Cell Biology: Study of Cells

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Cell Biology

• Cell biology, also known as cytology, is the study of cells

Preface

• The book presents cell biology concepts in a unique style, drawing from decades of teaching and research experience
• It aims to provide a compact, concise, and explicit work on the subject
• The book is intended for pre-degree and undergraduate students of Cell Biology
• It aims to be useful for teachers and students of higher degrees
• Cell biology is appreciated for its understanding of events in the cell, which unlocks the understanding of processes in complex organisms
• The book accounts for diversities in structure, function, and evolutionary advancement among different organisms

Historical Development of Cell Study

• Cytology has been a slow, accumulative scientific process spanning decades and featuring eminent scientists
• Robert Hooke discovered cells in 1665 while examining bark with a crude microscope, naming the empty compartments "cells"
• Hooke observed only the cell wall
• Van Leeuwenhoek conducted studies on bacteria and protozoa using his microscope in 1667
• In 1675, Grew and Malpighi studied plant tissues, discovering fibers and vessels
• In 1831, Robert Brown observed the nucleus in orchid epidermal cells
• Dujardin observed in 1834 living substances in cells, calling it 'sarcode'
• Purkinje (1839) and VonMohl (1846) later renamed 'sarcode' to 'protoplasm'
• Mathias Schleiden and Theodore Schwann proved in 1838-1839 that plant and animal bodies are made of cells
• The Cell Theory was proposed in 1839
• In 1846, Nageli showed new cells arise from pre-existing cells through cell division
• In 1855, Virchow stated new cells are offspring of older ones: 'omnis cellula e cellula'
• In 1861, De Bary and Schultze established the protoplasm theory
• The theory is that cells are tiny masses of protoplasm, each enclosing a nucleus
• In 1880, Strasburger showed the structure and mode of nucleus division
• Boveri confirmed the constancy of chromosome number in 1900
• Schleiden and Schwann's cell theory was reaffirmed, with basic tenets summed up as follows:
• Cells are the unit of structure and function in all living things
• Cells come from pre-existing cells through mitosis
• Organism activity is the summation of individual cell activities
• Wilson in 1925 stated the key to every biological problem lies in the cell
• The Organismal Theory proposes the whole organism is the basic entity, cells being merely incidental sub-units
• Scientists agree the cell is the basic unit of structure and function in all living organisms
• Cell biology integrates biotechnology, molecular biology, genetic engineering, and recombinant DNA technology

Techniques in Cell Studies

• Cell study uses sophisticated and expensive tools
• Progress has resulted from the development of newer, refined, analytical techniques since the 19th Century
• The electron microscope was invented by Knoll and Rusta in 1932
• Tissue and cell culture techniques found wide applications in cell study

Microscopy

• Jansen first invented the microscope in 1590
• Microscopes increase the resolution of objects by magnifying them
• Modern microscopes use multiple lenses and are called compound microscopes
• Different types of microscopes with modifications are in use for cytological investigations

Light Microscopes

• They use a light beam for illumination
• They can magnify objects to a maximum of 200nm (0.2µm)
• Several types are used for different examinations
• Ordinary Light Microscopes do not depend on light diffraction but are best for stained specimens
• Phase Contrast Microscopes depends on the specimen's components
• They examine unfixed and unstained specimens
• Polarizing Microscopes examine crystalline components, making them exceptionally visible
• They are best to observe calcium oxalate crystals in cells

Electron Microscopes

• Use high speed electrons instead of light beam for illumination
• They use magnetic lenses instead of glass lenses
• Different parts of the cell absorb or scatter electrons
• They form images on an electron-sensitive photographic plate or fluorescent screen
• Can magnify objects with gaps lesser than 200nm
• Light rays passing through objects below 200nm tend to overlap, blurring the image
• Several types and modifications are frequently used in cytology
• Transmission Electron Microscope (TEM) passes electrons through thin sections, allowing examination of structures
• TEM allows higher resolution of up to 2nm (20A)
• TEM shows arrangement of cell components and filaments
• The shape of individual macromolecules can also be observed using TEM
• Scanning Electron Microscope (SEM) allows electrons to pass over the surfaces of the specimen
• SEM probes the structures and reflects them on a screen for 3-dimensional appearance

Freeze-Fracture Electron Microscope

• Examines the interior of the membranes of cells
• Reveals the various invaginations of such membrane

Etch-Electron Microscope

• Used to examine the contours of cell surfaces
• Reveals the sculpturings and ornamentations of such surfaces

Cell Fractionation

• Various techniques to isolate and characterize either whole cells, its parts, components or constituents
• The methods include cell fractionation, flow cytometry, immuno-precipitation, x-ray diffraction, auto-radiography, chromatography, electrophoresis and dialysis
• Possible to grind (homogenate) cells with devices to enable single cells (isolates) or components for examination
• Using an electronic fluorescence-activated cell sorter sorts individual cells from a mixed suspension
• Other sorting methods: plating in culture media, centrifugation, and using cell adherence or antibody-binding properties
• It is possible to burst cells with mortar and pestle, releasing contents into solution for analysis
• Usually separated (fractionated) into different density-dependent components using an ultracentrifuge
• Each component is studied for chemical structure, content, or specific chemical activity
• Further possible to fractionate proteins and other macromolecules using partition chromatography
• Fractionation is possible via paper, thin-layer, or column chromatography, or electrophoretic techniques and amino acid analyzers
• Analyzed molecules are studied in detail using more specific techniques
• Radioisotope methodology and autoradiography are also adopted in the analysis

Cell Cultures

• Living cells of plants, animals, and humans can be cultured like bacteria and fungi
• Cultured cells can be picked with microneedles, dissected by microsurgery, or injected with solutions using micropipettes
• Cells can be induced to fuse, forming "hybrid cells" for understanding inheritance
• These new frontiers led to recombinant DNA technology, known as Genetic Engineering
• Now possible to maintain and manipulate cultures of anthers, embryos, tissues and organs
• Cell and tissue cultures can be induced to secrete secondary chemical products characteristic of their sources
• DNA technology and culture techniques fall under the broader field of BIOTECHNOLOGY

Prokaryotic versus Eukaryotic Cells

• Cells are fundamental units of structure and function in plants and animals
• All cells share structural similarities regardless of origin
• Living cells are grouped into prokaryotes or eukaryotes based on evolutionary development and structural complexity
• Prokaryotes are thought to be the ancestors of eukaryotes

Prokaryotes

• Generally simple, primitive, and unicellular, lacking cell differentiation
• Include mycoplasmas, bacteria, blue-green algae (cyanobacteria)
• Usually very minute, ranging in size from 1-10µm
• Lack organelles such as mitochondria, plastids, cytoskeleton, dictyosomes
• Nucleus is poorly developed, lacking a nuclear membrane, nucleolus, nucleoplasm, and chromosomes
• Genetic material is simple, naked, circular DNA in the cytoplasm
• Protein and RNA synthesis occur in the same compartment
• Cytoplasm lacks cytoplasmic streaming, endocytosis, or exocytosis
• Mitosis and meiosis are completely absent; cell multiplication is usually by fission

Eukaryotes

• Advanced cells found among higher algae, fungi, protozoans, plants, and animals
• Range from 10-100um in size
• Can be unicellular or multicellular
• Have a well-developed nucleus consisting of a nuclear membrane, nucleoplasm, nucleolus, and chromosomes with double stranded DNA
• RNA is formed in the nucleus, while protein synthesis occurs in the cytoplasm
• Cytoplasm consists of a complex cytoskeleton of protein filaments
• Contain cytoplasmic streaming, endocytosis, and exocytosis
• Multiplication of these cells is by mitosis and/or meiosis

The Structural Framework of Eukaryotic Cells

• There is no 'typical cell' in structure since no particular cell possesses all features
• All cells share attributes in common
• A hypothetically typical cell consists of the outer covering coat or envelope made up of the cell wall and/or cell membrane, and the protoplasm
• Protoplasm consists of the cytoplasm and the nucleus

The Cell Wall

• Secreted by the protoplasm
• It is found only in plant cells
• Original cell wall material during cytokinesis becomes the middle lamella
• Middle lamella is mainly calcium pectate, holding neighboring cells
• As the cell grows, new walls of pectose, hemicellulose, and cellulose deposit on the middle lamella
• Referred to as the primary cell wall
• The main and only cell wall in plant cells not undergoing secondary thickening or growth
• Primary cell walls are living and extensible
• In cells undergoing secondary growth, primary cell walls further deposit with cellulose, making secondary cell walls
• Woody tissues have secondary walls impregnated with materials like lignin, suberin, and waxes, killing and toughening the cells

Functions of the Cell Wall

• Gives protection to the protoplasm
• Confers shape and form to the cell
• The middle lamella cements adjoining cells at division
• Flexible and extensible, allowing growth without distortions in young and non-woody plants
• Secondary cell walls confer strength and rigidity to cells
• The cell wall allows material exchange between cells via the plasmodesmata, aiding metabolite transport

The Cell Membrane

• Also known as plasmalemma or plasma membrane
• Living hyaline material, covers protoplasm in all cells
• Consists of a phospholipid bilayer traversed by assorted proteins, forming a LIPO-PROTEIN BILAYER
• The proteins are mobile, helping move materials across the nonpolar phospholipid layers
• Plasma membranes are selective or partial in their transport mechanism

Functions of the Cell Membrane

• Regulates movement of materials
• Enhances cell adhesion
• Forms the seat of biochemical activities
• Confers electrical resistance
• Vital in forming pseudopodia during movement in cells

The Protoplasm

• Living part of the cell
• In its living state, the protoplasm is transparent, colloidal, and semipermeable
• Water makes up most of the protoplasm (75-90% of the cell's content)
• Substances including food materials are dissolved in this water
• Processes including food synthesis, growth, respiration, and excretion, take place in the protoplasm
• The protoplasm is divided into the cytoplasm and the nucleus

The Cytoplasm

• Also called cell-protoplasm
• Consists of the covering hyaline ECTOPLASM and the granular ENDOPLASM
• Contains water, fats, carbohydrates, inorganic salts, enzymes, etc.
• "non-living" materials form the cytosol or cell inclusions
• Contains small compartments bounded by semipermeable membranes, known as 'cell organelles'

Mitochondria

• Occur in all eukaryotic cells as rods, filaments or spherical bodies
• Consist of a double membrane
• Have an outer membrane and a much-folded inner membrane
• Inner membrane called CRISTAE surrounds a granular matrix
• Believed to have arisen through endosymbiosis
• Principal components are protein and phospholipids

Functions of the Mitochondria

• The power house of the cell since it is the site of energy production in the form of ATP (adenosine triphosphate)
• Houses enzymes associated with respiration (especially Kreb's cycle and electron transport system) and photosynthesis
• Contain DNA with some genes capable of synthesizing proteins and exhibiting other genetic traits
• Plays a role in the oxidation of not only glucose but other food materials

Ribosomes

• Minute organelles found scattered freely in the cytoplasm
• Can be associated with the membrane of the rough endoplasmic reticulum only found in eukaryotes
• Made up of RNA and proteins, mainly histones
• The RNA component of the ribosome (called ribosomal RNA or rRNA) consists of two subunits which vary in size and molecular weights
• During peaks of intense protein synthesis, several ribosomes cluster together, forming polyribosomes (polysomes) which enhances the synthesis of proteins

Endoplasmic Reticulum

• Network of tubules or membrane-bound cavities which connects the nuclear membrane, cytoplasm, and the cell membrane
• Present only in eukaryotic cells and visible with an electron microscope
• Rough ER (with attached ribosomes)
• Smooth ER ribosomes) on the membrane surface

Functions of ER

• Increases surface area of the cytoplasm, enhancing metabolic reactions in the cell
• Provide attachments for enzymes participating in biochemical reaction
• The rough ER helps in protein synthesis, and the transport and storage of these proteins
• The smooth ER assists in the formation and storage of enzymes especially involved in lipid and steroid metabolisms
• In liver cells, smooth ER is involved in detoxification of drugs
• In plants, the ER is involved in the formation of interconnecting cytoplasmic strands called plasmodesmata
• ER is involved in the formation of lysosomes
• They contribute in the formation of cell plate during cytokinesis

Golgi Body Apparatus

• First discovered in nerve cells of animals
• Located within animal cells
• Physician Camilo Golgi discovered it
• Network of flattened, parallel, double membrane sacs with pinched off vesicles
• The sacs are about 10-20 in animals but up to 100 in plants (where they are called dictyosomes)
• Entire stack of Golgi bodies is referred to as the Golgi complex or region
• Golgi bodies are absent in prokaryotes
• Extensions of the smooth ERs

Functions of the Golgi Body Apparatus

• Involved in storage, processing and transport of molecules formed in the cell (especially proteins and lipids) around the cytoplasm
• In glandular cells of animals, they are associated with the secretion of enzymes and hormones

Lysosomes

• Often regarded as 'suicide bags' due to powerful hydrolytic enzymes stored
• Tiny spherical structures found in the cytoplasm of eukaryotic cells
• Thought to originate from the ER
• A single outer membrane
• Two kinds of lysosomes, primary and secondary lysosome
• Primary: when the lysosome has not merged with a food vacuole or another organelle when it remains inactive
• Secondary: when they fuse with any other structures, its enzymes are activated making it active

Functions of Lysosomes

• Contains digestive (hydrolytic) enzymes, ex: proteases, nucleases, lipases, for cellular digestion of proteins, nucleic acids, and lipids
• Defends the cell by destroying foreign and dead structures
• Involved in fertilization of ovum releases lysosomal enzymes
• Involved in morphogenesis, ageing and selective destruction of unwanted parts of cells
• Kills the cell by autodigestion (hence the name suicide bags)

Centrosomes

• Minute bodies near the nucleus in animal cells and lower organisms
• Contain a pair of granules called centrioles, which are responsible for forming microtubules
• Plants, including algae and fungi, lack centrioles; microtubules are organized by amorphous structures

Functions of Centrosomes

• Organizes cytoskeletal materials (microtubules) which influence cell shape and turgidity
• Forms the spindle fibers that move the chromosomes during cell division
• Assists in forming flagella and cilia in motile organisms

Plastids

• Discoidal or spherically-shaped structures with double unit membranes
• Consist of a protein matrix called stroma
• Contain disk-shaped vesicles called thylakoids
• Each stack of several thylakoids is a granum
• Occur only in plant cells
• Usually absent in cells of fungi, bacteria, and blue green algae
• Vary in form, size and the pigments they contain
• All types arise from an immature Proplastid

Functions of Plastids

• Chloroplasts are green
• They contain chlorophyll to absorb light energy during the photosynthesis of sugar and starch
• Chloroplast contain enzymes and systems responsible for carbon dioxide fixation to form sugars
• Chromoplasts include Xanthophylls (yellow) and carotene (orange-red)
• Usually linked with colouration of flowers, fruits, leaves, and photosynthesis
• Leucoplasts are associated with mainly the storage of food materials
• Leucoplasts that store starch are called amyloplasts
• Protein and fat storing leucoplasts are proteinoplasts and elaioplasts

Spherosomes

• Occur as lipid droplets
• Not bounded by unit membranes.
• Some are bounded by single unit membranes.

Functions of Spherosomes

• Spherosomes are involved in the production and storage of fats (lipids) and oils
• Rich in fatty bodies

Microbodies

• Membrane-bound, enzyme-storing vesicles found in the cytoplasm of most eukaryotic cells

Peroxisomes

• Referred to as microbodies of animals
• They also occur in plants and have enzymes such as catalase
• Catalase metabolize hydrogen peroxide
• In plants, most of the enzymes required for the glycolate pathway of photosynthetic cycle is in leaves

Glyoxysomes

• Called plant microbodies
• Occur in storage tissues of fatty seeds
• Contain enzymes for both the beta oxidation of fatty acids in the glycoxylate cycle and for redirecting carbon into photosynthesis

Microtubules and Microfilaments

• Structures are the main cytoskeletal material of eukaryotic cells.
• Made of special protein fibres that polymerize into long chains
• Criss-cross the entire cytoplasm
• Protein present determines the type of cytoskeletal structure

Functions of Microtubules and Microfilaments

• Provides mechanical support and structures for maintenance of cell shape.
• Act as anchors that fixes organelles, ex: ribosomes and nucleus.
• Offer binding surfaces for enzymes and other macromolecules during metabolic reactions.
• Controls cellular movements through their vibrations and contractions.
• Involved in chromosome movements during cell division.
• Directs orientation of cell wall synthesis and positioning of newly formed walls during cytokinesis

Paramural Bodies

• Invaginations of the plasmalemma, appearing as pockets between cell wall and cytoplasm
• Existence of tubules and vesicles suggest that they are functional organelles involved in cell wall growth
• Two types: Plasmalemmasomes and Lisosomes.
• When detached from the plasmalemma, they form multivesiculate bodies.

Vacuoles

• Sacs or cavities surrounded by a single membranes known as tonoplast.
• More prominent in plants where they occupy a large central position.

Functions of Vacuoles

• Storage of food: sugars, aminos acids, salts, and other nutrients.
• Dissolved substances absorb water, developing a high osmotic pressure that preserves the turbidity of the cell.
• Stores certain pigments, specifically anthocyanins, that impart many colours.
• Acts as storehouses.
• Contributes to regulating the process of excretion.

The Nucleus

• The nucleus is the largest organelle of a eukaryotic cell but is usually of an oval or spherical shape
• Nuclei is present in all eukaryotic cells
• It consists of nucleic acids (DNA) and proteins

The Nuclear Membrane

• Has a double layer of membranes
• Found only in eukaryotic cells
• Thought to have been formed by the endoplasmic reticulum
• Dissolves in the prophase and metaphase stage in cell division.

The Nucleoplasm

• Made up of fluid that is similar to the cytoplasm that fills the nucleus
• Has many substances like nucleoproteins and phospholipids

Chromatin Network

• Fine and loose structures found in the nucleoplasm
• These structures turn into chromosomes following cell division.

Structures of the Cell Surface

• Microvilli or minute hair-like projections are present to allow for absorptive substances to be moved into folding of the cell.
• While Cilia assist the lining of the stomach and the trachea
• The fibres are arranged through a 9 + 2 fibre format.

Difference Between the Animal and Plant Cell

• The plant cell will always have the middle Lamela while the animal will not.
• Plants contain plastids which leads to pigmentation.

The Cell Wall

• It gives the cells definite shape and form.
• It imparts a rigid construction.

Association of Cells and Levels of Cellular Organization

• All Cells Are Different
• Cells can exist in Single Association
• colony association: volvox
• Filamentous association: spirogyra, ulothrix
• Complex: higher animals
• Cellular and Organ tissues.

The Cell and its Biophysical Environment

• The cell is not isolated but exchanges materials
• The exchange of gases across alveolus membrane is by diffusion
• Exchange of materials between a mother and the foetus across the placenta is diffusion
• Hormones move from endocrine systems into bloodstreams by diffusion

Osmosis

• The movement of solvent from L à H across osmotic membrane
• The cell builds up hydrostatic or turgor pressure

Respiration

• This is the process in which the cell obtains its energy
• ATP adenosine triphosphate is produced

Definition of the cell

• The cell is not isolated but exchanges materials. The exchange gasses is diffusion. Hormones move from one place to another.

Metabolism

• Cells need substances in large amounts such as carbon, oxygen, and potassium that are macro elements. However, microelements are chlorine, aluminum. Enzymes break down lipids carbohydrates, water, fats, mineral salts.
• Green plants convert elements into complex food substance
• Energy provided in cell division is ATP

Cellular Activities

• The activity in which cells get their energy is respiration. Where cell obtains energy. Cells break down carbohydrates protein etc released is the cells.
• The removal of waste of metabolism is excretion. Growth is assisted by assimilation of food by the cell. It proceeds to convert protoplasm within the cell.

Light microscopes types:

• Ordinary light Microscopes
• Phase Contrast Microscopes
• Polarizing microscopes

4.1 Cell cycle and mitosis

• Traditionally, the processes that take place during the division of a eukaryotic cell are represented as the cell cycle.
• The cell cycle consist of the i