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Cell Structure and Function
Transcript of Cell Structure and Function
The Discovery of Cells!
The accumulated research of the men from the previous slide can be summarized in the cell theory, one of the first unifying concepts developed in biology. The major principles of the cell theory are the following:
1. All organisms are made of cells
2. All existing cells are produced by other living cells
3. The cell is the most basic unit of life
The variety of cell types found in living things is staggering. Your body is made up of trillions of cells of many different shapes, sizes, and functions. You have long, thin nerve cells that transmit sensory information, as well as short, blocky skin cells that cover and protect the body.
Despite the variety of cell types, the cells in your body share many characteristics with one another and with the cells that make up every other living organism.
In general, cells tend to be microscopic in size, have similar building blocks, and are enclosed by a membrane that controls the movement of materials in and out of the cell.
Within the membrane, a cell is filled with cytoplasm. Cytoplasm is a jellylike substance that contains dissolved molecular building blocks.
2 Categories of Cells
Cells can be separated into 2 broad categories based on their internal structures:
Prokaryotic cells do not have a nucleus or other membrane-bound organelles. Instead, the cell's DNA is suspended in the cytoplasm. All prokaryotes are microscopic single-celled organisms.
Eukaryotic cells have a nucleus and other membrane-bound organelles. The nucleus, the largest organelle, encloses the genetic information. Eukaryotes may be multi-cellular or single-celled organisms.
Prokaryotic vs Eukaryotic
Both cell types are enclosed in a membrane.
Both cell types are filled with cytoplasm.
Prokaryotic does not have a nucleus, or organelles, and its DNA is suspended in the cytoplasm.
Eukaryotic has a nucleus, organelles, and its DNA is enclosed in the nucleus.
Eukaryotic cells are highly organized structures. Within the protective cell membrane are organelles that perform very specific tasks. These organelles are not floating around haphazardly. Rather, certain organelles are anchored to specific sites, which vary by cell type.
If the membrane was removed, the cell's contents would not ooze out in a puddle. Each eurkaryotic cell has a cytoskeleton, which is a network of proteins that is constantly changing to meet the needs of a cell. Three main types of fibers make up the cytoskeleton and allow it to serve a wide range of functions.
Give the cell its shape & act as tracks for the movement of organelles. When cells divide, microtubules form fibers (mitotic spindle) that pull half of the DNA into each new cell.
Give the cell its strength.
Tiny threads that enable cells to move and divide. They play an important role in muscle cells, where they help in muscle contraction and relaxation. Amoebas use these to crawl.
Just like the scientists we just discussed, modern biologists study cells using a microscope. However, today's biologists use microscopes and techniques more powerful than the pioneers of biology could have imagined. The microscopes we will use in class are like the one described in the video below.
Cells are a lot like factories...
Each organelle in a cell has specific functions that it performs. All of the functions of the different organelles is crucial to the overall task of sustaining life. Factories experience greater efficiency when they employ similar tactics through division of labor.
Dividing the Cell into 2 parts
A eukaryotic cell can be divided into 2 parts: the nucleus and the cytoplasm. The cytoplasm is the portion of the cell outside the nucleus. The nucleus is considered the control center of the cell (management) while the cytoplasm is the workspace for the factory. The nucleus and the cytoplasm work together in the business of life.
The nucleus is the control center of the cell and is usually the largest structure found within a cell. The nucleus contains nearly all the cell's DNA and with it coded info for making proteins and other important molecules.
The nucleus is surrounded by a nuclear envelope composed of two membranes. The nuclear envelope is dotted with thousands of pores, which allow material to move in and out of the nucleus.
Most nuclei also contain a small, dense region known as the nucleolus. The nucleolus is where the assembly of ribosomes begins.
Inside the nucleus, is a granular material called chromatin. Chromatin consists of DNA bound to protein. Chromatin is normally spread throughout the nucleus, but will condense to form chromosomes when the cell divides.
Chromosomes are distinct, threadlike structures that contain the genetic information that is passed from one generation of cells to the next.
One of the most important jobs carried out in the cellular "factory" is making proteins. Proteins are assembled on ribosomes. Ribosomes are created in the nucleolus and are small particles of RNA and protein found throughout the cytoplasm.
Ribosomes produce proteins by following coded instructions that come from the nucleus. Each ribosome, in its own way, is like a small machine in a factory, turning out proteins on orders that come from its "boss" - the nucleus.
Eukaryotic cells also contain an internal membrane system known as the endoplasmic reticulum, or ER. The ER is the site where lipid components of the cell membrane are assembled, along with proteins and other materials are exported from the cell.
Smooth & Rough ER
The portion of the ER involved in the synthesis of protein is called rough endoplasmic reticulum. It is given this name because of the ribosomes found on its surface.
Newly made proteins are inserted into the rough ER, where they may be chemically modified. Proteins that are exported from the cell are synthesized on the rough ER, as are many membrane proteins.
Smooth & Rough ER
The other portion of the ER is known as smooth endoplasmic reticulum (smooth ER) because ribosomes are not found on its surface. In many cells, smooth ER contains collections of enzymes that perform specialized tasks, including the syntheses of membrane lipids and the detoxification of drugs. Liver cells often contain large amounts of smooth ER.
Proteins produced in the rough ER move next into an organelle called the Golgi apparatus. The Golgi appears as a stack of closely apposed membranes. It looks very much like the ER that is near the nucleus.
The function of the Golgi apparatus is to modify, sort, and package proteins and other materials from the ER for storage in the cell or secretion outside the cell. The Golgi apparatus is like a customization shop, where the finishing touches are applied before they are ready to "leave" the factory.
Factories have cleanup crews and so does the eukaryotic cell. Lysosomes are small organelles filled with enzymes. One function of lysosomes is the digestion, or breakdown, of lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell. Enzymes are made of protein, so where do you think the lysosomes are made?
Lysosomes are also involved in breaking down organelles that have outlived their usefulness. Lysosomes perform the important function of removing "junk" from the cell. A number of serious human diseases, including Tays-Sachs disease, can be traced to lysosomes that fail to function properly.
Every factory needs a place to store things, and cells contain places for storage as well. Some kinds of cells contain sac like structures called vacuoles that store materials such as water, salts, proteins, and carbohydrates.
All living things need a source of energy. The factory is hooked up to the local power station and may have generators for back-up, but what about cells? Most cells get energy in one of two ways - from food molecules or from the sun.
Nearly all eukaryotic cells, including plant cells, contain mitochondria. Mitochondria are organelles that convert the chemical energy stored in food into compounds that are more convenient for the cell to use. Mitochondria are enclosed by two membranes- an outer and inner membrane. The inner membrane is folded up inside the organelle.
Unlike most organelles, mitochondria have their own ribosomes and DNA. This is evidence that mitochondria were once stand alone cells. Also, you inherit nearly all of your mitochondria from your mother.
Vacuoles are also found in some unicellular organisms and in some animals. The paramecium in the picture below, contains a vacuole called a contractile vacuole. This vacuole pumps excess water out of the cell. This control of water content is an example of a living organism maintaining its internal environment. We call this process..
Plants and some other organisms contain chloroplasts. Chloroplasts are organelles that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis. Chloroplasts are the biological equivalent of solar power plants.
Like mitochondria, chloroplasts are surrounded by two membranes, which contain the green pigment chlorophyl. Chloroplasts also contain their own genetic information, which suggests tat they may have descended from independent microorganisms.
In animal cells, the tubulin used to create microtubules also forms centrioles. Centrioles are located near the nucleus and help to organize cell division. Centrioles are not found in plant cells.
When you look at a map of the U.S., the map may have the counties and states outlined. These outlines represent the boundaries or borders for each county and state. It is important to know where something starts and ends. This same principle applies to cells.
All cells are surrounded by a thin, flexible barrier known as the cell membrane. The cell membrane is sometimes called the plasma membrane because many cells in the body are in direct contact with the fluid portion of the blood- the plasma. Many cells also produce a strong supporting structure layer around the membrane known as a cell wall.
The cell membrane regulates what enters and leaves the cell and also provides protection and support. The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer. The photo below shows that there are two layers of lipids. The lipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings.
In addition to lipids, most cell membranes contain protein molecule that are embedded in the lipid bilayer. Carbohydrate molecules are attached to many of these proteins. In fact, there are so many different molecules found in cell membranes that scientists sometimes describe them as "fluid mosaics". Some of the proteins form channels and pumps for moving materials across the membrane, while the carbs serve as chemical identification cards, allowing individual cells to recognize one another.
Cell walls are present in many organisms, including plants, algae, fungi, and many prokaryotes. Cell walls lie outside the cell membrane. The main function of the cell wall is to provide support and protection for the cell.
Most cell walls are made from fibers of carbohydrate and protein. These substances are created within the cell and then released at the surface of the cell membrane where they are assembled to form the wall. Cell walls are composed primarily from cellulose, which is also the primary component of wood and paper.
Every living cell exists in a liquid environment that it needs to survive. One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.
The cytoplasm of a cell contains a solution of many different substances in water. Remember, that a solution is a mixture of two or more substances, and that the dissolved substance is called the solute. The concentration of a solution is the mass of solute in a given volume of solution, or mass/volume.
In a solution, the particles are constantly moving and spread out randomly. Much like people, the particles have a desire for personal space and will attempt to move to an area that is "less crowded" with other particles. Diffusion is the movement of particles from an area of high concentration to an area of lower concentration. When the concentration of the solute is the same throughout a system, the system has reached equilibrium.
Diffusion in Cells
Suppose a substance is present in unequal concentrations on either side of the cell membrane. If the substances can cross the membrane, it particles will tend to move toward an area where it is less concentrated until equilibrium is achieved. At that point, there will be equal amounts of the substances on both sides of the membrane.
Diffusion in Cells
In diffusion, the particles are moving about by their own accord. Because diffusion depends upon random particle movements, substances diffuse across membranes without requiring the cell to use energy.
Although many substances diffuse across the cell membrane, some are too large or too strongly charged to diffuse. If a substance is able to diffuse, the membrane is said to be permeable to it. A membrane in impermeable to it if the substance cannot diffuse. Since our cell membranes allow some particles to pass, but not others, it is said to be semipermeable.
Water passes quite easily across most membranes, even though many solute molecules cannot. Osmosis is the diffusion of water through a selectively permeable membrane. Diffusion, as you know, is the movement of substances from an area of higher concentration to an area of lower concentration.
Water will continue to diffuse across the membrane until it reaches equilibrium. At this point, the solutions will be isotonic, which means "same strength". At the beginning, part of the solution was hypertonic or "above strength", while the other was hypotonic or "below strength".
Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane. A cell is almost always hypertonic to fresh water. This means that osmotic pressure should produce a net movement of water into the cell. This could be potentially dangerous for the cell. Do you know why?
Pressure is defined as the amount of force exerted per unit of area. Every day, we use pressure in many ways that go unnoticed (e.g. pressure keeps the tires on our vehicles inflated). Pressure is created by the collision of molecules against a surface. Just as we use pressure to make our lives easier, cells also use pressure to function.
If our cells were constantly exposed to fresh water, the danger of bursting would be greater. However, our cells are bathed in isotonic fluids, such as blood. These isotonic fluids (blood) have concentrations of dissolved materials roughly equal to those in the cells. Plant cells are often exposed to a lot of fresh water.... why don't they burst?
Some molecules are either too large or too strongly charged to cross the cell membrane through simple diffusion, yet they still make it to the party. How? Cell membranes have protein channels that act as carriers, making it easy for certain molecules to cross. Red blood cells, for example, possess protein channels on their membranes that allow glucose to pass through. Why is it important to get glucose into a cell?
As mentioned earlier, there are many protein channels throughout the cell membrane. Molecules often use specific protein channels that are designed for that particular molecule. In the end, it is still diffusion and the concentration differential is the key. Does this form of passive transport use energy?
As great as diffusion is, sometimes the cell needs to move materials in the opposite direction- against the concentration gradient. This is accomplished by a process known as active transport. As its name implies, active transport requires energy in order to occur. What is the name of the energy molecule created in the mitochondria & used by the cell?
The active transport of molecules across a cell membrane is normally carried out by transport proteins in the membrane. These proteins act like pumps and will "pump" the materials across the cell membrane.
Passive vs Active
Larger molecules obviously pose a larger problem in transporting across the membrane. Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. The resulting pocket breaks loose from teh outer portion of the membrane and forms a vacuole within the cytoplasm.
Phagocytosis & Pinocytosis
Two examples of endocytosis are phagocytosis and pinocytosis.
Extensions of cytoplasm surround a particle and package it within a food vacuole (like a sandwich bag). What organelle will help
this food particle?
Tiny pockets from along the cell membrane, fill with liquid, and pinch off the form vacuoles within the cell.
Many cells also release large amounts of material
the cell through a process called exocytosis. During exocytosis, the vacuole fuses with the cell membrane, forcing the contents out of the cell. This also requires energy, and is considered to be active transport.
Diversity of Life
Earth is sometimes called a living planet, and for good reason. Literally every corner of this planet is bursting with life. Living organisms are present in the deepest parts of the oceans, driest deserts, and coldest climates. Most of these living organisms all share the same characteristics of life, but they are not all the same.
As its name implies, unicellular organisms are made up of only one cell. Understand, cells are the fundamental unit with which organisms are made, but sometimes a cell
the organism. Unicellular organisms do everything you would expect a living organism to do: grow, respond to the environment, use energy, and reproduce.
Some of you may be inclined to dismiss the unicellular organisms as inconsequential. However, mankind has received some of his or her worst defeats at the hands of these tiny, simple organisms.