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© 2012 Pearson Education, Inc. Review ATP is the most important form of energy used by cells (“universal energy currency”) A solute is the material that is dissolved in a solution A solvent is the liquid in the solution that dissolves the solute
© 2012 Pearson Education, Inc. Cell Doctrine All living things are composed of cells A single cell is the smallest unit that exhibits all of the characteristics of life All cells come only from preexisting cells
© 2012 Pearson Education, Inc. Two Basic Cell Types Classified by Internal Organization Prokaryotic Cells Plasma membrane No nucleus Cytoplasm: fluid within membrane No true organelles (except for ribosomes, cilia, & flagella) Eukaryotic Cells Plasma membrane Nucleus: information center Cytoplasm: fluid within membrane Organelles: structures with specialized functions All human cells are eukaryotic
© 2012 Pearson Education, Inc. Figure 3.1a Nucleus Organelles Cytoplasm Plasma membrane a) A eukaryotic animal cell has a large nucleus and numerous small organelles. The cytoplasm is enclosed by a flexible plasma membrane.
© 2012 Pearson Education, Inc. Figure 3.1b b) Prokaryotic cells such as this bacterium have a rigid cell wall surrounding the plasma membrane. The genetic material is not surrounded by a membrane, and there are no organelles in the cell. The elongated bacterium in the center of the photo is about to divide in two, as its genetic material is concentrated at both ends of the cell. Cell wall Plasma membrane
© 2012 Pearson Education, Inc. Cell Structure Reflects Cell Function Though eukaryotic cells are remarkably similar, there are structural differences –Examples: Muscle cells –Contain numerous organelles providing energy needed for muscle contraction Nerve cells –Long and thin to carry impulses over distance Small size is efficient
© 2012 Pearson Education, Inc. Figure 3.2a a) A portion of several muscle cells of the heart (X 1,500).
© 2012 Pearson Education, Inc. Figure 3.2b b) Nerve cells of the central nervous system (X 830).
© 2012 Pearson Education, Inc. Figure 3.2c c) Cells lining a tubule of a kidney (X 250).
© 2012 Pearson Education, Inc. Cells Remain Small to Stay Efficient Small cells have a higher surface:volume ratio High surface:volume ratio promotes efficiency in –Acquisition of nutrients –Disposal of wastes
© 2012 Pearson Education, Inc. Figure 3.4a a)One large cell.
© 2012 Pearson Education, Inc. Figure 3.4b b) Eight small cells.
© 2012 Pearson Education, Inc. Figure 3.4c c) Cell with microvilli on one surface.
© 2012 Pearson Education, Inc. Plasma Membrane Surrounds the Cell Separates a cell from its environment Selectively permeable –Permits movement of some substances into and out of the cell, but blocks others Enables communication between environment and cell (an example is insulin, from the pancreas, binding to a part of a liver cell membrane, “telling” it to take up more glucose)
© 2012 Pearson Education, Inc. A Plasma Membrane Surrounds the Cell Plasma membrane is a phospholipid bilayer –Phospholipids: polar head and nonpolar tail –Cholesterol: makes membrane a bit more rigid –Proteins: provide means of transport through membrane & some are receptor proteins –Carbohydrates: recognition patterns for cells and organisms Nonrigid Fluid mosaic
© 2012 Pearson Education, Inc. Figure 3.5 Receptor protein Channel protein (always open) Gated channel protein (closed position) Carbohydrate groups Cytoskeleton filaments Phospholipid Cytoplasm Lipid bilayer Transport protein Glycoprotein Extracellular environment Cholesterol
© 2012 Pearson Education, Inc. Membrane Structure
© 2012 Pearson Education, Inc. Non-polar, small molecules (O 2, CO 2 ), & water can pass through the membrane without having to use special channels Polar molecules, larger molecules, & ions require specific channels to pass through the membrane
© 2012 Pearson Education, Inc. Molecules Cross the Plasma Membrane in Several Ways Passive transport –Cell does not need to expend energy for this Diffusion Osmosis Active transport – cell must expend energy Bulk transport –Involves membranous vesicles to move larger substances Endocytosis Exocytosis
© 2012 Pearson Education, Inc. Passive Transport Moves with the Concentration Gradient Passive transport is powered by the concentration gradient. In the cell it occurs as –Diffusion through lipid layer –Diffusion through protein channels –Facilitated transport Transport or carrier proteins in the membrane assist in moving molecules across the membrane, down the concentration gradient, without expending energy
© 2012 Pearson Education, Inc. Figure 3.8 Lower concentration Diffusion through the lipid layer. Lipid-soluble molecules such as O 2 and CO 2 diffuse freely through the plasma membrane. Diffusion through channels. Some polar and charged molecules diffuse through protein channels that span the membrane. Water is a typical example. Facilitated transport. Certain molecules bind to a protein, triggering a change in protein shape that transports the molecule across the membrane. Glucose typically enters cells by this method. Higher concentration
© 2012 Pearson Education, Inc. Active Transport Active transport moves substances from an area of lower concentration to an area of higher concentration –Requires a membrane protein (transporter) –Requires ATP or other energy source
© 2012 Pearson Education, Inc. Figure 3.9a a) In active transport using ATP, energy derived from the breakdown of ATP is used to change the shape of the carrier protein.
© 2012 Pearson Education, Inc. Figure 3.9b b) Some carrier proteins use energy derived from the downhill transport of one molecule to transport another molecule uphill. In this example, the energy to transport the square molecules comes from the facilitated transport of the spearhead molecules.
© 2012 Pearson Education, Inc. Passive and Active Transport
© 2012 Pearson Education, Inc. Endocytosis and Exocytosis Move Materials in Bulk Used to move larger molecules –Endocytosis: brings substances into the cell –Exocytosis: expels substances from the cell
© 2012 Pearson Education, Inc. Figure 3.10a a) Endocytosis. In endocytosis, material is surrounded by the cell membrane and brought into the cell. Extracellular environment Plasma membrane Cytoplasm Vesicle
© 2012 Pearson Education, Inc. Figure 3.10b b) Exocytosis. In exocytosis, a membranous vesicle fuses with the plasma membrane, expelling its contents outside the cell.
© 2012 Pearson Education, Inc. Endocytosis and Exocytosis
© 2012 Pearson Education, Inc. Information Transfer Across the Plasma Membrane Receptor proteins span membrane – required for transmission of information to and from cell Receptor sites (on receptor proteins) – interact specifically with signal molecules A change is triggered within the cell as a result of binding of signal molecule to receptor site Different cell types have different receptor proteins
© 2012 Pearson Education, Inc. Figure 3.11 Receptor site Extracellular environment Cytoplasm Substrate Product
© 2012 Pearson Education, Inc. The Sodium–Potassium Pump: Helps Maintains Cell Volume Sodium–potassium pump expels unwanted ions, keeps needed ones, and maintains cell volume ATP is used to expel 3 sodium ions for every 2 potassium ions brought into the cell Increase in cell volume = increase in water in cytoplasm by decreasing pumping and allowing more sodium inside cell Decrease in cell volume = less water in cytoplasm by increasing pumping and expelling more sodium ions
© 2012 Pearson Education, Inc. Figure 3.12a Most of the potassium diffuses out of the cell, but sodium diffuses in only very slowly Potassium is transported into the cell, and the sodium binding sites become exposed again. Potassium binding triggers another change of shape. The loss of sodium exposes two binding sites for potassium. Energy released by ATP causes the protein to change its shape, expelling the sodium ions. Cytoplasm Binding of three cytoplasmic Na + to the sodium-potassium pump stimulates the breakdown of ATP. Sodium ions bind to binding sites accessible only from the cytoplasm. a) The cell membrane contains Na + - K + pumps, and also channels that permit the rapid outward diffusion of K + but only a slow inward diffusion of Na +. 7 Extracelluar fluid 1 2 3 4 6 5
© 2012 Pearson Education, Inc. Figure 3.12b In the steady-state, the rate of outward sodium transport equals the rate of inward diffusion. When the rate of outward sodium transport exceeds inward diffusion, water diffuses out and the cell shrinks. When the rate of outward sodium transport is less than the rate of inward diffusion, water diffuses in and the cell swells b) The rate of transport by the Na + - K + pumps determines cell volume. Key: Active transport of Na + Diffusion of K + Diffusion of Na + Diffusion of H 2 O Sodium-potassium pump
© 2012 Pearson Education, Inc. Isotonic Extracellular Fluid Maintains Cell Volume Tonicity: relative concentration of solutes in two fluids Isotonic –Extracellular and intracellular ionic concentrations are equal –Cells maintain a normal volume in isotonic extracellular fluids –Regulatory mechanisms maintain extracellular fluid that is isotonic with intracellular fluid
© 2012 Pearson Education, Inc. Isotonic Extracellular Fluid Maintains Cell Volume Variations in tonicity –Hypertonic Extracellular ionic concentration higher than intracellular Water will diffuse out of cell Cell will shrink and die –Hypotonic Extracellular ionic concentration lower than intracellular Water will diffuse into cell Cell may swell and burst
© 2012 Pearson Education, Inc. Figure 3.13a a) Water movement into and out of human red blood cells placed in isotonic, hypertonic, and hypotonic solutions. The amount of water movement is indicated by the sizes of the arrows. 9 grams of salt in 1 liter of solution 18 grams of salt in 1 liter of solution Pure water Isotonic Hypertonic Hypotonic
© 2012 Pearson Education, Inc. Figure 3.13b b) Scanning of electron micrographs of red blood cells placed in similar solutions.
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