Cell level systems Revision Notes

What is Photosynthesis?

Photosynthesis is the process by which photosynthetic organisms, such as plants, algae, and some bacteria, convert light energy into chemical energy. This chemical energy is stored in organic compounds, primarily glucose (a type of sugar). The raw materials required for photosynthesis are carbon dioxide, which is absorbed from the air, and water, which is absorbed from the soil by roots. Light energy, typically from the sun, provides the energy needed for this reaction. Oxygen is released as a waste product into the atmosphere during this process.

Photosynthetic Organisms as Producers

Photosynthetic organisms are known as producers because they produce their own food using light energy. They do not need to consume other organisms to obtain energy. This ability to create organic matter from inorganic substances makes them the very first link in almost every food chain on Earth. Without them, there would be no initial source of energy to power the ecosystems.

The Importance of Photosynthesis for Food and Biomass

The glucose produced during photosynthesis serves as the primary food source for the photosynthetic organisms themselves, fuelling their growth and other metabolic activities. This stored energy and organic matter form their biomass. All other organisms, including humans, animals, and fungi, are consumers that depend on these producers for their food. Herbivores eat plants directly, and carnivores eat herbivores, thus indirectly obtaining energy that originated from photosynthesis. This makes photosynthetic organisms the ultimate source of food and biomass for nearly all life on Earth.

The Role of Oxygen Production

A vital by-product of photosynthesis is oxygen gas. Photosynthetic organisms release oxygen into the atmosphere, which is essential for aerobic respiration in most living organisms, including animals and humans. This constant replenishment of atmospheric oxygen ensures the survival of organisms that rely on it for their energy release processes. Therefore, photosynthetic organisms not only produce food but also maintain the habitable atmosphere.

What is DNA?

DNA, or deoxyribonucleic acid, is the molecule that carries genetic information in all living organisms. It contains the instructions for making proteins and is responsible for inherited characteristics. DNA is found primarily in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.

DNA as a Polymer

A polymer is a large molecule made up of many identical or similar smaller units linked together. DNA fits this description as it is a polymer composed of repeating monomer units called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases.

Structure of a DNA Nucleotide

Each DNA nucleotide is made of three parts: a deoxysribose sugar, a phosphate group, and one of four nitrogenous bases. The four bases are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). The sequence of these bases along the DNA strand is what encodes genetic information.

The Double Helix Structure

DNA typically forms a double helix structure, which looks like a twisted ladder. This structure is formed by two strands of nucleotides joined together. The 'rungs' of the ladder are made of pairs of nitrogenous bases held together by weak hydrogen bonds. Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This specific pairing rule is fundamental to DNA's function.

Core Practical: Investigating Photosynthesis Rate Using Pondweed

To investigate the rate of photosynthesis, an aquatic plant like pondweed (e.g., Elodea) is often used because it visibly produces oxygen bubbles, making the process easy to observe and measure. The setup typically involves placing the pondweed in a test tube filled with pond water containing dissolved sodium hydrogen carbonate, which provides a source of carbon dioxide. A lamp is used as a light source. The rate of photosynthesis is measured by counting the number of oxygen bubbles produced by the pondweed over a set period, such as one minute. This provides a quantitative measure of the reaction's speed. The number of bubbles counted per minute can be used as an indicator of the rate of oxygen gas production, and thus, the rate of photosynthesis.

Variables in Photosynthesis Experiments

When conducting an experiment to investigate photosynthesis, it is crucial to identify and control the variables. The independent variable is the factor that is deliberately changed by the experimenter, for example, light intensity (by varying the distance of a lamp). The dependent variable is the factor that is measured, such as the rate of oxygen bubbles produced per minute. Control variables are all other factors that must be kept constant to ensure a fair test and prevent them from affecting the results. These include temperature of the water, the concentration of carbon dioxide (e.g., using a consistent amount of sodium hydrogen carbonate solution), the type and mass/length of pondweed, and the duration of each measurement.

Safety and Practical Considerations

Safety precautions are important in any scientific experiment. When working with water and electrical equipment, care must be taken to prevent electric shocks; ensure hands are dry and electrical leads are in good condition. Eye protection should be worn to protect against splashes. In terms of practical setup, a heat shield (e.g., a beaker of water) can be placed between the lamp and the test tube to prevent unwanted temperature increases from affecting the results, as temperature is a control variable. Repeating experiments and calculating averages can improve the reliability of the results. It is also important to allow the plant to acclimatise to new conditions for a few minutes before starting measurements at each setting.

What is DNA?

DNA, or Deoxyribonucleic Acid, is the fundamental genetic material present in nearly all living organisms. It contains the instructions necessary for the construction and operation of a cell or organism. DNA dictates the characteristics and heredity shared across generations, acting as the blueprint for life. Understanding its structure is key to understanding how genetic information is stored and passed on.

The Double Helix Structure

The most defining characteristic of DNA is its 'double helix' structure. This can be visualized as a twisted ladder. The two long 'sides' of this ladder are formed by repeating sugar and phosphate groups, making up the 'backbone' of each strand. These two strands are then connected by 'rungs' made of paired nitrogenous bases. The double helix arrangement allows for efficient compaction of the lengthy DNA molecule within the cell nucleus.

Composition of DNA Strands

Each DNA strand is a polymer, meaning it's composed of many smaller repeating units called nucleotides. Every nucleotide consists of three main components: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. These bases are Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The sequence of these bases along a strand codes for genetic information. The specific pairing of these bases between the two strands is critical for DNA's function.

Base Pairing Rules

Within the double helix, the nitrogenous bases from one strand form specific pairs with bases from the complementary strand. This pairing is highly specific: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). These pairs are connected by weak hydrogen bonds, which allow the strands to be easily separated for processes like DNA replication and protein synthesis. This complementary base pairing rule is fundamental to how genetic information is copied accurately.