Chapter 4: Organization of the Cell (copy)
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.
Proposed by Theodor Schwann (zoologist) and Mathias Schleiden (botanist)
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.
Cells also contain DNA and genetic information
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 cellsare enclosed by a plasma membrane but
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.