Cell Structure and Microscopy: Study Guide

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

4.1 Microscopy and Cell Size

Differences Between Light and Electron Microscopes

Microscopes are essential tools in biology for visualizing structures too small to be seen with the naked eye. The two main types are light microscopes and electron microscopes, each with distinct capabilities.

Resolving Power: The ability to distinguish two close points as separate. Light microscopes have a lower resolving power (~200 nm) compared to electron microscopes (as low as 0.1 nm).
Magnification: Light microscopes typically magnify up to 1000–2000x, while electron microscopes can magnify up to 1,000,000x.
Image: Light microscopes produce color images of living or fixed specimens. Electron microscopes produce black-and-white images and require specimens to be fixed and dehydrated.
Types of Electron Microscopes:
- Transmission Electron Microscope (TEM): Provides detailed internal images by passing electrons through thin sections of specimens.
- Scanning Electron Microscope (SEM): Produces 3D images of specimen surfaces by scanning with a focused beam of electrons.

Example: TEM is used to view the internal structure of organelles, while SEM is used to observe the surface of a cell.

General Sizes of Biological Structures

Biological structures vary greatly in size, from whole cells to small molecules.

Cells: 1–100 μm (micrometers)
Organelles: 1–10 μm
Viruses: 20–300 nm (nanometers)
Macromolecules (e.g., proteins, DNA): 1–10 nm
Small molecules (e.g., H2O): ~0.1 nm

Additional info: 1 μm = 1000 nm; 1 nm = 10-9 m.

4.2 Cell Structure and Diversity

Basic Features Shared by All Cell Types

All cells, regardless of type, share certain fundamental features:

Plasma membrane: A selective barrier that encloses the cell.
Cytoplasm: The semi-fluid substance inside the cell.
Genetic material (DNA): Contains instructions for cell function.
Ribosomes: Structures that synthesize proteins.

Structure and Function of the Plasma Membrane

The plasma membrane is a dynamic structure that controls the movement of substances in and out of the cell.

Structure: Composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
Function: Maintains homeostasis, facilitates communication, and provides structural support.
Why well-suited: The amphipathic nature of phospholipids (hydrophilic heads, hydrophobic tails) creates a semi-permeable barrier.

Comparison of Prokaryotic and Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structural features.

Prokaryotic Cells:
- No membrane-bound nucleus; DNA is in the nucleoid region.
- Lack membrane-bound organelles.
- Generally smaller (1–10 μm).
- Examples: Bacteria and Archaea.
Eukaryotic Cells:
- Have a true nucleus enclosed by a nuclear envelope.
- Contain membrane-bound organelles (e.g., mitochondria, ER).
- Larger (10–100 μm).
- Examples: Plants, animals, fungi, protists.

Additional info: Eukaryotic cells can be unicellular or multicellular; prokaryotes are always unicellular.

Limits to Cell Size

Cell size is limited by the surface area-to-volume ratio, which affects the efficiency of material exchange.

As a cell grows, its volume increases faster than its surface area.
Efficient exchange of nutrients and waste requires a high surface area relative to volume.
Large cells may have difficulty transporting materials quickly enough to support life.

Equation:

(for a cube of side a)

Prokaryotic Cell Structures: Structure and Function

Prokaryotic Cell Wall: Rigid structure outside the plasma membrane; provides shape and protection. Composed mainly of peptidoglycan in bacteria.
Capsule: Gelatinous outer layer; protects against desiccation and immune attack, aids in adhesion.
Fimbriae: Short, hair-like appendages; help cells adhere to surfaces or other cells.
Prokaryotic Flagella: Long, whip-like structures; enable motility.
Nucleoid: Region containing the cell's DNA; not membrane-bound.
Ribosomes: Sites of protein synthesis; smaller (70S) than eukaryotic ribosomes (80S).

Complexity of Eukaryotic Cells

Eukaryotic cells are more complex due to compartmentalization and specialized organelles.

Membrane-bound organelles allow for specialized functions.
Internal membranes increase surface area for metabolic processes.
Greater structural and functional diversity.

Comparison of Plant and Animal Cells

Plant and animal cells share many features but also have key differences.

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large, central	Small or absent
Lysosomes	Rare	Common
Shape	Rectangular	Round/irregular

4.3 Eukaryotic Cell Structures

Nucleus

The nucleus is the control center of the eukaryotic cell, containing most of the cell's genetic material.

Structure: Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for transport.
Function: Stores DNA, coordinates cell activities such as growth and reproduction.

Nuclear Envelope and Nuclear Lamina

Nuclear Envelope: Double membrane that separates the nucleus from the cytoplasm; contains nuclear pores for molecular exchange.
Nuclear Lamina: Network of protein filaments that supports the nuclear envelope and maintains nuclear shape.

Nucleolus

Structure: Dense region within the nucleus.
Function: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Chromatin/Chromosomes

Chromatin: Complex of DNA and proteins (histones); exists in a less condensed form during interphase.
Chromosomes: Condensed chromatin visible during cell division; carry genetic information.

Ribosomes: Location and Function

Location: Found free in the cytoplasm or bound to the rough endoplasmic reticulum (ER) and nuclear envelope.
Function: Synthesize proteins by translating messenger RNA (mRNA).

Additional info: Ribosomes are composed of rRNA and proteins; they are not membrane-bound organelles.