Download MADANIA (High School) Grade 10-Biology

MADANIA (High School)
Grade 10-Biology - Handout 3
Transport through Membrane
22/10/2010
Membranes are a fluid mosaic of phospholipids and proteins
 Plasma membrane forms a
boundary between the living
cell and its surroundings and
controls the traffic of materials
into and out of the cell.
 Phospholipids are the main
components
of
biological
membranes. A phospholipid
has two different regions: a
hydrophilic phosphate group (head) and two non polar hydrophobic fatty acid tails. Phospholipids form
a two-layer sheet called a phospholipid bilayer.
 The proteins are embedded in a framework of phospholipids to help move molecules in and out of the
cell.
 In animal cells the steroid cholesterol helps stabilize the membrane at warm temperatures and also
keep the membrane fluid at lower temperatures.
 Carbohydrates act as cell identification tag.
A cell membrane exhibits selective permeable, which is, allows a certain molecules to pass through and
prevents other molecules from crossing. There are several ways by which this is done. This process can be
classified as passive and active transport.
A. Passive Transport
Passive transport is the movement of a substance from a region of higher concentration to a region of
lower concentration across a cell membrane without the input of
energy from the cell via respiration.
Diffusion
Diffusion is the most common form of passive transport. Diffusion
is the movement of molecules from a region where they are highly
concentrated to a region where they are less concentrated, down a
concentration gradient (the difference in concentration between
Passive transport of one type of molecule
two regions). When the molecules have been dispersed evenly,
there is no concentration gradient and a state of equilibrium has
been reached.
Example of diffusion: In our lungs, oxygen enters red blood cells
and carbon dioxide passes out of them. Both oxygen and carbon
dioxide are small, non polar molecules that diffuse easily through
Passive transport of two types of molecules
the phospholipid bilayer of a membrane.
S1/W13/Handout 3/G.10/DIN/Biology/2010-2011
Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane. Osmosis will occur in response
to the concentration of solutes dissolved in water. A solution with higher
water potential has a lower concentration of solute than the
concentration of solutes in a cell (hypotonic). In a solution with lower
water potential, the concentration of solutes is higher than that inside the
cell (hypertonic). A zero water potential solution has the same
concentration of solute as that of the cell (isotonic).
What is water potential?
The water potential of a solution is a measure of whether it is likely to
lose or gain “free” water molecules from another solution. A “free” water
molecule is one which is not bonded to other substances and is available
for osmosis.
Water balance between cells and their surroundings is crucial to
organisms.
A
B
C
Part A. The solute concentration of a cell and its
isotonic environment are essentially equal, and
the cell gains water at the same rate that it loses
it.
Part B. In hypotonic solution the concentration
of solute is lower than that of the cell causing an
animal cell to swell or burst (lyse). Though, for
plant, cell wall prevents the cell to burst. The
plant cell becomes turgid (very firm), which is
the healthy state for most plant cells.
Part C. In hypertonic solution the concentration of solute is higher than that of the cell causing an animal
and a plant cell to shrivel due to the water loss.
Facilitated Diffusion
There are some substances do not diffuse freely across a membrane
because of their polarity or charge. Therefore, they need specific
transport proteins to help them. Facilitated diffusion is a process in
which protein makes certain substances to move down its
concentration gradient.
Substances that use facilitated diffusion for crossing cell membranes
include a number of sugars, amino acid, ions and even water
Transport
protein
S1/W13/Handout 3/G.10/DIN/Biology/2010-2011
Solute molecule
B. Active Transport
Active transport is the movement of substances from a
region of lower concentration to a region of higher
concentration against the concentration gradient. It needs
energy from cell respiration. Many ions need to be moved
into or out of cells faster than the diffusion alone can
permit. Such substances can undergo active transport.
Example: the inside of an animal cell has a higher
concentration of potassium ions (K+) and a lower
concentration of sodium ions (Na+) than the solution
outside the cell. A transport protein called the sodium
potassium pump helps cell maintain these steep gradients
by shuttling Na+ and K+ across the membrane against their
concentration gradient.
C. Endocytosis and Exocytosis
Endocytosis is the process by which cells absorb large molecules (such as
proteins) from outside the cell by engulfing them with their plasma
membrane. It is used by all cells of the body because most substances
important to them are large polar molecules that cannot pass through the
hydrophobic plasma or cell membrane. The process opposite to
endocytosis is exocytosis. There are three kinds of
endocytosis, which are:
Phagocytosis - A cell engulfs a solid particle by
wrapping extensions called pseudopodia around it
and packaging it within a membrane-enclosed sac
large enough to be called a vacuole. The classic
example is an amoeba eating a bacterium.
Pinocytosis – The cell gulps droplets of fluid into
tony vesicles. It takes in any and all solutes
dissolved in the droplets.
Receptor-mediated endocytosis - Receptor-mediated endocytosis selectively internalizes specific
molecules that are found in low concentrations in the extracellular space, such as hormones , growth
factors, antibodies, iron, enzymes, vitamins, and cholesterol. The plasma membrane has indented to form a
coated pit then it pinches closed to form a vesicle that carries the molecules into the cytoplasm.
Exocytosis is the process in which intracellular vesicles in the cytoplasm
fuse with the plasma membrane and release or "secrete" their contents
into the extracellular space.
All pictures are taken from Campbell, Neil A. 2004. Biology, Concepts and Connections. Chapter 5. The Working Cell.
Pearson Education, Inc.
Other resources:
www.wikipedia.com
www.biologyreference.com/Dn-Ep/Endocytosis.html
Lum How Kee.2000.Biology the Living Science.Pearson Education South Asia Pte Ltd.
S1/W13/Handout 3/G.10/DIN/Biology/2010-2011