Biology 3a

Osmosis
1. Osmosis is a Special Case if Diffusion
  1. Osmosis is the movement if water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration
  2. A partially permeable membrane is just one with very small boles in it. So small that only tiny molecules (like water) can pass through them, and bigger molecules can't
  3. The water molecules usually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time
    1. But because there are more water molecules on one side than on the other, there's a steady net flow of water into the region with fewer water molecules, i.e. into the stronger sugar solution
      1. This means the strong sugar solution gets more dilute. The water acts like it's trying to "even up" the concentration either side of the membrane
  4. Osmosis is a type of diffusion - passive movement of water particles from an area of high water concentration to an area of low water concentration
2. Water moves into and out of cells by osmosis
  1. Tissue fluid surrounds the cells in the body - this is basically just water with oxygen, glucose and other stuff dissolved in it. It's squeezed out of the blood capillaries to supply the cells with everything they need
    1. The tissue fluid will usually have a different concentration to the fluid inside a cell. This means that water will either move into the cell, or out of the cell, by osmosis
      1. If a cell is short of water, the solution inside will become quite concentrated. This usually means the solution outside is more dilute, and so water will move into the cell by osmosis
        If a cell has lots of water, the solution inside will be more dilute, and water will be draw out of the cell and into the fluid outside by osmosis
  2. Osmosis experiment
    1. You cut up potato into individual cylinders, and you get some beakers with different sugar solutions in them. One should be pure water, another should be a very concentrated sugar solution. Then you can have a few others with concentrations in between
      1. You measure the length of the cylinders, then leave a few cylinders in each beaker for half an hour or so. Then you take them out and measure their lengths again. If the cylinders have drawn in water by osmosis, they'll he a bit longer. If water has been drawn out, they'll have shrunk a bit
Gas and Solute Exchange
1. Substances move by diffusion, osmosis and active transport
  1. 1) Life processes need gases or other dissolved substances before they can happen
  2. 2) For example, for photosynthesis to happen, carbon dioxide and water have to get into plant cells and for respiration to take place, glucose and oxygen both have to be inside the cell
  3. 3) Waste substances also need to move out of the cell so that the organism can get rid of them
  4. 4) These substances move where they need to be by diffusion, osmosis and active transport
    1. Diffusion is where particles move from an area of high concentration to an area of low concentration. For example, gases can just just diffuse through one another, like a smell spreading through a room. Alternatively, dissolved particles can diffuse in and out of cells through cell membranes
      1. Diffusion and osmosis both involve stuff moving from an are of high concentration to an area of lower concentration. Sometimes substances need to move in the other direction - which is where active transport comes in
  5. 5) In life processes, the gases and dissolved substances have to move through some sort of exchange surfaces. The exchange surface structures have to allow enough of the necessary substance to pass through
  6. 6) Exchange surfaces are ADAPTED to maximise effectiveness
    1. They are thin, so substances only have a short distance to diffuse
    2. They have a large surface are so lots of a substance can diffuse at once
      1. Exchange surfaces in animals have lots if blood vessels, to get stuff into and out of the blood quickly
        Gas exchange surfaces in animals are often ventilated too - air moves in and out
  7. 7) Exchanging substances get more difficult in bigger and more complex organisms - the place where the substances are needed ends up being a long way away exchange surfaces
2. The structure of leaves lets gases diffuse in and out of cells
  1. Carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leaf's structure is adapted so that this can happen easily
  2. The underneath of the leaf is the exchange surface. It's covered in little holes called stomata which the carbon dioxide diffuses through
    1. Oxygen (produced in photosynthesis) and water vapor also diffuses in and out through the stomata
  3. The size of the stomata are controlled by the guard cells. These close the stomata if the plant is loosing water faster than it is being replaced by the roots. Without these guard cells the plant will soon wilt
  4. The flattened shape of the leaf increases the area of this exchange surface so that it's more effective
  5. The walls of the cells inside the leaf form another exchange surface. the air spaces inside the leaf increase the area of this surface ares so there's more chance of carbon dioxide to get into the cells
  6. The water vapor evaporates from the cell inside the leaf. Then it escapes by diffusion because there's a lot of it inside the leaf and less of it in the air outside. Evaporation is quickest in hot, dry, windy conditions
The Breathing System
1. The lungs are in the thorax
  1. The thorax is the top part of your 'body'
    1. It's separated from the lower part of the body (the abdomen) by the diaphragm
2. The lungs are like big pink sponges and are protected by the ribs
  1. The air that you breath in goes through the trachea. This splits into two tubes called the bronchi (each one is called 'a bronchus'), one going into each lung
    1. The Bronchi splits into progressively smaller tubes called bronchioles
      1. The bronchioles finally ends at small bags called alveoli where the gas exchange takes place
3. Ventilation is breathing in and out
  1. Breathing in
    1. Intercostal muscles and diaphragm contract
      1. Thorax volume increases
        This decreases pressure and draws air in
  2. Breathing out
    1. Intercostal muscles and diaphragm relax
      1. Thorax volume decreases
        This increases the pressure, so air is forces out
4. Artificial ventilators help people to breath
  1. Ventilators are machines that move air (often with extra oxygen) into or out of the lungs, They help people who can't breath by themselves, e.g. if they're under general anaesthetic, or have a lung injury or disease
    1. They used to be a giant case (an 'iron lung') from the neck to the abdomen, with only the patient's head poking out. Air was pumped out of the case, pressure dropped, the lung expanded so air was drawn into the lungs. Air pumped into the case has the opposite effect, forcing air out of the lungs. However, they could interfere with the blood flow to the lower body
      1. Nowadays , most ventilators work by pumping air into the lungs. This expands the ribcage - when they stop pumping, the rubcage relaxes and pushes air back out of the lungs. This doesn't interfere with the blood flow but it can occasionally cause damage (e.g. burst alveoli) if the lungs can't cope with the artificial air flow
Diffusion Through Cell Membranes
1. Gas exchange happens in the lungs
  1. 1) The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it.
    1. 2) To do this the lings contain millions of little air sacs called alveoli where gas exchange takes place
      1. 3) The alveoli are specilised to maximise the diffusion of oxygen and carbon dioxide. They have:
        An enormous surface area
        A moist lining for dissolving gases
        Very thin walls
        A good blood supply
2. The Villi provide a really big surface area
  1. 1) The inside of the small intestine is covered in millions of these tiny little projections called villi
    1. 2) they increase the surface area in a big way so that digested food is absorbed much more quickly into the blood
      1. Notice they have
        A single layer of surface cells
        A very good blood supply to assist quick absorption
Active Transport
1. Root hairs are specilised for absorbing water and minerals
  1. The cells on the surface of plant roots grow into long "hairs" which stick out into the soil
    1. This gives the plant a big surface area for absorbing water and minerals from the soil
      1. Most of the water and mineral ions that get into a plant are absorbed by the root hair cells
2. Root hairs take in minerals using active transport
  1. The concentration of minerals is usually higher in the root hair cell than in the soil around it
    1. So normal diffusion doesn't explain how minerals are taken up into the root hair cells
      1. They should go the other way if they follow if they followed the rules of diffusion
        The answer is that a conveniently mysterious process called "active transport" is responsible
  2. Active transport allows the plant to absorb minerals from a very dilute solution, against a concentration gradient. This is essential for it's growth. But active transport needs energy from respiration to make it work
  3. Active transport also happens in humans, for example in taking glucose from the gut and from the kidney tubules
3. We need active transport to stop us starving
  1. Active transport is used in the gut when there is a low concentration of nutrients in the gut but a high concentration of nutrients in the blood
  2. When there is a higher concentration of glucose and amino acids in the gut the diffuse naturally into the blood
    1. But - sometimes there's a lower concentration of nutrients in the gut then there is in the blood
      1. This means the concentration gradient is the wrong way
        Active transport allows nutrients to be taken into the blood despite the fact that the concentration gradient is the wrong way round
Water Flow Through Plants
1. Flowering plants have 2 separate types of vessel - Xylem and Phloem - for transporting stuff around. Both types of vessel go to every part of the plant, but they are totally separate
  1. Phloem Tubes transport food
    1. Made of columns of living cells with small holes in the ends to allow stuff to flow through them
      1. They transport food substances (mainly dissolved in sugars) made in the leaves to growing regions (e.g. new shoots) and storage organs (e.g. root tubers) of the plant
        The transport goes in both directions
  2. Xylem tubes take water up
    1. Made of dead cells joined end to end with no end walls between them and a hole down the middle
      1. They carry water and minerals from the roots to the stem and leaves in the transpiration stream
2. Transpiration is the loss of water from the plant
  1. Transpiration is caused by the evaporation and diffusion of water from inside the leaves
    1. This crates a slight shortage of water in the leaf, and so more water is drawn from the rest of the plant through the xylem vessels to replace it
      1. This in tern means more water is drawn up from the roots, and so there's a constant transpiration stream of water through the plant
        Transpiration is just a side-effect of the way leaves are adapted for photosynthesis. They have to have stomata in them so that gases can be exchanged easily. Because there's more water inside the plant then in the air outside, the water escapes from the leaves through the stomata
Circulatory System - The Heart
1. The double circulatory system
  1. Humans have a double circulatory system - two circuits joined together
    1. The first one pumps deoxygenated blood to the lungs to take in oxygen. The blood is then returned to the heart
      1. The second one pumps oxygenated blood around all the other organs of the body. The blood gives up it's oxygen at the body cells and the deoxygenated blood returns to the heart to be pumped out to the lungs again
2. The heart contracts to pump blood around the body
  1. the heart is a pumping organ that keeps the blood flowing around the body. The walls of the heart are mostly made of muscle tissue

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	Created by francesca graham about 8 years ago