UNIT 1: Biochemistry
Chapter 2: Cell Structure and Function
pg. 70 - 107
2.4: Transport across Membranes
pg. 89 – 97
Characteristics of life state that living things must take in nutrients and
remove wastes, needed for metabolic reactions and the products.
The exchange of substances between the external and internal environments
of a cell is a complex process and highly selective.
Trans-membrane exchange does not just occur in the cell membrane but also
with all membrane bound organelles. Reactants must be able to enter the
organelles so metabolic reactions can occur and their products must be
removed.
Passive Membrane Transport
Passive transport – is the movement of a substance across a membrane
without expending energy.
Dynamic equilibrium – is the state in which continuous action results in
balanced conditions.
This passive motion occurs because molecules are always in constant motion,
and in a closed environment become uniformly distributed.
Diffusion is the primary mechanism of solute movement within a cell and
between cellular compartments separated by a membrane.
If molecules are more concentrated on one side of a membrane, these
molecules will eventually travel across the membrane creating a dynamic
equilibrium.
The rate of diffusion is controlled by the concentration gradient that exists
between the two sides or across the membrane.
The larger the concentration gradient the faster the rate of diffusion occurs.
Membranes are selectively permeable, which means that some substance can
pass through freely while others can not or require assistance. The size and
charge of the molecules determines whether or not a molecule will pass
across the membrane.
Figure 1: Diffusion is the process in which molecules move from a
region of higher concentration to a region of lower concentration, pg. 89
Simple Diffusion
Simple diffusion – is the ability of a small and non-polar substance to move
across a membrane unassisted.
Facilitated Diffusion
Facilitated diffusion – is the facilitate transport of ions and polar molecules
through a membrane via protein complexes.
Transport protein – is an integral membrane protein that provides a pathway
for molecules to cross a membrane.
Channel protein – is a hydrophilic pathway in a membrane that enables
water and ions to pass through.
Carrier protein – is a protein that binds to a molecule and transports it
across the lipid bi-layer.
Osmosis
Water is able to pass freely across the membrane. This allows the cell to
either increase or decrease in size.
Water always passes from an area of low solute concentration to areas of
high solute concentration. This is because the solutes are unable to pass
across the membrane freely.
solution → solvent + solute
The solute is dissolved in a solvent. The universal solvent is water.
Osmosis – is the passive diffusion of water across a membrane.
There are three situations that influenced by changes in solute concentration;
Hypotonic – is the property of a solution that has a lower solute
concentration than another solution.
Hypertonic – is the property of a solution that has a higher solute
concentration than another solution.
Isotonic – is a property of a solution that has the same concentration as
another solution.
Active Membrane Transport
There are some substances that are unable to pass freely across the cell
membrane but they are important to cellular function or molecules that need
to be moved against the concentration gradient. This is done through a
process of active transport, which requires the use of cellular energy (ATP).
There are specialized protein pumps which facilitate this process.
Active transport – is the movement of substances across membranes against
their concentration gradient using pumps.
Most active transport pumps move positively charged ions, such as; H+,
Ca2+, Na+, and K+, across the cell membrane. (proton pump, calcium pump,
and sodium-potassium pump). These charge ions create an unequal
distribution of positive cations and negative anions on either side of the
membrane, creating an electrochemical gradient.
Electrochemical gradient – is the combined effects of a difference in
electrical potential energy and a difference in the concentration gradients of
ions.
Exocytosis and Endocytosis
Large molecules that are unable to pass across the membrane by either
passive or active transport, but are still required by the cell, must have a
method of entering the cell.
Exocytosis is the movement of large molecules out of the cell, and
Endocytosis is the movement of molecules into the cell. These processes
actually require the membrane to form around materials entering and the
membrane reattaching as material leaves.
Chapter 2: Summary
pg. 100
Chapter 2: Self-Quiz
pg. 101
Chapter 2: Review
pg. 102 – 107
Unit 1: Self- Quiz
pg. 110 – 111
Unit 1: Review
pg. 112 – 119