January exams

(HL) Prebiotic conditions on early Earth

Early Earth lacked free oxygen and therefore ozone, had higher concentrations of carbon dioxide and methane, higher temperatures, and high levels of ultraviolet radiation

(HL) Absence of ozone layer

Without ozone in the stratosphere, ultraviolet radiation reached Earth’s surface, providing energy for chemical reactions

(HL) Effect of UV radiation on early Earth

UV radiation supplied energy that could drive spontaneous chemical reactions forming organic molecules

(HL) Atmospheric composition of early Earth

Dominated by carbon dioxide, methane, water vapor, and nitrogen with little or no oxygen

(HL) Prebiotic formation of carbon compounds

Early Earth conditions allowed carbon compounds to form spontaneously through chemical processes that do not occur today

(HL) Cells as the smallest units of life

Cells are the smallest structures capable of carrying out all functions of life independently

(HL) Characteristics of living organisms

Metabolism, homeostasis, response to stimuli, growth, reproduction, and self-sustaining chemical reactions

(HL) Why viruses are considered non-living

Viruses lack metabolism, cannot reproduce independently, and rely entirely on host cells

(HL) Challenge of explaining spontaneous origin of cells

Cells are highly complex and currently can only arise from pre-existing cells

(HL) Requirement: catalysis

Chemical reactions needed to be accelerated by catalysts to sustain early metabolic pathways

(HL) Requirement: self-replication

Molecules capable of copying themselves were necessary for inheritance and evolution

(HL) Requirement: self-assembly

Molecules needed to spontaneously organize into larger structures

(HL) Requirement: compartmentalization

Separation of internal chemistry from the environment was essential for early cells

(HL) NOS limitation in origin-of-life studies

Exact prebiotic conditions cannot be replicated and protocells did not fossilize, making hypotheses difficult to test

(HL) Miller–Urey experiment

Simulated early Earth conditions using gases and electrical sparks to test spontaneous formation of organic molecules

(HL) Results of Miller–Urey experiment

Amino acids and other organic compounds formed spontaneously

(HL) Evaluation of Miller–Urey experiment

Supports abiotic synthesis of organic molecules but does not explain origin of cells or genetic systems

(HL) Limitation of Miller–Urey experiment

Atmospheric composition used may not fully represent early Earth conditions

(HL) Formation of vesicles

Fatty acids spontaneously coalesce into spherical bilayers in aqueous environments

(HL) Importance of vesicle formation

Membrane-bound compartments allow internal chemistry to differ from the environment

(HL) Fatty acid bilayers

Simple membranes that can form without enzymes

(HL) RNA as first genetic material

RNA can store information, self-replicate, and catalyse reactions

(HL) Ribozymes

RNA molecules with catalytic activity

(HL) Evidence for RNA world

Ribozymes in ribosomes still catalyse peptide bond formation today

(HL) LUCA

The last universal common ancestor from which all current life descended

(HL) Evidence for LUCA

Universal genetic code and shared genes across all organisms

(HL) Universal genetic code

All organisms use the same codons to specify amino acids

(HL) Possibility of extinct early life forms

Other life forms may have evolved but were outcompeted by LUCA and its descendants

(HL) Estimating age of first living cells

Uses geological evidence, isotopes, and molecular data

(HL) Isotope evidence

Ratios of carbon isotopes in ancient rocks indicate biological activity

(HL) Stromatolites

Layered sedimentary structures formed by ancient microbial communities

(HL) Cyanobacteria

Photosynthetic prokaryotes associated with stromatolite formation

(HL) Molecular clock

Estimates evolutionary divergence based on mutation rates in DNA or proteins

(HL) Genomic analysis

Comparison of conserved genes across species to infer evolutionary relationships

(HL) Evidence for hydrothermal vent origin of life

Fossil evidence from ancient vent precipitates and conserved genetic sequences

(HL) Hydrothermal vents

Deep-sea environments rich in minerals and chemical energy

(HL) Advantages of hydrothermal vents

Provide stable energy sources and protected environments for early life

(HL Exam) Discuss why explaining the origin of cells is challenging

Cellular complexity, lack of fossils, and inability to replicate exact early Earth conditions

(HL Exam) Evaluate the evidence for abiotic synthesis of organic molecules

Supported by experiments like Miller–Urey but incomplete for explaining full cellular life

(HL Exam) Explain why RNA is a strong candidate for the first genetic material

RNA can both store information and catalyse reactions

(HL Exam) Explain how compartmentalization contributed to early cell evolution

Membranes allowed controlled internal chemistry and natural selection to act

(HL Exam) Explain the significance of LUCA

Demonstrates shared ancestry and explains universal molecular features of lif

Cell specialization

The process by which unspecialized cells develop specific structures and functions to perform particular roles in an organism

Zygote

A single diploid cell formed by the fusion of male and female gametes at fertilization

Morula

A solid ball of cells formed by repeated mitotic divisions of the zygote

Blastocyst

An early-stage embryo consisting of an inner cell mass and an outer trophoblast layer

Embryo

A developing multicellular organism formed after fertilization

Differentiation

The process by which cells become specialized through changes in gene expression

Gene expression

The activation of specific genes to produce proteins that determine cell structure and function

Production of unspecialized cells after fertilization

Mitotic divisions produce genetically identical cells that initially remain unspecialized

Impact of gradients on early embryos

Gradients of signaling molecules influence gene expression, leading to different cell fates

Morphogens

Signaling molecules that form concentration gradients and regulate gene expression during development

Fate map

A diagram showing which parts of an early embryo develop into specific tissues or organs

Stem cells

Undifferentiated cells that can divide endlessly and differentiate into specialized cell types

Properties of stem cells

Ability to self-renew through division and differentiate along different pathways

Stem cell niche

A specialized microenvironment that maintains stem cells or stimulates their differentiation

Function of stem cell niches

Regulate stem cell behavior by controlling division, maintenance, and differentiation

Stem cell niche in bone marrow

Maintains hematopoietic stem cells that produce blood cells

Stem cell niche in hair follicles

Regulates stem cells involved in hair growth and regeneration

Totipotent cells

Stem cells that can form all cell types including extraembryonic tissues

Pluripotent cells

Stem cells that can form all body cell types but not extraembryonic tissues

Multipotent cells

Stem cells that can differentiate into a limited range of related cell types

Change in potency during development

Cells are totipotent early, then become pluripotent, and later multipotent in adult tissues

Cell size as an aspect of specialization

Different cell functions require different sizes and shapes

Examples of variation in human cell size

Sperm cells are small and motile, egg cells are large and nutrient-rich, neurons are long, and muscle fibers are very large

Erythrocytes

Red blood cells specialized for oxygen transport

White blood cells

Cells of varying size specialized for immune defense

Surface area-to-volume ratio

A measure comparing a cell’s surface area to its volume

Importance of surface area-to-volume ratio

Exchange depends on surface area, while metabolic demand depends on volume

Constraint on cell size

As cell size increases, volume increases faster than surface area, limiting exchange efficiency

Model of surface area-to-volume ratio (NOS)

Cubes are used to model how surface area and volume scale, even though real cells are more complex

(HL) Adaptations to increase surface area-to-volume ratio

Cells may be flattened, form microvilli, or have invaginations to increase surface area

(HL) Flattened cells

Thin, flat cells reduce diffusion distance and increase surface area relative to volume

(HL) Microvilli

Microscopic membrane projections that greatly increase surface area for absorption

(HL) Invaginations

Infoldings of the cell membrane that increase surface area

(HL) Erythrocyte adaptations

Biconcave disc shape increases surface area and reduces diffusion distance for oxygen

(HL) Biconcave disc

A shape with a depressed center that increases surface area-to-volume ratio

(HL) Proximal convoluted tubule cells

Kidney cells with microvilli that increase surface area for reabsorption

(HL) Squamous epithelium

Thin, flat epithelial tissue adapted for rapid diffusion

(HL) Alveolus

An air sac in the lungs specialized for gas exchange

(HL) Type I pneumocytes

Extremely thin cells that reduce diffusion distance for oxygen and carbon dioxide

(HL) Type II pneumocytes

Cells containing many secretory vesicles that release surfactant

(HL) Role of surfactant

Reduces surface tension in alveoli, preventing collapse

(HL) Alveolar epithelium as a tissue

Contains more than one cell type because different functions require different adaptations

(HL) Adaptations of cardiac muscle cells

Branched cells with contractile myofibrils and typically one nucleus

(HL) Adaptations of striated muscle fibers

Long, unbranched fibers with many nuclei and abundant myofibrils

(HL) Myofibrils

Contractile structures composed of actin and myosin

(HL) Difference between cardiac and skeletal muscle

Cardiac muscle is branched and coordinated, skeletal muscle is long and multinucleate

(HL) Are striated muscle fibers cells?

They are considered cells despite being multinucleate because they form a continuous membrane and function as a unit

(HL) Adaptations of sperm cells

Flagellum for movement, mitochondria for ATP, and streamlined shape

(HL) Adaptations of egg cells

Large size, nutrient-rich cytoplasm, and protective layers to support early development

(Exam) Explain how differentiation occurs despite identical DNA

Cells express different genes due to signaling gradients and regulatory mechanisms

(Exam) Explain the role of morphogens in development

Morphogens form gradients that activate different genes at different concentrations

(Exam) Compare totipotent, pluripotent, and multipotent stem cells

Totipotent form all tissues, pluripotent form all body tissues, multipotent form limited related cell types

(Exam) Explain why stem cells are important in adult tissues

They allow tissue repair, maintenance, and regeneration

(Exam) Explain how surface area-to-volume ratio limits cell size

Volume increases faster than surface area, reducing efficiency of exchange in large cells

(HL Exam) Explain how erythrocyte structure supports function

Biconcave shape increases surface area and flexibility for oxygen transport

(HL Exam) Explain why alveoli require two pneumocyte types

Type I cells optimize diffusion, while Type II cells secrete surfactant to maintain alveolar stability

(HL Exam) Compare adaptations of sperm and egg cells

Sperm are specialized for movement and delivery of DNA, eggs are specialized for nourishment and early development

(NOS Exam) Explain why cubes are useful models for surface area-to-volume ratio

They simplify complex shapes while preserving scaling relationship