Barron's AP Biology - The Cell

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Prokaryote

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1

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

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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

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3

Form and Function Go Together

  • All cells do not look alike. Their function dictates their form and vice versa

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4

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

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5

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

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Typical Plant Cell

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7

Nucleolus

  • Prominent region seem in the nucleus during interphase

  • Where ribosome components are synthesized and assembled

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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

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9

Ribosomes

  • Site of protein synthesis

  • Found free in the cytoplasm or attached to the endoplasmic reticulum

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10

Golgi Apparatus

  • Packages and secretes substances produced in the ER

  • Lies near nucleus; consists of flattened membranous sacs

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11

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

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12

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

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13

Peroxisomes

  • Contain enzyme that converts hydrogen peroxide to harmless water

  • In liver cells, detoxify alcohol

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14

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

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15

Food Vacuoles

  • Formed by phagocytosis

  • Surrounded by a single membrane

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Central Vacuoles

  • Found in mature plant cells enclosed in a specialized membrane called a tonoplast

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17

Contractile Vacuoles

  • Found in freshwater protista like amoeba and paramecia

  • Pump out excess water that diffuses inward because organisms live in a hypotonic environment

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18

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

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19

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

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Cytoskeleton

  • Complex network of protein filaments that extends through the cytoplasm and gives cell its shape and ability to move

  • Ex: microtubules, microfilaments

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21

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

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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

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23

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

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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

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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

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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

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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

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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

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29

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

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Active Transport

  • Movement of molecules against a gradient from a region of low concentration to high concentration

  • Requires energy (ATP)

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31

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

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32

Pinocytosis

  • Cell drinking

  • The uptake of large, dissolved particles

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33

Phagocytosis

  • Engulfing of large particles or small cells by pseudopods

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Ion-channel receptors

  • Allosteric receptors that opens and shuts a gate in a membrane allowing an influx of ions

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35

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

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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

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37

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

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38

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

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