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Cell level systems Revision Notes
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Cell level systems
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The Structure of DNA: The Molecule of Life Revision Notes
What is DNA?
DNA, or deoxyribonucleic acid, is a complex molecule that contains all the genetic information for building and maintaining an organism. It is found in the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells. DNA carries the instructions that living organisms need to grow, develop, survive, and reproduce. It is a polymer, which means it is made up of many repeating smaller components called monomers.
The Basic Unit of DNA: The Nucleotide
The basic repeating unit, or monomer, of DNA is called a nucleotide. Every single DNA molecule is constructed from billions of these nucleotides linked together. There are four different types of nucleotides in DNA, and each type is distinct due to a specific component. Despite their differences, all nucleotides share a common structural pattern.
Components of a DNA Nucleotide
Each DNA nucleotide is made up of three sub-components: a phosphate group, a deoxyribose sugar, and a nitrogenous base. The phosphate group is a negatively charged molecule. The deoxyribose sugar is a five-carbon sugar that acts as a central linker. The nitrogenous base is the variable part of the nucleotide and is responsible for encoding genetic information. These three parts are joined together chemically.
The Four Different Bases
There are four different types of nitrogenous bases that can be attached to the deoxyribose sugar in a DNA nucleotide. These are adenine (A), guanine (G), cytosine (C), and thymine (T). The difference between these four bases is their specific chemical structure. Therefore, the four different DNA nucleotides are distinguished by which one of these four bases they contain: an A-nucleotide, a G-nucleotide, a C-nucleotide, or a T-nucleotide.
Understanding Photosynthesis: The Basis of Life Revision Notes
The Process of Photosynthesis
Photosynthesis is a key biological process carried out by plants, algae, and some bacteria. It involves converting light energy, usually from the sun, into chemical energy in the form of glucose. This stored energy is crucial for the organism's survival and growth. The process also produces oxygen as a byproduct, which is released into the atmosphere.
Inputs and Products of Photosynthesis
The essential raw materials for photosynthesis are carbon dioxide and water. Carbon dioxide is taken in from the air, and water is absorbed from the soil. The light energy is absorbed by chlorophyll, a green pigment found in chloroplasts, which acts as the energy source for the reaction. The main products formed are glucose, a sugar used by the plant for energy and growth, and oxygen, which is released into the atmosphere.
Overall Word Equation
The entire process of photosynthesis can be summarised by a simple word equation: Carbon dioxide + Water → (light energy) → Glucose + Oxygen. This equation highlights the reactants (carbon dioxide and water), the energy input (light energy), and the products (glucose and oxygen). Understanding this equation is fundamental to grasping how plants create their own food.
Importance of Photosynthesis
Photosynthesis is not only vital for the plants that perform it but also for nearly all life on Earth. It provides the primary source of organic compounds (food) that underpin most food chains. Furthermore, it continuously replenishes the oxygen in the Earth's atmosphere, which is essential for aerobic respiration in animals, humans, and many microorganisms. Without photosynthesis, life as we know it would not exist.
Sub-cellular Structures and Their Functions in Eukaryotic and Prokaryotic Cells Revision Notes
Eukaryotic Cell Structures and Functions (Animals)
Animal cells are eukaryotic and contain several key sub-cellular structures, each with a specialised role. The nucleus houses the cell's genetic material (DNA) organised into chromosomes and controls all cell activities by regulating gene expression. Mitochondria are known as the 'powerhouses' of the cell; they are the sites of aerobic respiration, where glucose is broken down with oxygen to release energy in the form of ATP. The cytoplasm is a jelly-like substance where most chemical reactions, such as anaerobic respiration, take place. The cell membrane is a partially permeable barrier that surrounds the cell, controlling which substances enter and leave. Ribosomes are small organelles responsible for synthesising proteins based on instructions from the nucleus. These structures collectively enable animal cells to carry out essential life processes.
Eukaryotic Cell Structures and Functions (Plants)
Plant cells are also eukaryotic, containing the same structures as animal cells (nucleus, mitochondria, cytoplasm, cell membrane, ribosomes) but with additional specialised organelles. The cell wall, made of cellulose, surrounds the cell membrane and provides structural support to the cell, preventing it from bursting and maintaining its shape. Chloroplasts contain chlorophyll and are the sites of photosynthesis, the process by which light energy is converted into chemical energy (glucose). Plant cells also feature a large, permanent central vacuole filled with cell sap, which helps maintain turgor pressure against the cell wall, supporting the cell and storing water, nutrients, and waste products. These structures allow plant cells to perform specific functions like photosynthesis and support.
Prokaryotic Cell Structures and Functions
Prokaryotic cells, such as bacteria, are generally much simpler and smaller than eukaryotic cells. They lack a true nucleus; their genetic material (a single, circular loop of DNA) is free in the cytoplasm. They also lack most other membrane-bound organelles. Essential structures include the cell membrane, which regulates the passage of substances; cytoplasm, the site of chemical reactions; and ribosomes, for protein synthesis. Many prokaryotes have a cell wall (often made of peptidoglycan) for structural support and protection. Some possess a capsule outside the cell wall for additional protection, a flagellum for movement, and plasmids, which are small, extra rings of DNA that can carry genes for antibiotic resistance or other traits. Their simpler structure allows for rapid growth and division.
How Structure Relates to Function
The relationship between sub-cellular structure and function is crucial for cell survival. For example, mitochondria have a folded inner membrane to increase their surface area for the enzymes involved in aerobic respiration, maximising energy production. Chloroplasts contain stacks of thylakoids (grana) to provide a large surface area for chlorophyll and the light-dependent reactions of photosynthesis. The cell wall in plants and prokaryotes is rigid to provide structural support. The cell membrane is selectively permeable, allowing it to control what enters and exits the cell. Ribosomes are tiny to fit into the cell and are numerous, reflecting the constant need for protein synthesis. This strong link ensures that each part of the cell is optimised for its specific role.
Photosynthesis as an Endothermic Reaction Revision Notes
What is Photosynthesis?
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy. This chemical energy is stored in glucose. The process uses carbon dioxide and water as reactants and produces glucose and oxygen as products. Chlorophyll, found in chloroplasts, is essential for absorbing light energy.
The Photosynthesis Equation
The balanced chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This equation shows that carbon dioxide and water are consumed, and glucose (sugar) and oxygen are produced. Light energy is required to make this reaction happen.
Photosynthesis as an Endothermic Reaction
An endothermic reaction is a chemical reaction that absorbs energy from its surroundings. Photosynthesis is a prime example of an endothermic reaction because it absorbs light energy from the sun. This absorbed energy is necessary to drive the reaction and is ultimately stored in the chemical bonds of the glucose molecules produced.
Energy Transfer in Photosynthesis
The light energy absorbed during photosynthesis is converted into chemical energy. This chemical energy is then stored in the glucose molecules. This stored energy is crucial for the plant's growth and metabolic processes, and it also forms the base of most food chains, as other organisms consume plants to obtain this energy.
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