Circulatory system
The heart pumps blood through vessels, transporting oxygen, nutrients, hormones, heat, and wastes.
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The cell: life's basic unit
Every known living thing is made of one or more cells. Your body contains trillions of specialized cells working together.
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Nucleus
Nucleus
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Mitochondria
Mitochondria
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Ribosomes
Ribosomes
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Membrane
Membrane
A simplified animal cell
Tiny parts, different jobs
Cell membrane: controls what enters and leaves. Nucleus: stores most of the cell's DNA. Ribosomes: build proteins. Mitochondria: help release usable energy from food molecules.
Real cells are crowded, dynamic places—not empty bags with a few floating parts.
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DNA stores instructions
DNA is a very long molecule. Its sequence uses four chemical bases—A, T, C, and G. A pairs with T, while C pairs with G.
A tiny DNA ladder
The real molecule twists into a double helix. Genes are stretches of DNA that can contain instructions for making functional molecules, often proteins.
AT
CG
GC
TA
AT
GC
DNA influences traits, but environment and development matter too. Living things are not determined by a single “blueprint” in a simple way.
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Your body is a network
Organs belong to systems, but no system works alone. A sprint requires the nervous, muscular, skeletal, respiratory, and circulatory systems together.
Respiratory system
In the lungs, oxygen moves into the blood and carbon dioxide moves out.
Nervous system
The brain, spinal cord, nerves, and sense organs send and process rapid electrical and chemical signals.
Musculoskeletal system
Muscles pull on bones across joints, creating movement while the skeleton supports and protects the body.
Digestive system
Food is broken into smaller molecules that can be absorbed and used by cells.
Immune system
Cells, tissues, and molecules recognize many threats and help protect the body.
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Life connects to life
No organism lives completely alone. Energy flows through food webs, while matter such as water and carbon cycles again and again.
Producers
Plants and algae capture light energy through photosynthesis and store some of it in chemical bonds.
Consumers
Animals and many other organisms obtain energy and materials by eating organisms or their products.
Decomposers
Fungi, bacteria, and other decomposers break down dead material and return nutrients to ecosystems.
One of biology's biggest ideas
Populations change over generations
Individuals vary. Some inherited variations affect survival and reproduction in a particular environment. Over many generations, natural selection can make helpful variations more common in a population.
variation + inheritance + selection + time
Evolution does not aim for “perfect.” Environments change, trade-offs exist, and different solutions can work.
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Plants are living solar-powered factories
Plants sense their environment, transport materials, reproduce, defend themselves, and make sugars using light energy.
Photosynthesis
Chloroplasts use light energy to help turn carbon dioxide and water into glucose and oxygen. The glucose stores chemical energy.
Roots and stems
Roots anchor and absorb water and minerals. Vascular tissues in roots, stems, and leaves transport water and dissolved sugars.
Flowers and seeds
Flowers are reproductive structures in flowering plants. Pollination can lead to fertilization and seeds containing young plants.
Stomata
Tiny leaf openings allow carbon dioxide to enter and oxygen and water vapour to leave. Guard cells adjust the openings.
Tropisms
Plants change growth direction in response to clues such as light and gravity, even though they do not walk from place to place.
Respiration too
Plants perform cellular respiration day and night to release usable energy from food. Photosynthesis and respiration are different processes.
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A mostly microscopic world
Many living things are too small to see without magnification, yet they shape every ecosystem.
Bacteria
Bacteria are single-celled organisms without a nucleus. Many are harmless or helpful; some cause disease.
Fungi
Yeasts, moulds, and mushrooms are fungi. They absorb nutrients and are vital decomposers; yeast also helps bread rise.
Protists
This diverse group includes many microscopic organisms such as amoebas and some algae. They do not all fit one simple description.
Viruses
Viruses carry genetic material inside a protein coat and reproduce only using host cells. Scientists debate whether to call them alive.
Helpful microbes
Microorganisms help digestion, recycle nutrients, make foods, produce medicines, and support plants.
Preventing infection
Handwashing, clean water, food safety, vaccination, and appropriate medical care interrupt different routes of disease spread.
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How biologists organize life
Classification helps scientists compare organisms and communicate about their evolutionary relationships.
From broad groups to one species
One common sequence is domain, kingdom, phylum, class, order, family, genus, species. Modern classification also uses DNA evidence to build branching family trees.
A scientific name contains genus and species, such as Homo sapiens. Shared traits can come from shared ancestry.
cell → tissue → organ → organ system → organism
Levels of organization reveal how smaller living structures cooperate to produce new abilities at larger scales.
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Quick knowledge check
Think first, then open the explanation.
Do all cells have a nucleus?
No. Bacteria and archaea lack a nucleus, and some specialized animal cells, such as mature human red blood cells, lose theirs.
Do plants get their food from soil?
Plants absorb water and minerals from soil, but they build sugars mainly from carbon dioxide and water using light energy.
Does one gene always control one trait?
No. Many traits involve several genes plus environmental and developmental influences.
Does evolution mean an individual changes because it needs to?
No. Evolution describes inherited changes in populations over generations, not purposeful change within one individual.