Introduction to Biology

• Biology is the scientific study of life.
• It encompasses the structure, function, growth, origin, evolution, and distribution of living organisms.
• Biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the creation and extinction of species.
• Living organisms are open systems that survive by transforming energy and decreasing their local entropy.

Subdisciplines of Biology

• Biochemistry examines the fundamental chemistry of life.
• Molecular biology studies the complex interactions of biological systems.
• Botany studies plants.
• Zoology studies animals.
• Microbiology studies microorganisms.
• Genetics studies heredity.
• Evolutionary biology studies the processes that produced the diversity of life on Earth.
• Ecology studies how organisms interact with their environment.
• Physiology studies the functions and mechanisms in a living system.

Cell Biology

• Cell biology is the study of cells, their physiological properties, structure, organelles, interactions with their environment, life cycle, division, and death.
• Cell biology examines both prokaryotic and eukaryotic cells.
• Research in cell biology includes studying cell signaling pathways, cell cycle, and cellular composition.
• Understanding the components and function of cells is fundamental to all biological sciences.
• Sub-areas of cell biology include the study of cell metabolism, cell communication, and cell growth.
• The central dogma of molecular biology (DNA -> RNA -> Protein) is a key concept in cell biology; it describes the flow of genetic information within a cell.

Cell Theory

• Cell theory states that the cell is the fundamental unit of life, all living organisms are composed of one or more cells, and all cells arise from pre-existing cells.
• Matthias Schleiden and Theodor Schwann proposed the initial two tenets of the cell theory in 1839.
• Rudolf Virchow added the third tenet "Omnis cellula e cellula" (all cells come from cells) in 1855.
• Cell theory unified botany and zoology by highlighting the common cellular basis of plant and animal life.

Prokaryotic Cells

• Prokaryotes are cells without a nucleus or other membrane-bound organelles.
• Bacteria and Archaea are the two domains of prokaryotes.
• Prokaryotic cells are generally smaller than eukaryotic cells, typically ranging from 0.5 to 5 micrometers in diameter.
• The prokaryotic cell's DNA is located in the nucleoid region, which has no surrounding membrane.
• Prokaryotes have a cell wall that provides structure and protection.
• Many prokaryotes have flagella for movement or pili for attachment.
• Ribosomes are present in the cytoplasm for protein synthesis, but they are smaller and differ in composition compared to eukaryotic ribosomes.
• Prokaryotes reproduce primarily through binary fission, a form of asexual reproduction.

Eukaryotic Cells

• Eukaryotes are cells that contain a nucleus and other membrane-bound organelles.
• Eukaryotic cells are structurally more complex than prokaryotic cells.
• Eukaryotes comprise protists, fungi, plants, and animals.
• Eukaryotic cells are typically larger than prokaryotic cells, ranging from 10 to 100 micrometers in diameter.
• The nucleus houses the cell's DNA, organized into chromosomes.
• Organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes perform specific functions within the cell.
• Eukaryotic cells can reproduce sexually through meiosis or asexually through mitosis.
• The cytoskeleton provides structural support and facilitates cellular movement and intracellular transport.

Cellular Organelles

• Organelles are specialized subunits within a cell that perform specific functions.
• Nucleus contains the cell's genetic material (DNA) and controls the cell's growth and reproduction.
• Mitochondria are responsible for generating energy (ATP) through cellular respiration (powerhouse of the cell).
• Endoplasmic Reticulum (ER) is involved in protein and lipid synthesis and transport; Rough ER contains ribosomes, while smooth ER does not.
• Golgi Apparatus processes and packages proteins and lipids for transport to other locations inside or outside the cell.
• Lysosomes contain enzymes that break down cellular waste and debris (recycling center).
• Ribosomes are responsible for protein synthesis.
• Chloroplasts (in plant cells) conduct photosynthesis, converting light energy into chemical energy.
• Vacuoles store water, nutrients, and waste products; they are particularly large in plant cells.
• Peroxisomes break down fatty acids and detoxify harmful substances.

Cell Membrane

• The cell membrane (plasma membrane) separates the interior of the cell from the external environment.
• It is composed primarily of a phospholipid bilayer with embedded proteins.
• The phospholipid bilayer is selectively permeable, controlling the movement of substances into and out of the cell.
• Membrane proteins can function as transporters, receptors, enzymes, or structural anchors.
• The fluid mosaic model describes the cell membrane as a dynamic structure with lipids and proteins that can move laterally.
• Cell membranes play critical roles in cell signaling, cell adhesion, and maintaining cell shape.

Cell Wall

• The cell wall is a rigid layer located outside the cell membrane in plant cells, bacteria, fungi, and algae.
• It provides structural support, protection, and shape to the cell.
• In plant cells, the cell wall is primarily composed of cellulose.
• In bacteria, the cell wall is made of peptidoglycan.
• Fungal cell walls are composed of chitin.

Cytoskeleton

• The cytoskeleton is a network of protein fibers that provides structural support, facilitates cell movement, and enables intracellular transport.
• The three main types of cytoskeletal filaments are:
  • Microfilaments (actin filaments): involved in cell movement, muscle contraction, and cell division; they are the thinnest filaments.
  • Intermediate filaments: provide structural support and mechanical strength; they are intermediate in size.
  • Microtubules: involved in cell division, intracellular transport, and cell motility; they are the thickest filaments.
• The cytoskeleton is a dynamic structure that can be rapidly assembled and disassembled in response to cellular signals.

Cell Communication

• Cell communication is the process by which cells interact and communicate with each other.
• Cells communicate through chemical signals, such as hormones, neurotransmitters, and local mediators.
• Cell signaling pathways involve a series of steps that transmit a signal from the cell surface to the interior, leading to a cellular response.
• Types of cell signaling include:
  • Endocrine signaling: hormones are secreted into the bloodstream and travel to distant target cells.
  • Paracrine signaling: signals are released and act on nearby cells.
  • Autocrine signaling: cells respond to signals that they themselves produce.
  • Direct contact signaling: cells communicate through direct physical contact.
• Receptor proteins on the cell surface or inside the cell bind to signaling molecules and initiate a signaling cascade.

Cell Cycle

• The cell cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (replication) to produce two new daughter cells.
• The cell cycle consists of two main phases: interphase and the mitotic (M) phase.
• Interphase includes:
  • G1 phase (gap 1): the cell grows and synthesizes proteins and organelles.
  • S phase (synthesis): DNA is replicated.
  • G2 phase (gap 2): the cell prepares for division.
• M phase (mitotic phase) includes:
  • Mitosis: the nucleus divides, resulting in two identical nuclei.
    • Prophase: chromosomes condense.
    • Metaphase: chromosomes line up in the middle of the cell.
    • Anaphase: sister chromatids separate and move to opposite poles.
    • Telophase: chromosomes arrive at the poles, and the nuclear envelope reforms.
  • Cytokinesis: the cytoplasm divides, resulting in two separate daughter cells.
• The cell cycle is tightly regulated by checkpoints that ensure proper DNA replication and chromosome segregation.

Cell Death

• Apoptosis is programmed cell death, a normal physiological process that eliminates damaged or unnecessary cells.
• Necrosis is uncontrolled cell death caused by injury or infection.
• Apoptosis is characterized by distinct morphological changes, including cell shrinkage, DNA fragmentation, and formation of apoptotic bodies.
• Caspases are a family of proteases that play a central role in apoptosis.
• Dysregulation of apoptosis can contribute to diseases such as cancer and neurodegenerative disorders.