Cells: The Living Units

Cell Theory

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

• All living organisms are composed of one or more cells.

• The cell is the basic structural and functional unit of life.

• All cells arise from pre-existing cells by division.

• Cells contain hereditary information (DNA) that is passed from cell to cell during division.

Major Regions of a Eukaryotic Cell

Eukaryotic cells are complex and have three main regions:

• Plasma Membrane: The outer boundary that separates the cell from its environment.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing cytosol, organelles, and inclusions.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. DNA is found in a nucleoid region. Example: Bacteria.

• Eukaryotic Cells: Have a true nucleus and numerous membrane-bound organelles. Example: Animal and plant cells.

Compartmentalization in Eukaryotic Cells

Compartmentalization refers to the presence of membrane-bound organelles in eukaryotic cells, allowing specialized functions to occur in separate areas. This increases efficiency and organization within the cell.

Generalized Eukaryotic Cell Structure

• Plasma membrane

• Cytoplasm (with organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes)

• Nucleus (with nuclear envelope, nucleolus, chromatin)

Example: A typical animal cell diagram would include all the above structures, with labels for each organelle.

Plasma Membrane Organization

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed mainly of lipids and proteins.

• Phospholipid bilayer: Two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Proteins: Embedded within or attached to the bilayer, serving various functions.

• Cholesterol: Interspersed within the bilayer, stabilizing membrane fluidity.

Hydrophilic and Hydrophobic Portions

• Hydrophilic heads: Face the aqueous environment inside and outside the cell.

• Hydrophobic tails: Face inward, away from water, forming the core of the membrane.

Functions of the Lipid Portion

• Acts as a barrier to most water-soluble substances.

• Provides fluidity and flexibility to the membrane.

Role of Cholesterol

• Stabilizes the membrane by reducing fluidity at high temperatures and preventing rigidity at low temperatures.

Membrane Proteins

• Integral proteins: Span the membrane; involved in transport and cell signaling.

• Transmembrane proteins: A type of integral protein that completely crosses the membrane; often function as channels or carriers.

• Peripheral proteins: Loosely attached to the membrane surface; involved in signaling or maintaining cell shape.

• Glycoproteins: Proteins with carbohydrate chains; play roles in cell recognition and signaling.

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer, allowing lateral movement of components.

Cell Junctions

Types of Cell Junctions

• Desmosomes: Anchoring junctions that hold adjacent cells together, providing mechanical strength. Example: Skin cells.

• Tight junctions: Seal adjacent cells to prevent leakage of molecules between them. Example: Intestinal lining.

• Gap junctions: Allow direct communication between cells through channels. Example: Cardiac muscle cells.

Nucleus

Structure and Function

• Nuclear membrane (envelope): Double membrane with nuclear pores for transport of materials.

• Nucleolus: Site of ribosome synthesis.

• Chromosomes and chromatin: DNA is organized as chromatin (loose) or chromosomes (condensed during cell division).

Mitochondria

Structure and Function

• Double-membraned organelle with inner folds called cristae.

• Site of ATP (energy) production via cellular respiration.

Endomembranous System

Components and Functions

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

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes and modifies proteins.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

Cytoplasm

Structure and Function

• Cytosol: Gel-like fluid where organelles are suspended.

• Inclusions: Non-living substances (e.g., glycogen, lipid droplets) stored in the cytoplasm.

• Organelles: Specialized structures performing specific functions.

Cytoskeleton

Structure and Function

• Microtubules: Hollow tubes that maintain cell shape and aid in intracellular transport.

• Intermediate filaments: Provide mechanical support for the cell.

• Actin filaments (microfilaments): Involved in cell movement and shape changes.

Membranous vs. Non-Membranous Organelles

Cell Cycle

Purpose and Basic Parts

• Purpose: To allow cells to grow, replicate DNA, and divide, ensuring tissue growth and repair.

• Phases:

  • Interphase: Cell grows and DNA replicates (G1, S, G2 phases).

  • Mitotic phase (M phase): Cell divides into two daughter cells (mitosis and cytokinesis).

Transcription and Translation

Comparison and Basic Steps

Key Equation:

Additional info: The above notes expand on the provided outline with academic context and examples to ensure a comprehensive understanding of cell structure and function.