Cell Structure, Membrane Transport, and Protein Synthesis: Study Guide for Anatomy & Physiology

Cell Structure and Function

Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life.

• All living things are composed of one or more cells.

• Cells are the basic (smallest) unit of life and conduct metabolism (chemical reactions necessary for life).

• All cells arise from pre-existing cells by division (DNA is copied and passed to daughter cells).

Cell Anatomy

Cells have three main structural components, each with specialized functions.

• Cell Membrane (Plasma Membrane): Outer boundary that regulates entry and exit of substances.

• Cytoplasm: Gel-like substance inside the cell containing organelles.

• Nucleus: Control center containing genetic material (DNA).

Cell Diversity: Human cells vary greatly in appearance and size to perform specific functions. Examples include:

• Red Blood Cell

• Nerve Cell

• Skeletal Muscle Cell

• Epithelial Cell

Plasma Membrane Structure and Function

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins.

• Phospholipids: Each molecule has a polar (hydrophilic) phosphate head and two non-polar (hydrophobic) lipid tails.

• Arrangement: Polar heads face outward toward water; non-polar tails face inward, away from water.

Fluid Mosaic Model: The membrane is a dynamic structure with proteins, lipids, and carbohydrates moving laterally within the layer.

Membrane Proteins

• Integral Proteins: Span the width of the membrane; involved in transport and signaling.

• Peripheral Proteins: Located on one side of the membrane; often involved in signaling or maintaining cell shape.

Other Membrane Components

• Cholesterol: Stabilizes membrane fluidity, especially during temperature changes.

• Glycolipids/Glycoproteins: Involved in cell recognition and immune response.

Transport Across the Cell Membrane

Selective Permeability

The plasma membrane allows some substances to pass while restricting others, maintaining homeostasis.

Solutions and Solutes

• Solvent: The liquid that dissolves solutes (in the body, this is water).

• Solute: The dissolved substance (e.g., sugar, salt).

Types of Membrane Transport

• Passive Transport: Does not require energy (ATP). Includes diffusion and osmosis.

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient.

Passive Transport Mechanisms

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

• Concentration Gradient: The difference in concentration across a membrane drives diffusion.

• Equilibrium: Achieved when solute is evenly dispersed.

• Rate of Diffusion: Influenced by particle size, concentration gradient, and temperature.

• Simple Diffusion: Direct movement of molecules across the membrane.

• Osmosis: Movement of water from high to low concentration across a selectively permeable membrane.

• Water moves through protein channels (aquaporins) or directly through the phospholipid bilayer if small enough.

Tonicity

Tonicity describes a solution's ability to cause osmosis across a membrane.

• Isotonic Solution: Equal solute concentrations inside and outside the cell; no net water movement.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, causing cell shrinkage.

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, causing cell swelling.

Example Table: Tonicity Scenarios

The following table summarizes the effect of different external solutions on a cell:

Solution Type	Solute Concentration (Outside)	Solute Concentration (Inside)	Water Movement	Cell Effect
Isotonic	Equal	Equal	No net movement	Cell remains the same
Hypertonic	Higher	Lower	Out of cell	Cell shrinks (crenation)
Hypotonic	Lower	Higher	Into cell	Cell swells (lysis)

Active Transport Mechanisms

• Sodium-Potassium Pump: Uses ATP to move 3 Na+ ions out and 2 K+ ions into the cell, maintaining electrochemical gradients.

Equation:

$3\,\mathrm{Na}^+_{(in)} + 2\,\mathrm{K}^+_{(out)} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{(out)} + 2\,\mathrm{K}^+_{(in)} + \mathrm{ADP} + \mathrm{P_i}$

• Secondary Active Transport: Uses the energy from primary active transport (e.g., Na+ gradient) to move other substances against their gradient.

Bulk Transport

• Endocytosis: Bulk transport into the cell.

  • Phagocytosis: "Cell eating"; ingestion of large particles (e.g., bacteria).

  • Pinocytosis: "Cell drinking"; ingestion of fluid and dissolved substances.

  • Receptor-Mediated Endocytosis: Specific ligands bind to receptors to initiate uptake.

• Exocytosis: Bulk transport out of the cell (e.g., hormones, enzymes, neurotransmitters).

Cytoplasm and Organelles

Cytoplasm

• Area between the plasma membrane and nucleus; contains cytosol (gel-like fluid) and organelles.

Major Organelles and Their Functions

• Mitochondria: Site of ATP production; contains its own DNA and ribosomes.

• Ribosomes: Site of protein synthesis; can be free in cytoplasm or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, packages, and ships proteins and lipids from ER.

• Lysosomes: Contain digestive enzymes; break down waste and old organelles.

• Cytoskeleton: Network of protein filaments; maintains cell shape, enables movement, and organizes organelles.

Cellular Extensions

• Microvilli: Increase surface area for absorption (e.g., digestive tract, kidney cells).

• Cilia: Move substances across cell surface (e.g., respiratory tract, uterine tubes).

• Flagella: Enable cell movement (e.g., sperm cells).

Nucleus and Genetic Material

Nucleus

• Nucleolus: Site of ribosome production.

• Chromatin: Long strands of DNA and protein; condenses to form chromosomes during cell division.

• Chromosomes: Packaged DNA for cell division.

DNA Structure

• Double Helix: Two strands of nucleotides wound around each other.

• Nitrogen Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

Protein Synthesis

Overview

Protein synthesis involves two main processes: transcription and translation.

Transcription

• DNA is transcribed into messenger RNA (mRNA) in the nucleus.

• mRNA: Single-stranded; carries genetic code from DNA to ribosome.

• mRNA Nitrogen Bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C).

• Codon: Sequence of three mRNA bases that codes for a specific amino acid.

• mRNA is processed: introns (non-coding regions) are removed, exons (coding regions) are spliced together.

Translation

• Occurs at the ribosome in the cytoplasm.

• tRNA brings amino acids to the ribosome, matching its anticodon to the mRNA codon.

• Polypeptide chain is formed, which folds into a functional protein.

Example Table: Decoding Protein Synthesis

DNA triplet	TTC	CAA	AGG
mRNA codon	AAG	GUU	UCC
tRNA anticodon	UUC	CAA	AGG
Amino acid	Lysine	Valine	Serine

Additional info: The amino acids are inferred based on the standard genetic code.

Summary Table: Major Cell Organelles and Functions

Organelle	Function
Nucleus	Stores genetic material; controls cell activities
Mitochondria	Produces ATP (energy)
Ribosomes	Protein synthesis
Endoplasmic Reticulum (Rough/Smooth)	Protein and lipid synthesis; detoxification
Golgi Apparatus	Modifies, packages, and ships proteins/lipids
Lysosomes	Digests waste and old organelles
Cytoskeleton	Maintains cell shape; enables movement