Unit 2 & 3: Life & Evolution

Role of Pigments in Vision

Pigment molecules play a crucial role in vision by absorbing light energy. Their molecular structure enables them to interact with photons, initiating the process of visual perception.

• Pigments are molecules that absorb specific wavelengths of light due to their unique molecular structures.

• Despite structural differences, all pigments share the ability to absorb light and undergo a change in their configuration.

• Example: Rhodopsin in rod cells and photopsins in cone cells are key visual pigments.

Light as a Source of Information

Light provides essential information for vision, and the retina is organized to efficiently capture and process this information.

• Cells in the retina are arranged in layers, with photoreceptors located at the back.

• Rods and cones are the two main types of photoreceptor cells, each containing light-sensitive pigments in their membranes.

• Photoreceptors are the only retinal cells that contain pigments.

• When a photon is absorbed, a conformational change occurs in the pigment molecule, triggering a signal to the brain.

• Example: The absorption of light by rhodopsin leads to a change in its structure, initiating the visual signal cascade.

Colour (Chromatic) Vision: Cone Cells

Cone cells in the retina enable the perception of color by responding to different wavelengths of light.

• Three types of cone cells exist, each sensitive to different wavelengths: short (SWS), medium (MWS), and long (LWS).

• These cells allow the brain to interpret light of different wavelengths as color.

• Example: SWS cones are most sensitive to blue light, MWS to green, and LWS to red.

The Role of Opsins in Color Vision

Opsins are proteins in photoreceptor cells that determine wavelength sensitivity and thus color vision.

• Three main opsins: Short Wave Sensitive (SWS), Medium Wave Sensitive (MWS), Long Wave Sensitive (LWS).

• Individuals with only SWS and MWS opsins have dichromatic vision; those with all three have trichromatic vision.

• Example: Most humans are trichromatic, while some animals are dichromatic.

Fundamental Characteristics of Life in All Organisms

All living organisms share certain fundamental characteristics that define life.

• Composed of cells

• Capable of replication

• Undergo evolution

• Process information

• Require energy

Theory in Science vs. Everyday Use

In science, a theory is a well-substantiated explanation based on evidence, not just a guess.

• Scientific theories are broad explanations supported by multiple lines of evidence.

• Theories generate new hypotheses and are rigorously tested.

• Example: The theory of evolution explains the diversity and unity of life.

Major Unifying Concepts of Biology

Two major concepts unify biological understanding: evolution by natural selection and the unity/diversity of life.

• Theory of Evolution by Natural Selection: Populations change over time as individuals with heritable traits that confer advantages reproduce more successfully.

• All organisms alive today descended from a common ancestor.

• Phylogenetic trees illustrate evolutionary relationships, showing both unity and diversity.

The Genetics of Color Vision

Color vision is determined by genes encoding opsin proteins, which are expressed in photoreceptor cells.

• A section of DNA on a chromosome that codes for an opsin protein is called an opsin gene.

• Gene → Protein → Functional Unit (Photoreceptor) → Cell → Organism

• Example: Mutations in opsin genes can lead to color blindness.

Key Points about Selection

Natural selection drives evolution by favoring traits that enhance survival and reproduction.

• Survival to pass on genes to the next generation is key.

• Successful reproduction is essential.

• Populations evolve, not individuals.

• Selection is directed by environmental circumstances, which can change over time.

Unity of Life: Shared Characteristics Across Domains

Cells in all three domains of life (Bacteria, Archaea, Eukarya) share fundamental characteristics.

• Basic building blocks (chemical molecules to make a functional cell)

• Genetic information flow: DNA → RNA → Proteins

• Plasma membrane

• Use and need for energy (metabolism)

Proteins and Their Functions

Proteins are essential biomolecules that carry out many of life's critical functions, including catalysis, structure, transport, and signaling.

• Proteins are polymers made of amino acids linked by peptide bonds.

• Each amino acid has a unique side chain (R group) that determines its chemical and physical properties.

• The primary structure of a protein is its unique amino acid sequence.

• Example: Collagen provides structural support in connective tissues.

Protein Structure and Formation

Proteins are formed by linking amino acids through peptide bonds, resulting in a specific sequence that determines their function.

• Amino acids are joined by peptide bonds via a condensation reaction.

• The sequence of amino acids (primary structure) dictates the protein's final shape and function.

Peptide Bond Formation Equation:

Primary Structure Representation:

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.