Transporting Substances Across Cell Membrane
All cells are constantly moving substances in / out of cell to maintain homeostasis.
*It is the job of the cell membrane to regulate this function.
*Substances such as oxygen, carbon dioxide, water, electrolytes (charged ions such as Mg+, Na+, etc.),
glucose molecules, and waste are necessary to travel in / out of cell.
*There are two types of transport, depending on size of molecule:
1) Passive Transport
2) Active Transport
Passive Transport
*requires NO input of energy from the cell (no ATP)to get substances in/out of cell; process occurs
naturally.
WHY?
substances getting in/out are either very small (carbon dioxide, oxygen, water) or the flow is going
with the natural force of gravity.
-
the “natural flow” of substances across the cell membrane is based upon the concentration.
Oxygen, Carbon Dioxide, and Water travel from where there is more to where there is less.
(from high to low concentration).
Types of Passive Transport
Diffusion – movement of molecules from an area of high concentration to an area of low concentration;
substances that easily diffuse through cell membrane include water, carbon dioxide, and oxygen.
- the substances will continue to spread out to a lower concentration until it is evenly distributed
throughout the space; this even distribution is called equilibrium.
- not ALL substances has the ability to easily diffuse across the cell membrane; it depends on the type
and size of substance.
Osmosis: A Special Type of Diffusion
Osmosis - a special type of diffusion in which WATER is moved from a region of high concentration to a
region of low concentration.
- water moves in or out of the cell depending on where the water level is the greatest/least  water is
always competing for space with larger molecules, such as Na+, Mg+, Cl-!!
naturally, cells’ liquid environment has these dissolved solutes (electrolytes) in their liquid (ex: Na+, K+,
Mg+).
Areas of the cell that have high solute concentration have low water levels.
- To determine which way water will move, you need to establish the environment on BOTH sides of cell
membrane (extracellular and intracellular):
-
water tends to naturally move to levels where water is low (via osmosis); therefore....
water tends to diffuse in/out of cell where solute concentration is the highest!!!
The aqueous environment that cells are “bathed” in can be described as....
1) Hypotonic
2) Hypertonic
3) Isotonic
Hypertonic – outside solution contains more solutes than water compared to inside cell.....water rushes
out of the cell.
(draw scenario)
Hypotonic – outside solutions contain less solutes than water compared to inside cell......water rushes
into the cell.
(draw scenario)
Isotonic – both the outside and inside solution is equal in solute and water concentration; no net
movement.
(draw scenario)
Will Diffusion or Osmosis Ever Stop
Theoretically, diffusion / osmosis will stop when both sides are equal in concentration. This is known as
equilibrium.
Stop – End of Quiz material Trimester 1 2013
Facilitated Diffusion – a type of diffusion in which substances traveling across the cell membrane need
help; a special protein called a carrier protein (a type of peripheral protein in the cell membrane) helps
the substance travel across;
- no energy in needed by the cell; therefore facilitated diffusion is still passive transport.
Example:
Glucose molecules are too big to diffuse across, but are too small for the cell to waste its energy
on.
2) Active Transport:
- unlike passive transport, active transport carries substances across the cell membrane
AGAINST the natural flow of substances (against a concentration gradient from LOW to HIGH
concentration).
- requires input of energy from cell (in the form of ATP).
- substances are usually too large to passively cross cell membrane.
- two types of active transport:
Cellular Pumps
Endo/Exocytosis
Sodium-Potassium Pumps
-
type of active transport in which special carrier proteins of the cell membrane “carry” charged
ions (Na+, K+, etc.) across the membrane, AGAINST a concentration gradient.
-
these charged ions are very important to the cell because they are responsible for conducting
electrical impulses from one body part to another.
- Dehydration can lead to an imbalance of these ions causing fatigue, delirium, loss of muscle control,
coma, and eventually death.
Endocytosis / Exocytosis
-
VERY large substances that are not charged ions and cannot move across the cell membrane use
this process (large lipids, proteins, etc.)
-
Since large molecules are moving against the concentration gradient, cell has to use ATP.
-
endo -/exocytosis is a process in which the membrane “engulfs” (by use of vesicles) the
substance to get it across the cell membrane.
-
Endocytosis – a type of active transport in which substances are transported into the cell.
-
Exocytosis – a type of active transport in which substances are transported out of the cell.
-
endo - / exocytosis can be further categorized by the TYPE of substance being transported
in/out of cell:
-
Pinocytosis – transport of fluids (NOT WATER!!!) in/out of cell by endo/exocytosis.
-
Phagocytosis – transport of large, solid substances in/out of cell by endo/exocytosis.