The important stuff for Biology: Unit 3


Transport Across Membranes


  • Passive process due to a gradient
  • High conc. to low conc. because of the random movement of particles until they are evenly spread.
  • Equal Conc. = Dynamic Conc. (meaning the particles are still moving)


  • Passive process due to a gradient
  • Diffusion of water molecules down a water potential gradient through a membrane
  • Net movement of water from a dilute solution to a concentrated solution through a membrane, along a conc. gradient until conc. are equal
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Active Transport:

  • Allows cells to move substances from an area of low conc. to an area of high conc. because substances move against the conc. gradient
  • Cells then absorb ions from very dilute solutions
  • Uses energy
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Surface Area:Volume

  • For a multi-cellular organism:
    • Diffusion through surface area is inadequate
    • Gills and lungs increase surface area for gas exchange, when SA:V is v. small
    • Insects; tracheal system

Inspiration - Breathing In

  • Ribs move out and upwards, intercostal muscles contract
  • Diaphragm contracts and flattens
  • Volume increases, pressure decreases
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Expiration - Breathing Out

  • Ribs move in and downwards, intercostal muscles relax
  • Diaphragm relaxes and curves
  • Volume decreases, pressure increases


  • Ventilation maintains a conc. gradient for diffusion
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The Lungs

Ventilation System of Mammals


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  • "air sac"
  • High conc. of oxygen -> blood
  • High conc. of Carbon Dioxide; blood ->Alveolus
  • Constant amount of blood and Carbon Dioxide are exchanged
  • Wall = 1 cell thick


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Absortion in Small Intestine

Small Intestine

  • Villus -> microvillus = Large surface area
  • Area of gut = 2000 meters squared
  • Big area is important. Small intestine is where digested food (glucoses, almino acids etc.) enter the blood stream by diffusion and active transport
  • Rich blood supply; steep conc. gradient for efficient diffusion
  • Large SA for diffusion
  • One cell thick wall, short distance for diffusion

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Uptake of Minerals


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Gas Exchange: Fish & Amphibians


  • Bodies; covered in protective scales; prevents oxygen being taken straight from the water
  • Thin gills, reduced distance of diffusion
  • Moist
  • Rich blood supply
  • Gills don't work in the air as a lack of water causes gills to stick together, reducing surface area so it's inadequate to take oxygen from the air


  • Tadpoles; frilly external gills, large SA, rich blood supply
  • Receive oxygen, and rifs Carbon Dioxide through diffusion
  • Tadpole -> frog. External gills -> absorbed. Metamporphisis
  • Frog's gas exchange, through skin, v.moist, rich blood supply
  • Mouth = large, thick skinned. Contains pair of v. simple lungs -> increase SA
  • In water, all gas exchange occurs through skin
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Gas Exchange: Insects & Plants


  • Tough outer coating -> little gas exchange
  • Need oxygen to power muscles -> internal respiratory system
  • Spiracles (system of tubes - cells of tissue) which open and close according to needs (open = need oxygen close = don't need oxygen. Prevents water loss)
  • Gas exchange occurs in trachuoles -> tubes
  • V. moist are pumped in and out of insect. Maintain conc. gradient
  • No blood supply within insect. Tracheoles = v.large SA, close contace with body cells


  • Need Carbon Dioxide (diffusion through leaves) & water for photosynthesis
  • SA of leaves, increased by flatness; thin, distance of diffusion is minimal
  • Air spaces allow Carbon Dioxide to be in contace with cell -> stomata
  • Carbon Dioxide not always needed. Leaves; are covered in a waxy cuticle, a waterproof, gasproof layer
  • have stomata; open when Carbon Dioxide is needed, close when not. Prevent water loss
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Uptake of Minerals

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The Circulatory System & Blood Vessels

The Circulatory System

Mammals have a double circulatory system:

  • Lungs <-> heart <->body

Fish have a single circulatory system

  • Heart -> guts -> body -> heart

Blood Vessels

  • Artery- Carry oxygenated red blood at v. high pressure; heart -> lungs
    - Thick walls, thick layer of muscle & elastic fibres
  • Vein- Carry deoxygenated purple blood; body -> heart.
    - Valves - prevent backflow
  • Capillary- No cell is more than 0.05mm away
    - narrow, v. think walls, diffusion of oxygen and glucose
    - Carbon Diocide cell -? capillary

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The Heart

  • (
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  • Right side; deoxygenated
  • Left side; oxygenated, thickest wall
  • Aorta - carries oxygenated blood around the body
  • Pulmonary Vein - oxygenated blood from the lungs
  • Pulmonary artert - deoxygenated blood to the lungs
  • Vena Cava - deoxygenated blood to the heart
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