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The cell Topic 1 Introduction • • • • • • The study of cells is called Cytology The cell is the smallest functional unit of life Cell theory Prokaryote vs Eukaryote Cell parts and their functions Normal cell reproduction 1.1 Cell Theory 1. All organisms are composed of one or more cells. 2. Cells are the smallest units of life. 3. All cells come from pre-existing cells Contributors to part 1 • Robert Hooke first described cells in 1665 using microscope he built. • A few years later, Antonie van Leeuwenhoek sees living cells and calls the animalcules. • In 1838, Matthias Schleiden states that plants are made of cells. • In 1839, Theodor Schwann states that animals are made of cells. Contributors to part 3 • Louis Pasteur, in the 1880’s, chicken broth experiment showed that living organisms would not spontaneously appear. Functions of life • All organisms, whether single cell (unicellular) or multicellular, carry out all the functions of life. • Metabolism nutrition • Growth excretion • Reproduction • Response • Homeostasis • Metabolism – all the chemical reactions that occur in an organism. Energy conversion • Growth may be limited but is always there. • Reproduction involves hereditary molecules being passed to offspring. • Responses to stimuli in the environment. • Homeostasis is maintenance of constant internal environment. • Nutrition provides compounds to convert to energy. • Excretion is the release of unneeded or toxic chemicals from an organism. Cells and Size • Cells are small enough to usually need a microscope to study them. • Light microscope is the most common and used light passing through the cells to see them. • Electron microscopes use electrons to form an image and have greater magnification than light microscopes. Relative size • • • • • • Cells (eukaryotes) 100 micrometers um Organelles 10 um Bacteria 1 um Viruses 100 nanometers nm Membranes 10 nm Molecules 1 nm Calculating size • Formula Magnification = size of the image / actual size Limiting cell size • The surface area of a cell, compared to the volume of the cell, limits how large a cell will get. • The amount of resources a cell needs to take in, as well as the amount of waste and heat that a cell needs to get rid of, depends on the volume of the cell. • The surface of the cell, the cell membrane, controls how fast this can occur. • As a cell gets larger in volume, its surface area also increases, but at a much slower rate. • Volume involves cubing the radius while surface area involves squaring the radius. • This means that the larger a cell gets, the smaller the surface area to volume ratio. • Eventually, the volume gets so large that the surface area can’t handle the traffic in and out and the cell stops growing. Cell reproduction and Differentiation • Single cell (unicellular) organisms need to reproduce to create new organisms. • Multicellular organisms need to create new cell to grow and replace damaged or old cells. • New cells in multicellular organisms need to differentiate, which means change into a particular type of cell. • All cells have the same DNA, but they use or express different parts of it to become different types of cells. • Once they become specialized, some cells (nerve, muscle) lose the ability to reproduce themselves. • Different types of cells can accomplish more as a group than they could as individuals, the group can do more than the sum of its parts. • This is referred to as an Emergent Property Stem Cells • There are cells within organisms that retain their ability to divide and differentiate into different types of cells. • These cells are called Stem Cells • Scientists discovered pluripotent or embryonic stem cells in the early 1980s. • Stem cells can produce different types of cells and also more stem cells. Stem Cell Research • Therapeutic cloning is using stem cells to grow cells to replace cells damaged through disease or accident. • Parkinson’s and Alzheimer’s are two diseases being treaded with nerve cells grown from stem cells. • Pancreatic cells are being grown to treat Diabetes Tissue specific Stem Cells • Certain tissue types have their own stem cells. • Leukemia treated with blood stem cells • Stargardt’s disease with photo receptor cells in the eye. Ethical issues • Pluripotent stem cell research is being done using stem cells from embryos through in vitro fertilization. • Some argue that this is taking a life. 1.2 Cell Structure Prokaryotes • • • • Much smaller than eukaryotes- 1 um or less Much simpler than eukaryotes- no organelles Appeared on Earth first Bacteria are the major members of this group Prokaryote structures • • • • • • Cell wall Plasma membrane Flagella – not all have them Pili Ribosomes Nucleoid region Cell Wall and Plasma Membrane • Cell wall protects and maintains shape of cell • Made of a carbohydrate-protein material called peptidoglycan. • Plasma membrane is just inside the cell wall. • Controls movement of material in and out of the cell and plays a role in binary fission. • Inside the membrane is the cytoplasm. No compartments in cytoplasm. Pili and flagella • Pili are hair like growths on the outside of the cell, used for attachment as well as reproduction. • Some prokaryotes have a flagellum or flagella (plural) which are longer than pili and are used for movement Ribosomes • All Prokaryotes have them • Site of protein synthesis • Smaller than Eukaryote ribosomes, 70s vs 80s Nucleoid Region • The area (no compartment) where the single circular DNA chromosome is located. • Prokaryote DNA is not attached to proteins like eukaryote DNA are. • Some prokaryotes also contain plasmids, small circular pieces of DNA that are separate from the main chromosome. • Plasmids replicate independently of the main chromosome and are not needed for day to day activities. Binary Fission • Prokaryotes divide by process called binary fission. • First the chromosome (DNA) is copied. • The two daughter chromosomes attach to different areas of the plasma membrane. • The cell elongates, fibers called FtsZ fibers separate the two chromosomes, then the cell divides into two identical daughter cells. Eukaryote Animal Cell Eukaryotes • Size ranges from 5-100 um • Contain organelles, membrane bound structures which perform specific functions for the cell. Eukaryote Plant Cell Eukaryote Organelles • • • • • • • • • Endoplasmic reticulum Ribosomes Lysosomes ( not usually found in plant cells) Golgi apparatus Mitochondria Nucleus Chloroplasts ( only in plants and algae) Centrosomes vacuoles • Cytoplasm – the area inside the plasma membrane. The fluid part is called the cytosol. • Endoplasmic Reticulum (ER) – A series of tubes/channels that transport materials throughout the cell. • Two types – Smooth ER and Rough ER Smooth ER • • • • • • Contains enzymes on its surface which: Production of lipids and phospholipids Production of sex hormones Detoxification of drugs in the liver Storage of calcium ions in muscle cells Release of glucose by liver Rough ER • Contain ribosomes on their surface which give a rough appearance. • The ribosomes of the Rough ER make proteins to be transported out of the cell. • There are ribosomes not attached to the Rough ER which float freely in the cytosol. • These free ribosomes make proteins to be used by the cell. • Ribosomes are composed of two units of RNA that are made in the nucleolus Lysosomes • Lysosomes – sacs of digestive (hydrolytic) enzymes, made by the Golgi apparatus, that help break down proteins, carbohydrates, lipids and nucleic acids. • They attach to old organelles or materials brought into the cell by phagocytosis. Golgi Apparatus • Made of flattened sacs called cisternae. • It collects, modifies, packages and distributes materials made by the cell. • The cis side of the Golgi apparatus is near the Rough ER and it receives products of the ER. • These products leave the Golgi Apparatus in small sacs called vesicles, at the trans side. Mitochondria • Rod shaped organelle close in size to a bacteria • Contain their own DNA, a single circular chromosome, similar to bacterial cells. • Contain their own ribosomes that are 70s, similar to bacteria. • They have a double membrane, the outer being smooth and the inner having many folds called cristae. • Inside the inner membrane is called the Matrix. Mitochondria • The area between the two membranes is called the inner membrane space. • The folds (cristae) allow for increased surface area for the reactions of the mitochondria to take place. • The reactions of the mitochondria are called cellular respiration and create molecules called ATP. Mitochondria Nucleus • Double membrane enclosed area, called the nuclear envelope, where the DNA is located. • Contains pores to allow movement into and out of the nucleus. • DNA is in pieces called chromosomes. • Different species have different numbers of chromosomes. • DNA is the genetic material of the cell and allows traits to be passed to offspring. • When not dividing, DNA is in a form called chromatin, which is DNA and proteins called histones. • Where the DNA wraps around 8 histones, an area is formed called a nucleosome. • Most eukaryote cells have a single nucleus. • Some, like red blood cells, have no nucleus. • Most nuclei also contain a dark area called a nucleolus which makes ribosomes. Chloroplast • Found only in algae and plant cells (autotrophs). • Surrounded by a double membrane and about the size of a bacteria. • Has its own DNA and 70s ribosomes, just like the Mitochondria. • Contain flattened sacs called thylakoids where the light absorption part of photosynthesis occurs. • Thylakoids are in stacks called Grana/ Granum • The fluid part of the chloroplast is called Stroma which contains enzymes for photosynthesis. • Like Mitochondria, Chloroplasts can reproduce themselves independently of a cell. Chloroplast Centrosome • Found in all eukaryotic cells. • Consists of a pair of centrioles at right angles to each other. • Centrioles make microtubules which provide structure and allow movement. • Also important for cell division. • Some higher evolved plants produce microtubules even though they lack centrioles. • Located close to the nucleus. Vacuoles • Membrane bound storage organelles made by the Golgi Apparatus. • Store many different substances including food, water, waste, and toxins. • Plant cells have a huge one filled with water and enzymes, allows rigidity for the plant. Prokaryote vs Eukaryote Eukaryote Plant vs Animal cells • Plant cell Animal cell cell wall no cell wall chloroplasts no chloroplasts Large central vacuole small or no vacuoles carbs stored as starch carbs stored as glycogen no centrioles centrioles Fixed angular shape rounded shape Extracellular Matrix (ECM) • Surrounds the outside of animal cells, composed of collagen fibers and glycoproteins. • Allows cells to attach to each other. • Allows communication and coordinated action between cells ECM 1.3 Membrane Structure History • Since 1915, scientists have known membranes are made of proteins and phospholipids. • In 1935, Davson-Danielli model suggested a phospholipid bilayer covered on both sides by a thin layer of protein. • In 1972, Singer-Nicolson model suggested proteins were inserted into the phospholipid bilayer and not a continuous layer. • They believed the phospholipids were fluid, meaning they were very loosely attached to each other and could move around, and the proteins formed a mosaic pattern within the bilayer. • Most of the new evidence came from the use of the electron microscope. Phospholipids • The main “backbone” of the membrane is made of a double layer of phospholipids. • Each phospholipid is composed of a three carbon molecule called glycerol • Two of the carbons have fatty acid (lipid) molecules attached to them. • The third carbon is attached to a highly polar alcohol/phosphate group. • The fatty acids are non-polar and not soluble in water. (Hydrophobic) • The alcohol/phosphate group in polar and soluble in water. (Hydrophilic) • This causes the phospholipids to align themselves with their polar “heads” outward and their non-polar “tails” inward. Phospholipid Cholesterol • Membranes must be fluid or flexible to function properly. • At various locations in the hydrophobic, or tail region of animal cells, there are cholesterol molecules. • They allow membranes to be properly flexible at a large range of temperatures. • Plant membranes do not have cholesterol. Proteins • It is the proteins found in and on the membranes that allow for the great diversity of membrane function. • There are two major types of membrane proteins, integral and peripheral. • Integral proteins are amphipathic, meaning they have both polar and non-polar regions. Proteins • These proteins will have their non –polar hydrophobic regions near the middle of the membrane (by the tails) and their polar hydrophilic regions near the edges. • Peripheral proteins are attached to the surface of the membrane, often attached to an integral protein. Protein Functions • There are many different proteins which have 6 general functions: • Sites for hormone binding – have a specific shape exposed to the outside that will only fit a specific hormone. When hormone attaches, it changes the shape of the protein, relaying a message inside the cell • Enzymatic action – proteins act as enzymes to catalyze reactions, attached to outside or inside of the membrane • Cell adhesion – allow cells to attach to each other permanently or temporarily. Gap and tight junctions Protein Function • Cell to cell communication - proteins with carbohydrates attached that provide identification for different species. • Channel for passive transport – allow substances to travel through the membrane from high to low concentration. • Pump for active transport – moves substances through membrane by changing shape using ATP for energy. Moves from Low to High 1.4 Membrane Transport • Passive and Active Transport Passive Transport • Does NOT require energy • Movement is from an area of higher concentration to an area of lower concentration. • Usually referred to as moving down the concentration gradient. • Osmosis (water), or diffusion Simple Diffusion • Particles moving from high to low concentration. In biology, usually across a membrane. • Examples: Oxygen moving from the blood into cells, Carbon Dioxide moving from cells into the blood. Facilitated Diffusion • Movement across a membrane is facilitated by a integral carrier protein. • Carrier protein changes shape but does not require energy. Osmosis • Osmosis is a type of diffusion that involves the passive movement of WATER across a partially permeable membrane. • Water will move from an area of lower solute concentration (hypotonic) to an area of higher solute concentration (hypertonic). Ease of movement • The size and polarity of a molecule determines how easily it can cross a membrane. • Small non-polar molecules cross easiest (gases) • Large polar molecules cross hardest.(ions and sugars) Active Transport • Requires work to be performed and therefor requires energy (ATP) • It is the movement of a substance AGAINST the concentration gradient, from an area of low concentration to an area of high concentration. • Example: animal cells have a high concentration of P+ inside and high Na+ outside. Sodium-Potassium Pump • 5 stages involved in moving Na+ and K+ against their concentration gradients: • 1- A specific integral protein binds to 3 sodium ions inside the cell • 2. The binding of the sodium to the protein causes phosphorylation to occur. ATP becomes ADP • 3- Phosphorylation causes the protein to change shape, expelling the Na+ from the cell • 4- Two Phosphate ions outside the cell bind to the protein, which causes the release of the phosphate. • 5- The loss of the phosphate returns the protein to its original shape, releasing the K+ into the cell. • The Na+ P+ pump is used extensively in nerve signal transport. Endo and Exocytosis • These are processes that allow larger substances in and out of cells. • Endo is into the cell, while exo is out of the cell. • Both processes depend on the fluidity of the cell membrane. Endocytosis • The membrane of a cell surrounds and engulfs an object, leaving it within a membrane inside the cell. Exocytosis • The opposite of endocytosis, how the cell exports proteins is a good example of how this works. • 1- Proteins produced by ribosomes on the Rough ER enter the lumen (inside) of the ER • 2- Protein exits the ER inside a vesicle and enters the cis end of the Golgi Apparatus. • 3- As protein moves through Golgi, it is modified then exits the trans end inside a vesicle. • 4. This vesicle moves to the cell membrane and fuses to release (secrete) the protein from the cell. • https://youtu.be/DuDmvlbpjHQ 1.5 The Origin of Cells • • • • Cell Theory: All organisms are made of 1 or more cells Cells are the smallest unit of life All cells come from other pre-existing cells • There are some problems with and exceptions to this theory. • How did the first cell arise? • Pasteur showed that cells don’t spontaneously arise. His experiment: • He boiled nutrient broth • He put sterile broth into 3 flasks, one open, one closed and one with a water trap type seal. He allow time for incubation • A sample from each flask was transferred to solid growth media (petri dish) and incubated. • The only flask that showed growth was the open one. Exceptions to normal • Multi nucleated cells of striated muscle, fungal hyphae and giant algae. • Very large cells with no compartments • Viruses • Where did the first cells come from. • Before that discussion we need to understand the transition from simple prokaryotes to complex eukaryotes. Endosymbiotic Theory • Presented by Lynn Margulis in 1981 • About 2 billion years ago, a bacterial cell began living inside another cell that had engulfed it (endocytosis) • This eukaryotic cell and the bacterial cell began a symbiotic relationship. • Over time, the bacteria cell evolved into a mitochondria. Evidence • Mitochondria: are about the size of bacteria • divide by binary fission • divide independently of their cell. • have their own 70s ribosomes • have their own DNA, single strand loop like bacteria • have a double membrane from being engulfed. • Chloroplasts also show the same evidence for endosymbiotic theory. • The protist hatena gets its energy by ingesting organic matter. When it eats algae, it starts photosynthesis to get energy. • The slug Elysia chlorotica is brown and eats things for energy when it is young. • As adults they are green due to eating algae and using them to do photosynthesis. • DNA of mitochondria more closely resembles that of bacteria than eukaryotes. 1.6 Cell Division • The life of a cell is described by the cell cycle. • In most cases, a cell will grow, then divide into two identical cells called daughter cells. • The cell cycle has a growth part as well as a division part. Cell Cycle Interphase • Longest phase of most cell cycles • Composed of three parts, G1 (Gap 1) , S and G2 (Gap 2). • G1 major event is growth of the new cells. • S major event is replication of the DNA • G2 major events are further growth as well as preparing for Mitosis (M phase). DNA starts to condense, organelles are copied, microtubules begin to form. Cyclins • Cyclins are proteins that help control a cells movement through the cell cycle. • Cyclins bind to cyclin-dependent protein kinases (CDKs), enabling them to act as enzymes. • These enzymes allow the cell to move from G1 to S and from G2 to M. • Some cells will stop at G1 and stay there (G0), such as nerve and muscle cells. Mitosis – Nuclear division • During Mitosis, the replicated chromosomes are divided into two identical groups in preparation for the cell division (cytokinesis). • The two new cells should be identical to each other and are called daughter cells. • Prior to Mitosis beginning, the DNA coils to form chromosomes. • Chromatin – nucleosomes- solenoids – looped domains - supercoil chromosomes • During Interphase, each DNA molecule is wrapped around histone proteins to form nucleosomes. • Nucleosomes coil into solenoids, then form looped domains and finally supercoil to chromosomes. • After replication during the S phase of interphase, each chromosome has an identical twin, attached to the original by a Centromere. Each molecule is called a chromatid and together they are called sister chromatids. Phases of Mitosis • • • • Prophase Metaphase Anaphase Telophase Prophase • Chromatin fibers continue to supercoil and form chromosomes. • Nuclear envelope (membrane) disintegrates and nucleolus disappears. • Mitotic spindle (fibers) forms. • Each chromosome has a region of its centromere called a kinetochore that the fibers attach to. • Centrosomes move to opposite poles Metaphase • Chromosomes move to the middle (equator) of the cell, this is called the metaphase plate. • The chromosomes centromeres lie on the plate • The microtubules, or spindle fibers do this. • The centrosomes are now at opposite poles. Anaphase • The shortest phase begins when the two sister chromatids of each chromosome split. • The chromatids, now chromosomes, move toward opposite poles, toward centrosomes. • The shortening of the microtubules causes the movement. • At the end of this phase, each pole of the cell has a complete, identical set of chromosomes. Telophase • • • • • • Chromosomes are at the poles Nuclear envelope reforms Chromosomes begin to uncoil Nucleolus reappears Spindle fibers disappear The cell elongates and is ready for cytokinesis Cytokinesis • In animal cells, a cleavage furrow forms • In plant cells, a cell plate forms • Both processes result in the cell being divided into two daughter cells with identical nuclei • Growth, development of embryos, tissue repair and asexual reproduction all involve Mitosis Cancer • Cancer is when the cell cycle runs out of control. • The mass of cells created is called a tumor. • A primary tumor is located at the original site of formation. • A secondary tumor forms as a result of metastasis or spreading to other areas. • Mitotic index is ratio of cells undergoing mitosis compared to the number not. The larger the number, the faster the cancer is growing. Causes of Cancer • Areas of a gene can change (mutate) to become oncogenes. • Oncogenes contribute to cancer formation. • Oncogenes can be formed by outside agents called mutagens. • Cigarette smoke is a mutagen. • There is a positive correlation between smoking and cancer.