Cell Structure, Function, and Processes: Study Notes for Anatomy & Physiology

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Cell Chemistry & Cell Components

Basic Processes in Cells

Cells perform a variety of essential processes to maintain life, including metabolism, cellular respiration, protein synthesis, cell division, transport, and communication. These processes ensure cells can grow, reproduce, and respond to their environment.

Metabolism: The sum of all chemical reactions in a cell, divided into:
- Anabolism: Building larger molecules from smaller ones (e.g., synthesis of proteins).
- Catabolism: Breaking down molecules to release energy (e.g., glucose breakdown).
Cellular Respiration: Cells convert glucose and oxygen into ATP, carbon dioxide, and water.
Protein Synthesis: DNA instructions are used to make proteins via transcription and translation.
Cell Division: Cells replicate DNA and divide (mitosis or meiosis).
Transport: Movement of substances across membranes by diffusion, osmosis, and active transport.
Communication: Cells send and receive signals to coordinate activities.

Cellular Respiration Equation:

Overview of Cell Structure

Cells are composed of various structures, each with specialized functions. The cell membrane, cytoplasm, nucleus, and organelles work together to maintain cellular activities.

Cell Membrane: Phospholipid bilayer controlling entry and exit of substances.
Cytoplasm: Jelly-like fluid where organelles are suspended and reactions occur.
Nucleus: Contains DNA; directs cell activities.
Organelles: Specialized structures (e.g., mitochondria, ribosomes, ER, Golgi apparatus, lysosomes).
Cell Wall: Provides support (in plant cells).
Chloroplasts: Photosynthesis (in plant cells).

Cell Membrane Structure & Function

Components of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed mainly of a phospholipid bilayer, proteins, cholesterol, and carbohydrates.

Phospholipids: Form a bilayer with hydrophilic heads outward and hydrophobic tails inward.
Proteins: Integral (transmembrane) and peripheral proteins serve as channels, transporters, receptors, and structural support.
Cholesterol: Maintains membrane fluidity and stability.
Carbohydrates: Attached to proteins/lipids; important for cell recognition and signaling.

Types of Membrane Proteins and Their Functions

Integral (Transmembrane) Proteins: Span the membrane; act as channels, transporters, or receptors.
Peripheral Proteins: Attached to membrane surface; involved in signaling and structural support.
Glycoproteins: Proteins with carbohydrate chains; cell recognition and immune response.
Channel Proteins: Form pores for diffusion of specific molecules/ions.
Carrier Proteins: Shuttle substances across the membrane (passive or active transport).
Receptor Proteins: Bind signaling molecules and trigger cellular responses.
Enzymatic Proteins: Catalyze reactions at the membrane surface.
Anchoring Proteins: Attach membrane to cytoskeleton or other cells.

Agonist and Antagonist (Drug Action)

Agonist: Binds to a receptor and activates it, mimicking natural substances (e.g., morphine at opioid receptors).
Antagonist: Binds to a receptor but blocks activation, inhibiting agonists (e.g., naloxone at opioid receptors).

Key Point: Agonists activate receptors; antagonists block them.

Cellular Organelles & Their Functions

Functions of Cellular Organelles

Nucleus: Stores DNA; controls cell activities.
Mitochondria: Generate ATP via cellular respiration.
Ribosomes: Protein synthesis.
Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis, detoxification).
Golgi Apparatus: Modifies, sorts, and packages proteins/lipids.
Lysosomes: Digest waste and foreign substances.
Peroxisomes: Break down fatty acids, detoxify substances.
Cytoskeleton: Structural support, movement, organelle organization.
Centrioles: Organize microtubules during cell division.
Plasma Membrane: Controls entry/exit; homeostasis.
Vesicles: Transport materials.
Chloroplasts: Photosynthesis (plants).
Cell Wall: Support and protection (plants).

Structure of the Nuclear Envelope

Double Membrane: Outer membrane (continuous with ER), inner membrane (supported by nuclear lamina).
Nuclear Pores: Allow selective transport between nucleus and cytoplasm.
Nuclear Lamina: Provides structural support and organizes chromatin.

Chromatin vs. Chromosome

Chromatin: Uncondensed DNA; allows gene expression and replication.
Chromosome: Condensed DNA during cell division; ensures accurate genetic distribution.

Cell Membrane Transport

Selectively Permeable Membrane

A selectively permeable membrane allows certain molecules or ions to pass while restricting others, maintaining homeostasis.

Example: Plasma membrane lets water, oxygen, and small nonpolar molecules pass easily, but restricts large or charged substances.

Passive vs. Active Transport

Passive Transport: No energy required; molecules move down concentration gradient (e.g., diffusion, osmosis).
Active Transport: Requires energy (ATP); molecules move against gradient (e.g., sodium-potassium pump).

Diffusion

Definition: Movement from high to low concentration.
Simple Diffusion: Direct movement across membrane (small, nonpolar substances).
Facilitated Diffusion: Movement via transport proteins (larger or polar substances).

Osmosis

Definition: Movement of water across a selectively permeable membrane from low to high solute concentration.
Channel Protein: Aquaporins facilitate osmosis.

Tonicity: Isotonic, Hypotonic, Hypertonic Solutions

Solution Type	Solute Concentration	Water Movement	Effect on Cell
Isotonic	Equal inside/outside	No net movement	Cell stays same size
Hypotonic	Lower outside	Into cell	Cell swells/lyses
Hypertonic	Higher outside	Out of cell	Cell shrinks/crenates

Primary Active Transport: Sodium-Potassium Pump

Uses ATP to move ions against their gradients.
Steps: 3 Na+ bind inside, ATP used, Na+ released outside, 2 K+ bind outside, K+ released inside.
Result: Maintains cell’s electrical balance; essential for nerve/muscle function.

Equation:

Secondary Active Transport

Uses energy from ion gradients (e.g., Na+) to transport other molecules (e.g., glucose).
Types: Symport (same direction), Antiport (opposite direction).

Vesicular Transport

Endocytosis: Cell takes in materials via vesicles (phagocytosis, pinocytosis, receptor-mediated).
Exocytosis: Cell releases materials by fusing vesicles with membrane.

Genetics & Protein Synthesis

Terminology Related to Genes and Genetic Code

Gene: DNA segment coding for a protein.
Allele: Variant of a gene.
Genotype: Genetic makeup.
Phenotype: Physical expression.
Genetic Code: Rules for translating DNA/RNA into proteins.
Codon: Three-nucleotide sequence coding for an amino acid.
Transcription: DNA to mRNA.
Translation: mRNA to protein.
Mutation: Change in DNA sequence.

DNA Triplet, mRNA Codon, tRNA Anticodon

DNA Triplet: Three DNA nucleotides; codes for amino acid.
mRNA Codon: Three RNA nucleotides; specifies amino acid during translation.
tRNA Anticodon: Complementary to mRNA codon; ensures correct amino acid delivery.

The Genetic Code in Detail

Universal: Nearly same in all organisms.
Redundant: Multiple codons for same amino acid.
Unambiguous: Each codon codes for one amino acid.
Start Codon: AUG (methionine).
Stop Codons: UAA, UAG, UGA.

Transcription Steps

Initiation: RNA polymerase binds promoter; DNA unwinds.
Elongation: RNA polymerase adds RNA nucleotides.
Termination: RNA polymerase reaches termination sequence; mRNA detaches.
mRNA Processing: 5' cap, poly-A tail, splicing (introns removed, exons joined).

RNA Processing

5' Capping: Modified guanine added to 5' end.
Poly-A Tail: Adenine nucleotides added to 3' end.
Splicing: Introns removed, exons joined.

Translation Steps

Initiation: mRNA binds ribosome; start codon recognized.
Elongation: tRNAs bring amino acids; peptide bonds form.
Termination: Stop codon reached; polypeptide released.

Cell Cycle & Division

Phases of the Cell Cycle

Phase	Description
Interphase	Cell grows, replicates DNA, prepares for division (G1, S, G2)
M phase	Nuclear and cytoplasmic division (mitosis, cytokinesis)
G0 phase	Resting phase; cells not preparing to divide

DNA Replication

Occurs during S phase of interphase.
Steps: Helicase unwinds DNA; DNA polymerase adds nucleotides; semi-conservative replication.

Steps of Mitosis (PMAT)

Prophase: Chromatin condenses; nuclear envelope breaks down; spindle forms.
Metaphase: Chromosomes align at equator; spindle attaches.
Anaphase: Sister chromatids pulled apart.
Telophase: Chromatids de-condense; nuclear envelopes reform.

Cytokinesis

Animal Cells: Contractile ring forms cleavage furrow; cell splits.
Plant Cells: Vesicles form cell plate; new cell wall forms.

Cell Cycle Regulation

Checkpoints: G1, G2, M; ensure errors are repaired or trigger apoptosis.
Regulatory Proteins: Cyclins and CDKs control progression.
External Signals: Growth factors stimulate/inhibit division.
Tumor Suppressor Genes: Monitor DNA integrity (e.g., p53).

Consequences of Unchecked Cell Cycle

Uncontrolled Division: Tumor formation.
Cancer: Cells ignore stop signals; invade tissues; metastasis.
Genetic Mutations: Accumulate, disrupt regulation.
Loss of Function: Affects tissue/organ health.

Cellular Structures: Nucleus, Nucleolus, Centrioles

Nucleus

Control center; contains DNA.
Double membrane with pores.

Nucleolus

Dense structure inside nucleus.
Produces rRNA and ribosome subunits.

Centrioles

Cylindrical microtubule structures.
Organize spindle during cell division; form cilia/flagella.

Cellular Structures: Mitochondria, Golgi Apparatus, Lysosome

Mitochondria

Double membrane; inner membrane forms cristae.
ATP production; contains own DNA.

Golgi Apparatus

Stacked, membrane-bound sacs.
Modifies, sorts, packages proteins/lipids.

Lysosome

Membrane-bound; contains digestive enzymes.
Breaks down waste, debris, foreign substances.

Cellular Structures: Smooth ER, Rough ER, Ribosomes

Smooth ER

No ribosomes; synthesizes lipids, detoxifies, stores Ca2+.

Rough ER

Ribosomes attached; synthesizes proteins.

Ribosomes

Made of rRNA and proteins; site of protein synthesis.

Cellular Structures: Plasma Membrane, Cytoplasm, Cilia/Flagella

Plasma Membrane

Phospholipid bilayer; embedded proteins, cholesterol, carbohydrates.
Controls entry/exit; selectively permeable.

Cytoplasm

Jelly-like substance; contains organelles and cytosol.
Site for metabolic reactions.

Cilia and Flagella

Cilia: Short, numerous; move substances across cell surface.
Flagella: Long, singular; cell movement (e.g., sperm).

Phospholipid Bilayer Formation

Phospholipids: Hydrophilic head, hydrophobic tails.
Self-assembly: Heads face water, tails face inward.
Bilayer: Foundation of cell membranes; acts as barrier.

Cytoskeleton: Types of Filaments

Filament Type	Protein	Diameter	Main Functions
Microfilaments	Actin	~7 nm	Support, movement, muscle contraction
Intermediate Filaments	Keratin, vimentin	8–12 nm	Strength, stability, anchor organelles
Microtubules	Tubulin	~25 nm	Shape, tracks for movement, cilia/flagella, mitotic spindle

Passive Transport

Simple Diffusion: Small, nonpolar molecules move directly through bilayer.
Facilitated Diffusion: Larger/polar molecules move via proteins.
Osmosis: Water moves via aquaporins.

Sodium-Potassium Pump

Active transport; uses ATP.
Moves 3 Na+ out, 2 K+ in per cycle.
Maintains membrane potential, cell volume.

Aquaporins

Channel proteins for water transport.
Facilitate rapid osmosis.

Tonicity in Detail

Type	Solute Concentration	Water Movement	Cell Effect
Isotonic	Equal	None	Normal
Hypertonic	Higher outside	Out	Shrinks (crenation)
Hypotonic	Lower outside	In	Swells (lysis)

Cell Cycle Control and Cancer

Control Center: Cyclins, CDKs, checkpoints regulate division.
Apoptosis: Programmed cell death prevents cancer.
Benign Tumor: Non-invasive, non-spreading.
Malignant Tumor: Invasive, can metastasize.
Metastasis: Spread of cancer cells to other tissues.