Cells & the Cell Cycle: Structure, Function, and Membrane Transport

Cells & the Cell Cycle

Introduction

The cell is the fundamental unit of life in all living organisms. Understanding cell structure, organelles, and the cell cycle is essential for studying anatomy and physiology. This guide covers the identification and function of major cell components, the stages of the cell cycle, and principles of membrane transport and tonicity.

Cell Structure & Components

Nucleus

The nucleus is the control center of the cell, containing genetic material and regulating cellular activities.

• Chromatin / DNA: Chromatin is the uncondensed form of DNA and proteins found within the nucleus. It condenses to form chromosomes during cell division.

• Nucleolus: A dense region within the nucleus responsible for ribosome synthesis.

• Nuclear Envelope: A double membrane that surrounds the nucleus, containing nuclear pores for transport of molecules.

Cytoplasm & Cell Membrane

The cytoplasm is the gel-like substance within the cell membrane, containing organelles and cytosol. The cell membrane, composed of a phospholipid bilayer, regulates entry and exit of substances.

• Cell Membrane (Plasma Membrane): A selectively permeable barrier made of a phospholipid bilayer with embedded proteins.

• Cytosol: The fluid portion of the cytoplasm where metabolic reactions occur.

Organelles

Organelles are specialized structures within the cell that perform distinct functions necessary for cellular survival.

• Mitochondria: The "powerhouse" of the cell, responsible for ATP production through cellular respiration.

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

• Smooth Endoplasmic Reticulum (Smooth ER): Involved in lipid synthesis and detoxification.

• Rough Endoplasmic Reticulum (Rough ER): Studded with ribosomes; synthesizes and processes proteins.

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

• Vesicles: Membrane-bound sacs for transport and storage.

  • Vacuole: Stores substances; larger in plant cells.

  • Lysosome: Contains digestive enzymes for breaking down waste.

  • Peroxisome: Breaks down fatty acids and detoxifies harmful substances.

• Cytoskeleton: Provides structural support and facilitates movement.

  • Centrioles: Involved in cell division and organization of microtubules.

  • Microfilaments, Microtubules, Intermediate Filaments: Components of the cytoskeleton with roles in cell shape, movement, and transport.

Cellular Extensions

• Flagellum: Long, whip-like structure for cell movement (e.g., sperm cells).

• Cilia: Short, hair-like projections for movement of substances across cell surfaces.

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

The Cell Cycle & Mitosis

Interphase

Interphase is the period of cell growth and DNA replication, preparing the cell for division. It consists of three sub-phases:

• G1 phase: Cell grows and carries out normal functions.

• S phase: DNA replication occurs.

• G2 phase: Cell prepares for mitosis.

During interphase, chromatin is uncondensed and the nuclear envelope is intact.

Mitosis

Mitosis is the process of nuclear division resulting in two genetically identical daughter cells. It consists of four main phases:

• Prophase: Chromatin condenses into visible chromosomes; mitotic spindle forms; nuclear envelope and nucleolus disappear.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Chromosomes decondense; nuclear envelopes reform; spindle disappears; nucleoli reappear.

Cytokinesis is the division of the cytoplasm, resulting in two separate cells. It begins during late anaphase and continues through telophase, involving the formation of a cleavage furrow by contracting microfilaments.

Cell Membrane Function: Transportation & Tonicity

Diffusion

Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration, resulting in even distribution.

• Key Principle: Molecules move down their concentration gradient without energy input.

• Example: Oxygen and carbon dioxide exchange in the lungs.

Formula:

$J = -D \frac{dC}{dx}$

Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane from an area of lower solute concentration to higher solute concentration.

• Key Principle: Water moves to balance solute concentrations across the membrane.

• Example: Water absorption in kidney tubules.

Formula:

$\Pi = iMRT$

Where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Tonicity

Tonicity describes the effect of a solution on cell volume due to osmosis.

Type of Solution	Solute Concentration	Effect on Cell
Isotonic	Equal to cell	No net water movement; cell remains unchanged
Hypotonic	Lower than cell	Water enters cell; cell swells and may burst (lysis)
Hypertonic	Higher than cell	Water leaves cell; cell shrinks (crenation)

Summary Table: Major Cell Organelles & Functions

Organelle	Function
Nucleus	Stores genetic material; controls cell activities
Mitochondria	Produces ATP via cellular respiration
Ribosomes	Protein synthesis
Endoplasmic Reticulum (Smooth & Rough)	Lipid synthesis (Smooth); Protein processing (Rough)
Golgi Apparatus	Modifies, sorts, and packages proteins/lipids
Lysosome	Digests cellular waste
Peroxisome	Detoxifies harmful substances
Vesicle/Vacuole	Transport/storage
Cytoskeleton	Structural support; movement
Centrioles	Cell division
Flagellum/Cilia/Microvilli	Movement/absorption

Additional info: Academic context and definitions have been expanded for clarity and completeness. All major checklist items from the lab have been covered and grouped logically for study purposes.