Movement of molecules in and out of cells

Dissolved substances
1. Diffusion
  1. Is the process by which particles spread out from a region of high concentration to low concentration (concentration gradient)
    1. This happens in the case of gases and dissolved substances
    2. This is a very important exchange process, common in living organisms
      1. Diffusion allows substances to move in and out of cells across a cell membrane providing they are dissolved and there is a concentration gradient
      2. For example
        Gas exchange in lungs; oxygen, via inhaled air in the alveoli , is diffused into the blood circulation
        Gas exchange in lungs; Carbon Dioxide, via bloodstream, diffuses into the lungs (alveoli)
        Gas exchange in a leaf; Carbon dioxide, outside the leaf, diffuses into the leaf and then into its cells
        Exchange in small intestine; Digested food molecules such as amino acids and glucose diffuse from the small intestine into the bloodstream, via the villa capillaries
2. Active transport
  1. Is a transport process by which dissolved molecules move against a concentration gradient
    1. This requires energy from repiration
    2. This is carried out out by a series of protein carriers within the cell membrane which have a binding site, allowing a specific dissolved substance to bind to the side of the membrane where it is at a lower concentration
      1. Energy from respiration then changes the shape of the protein so that it releases the substance onto the other side of the membrane
  2. It has the advantage of allowing cells to absorb dissolved substances from very dilute solutions
  3. For example
    1. Root hair cells in plant roots use active transport to absorb mineral ions (such as nitrates) from the soil- even though there are lower concentrations of minerals in the soil than there are in the root hair cell
    2. Small intestine villi cells use active transport alongside diffusion to maximise the absorption of glucose and other substance e.g., minerals
3. Osmosis
  1. Only applies to the movement of water from a high proportion of water to a low proportion through a partially permeable membrane
    1. A partially permeable membrane allows only small molecules like water to pass
    2. The idea is, if water from a concentrated solution drops but rises from a dilute solution they will eventually equalise
    3. In animal cells
      1. If the concentration of water outside of the cell is higher, water will enter the cell causing it to swell and eventually burst due to the lack of cell wall
      2. If the concentration of water outside the cell is lower then it will shrivel (no cell wall)
    4. In plant cells
      1. If the concentration of water is higher outside the cell, the cell will swell making it turgid
      2. If the concentration of water outside the wall is lower then the cytoplasm will shrink away from the cell wall
4. Parts of organisms are adapted to enhance the movement of molecules
  1. Surface area
  2. Minimal cell thickness
  3. (Animal) good blood supply due to extensive capillary network in exchange organs eg. villi
Exchange systems in plants
1. Exchange in the roots
  1. Roots absorb water from soil by osmosis and dissolve mineral ions from the soil by active transport
    1. Mineral ions are transported round the plant where they serve a variety of functions, whilst the water is transported to be used as a reactant in photosynthesis, as well as to cool the leaves by evaporation and support the leaves and shoots by keeping cells rigid
      1. To maximise the efficiency of absorption, roots have specialised cells called root hair cells which are found just behind the tip of the root
        Root hair cells have several adaptations:
        The tube-like protusion provides a greater surface area across which water and mineral ions can be exchanged
        The tube-like protusion can penetrate between soil, particles, reducing the distance across which water and mineral ions must move
        The root hair cell contains lots of mitochondria, which release energy from glucose during respiration in order to provide the energy needed for active transport
2. Exchange in the leaves
  1. The main function of the leaves is photosynthesis: the production of glucose with the input of energy from sunlight
    1. To do this effectively, leaves are adapted for maximum absorption of carbon dioxide and sunlight. For example:
      1. Flattened shape produces a large surface area in order to absorb light and carbon dioxide
      2. Their thin build means that only a short distance is travelled in diffusion of carbon dioxide into the leaf, and oxygen out of the cells
      3. Stomata are holes on the surface of the leaf that open and close to allow diffusion of carbon dioxide into the leaf and oxygen out the the size of the opening is controlled by guard cells which become turgid in order to open
      4. Internal air spaces in the spongy mesophyll layer increase surface area of leaf in which to absorb more carbon dioxide
Gaseous exchange in the lungs
1. The breathing system is used get oxygen into the bloodstream and carbon dioxide out of the body
  1. The lungs are like two big sponges protecting
    1. When we breathe in air, it goes down the trachea into the bronchi of which there is one in each lung
      1. The bronchi then split progressively into bronchioles which lead to the alveoli where gas exchange takes place
        The blood that comes into the alveoli has a higher carbon dioxide concentration which it has gained from respiration around the body
        Therefore carbon dioxide diffuses out of the blood into the alveoli where it can be expelled from the body
        The air we breathe in contains oxygen of which there is a higher concentration of in the alveoli than in the blood that flows past the alveoli
        Therefore alveoli diffuses from the alveoli into the bloodstream so it can go round the body for respiration in the muscles
        The alveoli is adapted for this job by:
        Thin cell walls which decrease distance for diffusion
        Folded walls which increase surface area
2. Ventilation is the process of breathing in and out
  1. When breathing in the intercostal muscles and diaphragm contract allowing the volume of the thorax to increase
    1. This decreases air pressure allowing air to draw in
      1. When breathing out the intercostal muscles and diaphragm relax decreases the volume in the thorax
        This increases the pressure forcing the air out of the body
        Artificial ventilators can be used to help people do this job

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Movement of molecules in and out of cells

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