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CHAPTER 9
MEMBRANES, DIFFUSION, OSMOSIS, ACTIVE TRANSPORT,
ETC.
• Membranes organize the chemical reactions making up metabolism
Membranes organize the chemical activities of cells
MEMBRANE STRUCTURE AND FUNCTION
Cytoplasm
• 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
• Phospholipids are the main structural components of membranes
• They each have a hydrophilic head and two hydrophobic tails
Membrane phospholipids form a bilayerHead
Symbol
Tails
• In water, phospholipids form a stable bilayer
Hydrophilicheads
Hydrophobictails
Water
Water
– The heads face outward and the tails face inward
• Phospholipid molecules form a flexible bilayer– Cholesterol and protein molecules are embedded in it – Carbohydrates act as cell identification tags
The membrane is a fluid mosaic of phospholipids and proteins
• The plasma membrane of an animal cell
Fibers of the extracellular matrix
Glycoprotein Carbohydrate (of glycoprotein)
Microfilaments of the cytoskeleton
Phospholipid
Cholesterol
Proteins
CYTOPLASM
Glycolipid
• Some membrane proteins form cell junctions
• Others transport substances across the membrane
Proteins make the membrane a mosaic of function
Transport
• 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
Enzyme activity Signal transduction
Messenger molecule
Receptor
Activated molecule
• In passive transport, substances diffuse through membranes without work by the cell– They spread from
areas of high concentration to areas of lower concentration
Passive transport is diffusion across a membrane
EQUILIBRIUMMolecule of dye Membrane
EQUILIBRIUM
• In osmosis, water travels from an area of lower solute concentration to an area of higher solute concentration
Osmosis is the passive transport of water
Hypotonicsolution
Solutemolecule
HYPOTONIC SOLUTION
Hypertonic solution
Selectivelypermeablemembrane
HYPERTONIC SOLUTION
Selectivelypermeablemembrane
NET FLOW OF WATER
Solute molecule with cluster of water molecules
Water molecule
• 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
Water balance between cells and their surroundings is crucial to
organisms
ISOTONIC SOLUTION
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
(1) Normal
(4) Flaccid
(2) Lysing
(5) Turgid
(3) Shriveled
(6) Shriveled
ANIMALCELL
PLANTCELL
Plasmamembrane
• Small nonpolar molecules diffuse freely through the phospholipid bilayer
• Many other kinds of molecules pass through selective protein pores by facilitated diffusion
Transport proteins facilitate diffusion across membranes
Solutemolecule
Transportprotein
• Transport proteins can move solutes across a membrane against a concentration gradient– This is called active transport – Active transport requires ATP
Cells expend energy for active transport
• Active transport in two solutes across a membrane
Transportprotein
1
FLUIDOUTSIDECELL
Firstsolute
First solute, inside cell, binds to protein
Phosphorylated transport protein
2 ATP transfers phosphate to protein
3 Protein releases solute outside cell
4 Second solute binds to protein
Second solute
5 Phosphate detaches from protein
6 Protein releases second solute into cell
• To move large molecules or particles through a membrane– a vesicle may fuse with the membrane and expel its
contents (exocytosis)
Exocytosis and endocytosis transport large molecules
FLUID OUTSIDE CELL
CYTOPLASM
– or the membrane may fold inward, trapping material from the outside (endocytosis)
• Three kinds of endocytosis
Pseudopod of amoeba
Food being ingested
Plasma membrane
Material bound to receptor proteins
PIT
Cytoplasm
• Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors
Connection: Faulty membranes can overload the blood with
cholesterol
LDL PARTICLEPhospholipid outer layer
Protein
Cholesterol
Plasma membraneCYTOPLASM
Receptor protein
Vesicle
• Enzymes and membranes are central to the processes that make energy available to the cell
• Chloroplasts carry out photosynthesis, using solar energy to produce glucose and oxygen from carbon dioxide and water
• Mitochondria consume oxygen in cellular respiration, using the energy stored in glucose to make ATP
Chloroplasts and mitochondria make energy available for cellular
work
• Nearly all the chemical energy that organisms use comes ultimately from sunlight
• Chemicals recycle among living organisms and their environment
Sunlight energy
Chloroplasts,site of photosynthesis
CO2
+H2O
Glucose+O2
Mitochondriasites of cellular
respiration
(for cellular work)
Heat energy