Cell Structure: Anatomy & Physiology Study Notes

Cells – Part 1: Structure

Overview of Cell Structure

Cells are the fundamental units of life, and their structure underpins all physiological functions. Understanding cell structure is essential for grasping how tissues and organs operate in the human body.

• Cell Theory: All cells are the structural and functional units of life. The activity of an organism depends on the collective activities of its cells.

• Cell Diversity: Over 250 different types of human cells exist, varying in size, shape, and subcellular components.

• Three Major Regions of a Generalized Cell:

  1. Plasma Membrane: Flexible outer boundary

  2. Cytoplasm: Intracellular fluid containing organelles

  3. Nucleus: DNA-containing control center

Cells – (1) Plasma Membrane

Structure and Function of the Plasma Membrane

The plasma membrane acts as a selective barrier, separating the intracellular fluid (ICF) from the extracellular fluid (ECF). It controls the entry and exit of substances, maintaining cellular homeostasis.

• Functions:

  • Acts as an active barrier

  • Plays a dynamic role in cellular activity

  • Consists of a flexible lipid bilayer

  • Decreases water solubility

• Structure:

  • Phospholipid bilayer with embedded proteins

  • Fluid mosaic model: membrane is made up of many pieces

  • Surface sugars form the glycocalyx

  • Membrane structures help hold cells together through cell junctions

Membrane Lipids

Lipids are the main structural components of the plasma membrane.

• Phospholipids: Have polar (hydrophilic) heads and nonpolar (hydrophobic) tails

• Cholesterol: Stabilizes membrane fluidity

• Glycolipids: Lipids with sugar groups on the outer membrane surface

Membrane Proteins

Membrane proteins allow cells to communicate with their environment and perform specialized functions.

• Integral Proteins: Inserted into the membrane; span the membrane; have hydrophobic and hydrophilic regions; function as transport proteins, enzymes, or receptors

• Peripheral Proteins: Attached to integral proteins or membrane surface; function as enzymes, motor proteins, or for cell shape and movement

Glycocalyx

The glycocalyx is a sugar coating on the cell surface, consisting of glycoproteins and glycolipids. It functions in cell recognition and protection.

• Acts as a biological marker for cell-to-cell recognition

• Allows immune system to recognize self vs. nonself

Cell Junctions

Cell junctions are specialized structures that connect cells together, forming tissues and organs.

• Tight Junctions: Impermeable junctions that prevent fluids and molecules from moving between cells (e.g., digestive tract)

• Desmosomes: Anchoring junctions that bind cells together like a zipper, providing mechanical strength (e.g., skin, heart muscle)

• Gap Junctions: Communicating junctions that allow small molecules to pass between cells (e.g., cardiac and smooth muscle cells)

Transport Processes Across the Plasma Membrane

The plasma membrane is selectively permeable, allowing only certain molecules to cross.

• Passive Transport: No energy required

• Active Transport: Energy (ATP) is required

Cells – (2) Cytoplasm

Structure and Components of Cytoplasm

The cytoplasm is the cellular material located between the plasma membrane and the nucleus. It consists of cytosol, inclusions, and organelles.

• Cytosol: Gel-like solution made up of water and soluble molecules

• Inclusions: Insoluble molecules; vary with cell type (e.g., glycogen granules, pigments, lipid droplets)

• Organelles: Metabolic machinery of the cell; each with specialized function

Cytoplasmic Organelles

• Membranous Organelles:

  1. Mitochondria

  2. Endoplasmic reticulum

  3. Golgi apparatus

  4. Peroxisomes

  5. Lysosomes

• Non-membranous Organelles:

  1. Ribosomes

  2. Cytoskeleton

  3. Centrioles

Mitochondria

Mitochondria are the power plants of cells, producing most of the cell's energy (ATP) via aerobic cellular respiration.

• Double membrane structure; inner membrane has many folds (cristae)

• Contain their own DNA, RNA, and ribosomes

Ribosomes

Ribosomes are the sites of protein synthesis.

• Free Ribosomes: Float freely in cytosol; synthesize soluble proteins

• Membrane-bound Ribosomes: Attached to endoplasmic reticulum; synthesize proteins for membranes or export

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubes and sacs. There are two types:

• Rough ER: Studded with ribosomes; site of protein synthesis for secretion and membranes

• Smooth ER: No role in protein synthesis; involved in lipid metabolism, detoxification, and calcium storage

Golgi Apparatus

The Golgi apparatus modifies, concentrates, and packages proteins and lipids received from the rough ER.

• Acts as the cell's "traffic director"

• Final transport vesicles can take one of three pathways: secretion, membrane incorporation, or lysosomal digestion

Peroxisomes

Peroxisomes contain enzymes that detoxify harmful substances and neutralize free radicals.

• Oxidase: Uses oxygen to convert toxins to hydrogen peroxide ($H_2O_2$)

• Catalase: Converts $H_2O_2$ to water and oxygen

• Also involved in breakdown and synthesis of fatty acids

Lysosomes

Lysosomes contain digestive enzymes that break down bacteria, viruses, and worn-out organelles.

• Isolate potentially harmful intracellular digestion

• Degrade nonfunctional organelles

• If ruptured, can digest the cell itself (autolysis)

Endomembrane System

The endomembrane system consists of membranous organelles that work together to produce, degrade, store, and export biological molecules, and degrade harmful substances.

• Includes ER, Golgi apparatus, secretory vesicles, lysosomes, and nuclear/plasma membranes

Cytoskeleton

The cytoskeleton is an elaborate network of rods running throughout the cytosol, providing structural support and enabling cell movement.

• Microfilaments: Thinnest; involved in cell motility and changes in cell shape

• Intermediate Filaments: Tough, insoluble protein fibers; resist pulling forces

• Microtubules: Largest; hollow tubes; determine cell shape and organelle distribution

Cells – (3) Nucleus

Structure and Function of the Nucleus

The nucleus is the control center of the cell, containing DNA and regulating gene expression and cell division.

• Surrounded by a nuclear envelope

• Contains nucleoli (sites of ribosome production)

• Contains chromatin (DNA and associated proteins)

Table: Comparison of Cell Junctions

Junction Type	Structure	Function	Example
Tight Junction	Integral proteins fuse adjacent cells	Prevents passage of molecules	Digestive tract
Desmosome	Linker proteins interlock cells	Provides mechanical strength	Skin, heart muscle
Gap Junction	Transmembrane proteins form channels	Allows passage of small molecules	Cardiac muscle

Key Equations

• Cellular Respiration (Mitochondria):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Detoxification (Peroxisomes):

$2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$

Summary Table: Major Cell Organelles and Functions

Organelle	Structure	Main Function
Mitochondria	Double membrane, cristae	ATP production
Ribosomes	Protein and rRNA	Protein synthesis
ER (Rough)	Membranous sacs with ribosomes	Protein synthesis
ER (Smooth)	Membranous sacs without ribosomes	Lipid metabolism, detoxification
Golgi Apparatus	Stacked membranous sacs	Protein modification and packaging
Peroxisomes	Membranous vesicles	Detoxification
Lysosomes	Membranous vesicles	Digestion of macromolecules

Additional info: Academic context and full explanations have been added to ensure completeness and clarity for exam preparation.