Lecture 7: Cell Structure, Function, and Microscopy

Review Concepts

This section reviews key concepts from previous material, focusing on acid-base behavior and protein structure.

• Acidic Molecules: Molecules like C2H5COOH behave as acids when added to aqueous solutions, donating protons (H+).

• Hydrophobic Interactions: These interactions influence protein tertiary structure by causing nonpolar side chains to aggregate away from water.

Microscopy

Types of Microscopes

Microscopes are essential tools for visualizing cells and their components. Different types offer varying levels of resolution and capabilities.

• Electron Microscopy: Uses a beam of electrons and electromagnetic lenses for imaging. Offers up to 0.2 nm resolution, about 1,000 times better than light microscopy.

• Light Microscopy: Uses visible light and stains to visualize living cells and their movement. Staining can kill cells but enhances visibility of structures like nuclei.

Electron Microscopy Types

• Scanning Electron Microscopy (SEM): Used for examining the external surface of cells, providing detailed images of cell morphology.

• Transmission Electron Microscopy (TEM): Used for viewing internal cell structures, such as organelles and membranes.

Cell Surface Area and Volume

Surface Area to Volume Ratio

The surface area-to-volume (SA/V) ratio is critical for cell survival, as it affects nutrient supply and demand.

• Formula:

• If SA/V ratio < 1, the cell cannot survive due to insufficient nutrient exchange.

Prokaryotes vs. Eukaryotes

Comparison of Cell Types

Cells are classified as prokaryotic or eukaryotic based on their structure and complexity.

Animal vs. Plant Cells

• Animal Cells: Separate incompatible reactions, cluster enzymes, and minimize diffusion distances for substrates.

• Plant Cells: Contain chloroplasts, central vacuole, and cell wall (absent in animal cells). Site of photosynthesis.

Cell Structure and Organelles

Bacterial Cell Components

• Ribosomes: Synthesize polypeptides (proteins).

• Cytoplasm: Site of metabolism.

• Pili: Allow attachment to surfaces and other bacteria.

• Flagella: Enable motility in certain bacteria.

• Nucleoid: Region where DNA is found.

• Plasma Membrane: Encloses cytoplasm.

• Cell Wall: Provides support and protection.

• Glycocalyx: Outer gelatinous covering for protection.

Universal Cell Features

• DNA: Genetic material is centrally located.

• Ribosomes: Synthesize proteins.

• Cytosol: Fluid filling the cell, site of metabolic processes.

• Cytoskeleton: Provides cell shape, motility, and transport.

• Plasma Membrane: Separates cell interior from exterior.

Cytosol and Cytoskeleton

Cytosol

The cytosol is the intracellular fluid found in all cells, containing water, molecules, and ions. It is the site of metabolic processes, signal transduction, and protein synthesis.

• Hydrophilic substances dissolve in cytosol.

• Supports metabolic activities and protein production.

Cytoskeleton

The cytoskeleton is a network of protein filaments that supports cell shape, anchors organelles, and provides pathways for material transport.

• Composed of distinct protein subunits.

• Includes microfilaments, intermediate filaments, and microtubules.

Microfilaments (Actin Filaments)

• Diameter: 7 nm

• Structure: Thin, entwined threads, often bundled

• Protein subunit: Actin

• Functions: Maintains cell shape, provides cell mobility, involved in cell division (cleavage furrow)

Intermediate Filaments

• Diameter: 8-12 nm

• Structure: Stretchy protein cables (laminin, vimentin, keratin, neurofilaments)

• Functions: Maintain cell shape, anchor nucleus and organelles, form nuclear lamina

Microtubules

• Diameter: 25 nm (largest)

• Structure: Hollow tubes, protein subunits α- and β-tubulin

• Functions: Maintain cell shape, provide cell motility (flagella, cilia), organize chromosome movement during cell division, serve as roadways for intracellular transport

Motor Proteins

• Myosin: Associated with actin filaments

• Kinesin and Dynein: Associated with microtubules; involved in cargo transport and organelle repositioning

Eukaryotic Flagella and Cilia

• Flagella are longer than cilia; usually 1-2 per cell

• Cilia are shorter and more numerous

• Both have the same internal structure (axoneme): 9 microtubules around the outside, 2 central microtubules inside

• Dynein bends microtubules for movement

Organelles and the Endomembrane System

Secretory Pathway

Eukaryotic cells have a complex system for protein synthesis and trafficking.

1. Nucleus

2. Endoplasmic Reticulum (ER)

3. Golgi Apparatus

Endoplasmic Reticulum (ER)

• Composed of phospholipid membranes

• Contains ER lumen (internal space) and cisternae (folds)

• Rough ER: Closer to nucleus, site of protein synthesis (ribosomes attached)

• Smooth ER: Makes carbohydrates and lipids, stores calcium ions, detoxifies substances (e.g., alcohol)

Golgi Apparatus

• Stack of flattened membranes

• Three compartments: CIS (receiving), MEDIAL (processing/glycosylation), TRANS (shipping)

• Functions: Protein processing, sorting, secretion

Endomembrane System Components

• Nucleus

• Nuclear envelope

• Lysosome (animal cells)

• Peroxisome

• Vacuole (plant cells)

• Endoplasmic reticulum

• Golgi apparatus

• Plasma membrane

Nucleus and Nucleic Acids

Nucleus

The nucleus is the core organelle containing genetic material and is the site of ribosomal RNA synthesis.

• Chromatin: Complex of DNA and protein

• Chromosomes: DNA is organized into chromosomes

• Nuclear envelope: Double phospholipid membrane separating nucleus from cytoplasm

• Nuclear pores: Passageways for RNA and other molecules

Nucleic Acids

• DNA: Stores hereditary information; cannot leave nucleus

• RNA: Carrier of genetic information; can leave nucleus

• Nucleotides: Monomers of nucleic acids, composed of pentose sugar, nitrogenous base, and phosphate group

Pentose Sugars

• DNA: Contains H at the 2' position

• RNA: Contains OH at the 2' position

Nitrogenous Bases

• Purines: Adenine, Guanine

• Pyrimidines: Cytosine, Thymine, Uracil

• Purines always pair with pyrimidines

• Pyrimidines are smaller

Nucleic Acid Bonding

• Phosphodiester Bonds: Strong covalent bonds linking nucleotides in a polymer

• Hydrogen Bonds: Hold nitrogenous bases together between strands

Other Organelles

Lysosomes and Peroxisomes

• Lysosomes: Contain enzymes for degrading proteins, lipids, nucleic acids, and carbohydrates; important for macromolecule breakdown and recycling

• Peroxisomes: Important for fatty acid metabolism and breakdown of reactive oxygen species

Semiautonomous Organelles

• Have their own DNA, can grow and reproduce independently

• Divide by binary fission (genome replicates, then organelle divides)

• Examples: Mitochondria (site of cellular respiration, energy conversion), Chloroplasts (site of photosynthesis in plants)

Summary Table: Key Cell Structures and Functions

Additional info: Some explanations and table entries have been expanded for clarity and completeness based on standard General Biology curriculum.