Cell Structure and Function: Study Notes for Anatomy & Physiology

Cells: Structure and Function

An Introduction to Cells

Cells are the fundamental units of life, forming the basis of all plants and animals. Understanding cell structure and function is essential for studying anatomy and physiology.

• Cell Theory:

  • Cells are the building blocks of all living organisms.

  • All cells arise from the division of preexisting cells.

  • Cells are the smallest units capable of performing vital physiological functions.

  • Each cell maintains homeostasis at the cellular level.

• Cytology: The branch of biology that studies cells.

• Types of Cells:

  • Sex Cells (Germ Cells): Reproductive cells (male sperm, female oocyte).

  • Somatic Cells: All body cells except sex cells.

• Basic Cell Structure:

  • Plasma (Cell) Membrane

  • Cytoplasm

    • Cytosol

    • Organelles

  • Nucleus

    • Chromosomes

    • Genes

Plasma Membrane

The plasma membrane is a thin, flexible barrier that separates the cell's internal environment from the external extracellular fluid.

• Extracellular Fluid (Interstitial Fluid): Watery medium surrounding the cell.

• Functions of the Plasma Membrane:

  • Physical Isolation: Separates cytoplasm (intracellular fluid, ICF) from extracellular fluid (ECF).

  • Regulation of Exchange: Controls entry of ions and nutrients, elimination of wastes, and release of cellular products.

  • Sensitivity: First part of cell affected by changes in ECF; receives signals from other cells.

  • Structural Support: Anchors cells and tissues.

• Structure:

  • Thickness: 6–10 nm

  • Composed of lipids, proteins, and carbohydrates

Membrane Lipids

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Stiffens membrane, reduces fluidity and permeability.

• Glycolipids: Contribute to membrane structure and cell recognition.

Membrane Proteins

• Integral Proteins: Embedded within the membrane; include transmembranous proteins that span the entire membrane.

• Peripheral Proteins: Attached to inner or outer surface; include anchoring proteins (stabilizers).

• Recognition Proteins: Identify cells as normal or abnormal.

• Enzymes: Catalyze reactions in ECF or cytosol.

• Receptor Proteins: Bind and respond to ligands (ions, hormones).

• Carrier Proteins: Transport specific solutes; may require ATP.

• Channels: Permit flow of water and solutes; gated channels regulate passage.

Membrane Carbohydrates

• Include proteoglycans, glycoproteins, and glycolipids.

• Extend outside the cell membrane, forming the glycocalyx (sticky "sugar coat").

• Functions of Glycocalyx:

  • Lubrication and protection

  • Anchoring and locomotion

  • Specificity in binding (receptors)

  • Genetically determined; immune system recognizes self glycoproteins/glycolipids

Organelles and the Cytoplasm

Cytoplasm

The cytoplasm includes all materials inside the cell and outside the nucleus. It consists of cytosol and organelles.

• Cytosol (Intracellular Fluid):

  • Contains dissolved nutrients, ions, proteins, and waste products.

  • High protein and potassium; low carbohydrate, lipid, amino acid, and sodium.

• Organelles: Structures with specific functions.

• Inclusions: Masses of insoluble material (e.g., glycogen, lipid droplets).

Non-Membranous Organelles

Non-membranous organelles lack a surrounding membrane and are in direct contact with cytosol.

Organelle	Structure	Function
Microvilli	Finger-shaped projections; actin filaments	Increase surface area for absorption; common in digestive system
Centrosome & Centrioles	Paired cylinders; nine triplets of microtubules	Form spindle apparatus during cell division; bases of cilia and flagella
Microfilaments	Thin actin filaments	Mechanical strength; interact with myosin for muscle contraction
Intermediate Filaments	Mid-sized filaments	Strengthen cell; maintain shape; stabilize organelles
Microtubules	Large, hollow tubulin tubes	Strengthen cell; move organelles; form spindle apparatus, centrioles, cilia
Cilia	Slender extensions; microtubules anchored to basal body	Motile cilia move substances; primary cilium senses stimuli
Flagella	Whip-like extension	Propels sperm cell
Ribosomes	Small & large subunits; rRNA	Synthesize proteins; free in cytoplasm or fixed to ER
Proteasome	Enzyme complexes	Disassemble damaged proteins for recycling

Membranous Organelles

Membranous organelles are surrounded by a membrane, isolating them from cytosol.

Organelle	Structure	Function
Rough Endoplasmic Reticulum (RER)	Membrane system with ribosomes	Protein and glycoprotein synthesis; folds proteins; packages in vesicles
Smooth Endoplasmic Reticulum (SER)	Membrane sacs/tubules; no ribosomes	Synthesizes phospholipids, cholesterol, steroid hormones, glycerides, glycogen
Golgi Apparatus	Stack of membrane sacs	Modifies/packages secretions; renews plasma membrane; produces lysosomes
Mitochondria	Rod-like, double membrane; cristae	Produces ATP from glucose; "powerhouse" of cell
Lysosomes	Enzyme-containing vesicles	Digest bacteria, molecules, damaged organelles; autophagy; autolysis
Peroxisomes	Small enzyme vesicles	Break down fatty acids; detoxify substances (e.g., hydrogen peroxide)

Nucleus

Structure and Function

The nucleus is the largest organelle and serves as the cell's control center, directing protein production and determining cell structure and function.

• Most cells have one nucleus; skeletal muscle cells have multiple, red blood cells have none.

• Cells cannot survive long if the nucleus is damaged.

Contents of the Nucleus

• Nucleoplasm: Fluid portion of the nucleus.

• Nuclear Matrix: Support filaments.

• Nucleoli: Nuclear organelles related to protein production; prominent in cells producing large amounts of protein (e.g., liver, muscle, nerve).

• DNA: Stores instructions for protein synthesis.

  • Nucleosome: DNA coiled around histone proteins; allows compact storage.

  • Chromatin: Loosely coiled nucleosomes; seen in non-dividing cells.

  • Chromosomes: Tightly coiled DNA; seen during cell division (mitosis).

Information Storage in the Nucleus

• Proteins: Made of amino acid sequences; blueprint stored in DNA.

• Genetic Code: Chemical language of DNA instructions; sequence of bases (A, T, C, G).

• Triplet Code: Three bases specify one amino acid.

  • $\text{Codon} = \text{Three DNA bases} \rightarrow \text{One amino acid}$

• Gene: Functional unit of heredity; DNA instructions for producing one protein.

Example: Protein Synthesis

• DNA sequence is transcribed into mRNA, which is then translated by ribosomes into a specific protein.

• Each gene codes for a unique protein, determining cell function and structure.

Additional info: The cell's ability to maintain homeostasis, respond to environmental changes, and perform specialized functions depends on the integrity and regulation of its organelles and genetic material.