The Variety of Life

Biology AQA new AS level unit 2, variety of life

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Haemoglobin - globular protein molecule in blood that combines with oxygen. It compromises 4 polypeptide chains around an iron-containing haem group. Structure:

  • Primary - 4 polypeptide chains.
  • Secondary - each chain coiled into a helix.
  • Tertiary - each chain folded into precise shape which important for carrying oxygen.
  • Quaternary - all 4 chains linked to form spherical molecule, each ferrous ion combines with 1 oxygen molecule (4 oxygens carried by 1 haemoglobin).

Role of Haemoglobin - transport oxygen efficiently, so it must:

  • readily associate with oxygen at gas exchange surface.
  • readily dissociate from oxygen at those tissues requiring it.

Haemoglobin changes its affinity for oxygen under different conditions due to its shape which changes in the presence of certain substances e.g. CO2 where it will bind more loosely to O2.

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Haemoglobin Affinity for Oxygen.

There are different types of haemoglobin with different properties:

  • 1. Hb with high affinity for oxygen - take it up easily.
  • 2. Hb with low affinity for oxygen - release it readily.

An organism living in an environment with little O2 requires Hb that readily combines with O2 to absorb enough. An organism with a high metabolic rate needs to release oxygen readily to tissues, provided there is lots of O2 in environment it is more important to release it easily.

Hb molecules have a slightly different sequence of amino acids so slightly different shapes which affects their affinity for O2.

Loading (associating) - Hb combines with O2, in the lungs in humans.

Unloading (dissociating) - Hb releases its O2, in tissues.

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Oxygen Dissociation Curves.

Partial Pressure - the amount of a gas that is present in a mixture of gases is measured by the pressure it contributes to the total pressure of the mixture (pO2)

When Hb is exposed to different pO2 it does not absorb it evenly. At very low concentrations of O2 the polypeptide chains are closely united so it is difficult to absorb the 1st O2 molecule. Once loaded it causes the polypeptides to absorb the rest easily. The graph of this relationship is an oxygen dissociation curve, they all have a similar shape but differ in their positions on the axes. The further to the left, the greater the affinity; further to the right, the lower the affinity. CO2 concentration effects it by:

  • Hb has a reduced affinity in the presence of CO2.
  • The greater the concentration the more readily it releases O2 (Bohr Effect).
  • At the gas exchange surface CO2 levels are low so O2 affinity is increased, curve shifts to the left.
  • In rapidly respiring tissues CO2 levels are high (and O2 levels low) so affinity is reduced & O2 is readily unloaded, curve shifts to right.

Dissolved CO2 is acidic and the low pH causes Hb to change shape.

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Loading, Transport and Unloaading of Oxygen.

  • At the gas exchange surface CO2 is constantly removed so pH is raised.
  • The higher pH changes the shapeof the Hb so it can load O2 easily.
  • The shape also increases the affinity so it is not released while being transported.
  • In tissues, CO2 is produced by respiration so the pH is lowered which changes the shapeof the Hb into 1 that has a lower affinity for oxygen. Hb releases its O2.
  • The higher the respiration rate, the more CO2 the tissue produces, the lower the pH, the greater shape change, the more readily O2 is unloaded, the more O2 is available for respiration.
  • Hb normally becomes saturated as it passes through the lungs, only 1 O2 is released at tissues. If the tissue is very active then 3 will be released.

Where you live, the size of the animal and the activity of it all contribute to O2 affinity.

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Starch, Glycogen and Cellulose.

Starch - a polysaccharide found in plants in the form of small grains, especially in seeds and storage organs. Made of chains of a-glucose linked by glycosidic bonds through condensation. Unbranched chain wound into a tight coil so very compact. It is the major energy source and is used for storage because: it is insoluble so doesn't draw water in by osmosis or diffuse out of cells; it is compact so a lot can be stored in a small place; and when hydrolysed it forms a-glucose with is easily transported and used in respiration.

Glycogen - serves the same role as starch but is in animal cells. It has shorter chains and is highly branched. Major carb storage product of animals. It is even more readily hydrolysed because of smaller chains.

Cellulose - made of b-glucose not a-. The positions of -H and -OH groups are reversedso to form a glycosidic bond it must rotate 180 degrees, -CH20H group alternates position. It is a straight and unbranched chain, they run parallel to 1 another so H bonds form cross-linkages. Molecules grouped to form microfibrils which are arranged in parallel groups. Major component of plant cell walls and provides rigidity. Prevents cell bursting by exerting inward pressure that stops influx of water. Cells are turgid so plants are semi-rigid which maintains stems for max s.a. for photosynthesis.

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Palisade Cells and Chloroplasts.

Leaf Palisade Cell - a typical plant cell that carries out photosynthesis. It is long and thin to form a continuous layer for absorbing sunlight. It has numerous chloroplasts that arrange themselves to absorb max light. It has a large vacuole to push chloroplasts and cytoplasm to edge of cell.

Chloroplasts - disc shaped (2-10microm long, 1microm diameter). Have:

  • Chloroplast envelope - highly selective double plasma membrane.
  • Grana - stacks of thylakoids, within these are chlorophyll. Some have tubular extensions to join up with adjacent grana. 1st stage of photosynthesis.
  • Stroma - fluid-filled matrix, 2nd stage of photosynthesis, contain starch grains.

Adapted to function of photosynthesis as:

  • Granal membranes provide large s.a. for attachment of chlorophyll, electron carriers and enzymes used in stage 1. Attached in highly ordered way.
  • Stroma fluid possesses all enzymes for stage 2.
  • Contain DNA and ribosomes to quickly manufacture proteins needed.
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Cell Walls and Specialised Plant Cells.

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Molly AK

Very good :)


very good :) thanks


how come your "Cell Walls and Specialised Plant Cells" revision card is blank?

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