Transcript Cell Membranes

Cell Membranes
Recall that the smallest unit of life is the cell.
All cells are surrounded by a plasma membrane that
is vital in various aspects of the cell’s biology.
General functions:
1. Selective isolation of cell’s contents from the external environment.
2. Regulation of material that moves in and out of the cell.
3. Communication with other cells.
Cell Membranes
membrane structure
Cellular membranes are composed of phospholipids that have
hydrophilic heads and hydrophobic tails.
How do you think these phospholipids
would line up in a water environment?
And Why?
They form a bilayer.
Note how the hydrophobic tails are all
Lined up together towards the inside,
Whereas the hydrophilic heads are lined
Up towards the outside watery environment.
Most biological molecules are water soluble
(i.e. polar) and can not pass this membrane.
The cell has effectively isolated the inside from
the outside!!
Cell Membranes
membrane structure
The cellular membrane (or plasma membrane) is more complex than that…
Membranes contain varying amounts of cholesterol… They regulate the fluidity and
flexibility of the membrane… (remember that the membrane is “fluid”)
There are also a lot of proteins and glycoproteins (proteins with sugar moieties) throughout
the membrane. We will discuss their function separately.
Cell Membranes
membrane proteins
Membrane protein types and functions:
1. Transport proteins - these proteins are involved in regulating transport of
various substances in and out of the cell.
2. Receptor proteins – these proteins recognize “signals” or “messages” from
their environment and can induce biological responses within the cells.
3. Recognition proteins - these proteins allow the cell to be “recognized” by
other cells. In multicellular organisms, these proteins are involved
in self recognition.
Recognition proteins are often glycoproteins.
Cell Membranes
Transport or material across plasma membranes
1. Simple diffusion:
The net movement of molecules down a concentration gradient. The greater
the gradient, the faster the movement (until the gradient is eliminated).
Note that diffusion is a very slow process.
Simple diffusion is involved in transport of water, dissolved gases, and small,
Lipid soluble substances (why do you think that is?)
Cell Membranes
Transport or material across plasma membranes
2. Facilitated diffusion:
Diffusion of molecules through a channel or carrier protein. Why would
you think there would be a need for “channels” or a “carrier” proteins?
Cell Membranes
Transport or material across plasma membranes
2. Facilitated diffusion: (cont.)
Diffusion of molecules through a channel or carrier protein. Why would
you think there would be a need for “channels” or a “carrier” proteins?
Cell Membranes
Transport or material across plasma membranes
3. Osmosis:
Diffusion of water across a differentially permeable membrane - this means
a membrane that allows water to pass through freely, but not various
dissolved molecules. (e. g. human blood cells)
Cell Membranes
Transport or material across plasma membranes
Vocabulary review for osmosis:
Isotonic solution - concentrations of solutes is same inside and outside
the membrane. Which direction do you think water
will flow?
Hypertonic solution – concentration of solutes is higher outside the membrane.
Which direction will water flow?
Hypotonic solution – concentration of solutes is lower outside the membrane.
Which direction will water flow?
Cell Membranes
Transport or material across plasma membranes
In case of simple diffusion, facilitated diffusion, and osmosis, substances were moving
from areas of high concentration to low concentration. There was no need for energy
(i.e. transport was passive).
Cells however sometimes need to transfer molecules from areas of low concentration
to areas of higher concentration. This processes are energy-requiring transport
systems (i.e. active transport).
Cell Membranes
Transport or material across plasma membranes
Active transport – uses ATP as a source of energy to transfer material (in this example
Calcium ions) up a concentration gradient. In this process, the ATP is broken
down to ADP.
Cell Membranes
Transport or material across plasma membranes
Cells can also use energy to actively engulf extracellular fluid or small particles. This
process is referred to as endocytosis, and there are three general forms:
Cell Membranes
Transport or material across plasma membranes
Review of three types of endocytosis:
Pinocytosis – pinching off of a small amount of the fluid around the cell.
Receptor-mediated endocytosis – this is the process where the cell selectively
bind to and engulf material found outside the cell.
Phagocytosis – process by which cells pick up large particles, including
whole microorganisms.
Cell Membranes
Transport or material across plasma membranes
Review of different forms of transport:
Cell Membranes
Transport or material across plasma membranes
Note that both active and passive transport can be for material being transferred both
into as well as out of the cells. In case of endocytotic processes, the release of material
out of the cell is referred to as exocytosis.
Cell Membranes
In case of plants, cells secrete one or two additional primarily cellulose cell walls around
their plasma membrane.
Cell Structure and Function
Next lecture…