Cell level systems Revision Notes

Process Overview

Aerobic respiration involves the complete breakdown of glucose in the presence of oxygen. This highly efficient process occurs in the mitochondria and produces a large amount of energy (ATP), carbon dioxide, and water. It is the primary way living organisms, including humans, generate energy for most activities. Anaerobic respiration, however, occurs without oxygen. It is a less efficient process that only partially breaks down glucose, resulting in a much smaller energy yield. Its location varies; glycolysis occurs in the cytoplasm, and the rest occurs in the cytoplasm as well.

Reactants and Products

For aerobic respiration, the reactants are glucose and oxygen. The products are carbon dioxide, water, and a large amount of ATP (energy). The word equation is: Glucose + Oxygen → Carbon dioxide + Water + Energy. For anaerobic respiration in animals, the reactant is glucose, and the product is lactic acid along with a small amount of energy. The word equation is: Glucose → Lactic acid + Energy. In yeast and plants (alcoholic fermentation), the reactants are glucose, and the products are ethanol, carbon dioxide, and a small amount of energy. The word equation is: Glucose → Ethanol + Carbon dioxide + Energy.

Energy Yield and Oxygen Requirement

A key difference is the energy yield. Aerobic respiration generates a significantly larger amount of ATP (around 30-32 molecules per glucose molecule) compared to anaerobic respiration (only 2 ATP molecules per glucose molecule). This difference is directly linked to the presence or absence of oxygen. Aerobic respiration absolutely requires oxygen to function, allowing for the complete oxidation of glucose. Anaerobic respiration occurs specifically in the absence of oxygen, serving as an energy-generating pathway when oxygen is scarce or unavailable, such as in intensely working muscles or oxygen-deprived environments for some microorganisms.

Location and Significance

Aerobic respiration primarily takes place in the mitochondria of eukaryotic cells, following an initial glycolysis phase in the cytoplasm. It is vital for sustaining long-term, high-energy activities. Anaerobic respiration always begins with glycolysis in the cytoplasm. The subsequent steps also occur in the cytoplasm. It is crucial for providing rapid bursts of energy when oxygen supply cannot meet demand, as seen in sprinting, or for organisms living in anaerobic environments. However, its products, like lactic acid, can be detrimental if they accumulate.

Limitations of Light Microscopy

Light microscopes use visible light and glass lenses for magnification. The resolution, which is the ability to distinguish between two separate points, is limited by the wavelength of light. Structures smaller than approximately 200 nanometres cannot be clearly seen or differentiated with a light microscope. Many sub-cellular organelles are smaller than this limit, appearing only as indistinct shapes or not visible at all. This prevents detailed study of their internal architecture.

Advantages of Electron Microscopy

Electron microscopes utilise a beam of electrons instead of light. Electrons have a significantly shorter wavelength than visible light. This fundamental difference allows electron microscopes to achieve a much higher resolution, enabling the visualisation of much finer details. The resolution can reach about 0.2 nanometres, which is approximately 1000 times greater than that of light microscopes.

Impact on Understanding Sub-cellular Structures

The superior resolution of electron microscopy has enabled scientists to observe the intricate internal structures of organelles that were previously invisible or unclear with light microscopes. For example, the detailed internal membranes (cristae) within mitochondria, the extensive network of the endoplasmic reticulum, the ribosomes on the rough endoplasmic reticulum, and the internal stacks (grana) within chloroplasts have all been clearly observed. These observations have been crucial for understanding their specific functions within the cell.

What are Photosynthetic Organisms?

Photosynthetic organisms are living things that can make their own food using light energy. The most common examples are plants, but algae (like seaweed) and certain types of bacteria also perform photosynthesis. They are crucial because they convert inorganic substances into organic compounds, forming the base of most food webs.

The Process of Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy. It happens inside specialized organelles called chloroplasts in plant and algal cells. The main ingredients are carbon dioxide (from the air) and water (from the soil or environment). Light energy is absorbed, and converted into glucose (sugar) and oxygen is released as a waste product. The general word equation is: Carbon dioxide + Water → Glucose + Oxygen (in the presence of light energy and chlorophyll).

Importance as Primary Producers

Photosynthetic organisms are called primary producers because they are the first organisms in a food chain to produce organic compounds from simple inorganic ones. They 'produce' the initial food source. All other organisms, directly or indirectly, depend on these producers for their energy and nutrient supply. For example, herbivores eat plants, and carnivores eat herbivores, showing this dependency across food chains.

Biomass Production

The glucose produced by photosynthesis is used for the plant's own energy needs and growth. Any excess glucose is converted into complex organic molecules like starch, cellulose, and proteins, which build up the plant's body. This accumulated organic matter is known as biomass. When organisms consume producers, this biomass, and the energy stored within it, is transferred, forming the foundation of biomass for the entire planet.

The Process of Photosynthesis

Photosynthesis is a chemical reaction used by plants and other producers to convert light energy into chemical energy, in the form of glucose. This process primarily takes place in the chloroplasts of plant cells, which contain the green pigment chlorophyll. Chlorophyll is essential for absorbing the light energy needed to drive the reaction. The raw materials required for photosynthesis are carbon dioxide from the atmosphere and water absorbed from the soil. The products of photosynthesis are glucose, which serves as the plant's food, and oxygen, which is released as a waste product into the atmosphere.

Word Equation for Photosynthesis

The word equation for photosynthesis clearly shows the reactants and products involved: Carbon dioxide + Water → (light energy) → Glucose + Oxygen. This equation highlights that light energy is a necessary condition for the reaction to occur, as it provides the energy required to convert the simple inorganic molecules (carbon dioxide and water) into complex organic molecules (glucose) and oxygen. Glucose is a sugar, which the plant can use immediately for respiration, convert to starch for storage, or use to build other complex organic molecules.

Importance of Photosynthesis

Photosynthesis is crucial for life on Earth for several reasons. Firstly, it produces glucose, which is the primary source of energy for plants themselves. This makes plants the base of almost all food webs, providing energy for herbivores and subsequently carnivores. Secondly, photosynthesis releases oxygen into the atmosphere, which is essential for aerobic respiration in most living organisms, including humans and animals. Without this continuous production of oxygen, aerobic life as we know it would cease to exist. It also plays a vital role in removing carbon dioxide from the atmosphere, helping to regulate global temperatures.

Key Components and Their Roles

Chloroplasts are the organelles within plant cells where photosynthesis occurs. They contain chlorophyll, the green pigment responsible for absorbing light energy. Stomata, tiny pores on the surface of leaves, allow for the uptake of carbon dioxide and the release of oxygen and water vapour. Roots absorb water and mineral salts from the soil, which are then transported to the leaves. The stem transports water to the leaves and glucose from the leaves to other parts of the plant. Each component plays a specific and interdependent role in supporting the overall process of photosynthesis.