Download The cell membrane

Lecture 5
The Cell
The cell membrane
and
Membrane Proteins
The cell
Ameoba-­‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ The cell
chromosomes
ribosome
Plasma
membrane
Ameoba-­‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ The cell
chromosomes
ribosome
Cytoplasm
Plasma
membrane
Ameoba-­‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ 2 Main cell types: Prokaryotic and Eukaryotic
3.5 billion yrs ago
1.5 billion yrs ago
No organelles
Only ribosomes
Thick cell wall
made of peptidoglycan
Genetic material floats free
Eukaryotic cells are ~ ten times larger than prokaryotic cells.
No cell wall (animals)
Or
Cell wall made from
cellulose (plants)
eukaryotic cell
An organelle is a specialized part of a cell Analogous to the organs of mul4cellular animals. All eukaryotic cells have a number of features in common.
These include: nucleus
cytoplasm
cell membrane
mitochondria
endoplasmic reticulum
Golgi apparatus
ribosomes
cytoskeleton
centriole
eukaryotic cell
The three main parts of every eukaryo4c cell are: • The cell membrane (its outside or coa4ng) • The nucleus (its center or brain) • The cytoplasm (everything in between), which is also called cytosol The cell membrane: -­‐ keeps the cell together. Separates living cell from non-­‐living surrounds -­‐  Is not solid. It has 4ny openings that let things in and out. -­‐ exhibits selec4ve permeabilty = allowing some substances to cross more readily than others This is very important for life – allows nutrients in -­‐ allows waste out Macrophage
Macrophage
T-cells
Nerve cells
Cellular membranes
•  Cell membranes are
fluid mosaics of lipids
and proteins
•  Cell membrane is made
up of a phospholipid
bilayer
Phospho-lipids
Amphipathic
molecules
Plasma membrane is made up of a phospholipid bilayer Creates a stable boundary between 2 aqueous compartments
Discovered in the 1900’s
Model of the cell membrane
A plasma membrane will adhere to water
Two schools of thought on how
1930’s
1960’s
Model of the cell membrane
1. not all membranes have the same % of proteins associated with them
2. membrane proteins are not very soluble in water
Singer and Nicholson
Cell membrane
Mosaic of proteins
• Lipids are always moving sideways • Proteins driL slowly • Cholesterol act as a temperature buffer keeping the membrane fluid at moderate temperatures • Membranes must be fluid to work Membrane proteins
• 
• 
• 
• 
Phospholipids- fabric of the cell membrane
Proteins –determine membranes specific functions
Different types of cells-different types of proteins
Different organelles within a cell-different proteins
Membrane proteins
•  2 types of membrane protein1.  Integral proteins penetrate the hydrophobic core, many are “transmembrane” 2. Peripheral proteins are not embedded in the membrane at all, they are oLen bound to exposed integral proteins or loosely to the surface of the membrane Membrane proteins Transmembrane protein
eg Bacteriorhodopsin
Transmembrane,
hydrophobic
regions are
usually alpha
helical
6 major Membrane protein func4ons Hydrophilic channel or Carrier protein Tight junc4ons formed between cells Team of enzymes Glycoprotein-­‐ ID tag Stablises loca4on of certain proteins Transport across the membrane 3 modes of transport:
Transport across a membrane
•  Regulation of transport across the cell
membrane-essential its to existence
Eg. Muscle cells Sugars, amino acids, oxygen, ions in out Carbon dioxide, ions, metabolic waste •  Cell membrane is selectively permeable
Diffusion-passive transport
•  What dictates the direction of transport?
•  Diffusion- tendency for molecules of any substance to
spread evenly into available space
•  Any substance will diffuse down its concentration
gradient- spontaneous process – no energy required
•  Small hydrophobic molecules will diffuse across the
membrane
Eg. Oxygen crosses into cells performing cellular respira4on Selective Permeability
•  Membrane proteins – key role in regulating transport
}
Need protein
transporters
Selective permeability
•  Charged and polar molecules can cross the
membrane by passing through transport proteins
•  Transport proteins span the membrane
•  Transport proteins are very specific
•  2 types- channel and carrier
Channel protein eg. aquaporin Carrier protein eg glucose Facilitated Diffusion-Passive Transport
•  Diffusion of hydrophilic
solutes across the
membrane must be
facilitated by transport
proteins
High conc. of solute
•  Transport proteins are
very specific
•  Channel proteins
provide hydrophilic
corridors
Eg.s
•  Ion channels•  Gated ion channelsstimulus regulated
channel transporter: eg. Aquaporin for water or Sodium channel for Na+ Low conc. of solute
Na+ Na+
Na+ Na+
Na+
Ligand gated ion channel eg. neurotransmi<ers Facilitated Diffusion-Passive Transport
•  Carrier proteinschange shape in
order to translocate
substances across
the membrane
•  Changes in shape –
triggered by the
binding and release of
the transported
molecule
Carrier protein eg. Glucose transporter Facilitated Diffusion is s@ll passive because the solute moves down the concentra@on gradient Active transport-uses energy
•  Movement of substances against their concentration
gradient across the membrane
•  From a low concentration to a high concentration
•  Requires work- cell uses energy- active
•  All active transporters are carrier proteins
•  Active transport enables a cell to maintain higher internal
concentrations of molecule compared to the external
environment
Active transport
•  ATP supplies the energy for active transport
•  ATP can transfer its terminal phosphate group
directly to the transport protein:
transfer of energy
phosphorylation
•  Eg of a carrier protein that requires ATP to
transport is the sodium-potassium pump
Adenosine triphosphate (ATP)
is the cells energy molecule Sodium-potassium protein pump
High in Na+
High in K+
Sodium and Potassium are being pumped against their concentration gradient
Sodium is already higher outside the cell and potassium is already higher inside the cell
Sodium-potassium protein pump
Sodium-­‐potassium pump = electrogenic pump = Generates a voltage across the membrane -­‐More nega@vely charged inside a cell than outside -­‐creates an “electrochemical” gradient Membrane potential
•  Voltage across the membrane =
“membrane potential”
-storage of electrical potential
energy
•  This potential energy can be
tapped into by the cell to carry
out work, instead of using ATP
•  eg.:
the diffusion of H+ ions back
down their electrochemical
gradient can be coupled to the
active transport of Sucrose
against its concentration
gradient
In Summary...Transport across the membrane Biology
Campbell and Reece
•  Chapters 5, 7 and 8
http://www.tcd.ie/Biology_Teaching_Centre/local/junior-freshman/