Post on 01-Apr-2015
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CP BIO: Ch. 7 The Cell Membrane
and transport across the membrane
The Plasma membraneBoundary between the cell and its environment
FLUID MOSAIC MODEL - Describes structure of cell membranes
• “mosaic” – sea of lipids with scattered proteins • “fluid” – molecules float and move around within
the layer
PhospholipidPhospholipid = lipid molecule with phosphate on one end
• glycerol + two fatty acids + phosphate group
• phosphate group = polar end
(hydrophilic “head”)
• Fatty acids = nonpolar end
(hydrophobic “tails”)
Phospholipids form a double layer
Nonpolar tails are on the inside (away from water)Polar heads are on the outside ( touch water)
Features of the Cell Membrane
• Semipermeable = some substances can pass through - some cannot- depends on: molecule size, charge, polar or nonpolar
• Cholesterol – molecules scattered among the phospholipids
- in animal cells
- keep membrane flexible in changing temperatures • Carbohydrates – glucose chains on outside of cell - Identification “tags”
- Receptor sites for messenger molecules
Fluid Mosaic Model
Membrane Proteinshave many functions
Transport Enzyme Receptor Allows a specific Catalyzes a Site for molecule to reaction inside messenger pass through the cell molecule to the membrane attach
Membrane proteins (2)
Identification Junctions Structure“SELF” – cell belongs Cells join to Keeps internal in this organism, form tissues, parts organized immunity communicate
How do membranes keep homeostasis?
Cell membranes are selectively permeable• The lipid layer blocks most
substances• Some molecules can cross the
membrane– By passive or active transport
• Some are too big to cross at all
PASSIVE TRANSPORTUSES NO CELL ENERGY
Molecules move randomly – spread out until evenly distributed• From an area of higher concentration to an area of
lower concentration
DIFFUSION Diffusion: movement of particles from an area
of high concentration to an area of lower concentration
• Two areas with different concentrations = concentration gradient
• Particles move in all directions (random)• NET movement is from high concentration to low• “Down the concentration gradient”
Diffusion
• Particles spread out until evenly distributed equilibrium (homogeneous)• AT equilibrium, particles continue moving, but
in all directions equally NO further change in concentration
Cell Membranes allow some particles to cross
Particles can diffuse across the lipid bilayer if they are:• Small • Nonpolar (lipid-soluble) Examples: CO2 , O2, fatty acids can diffuse easily
FACILITATED DIFFUSIONTransport proteins “help” particles move across
the membrane – DOWN their gradient • PASSIVE transport – no cell energy used• Proteins are SPECIFIC
– each allows only a certain substance to pass
Particles can cross by facilitated diffusion if they are:• Small • POLAR (water soluble) or CHARGED (ions)Examples: H2O, glucose, Ca+2, Cl-
Transport Proteins• Pores and Channels – open path through membrane• Carrier proteins – take particle on one side of
membrane and release on the other
OSMOSISDiffusion of water across a membrane Important process in cell homeostasis
Water crosses the cell membrane easily- Small enough to pass between lipid molecules- Also pass through special proteins, aquaporins
Why osmosis matters• Water crosses membrane easily, faster than
many solutes• Will try to reach equilibrium • If NET water moves into or out of cell changes
homeostasis• Unequal water on one side of cell membrane =
osmotic pressure
TONICITY = osmotic pressure in cells
ISOTONIC Equal concentrations of solutes inside and outside cell– Equal concentrations of water– Water goes in and out of cell at equal rates
Isotonic pressure in cells
• No NET movement of water into or out of cell
• Normal water pressure in animal cells
• Wilted (“flaccid”) water pressure in plant cells
When solute concentration is different on two sides of a membrane
lower solute concentration = hypotonicHigher solute concentration = hypertonic
Solutes will move down their gradient IF THEY CAN CROSS THE MEMBRANE
Water concentration is OPPOSITE of solutes
Low solutes HIGH WATER concentration High solutes LOW WATER concentration
Water WILL diffuse down its gradient and crosses the cell membrane easily
Goes TO whichever side has more solutes
Cells in hypotonic solutions
Cytoplasm swells - animal cells: swell, may burst (“lyse”) - plant cells: high osmotic pressure “turgor” - won’t burst - cell wall - “Turgid” – stiff and firm, upright stem
Solutes are lower outside cell, water is higherWater enters cell by osmosis
Cells in hypertonic solutionsSolutes are higher outside cell, water is lower
Water leaves cell by osmosis
Cytoplasm shrinks - “plasmolysis” - animal cells: shrivel - plant cells: low turgor pressure - cytoplasm pulls away from cell wall - but cell wall does not shrink
Osmosis in Animal Cells Animal cells like ISOTONIC conditions best
Osmosis in Plant Cells Plant cells like HYPOTONIC conditions best
Contractile Vacuoles
Fresh-water protists (like Paramecia or Amoeba) must constantly remove water that comes into the
cell by osmosis
ACTIVE TRANSPORT – uses cell energy
Two kinds:1. Molecule transport against the gradient
a. from low concentration to highb. pushed across by membrane proteins (pumps)
c. uses energy ATP d. small molecules and ions
Why would cells use active transport?1) To concentrate substances: Examples: kidneys concentrate wastes in urine, - intestine concentrates nutrients in blood2) To maintain an ion concentration
- sodium-potassium pump – allows nerve impulses
2) Bulk Transport – uses energy
To bring larger particles into or out of cellEndocytosis = brings material into cell - fold cell membrane around it form a vacuole a. Phagocytosis = “cell eating” - large particles or whole cells - examples: amoeba, white blood cells b. Pinocytosis = “cell drinking” - small folds of membrane take in liquids
Pinocytosis – “cell drinking”
Example: small intestine absorbs some water this way
Exocytosis – send OUT of cellVacuole containing substance fuses with cell membrane opens to outside of cell - for cell secretions - Examples: hormones from endocrine glands digestive juices from pancreas or intestine