Prokaryote
Cells with no nuclear or internal membranes; i.e, lysosomes, vacuoles, and mitochondria
Evolved 3.6 billion years ago
Prokaryotes are classified in two domains: Archaea and Bacteria
Contain small ribosomes
Cells are small
Contain naked, circular DNA
Eukaryotes
Cells with internal membranes
Evolved a billion years ago
According to the theory of endosymbiosis, chloroplasts and mitochondria were formerly tiny prokaryotes that took up residence inside larger cells and formed permanent symbiotic relationship
DNA is wrapped with histone proteins into chromosomes
Metabolism is aerobic
Cells are larger than prokaryotes
Form and Function Go Together
All cells do not look alike. Their function dictates their form and vice versa
Why Cells Are So Small
The surface area of the plasma membrane, which controls what enters and leaves a cell, limits the overall size of the cell. This is because while the surface area of a sphere increases at one rate, the volume increases at a much faster rate
Cell Organelles
Typical animal cell: A. Endocytotic vacuole B. Nucleolus C. Smooth endoplasmic reticulum D. Cell membrane E. Cytoplasm (cytosol) F. Rough endoplasmic reticulum G. Ribosomes H. Vacuole I. Nuclear membrane J. Mitochondria
Typical Plant Cell
Nucleolus
Prominent region seem in the nucleus during interphase
Where ribosome components are synthesized and assembled
Nucleus
Contains chromosomes (DNA) wrapped into a chromatin network
Surrounded by selectively permeable membrane that contains nuclear pores for the passage of large molecules like mRNA
Ribosomes
Site of protein synthesis
Found free in the cytoplasm or attached to the endoplasmic reticulum
Golgi Apparatus
Packages and secretes substances produced in the ER
Lies near nucleus; consists of flattened membranous sacs
Endoplasmic Reticulum (ER)
Membranous system of channels and flattened sacs that traverses the cytoplasm
Rough ER
Site of protein synthesis
Smooth ER
Site of protein synthesis
Connects rough ER to Golgi apparatus
Carries out various detoxification processes
Lysosomes
Sacs of hydrolytic enzymes surrounded by a single membrane
Principal site of intracellular digestion of macromolecules
Carry out programmed destruction of cells, apoptosis, using their hydrolytic enzyme
Found in large numbers in phagocytic white blood cells
Absent from plant cells
Peroxisomes
Contain enzyme that converts hydrogen peroxide to harmless water
In liver cells, detoxify alcohol
Mitochondria
Site of aerobic respiration, the process that generates ATP
Internal membranes are called cristae membranes
Enclosed in a double membrane because in ancient times these were tiny free-living cells that took up residence inside larger organisms. This is the theory of endosymbiosis
Food Vacuoles
Formed by phagocytosis
Surrounded by a single membrane
Central Vacuoles
Found in mature plant cells enclosed in a specialized membrane called a tonoplast
Contractile Vacuoles
Found in freshwater protista like amoeba and paramecia
Pump out excess water that diffuses inward because organisms live in a hypotonic environment
Chloroplasts
Present in all living plant cells
Site of photosynthesis
Contains the green photosynthetic pigment chlorophyll that absorbs light energy for photosynthesis
According to the Theory of Endosymbiosis, chloroplasts were once free-living prokaryotes
Cilia / Flagella
Appendages that protrude from eukaryotic cells for locomotion
Consist of special arrangement of microtubules: 9 pairs of microtubules + 2 singles (9 + 2)
Cilia are short; flagella are long
In paramecia, euglena, sperm, and human respiratory system
Cytoskeleton
Complex network of protein filaments that extends through the cytoplasm and gives cell its shape and ability to move
Ex: microtubules, microfilaments
Cell Wall
Found in plant cells; not in animal cells
In plants algae, it consists of cellulose
In fungi, it consists of chitin
Primary cell wall: immediately outside plasma membrane
Secondary cell wall: where found, located outside primary wall
Plasma Membrane
Selectively-permeable - fluid mosaic model
Consists of a lipid bilayer with proteins dispersed throughout
In vertebrates, cholesterol molecules are embedded in the interior of membrane for stability
External surface has glycoproteins that functions in cell-to-cell communications
Contains protein channels, pumps, and enzymes
Transport
Movement of a substance into or out of a cell
Can be active or passive
Passive: requires no expenditure of energy
Active: requires energy
Passive Transport
Movement of molecules down a gradient from a region of high concentration to a region of low concentration; no energy (ATP) required
Simple diffusion: no membrane required
Facilitated diffusion: molecules diffuses through membrane channels
Osmosis: type of diffusion where water diffuses across a membrane
Countercurrent exchange: special case of simple diffusion - flow of adjacent fluids in opposite direction to maximize rate of diffusion
Water Potential
Symbol is Greek letter psi = Ń°
Water moves across a membrane from a solution with higher water potential to a solution with lower water potential
Water potential of pure water = 0
Addition of solute to water lowers Ń°
Water flows from hypotonic solutions to hypertonic solutions
What Happens to a Cell in a Hypertonic Solution?
Cell shrinks because water flows from higher water potential to lower water potential
Hypertonic means higher concentration of solute
Cell shrinking is called plasmolysis
What Happens to a Cell in a Hypotonic Solution?
Animal cells swell or burst because water moves from higher water potential to lower water potential
Hypotonic means lower concentration of solute
Plant cells do not burst; they swell and become turgid
What Happens to a Cell in an Isotonic Solution?
Nothing happens to the cell because the concentrations of the solutions outside and inside the cell are equal
Aquaporins
Special water channel proteins in certain cells that facilitate the rapid diffusion of massive amounts of water across a cell membrane
They do not alter the water potential gradient; they only speed the rate of diffusion
Active Transport
Movement of molecules against a gradient from a region of low concentration to high concentration
Requires energy (ATP)
Sodium-Potassium Pump
Pumps Na+ and K+ ions across axon membranes against a gradient
Returns the neuron to its resting state - polarized
While the pump is working, no impulse can pass along the axon - an interval called the refractory period
Pinocytosis
Cell drinking
The uptake of large, dissolved particles
Phagocytosis
Engulfing of large particles or small cells by pseudopods
Ion-channel receptors
Allosteric receptors that opens and shuts a gate in a membrane allowing an influx of ions
Cytoplasmic Receptors
Reception: small, non-polar ligands diffuse directly through the plasma membrane and into the cytoplasm where they bind with an intracellular receptor
Transduction: Once activated, the receptor converts a molecular signal to a cellular response
Response: Can be a single step of a more complex signal transduction pathway
Signal Transduction Pathway
A multistep process in which a small number of extracellular signaling molecules produce a major cellular response via a cascade effect
The similarity in these pathways in bacteria, plants, and animals suggests that the pathways evolved hundreds of millions of years ago in a common ancestor
Advantage of a multistep pathway: it provides many opportunities for coordination and regulation
Gap Junctions
Another example of cell-to-cell communication
Provide cytoplasmic channels between adjacent cells in animals
Large molecules can pass from cell to cell
In the heart, they enable cells to coordinate contractions of muscle tissue
Plasmodesmata - channels between cells in plants
Apoptosis
Programmed cell death, brought about by signals that activate a series of suicide proteins
Essential to the development of the nervous system in embryos and to normal operation of the immune system
Different pathways involving about 15 different caspases carry out apoptosis
Triggering signals can come from the nucleus of a cell when the DNA is irreparably damaged or from the endoplasmic reticulum when excessive protein misfolding occurs
Signals can originate outside the cell and bind with all cell-surface receptor that triggers a signal transduction pathway to initiate apoptosis
Mitochondria can initiate and apoptotic pathway by leaking relay protein into the cytoplasm
An important example of cell communication