AQA Categories Archives: B3

Fuels from fermentation

Fuels from fermentation

In India only animal waste is fed in (there are taboos against using human waste), this produces high quality gas but little amounts of fertiliser.

Biogas can be produced on a large scale, or on a small scale.

For example, a small biogas generator can be used to supply the energy needs of a family, or the waste of a sugar factory or sewage works can be used to power a village or town.

Ethanol-based fuels

Sugar cane crops can grow fast – 4-5m per year. Their juice is rich in carbohydrates like sucrose. The starches of Maize can be broken down into glucose by using carbohydrase enzymes. If the sugar rich products of sugar cane or maize are fermented with yeast anaerobically, the sugars break down into ethanol and water.

Ethanol is extracted by distillation and used as a car fuel. Some cars (e.g. in Brazil) run on a mixture of ethanol and petrol (called gasohol – 90% petrol, 10% ethanol). Ethanol does not produce toxic gases when burned and so does not pollute as much as other fuels (which produce CO and SO2).

Countries like America do not have enough maize crops to produce large amounts of ethanol, and poorer countries that do have the crops don’t have the money to buy the equipment.

Using a fuel like ethanol is called carbon neutral because you are not contributing to carbon dioxide levels in the air by using it.

Biogas generators

There are two main types:

Factors to consider when designing a generator:

Cost – Continuous generators are more expensive as waste has to be mechanically pumped in, and digested material has to be mechanically removed constantly

Convenience – Batch generators are less convenient, as they have to be continually loaded, emptied and cleaned

Efficiency – Gas is produced most quickly at 35°C. If the temp. falls below this, gas production will be slower. Generators in some areas will need to be insulated or kept warm. The generator shouldn’t have any leaks, or gas will be lost.

Position – The waste will smell, so generators should be sited away from homes. The generator is also best located close to the waste source.

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The kidneys

The kidneys

Our bodies need to get rid of waste products. Three waste products our bodies must excrete are CO2, urea and sweat. This is known as homeostasis (controlling conditions inside the body).



Urea (a waste product from the breakdown of amino acids) is produced in the liver. Urea is toxic in high concentrations, although the liver releases it into the blood stream to be filtered out by the kidneys.


We take in water from food and drink, and water is a waste product of respiration. We lose water in sweat, faeces, urine and breathing out.

For our cells to work properly their water content must be maintained at the correct level. Our kidneys help us to maintain that balance.

Stages of blood filtration in the kidneys:

Stage 1: Ultrafiltration. Blood is brought to the kidneys to be filtered – blood passes through tiny tubules and water, salt, glucose and urea are squeezed out.

Stage 2: Selective reabsorption. The kidneys send all of the glucose and as much water and salt as the body needs into the blood. Sugar and dissolved ions may be actively absorbed against a concentration gradient.

Stage 3: Waste. Water, salt and urea are left – this is urine. Urine is sent to the through the ureter to the bladder where it is stored before being excreted.


Kidney failure:

Sometimes the kidneys can fail due to infections, toxic substances or genetic reasons. A patient with kidney failure will soon die unless there is a way to rid the body of the urea and excess salt.

A kidney dialysis machine provides an artificial kidney for the sufferers of kidney failure. The patient must use a dialysis machine for 3-4 hours three times a week.

The patients’ blood flows alongside a partially permeable membrane, surrounded by dialysis fluid which contains the same concentration of dissolved ions and glucose as the blood (this ensures that glucose and useful mineral ions are not lost)

Ions and waste can pass through, but big molecules like blood cells and proteins can’t pass through (like in the kidneys).

Dialysis removes urea and maintains blood sodium and glucose levels.

Instead of dialysis a kidney could be transplanted into the patient. This option is cheaper than dialysis but it requires a donor (a normal person can still function with one kidney). The new kidney might be rejected by the body’s immune system.

To prevent rejection of the transplanted kidney a donor kidney with a ‘tissue-type’ similar to the recipient is used and the patient can take immunosuppressant drugs.


Transplanted kidneys only work for around 9 years, then the patient has to return to dialysis.

Mycoprotein is a low-fat, protein-rich food suitable for    vegetarians. It

is made from the fungus Fusarium.The fungus grows and reproduces rapidly on a cheap   energy supply

(sugar syrup) in a fermenter.It requires aerobic conditions to grow. Its mass doubles    every 5 hours or so.The biomass is harvested, purified and dried to leave mycoprotein –colours and flavours are added to enhance it.





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The diffusion of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane.

Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid or stiff so that they are able to hold the plant upright.

What is a partially permeable membrane?

Some membranes in plant and animal cells allow certain particles to pass through them but not others. They are partially permeable membranes.

What is active transport?

Substances are sometimes absorbed against a concentration gradient. Particles move across a cell membrane from an area of low concentration to an area of high concentration. This requires energy from respiration and is called active transport.

How are human cells specialised to increase the rate of exchanging materials?


How are plant cells specialised to increase the rate of exchanging materials?

Leaf cells – carbon dioxide (needed for photosynthesis) enters leaf by diffusion, oxygen (by-product of photosynthesis) exits leaf by diffusion

Surface area of leaf increased by flat shape.

Internal air spaces give increased surface area for gas exchange.



Plants have stomata to obtain carbon dioxide from the atmosphere. Plants lose water vapour from the surface of their leaves – this is called transpiration.

The rate of transpiration is increased in hot, dry and windy conditions. Plants living in these conditions have a thick waxy layer to reduce water loss.

Most transpiration is through the stomata. The size of the stomata is controlled by guard cells. If plants lose water faster than it can be replaced by the roots, the stomata can close up to prevent wilting and to stop photosynthesis from slowing. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.

Root hair cells –Plants absorb water from the soil by osmosis through their root hair cells. Osmosis is the movement of water from a high water concentration to a low water concentration through a partially permeable membrane. Plants use water for several vital processes including photosynthesis and transporting minerals.

The long shape/large surface area, thin walls and proximity to the xylem cells used for transporting water up the plant means the root hair cells are specially adapted to gaining water from the soil.

You need to bear in mind:

Ÿ the direction of movement of water

  • the effect of water movement on the volume and therefore the pressure of the different solutions
  • that particles move in both directions through the membrane. Changing the pressure or the concentration on one side of the membrane will change the movement of the particles until equilibrium is reached.


How are dissolved materials transported around the body?

The heart pumps blood around the body. Blood flows from the heart to the organs through arteries and returns through veins. In the organs, blood flows through capillaries.

Substances needed by cells in the body tissues pass out of the blood, and substances produced by the cells pass into the blood through the walls of the capillaries.

There are two separate circulation systems, one to the lungs and one to all the other organs of the body.

The blood is a liquid tissue consisting of:

  • Plasma
  • Red blood cells
  • White blood cells
  • Platelets

Blood plasma transports:

  • carbon dioxide from the organs to the lungs
  • soluble products of digestion from the small intestine to other organs
  • urea from the liver to the kidneys
  • hormones


Red blood cells transport oxygen from the lungs to the organs. Red blood cells have no nucleus. They are packed with a red pigment called haemoglobin.

In the lungs oxygen combines with oxygen to form oxyhaemoglobin. In other organs oxyhaemoglobin splits up into haemoglobin and oxygen.

White blood cells ingest and destroy pathogens, produce antibodies to destroy pathogens, and produce antitoxins that neutralise the toxins released by pathogens.

Platelets are irregularly shaped bodies. They help to form clots to stop bleeding.



How does exercise affect the exhanges taking place within the body?

During exercise heart rate increases, rate and depth of breathing increases and arteries supplying the muscles dilate.

Blood flow to the muscles is increased (as well as the supply of sugar and oxygen), and the rate of removal of carbon dioxide is increased.

Energy released through respiration is used to enable muscles to contract. Respiration happens in mitochondria in cells. Glycogen stores in the muscles are used during exercise.

Poisonous lactic acid builds up in the muscles and causes painful cramps. When exercise stops there is an oxygen debt – you must keep breathing in order to get oxygen to the muscles and oxidise the lactic acid into carbon dioxide and water.









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Materials like oxygen, carbon dioxide and glucose enter and leave cells by diffusion. Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration.

Diffusion happens when the particles are free to move (gases and particles dissolved in solutions). Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. No energy is required.

Examples of diffusion in living systems:



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