A Level

Factors Affecting Photosynthesis A limiting factor limits the rate at which a process can take place. Processes such as photosynthesis are made up of a series of small reactions. It is the slowest of these reactions that determines the overall rate of photosynthesis....

Glycolysis is the initial stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of all living cells and is the process by which a hexose sugar, usually glucose, is split into two molecules of the 3-carbon molecule, pyruvate. There are four...

The pyruvate molecules produced during glycolysis possess potential energy that can only be released using oxygen in a process called the Krebs cycle. Before they enter the Krebs cycle, these pyruvate molecules must first be oxidised in a procedure known as the link...

The Krebs cycle involves a series of oxidation-reduction reactions that take place in the matrix of the mitochondria. 1. A 2 carbon acetylcoenzyme A from the link reaction combines with a 4 carbon molecule to produce a 6 carbon molecule 2. This 6 carbon molecule loses...

Hydrogen atoms and the electrons they possess are a valuable source of energy. These hydrogen atoms are carried by coenzymes NAD and FAD into the next stage of the process: the electron transport chain. This is the mechanism by which the energy of the electrons within...

If there is no oxygen (anaerobic conditions) then the final reaction to make water cannot take place. Electrons can’t leave the respiratory chain, and so NADH cannot unload any hydrogens to the respiratory chain. This means that there is no NAD in the cell; it’s all...

Summary of respiration to see how much ATP is made from each glucose molecule. ATP is made in two different ways: Some ATP molecules are made directly by the enzymes in glycolysis or the Krebs cycle. This is called substrate level phosphorylation (since ADP is being...

Nutrient Cycles All nutrient cycles have once sequence: Nutrient is taken up by producers as simple, inorganic molecules. Producer incorporates nutrient into complex organic molecules. When the producer is eaten, the nutrient passes into consumers. Nutrient then...

Energy and Ecosystems 5.1 Food Chains and Food Webs Ultimate source of energy is sunlight converted to chemical energy by photosynthesising organisms then passed as food to other organisms Producers Photosynthetic organisms that manufacture organic substances using...

Summary Questions p.90 The carbon dioxide concentration of the atmosphere is less on a summers’ day than on a winters’ day because more plants are photosynthesising in summer. This is because light is required for photosynthesis, and more light is available in summer,...

The light dependent reaction of photosynthesis involves the capture of light whose energy is used for two purposes: To add an inorganic phosphate molecule to ADP thereby making ATP To split water by the process of photolysis, into H+ ions and OH- ions The reaction...

Plants take up most of the nitrogen they need in the form of nitrate ions which are absorbed using active transport by the root hairs. Nitrate ions are very soluble and leach through the soil. In natural ecosystems, the nitrate levels are restored through recycling of...

All plants need mineral ions, especially nitrogen. Intensive food production makes large demands on the soil because mineral ions are continually taken up by the crops being grown on it. Mineral ions crops have absorbed are being removed. In natural ecosystems the...

Leaching Leaching is the process by which nutrients are removed from the soil. Rain water will dissolve any soluble nutrients, such as nitrates, and carry them deep into the soil and eventually beyond the reach of plant roots. The leached nitrates find their way into...

The many relationships between the members of a community in an ecosystem can be described by food chains and webs. Each stage in a food chain is called a trophic level, and the arrows represent the flow of energy and matter through the food chain. Food Chains Trophic...

The Sun is the source of energy for ecosystems. However, as little as 1% of this light energy may be captured by green plants and so made available to organisms in the food chain. These organisms in turn pass only a small fraction of the energy that they receive to...

Ecological pyramids show the mass or amount of energy stored by organisms at each trophic level. Pyramids of Number These show the numbers of organisms at each trophic level. The number of organisms at lower trophic levels is greater than the number of organisms at...

Agricultural ecosystems are made up largely of domesticated animals and plants used to produce food. There are considerable energy losses at each trophic level. This means that the energy we receive from the food we eat is often only a tiny proportion of that...

Productivity In natural ecosystems productivity is relatively low. The additional input to agricultural ecosystems is used to increase the productivity of a crop by reducing the effect of limiting factors on its growth. The energy used to exclude other species means...

Integrated Pest-Control Systems Integrated pest-control systems aim to integrate all forms of pest control rather than being reliant on one type. The emphasis is on deciding an acceptable level of the pest rather than trying to eradicate it altogether. Integrated...

Intensive rearing of livestock is designed to produce the maximum yield of meat, eggs and milk at the lowest possible cost. As energy passes along a food chain only a small percentage passes from one organism in the chain to the next. This is because much of the...

The greenhouse effect is a natural process that occurs all the time and keeps average global temperatures at around 17 degrees. The most important greenhouse gas is carbon dioxide. It is highly abundant and it remains in the atmosphere for much longer than other...

The concentrations of the greenhouse gases may be very small, but they are increasing. The result of these increases is the enhanced greenhouse effect, which is causing more heat to be trapped on Earth, leading to an increase in mean global temperature called global...