Chapter 4: Organization of the Cell

The Cell: Basic Unit of Life

• The cell theory holds that
  • (1) cells are the basic living units of organization and function in all organisms
  • (2) all cells come from other cells.
  • It explains that the ancestry of all the cells alive today can be traced back to ancient times.
  • Evidence that all living cells have evolved from a common ancestor is supported by the basic similarities in their structures and in their molecular composition.
• Every cell is surrounded by a plasma membrane that separates it from its external environment.
  • The plasma membrane allows the cell to maintain internal conditions that may be very different from those of the outer environment.
  • The plasma membrane also allows the cell to selectively exchange materials with its outer environment.
  • Cells have many organelles, internal structures that carry out specific functions.
• All cells have similar mechanisms for information transfer and chemical reactions that convert energy from one form to another.
• A critical factor in determining cell size is the ratio of the plasma membrane (surface area) to the cell’s volume.
  • The plasma membrane must be large enough relative to the cell volume to regulate the passage of materials into and out of the cell.
  • For this reason, most cells are microscopic.
• The size and shape of a cell are largely dictated by the functions it must perform.

Methods for Studying Cells

• Biologists use light microscopes, electron microscopes, and a variety of chemical methods, including the binding of specific antibodies, to study cells and learn about cell structure.
  • The electron microscope has superior resolving power, enabling investigators to see details of cell structures not observable with conventional microscopes.
  • Fluorescence microscopy can be used to track the locations and movements of specific tagged molecules within cells.
• Cell biologists use the technique of cell fractionation for purifying organelles as well as genetic methods to gain information about the function of cell structures.

Prokaryotic and Eukaryotic Cells

• Prokaryotic cells are enclosed by a plasma membrane but \n have little or no internal membrane organization.
  • They have a nuclear area rather than a membrane-enclosed nucleus.
  • Prokaryotic cells typically have a cell wall and ribosomes, and may have propeller-like flagella.
• Eukaryotic cells have a membrane-enclosed nucleus, and their cytoplasm contains a variety of organelles; the fluid component of the cytoplasm is the cytosol.
• Plant cells differ from animal cells in that plant cells have rigid cell walls, plastids, and large vacuoles, which are important in plant growth and development.
• Membranes divide the eukaryotic cell into compartments, allowing it to conduct specialized activities within small areas of the cytoplasm, concentrate reactants, and organize metabolic reactions.
  • Small membrane-enclosed sacs, called vesicles, transport materials between compartments.
• Membranes are important in energy storage and conversion.
• Membranes serve as work surfaces for certain chemical reactions.

The Cell Nucleus

• The nucleus contains genetic information coded in DNA.
  • The nucleus is bounded by a nuclear envelope, consisting of a double membrane perforated with nuclear pores that communicate with the cytoplasm.
• DNA in the nucleus associates with protein to form chromatin, which is organized into chromosomes.
  • During cell division, the chromosomes condense and become visible as threadlike structures.
• DNA transcribes its information in messenger RNA (mrNA) molecules, which enter the cytoplasm to provide information for protein synthesis by ribosomes.
• The nucleolus is a region in the nucleus that is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Membranes Organelles in the Cytoplasm

• The endoplasmic reticulum (ER) is a network of folded internal membranes in the cytosol.
  • Smooth ER is the site of lipid synthesis, calcium ion storage, and detoxifying enzymes.
  • Rough ER is studded along its outer surface with ribosomes that manufacture polypeptides.
  • Polypeptides synthesized on rough ER may be moved into the ER lumen, where they are assembled into proteins and modified by the addition of a carbohydrate or lipid.
  • These proteins may then be transferred to other compartments within the cell by small transport vesicles that bud off from the ER membrane.
• The Golgi complex consists of stacks of flattened membranous sacs called cisternae that process, sort, and modify proteins synthesized on the rough ER.
  • The Golgi complex also manufactures lysosomes.
• Glycoproteins are transported from the ER to the cis face of the Golgi complex by transport vesicles, which are formed by membrane budding.
  • The Golgi complex modifies carbohydrates and lipids that were added to proteins by the ER and packages them in vesicles.
• Glycoproteins exit the Golgi through vesicles that are formed at its trans face.
  • The Golgi routes some proteins to the plasma membrane for export from the cell. Others are transported to lysosomes or other organelles within the cytoplasm.
• Lysosomes contain enzymes that break down worn-out cell structures, bacteria, and debris taken into cells.
• Vacuoles store materials, water, and wastes.
  • They maintain hydrostatic pressure in plant cells.
• Peroxisomes are important in lipid metabolism and detoxify harmful compounds such as ethanol.
  • They produce hydrogen peroxide, but contain the enzyme catalase, which degrades this toxic compound.
• Mitochondria, organelles enclosed by a double membrane, are the sites of aerobic respiration.
  • The inner membrane is folded, forming cristae that increase its surface area.
• The cristae and the compartment enclosed by the inner membrane, the matrix, contain enzymes for the reactions of aerobic respiration.
  • During aerobic respiration, nutrients are broken down in the presence of oxygen.
  • Energy captured from nutrients is packaged in ATP, and carbon dioxide and water are produced as byproducts.
• Plastids are organelles that produce and store food in the cells of plants and algae.
• Chloroplasts are plastids that carry out photosynthesis.
• The inner membrane of the chloroplast encloses a fluid-filled space, the stroma.
• Grana, stacks of interconnected disclike membranous sacs called thylakoids, are suspended in the stroma.
• During photosynthesis, chlorophyll, the green pigment found in the thylakoid membranes, traps light energy.
  • This energy is converted to chemical energy in ATP and used to synthesize carbohydrates from carbon dioxide and water.

The Cytoskeleton

• The cytoskeleton is a dynamic internal protein fiber framework that includes microtubules, microfilaments, and intermediate filaments.
  • The cytoskeleton provides structural support and functions in various types of cell movement, including transport of materials in the cell.
• Microtubules are hollow cylinders assembled from subunits of the protein tubulin.
• In cells that are not dividing, the minus ends of microtubules are anchored in microtubule-organizing centers (MTOCs).
  • The main MTOC of animal cells is the centrosome, which usually contains two centrioles.
  • Each centriole has a 9 × 3 arrangement of microtubules.
• Microfilaments, or actin filaments, formed from subunits of the protein actin, are important in cell movement.
• Intermediate filaments strengthen the cytoskeleton and stabilize cell shape.
• Cilia and flagella are thin, movable structures that project from the cell surface and function in movement.
  • Each consists of a 9 + 2 arrangement of microtubules, and each is anchored in the cell by a basal body that has a 9 × 3 organization of microtubules.
  • Cilia are short, and flagella are long.

Cell Coverings

• Most cells are surrounded by a glycocalyx, or cell coat, formed by polysaccharides extending from the plasma membrane.
• Many animal cells are also surrounded by an extracellular matrix (ECM) consisting of carbohydrates and protein.
  • Fibronectins are glycoproteins of the eCM that bind to integrins, receptor proteins in the plasma membrane.
• Cells of most bacteria, archaea, fungi, and plant cells are surrounded by a cell wall made mainly of carbohydrates.
  • Plant cells secrete cellulose and other polysaccharides that form rigid cell walls.