Cell membrane
•Cells are highly dynamic and integrated

Cytoskeleton:
•Organize structure
•Activity in cell

•a network of protein fibers
•highly dynamic

supports and maintain cell shape

transport material

cell movement

microtubules (cell division in metaphrase)
< intermediate filaments
< microfilaments

not total random movement

Graph

Phospholipid bilayer:
highly dynamic

• Form cell’s boundary and intracellular compartments
• Provide a place of communication and transport between compartments
• Cellular membranes are fluid mosaics of lipids and proteins

Integral protein can absorb water faster in kidney cells rather than diffusion(too slow)

Integral protein - larger

Membrane fluidity - Factors
1) Saturated HC tails is well packed - more fluid
2) Cholesterol within cell membrane - black ramdom movement
3) Protein avoids movement

Peripheral protein -smaller

a) Transport


b) Enzymatic activity


c) Signal transduction


d) cell-cell recognition


e) intercellular joining


f) Atachement to the cytoskeleton and extracellular matrix

Selective permeability:
O2, CO2,N2(non-polar) >
H2O, glycerol(small uncharged polar) >
Glucose, sucrose(large uncharged polar) >
Cl- , K+, Na+(Ions)
(can use specialize protein to pass trough)

Mechanism of membrane transport

Passive transport
no energy investment

Active transport
use energy to move solutes against their gradients

allows cells to maintain concentration gradients that differ from their surroundings
e.g. Na+/K+ - ATPase in animal cell

Diffusion
• Substances diffuse down their concentration gradient
• At dynamic equilibrium, molecules cross the membrane in one direction as in the other

Facilitated diffusion
• Speeds the transport of a solute by providing efficient passage
• Channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
• Carrier proteins, bind to molecules and change shape to
shuttle them across the membrane

protein change its shape and gate is opened, release all Na+ ion

ATP hydrolysis
ATP => ADP + P
(graph)

Na+ / K+ ATPase
• Transport of 3 Na+ out of the cell and 2 K+ into the cell creates membrane potential o outside the membrane: more positive o Inside the cell: more negative
• When ions build up on one side of a plasma membrane, they establish both a concentration gradient and a charge gradient, collectively called the electrochemical gradient.
• Electrochemical gradients store energy for cellular work
e.g. drives movement of ions involved in nerve impulse transmission

Toxin and ion channels

Scorpion venom
interfere Na+-, K+- and Ca2+- channels of excitable cells, causing muscle paralysis麻痺(no breath) and death.

Cholera Toxin(霍亂菌)
activate the cystic fibrosis transmembrane conductance regulator (CFTR), causing a dramatic efflux(流出) of Cl- ions and water
from infected intestinal cells, leading to diarrhoea(腹瀉)

Botox: muscle contraction

Bulk Transport - require energy
Large molecules(polysaccharides/proteins) cross the membrane in bulk via vesicles

Exocytosis - transport vesicles migrate to the membrane, fuse with it, and release their contents

Endocytosis - cell takes in macromolecules by forming vesicles from the plasma membrane

Recetpor-medicated endocytosis
Binding of ligands to receptors triggers coated vesicles formation

Phagocytosis
A cell engulfs a particle(food/bacteria) in a vacuole (pseudopodium) which will hen fuse with a lysosome for digestion

Pinocytosis
Molecules are taken up when extracellular fluid is "gulped"大口地吃 into tiny vesicles