Download Membrane Proteins

A Closer Look at Cell Membranes
Chapter 5
阮雪芬
10/2/2012
Impacts, Issues:
One Bad Transporter and Cystic Fibrosis
 Transporter proteins regulate the movement of
substances in and out of cells; failure of one of
these proteins causes cystic fibrosis
Video: One bad transporter and cystic
fibrosis
5.1 Organization of Cell Membranes
 The basic structure of all cell membranes is the
lipid bilayer with many embedded proteins
 A membrane is a continuous, selectively
permeable barrier (選擇性通透)
Revisiting the Lipid Bilayer
 Phospholipid molecules in the plasma
membrane have two parts
• Hydrophilic heads interact with water molecules
• Hydrophobic tails interact with each other,
forming a barrier to hydrophilic molecules
Cell Membrane Organization
The Fluid Mosaic Model
 Fluid mosaic model
• Describes the organization of cell membranes
• Phospholipids drift and move like a fluid
• The bilayer is a mosaic mixture of phospholipids,
steroids, proteins, and other molecules
Fluid Mosaic Model
Variations on the Model
 Differences in membrane composition(組成差異)
• Different kinds and numbers of carbohydrates
attached to membrane proteins
• Different kinds of phospholipids(通常至少有一個
脂肪酸尾部不飽和而彎曲)
 Differences in fluidity(流體性差異)
• Some proteins are attached to the cytoskeleton，
因此固定不遊走; others drift around
• Archaeans have more rigid membranes than
bacteria or eukaryotes(因為磷脂尾部間形成共價
鍵，使古代菌細胞膜異常堅固)
Two Studies of Membrane Structure
5.2 Membrane Proteins
 Cell membrane function begins with the many
proteins associated with the lipid bilayer
Membrane Proteins
 Each type of protein in a membrane has a
special function
•
•
•
•
•
Adhesion proteins
Recognition proteins
Receptor proteins
Enzymes
Transport proteins (active and passive)
Common Types of Membrane Proteins
Adhesion
Protein
Enzyme
Receptor Protein
Recognition
Protein
Passive
Transporter
Active
Transporter
Stepped Art
Fig. 5-5, pp. 80-81
5.1-5.2 Key Concepts:
Membrane Structure and Function
 Cell membranes have a lipid bilayer that is a
boundary between the outside environment and
the cell interior
 Diverse proteins embedded in the bilayer or
positioned at one of its surfaces carry out most
membrane functions
5.3 Diffusion,
Membranes, and Metabolism
 Ions and molecules tend to move from one
region to another, in response to gradients
Membrane Permeability
 Selective permeability
• The ability of a cell membrane to control which
substances and how much of them enter or leave
the cell
• Allows the cell to maintain a difference between
its internal environment and extracellular fluid
• Supplies the cell with nutrients, removes wastes,
and maintains volume and pH
The Selectively Permeable Nature
of Cell Membranes
Concentration Gradients
 Concentration
• The number of molecules (or ions) of substance
per unit volume of fluid
 Concentration gradient
• The difference in concentration between two
adjacent regions
• Molecules move from a region of higher
concentration to one of lower concentration
Diffusion
 Diffusion
• The net movement of molecules down a
concentration gradient
• Moves substances into, through, and out of cells
• A substance diffuses in a direction set by its own
concentration gradient, not by the gradients of
other solutes around it (只跟自種粒子的濃度梯度
差有關，與他種粒子無關)
Examples of Diffusion
The Rate of Diffusion
 Rate of diffusion depends on five factors
•
•
•
•
•
Size(小分子移動耗能小，故擴散快)
Temperature
Steepness of the concentration gradient
Charge
Pressure
How Substances Cross Membranes
 Gases and nonpolar molecules diffuse freely
across a lipid bilayer (課本說水也是但…無視)
 Ions and large polar molecules require other
mechanisms to cross the cell membrane
• Passive transport
• Active transport
• Endocytosis and exocytosis
Membrane-Crossing Mechanisms
Fig. 5-8 (a-c), p. 83
Fig. 5-8 (d-e), p. 83
5.4 Passive and Active Transport
 Many types of molecules and ions diffuse across
a lipid bilayer only with the help of a specific
transport protein
Passive Transport
 Passive transport (facilitated diffusion)
• Requires no energy input
• A passive transport protein allows a specific
solute (such as glucose) to follow its
concentration gradient across a membrane
• A gated passive transporter changes shape
when a specific molecule binds to it. An openchannel transporter doen not. (葡萄糖的是gated，
因此運輸過程形狀會改變)
Passive Transport
Passive Transport
Passive Transport
Active Transport
 Active transport
• Requires energy input (usually ATP)
• Moves a solute against its concentration gradient,
to the concentrated side of the membrane
 Calcium pumps
• Active transporters move calcium ions across
muscle cell membranes into the sarcoplasmic
reticulum
Active Transport: Calcium Pump
Active Transport: Calcium Pump
Cotransport

Cotransport: Sodium-Potassium Pump
5.3-5.4 Key Concepts:
Diffusion and Membrane Transport
 Gradients drive the directional movements of
substances across membranes
 Transport proteins work with or against
gradients to maintain water and solute
concentrations
5.5 Membrane Trafficking
 By processes of endocytosis and exocytosis,
vesicles help cells take in and expel particles
that are too big for transport proteins, as well as
substances in bulk
 Membrane trafficking
• Formation and movement of vesicles formed from
membranes, involving motor proteins and ATP
Exocytosis and Endocytosis
 Exocytosis
• The fusion of a vesicle with the cell membrane,
releasing its contents to the surroundings
 Endocytosis
• The formation of a vesicle from cell membrane,
enclosing materials near the cell surface and
bringing them into the cell
Endocytosis and
Exocytosis
Exocytic Vesicle
Three Pathways of Endocytosis
 Bulk-phase endocytosis
• Extracellular fluid is captured in a vesicle and
brought into the cell; the reverse of exocytosis
• 不具選擇性
 Receptor-mediated endocytosis
• Specific molecules bind to surface receptors,
which are then enclosed in an endocytic vesicle
• 具選擇性
 Phagocytosis
• Pseudopods engulf target particle and merge as
a vesicle, which fuses with a lysosome in the cell
• 其實也有用到receptor，具選擇性
Receptor-Mediated Endocytosis
Phagocytosis
Membrane Cycling
 Exocytosis and endocytosis continually replace
and withdraw patches of the plasma membrane
 New membrane proteins and lipids are made in
the ER, modified in Golgi bodies, and form
vesicles that fuse with plasma membrane
5.5 Key Concepts:
Membrane Trafficking
 Large packets of substances and engulfed cells
move across the plasma membrane by
processes of endocytosis and exocytosis
 Membrane lipids and proteins move to and from
the plasma membrane during these processes
5.6 Which Way Will Water Move?
 Water diffuses across cell membranes by
osmosis
 Osmosis is driven by tonicity, and is countered
by turgor
Osmosis
 Osmosis
• The movement of water down its concentration
gradient – through a selectively permeable
membrane from a region of lower solute
concentration to a region of higher solute
concentration
 Tonicity
• The relative concentrations of solutes in two fluids
separated by a selectively permeable membrane
Osmosis
Tonicity
 For two fluids separated by a semipermeable
membrane, the one with lower solute
concentration is hypotonic, and the one with
higher solute concentration is hypertonic
• Water diffuses from hypotonic to hypertonic
 Isotonic fluids have the same solute
concentration
Experiment:
Tonicity
Effects of Fluid Pressure
 Hydrostatic pressure (turgor) (膨壓T.P.)
• The pressure exerted by a volume of fluid against
a surrounding structure (membrane, tube, or cell
wall) which resists volume change
 Osmotic pressure (滲透壓O.P.)
• The amount of hydrostatic pressure that can stop
water from diffusing into cytoplasmic fluid or other
hypertonic solutions
Hydrostatic Pressure in Plants
5.6 Key Concepts:
Osmosis
 Water tends to diffuse across selectively
permeable membranes, to regions where its
concentration is lower