Biology Notes: Key Concepts
1. Cell Theory
All living organisms are composed of one or more cells.
The cell is the basic unit of life.
All cells arise from pre-existing cells.
2. Types of Cells
Prokaryotic Cells: Simple cells without a nucleus (e.g., bacteria). DNA is free-floating in the
cytoplasm.
Eukaryotic Cells: Complex cells with a nucleus (e.g., plants, animals). They contain membrane-
bound organelles.
3. Cell Organelles and Their Functions
Nucleus: Contains the cell’s genetic material (DNA) and controls its activities.
Mitochondria: Powerhouse of the cell, responsible for energy production through cellular
respiration.
Ribosomes: Synthesize proteins.
Endoplasmic Reticulum (ER):
o Rough ER: Studded with ribosomes and helps in protein synthesis.
o Smooth ER: Involved in lipid synthesis and detoxification.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.
Chloroplasts (in plants): Site of photosynthesis, converting light energy into chemical energy
(glucose).
Cell Membrane: Semi-permeable membrane controlling movement of substances in and out of
the cell.
4. DNA and Genetics
DNA (Deoxyribonucleic Acid): Molecule that carries the genetic instructions for the
development, functioning, and reproduction of all living organisms.
Genes: Segments of DNA that code for specific proteins.
Chromosomes: Structures within the nucleus that contain tightly packed DNA. Humans have 46
chromosomes (23 pairs).
Mitosis vs. Meiosis:
o Mitosis: Cell division that results in two genetically identical daughter cells.
o Meiosis: Cell division that results in four genetically unique gametes (sperm or egg cells),
each with half the chromosome number.
5. Photosynthesis
The process by which plants convert light energy, carbon dioxide, and water into glucose (food)
and oxygen.
Occurs in the chloroplasts of plant cells.
Equation: 6CO2+6H2O+light⟶C6H12O6+6O26CO_2 + 6H_2O + light \longrightarrow
C_6H_{12}O_6 + 6O_26CO2+6H2O+light⟶C6H12O6+6O2
6. Respiration
Cellular Respiration: The process of breaking down glucose to release energy (ATP).
Takes place in the mitochondria.
Equation: C6H12O6+6O2⟶6CO2+6H2O+ATPC_6H_{12}O_6 + 6O_2 \longrightarrow 6CO_2 +
6H_2O + ATPC6H12O6+6O2⟶6CO2+6H2O+ATP
7. Evolution and Natural Selection
Evolution: The process by which different species of organisms develop and diversify from
earlier forms.
Natural Selection (Darwin): Organisms with traits better suited to their environment are more
likely to survive and reproduce.
8. Human Body Systems
Circulatory System: Includes the heart, blood, and blood vessels. Transports nutrients, oxygen,
and waste.
Respiratory System: Lungs and airways. Responsible for gas exchange (oxygen in, carbon dioxide
out).
Digestive System: Breaks down food into nutrients for energy (mouth, stomach, intestines).
Nervous System: Brain, spinal cord, and nerves. Controls and coordinates body activities.
Immune System: Defends the body against pathogens (white blood cells, antibodies).
9. Ecology
Ecosystem: A community of living organisms interacting with their physical environment.
Food Chain: A series of organisms each dependent on the next as a source of food (producers →
consumers → decomposers).
Biodiversity: The variety of life in the world or a particular habitat, crucial for ecosystem stability.
10. Genetics and Heredity
Dominant and Recessive Traits: Dominant traits mask the effects of recessive traits in offspring.
Punnett Squares: A diagram used to predict the outcome of a genetic cross.
