1.1 Nature of Viruses

Viruses occupy a distinctive position in biology because they are acellular and cannot reproduce independently. A virus consists essentially of genetic material—DNA or RNA—enclosed in a protein coat called a capsid. Some viruses also possess a lipid-containing envelope derived partly from host-cell membranes. Outside a suitable host cell, a virion does not carry out the complete set of metabolic activities associated with cellular life.

1.2 General Characteristics

Viruses are extremely small relative to most cells, lack cytoplasm and ribosomes, possess no independent energy-producing system, and depend on host cells for replication. Their host range may be broad or highly specific. Viral genetic material directs synthesis of viral components after entry into a susceptible cell. New virus particles are assembled from these components rather than produced by ordinary cell division.

1.3 Basic Viral Structure

1.4 Identification and Classification

Viruses may be identified and classified using the type of nucleic acid, capsid architecture, presence or absence of an envelope, replication strategy, host range and molecular characteristics. Classification based solely on the disease produced is inadequate because biologically different viruses may cause similar symptoms.

1.5 General Pattern of Viral Replication

1. Attachment to a susceptible host cell.
2. Entry of the virus or viral genome.
3. Uncoating where applicable.
4. Synthesis of viral nucleic acid and proteins using host resources.
5. Assembly of new viral particles.
6. Release, which may occur by cell lysis, budding or other mechanisms.

2.1 General Nature of Bacteria

Bacteria are unicellular prokaryotic organisms. Their cells lack a membrane-bound nucleus and the membrane-bound organelles characteristic of eukaryotes. Nevertheless, bacterial cells are highly organised and carry out complex metabolic activities. They occur in soil, water, air, living organisms and extreme environments, and their ecological roles include decomposition, nutrient cycling, symbiosis and disease.

2.2 Bacterial Cell Structure

A typical bacterium possesses a plasma membrane, cytoplasm, ribosomes and a nucleoid containing the main chromosome. Many bacteria have a cell wall, and some possess capsules, pili, flagella or plasmids. These additional structures influence protection, attachment, movement and genetic properties.

2.3 Bacterial Shapes and Arrangements

Cell morphology provides useful preliminary information. Cocci are approximately spherical, bacilli are rod-shaped, and spiral forms have curved or helical morphology. Cells may remain attached after division, producing pairs, chains, clusters or other arrangements. Shape alone cannot establish species identity, but it contributes to identification.

2.4 Identification of Bacteria

Bacterial identification can combine cell morphology, staining reactions, colony characteristics, biochemical tests, oxygen requirements, nutritional properties and molecular methods. A sound identification uses several independent characteristics because unrelated bacteria may share a single visible feature.

2.5 Reproduction and Genetic Variation

Most bacteria multiply by binary fission: DNA is replicated, the cell elongates, genetic material is partitioned and the cell divides. Binary fission is asexual. Bacteria can nevertheless acquire genetic variation through mutation and through mechanisms of horizontal gene transfer.

2.6 Biological Importance

Bacteria are not synonymous with disease. Many are indispensable decomposers and participate in nutrient cycles. Some live beneficially with plants and animals; others are used in food production, biotechnology and environmental processes. Pathogenic bacteria constitute only part of bacterial diversity.

3.1 General Characteristics

Fungi are eukaryotic organisms that obtain organic nutrients by absorption. They do not contain chlorophyll for photosynthesis. Many fungi secrete enzymes into their surroundings, digest complex materials externally and absorb the soluble products. Their cell walls contain chitin as an important structural component. Fungi may be unicellular, as in many yeasts, or multicellular, as in moulds and mushrooms.

3.2 Hyphae and Mycelium

The basic filament of many multicellular fungi is the hypha. A network or mass of hyphae forms a mycelium. Hyphae provide a large surface area for absorption. Some are divided by cross walls called septa, while others contain continuous cytoplasm with many nuclei. The visible mushroom is only a reproductive structure of certain fungi; much of the organism may exist as mycelium within the substrate.

3.3 Nutrition

Most fungi are heterotrophic. Saprotrophic fungi obtain nutrients from dead organic matter; parasitic fungi obtain resources from living hosts and may cause disease; mutualistic fungi participate in associations in which both partners benefit. External digestion followed by absorption is a defining nutritional strategy.

3.4 Reproduction

Fungi can reproduce asexually and sexually. Asexual methods include budding, fragmentation and formation of asexual spores. Sexual reproduction involves compatible nuclei and ultimately genetic recombination, though details vary among fungal groups. Spores are important for reproduction and dispersal.

3.5 Identification and Broad Classification

Fungi may be identified using macroscopic appearance, hyphal structure, spore-producing structures, reproductive features, physiological properties and molecular data. Traditional groupings rely strongly on reproductive structures, while modern classification increasingly reflects evolutionary relationships inferred from molecular evidence.

3.6 Importance of Fungi

• Decomposition of dead organic material and nutrient recycling.
• Mycorrhizal associations that improve mineral acquisition by many plants.
• Food and fermentation processes involving yeasts and edible fungi.
• Production of useful metabolites and pharmaceuticals.
• Plant, animal and human diseases caused by pathogenic species.
• Food spoilage and deterioration of materials.

4.1 General Characteristics of Plants

Plants are predominantly multicellular eukaryotes with cellulose-containing cell walls and chloroplasts in photosynthetic tissues. Most are photoautotrophic, using light energy to synthesise organic compounds from carbon dioxide and water. Their life cycles include reproductive stages that permit continuity and dispersal. Plant form ranges from relatively simple non-vascular organisms to complex vascular seed plants.

4.2 Basis of a General Plant Survey

A useful introductory survey considers presence of vascular tissue, production of seeds, production of flowers and fruits, dominant life-cycle features and structural organisation. These characteristics reveal major functional differences in transport, support, reproduction and adaptation to terrestrial environments.

4.3 Roots, Stems and Leaves

The root system commonly anchors the plant and absorbs water and mineral ions. Stems support leaves and reproductive structures and contain tissues that transport substances. Leaves are major photosynthetic organs in many plants. These organs are integrated: roots acquire resources from soil, leaves acquire carbon dioxide and light, and vascular tissues connect resource-producing and resource-consuming regions.

4.4 Vascular Tissues

Xylem conducts water and dissolved mineral ions principally from roots toward aerial organs and also contributes structural support. Phloem transports organic solutes, especially products of photosynthesis, between sources and sinks. Transport in plants is therefore not simply 'food moving down and water moving up'; direction in phloem depends on the positions of sources and sinks.

4.5 Flowering Plants

Angiosperms dominate many terrestrial ecosystems. Flowers are reproductive structures involved in sexual reproduction. Following fertilisation, ovules develop into seeds and the ovary commonly contributes to fruit formation. Seeds protect the embryo and contain stored resources or structures supporting establishment.

5.1 General Characteristics

Animals are multicellular eukaryotic heterotrophs that ingest or otherwise obtain organic nutrients from other organisms or organic material. Animal cells lack cellulose cell walls. Most animals possess specialised tissues, and many have organ systems for digestion, transport, gas exchange, excretion, coordination and reproduction. Mobility is common at least during part of the life cycle.

5.2 Criteria Used in Animal Classification

Introductory animal classification considers body symmetry, tissue organisation, body cavity, segmentation, embryological development and presence or absence of a vertebral column. These features are more informative than superficial resemblance. Modern animal systematics also relies strongly on molecular evidence.

5.3 Major Invertebrate Groups

5.4 Chordates and Vertebrates

Chordates possess defining features at least during development, including a notochord and dorsal hollow nerve cord. Vertebrates are chordates in which a vertebral column and complex internal skeleton are prominent. Major vertebrate groups commonly surveyed at introductory level include fishes, amphibians, reptiles, birds and mammals.

5.5 Plant and Animal Kingdoms Compared

6.1 Meaning of Adaptation

An ecological adaptation is a heritable structural, physiological or behavioural feature that contributes to survival and reproductive performance under particular environmental conditions. Adaptations arise across generations through evolutionary processes; they are not conscious responses made because an organism 'wants' to survive.

6.2 Structural, Physiological and Behavioural Adaptations

6.3 Adaptations of Xerophytes

Xerophytes are plants adapted to environments where water is frequently limiting. Depending on the species, useful features may include reduced leaf area, thick cuticle, stomatal regulation, water-storage tissues, extensive root systems and leaf hairs. No single feature defines every xerophyte; adaptations occur in combinations suited to particular environments.

6.4 Hydrophytes

Hydrophytes live in water or waterlogged environments. Floating leaves may have stomata concentrated on the upper surface, submerged tissues may possess reduced supporting structures, and large internal air spaces can aid buoyancy and gas movement. Because water is abundant, the selective problems differ greatly from those of dry habitats.

6.5 Animal Adaptations

Aquatic animals may show streamlined bodies, specialised respiratory surfaces and mechanisms for regulating salt and water balance. Terrestrial animals face risks of dehydration and must support their bodies against gravity. Desert animals may reduce daytime activity, conserve water physiologically or obtain water metabolically. Adaptation is therefore best understood by linking environmental challenge to biological function.

7.1 Meaning of Nutrition

Nutrition is the acquisition and utilisation of substances required for energy supply, growth, repair and regulation. Autotrophic organisms manufacture organic compounds from simpler inorganic substances using an external energy source. Heterotrophs obtain preformed organic materials from other organisms or their products.

7.2 Photosynthetic Nutrition

Photosynthesis converts light energy into chemical energy stored in organic compounds. In green plants it occurs principally in chloroplasts. Carbon dioxide and water provide raw materials, while light supplies energy. A simplified overall representation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light and photosynthetic pigments. This equation summarises a complex sequence rather than a single chemical step.

7.3 Mineral Nutrition in Plants

Plants also require mineral elements absorbed mainly from soil solution. Nitrogen contributes to amino acids and nucleic acids; magnesium is important in chlorophyll; phosphorus contributes to nucleic acids, ATP and membranes. Mineral deficiency can disrupt growth and metabolism, but visible symptoms alone may not identify a deficiency with certainty because different stresses can produce similar appearances.

7.4 Heterotrophic Nutrition in Animals

Animal nutrition commonly involves ingestion, digestion, absorption, assimilation and egestion. Digestion converts large food molecules into forms that can be absorbed. Absorbed nutrients enter internal transport systems and are assimilated into cellular metabolism or structures. Egestion removes undigested material and must not be confused with excretion, which removes metabolic wastes.

7.5 Meaning of Respiration

Cellular respiration comprises enzyme-controlled reactions that transfer energy from organic molecules to ATP and other usable forms. Aerobic respiration uses oxygen in the complete oxidation of suitable substrates and produces carbon dioxide and water. Anaerobic pathways release less usable energy per glucose molecule because oxidation is incomplete.

7.6 Aerobic and Anaerobic Respiration

7.7 Gas Exchange

Respiration is a cellular process; breathing and gas exchange are supporting processes and should not be treated as synonyms. Gas exchange supplies oxygen and removes carbon dioxide in many organisms. Effective exchange surfaces are typically thin, moist where required, sufficiently large and associated with mechanisms that maintain concentration gradients.

8.1 Need for Internal Transport

Very small organisms can exchange many materials directly across their surfaces because diffusion distances are short. As organisms become larger and more complex, specialised transport systems become necessary. Internal transport distributes nutrients, gases, hormones and wastes between exchange surfaces and cells.

8.2 Transport in Plants

Water and mineral ions absorbed by roots enter the xylem and are transported through the plant. Transpiration—the loss of water vapour principally from aerial surfaces—contributes to the upward movement of water through xylem. Phloem transports organic solutes between sources, where substances are produced or released, and sinks, where they are used or stored.

8.3 Circulation in Animals

A circulatory system typically includes a transport fluid, vessels or body spaces through which the fluid moves, and a pumping mechanism. In closed circulatory systems, blood remains largely within vessels; in open systems, circulating fluid enters body spaces more directly. Vertebrates possess closed systems.

8.4 Human Circulation as an Example

The mammalian heart generates pressure that drives blood through pulmonary and systemic circuits. The pulmonary circuit carries blood between heart and lungs; the systemic circuit connects the heart with other body tissues. Arteries carry blood away from the heart, veins return blood toward it, and capillaries provide thin exchange surfaces between blood and tissues.

8.5 Meaning of Excretion

Excretion is the removal of waste products generated by metabolism. Carbon dioxide from cellular respiration and nitrogen-containing wastes from amino-acid metabolism are examples. Egestion, by contrast, is elimination of material that was not digested and absorbed. Secretion is the useful release of substances such as enzymes or hormones.

8.6 Excretion in Animals

Different animals use different structures for excretion and osmoregulation. In vertebrates, kidneys regulate water and ion balance while eliminating nitrogenous wastes in urine. Lungs eliminate carbon dioxide and water vapour. The liver modifies or detoxifies various compounds and participates in formation of nitrogenous waste products.

8.7 Excretion in Plants

Plants produce metabolic wastes but lack specialised excretory organs comparable to animal kidneys. Gases can diffuse through stomata and other surfaces; some substances are stored in vacuoles or tissues, deposited in relatively inert forms, secreted, or removed when leaves, bark or other structures are shed.

9.1 Reproduction in Plants

Plants reproduce sexually and asexually. In flowering plants, flowers contain structures associated with gamete production and fertilisation. Pollination transfers pollen to a compatible receptive surface. Fertilisation follows successful pollen-tube growth and gamete fusion. The zygote develops into an embryo; the ovule develops into a seed, while the ovary commonly develops into a fruit. Vegetative reproduction produces new plants from non-seed structures such as stems, roots or leaves.

9.2 Reproduction in Animals

Sexual reproduction is widespread among animals and involves production of male and female gametes. Fertilisation may occur externally, as in many aquatic species, or internally. Development may occur within eggs deposited outside the parent or within the reproductive tract, depending on the group. Asexual reproduction also occurs in some animals through budding, fragmentation, parthenogenesis or related mechanisms.

9.3 Growth

Growth is a permanent increase in size, volume, cell number or dry mass resulting from biological processes. In multicellular organisms it depends on cell division, cell enlargement and synthesis of cellular material. Growth should be distinguished from temporary swelling caused simply by water uptake.

9.4 Plant Growth

Plant growth is strongly associated with meristems, regions containing actively dividing cells. Apical meristems contribute to primary growth in length, while lateral meristems contribute to increase in girth in plants that undergo secondary growth. Following division, cells enlarge and differentiate into specialised tissues.

9.5 Animal Growth

Animal growth patterns vary. In many animals, growth is more determinate than in plants, eventually slowing markedly after maturity. Growth requires nutrients, energy, genetic regulation and suitable environmental conditions. Hormones coordinate many aspects of growth and development.

9.6 Development

Development is the orderly sequence of changes in structure, function and organisation through the life of an organism. It includes differentiation, morphogenesis and maturation. Differentiation produces specialised cell types from less specialised precursors, while morphogenesis generates organised form.

9.7 Metamorphosis

Some animals undergo conspicuous post-embryonic transformations. In complete insect metamorphosis, development passes through egg, larva, pupa and adult. Amphibian metamorphosis transforms an aquatic larval form into a more terrestrial adult form in many species. Such changes involve coordinated structural and physiological reorganisation.

9.8 Growth and Development Compared

BIO 102 presents biological diversity and physiology as connected ideas. Viruses demonstrate a non-cellular strategy dependent on hosts; bacteria show the organisation and versatility of prokaryotic life; fungi demonstrate absorptive heterotrophy; plants and animals illustrate major eukaryotic body plans. Ecological adaptation connects structure and function to environment, while nutrition, respiration, transport, excretion, reproduction, growth and development explain how organisms maintain themselves and continue their lineages. Understanding these relationships is more valuable than memorising isolated lists of characteristics.