MEMBRANE STRUCTURE AND FUNCTION
• 2.4.2 Membranes organize the chemical activities of cells
– Membranes provide structural order for metabolism
• Form most of the cell's organelles
• Compartmentalize chemical reactions
– The plasma membrane forms a boundary between a living cell and its surroundings
• Exhibits selective permeability
• Controls traffic of molecules in and out
LE 5-10
Outside
of cell
Cytoplasm
LE 5-11a Hydrophilic head
Phosphate
group
Symbol
Hydrophobic tails
Membrane phospholipids form a bilayer
Phospholipids are the main structural components of membranes
Two nonpolar hydrophobic fatty acid "tails" One phosphate group attached to the hydrophilic glycerol "head"
Hydrophilic
heads
Hydrophobic
tails
Water
Water
In membranes, phospholipids form a bilayer Two-layer sheet Phospholipid heads facing outward and tails facing inward Selectively permeable
Polar lipid-soluble molecules pass through Nonpolar molecules not soluble in lipids do not pass through
• 2.4.3 The membrane is a fluid mosaic of phospholipids and proteins
– A membrane is a mosaic
• Proteins and other molecules are embedded in a framework of phospholipids
– A membrane is fluid
• Most protein and phospholipid molecules can move laterally
– Membrane glycoproteins and glycolipids function in cell identification
LE 5-12
Extracellular
matrix Glycoprotein
Carbohydrate
Plasma
membrane
Microfilaments
of cytoskeleton
Phospholipid
Cholesterol
Proteins
Cytoplasm
Glycolipid
1.3A.1 Cholesterol in mammalian membranes helps prevent extremes. Reducing fluidity, but also preventing fatty acid chain crystalizatioon
• 2.4.3 Proteins perform most membrane functions
• Transporters of substances across the membrane
• Receptors of chemical messages from other cells (signal transduction)
• Anchorage
• Cell recognition
• Intercellular junctions between adjacent cells
• Enzymes
LE 5-13a
Enzyme activity
LE 5-13b
Messenger molecule
Receptor
Activated
molecule
Signal transduction
LE 5-13c
Transport
ATP
• 2.4.4 Passive transport is diffusion across a membrane
– Diffusion is the tendency for particles to spread out evenly in an available space
• From an area of high concentration to an area of low concentration
– Passive transport across membranes occurs when a molecule diffuses down a concentration gradient
– Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane
LE 5-14a
Molecules of dye Membrane Equilibrium
LE 5-14b
Equilibrium
• 2.4.5 Transport proteins may facilitate diffusion across membranes
– In facilitated diffusion
• Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient
– To transport the substance, a transport protein may
• Provide a pore for passage
• Bind the substance, change shape, and then release the substance
LE 5-15
Solute
molecule
Transport
protein
• 2.4.6 Cells expend energy for active transport
– Active transport requires energy to move solutes against a concentration gradient
• ATP supplies the energy
• Transport proteins move solute molecules across the membrane
Transport
protein
Solute
ATP P
ADP
Protein
changes shape
P
Solute binding Phosphorylation Transport Protein reversion
Phosphate
detaches
P
2.4.7 Cells transport materials in vesicles
• Proteins made in ER are sent to the Golgi via vesicles
• Vesicles go from the Golgi to the plasma membrane
• 2.4.8 Exocytosis and endocytosis transport large molecules
– To move large molecules or particles through a cell membrane
• A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis)
• Membranes may fold inward, enclosing material from the outside (endocytosis)
LE 5-19a
Vesicle
Fluid outside cell
Protein
Cytoplasm
LE 5-19b
Vesicle forming
– Endocytosis can occur in three ways
• Phagocytosis ("cell eating")
–Surrounding food and engulfing it
• Pinocytosis ("cell drinking")
–Absorbing fluid into tiny vesicles
• Receptor-mediated endocytosis
–Like pinocytosis, but very specific
LE 5-19c
Pseudopodium of amoeba
Phagocytosis
Plasma membrane
Food being ingested
Material bound to receptor proteins
PIT
Cytoplasm
Receptor-mediated endocytosis Pinocytosis
• Osmosis is the diffusion of water across a membrane
– In osmosis water, molecules diffuse across a selectively permeable membrane
• From an area of low solute concentration
• To an area of high solute concentration
• Until the solution is equally concentrated on both sides of the membrane
– The direction of movement is determined by the difference in total solute concentration
• Not by the nature of the solutes
LE 5-16
Water
molecule
Selectively
permeable
membrane
Solute
molecule
H2O
Lower
concentration
of solute
Higher
concentration
of solute
Equal
concentration
of solute
Solute molecule with
cluster of water molecules
Net flow of water
• Water balance between cells and their surroundings is crucial to organisms
– Osmoregulation is the control of water balance
– Tonicity is the tendency of a cell to lose or gain water in solution
• Isotonic solution: solute concentration is the same in the cell and in the solution – No osmosis occurs
– Animal cell volume remains constant; plant cell becomes flaccid
• Hypotonic solution: solute concentration is greater in the cell than in the solution – Cell gains water through osmosis
– Animal cell lyses; plant cell becomes turgid
• Hypertonic solution: solute concentration is lower in the cell than in the solution – Cell loses water through osmosis
– Animal cell shrivels; plant cell plasmolyzes
LE 5-17
Isotonic solution Hypotonic solution Hypertonic solution
H2O H2O
(1) Normal (2) Lysed
H2O
H2O H2O H2O
Animal
cell
Plant
cell
(4) Flaccid (5) Turgid (6) Shriveled
(plasmolyzed)
(3) Shriveled
Plasma
membrane
H2O
H2O
The difference between passive and active transport is:
• A: Passive doesn’t take energy
• B: Active transport is for larger molecules
• C: Passive transport aloows things to come in and out of the cell
• D: All of the above are true
Which of the following is not a form of active transport?
• A: Exocytosis
• B: Endocytosis
• C: Facilitated diffusion
• D: Protein pumps
• Which of the following represents a picture of a cell placed in an environment that has more solutes outside of the cell?
A B C