Animal Science — KS3

Cell Organelles

All living things are made of cells — the smallest units of life. Animal cells contain several structures called organelles, each with a specific job:

• Cell membrane — controls what enters and leaves the cell; a thin, flexible barrier
• Cytoplasm — a jelly-like fluid where most chemical reactions take place
• Nucleus — the control centre; contains DNA with instructions for the cell
• Mitochondria — the powerhouses; release energy through aerobic respiration
• Ribosomes — tiny structures that build proteins from amino acids

Animal vs Plant Cells

Plant cells have all of the above plus three extra structures not found in animal cells:

• Cell wall — a rigid outer layer made of cellulose that gives shape and support
• Chloroplasts — contain chlorophyll and carry out photosynthesis
• Permanent vacuole — a large, fluid-filled sac that helps maintain shape

Magnification & Microscopy

Cells are far too small to see with the naked eye. Scientists use light microscopes and electron microscopes to view them. To calculate how much a microscope magnifies an image:

Example: a cell appears 3 mm wide in an image. The actual cell is 0.03 mm wide. Magnification = 3 ÷ 0.03 = ×100.

Rearranged: to find actual size, divide image size by magnification. To find image size, multiply actual size by magnification.

Levels of Organisation

In complex animals, cells do not work alone. They are organised into a hierarchy of increasingly complex structures:

• Tissue — a group of similar cells all doing the same job (e.g. muscle tissue)
• Organ — different tissues working together for a common function (e.g. the heart)
• Organ system — organs working together to perform a major body function
• Organism — the whole living animal (e.g. a galah, a hedgehog, a human)

Rosie the Galah

Rosie is a pink-and-grey galah from Australia. Her wings show perfectly how different tissues cooperate:

• Muscular tissue — contracts to power each wing beat during flight
• Epithelial tissue — covers the skin surface and lines internal organ passages
• Nervous tissue — carries electrical signals from Rosie's brain to her wing muscles
• Connective tissue — tendons, cartilage and bone that form the structural framework

Key Organ Systems

• Circulatory system — heart + blood vessels + blood; transports oxygen, nutrients and waste
• Digestive system — mouth + stomach + intestines; breaks down food and absorbs nutrients
• Respiratory system — lungs + trachea + diaphragm; gas exchange (O₂ in, CO₂ out)
• Nervous system — brain + spinal cord + nerves; coordinates all body responses
• Skeletal system — bones + joints; provides support, enables movement, protects organs

Types of Adaptation

An adaptation is any feature or behaviour that helps an organism survive and reproduce in its environment. Adaptations fall into three categories:

• Structural — a physical feature of the body (e.g. thick fur, a streamlined shape, long claws)
• Behavioural — an action or activity the animal performs (e.g. migrating, hibernating, displaying colour)
• Physiological — an internal body process (e.g. producing antifreeze proteins, storing fat reserves)

POTG Animals and Their Adaptations

Extreme Environments

• Polar regions — thick blubber (structural), huddling together for warmth (behavioural), producing antifreeze blood proteins (physiological)
• Deserts — large ears to radiate heat (structural), being nocturnal (behavioural), producing highly concentrated urine to conserve water (physiological)
• Deep ocean — bioluminescence to attract prey (structural / physiological), pressure-resistant cell membranes (physiological)

Adaptation and Evolution

Charles Darwin proposed the theory of natural selection: within any population, individuals vary. Those with traits that suit their environment are more likely to survive, reproduce and pass those traits to offspring. Over many generations this shifts the whole population — this is evolution.

Food Chains and Food Webs

A food chain shows a single pathway of energy transfer: grass → rabbit → fox. A food web links many overlapping food chains to show the full picture of feeding relationships in an ecosystem.

Trophic Levels

• Producers — plants and algae that manufacture food via photosynthesis; always at the base
• Primary consumers — herbivores that eat producers (e.g. rabbits, caterpillars, zooplankton)
• Secondary consumers — carnivores or omnivores that eat primary consumers (e.g. foxes, frogs)
• Tertiary consumers — top predators that eat secondary consumers (e.g. eagles, orcas)
• Decomposers — bacteria and fungi that break down dead organisms and return nutrients to the soil

The 10% Energy Rule

Energy is lost at every link in a food chain. On average only about 10% of the energy stored at one trophic level is transferred to the next. The rest is lost as heat or used by the organism for movement, growth and warmth.

This is why food chains rarely exceed four or five levels — there is too little energy left to sustain another.

Predator–Prey Relationships

In a balanced ecosystem, predator and prey numbers regulate each other. If prey increase (e.g. a mild winter), predator numbers follow (more food available). When predators increase, prey are hunted down faster, so prey numbers fall — which then causes predator numbers to fall too. This creates a repeating population cycle.

DNA, Genes and Chromosomes

Inside the nucleus of every cell is DNA (deoxyribonucleic acid) — a long, double-helix molecule that carries instructions for building and running an organism.

• DNA is tightly coiled into structures called chromosomes. Humans have 46 (23 pairs); hedgehogs have 44.
• A gene is a segment of DNA that codes for a specific protein or trait.
• An allele is a version of a gene (e.g. different alleles of the eye-colour gene produce different colours).

Sexual vs Asexual Reproduction

• Sexual reproduction — combines genetic material from two parents via gametes (sperm and egg). Offspring inherit a unique mix of alleles, producing genetic variation.
• Asexual reproduction — a single parent produces offspring that are genetically identical clones. Examples: bacteria binary fission, strawberry runners, spider plant offshoots.

Inherited vs Environmental Variation

• Inherited variation — determined by genes passed from parents (e.g. blood group, fur colour, eye shape)
• Environmental variation — caused by conditions during life (e.g. scars, language spoken, diet-influenced body size)
• Many traits are influenced by both: height has a genetic ceiling and floor, but diet and exercise determine where within that range you end up.

Dominant and Recessive Alleles

Organisms that reproduce sexually carry two alleles for each gene (one from each parent). When the two copies differ:

• The dominant allele is expressed (shown) even if only one copy is present — written as a capital letter (e.g. B)
• The recessive allele is only expressed when both copies are recessive — written in lower case (e.g. b)

An organism with BB or Bb shows the dominant trait. An organism with bb shows the recessive trait. A carrier (e.g. Bb) shows the dominant trait but can pass the recessive allele to offspring.