CELL STRUCTURE Credits Mann Shah and Reeba Sayed

CELL STRUCTURE Credits: Mann Shah and Reeba Sayed

Cell discovery The cell was first discovered and named by Robert Hooke, an English scientist. He chose cork as one of his samples. He remarked that it looked strangely similar to ‘cellula’ or small rooms which monks inhabited, thus deriving the name. Each cell appeared to be an empty box surrounded by a box to him. However, the cell walls observed by Hooke gave no indication of the nucleus and other organelles found in most living cells. The first man to witness a live cell under a microscope was Anton van Leeuwenhoek, who in 1674 described the algae Spirogyra. Hooke without realizing described the cell as the fundamental unit of all living things. Almost 200 years later, in 1839 a general cell theory emerged from the work of 2 German scientists, Theodor Schwann and Matthias Schleiden. The cell theory, or cell doctrine, states that all organisms are composed of similar units of organization, called cells. The concept has remained as the foundation of modern biology. To it has been added Virchow’s theory of 1855 that all cells arise from pre-existing cells by cell division.

Microscopy The Romans discovered that if you held one of these “lenses”, which to them was a piece of glass, over an object, the object would look larger. These early lenses were called magnifiers or burning glasses. The dramatic improvements were made in the quality of glass lenses, led to the development of cytology, a branch of biology where microscopes are designed and materials are prepared for examination under microscope.

The fundamentally different types of microscopes are Light microscopes Electron microscopes

Light / Optical Microscope Eyepiece Lens Body tube Nosepiece Objective lenses Condenser lens Condenser Iris Diaphragm Arm Stage Corse adjustment knob Fine adjustment knob Light source Base

Cell organelles Organelle Is a functionally and structurally distinct part of the cell. Is when organelles themselves are surrounded by membranes Compartmentalization so that their activities can be separated from the cytoplasm. Division of labor Is the sharing of work between different specialized cell organelles, since they have their own function

COMMON FEATURES: Cell surface membrane, Nucleus, Cytoplasm, Mitochondria, Golgi body DIFFERENCES: Centrioles & microvilli for plant cell; cell wall, chloroplast, Plasmodesma & vacuole for animal cell GENERALIZED PLANT CELL GENERALIZED ANIMAL CELL

Units of measurement Fraction of a meter Unit Symbol One thousandth= 0. 001= 1/1000= 10 -3 millimeter mm One millionth= 0. 000001= 1/100000= 10 -6 micrometer μm One thousandth millionth= 0. 00001= 1/100000= 10 -9 nanometer nm • 1μm = 10 -3 mm • 1 nm = 10 -3μm

Electron microscopy •

Electromagnetic spectrum of light Light travels in waves. The length of the waves of visible light varies from 400 nm – 700 nm. The whole range of different wavelengths is known as the electromagnetic spectrum. The longer the waves, the lower their frequency. Wavelength changes with energy. The greater the energy, the shorter the wavelength.

The mitochondria is large enough to interfere with the light waves. However, the ribosomes are too small to have any effect on the light waves. The general rule is that the limit of resolution is about one half the wavelength of the radiation. If an object is smaller than half the wavelength of the radiation, it cannot be seen separately from nearby objects. The best resolution that can be obtained from using a light microscope is 200 nm since the shortest wavelength of visible light is 400 nm, this corresponds to a maximum useful magnification of 1500 x. Ribosomes are approximately 25 nm in diameter. Therefore they can’t be seen using a light microscope. Also if an object is transparent it will allow light waves to pass through it & therefore will not be visible. For the same reason, specimens have to be stained before they can be seen.

Electron microscope • • • Biologists were unable to see anything smaller than 200 nm using a light microscope. The only solution was to use a radiation of smaller wavelength. Hence both UV and X-ray microscopes have been built, the latter with little success due to the difficulty faced to focus X-rays. A much better solution is to use electrons. When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits. Free electrons behave like electromagnetic radiation. They have a very short wavelength (the greater the energy, the shorter the wavelength) and higher frequency (the shorter the wavelength, the higher the frequency). Also they are negatively charged so they can be focused easily using electromagnets. Using an electron microscope, a resolution of 0. 5 nm can be obtained, 400 times better than a light microscope.

Types of electron microscopes Transmission electron microscope Scanning electron microscope

Ultrastructure of an animal cell Ribosome Rough ER Cell surface membrane Cytoplasm Centrioles Smooth ER Nuclear Envelope Rough ER Nucleolus Lysosome Nucleoplasm Chromatin Golgi Apparatus Mitochondria Microtubules Centrosome

Nucleus Nuclear envelope Chromatin Nucleolus Nucleoplasm Nuclear pores: Are micro pores present in the nuclear membrane that allow the exchange of substances between the cytoplasm and nucleoplasm. ATP Chromatin: Are chromosomes in a loosely coiled state in the nucleus (except during cell division) Hormones Entering the nucleus Ribosomes Chromosomes DNA Genes Nucleotides Proteins 1) Functional units 2) Control activities inside cells, and inheritance Leaving the nucleus m. RNA Ribosomes

Endoplasmic Reticulum and Ribosomes The ER is an extensive system of flattened compartments called sacs run through the cytoplasm. Ribosomes are structures of 25 nm made of RNA and proteins and are a sight for protein synthesis. They have a large and small subunit: Proteins are made in ribosomes on rough ER Enter the sacs and run through it (often modified here) Make lipids Smooth ER lack ribosomes and have different functions: Make steroids (like cholesterol) Make reproductive hormones (like estrogen and testosterone)

Golgi body These form a secretory pathway as proteins are exported outside the cell via the Golgi vesicles. Examples of proteins manufactured: Animals: Protein + Sugar = Glycoprotein Small sacs break off the ER to form vesicles Vesicles combine to form the Golgi body Collect, organizes and sorts molecules Golgi body breaks down to form Golgi vesicles. Transports proteins to other cell organelles or for secretion Removal of 1 st amino acid (methionine) from newly formed proteins to form functional proteins Plants: Sugars are converted to cell membrane components by enzymes

Lysosomes The spherical sacs contain hydrolytic enzymes, surrounded by a single membrane. They have no internal structure. The enzymes are kept separate from the rest of the cell to prevent damage Functions: Breakdown of old cell organelles Breakdown of old/ unnecessary cells (like mammary glands after lactation) Used to digest bacteria in WBC’s Secreted (Eg. the replacement of cartilage during bone replacement) The sperm head has a special lysosome called the acrosome that is used to digest the path to the ovum

Mitochondria : Structure Porin: It forms wide aqueous channels, allowing easy access for water-soluble molecules from the cytoplasm to the inter-membrane space The inner membrane is more selective as it precisely controls what ions and molecules enter the matrix

Mitochondria : Function These reactions / phases are: 1) Glycolysis – cytoplasm The matrix contains enzymes which supply hydrogen and electrons to the reactions that take place in the cristae. 2) Link reaction – matrix 3) Krebs cycle - matrix 4) Oxidative phosphorylation and electron transfer chain – intermembrane space The flow of electrons generates power to produce ATP molecules. Energy produced from oxidation of high-energy molecules is transferred to ATP. Folded cristae has greater surface area, increasing efficiency of respiration Energy is released by breaking down ATP to ADP, which is a hydrolytic reaction. ADP can be recycled into the mitochondrion. ATP leaves the mitochondrion and spreads rapidly, to other organelles, as it is a small and soluble molecule

Endosymbiont theory The size of a ribosome is measured in ‘s units’ which is sedimentation rate in a centrifuge Ribosomes present in chloroplasts and mitochondria are of 70 s, the same as bacteria Chloroplasts and mitochondria also have circular DNA in the stroma/ matrix Endo Inside Symbiont An organism that lives in a mutually beneficial relationship with another organism DNA present in these organelles is still responsible for coding and synthesis of vital proteins

Cell surface membrane and microvilli The cell membrane is about 7 nm thick and has a trilaminar surface (has 3 layer) Microvilli are finger-like extensions of the cell surface membrane, typical to certain epithelial cells. They are needed to increase surface area.

Microtubules and MTOC’s α and β Tubulin Dimers Polymerization Long protofilaments (5 nm) (x 13) Microtubule (25 nm) This organization is controlled by special locations in cells called MTOC’s The spindle used for separation of chromosomes during nuclear division is made of microtubules Microtubules form an intracellular transport system as secretory vesicles and cell organelles move along the outside of microtubules Microtubules + actin/ intermediate filaments cytoskeleton Membrane bound organelles are held in place by the cytoskeleton

Centrosome and centrioles Two centrioles are placed at right angles to each other in a region called the centrosome A centrosome is 500 nm by 200 nm and consists of nine triplets of microtubules Centrioles are MTOC’s in basal bodies, however their function in other cells remains unknown

Ultrastructure of a plant cell

Chloroplasts: Structure Lipid droplets Diameter: about 3 to 10μm Two membranes forming chloroplast envelope Self-replicate themselves 70 S ribosomes & circular DNA strand THYLAKOID: Fluid-filled sacs GRANA: Flat, disc- like structures stacked like a pile of coins STROMA: Like the matrix in mitochondria

Chloroplasts: Function LIGHT DEPENDENT STAGE LIGHT INDEPENDENT STAGE Light energy is absorbed Calvin cycle takes place in the stroma which uses the energy from the 1 st stage to convert CO 2 into sugars Some of this energy is used to manufacture ATP from ADP Breaking of water into H 2 and O 2 H 2 is oxidized to provide energy to make ATP Sugars are stored as in the form of starch grains in the stroma. Black spheres in stroma are reserves of lipid for making membranes or from the breakdown of membranes MAINLY CARRY OUT PHOTOSYNTHESIS

Two fundamentally different types of cell Prokaryotic cells Eukaryotic cells AVERAGE CELL 0. 5 – 5 μm DIAMETER Commonly up to 40 μm, 1000 – 10000 times the volume of prokaryotic cells DNA Naked, circular & lies free in the cytoplasm Associated with protein, forms chromosomes, is not circular & is contained in a nucleus which is surrounded by an envelope RIBOSOMES 70 S ribosomes, about 20 nm in diameter 80 S ribosomes, about 25 nm in diameter ER No ER present, ribosomes attached ORGANELLES Very few organelles - not membrane Many organelles present, bound unless formed by the in folding extensive compartmentalization of the cell membrane & division of labor CELL WALL Present - wall contains murein, a peptidoglycan Sometimes present - contains cellulose or lignin in plants & chitin in fungi

Prokaryotic v. Eukaryotic cell

Viruses In 1852, Dmitri Ivanovsky, a Russian microbiologist discovered that certain diseases could be transmitted by agents that could pass through the finest filters. This was the 1 st evidence for the existence of viruses (tiny structures much smaller than bacteria, between the living & the non-living). Viruses don’t have a cell structure. They are simple in structure. Most viruses consist only of • A self replicating molecule of DNA or RNA, its genetic code • A protective coat of protein molecules. A virus has a very symmetrical shape. Its protein coat (or capsid) is made of separate protein molecules each called a capsomere. Its size is about 20 – 300 nm. All viruses are parasitic because they can only reproduce by infecting & taking over living cells. The virus DNA or RNA takes over the protein synthesizing machinery of the host cell, which then helps to make new virus particles.