Biology 1 Chapter 1 & 2 PDF

Chapter 1: Structures and Functions of the Cell - Microscopy and the Discovery of the Cell - In 1665, Robert Hooke English botanist examined a thin slice of dried cork tissue using a very simple microscope. It appeared to be similar to honeycomb chambers, which reminded him of small mo...

Chapter 1: Structures and Functions of the Cell - Microscopy and the Discovery of the Cell - In 1665, Robert Hooke English botanist examined a thin slice of dried cork tissue using a very simple microscope. It appeared to be similar to honeycomb chambers, which reminded him of small monastery rooms. This is what gave birth to cell biology which also helped the invention of microscopes. He used a microscope to examine thin slices of cork. He observed that cork is made of tiny, hollow, compartments. They reminded him of small rooms so he gave it the name cells. The plants Hooke observed were actually dead. He was observing cell walls and empty space. - Overview of the Historical Department of the Microscope - 1590, Hans Janssen and his son Zacharias Janssen placed multiple (2) lenses in a tube and found out that objects seen through the tube appear greatly enlarged. - 1609, Galileo Galilei invented a compound microscope using convex and concave lenses. (This is the second modification in the history of the compound microscope.) - 1625, This was the first time the term "microscope" was used by Giovanni Faber to refer to the compound microscope of Galilei. He called it "little eye". - 1665, Robert Hooke coined the term "cell". He was the first to see a plant cell under a single lens microscope. - 1676, Antoine van Leeuwenhoek was the first to see living cells using his own single lens microscope. He examined blood cells, yeast, and insects. - 1830, Joseph Lister reduced spherical aberrations or the "chromatic effect" by using several weak lenses together at certain distances to get a good magnification without blurring the image. This was the prototype for the compound microscope. - 1874, Ernst Abbe introduced a mathematical formula that correlates resolving power to the wavelength of light. It made the calculation of the theoretical maximum resolution of a microscope possible. - 1931, Ernst Ruska and Max Knoll designed and built the first transmission electron microscope. The electron microscope does not depend on light but on electrons. It can visualize objects as small as the diameter of an atom. By 1933 they had produced a TEM with two magnetic lenses which gave 12,000 times magnification. - 1932, Frits Zernike invented the first phase contrast illumination which allows the imaging of transparent samples. - 1937, The SEM was invented by Manfred Von Ardenne. - 1942, Ernst Ruska invented the first scanning electron microscope (SEM). It transmits a beam of electrons across the surface of a specimen. - 1957, Marvin Minsky introduced the principle of confocal imaging which gives a resolution that is higher than that of conventional light. - 1972, Godfrey Hounsfield and Allan Cormack developed the Computerized Axial Tomography (CAT) scanner. It can generate cross-sectional views and three-dimensional images of internal organs and structures. - 1978, Thomas and Christoph Cremer developed the first practical confocal laser scanning microscope. This instrument uses focused laser beams to scan objects. - 1981, Gerd Binning and Heinrich Rohrer invented the scanning tunnelling microscope (STM). It can visualize individual atoms within materials and give three-dimensional images of objects down to the atomic level. - 1986, Ernst Ruska won the Nobel Prize for his contributions to the study of microscopy. - 1992, Douglas Prasher cloned the green fluorescent protein that he used in fluorescence microscopy. - 1993-1996, Stefan Hell pioneered the first super-resolution microscopy. - 2010, Researchers used a cryoelectron microscope to see the atoms of a virus. - 2014, Eric Betzig, Stefan Hell, and William Moerner got the Nobel Prize in Chemistry for the super microscopes they invented. It can see matter smaller than 0.2 um. - The Early Microscope - It was 1579 when Hans Janssen and Zacharias Janssen discovered that positioning a lens at each end of the tube allowed for the magnification of images when used to observe objects. This invention paved the way to more discoveries in the study of microbiology. - Seventeenth-Century Microscopes and the Discovery of the Cell - Galileo Galilei arranged two glass lenses in a cylinder and examined the compound eye of an insect. - The compound microscope by the Janssens was then modified by Robert Hooke. He is considered the "English Father of Microscopy." He saw these small room liked structures in the cell. What he actually saw was a dead plant cell. Antoine van Leeuwenhoek was the first to ever see a living cell in 1674. He was the first to study protozoa. He created an instrument that provided a magnification of 270x. - After 50 years, Robert Hooke and Antoine van Leeuwenhoek discovered that the shorter the focal lengths of the lenses, the greater the magnifying power. This led to the use of double convex or spherical lenses to create compound microscopes with better resolution and higher magnification. - Modern Microscopes - In an electron microscope, a beam of electrons produces an enlarged image of the specimen. - Modern electron microscopes have greater magnifying powers and greater resolutions. They can magnify objects by up to more than 200,000 times their actual size by using beams of electrons. Examples of electron microscopes are the Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM). - Transmission Electron Microscopes (TEM)- use magnets to aim a beam of electrons at thin specimens in a vacuum. They also produce a black and white image based on electron absorption. The specimen must be dead and stained with heavy metals. - Scanning Electron Microscope (SEM)- Can magnify up to 1 million times. Specimens can be in gross form. Image is made of the surface of the object. Electrons are bounced off an object and collected on a photographic plate. Specimens must also be dry, dead, and stained with heavy metals. - Scanning Tunneling Microscope (STM)- uses the charged tip of a probe to get very close to the specimen. Electrons "tunnel" between the probe and the specimen. Creates three dimensional computer images of live objects and even atoms. - The crafting of the Titan 80-300 Cubed, a scanning transmission electron microscope, has revolutionized nanotechnology. This microscope can identify atoms, measure their chemical states, and probe the electrons that bind them together. - The Parts of a Microscope and Their Functions - Most ordinary classroom laboratories use light compound microscopes to see living or preserved specimens by up to 1,000 times their original size, and specimens are often stained to make the images stand out. - Multiply the eyepiece with the objective lens to determine the total magnification. - Low-Power lens (objective 1) (yellow)-- Magnifies 10x and is marked at 10x. To find the total magnification when looking through ocular lens and the objective lens, you multiply the magnification of the two lenses together. An ocular lens usually has the magnifying power of 10x Ocular lens x Objective lens = Total Magnification. Ex. 10x multiplied by 10x =100x (100 times larger) - High-Power lens (objective 2) (blue) - Can magnify up to 40x on the common compound microscope model you are using. If examining an object with 40x high-power objective lens, together with an ocular lens of 10x, the result would be: 40x multiplied by 10x =400x (400 times larger) - Resolution -- the ability of a microscope to show the details of an object being examined. It refers to the shortest distance between two points on a specimen that can still be distinguished by the observer or the camera system. - Contrast -- refers to the darkness of the background with reference to the specimen. Usually, lighter specimens could be seen clearly on darker backgrounds. In order to see colorless/transparent specimens, you need to use a phase contrast microscope. - Magnification -- is the increase of an object's apparent size. - Discovery and Development of Fluorescent Proteins - Osamu Shimomura first isolated GFP from the jellyfish Aequorea Victoria in 1962. - Martin Chalfie expressed the gen in bacteria in 1994. - Roger Y. Tsien contributed to general understanding of how GFP fluoresces. A microscope with labels on it Description automatically generated - Parts of a Light Microscope - Compound light microscope: shines light through a specimen. - Magnifies up to 2,000 times. - The parts of a light microscope can be grouped into three: mechanical parts, illuminating parts, and magnifying parts. 1. **Mechanical Parts**- parts of a microscope that are involved in giving support or strength to the instrument. These are also parts that are movable and can be adjusted. a. Body tube- a hollow tube through which light passes from the object to the eyepiece. b. Revolving nosepiece- holds the objectives. It can be rotated to select the appropriate objective. The lenses must be "clicked" into place to successfully view a specimen. c. Arm- connects the base and the body tube together. It serves as a handle for carrying the microscope. d. Stage- the platform where the slide or specimen to be examined is placed. It has an opening at the center that allows light to pass from below to the specimen. e. Stage clips- holds the slide in place. f. Base- the part where the microscope is firmly anchored. It gives support to the whole microscope and is the part where the illuminators are attached. g. Inclination joint- a joint found in some microscopes at which the arm is attached to the pillar of the microscope. It is used for the tilting of the microscope. 2. Illuminating Parts **--** parts of the microscope that provide and capture the light for illumination. a. Mirror- reflects light from the surroundings to the specimen on the stage. It is planar on one side and concave on the other. The concave side of the mirror is used for natural light, while the flat side is used for artificial light. It is supported by the mirror rack. In more modern microscopes, this is already replaced with a light source or a bulb that provides light. b. Condenser- concentrates the light from the light source onto the object of specimen being studied. It is located below the stage, and it is held in place by a rack. c. Iris diaphragm- regulates the amount of light that reaches the specimen. It is attached beneath the condenser. 3. Magnifying Parts- parts of the microscope that are involved in magnifying the image of the specimens, including the resolution. a. Eyepiece or Ocular- the part through which an observer looks to view a specimen. It usually has a magnification of 10x, through eyepieces with 5x to 30x magnification are also available. b. Objectives- the main lenses that magnify the specimen being observed. Usually, the microscopes have three objectives, but more modern ones house four or even five objectives. Typical objectives have magnifying powers of 4x, 10x, 40x, and even 100x. - General Principles of Microscopy - Contrast - Differences in intensity between two objects, or between an object and background. - Important in determining resolution. - Staining increases contrast. - Use of light that is in phase increases contrast. - Cell Theory - Robert Brown first observed the spherical structure in plant cells. He called this structure the "nucleus" of the cell. - Theodore Schwann discovered the presence of cells in animal tissues. - Matthias Schleiden concluded that plant tissues are composed of cells. - Rudolf Virchow studied the growth and development of cells and discovered that all cells arise from preexisting cells. - The observations made by these scientists comprised of the cell theory which include (3 tenets of the cell theory): 1. Cells are the smallest unit of life. All living things are composed of one or more cells. 2. Cells are the basic unit of organization of all organisms. Unicellular organisms -- one cell Multicellular -- specialized regions called tissues. Cells -- Tissues -- Organs -- Organ Systems -- Organisms 3. Cells come from preexisting cells. -The ability of cells to divide to form new cells is the basis for all reproduction and for the growth and repair of all multicellular organisms. Modern cell theory adds two additional key points: a. Cells carry and pass on to the offspring hereditary units during the cell division. b. All cells are relatively the same in terms of chemical composition and metabolic activity. - Modern Cell Theory - Cells contain hereditary information in their DNA. This information is passed to new cells by cell division. - All cells have the same basic chemical composition. - Energy flow (metabolism and biochemistry) occurs within cells. - Why would we want cells to be small? -As cells grow too large, they become inefficient. -Cells that grow too large do not have enough surface area to take in nutrients to remove waste. -Cells that grow too large take longer to move material within the cell. -Small cells have more efficient transport systems because they have higher surface to volume ratio, S/V. - Cell Size - Most cells are microscopic. - Bacterial cells range about 1-10um (microns or micrometers) in diameter. - Most plant and animal cells are visible only under the microscope, ranging from a size of 10-50um in diameter. - Cell processes are more efficient and effective if the cell has a small surface area. The larger the cell, the greater the surface area required to maintain it. - As cell grows, its volume increases faster than its surface area increases by the square. This suggests that as cells grow bigger, its surface area to volume ratio becomes too small to maintain life functions such as exchanging materials with the environment. This is why cells are small. - Cell Shape - A cell's shape depends on it function. Ex. Most nerve cells are long, which are related to their functions of transmitting impulses from the central nervous system to the different parts of the body. Ex. Long extensions of nerve cell send and receive messages. - A neuron or nerve cells has cytoplasmic extensions, such as axons and dendrites, that are important in the performance of its functions. - Skin cells are flat cells that help cover the body from the external environment, white blood cells can change shape, helping them digest and kill disease-causing germs that invade the body. A diagram of cell body parts Description automatically generated - Cell Diversity - Cells vary in shape, size, and internal organization. - All cells have a specific job to do and look and function the best for that job. - Internal Organization - The structural characteristics of a particular cell are closely related to it function. The diversity in cells could be seen between different species. - A human body carries 7 different kinds of cells. Each cell in the human body is specialized and adapted for a particular job. Ex. Glandular cells are different from muscle cells. Glandular cells produce secretory materials such as mucus and hormones, which is why they are provided with more ribosomes and Golgi bodies. Muscle cells, on the other hand, are provided with more mitochondria to produce more energy needed in muscle contraction. Ex. A plant cell and an animal cell. - These two cells show great variations in parts because they function differently to perform specific tasks. Chapter 2: Cell Structures and Their Functions - Cells vary in many aspects. They vary in size, shape, and complexity. However, they are alike in a few basic characteristics. Every cell, except from bacteria, has three main parts: the nucleus, the cytoplasm, and the cell membrane. - Prokaryotic Cells - They are mostly microscopic, measuring from 1 to 10um in diameters, and exist in unicellular form. The terms prokaryote is derived from the words *pro* and *karyon,* which mean "before" and "kernel." This term describes cells that already exist before the evolution of the cell nucleus. - Prokaryotic cells do not have a membrane-bound nucleus. Their organelles are also not membrane-bound. There are two groups of bacteria in terms of evolution---the archaebacteria and the eubacteria. Both of these groups are Prokaryotic.  - Parts of a Prokaryotic Cells 1. Glycocalyx- an outer layer that provides protection. It is an important virulence factor since it protects disease-causing bacteria. It helps bacteria hold on to surfaces and protects them from being engulfed by marcophages. It may exist as a rigid capsule or a more unstructured slime layer. 2. Cell wall- a structure that confers rigidity and shape to the cell. It is found outside of the plasma membrane and is composed of peptidoglycan. 3. Plasma membrane- a structure that prevents the loss of water and electrolytes inside the cell. It also prevents the entry of unwanted substances into the cell. It is composed of phospholipid bilayer. 4. Plasmid- a small, circular, extrachromosomal DNA molecule found in the cytoplasm. It is separate from chromosomal DNA. 5. Nucleoid- the region where DNA is concentrated. 6. Cytoplasm- the whole inside region of the cell where chromosomes, ribosomes, and other cellular inclusions are suspended. 7. Ribosome- the site where proteins are synthesized or created. 8. Pilus (plural-pili)- a short, hairlike appendage on the surface of some bacteria. It helps bacteria adhere to the surface of host cells. It can also be used to transfer genetic material from one bacterium to another, in which case it is called sex pilus. 9. Flagellum (plural-flagella)- a long, threadlike structure that facilitates movement in bacteria. 10. Fimbriae- bristle-like fibers that are shorter than pili. It is primarily used for bacterial attachment to tissue surfaces. - Eukaryotic Cells - More complex than prokaryotic cells. A typic eukaryotic cell measures 10 to 100um in diameter. They are bigger than prokaryotic cells. These cells have components that are surrounded by membranes which are called organelles. The nucleus, the largest cell organelle, encloses the genetic material and is suspended in the cytoplasm. The most distinguishing feature of the type of cell is compartmentalization, which is achieved by the endomembrane system that occupies the interior of the cell, including the membrane-bound organelles. Most of these organelles independently perform their multiple biochemical jobs, which can proceed independently or simultaneously. Examples of eukaryotic cells are those that come from animals, plants, protists, and fungi. - The eukaryotic cells in your body and in other multicellular organisms vary in many aspects. It may vary in terms of size, shape, internal organization, and function. - Cell Structures and Their Functions - Cells vary in many aspects. They vary in size, shape, and complexity. However, they are alike in a few basic characteristics. Every cell, except for bacteria, has three main parts: the nucleus, the cytoplasm, and the cell membrane. A. Cell Membrane - The cell membrane, which is sometimes called the plasma membrane, is a thin layer that separates the cell from its external environment. It is the outermost covering of animal cells and functions as a selective barrier that regulates the entrance and exit of substances into and out of cells. It is said to be a selectively permeable membrane. It provides shape and flexibility for the cell. However, in plant cells, the cell wall is the outermost covering. The cell membrane can be described in two models: the classical model and the fluid mosaic model. - \(1930) Hugh Davidson and James Danielli hypothesized that the cell is covered by a thin, flexible envelope made up of phospholipid bilayer and proteins. This was the classical plasma membrane model. Through which the help of the early electron microscopists, it was confirmed that the phospholipid membrane has hydrophobic and hydrophilic ends. - \(1972) Jonathan Singer and Garth Nicholson proposed the fluid mosaic model of the plasma membrane which revolutionized our understanding of the nature of the membrane. This became more popular than the model of Davson and Danielli. This was made possible using a modern electron microscope, which is more precise in nature. At normal temperature, the plasma membrane behaves like a thin layer of fluid covering the surface of the cell and that individual phospholipids diffuse rapidly throughout the surface of the membrane. It is termed mosaic because it includes integral proteins that protrude above or below the lipid bilayer, peripheral proteins, glycolipid, cholesterol, and other molecules.  - The glyocalyx is the external coating of the cell membrane and is made up of glycoproteins and polysaccharides. It serves different functions as summarized below. 1. It provides protection 2. It enables cell-to-cell recognition 3. It contains receptor or contact sites for enzymes and hormones 4. It allows the cell to respond to changes in electrical potentials 5. It acts as a filtration barrier B. Cytoplasm - The region of the cell that surrounds the nucleus is the cytoplasm. It is a semifluid matrix and is the largest interior part of the cell where organelles and cellular inclusions are suspended. These are the organelles, and the other cellular inclusion found in the cytoplasm: - Cytoplasmic Organelles: 1. **The endoplasmic reticulum (ER)-** is a network of intercommunicating channels composed of membrane-enclosed sacs and tubules. It serves as an intracellular highway through which molecules can be transported from one part of the cell to another. The amount of ER varies in each cell depending on its activity. 2 forms: 1. Rough Endoplasmic Reticulum (RER) -Looks rough due to the presence of ribosomes on its membrane surface. A cell with more RER produces a large amount of proteins to be inserted into the membranes or exported to the outside. 2. Smooth Endoplasmic Reticulum (SER) -SER is more tubular and nongranular due to the absence of ribosomes. SER is usually involved in the synthesis of steroids in gland cells, breakdown of toxic substances by liver cells, and regulation of calcium levels in muscle cells. A diagram of a cell Description automatically generated 2. Golgi Apparatus \- similar to the endoplasmic reticulum. It is also a system of membranes. It appears as a series of flattened sacs with a characteristic convex shape. It is surrounded with numerous vesicles filled with fluid and suspended substances. It works in close association with the endoplasmic reticulum. It is responsible for the processing, packaging, and sorting of secretory materials for use within the cell or for exocytosis (cell secretion). Ex. After protein is synthesized by the ribosome, it passes into the interior of the rough endoplasmic reticulum. Then, it moves into the interior of the smooth ER, in which, the protein is enclosed by a membranous pouch that buds off from the smooth ER. Then it migrates and fuses with the Golgi apparatus where a new vesicle is formed and passed on to the cell membrane and expelled to the outside (exocytosis).  3. The Mitochondrion (plural-mitochondria) -Is the power plant of the cell. It varies in size, shape, and number depending on the degree of cellular activity. It contains enzymes that help in the chemical oxidation of food molecules and produces energy in the form of ATP. Studies have shown that active cells such as your liver cells have more mitochondria compared to less active cells such as your skin cells. A single liver cell may contain as many as 2,500 mitochondria, while a skin cell will have only a few hundred. -Mitochondria have their own ribosomes and DNA, which means that new mitochondria arise only when existing ones divide. A cross section of a plant cell Description automatically generated 4. Lysosomes -Small, spherical, membrane-bound organelles which contain various kinds of enzymes. Enzymes are molecules that digest proteins, nucleic acids, polysaccharides, and lipids. They protect a cell from invading bacteria and other pathogens. They also break down damaged or worn out cell parts. They can engulf and digest targeted molecules. When a molecule is broken down, the products pass through the lysosome membrane and are returned back into the cytoplasm to be recycled.  5. Secretory granules \- Large, dense granules with membranes. These fuse with the cell membrane to secrete substances such as enzymes, proteins, and signaling molecules out of the cell. 6. Lipid droplets -Store fatty acids and sterols. They take up much space and volume in adipocytes or fat cells. They appear as black spherical bodies of varying sizes when stained. - Cellular Macromolecules -Substances suspended in the cytoplasm with varying functions and are not membrane-bound structures. Their quantity is dependent on the cell type. 1. Ribosomes- not considered organelles because they are not surrounded by membranes. Each ribosome is an assemblage of two organic compounds, namely, proteins and RNA. In the cytoplasm, some ribosomes remain free, while others are attached to the endoplasmic reticulum. They are the molecules that synthesize proteins. The distribution of ribosomes inside the cell varies. This depends on how the proteins will be used. Proteins that are needed by the cell itself are produced by the freee ribosomes, while proteins that will be inserted into the cell membrane or exported outside of the cell are produced by those attached to the endoplasmic reticulum. 2. Centrosome- part of the cytoplasm that produces microtubules. In animal cells, it forms two small parts called the centrioles. The centrioles are small cylindrical structures made of short microtubules arranged in a circle. Though their main function is to assist in cell division, studies have shown that certain cells continue to divide even without them. 3. Cytoskeleton- provides mobility and strength for the cell. It is the collective term for the network of filaments and tubules that extends throughout the cell. The types of fibers compromising the cytoskeleton are as follows: a. Microtubules are long, slender, protein tubes. Together with microfilaments, they form the cytoskeleton or the framework of the cell. It is composed of linear polymers of tubulin. A network of microtubules forms the spindle apparatus that appears during cell division. These are also form the cores of the cilia and flagella of sperm cells and play a role in maintaining cell shape. b. Microfilaments support the cell to maintain its structure and shape, as it provides resiliency against forces that can alter its shape. Spindle fibers are examples of microfilaments that aid in the movement of chromosomes during cell division. They are also important in cytoplasmic streaming or cyclosis. A diagram of a structure Description automatically generated 4. Glycogen granules- abundant in liver cells. They play an important role in glucose metabolism 5. Biological pigments- abundant in plant cells, particularly un photosynthetic cells. These are usually found in plastids, such as chloroplastids, where chlorophyll pigments abound. In animals, pigments are mostly found in the cells of skin, eyes, hair, and feathers. C. Nucleus - The most visible part of the eukaryotic cell is the nucleus. In an animal cell, it is roughly spherical in shape and is generally located at the center of the cell. It is the site where nucleic acids are synthesized. The nucleus also serves as the site for the storage of hereditary factors. It is the source of ribonucleic acid (RNA), a molecule responsible for converting genetic instructions in DNA into functional substances such as proteins. Some cells, however, such as red blood cells and platelets, lose their nucleus as they mature.  - the **nuclear membrane** is composed of two layer which separate the nucleus from the cytoplasm. It contains ribosomes on its outer membrane. It is also continuous with the endoplasmic reticulum. The dense, protein-rich substance inside the nucleus is the **nucleoplasm** in which the spherical, unbound **nucleolus** (structures responsible for ribosome formation) is suspended. It is rich in proteins and nucleic acids, and is where rRNA is transcribed and assembled. - **Nuclear pores** are openings in the nuclear membrane. These act as selective channels between the cytoplasm and the inside of the nucleus, selectively allowing molecules that come in and out of the nucleus. The proteins that make up the nuclear pore complex are arranged radially with a large central hole. - Found inside the nucleus is the chromatin, which is made up of DNA and proteins and forms chromosomes during cell division. Chromosomes contain the genes inherited by the offspring from their parents. A diagram of a cell structure Description automatically generated - Humans have 46 chromosomes. Each organism has its own specific number of chromosomes. Abnormalities in the chromosome structure or aberration in the chromosome number can lead a genetic disorder or even death. - Cell Modifications and Adaptations - Cell modification refers to a process in which an ordinary or generic cell is transformed into a specialized cell in order to do a specific task for my body. The function that they perform after modification becomes different from their previous tasks. This process has contributed much to the adaptation and survival of organisms. For instance, not all cells in the small intestine are the same in structure and function. Some have microvilli, while others do not. **Microvilli** are cytoplasmic extensions that increase the surface area of a cell, hence, increasing the absorption of nutrients. - *Nerve cells* which are mostly elongated, facilitate the transmission of impulses from the brain to the spinal cord and different parts of the body. - *Red blood cells (RBCs) have a biconcave-disc shape and are highly deformable. The size of 2-3um in diameter allows for easy movement through the blood vessels. Red blood cells and platelets lose their nucleus as they mature. Losing the nucleus increases the surface area for gas exchange, enabling the optimal oxygenation of tissues in the body.* - The *Sperm cell* is another specialized cell with parts that help carry out its function. Sperm cells have a tail, the flagellum, which propels them toward the egg for fertilization. Sperm cells have plenty of mitochondria along their middle piece, which power the flagellum to move toward the egg cell. - The skin of some animals such as amphibians, squids, and octopuses contain *chromatophores.* These are star-shaped cells containing bioluminescent pigments that facilitate the changing of the color of the body. Jellyfish and *hydra* have *nematocysts* or stinging cells that contain needle-like structure used to inject a toxic substance into the prey of interest. - In plant cells, there are also specialized cells such as *root hairs*, which are elongated outgrowths from the outer layer of the root cells that help absorb water and minerals. They increase the absorption area and capacity of the roots. Cells on the surface of the area of the leaf are elongated in shape and are loaded with chloroplasts. Plant cells also have *plasmodesmata* or small pits that link one plant cell to another. They facilitate the movement of molecules between adjoining cells. *Guard cells* are specialized to regulate the opening and closing of the stomata. - Another specialized modification in eukaryotic cells is the presence of cell-cell junctions, the point where two cells come together. Through the cell junctions, the cells are joined in long term associations, thus, forming tissues and organs.  A diagram of a cell Description automatically generated  - Lesson Summary: - Cellular diversity is reflected in the different shapes and sizes of cells found in tissues - Cells are small to maximize cellular process - Prokaryotic cells and eukaryotic cells differ greatly in terms of internal organization and parts - Prokaryotic cells lack a nuclear envelope and membrane bound organelles. - Eukaryotic cells are greatly larger and have a very distinct nucleus that is clearly surrounded by a nuclear envelope. - The cell membrane, sometimes called the plasma membrane, the outermost covering of animal cells. - The materials in the cell that surround the nucleus make up the cytoplasm. - Organelles are tiny structures in the cytoplasm that are surrounded by a membrane. - Some organelles found in the cytoplasm include the endoplasmic reticulum, the Golgi apparatus, the mitochondrion, lysosomes, secretory granules, and lipid droplets. - Cytoplasmic inclusions are non-membranous substances and structures suspended in the cytoplasm. Some cytoplasmic inclusions in the cell include ribosomes, centrioles, microtubules, microfilaments, glycogen granules, and pigments. - Most eukaryotic cells have a nucleus. It houses an organism's genetic material. - Plant cells have cell walls and plastids. They also have larger water vacuoles than animal cells. - Cell specialization or modification refers to the process by which an ordinary or typical cell is converted into a specialized cell to do a different task for the body.