Download Science Chapter 2

Science Chapter 2
The Functioning Cell
Chapter 2.1
The Cell Membrane: Gatekeeper
of the Cell
The Functioning Cell
 In
order for our cells to function, they need
air, food, and water.
 If someone asked you why you need a
drink, you might say you are hot or thirsty,
but you could also say that your cells need
the water.
Cell Membrane
 Think
of a border separating two nations.
Customs officers check the items that
travelers are carrying. Some items, such
as firearms and plants are not allowed to
cross the border.
 A cell membrane works similarly in that it
allows some substances to enter or leave
a cell, but not others.
Cell Membranes
 When
a cell selects what items get to
come in and out it is called selectively
permeable.
 When a cell membrane allows anything to
cross it, it is called permeable.
 A membrane that allows nothing to cross
is call impermeable.
Example
 Think
of what happens when you pour
water into a plastic bag. The water is not
going to go through the plastic. The
plastic is impermeable to water.
 What if you pour water into a bag made of
cheesecloth? The water is going to go
through the cloth; therefore, it is
permeable.
Example
 Imagine
if you pour a mixture of water and
sand into a cheesecloth bag.
 Will the bag be permeable, impermeable
or selectively permeable?
Answer
 The
cheesecloth bag will be selectively
permeable to the mixture of water an
sand. Water will go through but most of
the sand will not.
Diffusion
 What
causes substances to move in the
first place?
 The particles in all liquids and gases are
constantly moving in every direction and
bumping into each other.
 Thus, in a concentrated blob, particles will
spread out to avoid more collisions.
 This spreading-out process is diffusion.
Diffusion
 Diffusion
also plays a part in moving
substances into and out of cells.
 Imagine an amoeba living in water and the
concentration of the carbon dioxide in the
water is the same as inside of the cell.
The carbon dioxide will move in and out of
the cell at the same rate.
Diffusion
 If
the amoeba is producing carbon dioxide
and there is more inside the cell than
outside the cell; then more carbon dioxide
cells will leave the cell and less will come
in.
 This process continues until the
concentration of carbon dioxide inside and
outside the cell are equal.
Osmosis
 The
most common substance found inside
and around the cell is water.
 About
 The
70% of a cell’s content is water.
diffusion of water though a selectively
permeable membrane is called osmosis.
Example
 If
you have ever left carrot sticks in the
fridge for awhile, you probably noticed that
they became limp.
 This is because they have lost their
moisture through osmosis.
 If you put the carrot sticks into water, then
24 hrs later, they are crisp again.
Osmosis
 Remember
when I said that when we are
hot we get thirsty?
 When
we are active, we lose moisture
from our body in our breath and our sweat.
Water is drawn out by osmosis. We need
water to restore cell water content.
Osmosis
 Glucose
(which cells use for energy)
dissolves in water to form a glucose
solution.
 When water moves out of a cell, the
dissolved substances inside the cell
become more concentrated.
 When water moves into a cell, the
dissolved substance becomes more
diluted.
Osmosis
 Water
tends to move from a dilute solution
to a more concentrated solution.
 Water
moves from an area of high
concentration to low concentration.
Transport
 Small
particles---such as water, carbon
dioxide, and oxygen---diffuse freely into
and out of cells through small openings in
the cell membrane.
 This only depends only on the
concentrations of the particles.
 It occurs without any use of energy by the
cell.
 Natural for these particles to equalize.
Active Transport
 When
certain substances need to have a
greater concentration inside the cell such
as glucose for energy. This is called active
transport and it needs the cell’s energy to
perform this task.
Active Transport
 Like
gates in a wall, large particles called
carrier proteins that are embedded in the
membrane controls the substances
moving through the wall.
 Each carrier protein attracts particles of a
certain substance.
 The protein attaches to the substance and
moves in through the membrane inside the
cell.
Questions
 Complete
questions 1-6 on page 47.