Cell Structure and Organization

• Cell biology involves studying cells, including their structure, function, and behavior.
• Cell biology aims to understand cells' composition, organization, and environmental interactions.
• Cell biology studies cell growth, division, differentiation, and programmed cell death.
• Cell biology has close links to molecular biology, biochemistry, and genetics.

Cell Structure and Organization

• Cells constitute the fundamental units of life.
• Cells are categorized mainly into prokaryotic and eukaryotic types.
• Prokaryotic cells do not possess a nucleus or other organelles enclosed by membranes.
• Eukaryotic cells are characterized by a nucleus and complex organelles.
• Eukaryotic cells contain key organelles: the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and peroxisomes.
• The plasma membrane encases the cell, defining its boundaries and controlling substance movement.
• Cytoplasm, located between the plasma membrane and nucleus, contains cytosol and organelles.
• The cytoskeleton offers structural support, facilitates movement, and enables intracellular transport.

Cell Membrane

• The plasma membrane consists of a phospholipid lipid bilayer with embedded proteins.
• The lipid bilayer exhibits selective permeability, allowing only certain molecules through.
• Membrane proteins mediate transport, signaling, and cell adhesion.
• In the fluid mosaic model, the membrane is dynamic, allowing lipids and proteins to move laterally.
• Cholesterol affects the membrane's fluidity and stability.
• Membrane carbohydrates are involved in cell signaling and recognition.

Nucleus

• The nucleus contains DNA, the cell's genetic material, organized into chromosomes.
• The nuclear envelope, a double membrane, regulates molecule transport between the nucleus and cytoplasm.
• Nuclear pores within the nuclear envelope selectively allow molecules through.
• The nucleolus is where ribosomes are assembled.
• Chromatin is formed by DNA and proteins (histones), creating a compact structure.

Endoplasmic Reticulum (ER)

• The ER is a comprehensive membrane network involved in protein and lipid synthesis.
• Rough ER (RER) contains ribosomes and is involved in protein synthesis and modification.
• Smooth ER (SER) lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.
• Proteins made on the RER move into the ER lumen to undergo modification and folding.

Golgi Apparatus

• The Golgi apparatus is responsible for the processing and packaging of proteins and lipids from the ER.
• It consists of flattened, membrane-bound sacs known as cisternae.
• Proteins and lipids are moved through the Golgi via vesicles and undergo modifications.
• The Golgi sorts and packages molecules into vesicles for transfer to the plasma membrane or other organelles.

Mitochondria

• Mitochondria produce ATP through cellular respiration and are the cell's powerhouses.
• They possess a double membrane, including an outer membrane and a highly folded inner membrane (cristae).
• The electron transport chain and ATP synthesis occur on the inner membrane.
• Mitochondria contain their own DNA and ribosomes, suggesting an endosymbiotic origin.

Lysosomes

• Lysosomes contain enzymes that break down cellular waste and debris.
• They maintain an acidic environment for optimal enzyme activity.
• Lysosomes participate in autophagy and phagocytosis.

Peroxisomes

• Peroxisomes are small organelles involved in detoxification and lipid metabolism.
• They contain enzymes that break down fatty acids, producing hydrogen peroxide.
• Catalase, an enzyme in peroxisomes, breaks down hydrogen peroxide into water and oxygen.

Cytoskeleton

• The cytoskeleton is a network of protein filaments providing structural support and facilitating cell movement.
• The three main types of filaments are microtubules, intermediate filaments, and actin filaments.
• Microtubules consist of tubulin dimers and are hollow tubes involved in cell division, intracellular transport, and cell shape.
• Intermediate filaments are rope-like structures providing mechanical strength and stability.
• Actin filaments consist of actin monomers and are flexible filaments involved in cell motility, muscle contraction, and cell shape.

Cell Cycle

• The cell cycle is a sequence of events from one cell division to the next.
• It includes interphase (growth and DNA replication) and mitosis/meiosis (cell division).
• Interphase includes G1, S, and G2 phases.
• During the G1 phase, the cell grows and prepares for DNA replication.
• During the S phase, DNA is replicated.
• During the G2 phase, the cell prepares for mitosis or meiosis.
• Mitosis results in two identical daughter cells.
• Meiosis results in four genetically diverse daughter cells (gametes).

Cell Communication

• Cells use signaling pathways to communicate.
• Signaling molecules (ligands) attach to receptors on target cells.
• Receptors may be located on the cell surface or inside the cell.
• Ligand binding to a receptor causes intracellular events, resulting in a cellular response.
• Types of signaling include endocrine, paracrine, autocrine, and direct contact.
• Key signaling pathways include receptor tyrosine kinases (RTKs), G protein-coupled receptors (GPCRs), and ion channels.

Cell Adhesion

• Cell adhesion is how cells interact and bind to each other and the extracellular matrix.
• Cell adhesion molecules (CAMs) mediate cell-cell and cell-matrix interactions.
• Types of CAMs include cadherins, integrins, selectins, and the immunoglobulin superfamily.
• Cadherins are calcium-dependent adhesion molecules involved in cell-cell adhesion.
• Integrins are transmembrane receptors binding to the extracellular matrix.
• Selectins bind to carbohydrates on other cells, mediating cell-cell interactions.
• Cell adhesion is important for tissue development, wound healing, and immune response.

Cell Junctions

• Cell junctions are specialized structures connecting cells.
• Types of cell junctions: tight junctions, adherens junctions, desmosomes, and gap junctions.
• Tight junctions seal cells together, preventing molecule leakage across epithelia.
• Adherens junctions link actin filaments in adjacent cells providing mechanical strength.
• Desmosomes link intermediate filaments in adjacent cells providing strong adhesion.
• Gap junctions allow direct communication between cells through cytoplasm-connecting channels.

Cell Differentiation

• Cell differentiation is the process of cells becoming specialized in structure and function.
• Cells receive signals determining their fate during development.
• Stem cells are undifferentiated cells that can self-renew and differentiate into specialized cells.
• Types of stem cells: totipotent, pluripotent, multipotent, and unipotent.
• Totipotent stem cells can differentiate into any cell type, including embryonic and extraembryonic tissues.
• Pluripotent stem cells can differentiate into any cell type of the body, but not extraembryonic tissues.
• Multipotent stem cells can differentiate into a limited range of cell types.
• Unipotent stem cells can only differentiate into one cell type.

Cell Death

• Cell death is a normal process to eliminate damaged or unwanted cells.
• Apoptosis and necrosis are the two main types of cell death.
• Apoptosis is programmed cell death, characterized by cell shrinkage, DNA fragmentation, and formation of apoptotic bodies.
• Necrosis is uncontrolled cell death, characterized by cell swelling, membrane rupture, and inflammation.
• Apoptosis is important for development, tissue homeostasis, and immune response.
• Dysregulation of apoptosis can lead to diseases such as cancer and autoimmune disorders.