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ENERGY AND THE CELL Living cells are compartmentalized by membranes Membranes are sites where chemical reactions can occur in an orderly manner Living cells process energy by means of enzyme-controlled chemical reactions
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5.10 Membranes organize the chemical activities of cells
MEMBRANE STRUCTURE AND FUNCTION 5.10 Membranes organize the chemical activities of cells Membranes organize the chemical reactions making up metabolism Cytoplasm Figure 5.10
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Membranes are selectively permeable
They control the flow of substances into and out of a cell Membranes can hold teams of enzymes that function in metabolism
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5.11 Membrane phospholipids form a bilayer
Phospholipids are the main structural components of membranes They each have a hydrophilic head and two hydrophobic tails Head Symbol Tails Figure 5.11A
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In water, phospholipids form a stable bilayer
The heads face outward and the tails face inward Water Hydrophilic heads Hydrophobic tails Water Figure 5.11B
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5.12 The membrane is a fluid mosaic of phospholipids and proteins
Phospholipid molecules form a flexible bilayer Cholesterol and protein molecules are embedded in it Carbohydrates act as cell identification tags
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The plasma membrane of an animal cell
Glycoprotein Carbohydrate (of glycoprotein) Fibers of the extracellular matrix Glycolipid Phospholipid Cholesterol Microfilaments of the cytoskeleton Proteins CYTOPLASM Figure 5.12
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5.13 Proteins make the membrane a mosaic of function
Some membrane proteins form cell junctions Others transport substances across the membrane Figure 5.13 Transport
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Many membrane proteins are enzymes
Some proteins function as receptors for chemical messages from other cells The binding of a messenger to a receptor may trigger signal transduction Messenger molecule Receptor Activated molecule Figure 5.13 Enzyme activity Signal transduction
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5.14 Passive transport is diffusion across a membrane
In passive transport, substances diffuse through membranes without work by the cell They spread from areas of high concentration to areas of lower concentration Molecule of dye Membrane EQUILIBRIUM EQUILIBRIUM Figure 5.14A & B
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5.15 Osmosis is the passive transport of water
In osmosis, water travels from an area of lower solute concentration (high water concentration) to an area of higher solute concentration (low water concentrations) Hypotonic solution Hypertonic solution Selectively permeable membrane Solute molecule HYPOTONIC SOLUTION HYPERTONIC SOLUTION Water molecule Selectively permeable membrane Solute molecule with cluster of water molecules NET FLOW OF WATER Figure 5.15
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5.16 Water balance between cells and their surroundings is crucial to organisms
Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution The control of water balance (osmoregulation) is essential for organisms ISOTONIC SOLUTION HYPOTONIC SOLUTION HYPERTONIC SOLUTION ANIMAL CELL (1) Normal (2) Lysing (3) Shriveled Plasma membrane PLANT CELL Figure 5.16 (4) Flaccid (5) Turgid (6) Shriveled
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5.17 Transport proteins facilitate diffusion across membranes
Small nonpolar molecules diffuse freely through the phospholipid bilayer Many other kinds of molecules pass through selective protein pores by facilitated diffusion Solute molecule Transport protein Figure 5.17
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5.18 Cells expend energy for active transport
Transport proteins can move solutes across a membrane against a concentration gradient This is called active transport Active transport requires energy (ATP)
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5.19 Exocytosis and endocytosis transport large molecules
To move large molecules or particles through a membrane a vesicle may fuse with the membrane and expel its contents (exocytosis) FLUID OUTSIDE CELL CYTOPLASM Figure 5.19A
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or the membrane may fold inward, trapping material from the outside (endocytosis)
Figure 5.19B
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Material bound to receptor proteins
Three kinds of endocytosis Plasma membrane Pseudopod of amoeba Material bound to receptor proteins Food being ingested PIT Cytoplasm Ingesting large particles/food Ingesting water/nutrient molecules Selective uptake of chemical molecules Figure 5.19C
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5.20 Connection: Faulty membranes can overload the blood with cholesterol
Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors Phospholipid outer layer LDL PARTICLE Receptor protein Protein Cholesterol Plasma membrane Vesicle CYTOPLASM Figure 5.20
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