Cell Theory and Contributors

Historical Foundations of Cell Biology

The cell theory is a fundamental concept in biology, stating that all living things are composed of cells, the cell is the basic unit of structure and function, and all cells arise from preexisting cells. Key contributors include Robert Hooke (first described cells), Van Leeuwenhoek (studied living cells), Schleiden and Schwann (formulated cell theory), and Singer & Nicolson (fluid mosaic model of cell membrane).

• Cell Theory: All living things are made of cells; cells are the basic unit of life; cells come from preexisting cells.

• Microscopic Anatomy: Cytology (study of cells) and Histology (study of tissues).

Types of Cells: Prokaryotic vs. Eukaryotic

Comparative Structure and Function

Cells are classified as prokaryotic (lacking a true nucleus, e.g., bacteria) or eukaryotic (possessing a nucleus, e.g., animal and plant cells). Eukaryotic cells are larger and more complex, with multiple linear chromosomes and membrane-bound organelles.

Animal vs. Plant Cells

Cell Diversity, Shapes, and Sizes

Variety of Cell Types and Functions

Human body contains over 200 types of cells, differing in shape, size, and function. These differences are essential for specialized roles in tissues and organs.

• Columnar: Long and narrow (e.g., lining of intestines)

• Cuboidal: Equal dimensions (e.g., kidney tubules)

• Discoid: Disc-shaped (e.g., erythrocytes)

• Stellate: Star-shaped (e.g., neurons)

• Fusiform: Spindle-shaped (e.g., muscle cells)

• Spherical: Round (e.g., leukocytes)

Structure of a Typical Animal Cell

Main Components and Their Functions

A typical animal cell consists of three main parts: nucleus, cytoplasm, and cell membrane. Each part contains specialized structures (organelles) that perform distinct functions.

• Nucleus: Largest organelle, contains genetic material (DNA), surrounded by nuclear envelope.

• Cytoplasm: Gel-like substance containing organelles; site of metabolic activity.

• Cell Membrane: Selectively permeable barrier, regulates entry and exit of substances.

Nucleus: Structure and Function

Genetic Control Center

The nucleus is the control center of the cell, housing DNA and coordinating activities such as growth, metabolism, and reproduction. It is surrounded by a double membrane with nuclear pores for transport.

• Nuclear Envelope: Double membrane with pores for exchange between nucleus and cytoplasm.

• Nucleolus: Site of ribosome synthesis, contains RNA.

• Chromatin: DNA and histone proteins, condenses to form chromosomes during cell division.

Cell Organelles

Membranous and Nonmembranous Structures

Organelles are specialized structures within the cytoplasm that perform specific functions. They are classified as membranous (e.g., mitochondria, endoplasmic reticulum) or nonmembranous (e.g., ribosomes, cytoskeleton).

• Membranous: Peroxisomes, lysosomes, endoplasmic reticulum, Golgi apparatus

• Nonmembranous: Cytoskeleton, centrioles, ribosomes

Cell Cycle and Division

Phases of the Cell Cycle

All cells undergo a life cycle consisting of interphase (growth and DNA replication) and mitotic phase (division). Interphase includes G1 (growth), S (DNA synthesis), and G2 (growth). Mitosis is the process by which somatic cells divide, producing two identical daughter cells.

• Interphase: Cell grows and replicates DNA.

• Mitosis: Division of nucleus (prophase, metaphase, anaphase, telophase).

• Cytokinesis: Division of cytoplasm.

Plasma Membrane Structure and Function

Fluid Mosaic Model and Membrane Components

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer, proteins, cholesterol, and carbohydrates. It separates the cell's interior from the external environment and facilitates communication and transport.

• Phospholipid Bilayer: Hydrophilic heads face outward, hydrophobic tails inward.

• Proteins: Integral (transmembrane) and peripheral; functions include transport, recognition, enzymatic activity, and cell joining.

• Glycocalyx: Sugar coating for cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Transport Across the Plasma Membrane

Passive and Active Processes

Substances move across the plasma membrane via passive (no energy required) or active (energy required) processes. Passive transport includes diffusion and osmosis; active transport uses ATP to move substances against concentration gradients.

• Passive Transport: Movement from high to low concentration (diffusion, osmosis).

• Active Transport: Movement from low to high concentration, requires ATP and carrier proteins.

• Vesicular Transport: Endocytosis (into cell), exocytosis (out of cell), phagocytosis (cell eating), pinocytosis (cell drinking), receptor-mediated endocytosis.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane. Tonicity describes the effect of a solution on cell volume: hypertonic (cell shrinks), hypotonic (cell swells), isotonic (no change).

Chromosome Structure and Cell Division

Chromosomes and Mitosis

Chromosomes are composed of DNA and proteins, separated during mitosis. Each chromosome consists of two sister chromatids joined by a centromere. The kinetochore is a protein structure essential for chromosome separation.

DNA Structure and Replication

Double Helix and Base Pairing

DNA is a double helix composed of nucleotides (adenine, thymine, cytosine, guanine). Replication is semiconservative, producing two identical DNA molecules. Key enzymes include helicase, primase, DNA polymerase, and ligase.

Protein Synthesis

Transcription and Translation

Protein synthesis involves two main steps: transcription (DNA to mRNA) and translation (mRNA to protein). Ribosomes, tRNA, and rRNA are essential for this process. Amino acids are assembled into polypeptides according to the genetic code.

• Transcription: DNA makes mRNA in the nucleus.

• Translation: mRNA is decoded by ribosomes and tRNA to synthesize proteins.

• Amino Acids: 20 types, each with amine, carboxyl, and R group.

Genetic Terms and Concepts

Key Definitions

• Genome: All DNA in a cell's nucleus.

• Gene: Segment of DNA coding for a trait.

• Exons/Introns: Coding/non-coding regions of DNA.

• Karyotype: Chart of chromosomes.

• Allele: Different forms of a gene.

• Mutation: Permanent change in DNA sequence.

• Genotype: Genetic makeup.

• Phenotype: Physical expression.

Summary

• Cell structure and function

• Transport mechanisms across membranes

• Cell cycle and division

• DNA structure and replication

• Protein synthesis

• Genetic terminology