Cytology: Cell Theory and Cell Structure

Cytology

• Cytology involves the study of cells, including their structure and function
• Cells are the building blocks of multicellular plants and animals
• Some cells are specialized to do specific functions

The Modern Cell Theory

• Robert Hook (1665) first studied biological material (cork cells) using a primitive microscope
• He recognized and named the different cork cells as little boxes
• Cellula is the Latin term for "rows of monks' cells in a monastery"
• Antonie van Leeuwenhoek (1674) crafted the first light microscope
• Schleiden (botanist) and Schwan (zoologist), formulated cell theory: all organisms consist of cells and cell products in 1838
• The cell is the basic unit of life
• Purkinje introduced protoplasm as the living substance of cells, in 1839
• Virchow introduced the concept of biogenesis in 1858
• All cells originate from existing cells; life comes from life
• The Modern Cell Doctrine: all organisms are composed of cells

Principles of the Modern Cell Theory

• Life occurs only in cells organisms consist of cells
• The cell is the basic unit through which matter and energy are obtained, converted, stored, and utilized
• Cells are self-producing
• Continuity of life resides on cell components like genes and chromosomes within the nucleus
• Cells originate from reproduction of previously existing cells
• Structure is related to function
• There exists a specific relationship between the structure and function of a cell
• The structure of cells is organized to execute its specific functions

Definition of a Cell

• A cell is the smallest structural and functional unit
• Contains all structures and molecules essential for life

Functions of Living Cells

• Responsiveness or Irritability: cells react to suitable stimuli
• Conduction: ability to conduct a stimulus along the cell surface
• Highly developed in nerve cells
• Respiration: oxidation of energy-rich molecules to release energy
• Water and CO2 are by-products
• Oxygen is essential (except in anaerobic organisms)
• Contractibility: ability to change form after a stimulus
• Highly developed in muscle cells
• Absorption: ability to take up substances from surroundings
• Energy extraction and synthesis of own molecules (metabolism and assimilation)
• Metabolism: chemical processes in the body
• Anabolic processes: larger molecules are built up
• Catabolic processes: large molecules are broken down
• Secretion and Excretion: release of essential or waste substances
• Growth and Reproduction: increase in protoplasm amount and/or increase in the number of cells after cell division
• Cell size reaches maximum
• Increase in cell number occurs via division

Metabolism

• Metabolism constitutes the total chemical changes and processes in a living cell
• Anabolism: synthetic or building chemical activities
• Catabolism: chemical activities that decrease chemical organization, the breakdown, and free energy therein
• Metabolism requires that foodstuff be absorbed by the cell, then it's broken down to build new products

Feeding

• Organisms constantly take in and assimilate materials for growth, maintenance, and energy
• Nutrients: inorganic (water and minerals from environment) or organic (carbohydrates, fats, proteins)
• Organic nutrients sources: autotrophic or heterotrophic nutrition
• Autotrophic nutrition: organisms like plants, algae, and some bacteria that synthesize organic foodstuff by using inorganic compounds to derive energy, e.g. photosynthesis or chemosynthesis from the sun
• Heterotrophic nutrition: organisms like animals, fungi, most bacteria, and non-green plants that feed on other plants or animals to degrade organic molecules

Respiration & Synthesis

• Respiration: oxidation of organic molecules to CO2 and H2O to obtain energy
• Organisms use absorbed organic foodstuff and worn-out body particles as fuel
• Synthesis: process by which complex cell components are built
• Organic compounds are used as building blocks
• These are usually reduction reactions, requiring energy

Cell Size

• Most cells are between 10-100 μm in diameter
• Too small to visualize without a microscope, with the exception of eggs of frogs or others such as bird eggs
• Cells are easily studied with a light microscope

Minimum Cell Size

• Mycoplasms are the smallest known cells
• Have a diameter of about 0.1 μm (100 nm)
• Obligate parasites that can only be observed with an electron microscope
• Living organisms can most probably not be smaller than this, because then the cells would not have enough proteins necessary to perform the functions of life
• About 200 different proteins are necessary for the most elementary form of life
• This number of proteins would not fit into a smaller space (or amount of protoplasm) as that of a Mycoplasm

Maximum Cell Size

• Cells are not generally large
• Multicellular organisms comprise millions of small cells
• Advantage of smaller cells lies in biological communication
• Cells need to communicate internally and externally
• Multicellular organisms usually consist of many small cells rather than a few large ones because small cells function more effectively

Surface Area to Volume Ratio

• Cells must absorb substances and excrete products via the cell surface membrane (plasmalemma)
• The cell surface must be large enough to meet the needs of its volume
• Surface/volume ratio limitations:
• Cell volume increases faster than its surface area
• Affects nutrient uptake and waste excretion
• Volume relationship between nucleus and cytoplasm exists
• Nucleus only has a limited amount of cytoplasm to maintain
• Cell volume affects diffusion of substances
• Smaller cells have a larger ratio of the surface area compared to the cell volume; more business can take place

Implications of Cell Size

• Smaller cells have a larger surface area relative to volume
• Cells can increase in size only until the surface area can meet volume needs for transport, secretion, and excretion
• If cell grows further, division into two smaller cells must occur

Specialization of Cells

• Cell specialization allows for optimizing cell metabolisms and is important in large multi-celled organisms; Large cells modify for special functions a) Some large plant cells
  • Have large vacuoles
  • Watery content does not take part in metabolism
  • Undergo cytoplasmic streaming a) Bird eggs
  • Extremely large with large amount of reserves and little metabolic activity a) Skeletal muscle cells
  • More than one nucleus
  • Extremely long with more than one nucleus for protein synthesis a) Nerve cells
  • Cytoplasm near cell surface in long narrow cylinders a) Epithelial cells
  • Microvilli increases surface area membrane of the epithelial cells without increasing the volume of the cells
  • Characteristic of the epithelial cells of the small intestine and kidney cells

Microscopy for Studying Cells

• A microscope is a very important instrument in the field of Health Sciences
• Light microscope:
  • Almost all cell structures had already been identified at the beginning of the twentieth century
  • Consists of a glass lens system and a mounted object
• Resolving power of a microscope is its ability to distinguish two points close to each other as separate points
• Many cell components (e.g. mitochondria and chloroplasts) are much smaller than the resolving power of a light microscope (usually about 200 μm) and will only show up as dots
• In order to study the fine structure (ultrastructure) of cells, a microscope with a smaller resolving power is required
• This means using an electron microscope (EM)
• EMs focus electron waves (instead of light waves) through a set of magnetic lenses
• Electron waves increase magnification to ±250 000x as well as the resolving power to 0.2 nm
• This is sufficient to show the internal structure of cell components

TEM

• A beam of electrons is focused on the specimen
• Some of the electrons are absorbed, and others are transmitted
• These transmitted electrons build an “image” of the specimen on a television screen
• The image is then photographed
• Photographs taken of ultrastructures are known as electron micrographs
• Internal structure (e.g. that of a mitochondrion) is only observed by means of an electron microscope and is referred to as its ultrastructure or fine structure
• TEMs take electron micrographs
• These show a two-dimensional cross-section (length and breadth) of the cell, magnified thousands of times

SEM

• Surface of a massive specimen is bombarded with a moving beam of electrons
• Reflected electrons from the surface of the specimen are absorbed by a detector
• 3D image of the specimen is created
• Because the SEM has a high focus depth, it allows the study of bulky specimens

Physical and Chemical Methods

• Cell fractioning using (differential centrifuging) separates cell components
• Mitochondria suspensions are separated, and their biochemical activity determined
• Tissues (e.g., a liver specimen) is pulverized with a high-speed mixer in a suitable buffer medium until cells rupture
• Contents separate into the buffer solution
• Homogenate is then consecutively centrifuged
• Increasing speed to precipitate the various cell components, separating them
• Larger cell components precipitate at lower G levels
• Smaller components precipitate at higher G levels
• Each fraction is then studied to determine its chemical composition, enzymes, and the different reactions associated with it
• Mitochondria contain 52 enzymes involved in cellular respiration
• Chloroplasts execute the complete process of photosynthesis

Cell Types

• All living organisms can be allocated to one of the following domains and kingdoms
• Cells can be divided into 2 groups based on internal organization: prokaryotes (all "bacteria") and eukaryotes

Prokaryotic Cells

• Typical of the Kingdoms Archaea and Bacteria
• Primitive and small (0.5-3 μm) wth a chromatin body (nucleus area) with no nuclear membrane - there is no true nucleus
• Chromatin body: network of filaments (approx 4.5nm) of DNA in the form of a single circular DNA thread
• Chromosome occurs free in cell in an area called the nucleoid
• Some bacteria have small amounts of “extra” DNA in circular plasmids that separate from the chromosome used in genetic manipulation:
• A desired gene is transferred to a plasmid
• Plasmid transferred to a plant cell and incorporated into its DNA
• Plant cell can express desired gene
• Cytoplasm is then followed by a cell wall if they aren't mycoplasms lacking it.
• Prokaryotics have no other membrane-bound organelles including mitochondria, chloroplasts, Golgi complex, lysosomes, or endoplasmic reticulum (ER)
• Prokaryotes have many ribosomes similar to those in mitochondria

Mesosome Function

• The only membranes within prokaryotes are a system of concentric membranes
• This is the mesosome, originates as invaginations of plasmalemma
• Functions:
• It enlarges the plasmalemma
• Because respiratory enzymes are situated on the membranes, the surface area for respiration is also increased
• It most probably takes part in the division of the DNA molecules

Prokaryotic Cell Walls

• Cell walls are unique in their chemical composition
• Composed of the polymer peptidoglycan (mucopeptide), along with polymers like proteins, polysaccharides, and lipids
• Polymers form a complex composed of a peptidoglycan (mucopeptide), along with other protein polymers polysaccharides, and lipids
• Ranges from 10 to 50 nm in thickness and can be arranged in single or multiple layers
• Chemical composition of cell walls differs between groups, specifically, Gram-positive and Gram-negative bacteria
• Gram-positive Bacteria
• Relatively thick walls with a homogenous composition of peptidoglycans
• Gram-negative Bacteria
• Thinner walls that are more complex
• Inner layer: peptidoglycans
• Outer layer: proteins, phospholipids, lipopolysaccharides (LPS)
• Chemical composition determines antibiotic susceptibility
• Penicillin inhibits peptidoglycan synthesis and can effectively target Gram-positive bacteria
• Penicillin is relatively ineffective against Gram-negative bacteria because the outer layer (with LPS) prevents the antibiotic from reaching peptidoglycan layer
• Gram classification determines therapy methods (drugs to use)
• Staining Gram-positive bacteria with an iodine solution that has been rinsed with alcohol or acetone stains the bacteria
• Iodine stain on Gram-negative bacteria is removed when rinsed with alcohol or acetone

Exterior Coverings

• In addition to the cell wall, coverings like slime (biofilm) or a capsule (polysaccharides and proteins) offer protection
• Protective layer due to strains of Streptococcus pneumoniae (causing pneumonia) without the capsule is engulfed by white blood cells
• Prokaryotes possess flagella or cilia for propelling, and pili (slender cytoplasmic extensions) to attach to the cell to surfaces and, during sexual reproduction, to each other.

Similarities between Prokaryotes and Eukaryotes

• Both types possess DNA and use the same genetic code
• Both synthesize proteins in the same via RNA (mRNA, rRNA and tRNA) using ribosomes
• Both use ATP as an energy source
• Specific molecules (coenzymes, e.g NAD, KoASH) execute identical functions
• Basic metabolism is the same
• From those similiarities, it follows that that Pro- and Eukaryotes have originated from common ancestors

Eukaryotic Cells

• Organisms consist of eukaryotic cells, except Archaea and Eubacteria including Cyanobacteria
• Have a demarcated nucleus by a nuclear membrane
• Larger and more complex than prokaryotes
• Evolved after prokaryotes

Eukaryotic Cell Anatomy

• Protoplasm constitutes the living cell composed of cytoplasm and nucleoplasm (inside the nucleus)
• Cytoplasm consists of cytosol, organelles, and inclusions
• Cytosol: translucent semi-gel fluid in which the organelles are suspended
• Organelles: cellular components
• Inclusions: colloidal system of proteins, lipids, carbohydrates, gasses, and mineral ions including the cell surface (cell wall and plasmalemma)

Cell Organelles

• A permanent structure within a cell with a specific function(s)
• All organelles are enclosed by membranes
• Exist as isolated compartments
• Membranes of the nucleus and the ER probably originate via invaginations and are continuous with the plasmalemma
• Endomembrane resembles plasmallema

Cytoskeleton

• Non-membranous structures in eukaryotic cells

The Cell Wall

• Cell walls surrounding the plasmalemma exist in porkaryotes and some eukaryotes
• Structure varies between different orgnaisms:
• Cellulose in plant cell walls
• Chitin in Fungal cell walls and bacteria
• Peptidoglycans and LPS (lipopolysaccharides)
• Functions to provide protection support, and shape while remaining permeable for substances

The Plasmalemma (Cell Membrane)

• Important cell organelle as the primary membrane surrounding the cell
• The membranes surrounding ER and nucleus are almost identical to the cell membrane
• Proteins "float", acting as icesbergs in the phospholipid sheet
• Carbohydrates extend from proteins
• Appears "trilaminar" using electron microscopy as two parallel-dark-electron dense lines bound together

Lipids in the Cell Membrane

_ Three Key components of the cell membrane are lipids, proteins, and carbohydrates, primarily fats

• Phospholipids are most abundant lipid in all membranes and consists of a double-layer made of phopholipids with less gylcolid and cholesterol:
• Contains polar part (phosphate) and a non-polar part (carbon chain)
• Polar part is hydrophilic
• Non-polar part is hydrophobic
• Function is differential permeability
• Cholesterol makes up 50% of animal cell membrane.
• Functions to keep the membrane fluid

Membrane Protéins

• Membranes float in a sea of lipids and depend of its ratio with the function of that same membrane membrane
• Membrane protein are classified on structure and function
• Integral protein-Stretch throught the membrane
• Transport functions can be act as enzymes, transport proteins and receptors as well as be involved with cell attachment and structure

Peripheral proteins

• Attached on the inside or outside of the membrane and phospholipid heads and CAN'T be transportes

Polysaccharide Chains in Cell Membrane

• Polysacchardine chains are found on integral outer surface of the cell membrane and form gycocalyx when abundent.

Carbohydrates

• Chains of oligosaccharides are linked to phospholipids and proteins
• Glycolipid is the term for carbohydrates linked to lipids protein are called gycoproteins
• Mediate cell intercell interactions with each other, used to ID REJECT tissue during transports
• Blood Type is a cell group based on presence a inherited antigenic substances on red blood cells and can be inherited

General Functions Of Plasmalemma

• Boundray/environement of Cell and integral, regulate out/in and allows high transfer in cells

Endocrine Interactions

• Membranes have receptors or hormone receptors and activate messangers for cAMP bonding with a goal in cell

Intercell Interactions

• The cell membrane is related to cell communication and attacement with other cells

Surface Communication

• Can Excrete extracellular such as polysacharides/cement/calcium
• Surface Molecules

The Nucleus

• largest obvious organelle, not on mature rbc cell or stems
• Can be mutlnucleated
• Contains the DNA instructions for the entire body (chromosomes) to allow proteins and activities as cell.

Nucleus Anatomy

• double membraned connected to GER, holds large pores allowing particles and plasm for the membrane, contains chromatin for chromosones in the form of amorpous.

Chromosones and Nucleioli

• Chromatin is thin and threadlike not divided complexed with DNHA and histone with proteins
• Chromosones contrain contain genes for DNA and replication
• Beside replication chromosone transcribe genes to mRNA allowing a templated symthesis for protide molecules
• Nucleioli are formed where protein and TRNA from cytoplassm combine with TRNA at the matrix and become protides
• These make the protiends and form a factory for the cell

What Are Ribosomes?

• Ribosomes are (±20 nm in diameter): solid, compact particles that are attached to endoplamosatic or float in cell made with riuboncilur RNA.
• Consists of 60L and 4S units, uses S value that relect the size and shpe of parts, this will be made later
• Function with protied and synthesis allowing and translation

The Endoplasmic Reticulum

• cytoplasm in all cells contain repeatedly folded plates with varying shapes and sizes, and cytoplasm is effectivly divided and same strcutrea as