cell structure.pdf

Raiyed M. Adeel 29/07/2018  If you look at living organisms under a microscope you will see they are made up of cells. The word cell was first used by Robert Hooke, a British biologist and early microscopeist. Hooke looked at thin slices of cork under a microscope. The structure he s...

Raiyed M. Adeel 29/07/2018  If you look at living organisms under a microscope you will see they are made up of cells. The word cell was first used by Robert Hooke, a British biologist and early microscopeist. Hooke looked at thin slices of cork under a microscope. The structure he saw looked like a honeycomb as it was made up of many tiny units. In 1665 Hooke published his book Micrographia. During the 1670s, the Dutch tradesman Antony van Leeuwenhoek, used microscopes to observe many microbes and body cells. Raiyed M. Adeel 29/07/2018  Later, biologists found cells everywhere. Biologists in the early part of the 19th century suggested that all living things were made of cells, but the role of cells as the primary building block of life was not discovered until 1839 when two German scientists, Theodor Schwann, a zoologist, and Matthias Jakob Schleiden, a botanist, suggested that cells were the basic unit of all living things. Later, in 1858, the German doctor Rudolf Virchow observed that cells divide to produce more cells. He proposed that all cells arise only from other cells. The collective observations of all three scientists form the cell theory. Raiyed M. Adeel 29/07/2018  Cell Theory: 1. All organisms are composed of one or more cells, and the life processes of metabolism and heredity occur within these cells. 2. Cells are the smallest living things, the basic units of organization of all organisms. 3. Cells arise only by division of a previously existing cell. Raiyed M. Adeel 29/07/2018  Cells are very small in size. If cells have such an important job, why are they so small? And why are there no organisms with huge cells? The answers to these questions lie in a cell’s need for fast, easy food. The need to be able to pass nutrients and gases into and out of the cell sets a limit on how big cells can be. The larger a cell gets, the more difficult it is for nutrients and gases to move in and out of the cell. Most cells are between 1 and 100 μm in diameter. One exception however, is eggs. Raiyed M. Adeel 29/07/2018 Raiyed M. Adeel 29/07/2018 STURUCTURES OF A CELL There are many different types of cells, but all cells have a few things in common. These are: a cell or plasma membrane. cytoplasm. ribosomes for protein synthesis. DNA (genetic information). Raiyed M. Adeel 29/07/2018  The cell membrane is the physical boundary between the inside of the cell (intracellular) and its outside environment (extracellular). It acts almost like the ”skin” of the cell.  Cytoplasm is the general term for all of the material inside the cell. Cytoplasm is made up of cytosol, a watery fluid that contains dissolved particles and organelles. Organelles are structures that carry out specific functions inside the cell.  Ribosomes are the organelles on which proteins are made. Ribosomes are found throughout the cytosol of the cell.  All cells also have DNA. DNA contains the genetic information needed for building structures such as Raiyed M. Adeel 29/07/2018 proteins and RNA molecules in the cell. Types of Cells There are two cell types: prokaryotes and eukaryotes. Prokaryotic cells are usually single-celled and smaller than eukaryotic cells. Eukaryotic cells are usually found in multicellular organisms, but there are some single-celled eukaryotes. Raiyed M. Adeel 29/07/2018 Raiyed M. Adeel 29/07/2018 Prokaryotes Prokaryotic cells are small, consisting of cytoplasm surrounded by a plasma membrane and encased within a rigid cell wall, with no distinct interior compartments and are organisms that do not have a cell true nucleus nor any organelles that are surrounded by a membrane. The bacteria, are the simplest organisms and are best example of prokaryotes. Raiyed M. Adeel 29/07/2018  Prokaryotes have ribosomes, which are not surrounded by a membrane but do have a specialized function, and could therefore be considered organelles. Most of the metabolic functions carried out by a prokaryote take place in the plasma membrane. Most prokaryotes are unicellular and have a cell wall that adds structural support and acts as a barrier against outside forces. Some prokaryotes have an extra layer outside their cell wall called a capsule, which helps them stick to surfaces or to each other. Raiyed M. Adeel 29/07/2018  Prokaryotic DNA usually forms a circular molecule and is found in the cell’s cytoplasm along with ribosomes. Prokaryotic cells are very small; most are between 1–10 μm in diameter. They are found living in almost every environment on Earth. Biologists believe that prokaryotes were the first type of cells on Earth and that they are the most common organisms on Earth today. Raiyed M. Adeel 29/07/2018 Eukaryotic Cells: A eukaryote is an organism whose cells are organized into complex structures by internal membranes and a cytoskeleton. The most characteristic membrane-bound structure of eukaryotes is the nucleus. This feature gives them their name, which comes from Greek and means ”true nucleus.” The nucleus is the membrane-enclosed organelle that contains DNA. Eukaryotic DNA is organized in one or more linear molecules, called chromosomes. Some eukaryotes are single-celled, but many are multicellular. Raiyed M. Adeel 29/07/2018 CELL STRUCTURE All living organisms contain a multitude of cell types, whose main functions are to maintain a proper homeostasis in the body, which is maintaining the internal environment of the body in relatively constant state. To perform this task, the cells possess certain structural features in their cytoplasm that are common to all. As a result, it is possible to illustrate a cell in a more generalized, composite form with various cytoplasmic organelles. It is essential to remember, however, that the quantity, appearance, and distribution of the cytoplasmic organelles within a given cell depend on the cell type and its function. Raiyed M. Adeel 29/07/2018 Raiyed M. Adeel 29/07/2018 Raiyed M. Adeel 29/07/2018 PLASMA MEMBRANE The plasma membrane (also called the cell membrane) has many functions. For example, it separates the internal environment of the cell from the outside environment. It allows only certain molecules into and out of the cell. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability. The plasma membrane also acts as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall. Raiyed M. Adeel 29/07/2018  The membrane that surrounds the cell consists of a phospholipid bilayer, a double layer of phospholipid molecules. The phospholipid molecules of the cell membrane are distributed as two layers. Their polar heads are arranged on both the inner and outer surfaces of the cell membrane. The nonpolar tails of the lipid layers face each other in the center of the membrane. Interspersed within and embedded in the phospholipid bilayer of the cell membrane are the integral membrane proteins and peripheral membrane proteins, which make up almost half of the total mass of the membrane. Raiyed M. Adeel 29/07/2018 Raiyed M. Adeel 29/07/2018  Phosphate Phospholipids  Fatty acid tails  hydrophobic  Phosphate group head Fatty acid  hydrophilic  Arranged as a bilayer  The phospholipid bilayer is permeable to such molecules as oxygen, carbon dioxide, water, steroids, and other lipid-soluble chemicals. Other substances, such as glucose, ions, or proteins, cannot pass through the cell membrane and cross it only by specific transport mechanisms. Some of these substances are transported through the integral membrane proteins using pump molecules or through protein channels that allow the passage of specific molecules. A process called endocytosis performs the uptake and transfer of molecules and solids across the cell membrane into the cell interior. In contrast, the release of material from the cell cytoplasm across the cell membrane is called exocytosis. Raiyed M. Adeel 29/07/2018  Pinocytosis is the process by which cells ingest small molecules of extracellular fluids or liquids. Phagocytosis refers to the ingestion or intake of large particles by the cells, such as bacteria, worn out cells, or cellular debris. Raiyed M. Adeel 29/07/2018  Cytoplasm The gel-like material within the cell that holds the organelles is called cytoplasm. The cytoplasm plays an important role in a cell, serving as a ”jelly” in which organelles are suspended and held together by a fatty membrane. The cytosol, which is the watery substance that does not contain organelles, is made up of 80% to 90% water. The cytosol plays a mechanical role by exerting pressure against the cell’s plasma membrane which helps keep the shape of the cell.  NUCLEUS The nucleus is a membrane-enclosed organelle found in most eukaryotic cells. The nucleus is the largest organelle in the cell and contains most of the cell’s genetic information. The genetic information, which contains the information for the structure and function of the organism, is found encoded in DNA in the form of genes.  The nuclear envelope is a double membrane of the nucleus that encloses the genetic material. It separates the contents of the nucleus from the cytoplasm. The nuclear envelope is made of two lipid bilayers, an inner membrane and an outer membrane. The outer membrane is continuous with the rough endoplasmic reticulum. Many tiny holes called nuclear pores are found in the nuclear envelope. These nuclear pores help to regulate the exchange of materials (such as RNA and proteins) between the nucleus and the cytoplasm.  NUCLEOLUS The nucleus of many cells also contains an organelle called a nucleolus. The nucleolus is mainly involved in the assembly of ribosomes. Ribosomes are organelles made of protein and ribosomal RNA (rRNA), and they build cellular proteins in the cytoplasm. The function of the rRNA is to provide a way of decoding the genetic messages within another type of RNA called mRNA, into amino acids. After being made in the nucleolus, ribosomes are exported to the cytoplasm where they direct protein synthesis.  MITOCHONDRIA A mitochondrion (mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells. Mitochondria are called the ”power house” of the cell because they use energy from organic compounds to make ATP. Mitochondria are round, oval, or elongated structures whose variability and number depend on cell function. Each mitochondrion (singular) consists of an outer and inner membrane.  Mitochondrion has two phospholipids membranes. The smooth outer membrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae. The fluid-filled inside of the mitochondrian, called matrix, is where most of the cell’s ATP is made. Mitochondria have their own DNA. ENDOPLASMIC RETICULUM The endoplasmic reticulum (ER) (plural, reticuli) is a network of phospholipid membranes that form hollow tubes, flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae. The ER has two major functions: Transport: Molecules, such as proteins, can move from place to place inside the ER, much like on an intracellular highway. Synthesis: make proteins and Lipids are also produced in the ER. There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Rough endoplasmic reticulum is studded with ribosomes which gives it a ”rough” appearance. These ribosomes make proteins that are then transported from the ER in small sacs called transport vesicles. The transport vesicles pinch off the ends of the ER. The rough endoplasmic reticulum works with the Golgi apparatus to move new proteins to their proper destinations in the cell. The membrane of the RER is continuous with the outer layer of the nuclear envelope.  Smooth endoplasmic reticulum does not have any ribosomes attached to it, and so it has a smooth appearance. SER has many different functions some of which are: lipid synthesis, calcium ion storage, and drug detoxification. The SER is made up of tubules and vesicles that branch out to form a network. In some cells there are dilated areas like the sacs of RER. Smooth endoplasmic reticulum and RER form an interconnected network. RIBOSOMES Ribosomes are small organelles and are the site of protein synthesis (or assembly). They are made of ribosomal protein and ribosomal RNA. Ribosomes can be found alone or in groups within the cytoplasm or attached to the endoplasmic reticulum. GOLGI APPARATUS The Golgi apparatus is a large organelle that is usually made up of five to eight cup-shaped, membrane covered discs called cisternae. The cisternae look a bit like a stack of deflated balloons. The Golgi apparatus modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell. The Golgi apparatus is found close to the nucleus of the cell where it modifies proteins that have been delivered in transport vesicles from the RER. It is also involved in the transport of lipids around the cell. VESICLES A vesicle is a small, spherical compartment that is separated from the cytosol by at least one lipid bilayer. Many vesicles are made in the Golgi apparatus and the endoplasmic reticulum, or are made from parts of the cell membrane. Vesicles are basic tools of the cell for organizing metabolism, transport, and storage of molecules. Vesicles are also used as chemical reaction chambers. They can be classified by their contents and function. LYSOSOME Lysosomes are vesicles that are formed by the Golgi apparatus. They contain powerful enzymes that could break down (digest) the cell. Lysosomes break down harmful cell products, waste materials, and cellular debris and then force them out of the cell. They also digest invading organisms such as bacteria. Lysosomes also break down cells that are ready to die.  CENTRIOLES Centrioles are rod-like structures made of short microtubules. Nine groups of three microtubules make up each centriole. Two perpendicularly placed centrioles make up the centrosome. Centrioles are very important in cellular division and found only in animal cell, where they arrange the mitotic spindles that pull the chromosome apart during mitosis. VACUOLES Vacuoles are membrane-bound organelles that can have secretory, excretory, and storage functions. Many organisms will use vacuoles as storage areas and some plant cells have very large vacuoles. CYTOSKELETON The cytoskeleton of a cell consists of a network of tiny protein filaments and tubules that extend throughout the cytoplasm. It serves the cell’s structural framework. Three types of filamentous proteins, microfilaments, intermediate filaments, and microtubules, form the cytoskeleton of a cell. Special Structures in Plant Cells Most of the organelles that have been discussed are common to both animal and plant cells. However, plant cells also have features that animal cells do not have; they have a cell wall, a large central vacuole, and plastids such as chloroplasts. Plants have very different lifestyles from animals. CELL WALL A cell wall is a rigid layer that is found outside the cell membrane and surrounds the cell. The cell wall made of cellulose and provides structural support and protection. Pores in the cell wall allow water and nutrients to move into and out of the cell. The cell wall also prevents the plant cell from bursting when water enters the cell. CENTRAL VACUOLE Most mature plant cells have a central vacuole that occupies more than 30% of the cell’s volume, but can also occupy as much as 90% of the volume of certain cells. The central vacuole is surrounded by a membrane called the tonoplast. The central vacuole has many functions. Aside from storage, the main role of the vacuole is to maintain turgor pressure against the cell wall. CHLOROPLAST Chloroplasts are the organelle of photosynthesis. They capture light energy from the sun and use it with water and carbon dioxide to make food (sugar) for the plant. The chloroplast is enclosed by an inner and an outer phospholipid membrane. Between these two layers is the inter membrane space. The fluid within the chloroplast is called the stroma, and it contains one or more molecules of small circular DNA. END