Chapter 3 (PART ONE)
3.1 Introduction
A cell is the smallest unit of life
A cell continually carries out metabolic activities essential for life, as well as specialized functions, and adapts to changing conditions.
Cells interact to form tissues, organs, and organ systems.
The human body consists of about 3.0\times10^{13} cells that vary considerably in shape and size, yet have much in common.
Differences in cell shape and arrangement make different functions possible.
Examples: Nerve cells have long extensions that conduct electrical impulses, while epithelial cells in the mouth are flat, thin, and densely packed, to provide a protective barrier.
Genes control a cell’s actions and responses.
3.2 Composite Cell
A composite cell includes many different cell structures; most cells contain most of these structures, but no cell contains all of them.
A cell consists of three main parts:
Nucleus: Contains DNA (genetic material) and directs cell’s activities
Cytoplasm: Organelles and fluids that make up the majority of the cell; between nucleus and cell membrane
Cell membrane: Boundary that encloses the cell
Within the cytoplasm are specialized organelles (little organs) that perform specific functions for the cell.
Organelles are suspended in a fluid called the cytosol.
3.3 Cell Membrane
The cell membrane is also called the plasma membrane.
Boundary that contains the cell contents.
Regulates the movement of substances in and out of the cell.
Participates in signal transduction, a method of communication with other cells.
Helps cells adhere to other cells.
General characteristics:
Extremely thin, flexible, and elastic.
Selectively permeable – regulates entry/exit of substances.
Has complex surface features, with adaptations to increase surface area (microvilli).
The cell membrane plays a critical role in maintaining cellular homeostasis and mediating interactions with the external environment.
3.4 Membrane Characteristics
Specific non-lipid-soluble molecules or ions pass through membrane channels; membranes are selectively permeable.
Lipid-soluble molecules (e.g., respiratory gases, steroid hormones) can pass through the phospholipid portion.
Water-soluble molecules are generally impermeable to the membrane without assistance.
Microvilli increase surface area to enhance absorption and exchange.
3.5 Membrane Structure: Lipids
The membrane is composed mainly of lipids and proteins, and some carbohydrates.
The basic framework is a bilayer (phospholipid bilayer): fatty acid tails inward and water-soluble heads facing the surfaces.
Phospholipids are fluid and can move, forming a stable fluid film.
Lipid-soluble molecules (respiratory gases, steroid hormones) can pass through the phospholipid portion.
The membrane is impermeable to water-soluble molecules.
Embedded cholesterol molecules stabilize the membrane and help make it less permeable to water-soluble substances.
3.6 Membrane Structure: Proteins
Many types of proteins are found in the cell membrane.
Proteins are classified by their relationship to the phospholipid bilayer:
Integral: span width of bilayer; may protrude on 1 or both sides.
Transmembrane: integral proteins that protrude on both sides.
Peripheral: associate with one side of bilayer.
Membrane proteins vary in shape: rod-like, globular, fibrous.
Functions:
Some proteins function as receptors or growth factors on the cell surface, starting signal transduction.
Other proteins transport ions or molecules across the membrane, such as ion channels.
Some proteins have carbohydrates attached (glycoproteins) and are used for cell identification; this helps identify cells as “self,” protecting them from attack by the immune system.
Proteins that protrude into the cell will anchor supportive rods and tubules, forming a cytoskeleton.
Membrane proteins called cellular adhesion molecules (CAMs) help determine a cell’s interactions with other cells.
3.7 The Cytoplasm
The cytoplasm makes up most of cell volume and consists of a clear liquid (cytosol), a supportive cytoskeleton, and networks of membranes and organelles.
Organelles perform specific functions for the cell.
Ribosomes:
Tiny, spherical structures composed of RNA and protein.
Provide structural support and enzyme activity for protein synthesis.
Not membranous.
Found in cytoplasm or bound to rough endoplasmic reticulum.
3.8 Endoplasmic Reticulum (ER)
Endoplasmic reticulum is made up of membrane-bound flattened sacs and vesicles and functions as a transport network throughout the cell.
Two types of ER:
Rough ER: contains ribosomes, and functions in protein synthesis.
Smooth ER: does not contain ribosomes, and functions in lipid synthesis, absorption of fats, and metabolism of drugs.
3.9 Golgi Apparatus
Vesicles: membranous sacs.
Golgi apparatus is composed of flattened, membranous sacs.
Refines, packages, and transports proteins formed in the rough ER.
Vesicles formed in the ER travel to the Golgi apparatus, which may modify their contents by adding sugar molecules to the proteins, to stabilize their structure or to enable folding.
A new vesicle pinches off the Golgi apparatus and may then move to the cell membrane to secrete its contents to the outside (exocytosis).
3.10 Mitochondria
Elongated fluid-filled sacs.
The inner membrane of a mitochondrion is folded into cristae, which increase surface area.
House many chemical reactions that extract energy from nutrients (cellular respiration) and produce ATP.
Store the energy in the chemical bonds of ATP.
Very active cells contain thousands of mitochondria.
Mitochondria have their own DNA and reproduce by dividing.
3.11 Lysosomes and Peroxisomes
Lysosomes:
Membranous sacs formed by budding off the Golgi apparatus.
Contain enzymes that break down nutrients, ingested materials, debris, worn-out cell parts, cholesterol (in some cells), toxins, and drugs.
Abundant in macrophages and liver cells.
Peroxisomes:
Membranous sacs similar in appearance to lysosomes.
Contain a different set of enzymes than lysosomes.
Enzymes function in the breakdown of fatty acids and hydrogen peroxide, and detoxification of alcohol.
Abundant in liver cells and cells of the kidney.
3.12 Other Cellular Structures 1: Cytoskeleton
In the cytoplasm, there are several structures, which are not organelles, but part of the cytoskeleton.
Microfilaments: made of the protein actin; cause various cellular movements; group together to form myofibrils in muscle.
Microtubules: made of the globular protein tubulin; are attached in a spiral to form a long tube; important in cell division.
Intermediate filaments: made of different proteins in different cells; abundant in skin cells and neurons.
3.12 Other Cellular Structures 2: Centrosome, Cilia, Flagella
Centrosome: a nonmembranous structure, made up of two hollow cylinders called centrioles, that function in the distribution of chromosomes during cell division; made of nine groups of three microtubules, i.e. 9\times 3 = 27 microtubules.
Cilia: motile extensions from the cell; short cilia are abundant on the free surfaces of certain epithelial cells and move in a wave (respiratory linings, for example) to move fluids or whole cells.
Flagella: another type of motile extension from the cell; usually a cell only has 1 flagellum; the only human cell with a flagellum is a sperm cell; the flagellum allows the sperm to swim toward the egg cell.
3.13 The Cell Nucleus
The nucleus contains genetic material, DNA, which controls cell activities.
The fairly large nucleus is surrounded by a double-layered nuclear envelope, containing relatively large nuclear pores that allow the passage of certain substances.
Contains a fluid called nucleoplasm.
The nucleolus is a small, dense body in the nucleus, composed of RNA and protein; it is the site of ribosome production.
Chromatin consists of loosely coiled fibers of protein and DNA found in the nucleus.
Condensed DNA is referred to as chromosomes; this form of DNA is present during cell division.