Cell level systems Revision Notes

What is Photosynthesis?

Photosynthesis is the process used by green plants and some other organisms to convert light energy into chemical energy. This chemical energy is stored in carbohydrate molecules, such as glucose, which are synthesised from carbon dioxide and water. The process releases oxygen as a byproduct. It is arguably the most important biochemical reaction on Earth, sustaining almost all life forms directly or indirectly.

Reactants and Products of Photosynthesis

The essential reactants for photosynthesis are carbon dioxide (CO₂) and water (H₂O). Carbon dioxide is taken in from the atmosphere through small pores called stomata on the leaves, while water is absorbed from the soil by the roots. The main products are glucose (C₆H₁₂O₆), a sugar used as an energy source and for structural components, and oxygen (O₂), which is released into the atmosphere. The word equation is: Carbon dioxide + Water → Glucose + Oxygen.

Energy Change in Photosynthesis

Photosynthesis is an endothermic reaction. This means that it absorbs energy from its surroundings. The energy absorbed is light energy, usually from sunlight. Without this input of light energy, the reaction would not occur. The absorbed light energy is converted into chemical potential energy, which is stored within the chemical bonds of the glucose molecules produced. This stored energy is then available for the plant's metabolic activities or for organisms that consume the plant.

What is Cellular Respiration?

Cellular respiration is the process by which living cells break down nutrient molecules, like glucose, to release energy. This energy is then used to power various cellular activities, such as muscle contraction, active transport, and synthesising new molecules. It is a continuous process essential for the survival of almost all life forms on Earth. The overall process converts chemical energy stored in organic compounds into ATP, the cell's main energy currency, and also releases heat.

Exothermic Nature of Respiration

An exothermic reaction is a chemical reaction that releases energy into its surroundings, typically in the form of heat. Cellular respiration is a prime example of an exothermic process. When glucose and oxygen react, the chemical bonds in these molecules are broken, and new, more stable bonds are formed in carbon dioxide and water. The energy released from forming these new bonds is greater than the energy absorbed to break the initial bonds, resulting in a net release of energy. This released energy powers cellular functions, but also warms the organism.

Energy Release and Heat

During cellular respiration, approximately 40% of the energy from glucose is captured in the form of ATP molecules, which are used to fuel cell activities. The remaining 60% of the energy is released as heat. This heat is not wasted; it plays a crucial role in maintaining the body temperature of warm-blooded animals (homeostasis). For example, when you shiver, your muscles rapidly respire to generate extra heat and increase your body temperature. The continuous release of heat underscores why respiration is classified as an exothermic process.

Understanding Limiting Factors

A limiting factor is a variable that restricts the rate of a process when it is in short supply. For photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration, and temperature. If any one of these is below the optimum level, it will limit the rate of photosynthesis, even if the other two factors are present in abundance. This concept is crucial for understanding how environmental conditions affect plant growth and crop yield. For example, on a dull day, light intensity is likely the limiting factor, while on a bright, cold day, temperature might be.

Impact of Light Intensity

Light provides the energy for photosynthesis. As light intensity increases, the rate of photosynthesis generally increases proportionally, up to a certain point. Beyond this point, the plant is receiving all the light it needs, and another factor (like carbon dioxide or temperature) will become limiting. If light intensity continues to increase, but another factor is limiting, the rate of photosynthesis will plateau. In the dark, photosynthesis cannot occur as there is no light energy to initiate the reactions.

Role of Carbon Dioxide Concentration

Carbon dioxide is a raw material for photosynthesis. An increase in carbon dioxide concentration generally leads to an increased rate of photosynthesis, provided that light intensity and temperature are not limiting. As with light, there is a maximum rate beyond which increasing carbon dioxide concentration will have no further effect, because another factor will become limiting. Plants absorb carbon dioxide from the atmosphere through their stomata. Low environmental carbon dioxide levels can significantly reduce photosynthetic efficiency.

Effect of Temperature

Temperature affects the rate of photosynthesis because it influences enzyme activity. Photosynthesis involves numerous enzyme-controlled reactions. As temperature increases from cold conditions, the rate of photosynthesis generally increases, reaching an optimum temperature where enzymes work most efficiently. Beyond this optimum, a further increase in temperature causes the enzymes to denature, dramatically reducing their activity and thus rapidly decreasing the rate of photosynthesis. Extreme temperatures, both too low and too high, can be detrimental to plant growth.

Recognising Mitochondria

Mitochondria supports Structure and Function of Sub-cellular Components in Eukaryotic and Prokaryotic Cells by giving students a diagram they can connect to the specification point being tested.

What To Explain

The answer should name the cell structure and link it to its function, such as control, respiration, photosynthesis or protein synthesis.

Syllabus Link

Primary private mapping: OCR GCSE Biology J250, Cell level systems.

Exam Answer Move

The mapped objective is: Account for the way the main sub-cellular structures of eukaryotic cells (plants and animals) and prokaryotic cells are related to their functions. Diagram marks usually come from linking a named structure to what it does.