Cell Structure and Function

• Biology is the scientific study of life, encompassing a wide range of topics from the molecular mechanisms within cells to the complex interactions of ecosystems.
• Cell biology, a subdiscipline of biology, focuses on the structure, function, and behavior of cells.
• The cell is the basic unit of life.

Cell Structure and Function

• Cells are broadly classified into two types: prokaryotic and eukaryotic.
• Prokaryotic cells (bacteria and archaea) lack a nucleus and other complex organelles.
• Eukaryotic cells (protists, fungi, plants, and animals) possess a nucleus and membrane-bound organelles.
• Key eukaryotic organelles include the nucleus, endoplasmic reticulum (ER), Golgi apparatus, mitochondria, lysosomes, and peroxisomes.
• The nucleus contains the cell's DNA, organized into chromosomes, and is the site of DNA replication and RNA transcription.
• The ER is a network of membranes involved in protein synthesis (rough ER) and lipid metabolism (smooth ER).
• The Golgi apparatus processes and packages proteins and lipids.
• Mitochondria are responsible for generating ATP through cellular respiration.
• Lysosomes contain enzymes that degrade cellular waste and debris.
• Peroxisomes are involved in various metabolic reactions, including detoxification.
• The plasma membrane is the outer boundary of the cell, regulating the passage of substances in and out.
• The cytoskeleton, composed of microtubules, intermediate filaments, and actin filaments, provides structural support and facilitates cell movement.

Cell Membrane

• The cell membrane is composed of a phospholipid bilayer with embedded proteins.
• Phospholipids have a hydrophilic (polar) head and hydrophobic (nonpolar) tails, which drive the formation of the bilayer in an aqueous environment.
• Membrane proteins perform various functions, including transport, signaling, and cell adhesion.
• Membrane fluidity is influenced by temperature and lipid composition.
• Cholesterol helps to maintain membrane fluidity at different temperatures.
• Selective permeability allows the cell membrane to control which molecules enter and exit the cell.

Cell Transport

• Passive transport does not require energy and includes diffusion, osmosis, and facilitated diffusion.
• Diffusion is the movement of molecules from an area of high concentration to low concentration.
• Osmosis is the diffusion of water across a semipermeable membrane from an area of low solute concentration to high solute concentration.
• Facilitated diffusion involves the transport of molecules across the membrane with the help of membrane proteins.
• Active transport requires energy (ATP) to move molecules against their concentration gradient.
• Examples of active transport include the sodium-potassium pump.
• Endocytosis is the process by which cells take up substances from the external environment by engulfing them in vesicles.
• Exocytosis is the process by which cells release substances to the external environment by fusing vesicles with the plasma membrane.

Cell Communication

• Cells communicate with each other through various signaling mechanisms.
• Direct contact, such as gap junctions, allows for direct communication between adjacent cells.
• Local signaling involves the release of signaling molecules that act on nearby cells (paracrine signaling) or on the same cell (autocrine signaling).
• Long-distance signaling involves the use of hormones that travel through the bloodstream to reach target cells.
• Signal transduction pathways convert extracellular signals into intracellular responses.
• Key components of signal transduction pathways include receptors, kinases, and second messengers.
• Receptors bind to signaling molecules and initiate a signaling cascade.
• Kinases phosphorylate proteins, activating or inactivating them.
• Second messengers, such as cAMP and calcium ions, amplify the signal.

Cell Cycle and Division

• The cell cycle is the series of events that a cell undergoes from its formation to its division into two daughter cells.
• The cell cycle consists of interphase (G1, S, and G2 phases) and mitosis (M phase).
• During interphase, the cell grows and replicates its DNA.
• DNA replication occurs during the S phase.
• Mitosis is the process of nuclear division, resulting in two identical daughter nuclei.
• Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase.
• Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.
• Cell cycle checkpoints ensure that the cell cycle progresses correctly.
• Meiosis is a type of cell division that produces four haploid gametes (sperm and egg cells) from a single diploid cell.
• Meiosis involves two rounds of division: meiosis I and meiosis II.
• Genetic variation is introduced during meiosis through crossing over and independent assortment.

Cell Metabolism

• Metabolism is the sum of all chemical reactions that occur within a cell or organism.
• Catabolism is the breakdown of complex molecules into simpler ones, releasing energy.
• Anabolism is the synthesis of complex molecules from simpler ones, requiring energy.
• Enzymes are biological catalysts that speed up chemical reactions.
• ATP (adenosine triphosphate) is the primary energy currency of the cell.
• Cellular respiration is the process by which cells generate ATP by oxidizing glucose and other organic molecules.
• Glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation are the main stages of cellular respiration.
• Photosynthesis is the process by which plants and other organisms convert light energy into chemical energy.
• The light-dependent reactions and the Calvin cycle are the two main stages of photosynthesis.

Genetics and Molecular Biology

• Genetics is the study of heredity and variation in organisms.
• DNA (deoxyribonucleic acid) is the genetic material that carries the instructions for building and maintaining an organism.
• DNA is composed of nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or thymine).
• RNA (ribonucleic acid) is involved in gene expression.
• RNA is composed of nucleotides, each consisting of a ribose sugar, a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or uracil).
• The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein.
• Transcription is the process by which RNA is synthesized from a DNA template.
• Translation is the process by which proteins are synthesized from an RNA template.
• Genes are segments of DNA that code for proteins or functional RNA molecules.
• Mutations are changes in the DNA sequence.

Cell Differentiation and Development

• Cell differentiation is the process by which cells become specialized in structure and function.
• During development, cells differentiate and organize into tissues and organs.
• Stem cells are undifferentiated cells that can divide and differentiate into various cell types.
• Totipotent stem cells can differentiate into any cell type, including embryonic and extraembryonic tissues.
• Pluripotent stem cells can differentiate into any cell type in the body, but not extraembryonic tissues.
• Multipotent stem cells can differentiate into a limited number of cell types.
• Apoptosis is programmed cell death, which is essential for normal development and tissue homeostasis.

Cell Interactions and Extracellular Matrix

• Cells interact with each other and with the extracellular matrix (ECM).
• Cell-cell junctions, such as tight junctions, adherens junctions, desmosomes, and gap junctions, mediate cell-cell adhesion and communication.
• The ECM is a network of proteins and polysaccharides that surrounds cells and provides structural support.
• Key components of the ECM include collagen, elastin, fibronectin, and laminin.
• The ECM influences cell behavior, including cell growth, differentiation, and migration.

Viruses

• Viruses are acellular infectious agents that can only replicate inside a host cell.
• Viruses consist of a nucleic acid genome (DNA or RNA) surrounded by a protein coat (capsid).
• Some viruses have an outer envelope derived from the host cell membrane.
• Viruses infect cells by attaching to specific receptors on the cell surface.
• Viral replication involves hijacking the host cell's machinery to produce more virus particles.
• Viruses can cause a variety of diseases in plants, animals, and bacteria.