[IV] Biology - B4

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  • Created by: Olivia
  • Created on: 29-03-14 11:00


All our food comes ultimately from PHOTOSYNTHESIS. Only green plants are able to convert energy in sunlight into energy in chemicals. Photosynthesis produces GLUCOSE (a simple sugar). Light energy is needed to drive photosynthesis, so it is often included in the equation.

carbon dioxide + water ---light energy---> glucose + oxygen

Oxygen is a waste product in the reaction. Plants can convert the glucose into a range of other chemicals, such as starch, cellulose and proteins. Starch and proteins are food molecules that provide energy to animals through food chains. For humans, the food part of rice is the grain. We can eat the grains because most of the grain is protein or starch. Humans can digest protein and starch. The leaves are mainly cellulose, which we cannot digest.

Photosynthesis is actually a series of chemical reactions. It is usually summarised as a single simple equation. A chemical in the plant called CHLOROPHYLL absorbs the light energy needed for photosynthesis. Chlorophyll is green, and this is what gives plants their green colour. The energy from the light is used to drive several reactions, which eventually build moleculs of glucose. The plant can then use the glucose to make other chemicals (such as cellulose for cell walls, starch for energy storage, oil for energy storage, respired to give energy, used to make chlorophyll or proteins for growth). It is only present in the parts of the plant that are in the light. It is found in small structures in the cells called CHLOROPLASTS.

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Other photosynthesisers

Not all plants that carry out photosynthesis are easy to see. The oceans are full of microscopic algae called phytoplankton. These single-celled plants are too small to see individually, but large populations of them can sometimes be seen from Earth orbit as a green smudge in the water. They produce about 60% of the oxygen in our atmosphere and provide food for krill, a small prawn-like animal, which in turn is food for baleen wales.

Carbon dioxide + water ---light energy---> glucose + oxygen

Water is essential in all living thing for:

- keeping chemicals in solution

- taking part in important reactions

- keeping cells the correct shape

- acting as a heat store.

The human body is more than 50% water. It takes a lot of energy to warm up and, once heated, water takes a long time to cool down. This helps to prevent the body from suffering from sudden changes in temperature.

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Water and living things

It is important to maintain a constant body temperature because the chemical reactions in our cells go much faster when temperatures rise and slow down too much when they get low. 

Percentage of normal water level (%) How do you feel?

100% Healthy and happy!

99% Thirsty

98% Very thirsty

97% Tired, moody and you may be sick

95% Pale and ill; very irritable and aggressive

90% Can't sweat so body temperature rises

85% Dangerously ill - doctors get worried

80% You're dead

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Toast burning in the kitchen can soon be smelled on the other side of the house. This is because the molecules of smoke in the air move by DIFFUSION around the whole house.

Diffusion happens because molecules are constantly and randomly bouncing around. This movement makes them spread out, so eventually they are evenly spread over the space available. This means that overall, molecules spread from an area where their concentration is higher to where their concentration is lower.

The rate at which molecules spread depends on:

- the temperature (particles bounce around more quickly as it gets hotter)

- any walls or barriers to their movement

- the difference in concentration in the two areas (the bigger the difference in concentration, the faster they spread).

Diffusion does not need any energy input - it happens because of the movement of molecules. Water diffuses between cells in living organisms without any need for the organism to do anything. The gases carbon dioxide and oxygen move in and out of the leaves of plants by diffusion.

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Osmosis is a special kind of water movement. Water moves by diffusion from areas of high concentration of water to areas of low concentration of water.

A solution with a high concentration of water is called a "dilute solution", and one with less water is called a "concentrated solution". So osmosis is the overall movement of water from a dilute to a more concentrated solution by diffusion.

Cells contain chemicals as well as water, so the solution inside is not as dilute as pure water. Each cell is surrounded by a membrane that lets small molecules through, but not large ones. This is called a PARTIALLY PERMEABLE MEMBRANE. Water molecules are very small and can pass into and out of cells by osmosis. 

If a cell is placed in pure water, the water molecules move from outside (the more dilute solution) to inside (the more concentrated solution). This makes the cell swell with the extra water and the membrane gets stretched like the skin of a balloon. If red blood cells are placed in pure water, they take in so much water they burst. If a cell is placed in a solution that is very concentrated, water leaves the cell by osmosis and the cell wrinkles up - like a dried prune. Water moves into plant root cells by osmosis. In a plant the forces produced by water moving into a cell can be very large. 

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Minerals in the soil

Plants need mineral salts to grow well. These minerals include metals and other chemical groups. The table shows some of the most important elements and what they are used for in the plant. Soil that has a low level of minerals is not fertile. 

Potassium is an element used for building proteins, cell growth, fruit development and resistance to diseases. Nitrogen is an element used to increase rapid growth which is essential for photosynthesis and all proteins. Phosphorus is the element used as an essential part of photosynthes that encourages rapid growth, especially in the roots. Magnesium is the element that is an essential part of chlorophyll used in photosynthesis (chlorophyll is the green chemical in plants that absorbs light energy). Magnesium is also an essential part of many other chemicals in cells. These elements are all taken up as parts of other chemicals. Nitrogen is taken up as nitrates. Plants use energy from respiration to react with nitrates with the sugar glucose to make AMINO ACIDS. These can then be built up into large protein molecules. This building up of larger molecules is called SYNTHESIS. 

Minerals in the soil are useful to plants. Some of these mineral nutrients contain metals. All plants will take these up from the ground to grow. Some plants can also take up toxic metals and lock them away in their cells to stop them causing damage. Plants growing on spoil heaps from lead mines can do this. Some plants end up with higher levels of toxic metal in their cells than exists in the soil.

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Hyperaccumulators and active transport

Some plants are so good at taking up metals that they are called "hyperaccumulators". They accumulate very high levels of metal. We are now planting these species in areas where other plants will not grow. The metal-tolerant plants take up the metals from the soil. The plants can then be harvested and the metal disposed of safely. Over years the level of dangerous metals in the soil goes down and it becomes safe for other species.

Water goes into plant roots by osmosis because there is usually more water in the soil than in the plant. Water moves from a dilute to a more concentrated solution. This needs no energy input from the plant. 

Minerals such as nitrates are usually more concentrated in plant cells than in the soil. To take these up, the plant has to use energy. Proteins in the cell membranes of root cells pump chemicals from the outside to the inside of the cell. This is called ACTIVE TRANSPORT.

Active transport is the movement of chemicals into or out of a cell, using energy from respiration. The cell controls the direction in which the chemicals move, not the difference in concentration. 

Nirate attracted to protein in cell membrane > Protein changes shape and pushes nitrate into cell. This needs energy from respiration.

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Speeding up photosynthesis

Photosynthesis produces water, oxygen and sugar, and uses carbon dioxide and water. The amount of oxygen produced is a good measure of the rate of photosynthesis. When photosynthesis is working quickly, lots of oxygen is produced. Photosynthesis needs water. There is usually enough water to keep photosynthesis working at full rate if the plant is healthy. Only plants living in very dry areas cannot get enough water for maximum photosynthesis. 

Photosynthesis also needs carbon dioxide. Carbon dioxide is rare. Only about 0.4% of the air is carbon dioxide. This is not usually enough to keep the photosynthesis at full speed. 

An increase in carbon dioxide increases the rate of photosynthesis - but only up to a point. The level part of a graph showing the rate of photosynthesis shows that the lack of something else is stopping the reaction going faster. This is called the LIMITING FACTOR because it is limiting the rate. With extra light, this limiting factor does not have an effect until much higher levels of carbon dioxide. But, eventually even with extra light, the rate of reaction stops increasing and the graph would level off.

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Measuring plant distribution

It is easy to tell the difference between a lush rainforest and a dry desert. How can we tell if an area in this country has a good amount of healthy plants? Ecologists need to identify the plants they find and then measure how common the plants are in an area.

An IDENTIFICATION KEY is a way to find the scientific name for an organism by answering yes/no questions. By examining a specimen carefully, it is possible to identify it and so find its correct scientific name. This name will be recognised by scientists all over the world - even if they do not speak the same language. So, a Malaysian researcher in the rainforests of Borneo may not recognise the name "stinging nettle", but knows what is meant by Urtica dioica. 

ABUNDANCE is a measure of how common a species is in an area. An abundant series is common; one that is less abundant will be rare. The easiest way to measure abundance is to count the plants. It is not possible to count all of the plants in an area so you need to take a sample. A QUADRAT is a frame that ecologists put on the ground. Then they can count the number of plants inside the frame. Once they have counted the plants in a number of quadrats, they can produce an average figure. An ecologist studying bluebells noticed that they tended to grow better nearer the edge of woodland. To gather data to test this idea, a sequence of quadrats was taken in a straight line from the centre of the woods out into the field beside it. A line of quadrats is called a TRANSECT. 

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We all need energy from food

To carry out movements, it requires a sequence of reactions in your eyes to see it, your brain to interpret what you see, nerve cells to carry impulses to your muscles and another set of reactions to contract the muscle fibres. All of these reactions need energy.

RESPIRATION is the chemical reaction used by all living organisms to produce the energy they need to make other reactions in their cells happen. These reactions may lead to movement (in muscles), building (synthesising) complex chemicals for growth and repair. Respiration using oxygen is called AEROBIC RESPIRATION. This type of respiration takes place in both animal and plant cells, and in some microorganisms.

Glucose + oxygen ---> carbon dioxide + water (+energy released)

Glucose is a type of simple sugar. The food we take in is converted to glucose to enable respiration to occur. Any energy left over is used to build fat, which stores energy for when we cannot get enough food.

Growth and repair of an organism need protein. Plants can make amino acids from nitrate ions absorbed through the roots and reacted with glucose produced by photosynthesis. Microorganisms can also synthesis amino acids in a similar way.

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Nitrates + glucose ---> amino acids ---> proteins

No animals can do this, so they need to get their protein from other living things. Digestion breaks down the protein in food to amino acids, and the body joins these together to make the type of protein needed for muscle building or tissue repair. Respiration provides the energy needed to drive these reactions.

When photosynthesis is fast, more sugar is produced than is needed. The plant uses energy from respiration to synthesise POLYMERS like starch from the excess glucose, which then act as a store of energy. These storage molecules depend on the exact species, but all of our food crops were chosen because they store energy as chemicals that we can digest. 

Another important polymer made by joining glucose molecules together is cellulose. This is a tough chemical that gives plant cell walls their strength. Humans cannot digest cellulose because we do not have the enzymes needed to break the bonds between the glucose units. 

Plants break down their energy stores when they need to. A potato tuber is a food store that gives up its energy when it starts to "chit". Chitting is when small shoots start to grow to become new potato plants.

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Anaerobic respiration

If the athlete needs more energy but cannot get anymore oxygen to the muscles, the body has a back-up plan... Respiration that does not require oxygen!

Glucose ---> lactic acid (+ energy released)

This is known as anaerobic respiration but it creates two problems, it does not give out as much energy and it produces lactic acid, which is poisonous.

Aerobic respiration is always the body's first choice. Respiration that does not need oxygen is called ANAEROBIC RESPIRATION.

Lactic acid is poisonous. Your muscles can cope with a small amount of it, but eventually if the levels rise too high, your muscles start hurting and thus stop working. After the race is over, the athletes body clears away the lactic acid in this equation:

Lactic acid + oxygen ---> water + carbon dioxide

The amount of oxygen needed to clear out all the lactic acid is called the OXYGEN DEBT. Runners breathe very deeply even after the race is finished to get in extra oxygen to clear the lactic acid. A good measure of fitness is to measure how quickly you recover from exercise, the faster you recover the fitter you are.

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Other anaerobic reactions

Some bacteria live in areas with low oxygen levels and have to use anaerobic respiration to get energy. Bacteria living in rivers or ponds polluted by sewage need to do this. Lactic acid is only one of the waste products they produce - you will notice some of the others with your nose!

Rice plants in paddy fields have their roots submerged in water. There is very little oxygen available, so the roots carry out anaerobic respiration.

Tetanus is a disease caused by anaerobic bacterium that is common in soils and manure. In the UK, the disease is rare, but it kills nearly 400,000 people every year across the world. Tetanus bacteria are inhibited by oxygen and can only get into the body through a puncture wound, perhaps from stepping on a dirty nail.

The wound is a low oxygen environment so the bacteria reproduce and release a poison that causes muscles to twitch, and finally to tense up completely.

About 40% of the people who are infected with tetanus, end up dying from it.

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The respiration reaction

Resperation is a series of chemical reactions that release a small amount of energy with each step. The AEROBIC respiration equation describes the overall change from the series of reactions.

glucose + oxygen ---> carbon dioxide + water (+ energy released)

C6H12O6 + 6O2 -à 6CO2 + 6H2O (+ energy released)

Respiration provides the energy to move chemicals into and out of cells by active transport. The nitrates a plant needs to make amino acids are taken up from the soil by active transport.

The respiration reaction is an exact reverse of the photosynthesis reaction - whereby the products for one are the reactants for the other. 

The athletes in endurance races make huge demands on the energy reserves of their bodies. They will use up over 40kJ in 8 hours. That is about four times the amount an average British teenager uses in a day. This energy comes from respiration. Respiration requires oxygen, so the athletes will be breathing deeply and pumping blood around their bodies very rapidly to supply the hard working muscles. Sometimes this is not quite enough.

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Organising the cell

Through an early light microscope, an animal cell looked a bit like a blob of grainy jelly. As microscopes became more powerful and scientists learned how to stain things in the cells, they were able to notice some structures in the jelly. These structures help to organise the reactions in the cell in the same way that a home is organised with different rooms for different jobs.

SIMILARITIES: the four typical cell types (animal, plant, bacterium and yeast) all have cytoplasm and a cell membrane. Cytoplasm is where most of the chemical reactions take place. This is what looked like the grainy jelly to early scientists. The outside of the cytoplasm is covered by the cell membrane, but this is far too thin to see using light telescopes. The cell membrane controls what enters and leaves the cell. Gases and water can pass in and out of the cell freely but the membrane is a barrier to other chemicals.

All the cells use DNA to store their genetic code. The genetic code carries information that the cells use to make enzymes and other proteins. In plant and animal cells, the genetic code is normally held in the NUCLEUS. The nucleus is an area of the cytoplasm separated off by a membrane. Plant and animal cells have MITOCHONDRIA embedded in the cytoplasm. Respiration takes place in the mitochondria, this process converts the energy in glucose to a form the cell can use. Anaerobic respiration uses enzymes present in the cytoplasm.

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Cell differences

DIFFERENCES: plants usually have a cell wall made up of CELLULOSE that is big enough to see with a light microscope. It let's water and other chemicals pass through easily. Some yeasts and bacteria also have cell walls but these are not usually made of cellulose. Animal cells do not have a cell wall.

Cells of green plants have CHLOROPLASTS embedded in the cytoplasm. These contain CHLOROPHYLL and carry out photosynthesis.

A yeast cell, like an animal and plant cell, has its DNA in the nucleus and it has enzymes for respiration in mitochondria. A bacterial cell has no nucleus or mitochondria. The DNA for the genes and the enzymes for respiration float free in the cytoplasm. This is less efficient than in animal and plant cells, where the important chemicals are held inside membrane bound cells.

ENZYMES are proteins, found in living things, that speed up reactions. Enzymes in biological washing powder speed up the breakdown of fat and protein. Many stains are protein (eg: blood or egg) or dirt held onto fabric by fat (eg: grease).

The enzymes break down the blood or dissolve the fat and let the mud drift away in the water. Enzymes work much better than soap for this - and at lower temperatures.

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How do enzymes work?

Enzymes are made in cells from a long chain of amino acids joined together. The correct order for the amino acids is coded by the genes. The chain is twisted around on itself to give a complicated three-dimensional shape. This shape is very important for how the enzyme works.

The lock and key model explains how an enzyme speeds up a reaction. The chemicals the enzymes work on are called the SUBSTRATES and the chemicals they produce are called the PRODUCTS. A part of the enzyme, called the ACTIVE SITE, is a special shape that allows the substrate to fit neatly into it - like a key in a lock. When the substrate is locked in, the enzyme brings about a change and the substrate is broken into two parts. These product molecules are then released from the enzyme because they do not fit. The enzyme can then be used again to break down more substrate molecules.

Some enzymes join chemicals together. Again, the molecules fit into active sites on the enzyme, but now the enzyme shape brings the important parts of the substrate molecules close together so they can react.

Once they react, they do not fit and are released. Sometimes the cell uses energy from respiration go help this reaction work. Sugar burns at over 1000oC to give out light and heat. Respiration in a cell is the same reaction but at a much lower temperature and with no energy wasted as light. Respiration is a series of mini reactions linked together in an ENZYME PATHWAY.

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Enzyme breakdown

Apples contain enzymes that react with oxygen in the air to turn the flesh into brown mushy goo. The speed at which this enzyme works is affected both by the temperature and pH. An increase in temperature tends to increase the rate of decay. When the enzyme is working as quickly as possible, the enzyme is at its OPTIMUM temperature. When the temperature goes above a certain level the enzyme is damaged and stops working altogether.

A good fruit salad stays bright and colourful even after it has been exposed to the air. A squeeze of lemon juice, or even lemonade, does the trick. The acid in the lemon juice slows down the reaction of the enzyme that makes the apple go brown.

Enzymes depend on the shape of their active site to work properly. As the temperature rises, the enzyme molecules are literally shaken apart. The active site is destroyed. Often the whole protein molecule is disrupted. You can see this when you boil an egg. The protein is disrupted so much that it forms a solid lump. This is caked DENATURING the enzyme.

A rise in temperature increases the rate of a reaction. In enzyme reactions, there is a balance between these two effects: damage to the enzyme by over-heating slows down the reaction and a rise in temperature increases the rate of reaction. A graph showing this will cause a hump.

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Preserving food

Many of the ways we preserve food depend on preventing enzymes in the food, or in micro organisms that land in it, from working. For example:
> drying - removes water from the food, making the enzymes come out of solution.
> pickling in vinegar - affects he pH, so damaging the enzymes
> freezing - slows down the spoilage reactions
> canning - sealing boiled food in an air tight container means that the boiling denatures the enzymes and the sealing stops micro organisms with new enzymes getting into the food.

Protein molecules are held together by bonds that bend the chain into the correct shape. These bonds, called hydrogen bonds, are damaged by changes in pH. This changes the shape of the active site the substrates need to link to for the enzyme to work.


Animals carry out aerobic and anaerobic respiration. Anaerobic respiration produces toxic chemicals and releases very little energy. Micro organisms have a more useful type of anaerobic respiration called fermentation. Fermentation is much more varied than aerobic respiration and different microbes produce a wide range of waste products.

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Biogas digesters

Yeast is an essential part of bread-making. The dough is mixed with a small amount of yeast in sugar and water and is then left to rinse in a warm place for up to an hour. During this time, thee yeast breaks down sugar in the mixture to make carbon dioxide and alcohol. The word equation is shown below.

Glucose ---> ethanol + carbon dioxide (+ energy released)

The carbon dioxide bubbles are trapped in the dough and make it light and fluffy. When the bread is cooked, the protein in the dough goes solid to give a network of tiny protein-coated bubbles. You can see these when you slice a loaf of bread. In China, a boiled mixture of grains is mixed with rice infected with a type of fungus. The fungus breaks down the starch to a sugary liquor that contains lactic acid. Yeast is added and fermentation begins, this can take several months but some of the most expensive wines may be left for decades.

In India, manure from cows is held in an air-tight steel container so that the microbes in the manure have to use anaerobic respiration by these microbes produces methane rather than carbon dioxide. The mixture warms up and methane bubbles up to be collected in the space in the container. The gas is drawn off at regular intervals and sold. It is used for cooking and heating. After a few weeks the microbes have been used up and the available food and respiration slows down. The manure in the digestif can be used for fertiliser on fields.

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