ch. 5 membrane structure and function membranes organize the chemical reactions making up metabolism
TRANSCRIPT
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CH. 5 MEMBRANE STRUCTURE AND FUNCTION
Membranes organize the chemical reactions making up metabolism
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Membranes are selectively permeable
–They control the flow of substances into and out of a cell
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Membrane phospholipids form a bilayer
• Phospholipids are the main structural components of membranes
• In water, phospholipids form a stable bilayer
The heads face outward and the tails face inward
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The membrane is a fluid mosaic of phospholipids and proteins
• Phospholipid molecules form a flexible bilayer– Cholesterol (only in animal cells)and protein
molecules are embedded in it – Carbohydrates act as cell identification tags (only on
outside of cell)
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Proteins make the membrane a mosaic of function
• Some membrane proteins form cell junctions
• Others transport substances across the membrane
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•Many membrane proteins are enzymes
Some proteins function as receptors for chemical messages from other cells
Enzyme activity
Signal transduction
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Movement through the channel
• Why do molecules move through membrane if you give them a channel?
?
?
HIGH
LOW
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Molecules move from high to low• Diffusion– move from HIGH to LOW concentration
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Diffusion• Move from HIGH to LOW concentration– passive transport– no energy needed
diffusion osmosis
diffusion of water
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Simple Diffusion • Move from HIGH to LOW
inside cell
outside cell
Which way will fat move?
fat
fat
fat
fat
fat
fat fat
fatfat
fat
fat
fat
fat
fat
LOW
HIGH
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Facilitated Diffusion• Move from HIGH to LOW through a channel
inside cell
outside cell
sugar
sugar
sugar
sugar
sugarsugar
sugarsugar
sugarsugar
sugar
Which way will sugar move?
sugar
sugar
LOW
HIGH
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Diffusion• Move from HIGH to LOW concentration – directly through membrane• simple diffusion• no energy needed
– help through a protein channel• facilitated diffusion (with help)• no energy needed HIGH
LOW
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Simple vs. facilitated diffusion
inside cell
outside cell
lipidinside cell
outside cell
H2O
simple diffusion facilitated diffusion
H2O
protein channel
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Passive Transport
• Requires no energy from cell• Movement down the
concentration gradient
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Osmosis is the passive transport of water across a cell membrane
• In osmosis, water travels from an area of lower solute concentration to an area of higher solute concentration – Hypertonic: [hyper=above, over]• Solutions in which more solute present (concentrated
solution)– Hypotonic: [hypo= below, under]• Solutions in which less solute is present (dilute
solution)– Isotonic: [ iso= equal]• Solutions have equal concentrations of substances
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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
ANIMALCELL
PLANTCELL
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Cells in an isotonic solution
• isotonic solution-
• (= concentrations)
• the concentration of dissolved substances in the solution is the same as the concentration of dissolved substances inside the cell.
H2OH2O
Water Molecule
Dissolved Molecule
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Cells in an isotonic solution
• water molecules move into and out of the cell at the same rate, and cells retain their normal shape.
H2O
H2O
Water Molecule
Dissolved Molecule
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Blood CellPlant Cell
Isotonic Solution
Cellular Structure and Function
Water and dissolved substances diffuse into and out of the cell at the same rate.
11,397x
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Cells in an isotonic solution
• A plant cell has its normal shape and pressure in an isotonic solution.
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Keeping right amount of water in cell• Balanced conditions (isotonic)– no difference in concentration
of water between cell & environment• cell in equilibrium• example: blood• problem: none–water flows across membrane
equally, in both directions– volume of cell doesn’t change
balanced
I couldbe better…
That’sbetter!
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Cells in a hypotonic solution• hypotonic solution:
dilute solution thus low solute concentration
• In a hypotonic solution, water enters a cell by osmosis, causing the cell to swell.
H2O
H2O
Water Molecule
Dissolved Molecule
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Cells in a hypotonic solution
• Plant cells swell beyond their normal size as pressure increases. (plants prefer this –it makes the leaves firm (turgid)
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Keeping right amount of water in cell• Freshwater -Hypotonic– a cell in fresh water– high concentration of water around cell• cell gains water
• example: Paramecium • problem: cells gain water,
swell & can burst– water continually enters
Paramecium cell
• solution: contractile vacuole – pumps water out of cell
freshwater
No problem,here
KABOOM!
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Controlling water
• Contractile vacuole in Paramecium
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Cells in a hypertonic solution• hypertonic solution: concentrated solution,
thus a high solute concentration
In a hypertonic solution, water leaves a cell by osmosis, causing the cell to shrink
H2OH2O
Water Molecule
Dissolved Molecule
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Cells in a hypertonic solution
• Plant cells lose pressure as the plasma membrane shrinks away from the cell wall. PLASMOLYSIS
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Plant Cell
Hypertonic SolutionCellular Structure and Function
Solute concentration is higher outside the cell.
Water diffuses out of the cell.
Blood Cell
13,000x
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Keeping right amount of water in cell
• Saltwater -Hypertonic– a cell in salt water– low concentration of water around
cell• cell loses water
– example: shellfish– problem: cell loses water• in plants: plasmolysis• in animals: shrinking cell
– solution: take up water
saltwater
I willsurvive!
I’m shrinking,I’m shrinking!
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Passive Transport• When a cell uses no energy to move particles
across a membrane passive transport occurs• Particles go DOWN their concentration gradient.• all diffusion & osmosis are passive transport.
Concentration gradientPlasma membrane
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Active Transport• active transport :Movement of materials through
a membrane against a concentration gradient and requires energy from the cell. ATP
Concentration gradient
Plasma membrane
Carrier proteins
Cellular energy
Cellular energy
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Active transport• Cells may need molecules to move against
concentration “hill”– need to pump “uphill”• from LOW to HIGH using energy
– protein pump– requires energy• ATP
ATP
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Exocytosis and endocytosis transport large molecules uses energy
1. Exocytosis [ exo = outside]movement of large materials OUT of the cell~a vesicle may fuse with the membrane and expel its contents
CYTOPLASM
FLUID OUTSIDE CELL
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Endocytosis2. Endocytosis [ endo = inside]
movement of large materials to INSIDE of the cell~membrane may fold inward, trapping materials from the outside
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Three kinds of endocytosis:All are Active Transport
1. Phagocytosis : “cell eating” solid” solid particles ingested by cell
2. Pinocytosis: “cell drinking” LiquidLiquid particles taken into cell
3. Receptor mediated: special receiver in membrane picks up specificspecific material