Comprehensive Biology Notes: Nature, Variety, and Structure of Living Organisms

The Nature and Variety of Living Organisms

1a. Characteristics of Living Organisms

Living organisms are distinguished from non-living matter by eight specific processes. To be classified as a living organism, an entity must demonstrate all of the following characteristics:

• Nutrition: Living organisms require a source of energy and raw materials to maintain life. Plants make their own food through the process of photosynthesis (autotrophic), while animals must consume other organisms to obtain their nutrients (heterotrophic).
• Respiration: This is a chemical process that takes place within the cells of all living organisms. It involves the breakdown of nutrient molecules (typically glucose) to release energy in the form of Adenosine Triphosphate ($ATP$). This energy is used for various metabolic processes.
• Excretion: Metabolism produces waste products (such as $CO_2$ from respiration or urea from the breakdown of proteins) which can be toxic if they accumulate. Excretion is the removal of these waste products from the body.
• Response to Stimuli (Sensitivity): Organisms have the ability to detect changes in their internal or external environment (stimuli) and coordinate a response to these changes to ensure survival.
• Movement: All living things show movement. This may involve the movement of the whole body (locomotion) or the movement of specific parts (such as a plant turning its leaves toward light).
• Control of Internal Conditions (Homeostasis): Living organisms maintain stable internal environments within restricted limits, such as the regulation of internal temperature, water content, and blood glucose levels.
• Reproduction: The process by which organisms produce offspring, ensuring the continuation of their species. This can occur sexually (two parents) or asexually (one parent).
• Growth: A permanent increase in size and dry mass. In multicellular organisms, this occurs through the increase in the number and size of cells.

1b. Variety of Living Organisms

Living organisms are classified into different groups based on their cellular structure and nutritional methods. They are broadly divided into eukaryotic and prokaryotic organisms.

Eukaryotic Organisms

• Plants: Multicellular organisms that contain chloroplasts and are capable of photosynthesis. They have cell walls composed of cellulose. They store carbohydrates as starch or sucrose. Examples include flowering plants like maize ($Zea\,mays$) or herbaceous legumes like peas.
• Animals: Multicellular organisms that do not contain chloroplasts and cannot photosynthesize. They lack cell walls and typically possess nervous coordination. They often have the ability to move from one place to another. Animals store carbohydrates as glycogen. Examples include mammals (e.g., humans) and insects (e.g., houseflies).
• Fungi: Organisms that cannot carry out photosynthesis. Their body is usually organized into a mycelium, which is made from thread-like structures called hyphae containing many nuclei. Some are single-celled (e.g., yeast). They have cell walls made of chitin and feed by saprotrophic nutrition (extracellular digestion where enzymes are secreted onto food and products are absorbed). They store carbohydrates as glycogen. Examples include $Mucor$ (multicellular) and yeast (unicellular).
• Protoctists: These are microscopic, single-celled organisms. Some, like $Amoeba$, live in pond water and have features like animal cells. Others, like $Chlorella$, possess chloroplasts and are more like plants. Some are pathogenic, such as $Plasmodium$, which causes malaria.

Prokaryotic Organisms

• Bacteria: Microscopic, single-celled organisms. They possess a cell wall, cell membrane, cytoplasm, and plasmids. They lack a nucleus but contain a circular chromosome of DNA. While some can photosynthesize, most feed on living or dead organisms. Examples include $Lactobacillus\,bulgaricus$ (used in yogurt production) and $Pneumococcus$ (a pathogen causing pneumonia).

Viruses

• Viruses: These are non-living, microscopic particles that are smaller than bacteria. They are parasitic and can only reproduce inside living cells (hosts). They infect every type of living organism. They have no cellular structure but possess a protein coat and one type of nucleic acid (either DNA or RNA). Examples include the Tobacco Mosaic Virus, the Influenza virus, and HIV (which causes AIDS).

Structure and Functions in Living Organisms

2a. Levels of Organization

Biological structures are organized into a hierarchy of increasing complexity:

• Organelles: Specialized structures within a cell that perform specific functions (e.g., mitochondria).
• Cells: The basic building blocks of all living organisms (e.g., a muscle cell).
• Tissues: A group of similar cells working together to perform a specific function (e.g., muscle tissue).
• Organs: A group of different tissues working together to perform a specific job (e.g., the heart).
• Organ Systems: A group of organs working together to perform body functions (e.g., the circulatory system).

2b. Cell Structure

The following structures are found in eukaryotic cells:

• Nucleus: Contains the genetic material (DNA) and controls the activities of the cell.
• Cytoplasm: A jelly-like substance where most chemical reactions take place. It contains organelles.
• Cell Membrane: Controls the entry and exit of substances into and out of the cell.
• Cell Wall: Found in plants, fungi, and bacteria; it provides structural support and protection. Plant cell walls are made of cellulose.
• Mitochondria: The site of aerobic respiration, where energy ($ATP$) is released.
• Chloroplasts: Found in plant cells; they contain chlorophyll and are the site of photosynthesis.
• Ribosomes: Small organelles responsible for protein synthesis.
• Vacuoles: Large permanent vacuoles in plants contain cell sap and help maintain cell turgidity.

2c. Biological Molecules

Living organisms are composed of organic molecules consisting of specific chemical elements:

• Chemical Elements:
  • Carbohydrates: Carbon ($C$), Hydrogen ($H$), Oxygen ($O$).
  • Lipids: Carbon ($C$), Hydrogen ($H$), Oxygen ($O$).
  • Proteins: Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), Nitrogen ($N$).
• Molecular Structure:
  • Carbohydrates: Large molecules like starch and glycogen are made from simple sugars (glucose).
  • Proteins: Large molecules made from long chains of amino acids.
  • Lipids: Composed of three fatty acid molecules and one glycerol molecule.
• Food Tests:
  • Glucose: Add Benedict’s solution and heat. Positive result: Blue to Brick-red.
  • Starch: Add Iodine solution. Positive result: Yellow-brown to Blue-black.
  • Proteins: Add Biuret solution. Positive result: Blue to Violet/Purple.
  • Lipids: Perform the Ethanol emulsion test. Positive result: Cloudy/Milky white emulsion.
• Enzymes: Biological catalysts that speed up metabolic reactions. They have an active site that fits a specific substrate. Factors like temperature and pH affect enzyme activity; excessive heat or extreme pH levels can cause the enzyme to denature (lose its shape).

2d. Movement of Substances Into and Out of Cells

Substances move across cell membranes via three primary mechanisms:

• Diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration. This is a passive process.
• Osmosis: The net movement of water molecules from a solution with a higher water potential (dilute) to a solution with a lower water potential (concentrated) across a partially permeable membrane.
• Active Transport: The movement of substances against a concentration gradient (from low to high concentration). This requires energy ($ATP$) from respiration.
• Factors Affecting Movement Rate:
  • Surface area to volume ratio.
  • Distance.
  • Temperature.
  • Concentration gradient.

2e. Nutrition

• Photosynthesis: Plants use light energy to convert carbon dioxide and water into glucose. The balanced chemical equation is:     $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
• Human Nutrition: Involves a balanced diet of carbohydrates, proteins, lipids, vitamins, minerals, water, and dietary fiber. The process includes:
  1. Ingestion (taking in food).
  2. Digestion (breaking down food physically and chemically).
  3. Absorption (moving nutrients from the gut to the blood).
  4. Assimilation (using absorbed nutrients in the cells).
  5. Egestion (removal of undigested waste).

2f. Respiration

Respiration is the process of transferring energy from glucose. It occurs in every cell.

• Aerobic Respiration: Occurs in the presence of oxygen. Yields more energy.     $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$
• Anaerobic Respiration: Occurs in the absence of oxygen. Yields less energy.
  • In animals: $Glucose \rightarrow Lactic\,acid$
  • In plants/yeast: $Glucose \rightarrow Ethanol + Carbon\,dioxide$

2g. Gas Exchange

• Plants: Gas exchange occurs via diffusion through the stomata. During the day, net exchange involves the uptake of $CO_2$ and the release of $O_2$ due to photosynthesis. At night, only respiration occurs, so $O_2$ is taken in and $CO_2$ is released.
• Humans: The respiratory system includes the trachea, bronchi, bronchioles, and alveoli. Gas exchange (diffusion of $O_2$ and $CO_2$) takes place in the alveoli, which have a large surface area, thin walls, and a rich blood supply. Ventilation is controlled by the diaphragm and intercostal muscles.

Assessment Information

• Question Styles: The examination consists of a variety of formats, including:
  • Multiple-choice questions.
  • Short-answer questions.
  • Calculations.
  • Extended open-response questions.
• Equipment: A calculator is permitted for use during the examinations to assist with calculations.