Chapter Three: Cell Structures

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Chapter outline

1. The cell membrane and the cell wall
   1. The cell membrane
      1. The cell membrane regulates the flow of materials into and out of a cell.
      2. It is selectively permeable, meaning that only certain materials can pass through the membrane.
      3. A cell membrane consists of a bilayer of phospholipids with protein molecules jutting through one or both layers.
   2. The cell wall
      1. Prokaryotic cell wall
         1. The prokaryotic cell wall consists of long polymers called peptidoglycans.
         2. Some prokaryotic cell walls have a second, outer layer composed of lipoproteins and lipopolysaccharides.
      2. Eukaryotic cell wall
         1. The eukaryotic cell wall has three parts: the primary cell wall, the middle lamella, and the secondary cell wall.
         2. The primary cell wall, composed mainly of cellulose, is the innermost layer of the wall.
         3. The middle lamella contains polysaccharides called pectins.
         4. The secondary cell wall is the outermost layer. It consists of cellulose and a strengthening material called lignin.
2. Movement through the cell membrane
   1. Simple diffusion
      1. Simple diffusion is when molecules simply pass through the phospholipids in the membrane.
      2. This only occurs for small, nonpolar molecules.
   2. Passive transport
      1. Protein channels
         1. Proteins can form tunnels through which molecules can diffuse.
         2. Since the molecules do not come in contact with the nonpolar fatty acids in the phosholipids, polar molecules can travel accross the membrane through a protein channel.
      2. Facilitated diffusion
         1. Carrier proteins bond to a molecule on one side of the membrane, travel accross, and then release it on the other side.
         2. Carrier proteins usually bond with a specific molecule, like an enzyme bonds to a specific substrate.
   3. Active transport
      1. Active transport accross the cell membrane works against a concentration gradient, so it requires an input of energy.
      2. A contractile vacuole is an example, as it forces excess water out of the cell, even if the water is more concentrated in the external environment.
3. Vacuoles, vesicles, lysosomes, and peroxisomes
   1. Vacuoles and vesicles
      1. Vacuoles are sacs within the cell often formed by the budding off of the cell membrane.
      2. Vacuoles can hold food particles, water, enzymes, and other substances.
      3. Vesicles are simply very small vacuoles, often formed by the budding off of parts of the Golgi bodies to hold protein molecules.
   2. Lysosomes and peroxisomes
      1. Lysosomes and peroxisomes are both sacs similar to vacuoles which contain enzymes.
      2. Lysosomes contain digestive enzymes which can break down large food particles or damaged organelles.
      3. Peroxisomes contain oxidizing enzymes which can neutralize many toxic substances, including hydrogen peroxide, by adding oxygen to them.
4. The nucleus, nucleolus, and ribosomes
   1. The nucleus
      1. The nucleus contains a cell's DNA.
      2. Since DNA controls the cell's production of proteins, and proteins affect the entire cell, the nucleus is sometimes called the cell's "brain."
      3. The nucleus is surrounded by the nuclear membrane, which has pores which allow RNA to pass through into the endoplasmic reticulum.
   2. The nucleolus
      1. The nucleolus is a structure within the nucleus responsible for producing ribosomes.
      2. Cells usually have several nucleoli.
   3. Ribosomes
      1. Ribosomes are tiny structures at which protein synthesis takes place.
      2. Ribosomes are composed of a large and small subunit, each composed of ribosomal RNA and protein.
      3. They are located in the endoplasmic reticulum and at the intersections of the microtrabeculae in the cytoplasm.
5. The endoplasmic reticulum and the Golgi bodies
   1. The endoplasmic reticulum
      1. The endoplasmic reticulum is a network of tunnels extending away from the nucleus to the Golgi bodies.
      2. Rough endoplasmic reticulum looks "rough" under a microscope because it contains ribosomes; smooth endoplasmic reticulum does not contain ribosomes.
      3. The endoplasmic reticulum is used to carry proteins to the Golgi bodies and synthesize new cell membrane.
   2. The Golgi bodies
      1. The Golgi bodies are stacks of membranous pouches found at the end of the endoplasmic reticulum.
      2. They receive proteins from the endoplasmic reticulum and send them to other organelles in the cell by packaging them into tiny vesicles which bud off of the Golgi bodies.
      3. The Golgi bodies can make changes to the proteins they receive before packaging them into vesicles.
6. The mitochondria
   1. Cellular respiration occurs in the mitochondria.
   2. Most eukaryotic cells have thousands of mitochondria.
   3. A mitochondrion's outer membrane separates it from the cytoplasm, and its folded inner membrane (the crystae) is where respiration actually occurs.
   4. Mitochondria have their own DNA and can replicate themselves, so scientists believe that they were once separate organisms which were incorporated into eukaryotes.
7. The cytoskeleton
   1. The cytoskeleton is a network of fibers made of protein which allows the cell to change its shape. The four types are microtubules, intermediate filaments, microfilaments, and the microtrabeculae.
   2. Microtubules
      1. Microtubules are 20 to 25 nanometers in diameter.
      2. They are involved in cell reproduction and are major components of cilia and flagella.
   3. Intermediate filaments
      1. Intermediate filaments are 7 to 10 nanometers in diameter.
      2. They are made of fibrous protein which cannot be as easily broken down as the other fibers in the cytoskeleton.
      3. Intermediate filaments help give the cell strength and shape.
   4. Microfilaments
      1. Microfilaments are 3 to 6 nanometers in diameter.
      2. Microfilaments are involved in the locomotion of cells which do not have specializes structures (like cilia or flagella) for movement.
   5. The microtrabeculae
      1. The microtrabeculae are very tiny fibers which interconnect all of the cell's organelles to help give the cell shape.
      2. Ribosomes are found at the intersections of the microtrabeculae.
8. Cilia, flagella, and pseudopodia
   1. Cilia and flagella
      1. Cilia are tiny hairs on the outside of the cell membrane, and flagella are large hairs. they are both used for locomotion.
      2. Both cilia and flagella are composed of microtubules arranged in a circle of nine pairs around a tenth pair in the center.
      3. At the connection of the cilia and flagella with the cell membrane is a structure called the basal body. The basal body is composed of microtubles arranged in a circle of triplets.
   2. Pseudopodia
      1. Pseudopodia are not true organelles; they are just extensions of the cytoplasm.
      2. The rest of the cytoplasm tends to flow toward a pseudopod, so pseudopodia can be used for movement.
      3. Pseudopopdia are also used to surround and capture prey.
9. Plastids
   1. Plastids are found only in eukaryotic autotrophs. The three different types are chloroplasts, chromoplasts, and leucoplasts.
   2. Chloroplasts
      1. Chloroplasts are green organelles (because they contain the green pigment chlorophyll) which have an outer membrane and a folded inner membrane.
      2. A solution called the stroma fills the space inside of the inner membrane.
      3. Within the chloroplast are stacks of flattened vesicles. The stacks are called grana, and the vesicles are known as thylakoids.
      4. Photosynthesis occurs within the thylakoids of a chloroplast.
   3. Chromoplasts
      1. Chromoplasts are like chloroplasts, but they do not contain the green pigment chlorophyll.
      2. The colored pigments in chromoplasts absorb different colors of light than chlorophyll does, allowing photosynthesis to occur in different lightning conditions.
   4. Leucoplasts
      1. Leucoplasts are colorless since they do not contain any pigments.
      2. They store starch, proteins, and lipids, releasing them when the cell requires them.

Terms to know

• active transport - Transport in which a cell must expend energy. Active transport occurs against a concentration gradient.
• basal body - A structure found at the connection of cilia and flagella with the cell membrane. It is composed of microtubules in a circular configuration of nine triplets.
• cell membrane - The structure which surrounds the cell and regulates the movement of materials into and out of the cell. It is composed mostly of phospholipids.
• cell wall - A structure found in most prokaryotes and some eukaryotes which gives the cell greater structure. In prokaryotes, it is composed of peptidoglycans, and in eukaryotes, it consists or polysaccharides, pectins, and lignin.
• cilia - Tiny hairs along the outside of the cell membrane which are used to move the cell and capture food particles.
• chloroplast - The organelle in which photosynthesis takes place. It contains chlorophyll.
• chromoplast - An organelle in which photosynthesis take place. It contains pigments other than chlorophyll, resulting in a color other than green.
• contractile vacuole - An organelle which pumps excess water of a cell to prevent it from bursting.
• endoplasmic reticulum - A network of tunnels which extend away from the nucleus, used for the transport of proteins.
• facilitated diffusion - A method of transport across the cell membrane by which carrier proteins bond to a molecule on one side of the membrane, move through the membrane, and then release it on the other side.
• flagella - Large hairs which can whip back and forth to propel a cell.
• Golgi body - Stacks of membranous pouches which act as a transport station, packaging proteins from the endoplasmic reticulum and placing them into tiny vesicles.
• intermediate filament - A part of the cytoskeleton with a strong, ropelike structure which gives the cell strength and helps it to maintain its shape.
• leucoplast - Colorless plastids in autotrophs which store starch, proteins, and lipids.
• lysosome - A sac similar to a vacuole which contains powerful digestive enzymes used to break down large food particles.
• microfilament - A part of the cytoskeleton which consists of actin and aids in cell movement.
• microtrabeculae - Tiny fibers which interconnect all of the structures within the cell and help to give the cell shape.
• microtubule - Fibers which extend from the center of the cell to the cell membrane. They are involved in cell reproduction and are part of the composition of cilia and flagella.
• mitochondria - The organelle in which cellular respiration occurs.
• nucleolus - A structure within the nucleus at which ribosomes are created.
• nucleus - The organelle in eukaryotes which contains the cells DNA and thus indirectly controls protein production and the rest of the cell.
• passive transport - A form of transport which allows highly polar molecules to move through the cell membrane without the expenditure of energy. This may occur either through protein channels or facilitated diffusion.
• peroxisome - An organelle similar to a vacuole which contains oxidizing enzymes which can help neutralize toxic substances.
• pseudopod - Extensions of the cytoplasm toward which the rest of the cytoplasm tends to flow. Pseudopodia can be used for movement and the capture of prey.
• ribosome - Structures found mainly in the endoplasmic reticulum whose function is to synthesize protein based upon the code of a messenger RNA molecule.
• vacuole - Membrane-bound sacs within a cell used to hold food particles, water, etc.
• vesicle - A tiny vacuole, often used to carry protein molecules packaged at the Golgi bodies.