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Cell Theory
Cells are the basic unit of life that makes up every living organism. Cells were discovered by an English scientist known as Robert Hooke; he observed the structure of a thinly sliced cork under the light microscope which he invented (Khan Academy, 2015). He observed in the dead plant specimen small structures which he believed were cells. Schneiden and Schwann later observed the cells of an animal and concluded that all animals have cells that live and breathe. The last discovery was by Rudolf Virchow, where he discovered that all cells come from other cells. The cell theory has the following concepts; they only arise by division from an existing cell, they are the building block of all organisms, cells contain inherited information which controls their functioning, with favourable conditions cells can survive independently, and they undergo metabolism.
Eukaryotic and Prokaryotic Cells
There are two types of cells: eukaryotic and prokaryotic cells. Prokaryotic is the simplest and most ancient type of cells; they do not contain a nucleus, nor do they have any membrane-bound organelle. Prokaryotes can be grouped into two groups: bacteria and archaea. In contrast, eukaryotic cells contain a nucleus within every one of them, and their organelles are membrane-bound.
Differences between Eukaryotic and Prokaryotic.
Typical animal
Animal cells are generally irregular in shape due to the absence of a cell wall. All animal cells comprise of the following organelles:
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Cytoplasm Yellow fluid composed of mainly water. The cytoplasm is continuously streaming, and it is the biggest part of a cell. Organelles are suspended in the cytoplasm and salts are dissolved in it.
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Nucleus It is the Control centre of the cell. It controls metabolism and reproduction activity of the cell. It contains the nucleolus which contains the genetic information of a cell.
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Endoplasmic reticulum Endoplasmic lined with ribosomes is called rough endoplasmic reticulum while the one not lined is called the smooth endoplasmic reticulum. The endoplasmic reticulum is a system of membranous canals filled with fluid that carry materials around the cell. Ribosomes synthesize proteins.
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Golgi body They temporarily store proteins which can then leave the cell via vesicles pinching off from the Golgi body.
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Lysosomes Contain strong digestive enzymes which break down organelles as well as food.
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Mitochondrion Organelle surrounded by a double membrane with the inner membrane being highly folded. Releases energy from the food inform of adenosine triphosphate.
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Vacuoles Fluid-filled organisms enclosed by a membrane and can store food, water minerals and waste products.
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Cell membrane Bi-lipid layer composed of protein and carbohydrate. The cell membrane separates the cell from its external environment, and it is selectively permeable. Selectively permeable meaning it controls what enters and what leaves.
Typical plant cell
Plant cells are regular in shape and rigid because they have cell walls. Although plant and animal cells have some similar organelles with the same functions, there are those distinct to only plants these include:
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Cell wall A rigid layer composed of cellulose polysaccharides and proteins which is found outside the cell membrane. The cell wall provides structural support to the cell and protects the cell from mechanical damage
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Chloroplast It is an elongated organelle filled with the green colouring pigment in plants called chlorophyll which is required for the photosynthesis. The organelle also contain stroma which comprises of circular DNA
Microscopy
Microscopes are generally built to magnify, increase resolution and contrast of cells being observed under the microscope. The reason we use it on cells is that cells are too small to be seen by the naked eye. There two types of microscopy we are concentrating on, optical microscopy and electron microscopy. Optical microscopy uses light under a system of lenses to magnify samples which can be viewed directly by the eye.
Light Microscope
A light microscope uses light rays that illuminate the specimen, making observation possible. The specimen is usually placed on a slide, stained with iodine and then covered with a slide and placed on the stage where clips secure it in position preventing the specimen from moving around. The light source produces light which passes through the condenser lens to the stage. The condenser lens mainly focuses light into a narrow beam to reach the specimen on the stage. On reaching the specimen, the light travels through the specimen to the objective lens, which magnifies the specimen. The objective lenses are usually four; hence the image can be viewed under different magnification factors. Light then travels to the eyepiece lens, which magnifies the image and converts the real enlarged image into an enlarged virtual image which is then projected to a receptor. The fine adjustment knob is turned to regulate the distance between the object and objective slightly to get the desired sharpness while the course adjustment knob which also moves the stage to regulate the distance between the specimen and the objective lens roughly and quickly
Magnification
Magnifications the size of an image divided by the size of an object
Magnification formula;
M= m1 ×m2
Where,
M=magnification
m1=linear magnification of the objective lens
m2=linear magnification of the eyepiece
Measuring a cell
Example 1
The image of a cell was measured to be 50mm, and the objective lens was set to *100.find the actual size of the cell.
Actual Size= Image Size / Magnification
50/100=0.5mm
Example 2
The actual size of a cell is 6 micrometers (µm), and the image of the cell was a measure to be 36 mm. Find magnifications?
Magnification = Image Size/Actual Size
1mm=1000µm therefore 36mm=36000µm
3600µm/6µm=×6000
Resolution
Resolution is the ability to distinguish between two distinct points. The greater the two points can be resolved, the greater the resolution.
Electron Microscope
This type of microscope uses electrons to view the image of the specimen. There are two types of electron microscope: The Transmission Electron Microscope (TEM) which allows electrons to pass through the specimen so one can see inside the cell and the Scanning Electron Microscope (SEM) collects the reflected and rebounded electrons off the specimen which gives the image of the outside surface.
The electrons released from the electron gun form a beam by the help of the anode which brings the electrons together, preventing scattering. The electron microscope is usually filled by vacuum inside to prevent coalition of electrons with air particles. The electron beam then flows to the condenser lens whose function is to direct the electron beams onto the specimen which is placed on a grid. Before the sample, is the objective lenses which magnify the image. The beam then passes the specimen to the fluorescent screen below where the image is formed. The fluorescent screen glows wherever the beam of electrons hit.
Advantages and disadvantage of an electron microscope over a light microscope
The electron microscope has higher resolution capacity over a light microscope; the electron microscope does require tuning the focus like the light microscope it focuses at once on the image. The only weakness of the electron microscope is that the cell has to be dead.
Methods of preparing a Slide
Slide preparation depends on the samples structure; these methods include dry mount, wet mount, staining smear and squash.
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Dry Mount The dry mount method can be used to observe dry specimens like pollen, hair, pollen and dead matter such as grasshoppers and aphids.
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Wet mount Wet mounts are usually used for aquatic samples, living organisms which require liquid suspensions brine, glycerin and immersion oil. The slide is prepared by placing a drop of fluid on the slide, place the specimen on the slide, lower the cover slowly to avoid bubbles and remove water with a paper towel.
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Smear slides The sample is usually liquid or slimes in this case. The sample is placed on a slide, and the second slide is used to smear the sample creating a thin layer. This is done slowly to prevent trapping of bubbles. The angle of the smearing slide determines the length of the smear.
A steep angle creates a shorter smear compared to a gentle angle.
Squash slides This method is usually designed for soft samples. The slides are prepared by placing the tissue on a wet mount and press the cover glass gently to remove excess water. Caution not to damage the sample.
Plasma Membrane
The plasma membrane is phospholipid layer that surrounds the surfaces of cells and organelles. They are evident in both eukaryotic and prokaryotic cells. The plasma membrane has many functions. Its principal function is to acts as a barrier protecting components of the cells and organelles from the outside environment. The plasma membrane is composed of two phospholipids bilayer making it able to maintain the structure of the cells and organelles. The bilayer is formed due to the properties of phospholipids. A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. The phosphate head is said to be hydrophilic because it is attracted to water and the tails are said to be hydrophobic because they repel water.
In an aqueous environment like the cells of our body, the phosphate heads position themselves to form the two outer rows because they are attracted to water molecules while the hydrophobic tails are protected by a non-aqueous core as a cluster to avoid the water. The plasma membrane is selectively permeable; it controls what goes in and out of the cell and organelles. Phospholipids membrane allows only oxygen, water and waste in and out of the cell but restricting entry of charged particles. The plasma membrane also contains other molecules that aid it in its functioning; these molecules include proteins, carbohydrates and cholesterol. Together the combination of these molecules form a fluid mosaic model.it is called the fluid mosaic model because the cell membrane is dynamic, the cell membrane moves on the soup, which is the phospholipid.
Membrane Proteins
There are several different proteins embedded in the membrane which serve a variety of function. They help with both facilitated diffusion and active transport. They connect cells; they participate in signal transduction and act as markers for cell identification. There are two types of membranes: intrinsic membranes and extrinsic membranes.
Integral Protein Integral proteins are proteins permanently embedded in the phospholipid bilayer. They include carrier proteins which are used to transport molecules in and out of the membrane by active transport and facilitated diffusion, channel protein which allows facilitated diffusion. The channel protein provides passage for molecules that cannot travel through the phospholipid layer such as ions and water.
Extrinsic Membrane
These types of membranes do not interact with the hydrophobic core. A good example is glycoproteins. Glycoproteins are Short chains of carbohydrates are attached to proteins; they serve as recognition signals to other cells and also act as cell receptors. They sometimes receive growth hormones or other enzymes that signal the cell to do things
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Glycolipids These are short chains of carbohydrates attached to membrane lipids; they offer cell recognition ability by other cells.
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Cholesterol Cholesterol molecules are also found in the plasma membrane. They provide support and fluidity to the plasma membrane structure. They allow it to change shape, making it fluid.
Factors Affecting Plasma Membranes
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Temperature Phospholipids are held together by hydrogen bonds that exist between the hydrophilic tails, them being held together tightly is what makes them semi-permeable. Hydrogen bonds are weak. When heat is applied, the molecules, they gain thermal energy and vibrate. The vibration causes the phospholipids to move more along the membrane and further apart from each other. The hydrogen bonds then break, causing the membrane to lose its structural integrity and the hydrophobic core to seize existing causing larger molecules to pass through easily. Besides, increasing the temperature also denatures the proteins. Denatured proteins can either prevent any particle from passing through or allow any particle to pass through depending on how they are denatured.
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Solvent type Phospholipids have a hydrophobic core meaning they cannot allow any polar solvent to pass through them. However, when placed in a non-polar solvent, the solvent seeps between the tails and some of the solvent molecules get embedded within the membrane. The non-polar solvent molecules break the hydrogen bonds wherever they get embedded, making the membrane much more permeable.
Active Transport
This is the movement of particles against a concentration gradient from a low concentration gradient to a high concentration gradient by using adenosine triphosphate (ATP) involving protein carriers.
In order to move a particle from a region of low concentration to a region of high concentration, the protein carrier should have specific binding site for the particle. Different protein carriers have different shapes of binding sites depending on the shape of the particle. Once the particle is bound, adenosine triphosphate (ATP) binds with the protein carrier on the other side that faces the cytoplasm. The ATP is hydrolyzed into adenosine di-phosphate and a phosphate group to release energy. The energy is used to change the shape of the carrier protein so that the protein faces the side of the cytoplasm after which the particle is released to the region of high concentration. The phosphate group is thereafter released, allowing the carrier to go back to its normal shape since its chemical composition is reverted to normal.
Passive Transport
Living organisms have to be able to exchange substances such as food, oxygen and waste products between cells and their environment to survive. Passive transport can be defined as the transfer of substances across a cell membrane without requiring metabolic energy. The substances move across the membrane by the use of their kinetic energy. The two types of passive transport are diffusion and osmosis.
Diffusion
Diffusion can be defined as the net movement of particles from a region of high concentration to a region of low concentration across a partially permeable membrane. Specific particles are able to do this; these particles have to be small and non-polar to undergo diffusion easily.
For diffusion to occur there has to be a concentration gradient. This means there has to be a region where there is more and a region where there is less. In regions of high concentration when particles move randomly, they collide. They move away from each other, and eventually, some of the particles will spread to regions of low concentration until they get evenly dispersed and reach equilibrium. At equilibrium, the particles still move freely because they still contain kinetic energy, but now there is no net movement but only equal movement in all directions.
In cells when particles reach equilibrium, there is still passage of particles through the plasma membrane but instead of distributing themselves equally on one side, they move across to the membrane to the other region. At equilibrium, there is no net movement of particles.
Osmosis
Osmosis like diffusion is a passive process; it does not require any energy to occur as it depends on the constant random movement of molecules. Osmosis is defined as the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Water potential can be defined as the potential of water molecules to move out of a solution. The more a solute is added to water, the more its water potential drops. Similar to diffusion, the random movement of water molecules across the membrane will continue until they reach equilibrium.
Animal cells and plant cells are affected by osmosis. When animal cells are placed in a solution that that is less concentrated to their cytoplasm (hypotonic solution), water molecules will move across the cell membrane to the cytoplasm by osmosis. The cell swells and eventually may burst. When this process occurs in red blood cells, it is referred to as haemolysis. When the cells are placed in a solution that is more concentrated than their cytoplasm (hypertonic solution), Water molecules move out of the cytoplasm to the surrounding solution by osmosis. The cell shrivels. When this process occurs in red blood cell, it is referred to as crenation. When the cell is placed in a solution whose concentration matches the cytoplasm (isotonic solution) equilibrium is reached, and the cell experiences no structural change
Due to the fact plants have a cell wall; they react differently from animal cells when placed under the same solutions. When plant cells are placed in a hypotonic solution, water molecules move into the cytoplasm, causing it to swell; hence the cell becomes turgid. The cell wall prevents the cell from bursting. Although, when placed in a hypertonic solution, water moves out of the cytoplasm by osmosis and the cytoplasm shrinks. The cell membranes move away from the cell wall; this process is referred to as plasmolysis. In an isotonic solution, there is no net movement of water molecules across the membrane causes the cell to be flaccid. Isotonic conditions cause plants to wilt.
Exchange Surfaces
All living organisms require exchange to function at optimal conditions. Materials exchanged can vary from food, oxygen to waste products. In this instance, we are going to concentrate on gaseous exchange of mammals, fish and insects.
Insect gaseous exchange
Although insects are small, they are usually highly active, which means they require a constant supply of oxygen and removal of carbon (IV) oxide. An insect poses exoskeleton, which is a skeleton outside their body. The exoskeleton offers protection and water retention ability. Because of this exoskeleton gases in the air cannot easily diffuse into their cells, they bounce off. The carbon (IV) oxide that builds in their cell cannot diffuse out easily, causing a building up. They have evolved a specific type of exchange surface called the tracheal system, which delivers oxygen to every tissue and removes carbon (IV) oxide as well. The tracheal system has openings on the skin known as spiracles. The spiracles are openings that lead to larger airways in the system, which are called trachea. The trachea then divides into a smaller tube called tracheoles.
Gaseous exchange in insects usually occurs in fluid the end of the tracheal system called the tracheal fluid. This fluid exits at the end of the tracheoles. The tracheal fluid is in contact with the tissues and cells of the insect. When insects are active, their muscles require energy. The muscles draw up tracheal fluid containing oxygen-filled air away from the airways. The movement of the fluid away from the tracheoles also lowers pressure in the tracheal system drawing more air into the spiracles. This movement also increases the overall surface area of the tracheoles, making it easier for oxygen-filled air entering the tracheoles to diffuse into the tissues easily and carbon dioxide filled air out of the tissues. The oxygen taken in is then used by cells for respiration. When the insect is dormant, the fluid seeps into the tracheoles from the surrounding cells and lining the airways.
Ventilating the tracheal system
Ventilation is a specialized breathing mechanism that tends to occur in larger insects. This occurs when air sacs which are swellings in the airways are squeezed by flight muscles altering the volume of thorax pushing air in and out. Ventilation action continually draws in air with more oxygen and pushes out air with more carbon dioxide.
Other large insects have a different type of specialized breathing mechanism. It is specialized in how it works. As their abdomen is expanded by different muscle, this causes the spiracles in their abdomen to close and the spiracles in their thorax to open. Oxygen filled air enters through the thorax spiracles while those in the abdomen are kept closed. In the next cycle, the spiracles on the thorax close and those in the abdomen open, letting out the carbon dioxide filled air.
Fish gaseous exchange
Fish need to perform gaseous exchange just like any other animal. The only difference is that they do it underwater. Unlike land organisms, they take oxygen that has been dissolved in the water. This has made fish evolve specialized structure called gills so that they can exchange gases effectively. As they swim, water enters through their mouth and exit through the gills as they move. The gills consist of a series of bony arches with two stacks of gill filaments.
Gill filaments have protruding rows of very thin lamellae. Lamellae are used in biology to describe something thin and flat. Each lamella consists of a network of capillaries covered by a single layer of epithelial cells. Capillaries carry blood rich in carbon (IV) oxide (deoxygenated blood) from the body and as they reach the lamellae the branch into smaller capillaries. Capillaries are the site at which gaseous exchange occurs. After gaseous exchange, the capillaries re-connect and take oxygen-filled air (oxygenated blood) to where it is needed. The thin layer of epithelial cells provides a very small diffusion pathway. Being that these structures are very delicate, they are protected by a bony structure known as the operculum.
Bucall pumping
Buccal pumping is the method fish use to ventilate their gills by making freshwater move through the gills and deliver dissolved gases to those capillaries. The purpose of this is to maintain a strong diffusion gradient so that efficient gaseous exchange can take place. To ventilate the gills, they use the coordinated opening of the opercular vents with the closing of the buccal cavity. The buccal cavity means the fishs mouth.
When the buccal cavity opens the opercular vents close and water is drawn into the buccal cavity which is then pumped over the gills providing oxygenated water to the gills. When the mouth is closed, the opercular vents become open and provides the pathway of water in the mouth to flow over the gills and escape out the vents taking deoxygenated water with it. Fish such as sharks do not have the ability of buccal pumping; therefore, they always need to swim to keep a fresh supply of water over their gills.
Counter current flow mechanism in the gills
The gill filament and the lamellae are oriented in such a way that they ensure water passes over them counter current to the flow of the blood in the capillaries. Counter-current can be defined as two liquids in proximity to each other flowing in the opposite direction. The counter-current flow ensures optimum gaseous exchange.
When water flows past the capillaries, it loses oxygen to the blood down the diffusion gradient. The oxygen in the water diffuses into the blood with low oxygen levels.
As the blood continues to flow, it continues to take in more oxygen from the freshwater entering the system. There are two steps in this: there is receiving oxygen from partially oxygenated water, and there is receiving more oxygen from freshwater entering the system. This maximizes efficiency
Mammal Gaseous Exchange
Mammals are large organisms; therefore, they require a large intake of oxygen; they also produce large amounts of carbon (IV) oxide. Therefore mammals evolved a specialized gaseous exchange system to satisfy the need for this requirement. They evolved the lungs which are generally found in the thorax. The lungs are encased in the protection of a group of ribs which make up the rib cage. The gaseous system is separated from the abdomen by a diaphragm. The diaphragm is flexible cartilage. Lungs are two in number, one for each side of the body.
The lungs are connected to the mouth and nose by the trachea and bronchi. The mouth and nose are the portals of entry and exit for these gases. When mammals breathe in, air travels down the oral cavity and descends down the trachea. The trachea then divides into two primary tubes named the bronchi (bronchus in singular). These tubes are lined down with C-shaped cartilage rings. Cartilages are stiff tissues that provide support and allow little flexibility to the tubes. They provide support by keeping the airway open. The inner walls of the tubes are lined with ciliated epithelial tissue; ciliated cells are basically cells with hair at the top. The function of the ciliated cells is to waft mucus, dirt and pathogens in a particular direction. The walls are also lined with smooth muscles which contract and relax adjusting the diameter of the tube regulating the volume of air flowing in the tubes. The epithelium tissues also contain goblet cells which produce mucus that traps dust
The bronchi divide into smaller and smaller branches as they go deeper into the lungs. These branches are called bronchioles. They lead down to the alveoli, which are the smallest unit. They mainly consist of smooth muscles and some epithelial cells. The lungs have two-process cycle ventilation; inspiration and expiration.
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