Membranes
passive and active transport animation
Membrane Structure and Function
The Plasma Membrane
--the fluid mosaic model (S.J Singer)
-- semi-permeable
--fluid portion is a double layer of phospholipids, called the phospholipid bilayer
Jobs of the cell membrane
Isolate the cytoplasm from the external environment
Regulate the exchange of substances
Communicate with other cells
Identification
The Plasma Membrane is also called the Phospholipid bilayer
Phospholipids contain a hydrophilic head and a nonpolar hydrophobic tail
Hydrogen bonds form between the phospholipid "heads" and the watery environment inside and outside of the cell
Hydrophobic interactions force the "tails" to face inward
Phospholipids are not bonded to each other, which makes the double layer fluid
Cholesterol embedded in the membrane makes it stronger and less fluid
Proteins embedded in membrane serve different functions
1. Channel Proteins - form small openings for molecules to difuse through
2. Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell
3. Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins)
4. Cell Recognition Proteins - ID tags, to idenitfy cells to the body's immune system
5. Enzymatic Proteins - carry out metabolic reactions
Transport Across Membrane
The membrane is differentiallty permeable (also called semipermeable ) - which means
Passive Transport
Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas of low concentration, down a concentration gradient
Facilitation Diffusion - diffusion that is assisted by proteins (channel or carrier proteins)
Osmosis -
-diffusion of water. Salt Sucks
Osmosis affects the turgidity of cells, different solution can affect the cells internal water amounts
Contractiles Vacuoles are found in freshwater microorganisms - they pump out excess water
Turgor pressure occurs in plants cells as their central vacuoles fill with water.
Active Transport
- involves moving molecules "uphill" against the concentration gradient, which requires energy
Endocytosis - taking substances into the cell (pinocytosis for water, phagocytosis for solids)
Exocytosis - pushing substances out of the cell, such as the removal of waste
Sodium-Potassium Pump - pumps out 3 sodiums for ever 2 potassium's taken in against gradient
Demo - Starch in the baggie, iodine in the beaker. What happens and why?
Observation of elodea cells in salt water. What happens and why?
Chapter 7 Guided Reading Assignment
What does selective permeability mean and why is that important to cells?
What is an amphipathic molecule?
What were the ideas concerning the plasma membrane models below:
a. Gorter and Grendel
b. Davson and Danielli
c. Singer and Nicolson
Describe the freeze fracture technique and why is it useful in cell biology.
How is the fluidity of cell membrane’s maintained?
Label the diagram below – for each structure – briefly list it’s function:
List the six broad functions of membrane proteins.
How do glycolipids and glycoproteins help in cell to cell recognition?
Why is membrane sidedness an important concept in cell biology?
How has our understanding of membrane permeability changed since the discovery of aquaporins?
What is diffusion and how does a concentration gradient relate to passive transport?
Why is free water concentration the “driving” force in osmosis?
Why is water balance different for cells that have walls as compared to cells without walls?
Label the diagram below:
What is the relationship between ion channels, gated channels and facilitated diffusion – write 1 -2 sentences using those terms correctly.
How is ATP specifically used in active transport?
Define and contrast the following terms: membrane potential, electrochemical gradient, electrogenic pump and proton pump.
What is cotransport and why is an advantage in living systems?
What is a ligand?
Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated endocytosis.