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Chapter 4 Reading Guide- Cells and Their Environment
Section 1 (pages 74-80)
Name__KEY_____________
Date___________ Period ___
1. All organisms and their cells must respond to external conditions to _maintain _ _stability_.
This is called maintaining _homeostasis__.
2. Movement across the membrane that does not require energy from the cell is called _passive_ __transport__.
A difference in the concentration of a substance across a space is called a _concentration _ _gradient____.
Eventually the concentration will be equal in the two spaces and this is called _equilibrium____________.
3. If there is a concentration gradient in the solution, the substance will move
from an area of _high_ concentration to an area of __low__ concentration.
4. The movement from an area of _high_ concentration to an area of
__low___ concentration is called _diffusion___________.
5. If this is allowed to continue, eventually the substance reaches equilibrium.
6. Only substances that can pass through a cell membrane will _diffuse___
down through the cell membrane.
7. The cell membrane is _selectively__ __permeable__________________
to substances. This means that it allows some things to pass through, but not others.
The _interior__ is nonpolar and it repels ions and other polar molecules.
Molecules that are very _small__ or nonpolar can diffuse across the membrane,
_down__ its concentration gradient.
Page 76
8. Water molecules are small and can diffuse through the cell membrane. This is called __osmosis__.
9. What causes _osmosis_? Water molecules can move freely, but the dissolved substances cannot. So water moves
across the membrane to _even_ out the concentration of the substance on both sides of the cell membrane.
Page 77
10. There are _3_ possibilities for water movement. 1. Water moves out. 2. Water moves in. 3. No net water movement.
11. When water diffuses out of the cell, it _shrinks_. The solution that causes the cell
to shrink is called a ___hypertonic____solution_______. This happens when the fluid
outside of the cell has a __higher___ concentration of particles and it means that it has a
lower concentration of water. This means the concentration of water inside is _higher___
and therefore it will flow _out_ _of_ _the_ _cell_. (opposite of substances-see picture in text)
12. When water diffuses into the cell, it _swells___. The solution that causes the cell
to swell is called a __hypotonic_ __solution_____. This happens when the fluid
outside of the cell has a _lower___ concentration of particles and it means that it has a
higher concentration of water. This means the concentration of water inside is __lower____
and therefore it will flow _into_ _the_ _cell_. (opposite of substances see picture in text)
13. If inside and outside of the cell have the _same___ concentration of free water molecules, water
diffuses inside and outside at ___equal__ __rates_. This means there is _no_ net movement
of water in or out of the cell. The cell stays the _same_ size and we say it is in a state of _equilibrium_.
If this happens, we say that this cell is in an _isotonic_ _solution__. (see picture in text)
14. If left unchecked, cells in a _hypo_tonic solution could _burst_. Different cells deal with this problem in
different ways. The cells of plants and fungi have rigid _cell_ _walls_ that keep the cell from getting too big. Some
single-celled organisms have a __contractile_ _vacuoles__, that collects excess water from inside the cell and forces
it out. Animal cells don’t have either of these, but they reduce swelling by removing _dissolved_ _particles_ from the
cytoplasm.
Page 78
15. Most _ions__ and ___polar____ molecules cannot pass across the cell membrane because the interior of the cell
membrane is ____nonpolar_______.
16. However, they can go across the membrane with the help of
____transport proteins _________ or ____channels______.
17. ___Permiability_______ is one of the most important properties of the cell
membrane because it lets the cell control what enters and leaves the cell.
18. Even though ions cannot diffuse through the nonpolar (uncharged) interior of the
lipid bilayer, they can go through _ion__ ____channels___
19. An _ion____ ___channel___ is a transport protein with a polar (charged) pore
through which _ions____ (charged molecules) can pass. Some pores are always
_open___. Others have __gates_____ that can be closed. None of these require any
energy because the ions always move __down_____ their concentration gradient.
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20. Some cells have different proteins that bind to the substance on one
_side, carry it across the _membrane___ and then release it on the other _side.
These are called ___carrier____ __protein__. If these things move
substances down their concentration gradient it is called _facilitated__
___diffusion_____. This is a type of _passive___ transport.
In your own words, write the three steps of facilitated diffusion from page 80.
1.
2.
3.
Molecule to
be carried
Section 2 (pages 81-86)
1. The transport of molecules across the cell membrane against its concentration gradient is
called __active_ ___transport____. This requires __energy___ in the form of _A _T _P.
Since it is going against the concentration gradient molecules move from _low__ to _high_.
Page 83
2. Some substances are too large to moved by transport proteins. Instead, they are moved
across the membrane by _vesicles__. The movement into a cell by a__vesicle_____is
called _endocytosis____. In your own words, write the 3 steps of endocytosis (page 83)
1.
Energy
Molecule
being carried
2.
3.
LABEL THESE PICTURES FROM PAGE 83
3. The movement of a substance by a vesicle to the __outside___ of the cell
is called __exocytosis___. During this process, vesicles in the cell _fuse___
with the ___cell___ __membrane_, releasing their contents.