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3. Organisms

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Understanding points

Species

A group of organisms that can interbreed to produce fertile offspring
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Binomial naming system: Genus species (e.g. Homo sapiens)
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*(AHL) Must have the same chromosome number or else problems in meiosis occur
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Different species have different genome sizes and base sequences

Population

A group of organisms of the same species, living in the same area, at the same time
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Two populations living in different areas are unlikely to interbreed
A3.1 Diversity of organisms

Understanding points

Taxonomy

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Taxa: groups used to classify organisms
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Hierarchy of taxa: Domains – Kingdom – Phylum – Class – Order – Family – Genus – Species
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Every organism that has evolved from a common ancestor is included in the same taxon
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In each taxon, all the species are evolved from the same common ancestor

Phylogeny

The study of evolutionary ancestry
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Phylogenetic classification is based on DNA sequence
A3.2 Classification and cladistics

Understanding points

Ventilation, cell respiration, gas exchange

Ventilation
Exchange of air between the atmosphere and the lungs
Achieved by the physical act of breathing
Cell respiration
Generation of ATP from organic molecules
Enhanced by the presence of oxygen
Gas exchange
Exchange of O₂ and CO₂ between the alveoli and bloodstream by passive diffusion
B3.1 Gas exchange

Understanding points

Blood components

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Glucose, Dissolved gas, Urea, Hormones, Antibodies, Proteins

Blood vessels

*(AHL)

Blood circulation

B3.2 Transport

Understanding points

Muscle structure

Muscle fibre structure

Sarcomere

Repeating contractile units composed of two myofilaments: actin and myosin
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The thick filament (myosin) contains small protruding heads which bind to the thin filament (actin)
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Movement of these filaments relative to one another causes lengthening /shortening of sarcomere
B3.3 Muscle and motility

Understanding points

Levels of organisation

Hormonal vs neuronal signaling

Central nervous system

Brain
Stores, processes, and integrates information received from the body
C3.1 Integration of body systems

Understanding points

Immunity

The ability of an organism to resist infection
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Pathogens: organisms that cause infectious disease

First line of defence

Phagocytosis

C3.2 Defence against disease

Understanding points

Modes of reproduction

Sexual reproduction in flowering plants

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Self-pollination: transfer of pollen from anther to stigma on the same plant
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Cross-pollination: transfer of pollen from anther of one flower to stigma of a flower on different plant

Self-incompatibility mechanisms

D3.1 Reproduction
D3.2 Inheritance

Understanding points

Homeostasis

Keeping a constant internal environment
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Positive Feedback - accelerates the concentration gap between the original and new level
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Negative Feedback - maintains the concentration of metabolites

Body temperature - hypothalamus

Temperature control is regulated by thyroxine
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Thyroxine is secreted by the thyroid gland in response to signals from the hypothalamus
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Essential to the proper development and differentiation of cells
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Increases basal metabolic rate (amount of energy the body uses at rest) → stimulates carbohydrate and lipid metabolism via oxidation of glucose and fatty acids → heat
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Deficiency of iodine in the diet leads to decreased thyroxine production, causing enlargement of the thyroid gland and goitres
D3.3 Homeostasis