These notes are highlighted from our A-level Biology exam focus textbook, for complete study materials on this course, refer to the textbook. For effective preparation, practice past questions on this course as much as possible.
Topic 25: Diversity and General Characteristics of Animals
Animals are living organisms that belong to the kingdom Animalia. Unlike plants, animals cannot make their own food and must obtain nutrition by consuming other organisms. Animals are also able to move from one place to another, at least at some stage of their life cycle, and they respond quickly to changes in their environment.
Animals share certain general characteristics that distinguish them from other kingdoms. They are multicellular, meaning their bodies are made up of many cells working together. Their cells are eukaryotic, meaning each cell has a true nucleus enclosed in a membrane. Animals do not have cell walls, unlike plants and fungi. They reproduce mostly sexually, though some can also reproduce asexually. Animals also go through a period of development after fertilization where a single cell grows into a complex organism.
The diversity of animals is enormous. Animals live in virtually every environment on earth including oceans, rivers, forests, deserts, grasslands, and even inside other organisms. Their lifestyles vary greatly as some are predators that hunt other animals, some are herbivores that eat only plants, some are scavengers that feed on dead matter, and some are parasites that live on or inside a host organism.
Animals are broadly categorized into two major groups which are invertebrates and vertebrates. Invertebrates are animals that do not have a backbone and they make up the vast majority of animal species on earth. Vertebrates are animals that have a backbone and include fish, amphibians, reptiles, birds, and mammals.
Topic 26: Systematics and Taxonomy of Animals
Animal taxonomy is the science of classifying animals into groups based on their shared structural features, evolutionary history, and biological characteristics. Understanding how animals are classified helps scientists communicate clearly about different species and understand their relationships to one another.
Animals are organized at different levels of classification. At the broadest level all animals belong to the kingdom Animalia. Below the kingdom level animals are divided into phyla, and each phylum contains animals that share a fundamental body plan.
The classification of invertebrates is based on several key features. Animals without tissues are the simplest animals and include sponges which belong to the phylum Porifera. Animals with tissues but without true organs include jellyfish and other members of the phylum Cnidaria. Animals that show bilateral symmetry have a body that can be divided into two equal halves along one plane and this includes most of the more complex animals. Animals with a body cavity called a coelom are called coelomates and this body cavity allows for the development of complex organs. Segmented animals have bodies divided into repeating units and the most familiar example is the earthworm. Animals with jointed appendages belong to the phylum Arthropoda which is the largest phylum in the animal kingdom. Animals with a backbone belong to the subphylum Vertebrata.
There are major and minor phyla in the animal kingdom. The major phyla include Porifera which are the sponges, Cnidaria which include jellyfish and corals, Platyhelminthes which are the flatworms, Nematoda which are the roundworms, Annelida which are the segmented worms, Mollusca which include snails and squids, Arthropoda which include insects and crustaceans, Echinodermata which include starfish and sea urchins, and Chordata which includes all vertebrates.
Animals also have different types of tissues and organ systems. The four basic tissue types in animals are epithelial tissue which covers body surfaces, connective tissue which supports and connects other tissues, muscle tissue which enables movement, and nervous tissue which transmits signals throughout the body. These tissues combine to form organs and organ systems that carry out specific functions in the body.
In practical class students identify and classify animal specimens from different phyla and also carry out dissections of selected animals such as cockroaches, fish, frogs, and rats to observe their internal structures firsthand.
Topic 27: Evolution of Animals
The evolution of animals refers to how animal life has changed and diversified over millions of years through the process of natural selection and adaptation. Animals have evolved to live successfully in three major environments which are water, land, and air, and each environment has shaped the features and body plans of the animals that live there.
Animals that live in water have developed several adaptations to help them survive in aquatic environments. Fish have streamlined bodies that reduce resistance as they move through water. They have gills for extracting dissolved oxygen from water. They also have fins for steering and a lateral line system for detecting vibrations in the water. Aquatic mammals like whales and dolphins have evolved from land-dwelling ancestors and returned to the sea, developing paddle-like limbs and the ability to hold their breath for extended periods.
Animals that live on land face different challenges such as the need to support their own body weight against gravity, the risk of drying out, and the need to breathe air directly. Land animals have evolved strong skeletons to support their bodies, waterproof skin or coverings to prevent water loss, and lungs for breathing air. Limbs evolved to allow movement on land and the reproductive system also changed to protect embryos from drying out.
Animals that live in air have evolved lightweight bodies, wings, and powerful flight muscles. Birds have hollow bones that reduce their weight, feathers that provide lift and insulation, and a highly efficient respiratory system that provides the large amounts of oxygen needed for flight. Some insects also fly and have evolved wings from extensions of their body wall.
Topic 28: Invertebrates
Invertebrates are animals that lack a backbone and they represent the largest and most diverse group of animals on earth. They are found in almost every habitat and play crucial roles in ecosystems as decomposers, pollinators, and food sources for other animals.
The study of invertebrates involves understanding their taxonomy, characteristics, diversity, lifestyles, morphology, and life cycles. It is important to know representative examples from each major invertebrate group.
Flatworms belong to the phylum Platyhelminthes and they have flat, ribbon-like bodies. Free-living flatworms like planarians live in water and feed on small organisms. Parasitic flatworms are of great medical and veterinary importance. Trematodes, also called flukes, are parasitic flatworms that infect the liver, lungs, and blood of vertebrate hosts. Cestodes, also called tapeworms, are parasitic flatworms that live in the intestines of vertebrates and can grow to enormous lengths. These parasites cause serious diseases in humans and animals and understanding their life cycles is important for controlling their spread.
The body plan of invertebrates is an important feature used in their classification. Some invertebrates have radial symmetry meaning their body parts are arranged around a central point like the spokes of a wheel. Others have bilateral symmetry meaning their body can be divided into two mirror-image halves. The presence or absence of a body cavity, the type of body covering, the number of body segments, and the arrangement of organ systems are all features used to distinguish between different invertebrate groups.
Arthropods are the most successful group of animals on earth in terms of the number of species and the diversity of habitats they occupy. Their success is due to several key features. They have a hard external skeleton called an exoskeleton that provides protection and prevents water loss. Their body is divided into segments and they have jointed appendages that allow for a wide range of movements. They also have a relatively advanced nervous system compared to other invertebrates. The arthropod group includes insects, spiders, crabs, centipedes, and millipedes.
Topic 29: Introduction to Chordates
Chordates belong to the phylum Chordata and they include some of the most familiar and complex animals on earth. All chordates share four key features at some stage of their development. These are a notochord which is a flexible rod that provides support, a dorsal hollow nerve cord which runs along the back and develops into the brain and spinal cord, pharyngeal slits which are openings in the throat region, and a post-anal tail.
Living in different habitats presents unique challenges and chordates have evolved remarkable adaptations to meet these challenges. Aquatic chordates must extract oxygen from water, control their buoyancy, and move efficiently through a dense medium. Terrestrial chordates must support their body weight, prevent desiccation, and breathe air. Aerial chordates must generate enough lift to overcome gravity and maintain flight.
The history of chordates is long and fascinating. The earliest chordates were simple aquatic animals that gradually evolved into more complex forms. Over millions of years different groups diverged and developed unique adaptations that allowed them to colonize new environments. This led to the incredible diversity of chordates we see today.
The major groups of vertebrate chordates are fish, amphibians, reptiles, birds, and mammals. Fish are the oldest and most diverse group of vertebrates and they are fully adapted to aquatic life. Amphibians like frogs and salamanders can live both in water and on land but they must return to water to reproduce. Reptiles were the first fully terrestrial vertebrates and they dominated the earth for millions of years during the age of the dinosaurs. Birds evolved from a group of dinosaurs and developed flight as their defining adaptation. Mammals are warm-blooded vertebrates that feed their young with milk and have hair or fur covering their bodies.
The rise and fall of dominant reptiles is an important topic in vertebrate evolution. During the Mesozoic era dinosaurs were the dominant land animals on earth. They existed in an enormous variety of forms ranging from small feathered creatures to enormous long-necked giants. Their extinction approximately 66 million years ago following a massive asteroid impact and subsequent environmental changes allowed mammals to diversify and become the dominant land vertebrates.
When classifying vertebrates into classes it is important to clearly state the taxonomic features that justify placing animals in each group. Fish are characterized by gills, scales, and fins. Amphibians are characterized by moist skin, metamorphosis, and dependence on water for reproduction. Reptiles are characterized by dry scaly skin, internal fertilization, and amniotic eggs. Birds are characterized by feathers, beaks, hollow bones, and endothermy. Mammals are characterized by hair, mammary glands, endothermy, and live birth in most species.
Topic 30: Ecological and Economic Importance of Animals
Animals play enormously important roles in both natural ecosystems and in human society. Understanding these roles helps us appreciate why animal conservation is so important and why the loss of animal species can have devastating consequences.
Animals benefit humans in many direct ways. They provide food in the form of meat, eggs, milk, and honey. They provide materials like leather, wool, and silk that are used to make clothing and other products. Animals are also used in scientific research to develop and test new medicines and medical procedures. In agriculture, animals like bees are essential pollinators that enable the reproduction of many crop plants. Animals like earthworms improve soil fertility by breaking down organic matter. Draft animals like oxen and horses have been used for centuries to help humans with farming and transportation.
Arthropods are particularly important economically. Insects like bees and butterflies are vital pollinators for both wild plants and agricultural crops. Silkworms produce silk which is a valuable textile material. Some insects are used in biological pest control to keep populations of harmful insects in check. However arthropods can also cause economic harm. Mosquitoes transmit diseases like malaria and yellow fever. Locusts can devastate crops. Ticks and mites cause disease in livestock and reduce agricultural productivity.
In natural ecosystems animals play critical roles in maintaining balance. Predators control the population of prey species preventing any one species from becoming too numerous. Herbivores shape plant communities by feeding on certain plants and allowing others to thrive. Decomposers break down dead organic matter and return nutrients to the soil. Scavengers clean up dead animals and prevent the spread of disease.
Topic 31: Physiological Processes
Physiology is the study of how the body and its organs function. In this section we look at the key physiological processes that keep animals alive and functioning including nutrition, respiration, the skeletal system, and reproduction.
Nutrition in animals refers to the processes by which animals obtain, digest, and absorb food to provide energy and raw materials for growth and repair. There are different types of nutrition in animals. Holozoic nutrition involves taking in solid food, digesting it internally, and absorbing the nutrients. This is the most common type in animals including humans. Parasitic nutrition involves obtaining food from a living host. Saprotrophic nutrition involves feeding on dead and decaying organic matter.
In humans nutrition involves the alimentary system which is a long tube running from the mouth to the anus. Food enters through the mouth where it is chewed and mixed with saliva. It then passes through the oesophagus to the stomach where it is churned and mixed with gastric acid. From the stomach food moves to the small intestine where most digestion and absorption takes place with the help of enzymes produced by the pancreas and intestinal walls and bile produced by the liver. The large intestine absorbs water from the remaining material and the waste is eventually excreted through the anus.
Digestion involves the use of many different enzymes. Amylase breaks down carbohydrates into simple sugars. Proteases break down proteins into amino acids. Lipases break down fats into fatty acids and glycerol. Each enzyme works best at a specific pH and temperature and this is why conditions in different parts of the digestive system vary.
Respiration in mammals takes place mainly in the lungs. Air is inhaled through the nose or mouth and travels down the trachea into the bronchi and then into the bronchioles which are smaller and smaller airways. At the end of the bronchioles are tiny air sacs called alveoli where gas exchange takes place. Oxygen from the air passes into the blood through the thin walls of the alveoli and carbon dioxide from the blood passes out into the air to be exhaled. The circulatory system plays a vital role in respiration by carrying oxygen from the lungs to all the cells of the body and bringing carbon dioxide back from the cells to the lungs.
The skeletal system provides the framework that supports the body, protects internal organs, and enables movement. The human skeleton is made up of bones and cartilage. Bone is hard and rigid and is made up of calcium phosphate and collagen fibres. Cartilage is softer and more flexible and is found at the ends of bones, in the nose, ears, and between vertebrae. The mammalian endoskeleton consists of the axial skeleton which includes the skull, vertebral column, and ribcage, and the appendicular skeleton which includes the limbs and the girdles that attach them to the axial skeleton.
Joints are points where two or more bones meet. Fixed joints allow no movement and are found in the skull. Slightly movable joints allow limited movement and are found between the vertebrae. Freely movable joints or synovial joints allow a wide range of movement and are found in the shoulder, hip, knee, and elbow. The type of movement a joint allows depends on its structure.
Reproduction in vertebrates can be sexual or in some cases asexual. In sexual reproduction genetic material from two parents is combined to produce offspring with a new combination of traits. The human male reproductive system produces sperm in the testes. The sperm travel through a series of ducts and are released during ejaculation. The human female reproductive system produces eggs in the ovaries. Each month during the menstrual cycle one egg is released in a process called ovulation. If the egg is fertilized by a sperm it implants in the wall of the uterus and develops into an embryo. If fertilization does not occur the lining of the uterus is shed during menstruation.
Topic 32: Transport of Substances Across Membranes
The transport of substances into and out of cells and across membranes is a fundamental process that keeps all living organisms alive. In this topic we examine how substances move across membranes, how the body excretes waste, and how the circulatory system transports materials throughout the body.
Excretion is the process by which metabolic waste products are removed from the body. In mammals the main excretory organ is the kidney. Osmoregulation is the control of water and salt balance in the body and the kidneys play a central role in this process. The functional unit of the kidney is called the nephron. Each kidney contains millions of nephrons and together they filter the blood and produce urine.
The nephron works through three main processes. Ultrafiltration occurs in a structure called the glomerulus where blood is filtered under high pressure and water, glucose, urea, and salts pass out of the blood into the nephron tubule. Selective reabsorption occurs as the filtrate moves along the tubule where useful substances like glucose and most of the water are reabsorbed back into the blood. The remaining fluid which contains urea and excess salts becomes urine which is collected and eventually excreted from the body. Weather and environmental conditions affect excretion because in hot conditions the body loses more water through sweating and therefore produces less urine while in cold conditions more urine is produced.
The circulatory system is responsible for transporting substances throughout the body. The human circulatory system is a closed double circulation meaning blood passes through the heart twice for each complete circuit of the body. The right side of the heart pumps blood to the lungs where it picks up oxygen and releases carbon dioxide. The left side of the heart pumps oxygenated blood to the rest of the body where oxygen is delivered to cells and carbon dioxide is collected.
Blood is the transport medium of the circulatory system and it is made up of several components. Red blood cells contain haemoglobin which binds to oxygen and carries it around the body. White blood cells are part of the immune system and defend the body against infection. Platelets are small cell fragments that help blood to clot when a blood vessel is damaged. Plasma is the liquid component of blood that carries dissolved substances like glucose, hormones, carbon dioxide, and waste products.
The functions of blood include transporting oxygen and nutrients to cells, carrying carbon dioxide and waste products away from cells, distributing hormones from glands to target organs, regulating body temperature, and protecting the body from infection through the immune response.
Osmosis is the movement of water molecules from a region of low solute concentration to a region of high solute concentration through a semi-permeable membrane. Osmotic pressure is the pressure that would need to be applied to a solution to prevent the inward flow of water by osmosis. Turgor pressure is the pressure exerted by water inside a cell against the cell wall and it is what keeps plant cells firm.
Active transport is the movement of substances across a membrane against their concentration gradient, meaning from an area of low concentration to an area of high concentration. This process requires energy in the form of ATP and special carrier proteins in the membrane. Active transport is used when cells need to absorb substances that are in lower concentration outside the cell than inside such as when root cells absorb mineral ions from the soil.
Growth and development in animals involves a series of carefully regulated stages. In embryology development begins with fertilization when a sperm cell fuses with an egg cell to form a zygote. The zygote then divides repeatedly through mitosis to form a ball of cells called a morula which develops into a blastula and then into a gastrula as cells begin to differentiate into different tissue types. The principles of development include growth, differentiation where cells become specialized, morphogenesis which is the shaping of the body form, and integration where different parts of the developing organism are coordinated.
In practical class students conduct experiments to demonstrate diffusion, osmosis, and plasmolysis. Diffusion is observed when a colored substance spreads through water without stirring. Osmosis is demonstrated using potato strips or visking tubing placed in solutions of different concentrations. Plasmolysis is the shrinking of a plant cell's contents away from the cell wall when the cell loses water by osmosis and can be observed under a microscope using onion or Elodea cells.