AQA AS Level Biology Unit 2 - The Variety of Life

The 4th topic in unit 2 AS biology AQA

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The haemoglobins are a group of chemically similar molecules found in a wide variety of organisms. The structure of haemoglobin molecules is made up of:

  • Primary structure: consisting of 4 polypeptide chains
  • Secondary structure - in which each of these 4 polypeptide chains is coiled into a helix
  • Tertiary structure - in which each polypeptide chain is folded into a precise shape - an inportant factor in its ability to carry oxyegen
  • Quaternary structure - in which all four polypeptide chains are linked together to form an almost spherical molecule. Each polypeptide is associated with a haem group - which contains a ferrous ion. Each Fe ion can combine with a single oxygen molecule making a total of 4 oxygen molecules that can be carried by a single haemoglobin molecule in humans.
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The role of haemoglobin

The role of haemoglobin is to transport oxygen. To be efficient at transporting oxygen, haemoglobin must:

  • readily associate with oxygen at the surface where gas exchange takes place
  • readily dissociate with oxygen at those tissues that require it

Haemoglobin changes its affinity for oxygen under different conditions. It achieves this because its shape changes(by altering the sequences of amino acids) in the presence of certain substances such as carbon dioxide, the new shape of haemoglobin molecule binds more loosely to oxygen. As a result haemoglobin releases its oxygen

Haemoglobins with high affinity - these take up oxygen more easily but release it readily ( required by organisms in an environment with low oxygen concentration)

Haemoglobins with low affinity for oxygen - these take up oxygen less easily but release it more readily (required by organisms with a high metabloic rate in an area with high concentrations of oxygen)

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Loading and unloading oxygen

The process by which haemoglobin combines with oxygen is called loading or associating. In humans this takes place in the lungs

The process by with haemoglobin releases its oxygen is called unloading or dissociating. In humands this takes place in the tissues.

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Oxygen dissociation curve

  • When haemoglobin is exposed to different partial pressures of oxygen is does not absorb the oxygen evenly.
  • At very low concentrationsof oxygen, the 4 polypeptides of the haemoglobin molecule are closely united and so it is difficult to absord the first oxygen molecule.
  • However, once loaded, this oxygen molecules causes the polypeptides to load the remaining three oxygen molecules very easily.
  • The graph of this relationship is known as the oxygen dissociation curve.

The graph shows a very small decreasei the partial pressure of oxygen leads to a lot of oxygen becoming dissociated from the haemoglobin. The graph tailsoff at very high concentrations simply because the haemoglobin is almost saturated with oxygen.

The shape of haemoglobin molecules can change under different conditions. Therefore there are a large number of different oxygen dissociation curves. They all have a roughlysimilar shape but differ in their position on the axis.

  • The further to the left the curve, the greater affinity of haemoglobin for o2
  • The further to the right the curve, the lower affinity of haemoglobin for o2
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Effects of CO2 concentration

Haemoglobin has a reduced affinity gor o2 in the presence of co2. The greater the concentration of CO2 the more readily haemoglobin releases its oxygen (the Bohr effect) - this explains why the behavious of the haemoglobin changes in different regions of the body

  • At the gas-exchange surface (e.g lungs) the level of carbon dioxide is low because it diffuses across the exchange surface and is expelled from the organism. The affinity of haemoglobin for oxygen in the lungs, means that oxygen is readily loaded by haemoglobin, The reduce CO2 level have shifted the OCD to the left
  • In rapidly respiring tissure (e.g muscles) the level of carbon dioxide is high. The affinity of haemoglovin for oxygen is reduced, which, coupled with the low concentration of oxygen in the muscles means that oxygen is readily unloaded fro the haemoglobin intothe muscle cells. The increase carbon dioxide level has shifted the ODC to the right.

The greater the concentration of CO2 the more readily haemoglobin releases its oxygen. This is because dissolved CO2 is acidic and the low in pH causes haemoglobin to change shape.

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Loading, transport and unloading of oxygen

  • At gas-exchange surfaces the CO2 is constantly being removed
  • The pH is raised due to the level of CO2
  • The higher pH changes the shape of the haemoglobin for O2, so it is not released whilebeing transportedin the blood to the tissues
  • In the tissues, CO2 is produced by respiring cells
  • CO2 is acidic in solution, so the pH of the blood withing the tissues is lowered
  • The lower pH changes the shape of the haemoglobin into one with a lower affinity for oxygen
  • Haemoglobin releases its oxygen into the respiring tissues

This is a flexible way of ensuring that there is always sufficient oxygen for respiring tissues. The more active a tissue, the more O2 is unloaded. This works as follows

The higher the rate of respiration ---> the more CO2 the tissues produce   ---> the lower the pH ---> the greater the haemoglobin shape change          --->the more readily oxygen is unloaded ---> the more O2 is available for respiration

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  • Starch is a polysaccharide that is found in many parts of a plant (never found in animals) in the form of small grains. Especially large amounts occur in seeds and storage organs such as potato tubers.
  •  It forms an important component of food and is themajor energy source in most diets.
  • Starch is made up of chains of a-gloucose monosaccharides linked by glycosidic bonds that are formed by condensation readtions. The unbranched chain is wound into a tight coil that makes the molecule very compact.

The main role of starch is energy storage, something it is especially suited for because:

  • it is insoluable and therefore doesn't draw water into the cells by osmosis
  • being insoluable, it does not easily diffuse out of cells
  • it iscompact, so a lot of it can be stored in a small space
  • when hydrolysed it forms a-glucose which is both easily transported and readily used in respiration
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Structure of a starch molecule


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  • Glycogen has a very similar structure to starch but has shorter chains and is more highly branched
  • It sometimes stores 'animal starch' because it is the major carbohydrate storage product in animales
  • in animlas it is stored as small granules mainly in the liver and in the muscles
  • It structure suirs it for storage for the same reasons as those given for starch. However, because it is made up of smaller chains, it is even more readily hydrolysed to a-glucose.
  • Glycogen is found in animal cells but never in plant cells.
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  • Celulose differs from starch and glycogen in one major respect - its is made with monomers of B-glucose rather than a-glucose
  • This seeminly small variation produces fundamental differences in structure and function of this polysaccahride
  • The main reason for this is that in the B-glucose units, the position of the -H group and the -OH group on a single carbon atom are reversed.
  • In B-glucose the -OH group is abover, rather than below the ring. This means that to form glycosidic links, each B-glucose molecule must be rotated by 1180 degrees compared to its neighbour,
  • The result is that the -CH2OH group on each B-glucose molecule alternates between being above and below the chain

Rather than forming coiled chian like starch, cellulose has straight unbranched chains. These run parallel to one another, allowing hydrogen bond to form cross-linkages between adjacent chains. While eachindividual hydrogen bond adds very little to the strength of the molecule the sheer overall number of them makes a considerable contricution to strengthening cellulose, making it the valuable structural material that it is.

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Cellulose (2)

The cellulose molecules are grouped together to form microfibrils which are arranged in parallel groups called fibrils.

Cellulose is a major component of plant cell walles and provides rigidity to the plant cell. The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis.

It does this by exerting an inward pressure that stops any further influx of water. As a result living plant cells are turgid and push against one another, making herbaceous parts of the plant semi-rigid. This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.

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Leaf palisade cell

The leaf palisade cell is a typical plant cell. Its function is to carry out photosynthesis. The main features that suit it to its function of photosynthesis are:

  • long, thin cells that form and continuous layer to absorb sunlight
  • numerous chloroplasts that arrange themselves in the best positions to collect the maximum amount of light
  • a large vacuole that pushed the cytoplasm and chloroplasts to the edge of the cell. Chloroplasts are organelles that carry out photosynthesis.


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Chloroplasts vary in shape and size but are typically disc-shaped 2-10 micro metres long a 1 micrometre in diameter. Their main features are:

  • The chloroplast envelope - is a double plasma membrane that surrounds the organelle. It is highly selective in what it allows to enter and leave the cholroplast
  • The grana are stacks of up to 100 disc like structures called thylakoids. Within the thylakoids is the photosynthetic pigment called chlorophyll. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. The grana are where the first stages of photosynthesis take place
  • The stroma - is a fluid filled matrix where the secong stage of photosynthesis takes place. Withing the stroma are a number of other structures, such as starch grains.

Chloroplasts are adapted to their function of harvesting sunlight and carrying uot photosynthesis in the following ways:

  • The granal membrances provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis. These chemicals are attached to the membran in a highly ordered fashion
  • The fluid of the stroma possesses all the enzymes needed to carry out the second stage of photosynthesis
  • Chloroplasts contain both DNA and ribosomes so they can quickly and easily maufacture some of the proteins needed for photosynthesis
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Cell Wall

Characteristic of all plant cells, the cell wall consists of microfibrils of the polysaccharide cellulose embedded in a matrix. Cellulose microfibrils have considerable strength and so contribute to the overall strength of the cell wall. Cell walls have the following features:

  • They consist of a number of polysaccharides, such as cellulose
  • There is a think layer, called the midle lamella, which marks the boundary between adjacent cell walls and so contribute to the movement of water through the plant

The functions of the cellulose cell wall are:

  • to provide mechanical strength in order to prevent the cel bursting under the pressure created by the osmotic entry of water
  • to give mechanical strength  to the plant as a whole
  • to allow water to pass along it and so contribute to the movvement of water through the plant
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Difference between plant and animal cells

There are a few fundamental difference between plant and animal cells

Plant cells:

  • Cellulose cell wall surrounds the cell as well as a cell-surface membrane
  • Chloroplasts are present in large numbers in most cells
  • Normally have a large, single, central vacuole filled with cell sap
  • Starch grains are used for storage

Animal cells

  • Only a cell-surface membrane surrounds the cell
  • Chloroplasts are never present
  • If vacuoles are present they are small and scattered throughout the cell
  • glycogen granules are used for storage
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The root hair cell

Each root hair is an extension of a root epidermal cell. Root hairs are the exchange surfaces in plants that are responsible for the absorption of water and mineral ions. These root hairs remain functional for a few weeks before dying back to be replaced by others nearer the frowing tip.

Root hairs absorbe water by the process of osmosis. The soil solution surrounds the particles that make up soil. It contains a very low concentration of mineral ions dissolved in water. The root hairs by comparison have a relatively high concentration of ions and sugars within their vacuoles and cytoplasm. Because the root hairs are in direct contact with the soil solution, water moves by osmosis from the soil solution into the root hair cells

The concentration of ions inside the root hair cell is normally greater than that in the soil solution. The uptake of mineral ions is therefore afainst the concentration gradient and requires active transport. This is achieved using special carrier proteins that use ATP This provides energy to transport particular ions from the soil solution (where they are in lower concentrations) to the root hair cytoplasm and vacuole (where they are in higher concentrations.)

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Xylem vessels

In flowering plants, xylem vessels are structures through which the vast majority of water is transported. Xylem vessels have thick cells walls.

These vessels may vary in appearance, depenfing on the type and amount of thickening of their cell walls. As they mature, their walls incorporate a substance called lignin and the cells die. The end walls break down, which allows the cell to preform a continuous tube. The lignin often forms rings or spirals around the vessel.

Specialised cells are those that are adapted to a particular function.

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