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  • Created on: 01-11-12 09:44









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Enzymes are biological catalysts.

There are optimum temperatures and pH values at which their activity is greatest. Enzymes are also proteins, and usually denatured above about 45ºC.

Enzymes are important in respiration. Aerobic respiration releases energy from glucose.

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What are Enzymes?

Enzymes are biological catalysts - catalysts are substances that increase the rate of chemical reactions without being used up. Enzymes are also proteins that are folded into complex shapes that allow smaller molecules to fit into them. The place where these substrate molecules fit is called the active site.

The animation shows how this works. In this example, two small molecules join together to make a larger one.

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Temperature and enzymes

As the temperature increases, so does the rate of reaction. But very high temperatures denature enzymes.

The graph shows the typical change in an enzyme's activity with increasing temperature. The enzyme activity gradually increases with temperature until around 37ºC, or body temperature. Then, as the temperature continues to rise, the rate of reaction falls rapidly, as heat energy denatures the enzyme.

Graph showing pH and enzyme activity. Between pH 4.5 and pH 8, enzyme activity increases steadily. It peaks at pH 8, then decreases fairly rapidly (

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pH and enzymes

Changes in pH alter an enzyme’s shape. Different enzymes work best at different pH values. The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5. Enzymes in the stomach have an optimum pH of about 2.

Graph showing temperature and enzyme activity. Between 0 and 40ºC, enzyme activity increases steadily. It peaks at 40°C (optimum temperature), then decreases rapidly (

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Enzymes and respiration

Enzymes in cells catalyse photosynthesis, protein synthesis - joining amino acids together, and aerobic respiration.

Aerobic respiration

Respiration is not the same thing as breathing. That is more properly called ventilation. Instead, respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the process happen in tiny objects inside the cell cytoplasm, called mitochondria.

This is the equation for aerobic respiration:

glucose + oxygen    →    carbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. In plants, for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids can then join together to make proteins. The energy is also used:

  • to allow muscles to contract in animals
  • to maintain a constant body temperature in birds and mammals

You may wish to check your understanding of how respiration affects the length of food chains, and the efficiency of food production. Visit the Food chains and cycles section for a reminder.

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a process through which plants produce their food, is of utmost importance to us humans and other living beings. A proper and indepth knowledge about this pivotal process will help you understand and preserve our beautiful environment in the long run. Click on the links below to load yourself with useful information regarding this naturally occuring chemical process, certainly the most important process that we know.

Libra ( 

Carbon dioxide + water produces -> glucose + oxygen

In chemical language, this is written as,

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Osmosis is the movement of water from a high water concentration to a low water concentration through a partially permeable membrane. Osmosis takes place in all living cells.

Plants absorb water from the soil by osmosis through their root hair cells. Plants use water for several vital processes including photosynthesis and transporting minerals.


Osmosis is one of the most challenging ideas in GCSE biology. You need to understand a particle model and be able to use it to explain this special type of diffusion that takes place through partially permeable membranes – membranes that let small particles pass through but not large particles.

Osmosis explains how water moves from high water concentration to low water concentration through a partially permeable membrane.

You need to remember:

  • 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.
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Osmosis in cells

Osmosis takes place in all cells. The cell membrane is partially permeable.

Osmosis in red blood cells.

If a red blood cell is placed in water, water enters the cell by osmosis. Because the membrane is quite weak the cell will burst as the volume and therefore the pressure in the cell increases. Red blood cells shrink when placed in concentrated solutions of sugar as water moves out of them by osmosis. This makes the cells appear wrinkled when viewed through a microscope.

This does not happen inside the body because the kidneys make sure the concentration of the blood stays about the same as the concentration of the solution inside the red blood cell.

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Osmosis in cells continued

Osmosis in plant cells

Plant cells have a strong rigid cell wall on the outside of the cell membrane. This stops the cell bursting when it absorbs water by osmosis. The increase in pressure makes the cell rigid. This is useful as plants do not have a skeleton. Instead the leaves and shoots can be supported by the pressure of water in their cells. If plant cells lose too much water by osmosis they become less rigid and eventually the cell membrane shrinks away from the cell wall.

Water entering the cell by osmosis inflates the cell and makes it rigidA plant cell that looks slightly bloated ( plant cell with walls that curve in slightly ( of water makes the cell limp and shrinks the cell membrane away from the cell wall.

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  • Lysis – bursting an animal cell by osmosis
  • Crenation – shrinking an animal cell by osmosis
  • Turgid – a plant cell fully inflated with water
  • Plasmolysed – a plant cell that has lost water causing the cell membrane to be pulled away from the inside of the cell wall
  • Flaccid – a plant cell that is limp through a reduction of pressure inside the cell
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Root hair cell (

Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis.

The absorbed water is transported through the roots to the rest of the plant where it's used for different purposes:

  • It's a reactant used in photosynthesis
  • It supports leaves and shoots by keeping the cells rigid
  • It cools the leaves by evaporation
  • It transports dissolved minerals around the plant
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Leaves are adapted for photosynthesis by having a large surface area, and contain stomata (openings) to allow carbon dioxide into the leaf. These design features can result in the leaf losing a lot of water. The cells inside the leaf have water on their surface. Some of this water evaporates, and the water vapour can then escape from inside the leaf by diffusion.

To reduce loss the leaf is coated in a wax cuticle to stop the water vapour escaping through the epidermis. Leaves usually have fewer stomata on their top surface to reduce this water loss.

Shows the waxy cuticle on top of the upper epidermis.Under this is the palisade mesophyll layer and spongy mesophyll layer, which has air spaces in it. At the bottom, is the lower epidermis and wax cuticle. Gases are exchanged through the stoma. On each side of the stoma there is a guard cell with chloroplasts. (

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Reducing water loss


Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells. Each stoma is surrounded by a pair of sausage-shaped guard cells. In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.


Most plant cells are turgid at all times. This supports the weight of the plant, which is especially important where there is no woody tissue, such as leaves, shoot and root tip. If the plant loses water faster than it can be absorbed the cells lose turgor pressure and become flaccid. This causes the plant to wilt.

You should be able to explain why most plants will wilt if they get flooded by sea water. (Hint: sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.)

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Dissolved substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

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. This is true in gases and for particles dissolved in solutions. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

Examples of diffusion

Two examples of diffusion down concentration gradients

gut  ,digested food ,products gut cavity blood in capillary of villus

Remember, particles continue to move from a high to a low concentration while there is a concentration gradient.

In the lungs, the blood will continue to take in oxygen from the alveolar air spaces provided the concentration of oxygen there is greater than in the blood. Oxygen diffuses across the alveolar walls into the blood, and the circulation takes the oxygen-rich blood away.

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diffusion cont

particles continue to move from a high to a low concentration while there is a concentration gradient.

In the lungs, the blood will continue to take in oxygen from the alveolar air spaces provided the concentration of oxygen there is greater than in the blood. Oxygen diffuses across the alveolar walls into the blood, and the circulation takes the oxygen-rich blood away.

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molocules and Diffusion

Water molocules diffuse accross the membrane from the weak sugar solution into the strong sugar solution.

This continues untill the concentratetion is the same both sides of membrane

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Active transport

Active transport is the process by which dissolved molecules move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient - and therefore require an input of energy from the cell.

Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient


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active tran (contin)

Movement of subsrance in and out by diffusion involves molocules moving down a concentration gradient from high to low concentraton.

Active tranport makes high concentration

Diffussion makes Low concentration

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mineral elements

plants need to mineral elements from the soil for healthy growth.minerals enter a plant through its roots.

The concentration of minerals in the soil is lower than that is inside a root hair cell.

Minerals enter a root cell by active transport.The plant uses energey to  move minerals up teh concentration gredient form the soil into its root cells.

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Nitrogen cycle

The nitrogen cycle is very important element,it is a major oart of proteins and all living things need proteins to make new cells.


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respiration is the process by which energy is released from glucose.

Aerobic respiration

Aerobic respiration requires oxygen. It happens in cells when glucose reacts with oxygen. Here are the word and symbol - higher only - equations:

glucose + oxygen    →    carbon dioxide + water (+ energy)

C6H12O6 + 6O2    →    6CO2 + 6H2O (+ energy)

Energy is shown in brackets in each equation because it is not a chemical substance.

Anaerobic respiration

Anaerobic respiration does not need oxygen. It happens when there is not enough oxygen for aerobic respiration. Here is the word equation:

glucose    →    lactic acid (+ energy)

Much less energy is released by anaerobic respiration than by aerobic respiration.

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