Cell Biology: Structure, Function, and Processes

Basic Processes of Cells

Cells carry out essential processes that sustain life, including metabolism, transport, communication, and reproduction. Understanding these processes is fundamental to anatomy and physiology.

• Cell Metabolism: The sum of all chemical reactions in a cell, including both anabolic (building up) and catabolic (breaking down) reactions.

• Anabolic Reaction: Synthesis of complex molecules from simpler ones; requires energy. Example: protein synthesis.

• Catabolic Reaction: Breakdown of complex molecules into simpler ones; releases energy. Example: glycolysis.

• Oxidation-Reduction Reaction: Chemical reactions involving the transfer of electrons; important in cellular respiration.

• Transport of Substances: Movement of molecules into, out of, or within cells.

• Communication: Cells communicate via chemical signals, often through receptors on their membranes.

• Cell Reproduction: Cells divide to produce new cells, essential for growth and repair.

• Cell Division: The process by which a cell splits into two daughter cells, typically via mitosis.

Animal Cell Structure

Animal cells are composed of various structures, each with specific functions.

• Plasma Membrane: The boundary of the cell, regulates entry and exit of substances.

• Cytoplasm: The region between the plasma membrane and nucleus, contains cytosol and organelles.

• Cytosol: The fluid portion of cytoplasm, site of many metabolic reactions.

• Organelles: Specialized structures within cells, each performing distinct functions.

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

• Nucleus: Contains genetic material (DNA), controls cellular activities.

Plasma Membrane Structure and Function

The plasma membrane separates the intracellular and extracellular environments and is crucial for maintaining cellular homeostasis.

• Intracellular Space: Area inside the cell.

• Extracellular Space: Area outside the cell.

• Intracellular Fluid (Cytosol): Fluid within the cell.

• Extracellular Fluid (ECF): Fluid outside the cell.

• Phospholipid Bilayer: Double layer of phospholipids forming the membrane; hydrophilic heads face outward, hydrophobic tails inward.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins embedded in a flexible lipid bilayer.

• Integral (Transmembrane) Protein: Proteins that span the membrane, involved in transport and signaling.

• Peripheral Protein: Proteins attached to the membrane surface, often involved in signaling or structural support.

• Channel: Protein that forms a pore for passive movement of substances.

• Carrier: Protein that binds and transports substances across the membrane.

• Receptor: Protein that binds signaling molecules, triggering cellular responses.

• Enzyme: Protein that catalyzes chemical reactions.

• Substrate: The molecule upon which an enzyme acts.

• Product: The result of an enzymatic reaction.

• Selective Permeability: The membrane allows certain substances to pass while restricting others.

Passive Transport

Passive transport involves the movement of substances across the membrane without energy input, driven by concentration gradients.

• Diffusion: Movement of molecules from high to low concentration.

• Concentration Gradient: Difference in concentration across a space.

• Equilibrium: State where concentrations are equal across the membrane.

• Simple Diffusion: Direct movement of small, nonpolar molecules through the membrane.

• Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers.

• Uniporter: Carrier protein transporting one substance in one direction.

• Antiporter: Carrier protein transporting two substances in opposite directions.

• Symporter: Carrier protein transporting two substances in the same direction.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Aquaporin: Channel protein facilitating water movement.

• Tonicity: The effect of a solution on cell volume.

• Isotonic Solution: Equal solute concentration; cell volume remains unchanged.

• Hypertonic Extracellular Solution: Higher solute concentration outside; cell shrinks.

• Hypotonic Extracellular Solution: Lower solute concentration outside; cell swells.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Pump: Protein that uses energy to transport substances.

• Primary Active Transport: Direct use of ATP to move substances.

• Secondary Active Transport: Uses energy from one substance's gradient to move another.

• Sodium-Potassium Pump: Maintains cellular ion balance by pumping Na+ out and K+ in. Equation: per ATP hydrolyzed.

• Vesicular Transport: Movement of large particles via vesicles.

• Transport Vesicle: Membrane-bound sac for transport.

• Endocytosis: Uptake of substances into the cell via vesicles.

• Endosome: Vesicle formed during endocytosis.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Phagocyte: Cell specialized for phagocytosis (e.g., macrophage).

• Lysosome: Organelle containing digestive enzymes.

• Exocytosis: Release of substances from the cell via vesicles.

• Transcytosis: Transport of substances across a cell by vesicle movement.

Cellular Organelles

Organelles are specialized structures within cells, each with unique functions.

• Mitochondrion: Site of ATP production; "powerhouse" of the cell.

• Outer Mitochondrial Membrane: Surrounds the mitochondrion.

• Intermembrane Space: Space between outer and inner membranes.

• Inner Mitochondrial Membrane: Contains proteins for ATP synthesis.

• Cristae: Folds of the inner membrane, increase surface area.

• Matrix: Innermost compartment, site of Krebs cycle.

• Ribosome: Site of protein synthesis; can be free or bound.

• Free Ribosome: Located in cytosol; synthesize proteins for use in the cell.

• Bound Ribosome: Attached to ER; synthesize proteins for export or membranes.

• Endoplasmic Reticulum (ER): Network of membranes; involved in protein and lipid synthesis.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids, detoxifies.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

Cytoskeleton

The cytoskeleton provides structural support, facilitates movement, and organizes cellular contents.

• Microvilli: Finger-like projections increasing surface area for absorption.

• Cilia: Hair-like structures for movement of substances across cell surface.

• Flagella: Long, whip-like structures for cell movement (e.g., sperm).

Nucleus and DNA

The nucleus houses genetic material and controls cellular activities through gene expression.

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Nuclear Pore: Openings allowing transport between nucleus and cytoplasm.

• Nucleoplasm: Fluid inside the nucleus.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and protein complex; condenses to form chromosomes.

• Chromosome: Condensed chromatin during cell division.

• Sister Chromatids: Identical copies of a chromosome joined at the centromere.

• Centromere: Region joining sister chromatids.

Protein Synthesis

Protein synthesis is the process by which cells build proteins, involving transcription and translation.

• Transcription: DNA is copied into messenger RNA (mRNA) in the nucleus.

• Translation: mRNA is decoded by ribosomes to assemble proteins.

• Gene: Segment of DNA coding for a protein.

• Deoxyribonucleic Acid (DNA): Genetic material; double helix structure.

• Ribonucleic Acid (RNA): Single-stranded nucleic acid; includes mRNA, tRNA, rRNA.

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosome.

Cell Cycle

The cell cycle describes the sequence of events in cell growth and division.

• Interphase: Period of cell growth and DNA replication; includes G1, S, and G2 phases.

• G1 Phase: Cell grows and prepares for DNA replication.

• S Phase: DNA is replicated.

• G2 Phase: Cell prepares for division.

• M Phase: Mitosis and cytokinesis occur.

• Mitosis: Division of the nucleus into two identical nuclei.

• Cytokinesis: Division of the cytoplasm, forming two daughter cells.

• G0 Phase: Resting phase; cells are not actively dividing.

Cell Cycle Table

Example: During the S phase, a liver cell duplicates its DNA to ensure each daughter cell receives a complete set of genetic instructions.

Additional info: The cell cycle is tightly regulated by checkpoints to prevent uncontrolled cell division, which can lead to cancer.