CHAPTER 2

THE F

UNCTIONIN

G CELL

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 organism- in 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.