Biology: Introduction to Cells and Life

What is Biology?

• The study of life and living organisms
• Involves the study of the structure, function, growth, evolution, and distribution of all living organisms

Characteristics of Living Organisms

• Made up of cells
• Respond to stimuli
• Grow and develop
• Reproduce
• Metabolize
• Evolve

Cell Biology

• Cell is the fundamental structural and functional unit of life
• All organisms are composed of one or more cells
• Cells are the basic units of life

Why is Cell considered a Structural Unit of Life?

• All organisms are composed of cells
• Cells are the building blocks of body
• Cells are organized to form tissues, organs, and systems
• Division of labor increases efficiency

Why is Cell considered a Functional Unit of Life?

• The activity of an organism is the sum total of all activities of its cells
• All cells possess the same genetic material, though mature cells may become specialized for certain functions
• Organelles perform different functions and control the activity of the cell

Discovery of Cell

• Zachris Janssen built the first microscope in 1590
• Galileo modified the microscope in 1610
• Robert Hooke observed honeycomb-like structures in cork and coined the term "cell" in 1665
• Anton Van Leeuwenhoek discovered bacteria, protozoans, and sperm cells in 1673

Cell Theory

• Proposed by Matthias Schleiden and Theodor Schwann in 1838-1839
• States that all organisms are composed of cells and their products
• Cells are the structural and functional units of all organisms
• Limitations: did not explain how new cells were formed

Modern Cell Theory

• Formulated by Rudolf Virchow in 1855
• States that all cells arise from pre-existing cells
• All organisms are composed of cells and their products
• Activities of an organism are the outcomes of the sum total of activities of its constituent cells

Cell Size and Shape

• Cells vary in size and shape
• Mycoplasma is the smallest cell, while ostrich egg cells are the largest
• Human neuron cells are the longest, and human RBCs are the smallest

Prokaryotic Cell

• General characteristics: small size, kingdom Monera, and bacteria are common members
• Structures: cell envelope, cell wall, cell membrane, and cytoplasm
• Functions: protection, support, and selective transport

Cell Wall

• Below glycocalyx and above cell membrane
• Rigid covering provides strength and support
• Made up of peptidoglycan or murein

Gram Staining

• Differentiates between Gram-positive and Gram-negative bacteria
• Gram-positive bacteria retain blue color, while Gram-negative bacteria do not

Cell Membrane

• Innermost to cell envelope
• Composed of phospholipid bilayer and proteins
• Functions: protective covering, selective transport, and formation of cytoplasmic structures

Cytoplasm

• Crystallo-colloidal complex
• High content of water
• Forms protoplast (CM + cytoplasm)
• Granular due to ribosomes

Cytoplasmic Structures

• Mesosomes
• Chromatophores
• Ribosomes
• Nucleoid
• Plasmid
• Inclusion bodies

Eukaryotic Cell

• Cell wall (non-living)
• Protoplasm (living)
• Cytoplasm
• Nucleus
• Cell membrane

Differences between Plant and Animal Cells

• Size: plant cells are larger

• Cell wall: present in plant cells, absent in animal cells

• Shape: plant cells have a definite shape, while animal cells often change shape

• Vacuole: large central vacuole in plant cells, smaller vacuoles in animal cells

• Nucleus: peripheral nucleus in plant cells, central nucleus in animal cells

• Plastid: present in plant cells, absent in animal cells### Cell Membrane Characteristics

• Also known as Plasma membrane, Biomembrane, or Plasmalemma

• Discovered by Schwann in 1838

• Tripartite/Trilaminar structure: 3 layered with outer dark, middle transparent, and inner dark layers

• Size/Thickness: Average 75 Å, ranging from 50 Å to 100 Å

• Dynamic, quasi-fluid, film-like, and semi-permeable

Composition

• Lipids (20%-79%), proteins (20%-70%), and carbohydrates (1%-5%)
• Lipid and protein composition can vary, e.g., human RBC: 52% protein and 40% lipids, myelinated neuron: 20% protein and 80% lipids

Fluid Mosaic Model

• Proposed by Singer and Nicholson in 1972
• Cell membrane shows a quasi-fluid structure with a mosaic pattern
• Protein "icebergs" in a sea of lipids

Protein Types

• Intrinsic (integral) proteins: 70% of the membrane, cannot be extracted without breaking the membrane
• Extrinsic (peripheral) proteins: 30% of the membrane, superficially placed and removable with a slight change in pH

Functions of Cell Membrane

• Compartmentalization
• Formation of intercellular junctions
• Secretion
• Semi-permeability
• Transport (passive and active)

Transport

• Passive transport: without energy, along the gradient, e.g., neutral solutes
• Active transport: with energy, against the gradient, e.g., Na+/K+ pump
• Carrier proteins are required for transport of polar solutes, large molecules, charged ions, and amino acids

Bulk Transport

• Enclosure of material in vesicles of membrane
• Carrier vesicles formed by membrane invagination
• Types: Pinocytosis (cell drinking) and Phagocytosis (cell eating)

Membrane Fluidity

• Ability of membrane components to move
• Movements: rotational, lateral diffusion, and flip-flop
• Degree of fluidity: should be optimized, as excessive or low fluidity is detrimental
• Regulation of fluidity: cholesterol acts as a fluidity buffer