Cell Structure and Function

• Cell biology involves studying their structure, function, and behavior.
• It includes cells physiological properties, interactions with the environment, and lifecycle.
• Understanding cell biology is fundamental to understanding tissues, organs, and entire organisms.

Cell Structure

• Cells are broadly classified into prokaryotic and eukaryotic cells based on their structure.
• Prokaryotic cells lack a nucleus and other membrane-bound organelles; Bacteria and Archaea are prokaryotes.
• Eukaryotic cells possess a nucleus and other membrane-bound organelles like mitochondria and endoplasmic reticulum; Eukaryotes include protists, fungi, plants, and animals.
• Key components of a cell include the plasma membrane, cytoplasm, and genetic material (DNA).

Plasma Membrane

• The plasma membrane is a selectively permeable barrier separating the cell's internal environment from the external environment.
• It comprises a lipid bilayer, mainly phospholipids, with embedded proteins and carbohydrates.
• The fluid mosaic model describes it as a dynamic structure where lipids and proteins can move laterally.
• Membrane proteins can function as transporters, receptors, enzymes, or anchors.
• Cholesterol within the membrane helps maintain fluidity and stability.

Cytoplasm

• The cytoplasm is the gel-like substance within the cell, excluding the nucleus.
• It contains various organelles, the cytoskeleton, and cytosol.
• Cytosol is the aqueous component of the cytoplasm, containing ions, small molecules, and macromolecules.

Nucleus

• The nucleus is the control center of the cell, housing genetic material (DNA) organized into chromosomes.
• It is surrounded by a double membrane called the nuclear envelope, containing nuclear pores for transport.
• The nucleolus within the nucleus is the site of ribosome synthesis.
• Chromatin, a complex of DNA and proteins, forms the chromosomes.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum is an extensive network of membranes involved in protein and lipid synthesis.
• There are two types of ER: rough ER (RER) and smooth ER (SER).
• RER is studded with ribosomes and is involved in protein synthesis and modification.
• SER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.

Golgi Apparatus

• The Golgi apparatus processes and packages proteins and lipids synthesized in the ER.
• It consists of flattened membranous sacs called cisternae.
• Proteins and lipids are modified, sorted, and packaged into vesicles for transport to other organelles or the plasma membrane.

Lysosomes

• Lysosomes are organelles containing digestive enzymes that break down cellular waste and debris.
• They are involved in autophagy (self-eating) and phagocytosis (cellular eating).
• Lysosomes maintain an acidic internal environment to facilitate enzyme activity.

Mitochondria

• Mitochondria are the powerhouses of the cell, responsible for generating ATP through cellular respiration.
• They have a double membrane structure: an outer membrane and an inner membrane folded into cristae.
• The inner membrane contains the enzymes of the electron transport chain.
• Mitochondria contain their own DNA and ribosomes, suggesting an endosymbiotic origin.

Chloroplasts

• Chloroplasts are organelles found in plant cells and algae, responsible for photosynthesis.
• They contain chlorophyll, a pigment that captures light energy.
• Like mitochondria, chloroplasts have a double membrane and their own DNA and ribosomes.
• Photosynthesis converts light energy into chemical energy in the form of glucose.

Cytoskeleton

• The cytoskeleton is a network of protein fibers that provides structural support, facilitates cell movement, and enables intracellular transport.
• There are three main types of cytoskeletal fibers: microfilaments, intermediate filaments, and microtubules.
• Microfilaments (actin filaments) are involved in cell shape, muscle contraction, and cell motility.
• Intermediate filaments provide structural support and mechanical strength.
• Microtubules are involved in cell division, intracellular transport, and the structure of cilia and flagella.

Cell Communication

• Cells communicate with each other through various signaling molecules and receptors.
• Types of cell signaling include direct contact, paracrine signaling, endocrine signaling, and synaptic signaling.
• Receptors can be located on the cell surface or inside the cell.
• Signal transduction pathways convert extracellular signals into intracellular responses.
• Common signaling pathways involve protein kinases, second messengers, and transcription factors.

Cell Cycle

• The cell cycle is the series of events that a cell undergoes from one division to the next.
• It consists of interphase (G1, S, G2) and the mitotic (M) phase.
• During interphase, the cell grows, replicates its DNA, and prepares for division.
• The M phase involves mitosis (nuclear division) and cytokinesis (cytoplasmic division).
• Mitosis is divided into phases: prophase, metaphase, anaphase, and telophase.
• Cell cycle checkpoints ensure that each stage is completed accurately before proceeding to the next.

Cell Division

• Cell division is the process by which a cell divides into two or more daughter cells.
• Mitosis produces two genetically identical daughter cells and is used for growth and repair.
• Meiosis produces four genetically distinct daughter cells with half the number of chromosomes and is used for sexual reproduction.
• During meiosis, homologous chromosomes exchange genetic material through crossing over, increasing genetic diversity.

Cell Differentiation

• Cell differentiation is the process by which cells become specialized in structure and function.
• During development, cells receive signals that activate specific genes, leading to differentiation.
• Differentiated cells have distinct patterns of gene expression.
• 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 tissues.
• Pluripotent stem cells can differentiate into any cell type in the body, but not embryonic tissues.

Cell Metabolism

• Cell metabolism encompasses all the chemical reactions that occur within a cell to sustain life.
• These reactions include catabolism (breaking down molecules) and anabolism (building up molecules).
• Enzymes catalyze metabolic reactions.
• ATP (adenosine triphosphate) is the primary energy currency of the cell.
• Key metabolic pathways include glycolysis, the Krebs cycle (citric acid cycle), and oxidative phosphorylation.

Cell Death

• Cell death is a normal and essential process in multicellular organisms.
• Apoptosis is programmed cell death, which is characterized by controlled self-destruction.
• Necrosis is uncontrolled cell death, often caused by injury or infection, leading to inflammation.
• Apoptosis plays a crucial role in development, tissue homeostasis, and immune function.

Cell Adhesion

• Cell adhesion is the process by which cells bind to each other and to the extracellular matrix.
• Cell adhesion molecules (CAMs) mediate cell-cell and cell-matrix interactions.
• Types of CAMs include cadherins, integrins, selectins, and immunoglobulin superfamily members.
• Cell adhesion is important for tissue structure, cell migration, and cell signaling.

Extracellular Matrix (ECM)

• The extracellular matrix is a network of proteins and polysaccharides secreted by cells.
• It provides structural support, regulates cell behavior, and mediates cell-cell communication.
• Major components of the ECM include collagen, elastin, fibronectin, and proteoglycans.
• The ECM influences cell adhesion, migration, differentiation, and proliferation.

Cell Transport

• Cell transport mechanisms facilitate the movement of molecules across the plasma membrane.
• Passive transport does not require energy and includes diffusion, osmosis, and facilitated diffusion.
• Active transport requires energy and includes primary active transport and secondary active transport.
• Vesicular transport involves the movement of large molecules or particles within vesicles, including endocytosis and exocytosis.