WARM UP
• Try to think of things that a border crossing would
have in common with cells.
• After thinking for a minute, turn and ask a partner if
they thought of anything the two have in common.
BORDER CROSSING
• When you cross the border into another
country, you may not be able to bring
certain items with you because of
different laws in certain countries.
• Each one of our cells have a border
crossing of their own. We call it the cell
membrane.
BORDER CROSSING
• Just like the customs checkpoint, a cell
membrane allows some substances to
enter or leave the cell, and it stops
other substances.
• The cell membrane is said to be
selectively permeable because it only
lets certain materials through.
If a membrane let any substance through, it
would be permeable. If a membrane doesn’t
let anything through, it is impermeable.
PERMEABILITY
How can a cell membrane be selectively
permeable?
Imagine a ziploc bag full of water. Assuming it
has no holes, it will hold the water. Now
imagine doing the same thing with a cloth
grocery bag. What will happen?
PERMEABILITY
Plastic (like ziploc bags) is impermeable to
water, but cloth isn’t.
The difference is in the structure of the material
from which the bags are made.
What would happen if you poured a mixture of
sand and water in to the two bags?
DIFFUSION
• The structure of a cell
membrane controls what
can move into and out of a
cell.
• What causes substances to
move in the first place?
• The picture is a clue!
DIFFUSION
• According to the particle theory, the particles in all
liquids and gases are constantly moving in every
direction and bumping into each other.
• These collisions explain why particles that are
concentrated in one area, such as a blob of ink in a
water glass, spread apart into areas where there are
fewer ink particles, and fewer collisions.
DIFFUSION
When particles spread out to avoid a collision,
we call it diffusion. Eventually, the ink
particles will be evenly distributed throughout
the container of water.
When the particles are evenly distributed, they
continue to move, but there is no change in
the overall distribution of the ink in the water.
DIFFUSION
• Diffusion also plays a part in moving
substances into and out of cells. For
example imagine an amoeba living in
water.
DIFFUSION
• Stop and think:
–Try and think of 3 new examples
of diffusion, then pair up with a
partner and compare notes. Be
ready to share your answers!
DIFFUSION
• The amount of dissolved carbon dioxide gas
in the water is the same as the amount of
dissolved carbon dioxide gas in the cytoplasm
of the amoeba.
• The carbon dioxide will move in and out of the
amoeba at the same time, passing through a
selectively permeable membrane.
DIFFUSION
• Now imagine the amoeba has been producing
carbon dioxide gas as a waste product inside
its single cell.
• There is now more carbon dioxide in the cell
than in the water surrounding it.
• As a result, the carbon dioxide particles move
out of the cell at a faster rate than they move
inside the cell.
DIFFUSION
• The carbon dioxide particles diffuse out
of the amoeba until the amoeba and
the water surrounding it have equal
amounts of carbon dioxide.
• Here’s an interesting explanation of
diffusion.
DIFFUSION, OR OSMOSIS?
• Diffusion is not to be confused with osmosis (we’ll
learn about osmosis next).
• Most of the examples of diffusion we’ve talked about
involve water, like the ink diffusing. However, diffusion
just refers to the way molecules move from areas of
high concentration, where there are lots of other
similar molecules, to areas of low concentration,
where there are fewer similar molecules.
• Water isn’t always involved in diffusion!
OSMOSIS
• The most common substance found inside
and around cells is water. About 70 percent of
a cell’s content is water, and most cells die
quickly without a supply of water.
• Water particles are small and can easily move
into and out of cells by diffusion.
• The diffusion of water through a selectively
permeable membrane is called osmosis.
OSMOSIS
• Water is important to living things
because it dissolves many of the
substances involved in cell processes.
• For example, glucose (which cells use
for energy) dissolves in water to make a
glucose solution.
OSMOSIS
• When water moves out of a cell, the dissolved
substances inside the cell become more
concentrated.
• When water moves into a cell, the dissolved
substances become more diluted.
• Water tends to move by osmosis from a dilute
solution to a more concentrated solution.
ACTIVE TRANSPORT
• Here’s a cool video about active transport.
ACTIVE TRANSPORT
• Small particles, like water, carbon
dioxide, and oxygen, can diffuse freely
in and out of cells through small
openings in the cell membrane.
• This process depends only on the
concentrations of the particles, and it
occurs without any energy by the cells.
ACTIVE TRANSPORT
•
•
•
However, cells also require certain substances in
greater concentrations or in lower concentrations
than can be obtained by diffusion alone.
For example, cells need large amounts of
glucose, which supplies them with energy.
To meet this need, glucose particles must move
from an area of low concentration (outside the
cell) to an area of high concentration (inside the
cell).
ACTIVE TRANSPORT
• This process reverses the usual
movement caused by diffusion.
• Unlike diffusion, this process requires
the use of energy by the cells.
• It is a bit like pushing a car uphill
instead of letting it roll down.
ACTIVE TRANSPORT
• The controlled movement of substances through the
cell membrane is carried out by the membrane itself.
• To understand how it does this, scientists have
studied the membrane structure in great detail.
• With the help of more powerful microscopes, the
discovered large particles called carrier proteins
embedded in the membrane.
ACTIVE TRANSPORT
•
Carrier proteins are like gates in a wall. They control
substances entering or leaving the cell.
•
Each carrier protein attracts particles of a particular
substance. The protein attaches to the substance,
moves it through the membrane, and releases it on
the opposite side.
•
This whole process is called active transport.
HOW DO CELLS GET ENERGY?
In the same way that energy runs a computer, food supplies the
energy that the cells of the body need to carry out their activities.
Cells cannot work without energy, and the energy they use comes
from food.
For animals, food may be a sandwich, a mouse, or a blade of grass.
For plants, food is carbohydrates made in their leaves by the
process of photosynthesis.
All foods have one thing in common: they all have particles that
contain chemical energy.
HOW DO CELLS GET ENERGY?
The energy in food can only be released after food particles have
entered the cells and have been broken down by a chemical
reaction. The process that releases the food energy is called
cellular respiration.
Do you think that respiration is breathing in and out? It turns out
there may be a bit more to it than that!
Remember that cells carry out all the functions of living things. Your
cells use the oxygen that you breathe in for cellular respiration,
and they produce the carbon dioxide that you breathe out.
Cellular respiration occurs in nearly all living cells in every organismin plants and micro-organisms, as well as in animals.
HOW DO CELLS GET ENERGY?
Inside cells, oxygen combines with food particles
(such as carbohydrates) in cellular respiration.
The word equation for this chemical reaction
is:
HOW DO CELLS GET ENERGY?
The chemical change of cellular respiration can be compared to the burning
of fuel. Like something being burned, much of the energy from the
reaction ends up as heat.
Think of what happens when your body needs more energy, such as when
you are running in gym class. First, you should eat a good meal of energy
rich carbohydrates. As you run, you breathe more quickly, pumping in
more oxygen for your cells to use. The oxygen and food particles react
inside your cells, producing energy for your muscles. At the end of gym
class, you probably feel pretty warm!
While you were running, every one of the millions of cells in your muscles
has been burning food particles at a higher rate, not to mention the
trillions of other cells in your body all working to carry out the respiration
reaction.
POWERHOUSES OF THE CELL
Cellular respiration does not take place everywhere inside the cell. It
occurs mainly inside the mitochondria.
Because energy is produced within the mitochondria, these
organelles are often called the “powerhouses” of the cell.
Different cells use different amounts of energy and have different
numbers of mitochondria. Active cells, such as those in muscles,
may contain several hundred mitochondria.
The energy produced by the mitochondria can be used by other
parts of the cell.
POWERHOUSES OF THE CELL
Why do cells need energy?
-Cell membranes need energy to move materials into and out of cells by active
transport.
-Muscle cells need energy to contract.
-Nerve cells use energy to send signals.
-Most cells also use energy to grow and reproduce.
DID YOU KNOW?
Have you ever felt sore after working your muscles harder than
usual? This happens because your muscle cells cannot get
oxygen fast enough to meet their demand for energy. So, instead
of using cellular respiration to release energy from glucose, the
muscle cells use a chemical reaction called fermentation. A
product of this reaction is a chemical called lactic acid. It is this
chemical that makes your muscles feel sore.
Regular exercise strengthens your muscles and improves the
efficiency of your heart and lungs. This helps you get more
oxygen to your cells faster and reduces the risk of soreness after
exercise.
HOW DO CELLS REPRODUCE?
All organisms, including you, begin life as a single cell. How does
one cell produce a multicellular organism?
A cell divides itself over and over again to make new cells. (Don’t
forget cell theory, which states that cells can only come from
previously existing cells!) During cell division, one cell divides to
become two cells. Then each of those cells divides into two more
cells, and so on.
In this way, and with amazing speed, one fertilized egg from which a
human develops eventually produces a baby consisting of
trillions of cells.
HOW DO CELLS REPRODUCE?
Cell reproduction doesn’t stop when a baby is born,
otherwise you would never have grown up to be as tall
as you are now!
Many of your cells keep dividing even when you are a
fully grown adult.
REGENERATION
The photo to the right
shows a pond organism
called a hydra. It is
named after a monster
from Greek mythology.
Although the monster hydra
doesn’t exist, there are
living things today that
share some of its
characteristics.
REGENERATION
Many living things today can replace damaged or missing body
parts, a process which is called regeneration.
Is it possible for humans to regenerate?
-Have you ever broken a bone?
-Have you ever scraped your knee or your elbow?
-The wound is able to heal because dead and damaged cells are
replaced by new ones. The new cells, produced by cell division,
grow over the injury and replace the wound.
REGENERATION
Like organisms, individual cells have a life cycle- they grow, divide,
and eventually die. Different cells grow and divide at different
rates, depending on their structure and function.
For example, the cells in your skin wear out quickly and last only a
few days or weeks. The longest-lived cells are nerve cells, which
can last a lifetime.
Regeneration is evidence that organisms can produce new cells
throughout their lives. For example, some lizards can grow a new
tail, and lobsters, newts, tadpoles and insects can grow new legs.
Deer shed their antlers every year, and new ones grow back to
replace them.
Can you think of any other examples of regeneration?
MITOSIS
How does one cell become two?
If you look at cells under a microscope, you may be able to observe them in
the process of dividing. The best places to look for dividing cells are in
parts of an organism that are growing rapidly, such as the root tips of
young seedlings.
Inside cells are things called “chromosomes”, which become visible only
when the cell is about to divide in two. Chromosomes contain the cell’s
genetic material (DNA), which gives the instructions to the new cell.
MITOSIS
During cell division, the genetic material duplicates and
then divides into two identical sets of chromosomes.
This process is called mitosis.
It is very similar in all forms of life, whether the organism
is a unicellular micro-organism or a multi-cellular plant
or animal. The two new cells are called “daughter
cells” and each gets one set of chromosomes.
CELL DIVISION
How rapidly do cells divide?
All healthy cells have regular rates of division. For example, certain bacterial cells
divide once every 20 minutes. Frog embryo cells divide in about an hour, cells
lining your intestine take about 48 hours to divide, and your liver cells divide only
once every 200 days.