A Tour of the Cell

Biologists Use Microscopes & Biochemistry to Study Cells

Understanding cells requires specialized tools and techniques. Microscopy and cell fractionation are foundational methods for visualizing and analyzing cellular structures and functions.

• Microscopy revolutionized biology by allowing scientists to observe cells and their components.

• Key historical figures include Robert Hooke (first to observe cell walls) and Antoni van Leeuwenhoek (developed advanced microscopes and observed 'animalcules').

Light Microscopy (LM)

Light microscopes use visible light to magnify specimens, making them essential for basic cell biology.

• Magnification up to ~1,000x life-size.

• Glass lenses refract light to magnify images.

• Most cells are too small to be seen by the unaided eye.

Parameters and Limitations of Microscopy

Three main parameters determine the effectiveness of a microscope:

• Magnification: Ratio of image size to actual size.

• Resolution: Clarity of the image; the minimum distance two points can be distinguished as separate.

• Contrast: Difference in brightness between light and dark areas.

Microscopy Techniques and Advances

Various techniques enhance contrast and allow for the visualization of cell components. Most organelles are too small to be resolved by light microscopy alone.

• Staining and labeling improve visibility of structures.

• Recent advances include fluorescent markers and confocal microscopy, improving resolution to 10-20 µm.

Electron Microscopes

Electron microscopes use beams of electrons for much higher resolution than light microscopes, allowing visualization of subcellular structures.

• Scanning Electron Microscope (SEM): Focuses electrons on the surface, producing 3D images.

• Transmission Electron Microscope (TEM): Focuses electrons through a specimen, revealing internal structures.

Cell Fractionation

Cell fractionation separates cellular components by size using centrifugation, allowing the study of individual organelles and their functions.

• Homogenization breaks cells apart.

• Centrifugation separates components by density and size.

Cell Types and Their Organization

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on structural differences.

• Prokaryotic cells: Bacteria and Archaea; lack a nucleus and membrane-bound organelles; DNA in nucleoid region; generally smaller (1-5 µm).

• Eukaryotic cells: Protists, fungi, animals, and plants; DNA in a nucleus; contain membrane-bound organelles; generally larger (10-100 µm).

• All cells share a plasma membrane, cytosol, chromosomes, and ribosomes.

Structural Features of Eukaryotic Cells

Eukaryotic cells have complex internal structures, including organelles that compartmentalize functions.

• Animal and plant cells share many organelles but also have unique features (e.g., chloroplasts in plants).

Plasma Membrane Structure and Function

The plasma membrane is a selective barrier composed of a phospholipid bilayer, allowing passage of oxygen, nutrients, and waste.

• Structure: Double layer of phospholipids with embedded proteins.

Surface Area to Volume Ratio

The ratio of surface area to volume is critical for cell function, as it affects the efficiency of material exchange.

• Smaller cells have a greater surface area relative to volume, facilitating diffusion.

Internal Structures and Organelles

Nucleus: Location of Genetic Instructions

The nucleus is the control center of eukaryotic cells, containing most of the cell's DNA organized into chromosomes.

• Enclosed by a double-membrane nuclear envelope with nuclear pores for molecular transport.

• Contains chromatin (DNA + proteins) and the nucleolus (site of rRNA synthesis).

Ribosomes: Protein Factories

Ribosomes are complexes of rRNA and protein that synthesize proteins using genetic information from DNA.

• Free ribosomes function in the cytosol; bound ribosomes are attached to the endoplasmic reticulum or nuclear envelope.

The Endomembrane System

The endomembrane system regulates protein trafficking and performs metabolic functions. It includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Organelles are connected directly or via vesicle transfer.

• Functions include protein synthesis, transport, lipid metabolism, and detoxification.

Endoplasmic Reticulum (ER)

The ER is a biosynthetic factory with two regions: smooth ER (lacks ribosomes) and rough ER (studded with ribosomes).

• Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Rough ER: Synthesizes and secretes proteins and glycoproteins, distributes proteins via vesicles, and is a membrane factory for the cell.

Golgi Apparatus: Shipping & Receiving Center

The Golgi apparatus consists of flattened sacs (cisternae) and modifies, sorts, and packages products from the ER for transport.

• Produces glycolipids and manufactures certain macromolecules.

Lysosomes: Digestive Compartments

Lysosomes are membranous sacs containing hydrolytic enzymes for digesting macromolecules.

• Participate in phagocytosis (engulfing food particles) and autophagy (recycling cell components).

• Acidic internal environment is essential for enzyme activity.

Vacuoles: Diverse Compartments

Vacuoles are large vesicles with varied functions, especially prominent in plant and fungal cells.

• Food vacuoles (formed by phagocytosis), contractile vacuoles (pump excess water), and central vacuoles (storage and support in plants).

Endomembrane System Review

The endomembrane system integrates the synthesis, modification, and transport of cellular products, ensuring cellular organization and function.

Energy-Related Organelles

Evolutionary Origins of Mitochondria & Chloroplasts

The endosymbiont theory proposes that mitochondria and chloroplasts originated as prokaryotic cells engulfed by ancestral eukaryotes.

• Both organelles have double membranes, their own DNA, and ribosomes, and can grow and reproduce independently within cells.

Mitochondria: Chemical Energy Conversion

Mitochondria are the sites of cellular respiration, converting oxygen and organic molecules into ATP.

• Structure: Smooth outer membrane, highly folded inner membrane (cristae), intermembrane space, and mitochondrial matrix (contains DNA and ribosomes).

Chloroplasts: Site of Photosynthesis

Chloroplasts, found in plants and algae, capture light energy to drive photosynthesis.

• Structure: Inner and outer membranes, thylakoids (stacked into grana), and stroma (internal fluid with DNA, ribosomes, and enzymes).

• Other plastids include amyloplasts (starch storage) and chromoplasts (pigment storage).

Peroxisomes: Oxidative Organelles

Peroxisomes are single-membrane organelles containing enzymes that detoxify harmful substances and break down fatty acids.

• Oxidase converts toxins to hydrogen peroxide (H2O2), which is then converted to water and oxygen by catalase.

Cytoskeleton and Cell Structure

Cytoskeleton: Support and Motility

The cytoskeleton is a network of protein fibers that organizes cell structure, provides mechanical support, and facilitates movement.

• Anchors organelles and molecules, maintains cell shape, and enables cellular motion via motor proteins.

Components of the Cytoskeleton

• Microtubules: Thickest; hollow rods made of tubulin; shape/support cell, guide organelle movement, separate chromosomes during division.

• Microfilaments (Actin Filaments): Thinnest; solid rods of actin; bear tension, support cell shape, involved in muscle contraction and cell movement.

• Intermediate Filaments: Middle diameter; reinforce cell shape, anchor organelles, more permanent structures.

Cell Walls and Extracellular Structures

Cell Walls of Plants

Plant cell walls are extracellular structures that provide protection, maintain shape, and prevent excessive water uptake.

• Composed mainly of cellulose microfibrils embedded in a matrix of polysaccharides and proteins.

• Layers: Primary cell wall (thin/flexible), middle lamella (pectin-rich glue), secondary cell wall (thick, strong, in mature cells).

Extracellular Matrix (ECM) of Animal Cells

The ECM is a network of glycoproteins (collagen, proteoglycans, fibronectin) outside animal cells, providing structural support and mediating cell communication.

• Integrins are cell-surface receptors that connect the ECM to the cytoskeleton.

Cell Junctions

Cell junctions allow neighboring cells to adhere, interact, and communicate.

• Plasmodesmata: Channels in plant cell walls for transport and communication.

• Tight junctions: Prevent leakage between animal cells.

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap junctions: Channels for communication between animal cells.

Emergent Properties of Cells

Cellular functions arise from the coordinated activity of all cellular components, demonstrating that the cell is greater than the sum of its parts.

• Example: Macrophage function requires the cytoskeleton, lysosomes, and plasma membrane working together.