  • thin walls to give a short diffusion distance 
  • good blood supply
  • constantly covered in water which maintains a high concentration of water