Biology: What is Life?

What is Life?

• Life is a complex and organized phenomenon characterized by five key qualities:
  • Organization into a hierarchical pattern
  • Requirement of energy
  • Maintenance of internal consistency through homeostasis
  • Reproduction, growth, and development
  • Evolution, adaptation through mutation and natural selection
• Biologists have settled on these five qualities as defining life.
• The Human Genome Project cost around 3billion,whileCelerasequencedthehumangenomefor3 billion, while Celera sequenced the human genome for 3billion,whileCelerasequencedthehumangenomefor300 million.
• D. radiodurans, a bacterium, can withstand up to 5 kGy of radiation.
• One Gray (Gy) equals one Joule of gamma radiation absorbed per kilogram.
• Synthetic life can now be created.

Life is Organized

• The smallest unit of a pure substance is an atom (e.g., carbon atom)
• A group of joined atoms forms a molecule (e.g., DNA)
• Organelles are membrane-bound structures with specific functions within a cell (e.g., chloroplast)
• The fundamental unit of life is a cell (e.g., a leaf cell).
• Tissues are collections of specialized cells functioning together (e.g., the epidermis of a leaf).
• An organ consists of interacting tissues for specific functions (e.g., a leaf).
• An organ system is a group of organs connected physically or chemically for joint function (e.g., the aboveground part of a plant).
• A single living individual is an organism (e.g., one acacia tree).
• A population is a group of the same species living in the same place and time (e.g., multiple acacia trees).
• A community encompasses all the populations in a region (e.g., all populations in a savanna).
• An ecosystem comprises the living and nonliving components of an area (e.g., the savanna).
• The biosphere encompasses the global ecosystem, including all parts of the planet and its atmosphere where life exists.

Life Maintains Internal Consistency

• Cells and organisms maintain an equilibrium state through homeostasis.
• Homeostasis is crucial for stability and survival in changing environments.

Life Reproduces, Grows and Develops

• Organisms can reproduce asexually or sexually.
• Sexual reproduction combines characteristics from different individuals.
• Reproduction, growth, and development are fundamental aspects of the life cycle.

The Chemistry of Life

• Living organisms consist of elements, but not all elements are essential for life.
• Only about 25 elements are essential to life.
• Bulk elements make up the majority of cells, while trace elements are present in smaller quantities.
• Atoms of essential elements bond to form molecules and compounds crucial for life.
• Water is essential for life due to its unique properties:
  • Polarity allowing hydrogen bonding
  • Cohesiveness
  • Solvent capabilities
  • Temperature regulation
  • Expansion upon freezing
  • Involvement in chemical reactions

The Organic Chemistry of Life

• Carbon-based organic molecules are essential for life, and they are often polymers of monomeric subunits.
• Monomers join to form polymers:
  • Nucleic acids are polymers of nucleotides.
  • Carbohydrates are polymers of monosaccharides.
  • Proteins are polymers of amino acids.
  • Lipids are not polymers but are large and diverse molecules.
• Macromolecules, composed of polymers, are critical for cell function:
  • Carbohydrates serve as energy storage and structural components.
  • Lipids are vital for membranes, energy storage, and signaling.
  • Proteins are essential for a wide variety of cellular functions, including structure, locomotion, transport, storage, and catalysis.
  • Nucleic acids store and transmit genetic information.

Carbohydrates

• Carbohydrates are sugars.
• Monosaccharides are simple sugars that can combine to form disaccharides.
• Polysaccharides are complex carbohydrates formed from multiple sugar units.

Lipids

• Lipids are hydrophobic and do not dissolve in water.
• They are also known as fats.
• Important lipid types include:
  • Cholesterol
  • Testosterone

Proteins

• Proteins are complex and highly versatile.
• Examples of proteins include:
  • Membrane proteins
  • Hemoglobin
  • Keratin
  • Polymerases
  • Antibodies
  • Enzymes
• A protein chain consists of amino acids linked together by peptide bonds to form one or more polypeptides.
• Of the 20 naturally occurring amino acids, some are essential and must be obtained from the diet.
• Protein shape is crucial for function and can be disrupted by factors like heat, pH, and salts.
• Denaturation can inactivate a protein.

Nucleic acids

• Nucleotide monomers form long nucleic acid polymers like DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
• DNA is the cell's data storage molecule, encoding genetic blueprints.
• RNA plays multiple roles in utilizing the genetic information stored in DNA.

Cells are the Units of Life

• Cell theory states:
  • All organisms are composed of one or more cells.
  • The cell is the fundamental unit of life.
  • Cells arise from preexisting cells.
• Cells are typically small.
• All cells share common features:
  • DNA and RNA
  • Ribosomes
  • Proteins
  • Cytosol/cytoplasm
  • Cell membrane
• Cell membranes are crucial for life, allowing for nutrient exchange and maintaining the surface area-to-volume ratio.

Different Cells for Different Domains

• Three domains of life:
  • Bacteria
  • Archaea
  • Eukarya
• Bacteria and archaea were previously grouped as prokaryotes (“before nucleus”), but now separate domains.
• Bacteria are the most abundant and diverse organisms on Earth.

Domain Bacteria

• Bacteria are structurally simple organisms.
• They possess a nucleoid where the circular DNA molecule congregates.
• Most bacteria have a rigid cell wall for protection, structural rigidity, and shape.
• Some bacteria possess flagella for movement.

Domain Archaea

• Archaea resemble bacteria in many ways, including:
  • Nucleoid
  • Cell wall
  • Flagella (some)
• Archaea have unique biochemical components differing from bacteria and eukaryotes.
• Their ribosomes are more similar to those of eukaryotes.
• Archaea are considered the closest relatives of eukaryotes.

Domain Eukarya

• Domain eukarya encompasses diverse organisms (animals, yeasts, fungi, plants, and protists).
• Eukaryotic cells are larger than bacteria and archaea, sharing several cellular features.
• The most notable difference is the presence of a membrane-bound nucleus and organelles.

The Eukaryotic Cell

• All eukaryotic cells have a cell membrane separating them from their surroundings.
• The cell membrane is composed of a phospholipid bilayer and proteins that act as selective barriers.
• The fluid mosaic model describes the composition and dynamics of the membrane, with various proteins floating within the lipid bilayer.

Eukaryotic Organelles

• Organelles have specialized functions within the cell.
• The endomembrane system (nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, and cell membrane) is a coordinated network.
• This system facilitates the production, packaging, distribution, and release of complex biochemicals.
• Vesicle transport connects different components of the endomembrane system.

The Nucleus

• The nucleus is the site of gene expression.
• mRNA exits the nuclear envelope through nuclear pores.
• The nucleolus produces ribosomes.

Endoplasmic Reticulum

• The rough endoplasmic reticulum (RER) synthesizes secreted proteins.
• Free ribosomes synthesize soluble proteins.
• The smooth endoplasmic reticulum (SER) synthesizes lipids.

Golgi Apparatus

• Proteins from the ER pass through the Golgi apparatus, where they undergo folding and maturation.
• Golgi enzymes add sugars to proteins and lipids.
• The Golgi also sorts and packages materials.

Lysosomes, Vacuoles, and Peroxisomes

• Eukaryotic cells break down molecules in specialized membranous compartments.
• Lysosomes contain break-down enzymes that are produced in the RER and directed by the Golgi.
• Vacuoles are large vesicles in plant cells, serving as “lysosomes” and filling with water to maintain turgor pressure.
• Peroxisomes contain enzymes that break down toxic substances.

Chloroplasts and Mitochondria

• Chloroplasts and mitochondria are involved in energy transduction and production.
• Chloroplasts carry out photosynthesis in plant cells.
• Mitochondria extract energy from nutrients.
• Mitochondria and chloroplasts have their own DNA, inherited maternally.

The Cytoskeleton

• The cytoskeleton is a network of protein tracks and tubules that provides structural support to eukaryotic cells.
• It includes:
  • Actin fibers for strength and resistance to stretching and compression.
  • Intermediate filaments for similar function and cell-cell binding.
  • Microtubules, with multiple functions.

Intercellular Communication

• Plant cells have plasmodesmata, channels connecting adjacent cells for intercellular communication and exchange of nutrients.