Bcm 202

BCM 202: General Medical Biochemistry II

The Cell

Theory, Types, Major Components and Diversity

The Cell Theory

• Definition: A widely accepted explanation that states that:
    - All living organisms are composed of cells.
    - New cells arise from the division of existing cells.
    - Cells are the fundamental building units of life.

• Historical Background:
    - Proposed initially by Theodor Schwann and Matthias Jakob Schleiden in 1838.
    - Marked a significant advancement in biology, directly affecting how living processes are examined at the cellular level.

• Microscopy and Observations:
    - The invention of the microscope in the 16th century was pivotal in allowing people to observe cells, increasing interest in this previously invisible aspect of life.

Origin of the term "Cell"

• First used by Robert Hooke (1635–1703) when examining cork with a simple microscope.

• Found structures that resembled small blocks (cells), reminiscent of the cells occupied by monks.

• Today, the term "cell" defines a microscopic unit of life, separated from its environment by a cell membrane.

Spontaneous Generation vs. Cell Theory

• Most biologists believe in spontaneous generation of life from inanimate matter, but details are unknown, and it is accepted that these processes took extensive time.

• Rudolf Virchow states “omnis cellula e cellula” meaning all cells arise from pre-existing cells, refuting the concept of spontaneous generation occurring within typical lifetimes.

Historical Discoveries in Cell Biology

• Discovered by Antonie van Leeuwenhoek starting in 1673:
    - Blood cells, spermatozoa, and a variety of unicellular organisms termed "animalcules."
    - These discoveries broadened understanding of life but initially did not reveal insights on their uniformity.

• Significant delay in formulating cell theory due to:
    - Inferior quality of early microscopes.
    - Enduring ancient beliefs regarding the definition of a fundamental living unit.

Key Discoveries in the 1830s

• Enhanced microscopes significantly contributed to the progression of cell theory with three major discoveries:
    1. The nucleus was observed by Robert Brown in 1833, identified as a constant component of plant cells.
    2. Recognition of nuclei in animal cells.
    3. Awareness of a living substance known as protoplasm capable of active movements, particularly in plants.

• Post these discoveries, cells could no longer be dismissed as mere voids in plant tissue, as they were understood to contain living materials.

Contribution of Schleiden and Schwann (1838-1839)

• Matthias Jakob Schleiden noted:
    - Lower plants consist of a single cell; higher plants consist of many cells.

• Theodor Schwann extended the theory to animals, bridging studies between botany and zoology.

• In 1839, they declared:
    - Cells are the elementary particles of both organisms and classified organisms as unicellular or multicellular.

• Their description of cells included: membrane, nucleus, and cytoplasm, leading to a comprehensive understanding of biological structure.

Structures and Functions of Major Cell Components

The Cell: Basic Unit of Life

• The cell is both the structural and functional unit of life, seen as the fundamental unit of biological activity—concept clarified by Schleiden and Schwann (1838).

• Post 1940, deeper complexities of cell structure were uncovered.

Prokaryotic vs. Eukaryotic Cells

• Division: Living cells fall into two main categories:
    1. Prokaryotes (
       - Greek: pro– before; karyon – nucleus)
       - Characteristics:
         - Lack a defined nucleus.
         - Simplistic structure (e.g., bacteria).
    2. Eukaryotes (
       - Greek: eu– true; karyon – nucleus)
       - Characteristics:
         - Possess a well-defined nucleus.
         - More complex structure and function found in higher organisms (plants and animals).

Comparison of Prokaryotic and Eukaryotic Cells

Eukaryotic Cell Characteristics

• The human body contains approximately 10^{14} cells, encompassing around 250 distinct types:
    - Examples include:
      - Erythrocytes (red blood cells)
      - Nerve cells
      - Muscle cells
      - Pancreatic cells

• Dimensions: Eukaryotic cells generally range from 10 to 100 µm.

• Differences between plant and animal cells include:
    - Plant Cells:
      - Possess a rigid cell wall mainly composed of cellulose.
      - Contains chloroplasts for photosynthesis.
    - Animal Cells:
      - Cells lack a rigid cell wall and chloroplasts.

Cell Organelles Overview

Animal Cell Components:

• Intermediate filaments, Ribosomes, Rough endoplasmic reticulum, Mitochondria, Plasma membrane, Nucleus, Nucleolus, Chromatin, Golgi apparatus, Golgi vesicle, Cytoplasm, Vacuole, Microtubule, Centrosome, Microfilament, Lysosome, Smooth endoplasmic reticulum, Secretory vesicle, Peroxisome.

Prokaryotic Cell Components:

• DNA (Nucleoid), Mesosome, Plasma membrane, Cell wall, Capsule, Ribosomes, Cytoplasm, Bacterial flagellum.

Plasma Membrane

• Alternatively known as Cell Membrane or Cytoplasmic Membrane.

• Selectively Permeable membrane formed from a lipid bilayer and proteins.

• Present in both plant and animal cells, providing:
    - Structural shape
    - Protection to internal contents

• According to the fluid mosaic model, plasma membranes consist of a lipid bilayer with embedded proteins, allowing movement and cellular communication.

Functions of the Plasma Membrane

• Apart from guiding cell contents, regulates movement of substances into and out of the cell.
    - Engages in passive and active transport,
    - Maintains homeostasis despite external environmental changes.

Cytoplasm

• A viscous substance between membrane and nucleus found in both plant and animal cells.

• Composed of:
    - Water, organic and inorganic compounds

• Essential in housing cell organelles which contain enzymes critical for regulating metabolic activities.

Nucleus

• Largest organelle, enveloped in a double membrane known as the nuclear envelope.

• Outer membrane connects to the endoplasmic reticulum.

• Contains nuclear pores (about 90 nm in diameter) for material exchange with surrounding cytoplasm.

• Houses DNA, organized with histones into nucleosomes.

• DNA organization leads to the formation of chromatin fibers, resulting in human chromosomes (Total 46 chromosomes).

• The nucleolus, rich in ribosomal RNA, synthesizes RNA entering the cytoplasm through nuclear pores.

• The nucleoplasm consists of various enzymes, including DNA and RNA polymerases.

Mitochondria

• Often referred to as the cell's powerhouses, they are crucial for cellular respiration and energy metabolism.

• Dimensions: Typically 1.0 x 3 µm, with around 2,000 mitochondria constituting about 1/5th of total cell volume.

• Comprised of a double membrane:
    - Outer membrane: Smooth and enveloping.
    - Inner membrane: Folded into cristae, increasing surface area.

• Functionality includes energy metabolism and ATP production, with enzymes for carbohydrate and lipid metabolism located in the matrix, supporting ATP production through oxidative phosphorylation.

• Contains circular double-stranded DNA (mtDNA), RNA, and ribosomes for protein synthesis, with about 10% of mitochondrial proteins synthesized internally.

Endoplasmic Reticulum (ER)

• Network of membrane-enclosed spaces throughout the cytoplasm, facilitating various cellular functions.

• Types:
    - Rough Endoplasmic Reticulum (RER):
      - Studded with ribosomes, appearing granular.
      - Critical for protein biosynthesis.
    - Smooth Endoplasmic Reticulum (SER):
      - Lacks ribosomes, synthesizes lipids, metabolizes drugs, and supplies Ca^{2+} ions for cellular activities.

Golgi Apparatus

• Unique cluster of vesicles (dictyosomes) in eukaryotic cells.

• Processes newly synthesized proteins by adding carbohydrates, lipids, or sulfate groups for protein transport, with certain proteins being enclosed in vesicles for secretion post-modification.

• Also participates in membrane synthesis for organelles (peroxisomes, lysosomes).

Lysosomes

• Spherical vesicles with a single membrane, functioning as the cell's digestive system.

• Contains hydrolases (digestive enzymes) responsible for degrading cellular substances (
     - Examples:
       - α-glucosidase (glycogen),
       - Cathepsins (proteins),
       - Lipases (lipids),
       - Ribonucleases (RNA)).

• Maintain dynamic conditions of cellular components and recycle degraded products.

• Accumulation of residual products known as lipofuscin, implicated in aging.

Peroxisomes

• Also termed microbodies, single-membrane organelles involved in detoxification (via catalase) and oxidation of fatty acids.

• Particularly in plants, specialized forms known as glyoxysomes enable the glyoxylate cycle.

Cytosol and Cytoskeleton

• The cytosolic matrix houses enzymes, metabolites, and salts in a gel-like medium.

• The cytoskeleton, a network of protein filaments, determines cell structure and organization.
    - Types of Filaments:
      - Microtubules
      - Actin filaments
      - Intermediate filaments

Vacuoles

• Defined as fluid-filled spaces within cells, primarily for storage, can hold:
    - Enzymes
    - Waste products
    - Water
    - Nutrients.

• Assist in waste export and maintaining internal pressure, with plant cells typically housing larger vacuoles.

Cilia and Flagella

• Cilia: Small, hair-like projections that move fluids or the cell itself, functioning similarly to oars.

• Flagella: Larger structures primarily responsible for cell movement.

• Both cilia and flagella consist of:
    - An axoneme structure (9 pairs of peripheral microtubules and central microtubules).
    - Central tubules interconnected by bridges and embedded in a central sheath.

Constancy/Uniformity in All Cells

• Despite cellular diversity, all living cells exhibit fundamental characteristics:
    - Genetic Material: All utilize DNA for information storage and RNA/proteins for execution of functions.
    - Basic Machinery: Commonality in metabolism, protein synthesis, energy conversions (e.g., ATP).
    - Plasma Membrane: All cells enclosed by a membrane regulating transport.
    - Common Ancestry: Universal traits indicate descent from a common ancestral cell.