Z

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.