OCR Categories Archives: B4: The Processes of Life

B4.3 How do living organisms obtain energy?

B4.3 How do living organisms obtain energy?

All living organisms require energy released by respiration for some chemical reaction in cells – the energy is used for:

  • Movement
  • Synthesising (making) larger molecules
  • Active transport

Large molecules, such as starch and cellulose are synthesised from smaller molecules such as glucose in plant cells. This involves joining the glucose molecules (monomers) together to form a polymer (made of many units)






Amino acids are synthesised from glucose and nitrates. Proteins are made in plant, animal and bacterial cells from strings of amino acids joined together


AEROBIC RESPIRATION releases energy through the breakdown of glucose molecules, by combining them with oxygen inside living cells. The majority of animal and plant cells  and some microorganisms respire aerobically.


ANAEROBIC RESPIRATION takes place in conditions of low oxygen or absence of oxygen to include:

  • Animals cells (e.g. in humans during vigorous exercise)
  • Plant cells (e.g. in plant roots in waterlogged soil)
  • Microbial cells (bacteria in puncture wounds)

The equation for anaerobic respiration in animal cells and some bacteria is:

The equation for anaerobic respiration in plant cells and some microorganism (including yeast, which is used in brewing and making bread) is:

Aerobic respiration releases more energy per glucose molecule than anaerobic respiration – a maximum of 18 times as much. In humans anaerobic respiration can only occur for a short period.

Animal cell:

  • Cell membrane – allows gases and water to enter and leave the cell freely while acting as a barrier to other, larger chemicals
  • Nucleus – contains the DNA that carries the genetic code for making proteins, including the enzymes needed in respiration
  • Cytoplasm – where proteins, including enzymes used in anerobic respiration are made
  • Mitochondria – where aerobic repiration occurs.

Microbial cells have similar structures, but with some important differences:

BACTERIA – an important feature of bacteria is that they do not have any membrane-bound organelles. Therefore they do not have a nucleus or mitochondria




YEAST – yeast is a type of fungus – it is used to make bread and alcohol. Unlike bacteria yeast does have membrane-bound organelles.



Applications of Anaerobic respiration – Biotechnology has enabled us to use the products of anaerobic respiration

Making bread – Yeast is added to a dough, made from flour, salt, water and other ingredients. The dough is effectively a source of glucose that is needed for anaerobic respiration

Brewing alcohol – Brewing involves a fermentation process:

Aerobic fermentation – the yeast is exposed to air and grows rapidly on the sugar provided. Some alcohol is produced but the majority of energy is used to produce more yeast cells.

Anaerobic respiration – this takes place in the absence of oxygen. The yeast respires anaerobically and produces alcohol and carbon dioxide instead of multiplying

Biogas – It is now possible to introduce bacteria to biodegradable substances such as manure, sewage and household waste in landfill sites. The anaerobic digestion leads to the production of methane (an explosive gas) and carbon dioxide. The methane can be used as a low-cost fuel.






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B4.2 How do plants make food?

B4.2 How do plants make food?

Photosynthesis can be written as


There are three main stages in photosynthesis:

❶ Light energy is absorbed by the green chlorophyll

❷ Energy used to bring about the reaction between carbon dioxide and water to produce glucose (a sugar)

❸ Oxygen produced as a waste product

Glucose is made up of carbon, hydrogen and oxygen atoms. Glucose made by the process of photosynthesis may be used in three ways:

❶ It can be converted into chemicals required for growth of plant cells such as cellulose

❷ It can be converted into starch, a storage molecule, that can be converted back to glucose when the plant requires it

❸ It can be broken down during the process of respiration, releasing energy stored in the glucose molecules

Plant cell:

  • Cell wall – provides support for cell
  • Cell membrane – allows gases and water to pass in and out of the cell freely while acting as a barrier to other, larger chemicals
  • Nucleus – contains DNA which carries the genetic code for making enzymes and other proteins used in the chemical reactions of photosynthesis
  • Vacuole – used by the cell to store waste materials and to regulate water levels
  • Cytoplasm – where the enzymes and other proteins are made
  • Mitochondria – where respiration occurs
  • Chloroplasts – contain chlorophyll and the enzymes for the reactions in photosynthesis



Plants need other chemicals in addition to glucose. The roots take up minerals from the soil in solution. Nitrogen, in the form of nitrates is absorbed and used by plant cells to make proteins.

Substances move through cells via the process of DIFFUSION. Diffusion is the overall movement of a substance from a region where it is in high concentration to an area where it is in lower concentration. Diffusion is a passive process as it does not need an energy input to happen.



OSMOSIS is a specific type of diffusion – it is the overall movement of water from a dilute to a more to a more concentrated solution through a partially permeable membrane. A partially permeable membrane allows water molecules through, but not solute molecules because they are too large.

The movement water into plant roots occur by osmosis.




ACTIVE TRANSPORT is the overall movement of a chemical substance across a cell membrane (from where the substance is in low concentration to where it is in higher concentration). This requires energy, which is provided by respiration. Active transport is used in the absorption of nitrates by plant roots.

There are several factors that can limit the rate the rate of photosynthesis:

  • Temperature – too low and photosynthesis stops until the temperature rises again. Too high and the enzymes stop working permanently
  • Carbon dioxide concentration – as carbon dioxide concentration increases, so does the rate of photosynthesis
  • Light intensity – light is needed for photosynthesis. The greater the availability of light the quicker photosynthesis will take place

To identify the effect of light on plants, biologists have to carry out fieldwork. This involves using a variety of techniques to measure the amount of available light and to see how this has affected the growth of plants.

  • A light meter can measure the amount of light that is hitting the leaf. The amount of light is measured in units of lux. Data-loggers can be fitted with a light meter and readings taken over a period of time.
  • A quadrat is a square shape, often divided up into smaller squares. The quadrat is placed randomly on sections of the area in question and the plants that fall inside the quadrat are vaunted
  • It is vital to use an identification key to ensure that plants are correctly identified. A key enables the rapid identification of plants and animals by asking questions such as ‘does the plant have parallel veins in its leaves?’

Sometimes it is preferable to measure the changes in plant life along a straight line – in this case a transect may be taken. A line is drawn and the quadrat is placed at set intervals along the line and the plants are counted. This gives a picture of the changes in plant life over the line of the transect.


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B4.1 How do chemicals reactions take place in living things

B4.1 How do chemicals reactions take place in living things

All living cells are made from basic units – these are called CELLS.

The processes of life carried out by all living cells depend on chemical reactions within cells – these reactions need energy released by RESPIRATION (the release energy from food). RESPIRATION is one of seven major life processes:

PHOTOSYNTHESIS – makes food molecules and energy available to living organisms through food chains.

Photosynthesis can be summarised by saying that carbon dioxide and water are combined to produce sugar and oxygen in the presence of light and chlorophyll.

Photosynthesis uses sunlight to build large molecules in plant cells and some microorganisms (e.g. phytoplankton)

Enzymes are biological catalyst; they are proteins that speed up chemical reactions. Cells make enzymes according to the instructions carried in genes. Enzymes are specific so only molecules with the correct shape can fit into the enzyme – this is called the lock and key model.


Once the enzyme and molecule (substrate) are linked the reaction takes place, the products are released and the process is able to start again.



For enzymes to work to their optimum they need a specific and constant temperature:

Different enzymes have different optimum working temperatures. For example, in the human body enzymes work best at 37˚C . Below this temperature their rate of action slows down however, if the temperature gets too high the enzyme is DENATURED and stops working.



The biological name for the process of permanent change in an enzyme’s shape is denaturing. The place where the substrate fits to the enzyme is called the ACTIVE SITE. When subjected to high temperatures, the shape of the active site changes irreversibly. This means molecules can no longer fit and the reaction stops


Enzyme activity at different temperatures in a balance between:

  • Increased rates of reaction as temperature increase
  • Changes to the active site at higher temperatures, including denaturing

The active site is influenced by pH.

Changes in pH in the enzyme’s environment can make and break intra- and intermolecular bonds in the enzyme. This changes the shape of the active site and its effectiveness.

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