Bio 100 Lab 1 UPDATED

Microscopy and the Cell

Introduction

Understanding cell structure is fundamental to biology. The cell theory states that all organisms are composed of cells and all cells arise from pre-existing cells. This chapter explores the diversity of cell types, their structures, and the use of microscopy to study them.

Cell Structure and Types

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their internal organization.

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. DNA is located in a region called the nucleoid and is typically a single, circular chromosome with few associated proteins. Prokaryotes do not divide by mitosis.

• Eukaryotic Cells: Possess a true nucleus surrounded by a nuclear envelope. DNA is organized into multiple linear chromosomes with histones and other proteins. Eukaryotes contain membrane-bound organelles (e.g., mitochondria, chloroplasts, Golgi apparatus, endoplasmic reticulum) and divide by mitosis.

Some organelles, such as mitochondria and chloroplasts, contain their own DNA, which is circular and protein-free, resembling prokaryotic chromosomes. This supports the endosymbiotic theory, which proposes that these organelles originated as free-living prokaryotes incorporated into ancestral eukaryotic cells.

Table 1-1. Differences between Prokaryotic and Eukaryotic Cells

Characteristic	Prokaryotic Cells	Eukaryotic Cells
Membrane-bound organelles	None	Mitochondria, Chloroplasts, Golgi, ER, etc.
Nucleus	No true nucleus (nucleoid region)	True nucleus with nuclear envelope
Organization of DNA	Few proteins, 1 circular chromosome	Multiple linear chromosomes with histones
Type of cell division	No mitosis	Mitosis
Size	Small	Large

Types of Prokaryotic Organisms

• Archaebacteria: Some are methanogenic or reduce sulfur.

• Pseudobacteria: Tiny, lack true cell walls (e.g., mycoplasmas).

• Bacteria: Have true cell walls.

• Cyanobacteria: Also known as blue-green algae; photosynthetic.

Prokaryotes can be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (most are saprophytic or parasitic). They may exist as single cells, in clusters, or as colonies, but are not considered multicellular organisms.

Structure of a Typical Prokaryotic Cell

• Cell wall: Provides strength and protection.

• Capsule: Additional protective layer.

• Plasma membrane: Surrounds cytoplasm.

• Nucleoid: Region containing DNA.

• Flagellum: Enables motility.

Eukaryotic Organisms

Eukaryotes include protists, fungi, plants, and animals. Their cells contain specialized organelles for various functions:

• Mitochondria and Chloroplasts: Energy metabolism.

• Rough Endoplasmic Reticulum (RER): Protein synthesis (with ribosomes).

• Golgi Apparatus: Processes and packages proteins.

Protists

Overview

Protists are the simplest eukaryotes, mostly unicellular, with some forming colonies. They are classified as non-photosynthetic (protozoa and fungus-like protists) or photosynthetic (algae).

Protozoa

• Amoeba: Flexible, unicellular organisms found in soil and water. Move and ingest food via pseudopodia. Some are parasitic (e.g., Entamoeba histolytica).

• Paramecium: Unicellular, ciliated, freshwater organisms. Possess a macronucleus (asexual reproduction) and micronuclei (conjugation/sexual reproduction). Food is ingested via an oral groove and digested in food vacuoles. Contractile vacuoles expel excess water.

Algae

• General Features: Photosynthetic, may be unicellular or colonial, most have cell walls.

• Euglena: Single-celled, photosynthetic in light but can be heterotrophic in darkness. Lacks a cell wall, has flagella for movement, and an eyespot for light detection. Some lack chloroplasts and are heterotrophic.

Locomotion in Protists

• Amoeba: Moves by extending pseudopodia (cytoplasmic streaming).

• Paramecium: Moves by beating cilia.

• Euglena: Moves using flagella.

All forms of movement involve cytoskeletal elements and require ATP.

Multicellular Organisms

Advantages of Multicellularity

• Surface-to-volume ratio: Limits cell size; multicellularity allows larger organisms by increasing surface area for nutrient uptake.

• Cell specialization: Enables formation of tissues and organs with distinct functions (e.g., red blood cells for oxygen transport, muscle cells for movement).

Multicellular organisms include plants, animals, and fungi. Different cell types express different proteins, as determined by their genes.

Plant Cells

• Specialized for photosynthesis: Contain chloroplasts, a cell wall, and a large central vacuole.

• Other features: Lack centrioles (except in some lower plants). The cell wall provides structural support; the vacuole stores water and nutrients.

• Root cells: Lack chloroplasts and a vacuole, specialized for absorption.

Protoplasmic streaming (movement of organelles) can be observed in leaf cells, mediated by the cytoskeleton.

Animal Cells

• Features: Lack cell walls, chloroplasts, and large central vacuoles. Contain centrioles, nucleus, mitochondria, Golgi apparatus, and endoplasmic reticulum.

• Example: Human cheek cells can be observed under a light microscope, typically stained for visibility.

Microscopy

Introduction to the Light Microscope

The light microscope revolutionized biology by allowing visualization of cells and microorganisms. Two key properties are:

• Magnification: Increases the apparent size of objects.

• Resolution: The smallest distance at which two points can be distinguished as separate. Human eye resolution is about 0.1 mm; light microscopes can resolve down to 0.1 μm, and transmission electron microscopes to 0.001 μm.

Resolution is more important than magnification for image clarity.

Measurement and Magnification

To estimate cell size under the microscope:

• Use a slide with a known scale (e.g., mm graph paper).

• Determine the diameter of the field of view at a given magnification.

• Estimate cell size by comparing it to the field diameter.

Example Calculation: If 1.5 boxes (each 1 mm) fit across the field, the diameter is 1.5 mm = 1500 μm. If a cell is 1/3 the diameter, its size is:

$\text{Cell size} = \frac{1}{3} \times 1500\ \mu m = 500\ \mu m$

Laboratory Observations

Onion Cells

• Thin sheets of onion tissue are observed under low and high power.

• Cell walls and nuclei are visible; staining (e.g., with acetocarmine) enhances contrast.

• Cell size can be estimated by counting the number of cells across the field of view.

Elodea

• Leaf cells are observed for cyclosis (movement of chloroplasts).

• Details of cell structure are visible under high power.

Potato Cells

• Thin slices are stained with Lugol's reagent to detect starch (red for straight chains, blue for branched, purple for both).

• Starch is packaged in organelles called amyloplasts.

Human Cheek Cells

• Cells are collected by scraping the inside of the cheek, stained with methylene blue, and observed under the microscope.

• Appear blue with a dark nucleus; lack cell walls and chloroplasts.

Living Cultures and Demonstrations

Brown Hydra

• Hydra are multicellular, visible to the naked eye, and display feeding behavior distinct from unicellular protists.

• Feeding involves capturing prey with tentacles; behavior is sensitive to environmental disturbance.

Student Procedures for Observing Protists

• Use depression slides to observe live cultures of Amoeba, Paramecium, and pond water samples.

• Observe and sketch cell structure, organelles, and movement.

Summary Table: Key Features of Observed Cells

Cell Type	Key Features	Visible Organelles/Structures	Motility
Prokaryote (e.g., Bacteria)	No nucleus, cell wall, nucleoid, sometimes capsule/flagella	Nucleoid, cell wall, sometimes flagella	Flagella (if present)
Amoeba	Unicellular, flexible, pseudopodia	Nucleus, pseudopodia, cytoplasm	Pseudopodia (cytoplasmic streaming)
Paramecium	Unicellular, cilia, oral groove, contractile vacuole	Macronucleus, micronucleus, cilia, food vacuoles	Cilia
Euglena	Unicellular, flagellum, eyespot, chloroplasts	Nucleus, chloroplasts, contractile vacuole, eyespot	Flagellum
Plant Cell (Elodea, Onion, Potato)	Cell wall, chloroplasts (except root cells), large vacuole	Nucleus, chloroplasts, vacuole, cell wall	None (but cytoplasmic streaming)
Animal Cell (Cheek cell)	No cell wall, no chloroplasts, nucleus present	Nucleus, cytoplasm, plasma membrane	None

Key Terms

• Cell Theory: All living things are made of cells; all cells come from pre-existing cells.

• Organelle: Specialized structure within a cell with a specific function.

• Endosymbiotic Theory: Mitochondria and chloroplasts originated as free-living prokaryotes.

• Magnification: Apparent increase in size of an object.

• Resolution: Ability to distinguish two points as separate.

• Pseudopodia: Temporary cytoplasmic projections for movement and feeding (Amoeba).

• Cilia: Short, hair-like structures for movement (Paramecium).

• Flagellum: Long, whip-like structure for movement (Euglena).

• Cytoplasmic Streaming: Movement of cytoplasm within a cell.

Additional info:

• Amyloplasts are specialized organelles in plant cells that store starch (not explicitly named in the text).

• Protoplasmic streaming is driven by actin filaments of the cytoskeleton.

• Electron microscopes provide much higher resolution than light microscopes, allowing visualization of subcellular structures.