Microscopy

Introduction to Microscopy

Microscopy is a fundamental technique in biology that allows scientists to observe cells and their components. The development of microscopes has enabled the discovery of cell structure and function.

• Cell walls were first seen on dead cells of oak bark by Robert Hooke in 1665.

• Microscopes were first used by Renaissance scientists to study living cells.

Types of Light Microscopy

Light microscopes use visible light to illuminate specimens. Several types exist, each with unique advantages:

• Brightfield

• Phase-contrast

• Differential interference contrast

• Fluorescence

• Confocal

• Deconvolution

• Super resolution

Electron Microscopy

Electron microscopes use beams of electrons for much higher resolution imaging.

• Scanning Electron Microscopy (SEM)

• Transmission Electron Microscopy (TEM)

• Cryo-electron Microscopy

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two major types based on their structure and organization.

• Prokaryotic cells: Found in Bacteria and Archaea.

• Eukaryotic cells: Found in protists, fungi, animals, and plants.

Shared Features of All Cells

• Plasma membrane: Encloses the cell, controlling movement of substances.

• Cytosol: Semi-fluid substance inside the cell.

• Chromosomes: Carry genetic information (DNA).

• Ribosomes: Synthesize proteins.

Major Differences

• Prokaryotic cells: DNA is concentrated in a region called the nucleoid, which is not membrane-bound.

• Eukaryotic cells: Most DNA is in the nucleus, which is bounded by a double membrane and contains internal membranes that split the cell into organelles.

Animal Cell Structure

Overview of Animal Cell Components

Animal cells have specialized structures that perform distinct functions necessary for life.

• Cell membrane: Semi-permeable barrier that regulates entry and exit of substances.

• Nucleus: Contains most of the cell's genes and is surrounded by a nuclear envelope with pores for RNA transport.

• Plasma membrane: Selective barrier for passage of oxygen, nutrients, and waste.

Nucleus

• Enclosed by a double membrane (nuclear envelope).

• Contains chromatin (DNA and proteins).

• DNA is organized into discrete units called chromosomes.

• Nucleolus: Site of ribosomal RNA synthesis.

Ribosomes

• Made of ribosomal RNA and protein.

• Build proteins in the cytosol (free ribosomes) and on the endoplasmic reticulum or nuclear envelope (bound ribosomes).

Endoplasmic Reticulum (ER)

• Smooth ER: Synthesizes lipids, stores calcium ions.

• Rough ER: Studded with ribosomes, secretes glycoproteins.

Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.

• Modifies, sorts, and distributes products of the ER.

Lysosomes

• Membranous sac of hydrolytic (digestive) enzymes.

• Autophagy: Recycles the cell's own organelles and macromolecules.

• Phagocytosis: Some cells engulf other cells, forming a food vacuole that fuses with lysosomes.

Endomembrane System

The endomembrane system consists of interconnected organelles that transfer materials via vesicles.

• Nuclear envelope

• Endoplasmic reticulum

• Golgi apparatus

• Lysosomes

• Plasma membrane

Peroxisomes

• Remove damaging chemical agents from the cell.

• Break down foreign elements and inactivate/destroy them.

Mitochondria

• Sites of cellular respiration, producing ATP using oxygen.

Evolutionary Origins of Mitochondria and Chloroplasts

• Both have similarities with bacteria, supporting the endosymbiont theory.

• Endosymbiont theory: Engulfed cell formed a relationship with the host cell and evolved into mitochondria or chloroplasts.

Plasma Membrane Structure and Function

Properties of Plasma Membrane Proteins

The plasma membrane is a dynamic structure composed of lipids and proteins, providing selective permeability and communication.

• Amphipathic: Proteins have both hydrophobic and hydrophilic regions.

• Proteins are diverse and selective in function.

Role of Carbohydrates on the Plasma Membrane

• Carbohydrates are attached to proteins and lipids, playing roles in cell recognition and signaling.

Types of Molecules Diffusing Through the Membrane

• Small, hydrophobic molecules diffuse most readily through the membrane.

• Large or charged molecules require transport proteins.

Passive and Active Transport

• Passive transport: Movement of substances down their concentration gradient without energy input.

• Active transport: Movement against the concentration gradient, requiring energy (usually ATP).

Key Equations:

• Diffusion is driven by a concentration gradient.

• Energy source for active transport:

Endocytosis Processes

Types of Endocytosis

Endocytosis is the process by which cells internalize substances from their environment.

• Phagocytosis: Cell engulfs large particles or cells (e.g., white blood cells engulfing bacteria).

• Pinocytosis: Cell engulfs extracellular fluid; non-selective in the molecules it brings into the cell.

• Receptor-mediated endocytosis: Highly selective; specific molecules are recognized and internalized by receptors.

Comparison Table: Pinocytosis vs. Receptor-Mediated Endocytosis

Summary Table: Prokaryotic vs. Eukaryotic Cells

Additional Info

• Organelle and membrane proteins are localized to enable specific cellular functions, defining cell types.

• Phagocytic white blood cells are used to study lysosomes due to their active role in engulfing and digesting foreign material.

• Disruption of nuclear pore complexes interferes with mRNA transport out of the nucleus.