Chapter3 - Cells

Cells

Overview

• Cells are small compartments filled with chemicals that form the building blocks for all organisms.

• Cells must be small to ensure rapid movement of molecules in and out to meet cell needs.

• Historical events, like the Plague, prompted investigations into disease and the search for tools to study smaller entities.

Microscopy

• The invention of the microscope in the 17th century allowed scientists to view cells in detail.

• Light does not shine directly on specimens but passes through, magnified by optical lenses.

• Example: Light micrograph (LM) of a white blood cell surrounded by red blood cells.

Robert Hooke and Cell Theory

• In 1665, Robert Hooke observed tissue through a simple microscope and discovered compartments he termed cells.

• Accumulation of evidence led to the conclusion that all living things consist of cells, culminating in cell theory:

  • All living things are made up of cells.

  • All cells arise from pre-existing cells.

Electron Microscopes

• Electron microscopes utilize focused beams of electrons to provide greater detail and higher magnifications than light microscopes, up to 100,000x.

• This high magnification enables the observation of intracellular structures and even viruses.

• Types of electron microscopes:

  • Scanning Electron Microscope (SEM): Used to examine surfaces of small objects, providing 3D images.

  • Transmission Electron Microscope (TEM): Useful for exploring internal structures of cells.

Cell Types

Prokaryotic and Eukaryotic Cells

• Cells generally fall into two categories:

  • Prokaryotic Cells:

    • Very small, lack a nucleus (contain a nucleoid region with circular DNA), lack organelles, usually single-celled.

  • Eukaryotic Cells:

    • Larger, contain a defined nucleus with linear DNA, possess structured organelles, may form multicellular organisms.

Cellular Structures

Generalized Prokaryotic Cell

• Major components:

  • Cell membrane

  • Cell wall

  • Capsule

  • Flagellum (for movement)

  • Nucleoid region with DNA

  • Cytoplasm and ribosomes.

Generalized Animal Cell

• Main components include:

  • Nucleus (with nucleolus and nuclear envelope)

  • Ribosomes

  • Mitochondria

  • Various endoplasmic reticulum types (smooth and rough)

  • Golgi apparatus

  • Cytoskeleton and associated fibers.

Generalized Plant Cell

• Key structures:

  • Nucleus (with nucleolus and nuclear envelope)

  • Chloroplasts (site of photosynthesis)

  • Cell wall and central vacuole

  • Various organelles as seen in animal cells.

Membrane Structure and Function

Plasma Membrane

• Separates the cell from its environment and is mainly composed of phospholipids and proteins.

• The phospholipid bilayer's arrangement creates a barrier between the interior of the cell and its surroundings.

Fluid Mosaic Model

• Describes how phospholipid and protein molecules drift freely within the membrane:

  • Fluid: Components can move within the membrane.

  • Mosaic: Variety of proteins embedded within.

Membrane Proteins Functions

• Membrane proteins contribute to:

  • Cell signaling

  • Enzymatic activity

  • Transport across the membrane

  • Intercellular joining and recognition.

Selective Permeability

• Membranes regulate which substances can enter or exit the cell at varying speeds.

• Certain molecules, like glucose, require transport proteins to facilitate their movement into the cell.

The Nucleus

Function of the Nucleus

• Serves as the control center containing genetic information essential for protein production.

• Surrounded by the nuclear envelope, which encapsulates chromosomes and contains the nucleolus (where ribosomal RNA is made).

DNA and Protein Synthesis

• DNA information is transcribed into mRNA and translated into proteins by ribosomes in the cytoplasm:

  1. Synthesis of mRNA in the Nucleus

  2. Movement of mRNA to Cytoplasm

  3. Synthesis of Protein

Endomembrane System

Overview of Organelles

• Organelles are membrane-bound structures within a cell, functioning in manufacturing and distribution.

• The endomembrane system includes:

  • Endoplasmic reticulum (ER)

  • Golgi apparatus

  • Lysosomes

Endoplasmic Reticulum (ER)

• Composed of:

  • Rough ER: Studded with ribosomes, synthesizes proteins and membrane molecules.

  • Smooth ER: Lacks ribosomes, involved in lipid production and calcium ion storage.

Golgi Apparatus

• Processes, packages, and ships proteins and lipids from the ER.

• Receives transport vesicles, refines contents, and modifies them for export.

Lysosomes

• Contain digestive enzymes for breaking down food and damaged organelles (fuses with vacuoles).

Vacuoles

• Membranous sacs with contractile vacuoles in protists and central vacuoles in plants.

Energy-Converting Organelles

Chloroplasts and Mitochondria

• Organelles responsible for energy processes:

  • Chloroplasts: Conduct photosynthesis in plants.

  • Mitochondria: Generate energy through cellular respiration in both plants and animals.

Cellular Movement Structures

Cilia and Flagella

• Flagella: Propel cells with a whiplike motion.

• Cilia: Shorter structures that create wave-like movements for propulsion.

Cell Communication and Junctions

Cell Junctions in Plants

• Plasmodesmata: Pathways allowing communication between adjacent plant cells.

Animal Cell Junctions

• Cells adhere through:

  • Tight junctions

  • Anchoring junctions

  • Communicating junctions (gap junctions).

Fluid Mosaic Model for Membrane Structure

• Conceptualizes the organization of membranes with embedded structures (proteins, cholesterol, glycoproteins).

Maintaining Cell Shape

• Internal network known as the cytoskeleton provides rigidity and allows for shape changes.