AP Biology: 2.B.1 – 2.B.3 Cell
Membranes
All cells have a cell
membrane. These membranes are all semipermeable, which means only certain
particles can pass through. A cell needs to be able to move things in and out
of the membrane so that it can survive, as all living things need energy and
nutrients. They serve the purpose of separating the inside of the cell from the
outside as well.
These cell membranes are made
of two layers of what is known as a phospholipid. These phospholipids are
amphipathic because they contain a hydrophilic head, which likes water due to
its polarity, and a fatty acid “tail” that’s nonpolar and detests water. The
heads line both the outer facing part of the membrane and the inner facing
membrane, with the tails facing one another.
Phospholipids also move
around. Once a month the outer phospholipids switch places with the inner
facing ones and are almost constantly moving around with other phospholipids
around them laterally.
In order to move larger and
polar substances into and out of the cell, protein channels are needed.
There are three types of
protein channels: Integral membrane, trans-membrane and peripheral proteins.
Integral membrane proteins
are proteins that are only in one half of the phospholipid bilayer.
Trans-membrane proteins go
all the way through and are key to the transport of large substances from one
side to the other. A good example of this is the sodium potassium pump.
Peripheral proteins are bound
to the surface of the bilayer.
Membrane proteins, oddly
enough, have hydrophilic and hydrophobic like phospholipids. Likely to stay in
the membrane and perform their tasks to the best of their abilities. They
function to transport things, produce and contain enzymatic activity, signal
transduction, cell-to-cell identification, intercellular joining and forming
attachments to the cytoskeleton & cellular matrix.
In addition to proteins,
there are also other things attached to the phospholipid bilayer, such as
membrane carbohydrates, glycolipids (which send and receive signals from other
cells) and glycoproteins (also are important with interacting with other cells.
They are very diverse).
Cholesterol, contrary to
“common belief” are absolutely necessary to the functioning of cells (and
bodily processes). These special molecules are integrated in the membrane and
prevent it from becoming overly fluid or overly firm.
Transport proteins, which are
often trans-membrane, serve the purpose of transporting substances in and out
of the cell. They have a hydrophilic channel that goes through it vertically
and connects the extracellular fluid and the cytoplasm. Certain molecules like
sugars can use this.
Although all cells have a
plasma membrane, not all have what is known as a cell wall. Only plants, fungi
and bacteria have this rigid structure that in addition to providing
protection, prevents the cell from absorbing too much water and bursting. Cell
walls are made up of a substance called cellulose. In fact, one of a few ways
scientists have discovered how to kill harmful bacteria is by destroying the
cell.
Prokaryotes have a cell wall
made up of a substance called peptidoglycan and fungi have cell walls made of
chitin, which is also the substance that makes the exoskeleton of arthropods.
Osmosis is the process of
water moving across a semipermeable membrane. As water is a polar molecule, it
can’t freely pass through the phospholipid bilayer without assistance. A
special protein called an aquaporin forms a protein channel in the membrane
that allows water to diffuse (move from a point of high concentration to low
concentration) into and out of the cell. This is an example of facilitated diffusion,
which occurs when a protein is required to move a certain substance across a
membrane.
When water inside a cell has
a higher concentration of water inside it than outside, the cell is hypotonic.
When that scenario is reversed, then it is hypertonic. When dynamic equilibrium
is achieved and the concentration inside and out is equal to the point of there
being no significant travel inside or outside by water, both the internal and
external cell are isotonic.
There are two ways molecules
move: through passive transport which requires no energy and active transport,
which requires energy (ATP) to move molecules from areas of low concentration
to high concentration. It is used whenever molecules need to be transported
against their concentration gradient.
Sometimes groups of ions move
through the plasma membrane through endocytosis and exocytosis in what is
called bulk transport. In exocytosis it is often seen in organelles called
vesicles that fuse with the lipid bilayer to remove wastes from the cell.
Phagocytosis is when the vesicles take in large substances from outside the
cell. Pinocytosis is when water and dissolved particles are taken in.
Like earlier stated, water
prefers to move from areas of higher water potential to low lower water
potential. Water potential is defined by the sum of the solute concentration
and water pressure.
To understand this equation, one
must first know the basics:
Yp = the pressure on the water. In an open container,
it is 0. In a turgid plant cell, it is greater than zero.
Ys = - iCRT
i = ionization constant (for
sucrose this is 1.0, for NaCl this is 2.0)
C = molar concentration
Molarity: Moles Solute/Volume of
Solution. M = Moles/L
R = pressure constant (R =
.0832 liter bars/mole K)
T = temperature in Kelvin
(273 + degrees Celsius)
Also, in order for eukaryotic
cells to function at best despite their large sizes, they have evolved to
contain many internal membranes (called organelles) that carry out specific
cell processes. They compartmentalize to efficiently carry out cellular
processes that need different environments to work. The digestion that occurs
in lysosomes requires an acidic environment, for example.
Internal membranes also
increase the surface area, which allows more reactions to occur because there
are more membrane bound organelles. The larger the surface area to volume ratio
a cell has, the easier it is to transport things in and out, and to the
organelles inside.
There are several membrane-bound
organelles:
The Rough Endoplasmic
Reticulum modifies secretory proteins that are then synthesized by ribosomes
and moved to the lumen of the Rough E.R.
The smooth endoplasmic
reticulum synthesizes lipids, phospholipids and steroids in addition to
carrying out cell metabolism and detoxifying drugs like alcohol.
The Golgi Apparatus, which is
known as the post office of the cell, packages and sorts proteins for
secretion. These packaged proteins are then sent to the cisface and are
modified as they move through the organelle’s cisternae (the inside of the
Golgi’s main body). They are then sent in vesicles to the cell’s membrane.
The nuclear membrane is a
bilayer with the same lumen as the E.R. It has many ribosomes imbedded into it and
houses the nucleus, nucleolus and DNA of the cell.
The vacuoles in plant cells
contain large amounts of water in addition to storing things. The vacuoles of
animal cells contain food particles and can merge with lysosomes to digest it.
In Bacteria and Archaea lack
inner membranes because they’re usually too small for that, but their cell
processes and enzymes are confined to different parts of the cell.
Citations:
http://upload.wikimedia.org/wikipedia/commons/thumb/7/76/Osmotic_pressure_on_blood_cells_diagram.svg/1280px-Osmotic_pressure_on_blood_cells_diagram.svg.png
