How Cells Work Flashcards
Membranes are composed of
a bilayer of phospholipids with
embedded and attached proteins,
in a structure biologists call a fluid mosaic.
Many phospholipids
are made from unsaturated fatty acids that have kinks in their tails.
These kinks prevent phospholipids from packing tightly together, keeping them in liquid form.
In animal cell membranes, cholesterol helps
stabilize membranes at warmer temperatures and
keep the membrane fluid at lower temperatures
Membrane proteins perform many functions.
Some proteins help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix.
Some proteins function as receptors for chemical messengers from other cells.
Some membrane proteins function as enzymes.
Some membrane glycoproteins are involved in cell-cell recognition.
Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other.
Membranes may exhibit selective permeability, allowing some substances to cross more easily than others.
Phospholipids
the key ingredient of biological membranes, spontaneously self-assemble into simple membranes.
The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells.
Diffusion
is the tendency of particles to spread out evenly in an available space.
Particles move from an area of more concentrated particles to an area where they are less concentrated.
This means that particles diffuse down their concentration gradient.
Eventually, the particles reach equilibrium where the concentration of particles is the same throughout
passive transport
Diffusion is the tendency of particles to spread out evenly in an available space.
Particles move from an area of more concentrated particles to an area where they are less concentrated.
This means that particles diffuse down their concentration gradient.
Eventually, the particles reach equilibrium where the concentration of particles is the same throughout.
osmosis
One of the most important substances that crosses membranes is water.
The diffusion of water across a selectively permeable membrane is called osmosis
If a membrane permeable to water but not to a particular solute separates two solutions with different concentrations of this solute,
water will cross the membrane,
moving down its own concentration gradient,
until the solute concentration on both sides is equal.
Tonicity
Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water.
Tonicity mostly depends on the concentration of a solute on both sides of the membrane.
How will animal cells be affected when placed into solutions of various tonicities?
When an animal cell is placed into
an isotonic solution, the concentration of solute is the same on both sides of a membrane, and the cell volume will not change,
a hypotonic solution, the solute concentration is lower outside the cell, water molecules move into the cell, and the cell will expand and may burst, or
a hypertonic solution, the solute concentration is higher outside the cell, water molecules move out of the cell, and the cell will shrink.
osmoregulation
For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance.
The cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different
The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment.
But in a hypertonic environment, plant and animal cells both shrivel.
facilitated diffusion
Hydrophobic substances easily diffuse across a cell membrane.
However, polar or charged substances do not easily cross cell membranes and, instead, move across membranes with the help of specific transport proteins in a process called facilitated diffusion, which
does not require energy and
relies on the concentration gradient
Some proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules.
Other proteins bind their passenger, change shape, and release their passenger on the other side.
In both of these situations, the protein (transporter) is specific for the substrate, which can be sugars, amino acids, ions, and water.
aquaporin
Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow.
The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin.
In active transport, a cell
must expend energy to
move a solute against its concentration gradient.
The Sodium-Potassium ATPase pump
