Chapter 6 STUDY GUIDE - Cell Structure and Function

Cell Structure and Function

6.1 Microscopy and Cell Study

Microscopy is essential for studying cells, as it allows scientists to observe structures not visible to the naked eye. Different types of microscopes provide varying levels of detail and are suited for different applications.

• Light Microscopy: Uses visible light to illuminate specimens. Suitable for viewing live cells and tissues.

• Electron Microscopy: Uses beams of electrons for much higher resolution, allowing visualization of subcellular structures.

• SEM (Scanning Electron Microscope): Provides detailed images of cell surfaces.

• TEM (Transmission Electron Microscope): Reveals internal cell structures in high detail.

• Atomic Force Microscopy: Investigates atomic and chemical bonds by scanning surfaces with a fine probe.

Key Points:

• Advantages of Light Microscopy: Can view living cells, relatively simple and inexpensive.

• Disadvantages of Light Microscopy: Limited resolution compared to electron microscopes.

• Advantages of Electron Microscopy: High resolution, can visualize organelles and macromolecular complexes.

• Disadvantages of Electron Microscopy: Requires fixed (dead) specimens, complex preparation.

• SEM vs. TEM: SEM images surfaces; TEM images internal structures.

Example: SEM is used to study the surface of pollen grains, while TEM is used to observe the arrangement of organelles within a cell.

6.1.4 Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components for individual study.

• Process: Cells are broken open and spun in a centrifuge to separate organelles by size and density.

• Purpose: Allows scientists to study the function of specific organelles in isolation.

Example: Isolating mitochondria to study their role in cellular respiration.

6.2 Cell Types and Organization

6.2.1 Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

Unique Features:

• Prokaryotes: Simpler structure, smaller size, reproduce by binary fission.

• Eukaryotes: Larger, more complex, compartmentalized functions.

Example: Escherichia coli (E. coli) is a prokaryote; human skin cells are eukaryotes.

6.2.2 Cell Size and Function

Cell size is influenced by the need to efficiently exchange materials with the environment. Surface area-to-volume ratio is a key factor.

• Why are some cells larger than others? Specialized function may require larger size (e.g., egg cells, neurons).

• Plasma Membrane: All cells have a plasma membrane that controls entry and exit of substances.

• Cytoplasm: The region between the plasma membrane and nucleus; contains organelles and cytosol.

Formula:

• Surface area of a sphere: $4\pi r^2$

• Volume of a sphere: $\frac{4}{3}\pi r^3$

Example: Red blood cells are small to maximize surface area for gas exchange.

6.2.3 Plant vs. Animal Cells

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

• Plant Cells: Have cell walls, chloroplasts, and large central vacuoles.

• Animal Cells: Lack cell walls and chloroplasts, have smaller vacuoles.

Table: Comparison of Plant and Animal Cells

Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Vacuole	Large central	Small or absent
Lysosomes	Rare	Common

6.3 Eukaryotic Cell Structure

6.3.1 Nucleus

The nucleus is the control center of the cell, housing genetic material and coordinating activities such as growth and reproduction.

• Nuclear Envelope: Double membrane that encloses the nucleus.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins.

6.3.2 Ribosomes

Ribosomes are molecular machines that synthesize proteins by translating messenger RNA (mRNA).

• Free Ribosomes: Float in the cytosol; make proteins for use within the cell.

• Bound Ribosomes: Attached to the endoplasmic reticulum; make proteins for export or for membranes.

6.4 Endomembrane System

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins.

• Components: Endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, plasma membrane.

• Function: Protein and lipid synthesis, transport, and processing.

Example: The rough ER synthesizes proteins, which are then modified in the Golgi apparatus and transported to their destination.

6.5 Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells.

• Mitochondria: Site of cellular respiration; converts glucose into ATP.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Endosymbiotic Theory: Proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotes.

Evidence for Endosymbiotic Theory:

• Both have their own DNA and ribosomes.

• Double membranes.

• Reproduce independently within the cell.

6.6 Cytoskeleton

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

• Microtubules: Hollow tubes; maintain cell shape, form spindle fibers, and serve as tracks for organelle movement.

• Microfilaments: Thin rods; involved in muscle contraction and cell movement.

• Intermediate Filaments: Provide mechanical support.

Motor Proteins: Such as dynein and kinesin, move along cytoskeletal tracks to transport vesicles and organelles.

6.7 Cell Connections and Communication

Cells interact and communicate through specialized structures and signaling pathways.

• Cell Junctions: Structures that connect cells to each other or to the extracellular matrix.

• Types of Cell Junctions:

Junction Type	Function
Tight Junctions	Seal cells together to prevent leakage
Desmosomes	Anchor cells together
Gap Junctions	Allow communication between cells
Plasmodesmata (plants)	Channels for transport and communication

6.8 Cell Function and Integration

Cells function as integrated units, with all parts working together to maintain homeostasis and perform life processes.

• Coordination: Organelles and structures must function together for the cell to survive and adapt.

• Diseases: Malfunction of any component can lead to disease (e.g., cystic fibrosis from faulty ion channels).

Example: In diabetes, improper function of insulin receptors affects glucose uptake by cells.