Chapter 3 Cells Flashcards
what is a cell
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 30 trillion cells that vary considerably in shape and size, yet have much in common
Differences in cell shape make different functions possible
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
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
organelles
Within the cytoplasm are specialized organelles (little organs) that perform specific functions for the cell
Organelles are suspended in a fluid called the cytosol
Cell membrane is also called 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:
of cell membrane
Extremely thin, flexible, and elastic
Selectively permeable – regulates entry/exit of substances
Has complex surface features, with adaptations to increase surface area
Cell Membrane Structure: Lipids
Composed mainly of lipids and proteins, and some carbohydrates
The basic framework of the cell membrane consists of a double layer (bilayer) of phospholipids, with fatty acid tails turned inward and the water-soluble heads facing the surfaces
The phospholipids can move, forming a stable fluid film
Lipid-soluble molecules (respiratory gases, steroid hormones) can pass through the phospholipid portion of the membrane
Cell membrane is impermeable to water-soluble molecules
Embedded cholesterol molecules stabilize the membrane, and help make it less permeable to water-soluble substances
Cell 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: rodlike, globular, fibrous
Membrane proteins vary in function:
Some proteins function as receptors or growth factors on the cell surface, starting signal transduction
Other proteins transport ions or molecules across cell membrane, such as ion channels
Cell Membrane structure: proteins
Cell Identification
Other 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
cytoskeleton
Proteins that protrude into the cell will anchor supportive rods and tubules, forming a cytoskeleton
cellular adhesion molecules
Membrane proteins called cellular adhesion molecules (CAMs) help determine a cell’s interactions with other cells
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:
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
Organelles: Endoplasmic reticulum (ER):
Made up of membrane-bound flattened sacs and vesicles
Functions as a transport network throughout the cell
2 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
Organelles:
Vesicles
Membranous sacs
Store or transport substances within or between cells
Golgi apparatus:
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)
Mitochondria
Elongated fluid-filled sacs
The inner membrane of the 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
Lysosomes:
Membranous sacs
Formed by budding off Golgi apparatus
Contain enzymes that break down nutrients, ingested materials, debris, worn out cell parts, cholesterol (in some cells), toxins, and drugs
Peroxisomes
Also membranous sacs
Contain a different set of enzymes than lysosomes
Their enzymes function in the breakdown of fatty acids and hydrogen peroxide, and detoxification of alcohol
cytoskeleton.
In the cytoplasm, there are several structures, which are not organelles, but rather part of the cytoskeleton.
Microfilaments and microtubules
Microfilaments and microtubules are thin, threadlike structures that serve as the cytoskeleton of the cell:
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 fibers
made of different proteins in different cells, are abundant in skin cells and neurons
Other Cellular Structures: Centrosome
A non-membranous 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
Other Cellular Structures: 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
Other Cellular Structures: Flagella
Another type of motile extension from cell; usually a cell only has 1 flagellum; the only human cell with a flagellum is a sperm cell; the flagellum allow the sperm to swim toward the egg cell
nucleus
The nucleus contains genetic material, DNA, which controls cell activities
nuclear envelope
The fairly large nucleus is surrounded by a double-layered nuclear envelope.
Containing relatively large nuclear pores that allow the passage of certain substances
nucleoplasm
Contains a fluid called nucleoplasm
The nucleolus
The nucleolus is a small, dense body in the nucleus, composed of RNA and protein; it is the site of ribosome production
Chromatin
Chromatin consists of loosely coiled fibers of protein and DNA found in the nucleus
chromosomes
Condensed DNA is referred to as chromosomes; this form of DNA is present during cell division
selective barrier
The cell membrane is a selective barrier, which controls which substances pass through it
Movements Into and Out of the Cell:
passive
Mechanisms of movement across the membrane may be passive, requiring no energy from the cell,
active
requiring cellular energy
passive transport mechanisms:
Examples of passive transport mechanisms: diffusion, facilitated diffusion, osmosis, and filtration
active transport mechanisms
Examples of active transport mechanisms: active transport, endocytosis, and exocytosis
Passive mechanisms:
Diffusion
Characteristics: Molecules move through the phospholipid bilayer from regions of higher concentration to regions of lower concentration.
Source of Energy: Molecular motion
Example: Exchange of oxygen and carbon dioxide in the lungs
Facilitated diffusion
Ions move through channels, or molecules move by carrier proteins, across the membrane from a region of higher concentration toward one of lower concentration.
Molecular motion
Movement of glucose through a cell membrane
Osmosis
Water molecules move through a selectively permeable membrane toward the solution with more impermeant solute (greater osmotic pressure).
Molecular motion
Distilled water entering a cell
Filtration
Smaller molecules are forced through porous membranes from regions of higher pressure to regions of lower pressure.
Hydrostatic pressure
Molecules leaving blood capillaries
Active mechanisms:
Active transport
Carrier molecules transport molecules or ions through membranes from regions of lower concentration toward regions of higher concentration.
Cellular energy (ATP)
Movement of various ions, sugars, and amino acids through membranes
Endocytosis:
Pinocytosis
Membrane engulfs droplets containing dissolved molecules from surroundings.
Cellular energy
Uptake of water and solutes by all body cells
Phagocytosis
Membrane engulfs particles from surroundings.
cellular energy
White blood cell engulfing bacterial cell
Receptor-mediated endocytosis
Membrane engulfs selected molecules combined with receptor proteins
Cellular energy
Cell removing cholesterol molecules from its surroundings
Exocytosis
Vesicle fuses with membrane and releases contents outside of the cell.
Cellular energy
Neurotransmitter release
Cell Cycle:
Main phases
interphase, mitosis, and cytokinesis (division of the cytoplasm)
checkpoints
Whether the cell cycle progresses at certain times, called checkpoints, is controlled by interaction of special proteins
restriction checkpoint
A restriction checkpoint determines fate of cell: to continue cell cycle and divide, to enter a non-dividing stage as a specialized cell, or to die
Interphase
Phase of the cell cycle, before cell division begins, in which the cell grows and synthesizes new molecules, membranes, DNA, and organelles
This is a time of great synthetic activity, rather than a time of rest
Cell Division:
2 types of cell division
Meiosis:
Only used for sperm & egg cell production
Ensures that mature gametes have ½ the normal number of chromosomes as normal body cells
Cell Division (Mitosis + Cytokinesis):
More common type of cell division, used to increase number of cells
Mitosis is division of the nucleus
Cytokinesis is the division of the cytoplasm
Regulated carefully, so that each of two daughter cells receives an exact copy of the original cell’s genetic material (DNA)
Mitosis
Mitosis is described as a series of four stages, but the process is actually continuous:
Prophase
The first stage of mitosis
DNA condenses into visible chromosomes (each consisting of two chromatids connected by a centromere)
Centrioles migrate to opposite poles
Microtubules of the cytoskeleton organize into spindle fibers
The nuclear membrane and nucleolus disassemble
Metaphase
Spindle fibers attach to centromeres on the chromosomes
Chromosomes randomly align midway between centrioles
Anaphase
Sister chromatids are pulled apart
Chromatids are now considered individual chromosomes
Chromatids are pulled along the shortening spindle fibers toward opposite centrioles
Telophase
The final stage of mitosis
Begins when the chromosomes have completed their migrations to opposite sides
A nuclear envelope and nucleolus assemble for each forming cell
The spindle fibers disassemble
The chromosomes unwind, and take back their chromatin form
Cytokinesis (Cytoplasmic Division)
Begins during anaphase of mitosis, when cell membrane constricts around middle of cell
cleavage furrow
Constriction, called the cleavage furrow, continues throughout telophase; it eventually pinches the daughter cells apart
Contractile ring
Contractile ring of microfilaments attaches to inside of cell membrane; it divides the cytoplasm
The two daughter cells may have slightly different amounts of cytoplasm and organelles, but they share identical genetic information
Cell Differentiation:
Differentiation
The process by which a cell develops/specializes into a specific type of cell with specialized functions
Humans have >290 types of differentiated cells
Cell differentiation allows cells to specialize by using different parts of the complete genome that is present in each cell; in each type of cell, some genes are “turned on” while others are “turned off”
Stem cells
retain the ability to divide without specialization; their presence in the cell allows for continuous growth and renewal
Progenitor cells
are daughters of stem cells that are partially specialized.
Stem and progenitor cells allow cells to retain the ability to produce cells that will become the differentiated cells in each area of the body
Cell Death
Not all cells divide or differentiate; some die
Apoptosis
is a form of programmed cell death that is a normal part of development, rather than from injury or disease
Removes overgrown tissues, damaged cells, extra cells in fetus
Steps of apoptosis:
Cell becomes rounded and bulges
Nuclear membrane breaks down
Chromatin condenses and enzymes cut up the chromosomes
Cell shatters into many membrane-bound pieces
Scavenger cells engulf and destroy the fragments
