B4: The Processes of Life

• All living things are made from basic units called cells.
• Chemical reactions take place inside cells to enable them to function.
• All chemical reactions need energy to take place which comes from respiration (release of energy from food).
• The energy in food ultimately comes from the sun.
• Plants (and some microorganisms) at the start of the food chain capture light energy through photosynthesis, making food molecules. Energy is then transferred through food chains.
• Photosynthesis is when carbon dioxide and water are combined to produce sugar and oxygen in the presence of light and chlorophyll.
• Photosynthesis is a series of reactions, some in the dark, some in the light.
• Respiration is when sugar and oxygen are combined to produce carbon dioxide and water, releasing energy in the process.
• Respiration is a series of reactions releasing energy from large food molecules in all living cells

• Enzymes are organic catalysts (speed up the rate of chemical reactions in living things).
• Enzymes are a type of protein coded for by instructions carried in genes.
• Only a molecule with the correct shape can fit into an enzyme similar to a key (e.g. the substrate) fitting into a lock (e.g. the enzyme)
• Once the enzyme and molecule 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. 
• The peak of the graph shows the maximum enzyme activity which is at the optimum temperature.
• Different enzymes have different optimum pH.
• Different enzymes have different optimum working temperatures (e.g. the human body enzymes work best at around 37 degrees. Below this temperature their rate of action slows down while above 40 degrees they denature and the active site will now be a different shape and the substrate won't fit anymore - this is an irreversible change).
• Enzymes can speed up catabolic reactions (where molecules are broken down) and anabolic reactions (where new molecules are built up)
• The pH also influences the shape and effectiveness of the active site.

carbon dioxide + water light energy  /> glucose + oxygen

There are 3 stages of photosynthesis :

• Light energy is absorbed by a green chemical (chlorophyll) in green plants which is not used up in the process as it is not a reactant.
• Within the chlorophyll molecule, the light energy is used to rearrange the atoms of carbon dioxide and sugar to produce glucose (a sugar).
• Oxygen is produced as a by-product, it exits the plant via the leaf or can be reused by the plant in respiration.

• Glucose is effectively a carbon skeleton that can be used to build many other molecules in living things.
• e.g. cellulose is a structural carbohydrate that makes cell walls in plants. It is very strong and helps to provide support. Cellulose is made of repeating molecules of a certain form of glucose
• Protein is made up of different amino acid molecules which were made from glucose. They are vital for growth and repair.
• Chlorophyll is the organic pigment where the first stages of photosynthesis take place. It is also produced using a skeleton based on carbon.
• To store glucose, it is converted into another carbohydrate suitable for storage called starch.
• Starch is a long chain of glucose units that can be packed together very efficiently.
• Glucose is used for energy which is released by respiration.

•  Chloroplasts - contain chlorophyll and the enzymes for some reactions of photosynthesis. These are only present in green parts of the plant.
• Cell Membrane - allows dissolved gases and water to enter and leave the cell freely while acting as a barrier to other, larger chemicals.
• Nucleus - contains DNA that carries the genetic code for making enzymes and other proteins.
• Cytoplasm - where enzymes and other proteins are made.
• Mitochondria - where respiration occurs.
• Vacuole - used by the cell to store waste materials and to regulate water levels.
• Cell wall - provides support for the cell.                                                                                                                                              

• 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 the plant cells to make new 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. (e.g. if the concentration of carbon dioxide in a plant cell is lower than the concentration outside, then carbon dioxide will diffuse into the cell).
• Diffusion is the main method by which gases enter and leave the plant.
• Gases such as carbon dioxide and oxygen exchange between the leaf and the surrounding air through small holes on the underside of a leaf, called stomata.

• Osmosis is a specific type of diffusion. It is the overall movement of water from a dilute to a more concentrated solution through a partially permeable membrane.
• A partially permeable membrane allows water molecules through but not solute molecules as they are too large.
• The effect of osmosis is to gradually dilute the concentrated solution.
• This is what happens at root hair cells, where water moves from the soil into the cells by osmosis due to a concentration gradient.

• Active transport is the overall movement of a chemical substance against a concentration gradient (e.g. from where the substance is in low concentration to where it is in a higher concentration). This requires energy, which is provided by respiration.
• An example of active transport is found in plant roots. Plants require nitrates. The nitrate concentration inside the plant is naturally higher than the outside. The plant cells use active transport to bring the nitrate from where it is in low concentration to where it is at a higher concentration in the root cell.

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

• Temperature - too low and photosynthesis stops until the temperature rises again. Too hot 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 availabilty of light, the quicker photosynthesis will take place.

• On each graph, the rate of reaction stays the same or, in the case of temperature, drops. This indicates that the rate of photosynthesis is now limited by one of the other factors (except for temperature, where enzymes are denatured so that photosynthesis stops).

• 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 see how this has affected the growth of plants.
• A light meter can measure the amount of light that is hitting a 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.
• It is impractical to count all of the plants growing in a given area so, biologists sample a proportion of the available land to accurately estimate how many plants there are.
• To do this, biologists use quadrats. 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 inside the area of the quadrat are counted.
• Alternatively, the area covered by the leaves of plants can be counted.
• A key is used to ensure the plants are correctly identified which enables rapid identification.
• The answers rapidly lead to an idea of what the plant or animal is.
• Sometimes it is preferable to measure the changes in plant life along a straight line so a transect may be taken. A line is drawn and the quadrat is placed at set intervals along the line and the plants counted as before which gives a picture of the changes in plant life.

All living organisms require energy for chemical reactions inside cells. This energy is released through the process of respiration. The energy is used for:

• movement.
• synthesising (making) larger molecules.
• active transport.

• Larger 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.
• Glucose + Oxygen ---------// carbon dioxide + water + energy released

• Anaerobic respiration takes place in animal cells (e.g. in humans during vigorous exercise), plant cells (e.g. in plant roots in waterlogged soil) and in some microbial cells (e.g. bacteria in puncture wounds) when there is no oxygen or oxygen supply is low.
• In animal cells and some bacteria: glucose---// lactic acid + energy released
• In plant cells and some microorganisms (including yeast, which is used in brewing and making bread): glucose -------// ethanol + carbon dioxide + energy released

• Aerobic respiration releases more energy per molecule than anaerobic respiration - a maximum of 18 times as much. Humans can only anaerobically respire for a short period.

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 respiration.
• The dough also provides an anaerobic environment for the yeast cells to grow and multiply.
• As they grow, they respire anaerobically - producing ethanol and carbon dioxide gas as waste products.
• The gas bubbles of carbon dioxide released by the yeast are trapped in a molecule called gluten. This makes the dough expand. The ethanol quickly evaporates.
• Once the dough has risen and has been baked, all that is left is the expanded structure of the bread.

Brewing involves a fermentation process. There are two stages:

• Aerobic Fermentation lasts about a week and is the stage where the yeast is exposed to air and grows very rapidly on the sugar provided. Some alcohol is produced but the majority of energy is used to produce more yeast cells.
• Anaerobic Fermentation lasts for weeks or months. This takes place in the absence of oxygen. The yeast respires anaerobically and produces alcohol and carbon dioxide instead of multiplying. Once brewed, the carbon dioxide escapes (unless it is needed by the brewer, e,g, in producing champagne).

• Approximately 70% of the fermentation takes place aerobically.
• The remaining 30% of the fermentation takes place anaerobically.
• You would not get more alcohol if it was left longer because the alcohol as a waste product is poisonous to the yeast and eventually it reaches a level high enough to kill the yeast.
• For stronger alcohol content, distillation has to be undertaken.

• 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.
• As the methane is generated from materials that were thrown away, it is regarded as being a renewable energy source.
• The United Nations consider the gas to be a key energy source for the future.