Section 3.1: Cells and Plasma Membrane

Cell Theory

The cell theory is a fundamental concept in biology, stating that all living organisms are composed of cells, which are the basic units of structure and function in life. The functioning of an organism depends on the activities of its cells.

• Cells arise from: Pre-existing cells through cell division.

Generalized Cell Structure

Most animal cells share three basic parts:

• Plasma membrane: The outer boundary of the cell, controlling entry and exit of substances.

• Cytoplasm: The internal fluid containing organelles.

• Nucleus: The control center containing genetic material (DNA).

Extracellular Material

Materials found outside the cell include:

• Extracellular fluid: Includes interstitial fluid, blood plasma, and cerebrospinal fluid.

• Cellular secretions: Substances such as saliva, mucus, and enzymes.

• Extracellular matrix: A network of proteins and polysaccharides that provide structural support.

Plasma Membrane

The plasma membrane is a selectively permeable barrier that separates the cell from its environment.

• Functions: Controls what enters and leaves the cell, provides protection, and facilitates communication.

• Structure: Composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

Membrane Lipids

• Phospholipids: Make up about 75% of the membrane; have hydrophilic heads and hydrophobic tails.

• Glycolipids: About 5%; lipids with attached carbohydrates, important for cell recognition.

• Cholesterol: About 20%; stabilizes membrane fluidity.

Membrane Proteins

• Integral proteins: Span the membrane; involved in transport and signaling.

• Peripheral proteins: Loosely attached to the membrane surface; function in support and signaling.

Membrane Protein Functions

• Transport: Move substances across the membrane.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Enzymes: Catalyze chemical reactions.

• Cell-cell recognition: Allow cells to identify each other.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

• Cell-to-cell joining: Form junctions between cells.

Glycocalyx

• Consists of glycoproteins and glycolipids on the cell surface.

• Functions in cell recognition and protection.

Cell Junctions

• Tight junctions: Prevent leakage between cells (e.g., intestinal lining).

• Desmosomes: Anchor cells together (e.g., skin cells).

• Gap junctions: Allow communication between cells (e.g., cardiac muscle).

Transport Across the Membrane

The plasma membrane is selectively permeable, allowing only certain molecules to pass through.

• Passive transport: No energy required; substances move down their concentration gradient.

• Active transport: Requires energy (ATP); substances move against their concentration gradient.

Passive Transport Mechanisms

• Simple diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer.

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

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

Osmosis and Tonicity

• Osmolarity: Measure of solute concentration.

• Tonicity: Effect of a solution on cell volume.

• Isotonic solution: No net water movement; cell remains unchanged.

• Hypertonic solution: Water moves out; cell shrinks.

• Hypotonic solution: Water moves in; cell swells.

Active Transport Mechanisms

• Primary active transport: Direct use of ATP to move substances (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from ion gradients created by primary active transport.

Vesicular Transport

• Endocytosis: Uptake of large particles or fluids via vesicles (includes phagocytosis, pinocytosis, receptor-mediated endocytosis).

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

Section 3.2: The Cell and Cytoplasmic Organelles

Cytoplasm

The cytoplasm is the cellular material inside the plasma membrane, excluding the nucleus. It consists of cytosol (fluid), inclusions (stored nutrients), and organelles (functional structures).

Cytoplasmic Organelles

• Membranous organelles: Mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes.

• Nonmembranous organelles: Ribosomes, cytoskeleton, centrioles.

Mitochondria

• "Powerhouse" of the cell; site of ATP production.

• Enclosed by a double membrane; inner membrane has folds called cristae.

• Contain their own DNA and ribosomes.

• Endosymbiont theory: Mitochondria originated from engulfed prokaryotes.

Ribosomes

• Nonmembranous; site of protein synthesis.

• Can be free in cytosol or bound to rough ER.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins for export or membranes.

• Smooth ER: Lacks ribosomes; involved in lipid metabolism, detoxification, and calcium storage.

Golgi Apparatus

• Stacked, flattened sacs; modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Peroxisomes

• Membranous sacs containing enzymes that detoxify harmful substances (e.g., hydrogen peroxide).

• Neutralize free radicals (highly reactive molecules).

Lysosomes

• Membranous sacs containing digestive enzymes.

• Break down waste, cellular debris, and foreign materials.

Cytoskeleton

• Network of protein filaments providing structural support, movement, and transport within the cell.

• Microfilaments: Thinnest; involved in cell movement and shape.

• Intermediate filaments: Provide tensile strength.

• Microtubules: Largest; form the mitotic spindle and cellular extensions.

Cellular Extensions

• Microvilli: Fingerlike projections that increase surface area for absorption.

• Cilia: Short, hairlike structures that move substances across the cell surface.

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

Section 3.3: The Nucleus and DNA

Nucleus

The nucleus is the largest organelle, containing the cell's genetic material and controlling cellular activities.

• Nuclear envelope: Double membrane with nuclear pores for material exchange.

• Nucleoli: Sites of ribosome assembly.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide.

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

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

• Cytokinesis: Division of the cytoplasm.

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

Protein Synthesis

Protein synthesis involves two main steps: transcription and translation.

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

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a protein.

Genetic Code

• Each three-base sequence on mRNA (codon) specifies one amino acid.

• There are 64 possible codons but only 20 amino acids (redundancy protects against mutations).

Key Equations and Concepts

• Osmosis: Where J is the flux, P is permeability, and C is concentration.

• Sodium-Potassium Pump: per ATP hydrolyzed.

• Central Dogma:

Table: Comparison of Cell Junctions

Additional info:

• Some details (e.g., specific protein names, full lists of organelles) were logically inferred based on standard Anatomy & Physiology curriculum.

• Tables and equations were added for clarity and completeness.