Cell Structure and Function

Introduction to Cells

Cells are the basic structural and functional units of all living organisms. They perform essential functions such as energy production, protein synthesis, transport of materials, and waste disposal. Understanding cell anatomy and physiology is fundamental for students of Anatomy & Physiology.

• Definition: A cell is the smallest unit of life capable of independent existence and reproduction.

• Functions: Includes creating fuel for the body, manufacturing proteins, transporting materials, and disposing of wastes.

• Cell Diversity: Over 200 different types of human cells exist, varying in size, shape, components, and function.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structure and complexity.

• Prokaryotic Cells: Found in bacteria and archaea. Lack membrane-bound organelles and a nucleus. Possess a single chromosome and reproduce asexually (cloning).

• Eukaryotic Cells: Found in animals, plants, fungi, and protists. Contain membrane-bound organelles and a nucleus. Have multiple chromosomes and reproduce sexually (genetic material from two parents).

Cell Anatomy

Common Structures of Eukaryotic Cells

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

• Cytoplasm: The internal fluid and organelles between the plasma membrane and nucleus.

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

Plasma Membrane

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

• Fluid Mosaic Model: Describes the membrane as a double layer of phospholipids with embedded proteins and cholesterol.

• Phospholipids: 75% of membrane; have polar (hydrophilic) heads and non-polar (hydrophobic) tails.

• Glycolipids: 5%; involved in cell communication.

• Cholesterol: 20%; provides stability.

• Proteins: Embedded within the membrane; function as transporters, receptors, enzymes, and structural components.

• Carbohydrates: Attached to proteins and lipids; involved in cell recognition.

Types of Membrane Proteins

• Transport Proteins: Facilitate movement of substances across the membrane.

• Receptor Proteins: Receive and transmit signals from the environment.

• Structural Proteins: Provide support and shape to the cell.

• Enzymes: Catalyze chemical reactions at the membrane.

Cell Junctions

• Tight Junctions: Fuse cells together, making them impermeable.

• Desmosomes: Anchor cells together, providing strength and reducing tearing.

• Gap Junctions: Allow communication between cells via channels for ions and small molecules.

Cytoplasm and Cytoskeleton

• Cytosol: Fluid portion containing water, proteins, salts, and sugars.

• Organelles: Specialized structures performing metabolic functions; can be membranous (e.g., mitochondria, ER) or non-membranous (e.g., ribosomes, cytoskeleton).

• Inclusions: Insoluble particles such as glycogen granules, lipid droplets, and pigments.

• Cytoskeleton: Network of rods and filaments providing shape, support, and transport within the cell.

Major Cytoplasmic Organelles

• Mitochondria: Double-membrane organelles; site of aerobic cellular respiration and ATP production.

• Ribosomes: Composed of protein and rRNA; site of protein synthesis. Free ribosomes synthesize proteins for the cytosol; bound ribosomes (on ER) synthesize proteins for membranes or export.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins and phospholipids; smooth ER (without ribosomes) involved in lipid metabolism, detoxification, and calcium storage.

• Golgi Apparatus: Stacked, flattened sacs; modifies, concentrates, and packages proteins and lipids from the ER.

• Lysosomes: Membranous sacs containing digestive enzymes; break down viruses, bacteria, toxins, and worn-out organelles.

• Peroxisomes: Membranous sacs containing oxidases; neutralize free radicals and perform metabolic functions.

• Cellular Extensions: Cilia move substances across cell surfaces; Flagella propel whole cells; Microvilli increase surface area for absorption.

Nucleus

• Nuclear Envelope: Double-membrane barrier enclosing nucleoplasm; continuous with rough ER.

• Nucleolus: Dense body involved in rRNA synthesis and ribosome assembly.

• Chromatin: DNA wound around histone proteins; condenses to form chromosomes during cell division.

Cell Physiology

Membrane Transport

The plasma membrane regulates the movement of substances into and out of the cell, maintaining homeostasis.

• Intracellular Fluid (ICF): Fluid within cells (40% of body water).

• Extracellular Fluid (ECF): Fluid outside cells (20% of body water).

• Selectively Permeable: Only certain materials can cross the membrane.

Types of Membrane Transport

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

• Active Processes: Require energy (ATP); substances move against their concentration gradient.

Passive Transport

• Simple Diffusion: Movement of lipid-soluble substances directly through the phospholipid bilayer (e.g., O2, CO2).

• Facilitated Diffusion: Movement of polar molecules via channel or carrier proteins (e.g., glucose, amino acids, ions).

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

• Filtration: Movement of water and solutes across a membrane due to hydrostatic pressure (e.g., in capillaries).

Osmolarity and Tonicity

• Osmolarity: Measure of total concentration of solute particles.

• Tonicity: Ability of a solution to alter cell water volume.

• Isotonic: Same solute/water concentration as inside cells.

• Hypertonic: Higher solute concentration outside the cell; causes cell to shrink.

• Hypotonic: Lower solute concentration outside the cell; causes cell to swell.

Active Transport

• Primary Active Transport: Direct use of ATP to move solutes against their gradient (e.g., sodium-potassium pump).

• Secondary Active Transport: Indirect use of ATP; uses ion gradients created by primary active transport.

• Vesicular Transport: Movement of large particles via vesicles (endocytosis, exocytosis, phagocytosis, pinocytosis, receptor-mediated endocytosis).

Sodium-Potassium Pump Equation

The sodium-potassium pump maintains electrochemical gradients essential for muscle and nerve function:

Cell Cycle

Phases of the Cell Cycle

• Interphase: Cell grows and carries out normal functions; consists of G1 (gap 1), S (synthesis), and G2 (gap 2) phases.

• Mitotic Phase: Cell division; includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

• Prophase: Chromatin condenses into chromosomes; nuclear envelope dissolves.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Division of cytoplasm, resulting in two daughter cells.

Control of Cell Division

• Go Signals: Critical cell volume, growth factors, hormones.

• Stop Signals: Contact inhibition, repressor genes, cell cycle checkpoints (G1, G2, spindle checkpoint).

Protein Synthesis

Genetic Code and Nucleic Acids

• DNA: Double-stranded molecule with nucleotides (adenine, guanine, cytosine, thymine).

• RNA: Single-stranded molecule with nucleotides (adenine, guanine, cytosine, uracil).

• Base Pair Rule: A pairs with T (DNA) or U (RNA); G pairs with C.

Types of RNA

• Messenger RNA (mRNA): Carries instructions from DNA to ribosomes.

• Ribosomal RNA (rRNA): Forms part of the ribosome and helps translate mRNA.

• Transfer RNA (tRNA): Transfers amino acids to the ribosome and decodes mRNA's message.

Steps of Protein Synthesis

• Transcription: Occurs in the nucleus; DNA is used as a template to synthesize mRNA.

• Translation: Occurs in the cytoplasm; ribosomes read mRNA and assemble amino acids into a polypeptide chain.

Transcription Equation

Translation Equation

Summary: From DNA to Protein

• DNA triplets are transcribed into mRNA codons.

• mRNA codons are translated into tRNA anticodons, which bring specific amino acids.

• Polypeptide chains are formed, resulting in functional proteins.

Cell Death and Growth

• Apoptosis: Programmed cell death; cell destroys itself.

• Hyperplasia: Excessive cell growth.

• Atrophy: Cell shrinkage and death.

Table: Major Cell Organelles and Their Functions

Additional info: The cartoon "If organelles could talk" by Beatrice the Biologist humorously illustrates the roles of major organelles, reinforcing their functions in a memorable way.