2A Cell Structure & Function - Copy

Foundation in Science: Cell Structure & Function

Learning Objectives

• State the cell theory

• Compare and contrast the structures of prokaryotic and eukaryotic cells.

• Describe the structure and function of organelles.

• Explain apoptosis.

An Introduction to Cells

Definition of a Cell:

The basic structural unit which serves the fundamental functions of an organism.

Basic Features of All Cells:

• Plasma Membrane: The lipid bilayer that encloses the cell and regulates the movement of substances in and out.

• Cytoplasm: The jelly-like substance within the cell that holds organelles in place and enables cellular processes.

• DNA: The genetic material that carries instructions for the cell's structure and function.

• Ribosomes: The cellular structures responsible for protein synthesis, which can be found free-floating in the cytoplasm or attached to the endoplasmic reticulum.

Structure of the Plasma Membrane

Composition:

• Lipid Bilayer: Primarily composed of phospholipids, which create a semi-permeable membrane.

• Proteins: Embedded within the lipid bilayer; function as channels, receptors, and markers for cell identification.

• Carbohydrate Chains: Attached to proteins and lipids on the extracellular side, aiding in cell communication and recognition.

Function:

• Acts as a barrier to protect the cell.

• Facilitates selective transport and communication through protein channels and receptors.

Cell Theory

Historical Background

• Robert Hooke (1665): First to observe and name cells, he described plant tissue as resembling the rooms (cells) in a monastery.

• Mathias Schleiden (1838) & Theodor Schwann (1839): Proposed that all organisms are made up of cells, forming the foundation of cellular biology.

• Rudolph Virchow (1855): Stated that all cells arise from existing cells, emphasizing that cells reproduce.

Components

• All organisms are composed of cells.

• The cell is the smallest unit of life.

• All cells arise only from pre-existing cells.

• Concept: All modern cells are descendants of the first living cells, having evolved over billions of years.

Microscopy

• Light Microscopes: Use visible light to magnify objects; can resolve structures as small as 200 nm apart.

• Electron Microscopes: Utilize electron beams to achieve much higher resolutions, capable of resolving structures as small as 0.2 nm, allowing for detailed visualization of cellular components.

Cell Size

Typical Sizes:

• Cells generally range from 5 to 50 µm in diameter; the size is critical for maintaining efficient transport and communication within the cell.

Understanding Micrometers:

• 1 micrometer (µm) = 1/1,000,000 m.

Comparison of Sizes:

Visual comparison of various cell sizes, such as those of bacteria, plant cells, and animal cells, demonstrates the diversity of cell types and functions.

Types of Cells

Prokaryotic Cells

• Characteristics: Unicellular organisms lacking a true nucleus; DNA is located in a nucleoid region not enclosed by a membrane.

• Organelles: Do not have membrane-bound organelles, and their cellular functions occur in the cytoplasm. Strong cell walls provide structure.

• Size: Typically range from 0.5-10 µm.

• Examples: Bacteria and Archaea, which exhibit a variety of forms and abilities.

Eukaryotic Cells

• Characteristics: Can be unicellular or multicellular; contain a true nucleus and membrane-bound organelles that perform specialized functions.

• Size: Typically range from 10-100 µm, accommodating their complex structures.

• Examples: Include protists, fungi, animals, and plants.

Unicellular vs Multicellular Eukaryotes

• Unicellular Eukaryotes: Organisms like protists that function independently as single cells.

• Multicellular Eukaryotes: Organisms such as plants and animals that are composed of many cells working together with specialized functions.

Structure of Eukaryotic Cells

Eukaryotic Animal Cell

• Composed of a thin plasma membrane enclosing a cytoplasmic matrix; includes a nucleus that houses the nucleolus and chromatin fibers.

• Major Organelles Include:

  • Mitochondrion: ATP production via cellular respiration.

  • Endoplasmic Reticulum (ER): Rough ER studded with ribosomes for protein synthesis and smooth ER for lipid synthesis.

  • Lysosome: Contains enzymes for digestion.

  • Centrioles: Involved in cell division.

Eukaryotic Plant Cell

• Contains a rigid cell wall, chloroplasts for photosynthesis, and a large central vacuole for storage and maintaining turgor pressure.

• Besides typical organelles present in animal cells, plant cells lack centrioles and incorporate additional structures like plasmodesmata for cell communication.

Organelles

Nucleus

• Control Center: Houses the majority of the cell’s genetic material.

• Nuclear Envelope: Double membrane with pores allowing regulated transport of molecules between the nucleus and cytoplasm.

Ribosomes

• Composed of ribosomal RNA (rRNA) and proteins, key for translating mRNA into proteins. Found freely in the cytoplasm or bound to the rough ER, each serves different roles in protein synthesis.

Endomembrane System

• Components: Include the nuclear envelope, endoplasmic reticulum (smooth and rough), Golgi apparatus, lysosomes, vacuoles, and the plasma membrane. These structures work together to process and transport proteins and lipids within the cell.

Golgi Apparatus

• Functions as a central hub for modifying, sorting, and packaging proteins and lipids received from the ER into transport vesicles for distribution.

Lysosomes

• Contain hydrolytic enzymes for breaking down macromolecules. Play crucial roles in intracellular digestion (phagocytosis) and recycling cellular components (autophagy).

Vacuoles

• Storage organelles in plant and fungal cells, hold food, water, salts, pigments, and waste products.

Energy-Converting Organelles

Mitochondria

• Known as the powerhouse of the cell, essential for ATP production through aerobic respiration. It has a double membrane, with the inner membrane extensively folded into structures called cristae, which increase surface area for reactions.

Chloroplasts

• Organelles in plants and some protists that perform photosynthesis. They contain chlorophyll and other pigments, enabling the conversion of light energy into chemical energy in the form of glucose.

Cytoskeleton

• A dynamic network of fibers providing structural support, shape, and facilitating intracellular transport. It comprises:

  • Microtubules: Tubular structures that support the cell and assist in cell division.

  • Microfilaments: Thin fibers involved in muscle contraction and cell motility.

  • Intermediate Filaments: Provide mechanical stability to the cell.

Extracellular Structures

Cell Walls

• Provide additional support and protection. In plants, they're primarily made of cellulose fibers organized into multiple layers including the primary wall, middle lamella for adhesion, and sometimes a secondary wall for added strength.

Plasmodesmata

• Channels embedded in plant cell walls, allowing for direct communication and transport of materials between adjacent cells.

Extracellular Matrix (ECM) in Animals

• Composed of glycoproteins (such as collagen and fibronectin) that support cell structure, facilitate cellular communication, and aid in tissue formation.

Intercellular Junctions

• Structures that allow cells to adhere to one another and communicate:

  • Tight Junctions: Seal cells tightly together, preventing leakage of fluids.

  • Desmosomes: Anchoring junctions that hold cells together in tissues that experience stress.

  • Gap Junctions: Allow the passage of ions and small molecules, enabling communication between neighboring cells.

Cell Death

Types of Cell Death

• Apoptosis: A programmed process of cell death essential for eliminating unnecessary or damaged cells without harming surrounding tissues.

• Necrosis: Cell death resulting from injury leading to inflammation and damage to nearby cells due to enzyme release.

Review Questions

1. Which structure aids in maintaining homeostasis in the cell? (Cell Membrane)

2. Where would water and dissolved minerals be stored? (Vacuole)

3. What provides rigidity in plant cells? (Cell Wall)

4. What structure is associated with the destruction of worn-out organelles? (Lysosome)

5. Which structure is found ONLY in animal cells? (Centrioles)

6. Which organelle is primarily responsible for protein synthesis? (Ribosome)

7. Which structure functions in intracellular transport? (Endoplasmic Reticulum)

8. What is true about the cell wall? (Rigid)

9. Which cell organelle does cyanide influence primarily? (Mitochondria)

10. Which structure is known as the control center of the cell? (Nucleus)