Cell Structure, Membrane Transport, and Cell Cycle: Study Notes for Anatomy & Physiology

Cell Structure and Organelles

Overview of Cell Organelles

Cells are the basic units of life, containing specialized structures called organelles that perform distinct functions necessary for cellular survival and activity. Recognizing the location, structure, and function of each organelle is essential for understanding cell biology.

• Nucleus: The largest organelle, contains genetic material (DNA) and controls cellular activities.

• Nucleolus: Dark-staining region within the nucleus, involved in ribosomal RNA synthesis.

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

• Endoplasmic Reticulum (ER): Network of membranes; rough ER is studded with ribosomes (protein synthesis), smooth ER lacks ribosomes (lipid synthesis, detoxification).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Powerhouse of the cell; site of ATP (energy) production via cellular respiration.

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

• Peroxisomes: Break down fatty acids and neutralize toxic compounds.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) providing structural support and facilitating movement.

• Centrosomes and Centrioles: Organize microtubules and play a role in cell division.

• Plasma Membrane: Phospholipid bilayer that encloses the cell, regulates entry and exit of substances.

Example: Muscle cells contain many mitochondria to meet high energy demands.

Extracellular Fluid (ECF)

Extracellular fluid refers to all fluid outside the cell, including interstitial fluid and plasma, which bathe each cell in fluid and provide nutrients.

Plasma Membrane Structure and Function

Phospholipid Bilayer

The plasma membrane is primarily composed of a bilayer of phospholipids, with embedded proteins, cholesterol, and small amounts of glycolipids.

• Phospholipids: Each molecule has a polar, hydrophilic head and two nonpolar, hydrophobic tails.

• Arrangement: Heads face outward toward water; tails face inward, away from water.

Example: The bilayer forms a selective barrier, allowing only certain substances to pass.

Membrane Proteins

Membrane proteins facilitate communication and transport between the cell and its environment. There are two main types:

• Integral (transmembrane) proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Loosely attached to one side of the membrane, often involved in signaling or structural support.

Six major functions of membrane proteins:

1. Transport

2. Enzymatic activity

3. Signal transduction

4. Cell-cell recognition

5. Intercellular joining

6. Attachment to the cytoskeleton and extracellular matrix

Glycocalyx

The glycocalyx is a carbohydrate-rich area on the cell surface, important for cell recognition and immune response.

Membrane Transport

Types of Membrane Transport

Cells must regulate the movement of substances across the plasma membrane. Transport can be passive (no energy required) or active (requires energy).

• Passive Transport: Movement of molecules from high to low concentration (down the concentration gradient).

• Active Transport: Movement of molecules from low to high concentration (against the gradient), requires ATP.

Passive Transport Mechanisms

• Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2).

• Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a semipermeable membrane.

Factors Affecting Diffusion: Concentration, molecular size, and temperature. Higher concentration gradients, smaller molecules, and higher temperatures increase the rate of diffusion.

Osmolarity and Tonicity

Osmolarity is the total concentration of solute particles in solution. Water moves from areas of low solute concentration to high solute concentration across membranes.

Tonicity describes the effect of a solution on cell volume:

• Hypertonic: Higher solute concentration outside the cell; cell loses water and shrinks.

• Hypotonic: Lower solute concentration outside the cell; cell gains water and swells.

• Isotonic: Equal solute concentration; no net water movement.

Cell Organelles: Structure and Function

Organelle	Membranous/Nonmembranous	Function
Nucleus	Membranous	Stores genetic material, controls cell activities
Ribosomes	Nonmembranous	Protein synthesis
Endoplasmic Reticulum (ER)	Membranous	Rough ER: protein synthesis; Smooth ER: lipid synthesis, detoxification
Golgi Apparatus	Membranous	Modification, sorting, and packaging of proteins/lipids
Mitochondria	Membranous	ATP production (energy)
Lysosomes	Membranous	Digestion of cellular waste
Peroxisomes	Membranous	Breakdown of fatty acids, detoxification
Cytoskeleton	Nonmembranous	Structural support, movement
Centrosomes/Centrioles	Nonmembranous	Organization of microtubules, cell division

Cell Surface Structures

• Cilia and Flagella: Aid in movement of the cell or movement of material across the cell surface (e.g., trachea, fallopian tube, sperm).

• Microvilli: Finger-like projections that increase surface area for absorption.

Nucleus and Chromatin

Nucleus

The nucleus contains the cell's genetic material and is the site of DNA replication and transcription. Most cells have one nucleus, but some (e.g., skeletal muscle cells) are multinucleated.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA synthesis.

Cell Cycle and DNA Replication

Cell Cycle Stages

The cell cycle consists of two main stages: interphase (cell growth and activity) and mitotic phase (cell division).

• Interphase: Includes three subphases:

  • G1 (gap 1): Vigorous growth and metabolism

  • S (synthesis): DNA synthesis

  • G2 (gap 2): Preparation for division

• Mitotic Phase: Division of the nucleus and cytoplasm to form two daughter cells

DNA Replication

During the S phase, DNA is replicated so that each daughter cell receives a complete copy of the genetic material.

• Semiconservative Replication: Each new DNA strand consists of one old strand and one newly synthesized strand.

Equation:

$\text{DNA}_{\text{new}} = \text{DNA}_{\text{old}} + \text{DNA}_{\text{synthesized}}$

Chromosomes

• Chromatids: After replication, each chromosome consists of two identical sister chromatids joined at the centromere.

• Centromere: Region where sister chromatids are attached.

Check Your Understanding

• The compartment between the plasma membrane and nuclear envelope is called the cytoplasm (contains organelles and cytosol).

• Cells with high energy demands (e.g., muscle cells) have increased quantities of mitochondria.

• Red blood cells do not contain a nucleus at maturity.

Summary Table: Key Cell Structures and Functions

Structure	Main Function	Example/Application
Nucleus	Genetic control center	Contains DNA for protein synthesis
Mitochondria	ATP production	Abundant in muscle cells
Ribosomes	Protein synthesis	Free or attached to rough ER
Golgi Apparatus	Protein/lipid modification	Secretion of hormones
Lysosomes	Digestion	Breakdown of cellular debris

Additional info: Academic context and definitions have been expanded for clarity and completeness. All major cell organelles, membrane transport mechanisms, and cell cycle stages are covered for exam preparation.