Cell Structure and Function: Study Notes for Anatomy & Physiology

Cell Theory and Diversity

Cell Theory

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

• Definition: A cell is the structural and functional unit of life.

• Key Points:

  • All living organisms are composed of cells.

  • Cells arise only from pre-existing cells.

  • Structure and function are complementary.

Cell Diversity

Human cells exhibit a wide range of diversity in size, shape, and subcellular components.

• Over 250 different types of human cells

• Types differ in size, shape, and specialized functions.

Basic Parts of a Human Cell

Main Components

Cells have three basic parts that contribute to their structure and function.

• Plasma membrane: Flexible outer boundary that separates the cell from its environment.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: DNA-containing control center.

Cell Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of the cell membrane as a dynamic and flexible arrangement of lipids and proteins.

• Separates the intracellular fluid (ICF) from extracellular fluid (ECF).

• Controls what enters and leaves the cell.

Membrane Proteins

Types of Membrane Proteins

Membrane proteins are essential for various cellular functions and are classified into two main types.

• Integral proteins: Span the membrane, embedded in the lipid bilayer; have both hydrophobic and hydrophilic regions.

• Peripheral proteins: Located on the intracellular or extracellular surface; support and cell connections.

Functions of Membrane Proteins

Membrane proteins perform diverse functions critical to cell survival and communication.

• Transport: Move substances across the membrane.

• Receptors: Bind chemical signals and initiate cellular responses.

• Enzymes: Catalyze chemical reactions.

• Cell recognition: Identify cells to the immune system.

• Attachment: Anchor cells to cytoskeleton or extracellular matrix.

• Cell joining: Form junctions between cells.

Cell Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect cells and facilitate communication and adhesion.

• Tight junctions: Impermeable joints that prevent molecules from passing between cells.

• Desmosomes: Anchoring junctions that bind adjacent cells together.

• Gap junctions: Communicating junctions that allow ions and small molecules to pass between cells.

Membrane Transport

Passive vs. Active Transport

Transport across the cell membrane can be passive or active, depending on energy requirements.

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

• Active transport: Energy (ATP) is required; substances move against their concentration gradient.

Diffusion and Types

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.

• Simple diffusion: Lipid-soluble molecules move directly through the phospholipid bilayer.

• Carrier-mediated facilitated diffusion: Protein carrier specific for one chemical; binding of solute causes protein to change shape.

• Channel-mediated facilitated diffusion: Through channel protein; mostly ions selected on basis of size and charge.

• Osmosis: Movement of water through a specific channel protein (aquaporin) or through the lipid bilayer.

Factors Influencing Diffusion

• Temperature

• Concentration gradient

• Size of particles

Osmolarity and Equilibrium

Osmolarity is the concentration of solute particles in a solution. Equilibrium is reached when solute and water concentrations are equal on both sides of a membrane.

Hydrostatic and Osmotic Pressure

• Hydrostatic pressure: Outward pressure exerted on cell side of membrane caused by increases in volume.

• Osmotic pressure: Inward pressure due to tendency of water to be "pulled" into a cell with higher solute concentration.

Tonicity

Tonicity describes the ability of a solution to change the shape or tone of cells by altering the cell's internal water volume.

• Isotonic: Solution has same osmolarity as inside the cell; no net water movement.

• Hypertonic: Solution has higher osmolarity than inside the cell; water flows out, cell shrinks.

• Hypotonic: Solution has lower osmolarity than inside the cell; water flows in, cell swells.

Types of Active Membrane Transport

• Primary active transport: Energy comes directly from ATP hydrolysis.

• Secondary active transport: Energy is derived indirectly from ion gradients created by primary active transport.

Sodium-Potassium Pump

The sodium-potassium pump is an enzyme (Na+-K+ ATPase) that pumps Na+ out of the cell and K+ back into the cell.

• Equation:

$3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Membrane Potential

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell when it is not excited.

• Essential for nerve impulse transmission and muscle contraction.

Vesicular Transport

Types of Vesicular Transport

• Endocytosis: Transport into the cell.

• Exocytosis: Transport out of the cell.

• Phagocytosis: "Cell eating"; engulfing large particles.

• Pinocytosis: "Cell drinking"; engulfing fluids.

• Receptor-mediated endocytosis: Specific molecules are taken in after they bind to a receptor.

Cell Adhesion Molecules (CAMs) and Plasma Membrane Receptors

Roles of CAMs

Cell adhesion molecules play key roles in embryonic development, wound repair, and immunity.

Roles of Plasma Membrane Receptors

• Contact signaling: Cells come together and touch; recognition of each other.

• Chemical signaling: Chemical messenger binds a specific receptor and initiates a response.

G Protein-Linked Receptor Functions

Steps in G Protein Signaling

• Ligand (chemical messenger) binds to the receptor.

• Receptor activates G protein.

• G protein activates an effector protein (enzyme or ion channel).

• Effector protein produces a second messenger (e.g., cyclic AMP or Ca2+).

• Second messenger activates other enzymes or cellular responses.

Cytoplasm and Organelles

Definitions

• Cytosol: Gel-like solution made up of water and soluble molecules such as proteins, salts, and sugars.

• Inclusions: Insoluble molecules; vary with cell type.

• Organelles: Metabolic machinery structures of cells with specialized function.

Membranous vs. Nonmembranous Organelles

Organelles are classified based on the presence or absence of a membrane.

Membranous	Nonmembranous
Mitochondria	Ribosomes
Endoplasmic reticulum	Cytoskeleton
Golgi apparatus	Centrioles
Peroxisomes
Lysosomes

Cell Cycle and Division

Cell Cycle

The cell cycle is the series of changes a cell undergoes from the time it is formed until it reproduces.

• Growth

• Growth and DNA synthesis

• Growth and final preparations for division

• Mitosis

Phases of Interphase

• G1 phase: Vigorous growth and metabolism.

• S phase: DNA replication occurs.

• G2 phase: Preparation for division.

DNA Replication

DNA replication is the process by which a cell makes an identical copy of its DNA.

• DNA polymerase attaches to primer and begins adding nucleotides to form new strand.

• Result: Two identical "daughter" DNA molecules are formed from the original.

Cytokinesis

Cytokinesis is the division of the cytoplasm to form two separate daughter cells after mitosis.

Cell Division Control

• Regulated by cyclins and cyclin-dependent kinases (Cdks).

• Contact inhibition prevents cells from dividing when they touch each other.

Gene Expression: Transcription and Translation

DNA Transcription

Transcription is the process of copying DNA into messenger RNA (mRNA).

• Initiation: RNA polymerase binds to promoter region.

• Elongation: RNA polymerase adds complementary RNA nucleotides.

• Termination: RNA polymerase reaches a stop signal and releases the mRNA.

mRNA Processing

• Intron: Non-coding region removed from pre-mRNA.

• Exon: Coding region retained in mature mRNA.

mRNA Translation

Translation is the process by which mRNA is decoded to produce a specific polypeptide.

• Initiation: Ribosome assembles around the mRNA.

• Elongation: tRNA brings amino acids to the ribosome; peptide bonds form.

• Termination: Ribosome reaches a stop codon and releases the polypeptide.

Additional info: Some steps and definitions were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.