Cells

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Bell RingerWoodruff is conducting an experiment to test how temperature affects plant growth. He sets up four different trials. In one trial, the temperature is set at 50 degrees F, another 60 degrees F, one 70 degrees F. and the last 80 degrees F. What is the independent variable in this experiment?

Prefixes and suffixes to know for this unit:

a- means “not.” The word atypical, for example means “not typical.”

Bio- means “life” or living.

-ology means “the study of,” so biology means “the study of living things.”

Uni- means “one.” A unicycle, for example, has only one wheel.

What characteristics are found in living things?

Cells form tissues.

Tissues form organs.

Organs form systems.

Systems form the organism.

Cells are the smallest unit of life on earth.

Most cells cannot be seen without the use of a microscope.

Almost all cells live in a liquid environment.

In order to survive, cells must carry on the basic functions of life such as obtaining food, responding to their environment, using energy, getting rid of waste, and reproducing.

There is nothing simple about a simple cell.

Cells contain a number of different structures. Some of these structures are called organelles. Each structure and organelle has a specific purpose or job.

Nucleus - The nucleus controls what the cell does and contains the DNA (genetic information) needed for reproduction. The nucleus is like the control room of the factory; it tells everything else what to do.

About half the cells on are planet do not have a nucleus. That’s because bacteria, the most numerous organism on the planet, do not have nuclei. Cells that do not have a nucleus are known as prokaryotic cells. We will discuss them further when we talk about bacteria.

Cytoplasm - The cytoplasm is the liquid in the cell that all the other organelles float around in.

Mitochondria The mitochondria store energy for the cell to use. They are like the power cells for the factory; they are the powerhouse of the cell. They are like rechargeable batteries, though.

Cell

membrane The cell membrane allows certain things to go in and out of the cell. It is like a screen door in a factory; it allows some things to go in and out, but not others.

Ribosomes - The ribosomes make and put together the different pieces of proteins that the cell makes. Ribosomes are like the assembly line in the factory; they use certain materials to put together a product. There are hundreds or even thousands in a cell.

Some ribosomes are free floating, but most of them are located on an organelle known as the Endoplasmic Reticulum, or ER.

The endoplasmic reticulum and the Golgi body are membrane systems within the cell whose jobs include making, transporting, and secreting proteins. Remember, all living things are made out of proteins. Note: in cell diagrams these two structures often look the same. The ER is always near the nucleus and is dotted with ribosomes.

Lysosome - The lysosomes secrete powerful enzymes that digest food into food vacuoles in the cell. Once the food has been digested, the lysosomes help the cell get rid of the waste by moving it to the cell membrane where it can be dumped.

The smaller organelles and tiny particles inside the cells like proteins don’t just float around in the cytoplasm all willy nilly. Inside each cell is a series of roads and bridges that make up what is called the cytoskeleton. These roads and bridges consist of tiny filaments and tubules.

Transport in cells

Plant cells are similar in most ways to animal cells, however, plants do not have to “eat” in order to grow and reproduce. Plants are able to make their own food by using sunlight to covert carbon dioxide and water into sugars which the plant stores and uses for food. This process is known as photosynthesis.

The prefix photo- means “light.”

So, let’s review:

plants are able to make their own food through photosynthesis because they contain an amazing substance known as chlorophyll. In plants, chlorophyll is stored in a special organelle known as a chloroplast.

What other organelles or structures are found in plants cells that we did not see in animal cells?

Central

The cell wall is a thick layer of cellulose that surrounds the cell and gives the plant support.

The large central vacuole stores water and essential minerals and helps with support by exerting outward pressure on the cell wall.

Cellular Functions

All living things must be able to obtain food and eliminate waste.

Cells can obtain food and eliminate waste in two primary ways:Active Transport andPassive Transport

Remember…

the cell membrane controls what enters and leaves the a cell.

PASSIVE TRANSPORT

One way the cell does this is through a process known as osmosis. Osmosis is the process where liquids are able to pass through a membrane like a cell membrane.

Cell membranes are semi-permeable, meaning they allow only water and certain molecules that are dissolved in the water (solutes) to pass through the cell membrane. Oxygen and carbon dioxide gases both dissolve in water and can move back and forth through the cell membrane with the water.

Animal cells need water and oxygen but need to get rid of carbon dioxide.

Plant cells need water and carbon dioxide for photosynthesis (and oxygen for cellular respiration. We’ll get to that later), but they also release oxygen as a waste material.

Because so much water, oxygen, and carbon dioxide are constantly passing into and out of a cell, it would totally exhaust the cell if the cell had to use its own stored energy (in the mitochondria) for osmosis.

Luckily for the cell, osmosis is a type of passive transport. This means that the cell exerts no energy during osmosis.

Remember that most cells live in a liquid environment. There is liquid inside the cell and liquid outside the cell. Since the liquid inside the cell and the liquid outside the cell usually have a bunch of other stuff dissolved in them, we refer to these liquids as solutions.

Let’s see how this works.

When the concentration of the dissolved particle (indicated by a %) is greater outside the cell than it is inside the cell, it is a hypertonic solution.

The prefix hyper means “excessive” or “above normal.”

Think (but don’t act) about hyperactivity.

When the concentration of the dissolved particle (indicated by a % is greater inside the cell than it is outside the cell, it is a hypotonic solution.

The prefix hypo means “under” or “below normal.”

Think of (if you dare) a hypodermic needle. Needles go “under” or “below” your skin. Since water will want to move into the cell, think of the needle injecting it in in a hypotonic situation.

Let’s use salt as an example. Salt is one of those substances that can dissolve in water. Salt, or sodium, is necessary in plants and animals, but too much can cause a lot of problems.

Inside of cell.High concentration of water (blue), but low concentration of salt (green)

Outside of cell. High concentration of salt (green), but low concentration of water (blue)

If water always tries to move to the region where the concentration of solutes is higher, in which direction will the water flow?

So, how can this affect a cell then?

(Hypertonic)(Hypotonic)

Water flows into the plant cell. If too much water flows into the central vacuole, the cell wall can burst.

Hypotonic because the solution inside the cell is higher than the solution outside the cell.

Hypertonic because the solution outside the cell is higher than the solution inside the cell.

Water flows out of the plant cell. If too much water flows out of the central vacuole, the cell wall starts to buckle and droop..

In a hypotonic situation, cells can swell so much with water that they burst.

Farmers often fear this will happen after a heavy rain. Fruit crops like tomatoes often burst if they get too much water.

When the concentration of the solution inside and outside the cell is the same, the cell has reached equilibrium, and the solution is called an isotonic solution.

So, what’s to keep the dissolved particles from clumping up in one particular area either inside the cell or outside the cell?

That’s where the process of diffusion comes in. Diffusion is a process where particles in a crowded area want to move to a less crowded area, spread out.

Think about what happens when you make the favorite drink of Southerners: sweet tea. What happens when you first put the tea bag in the hot water?

And the sugar? Why do we have to stir the mix once we put the sugar in?

Through diffusion, the particles will continue spreading out until they are distributed uniformly through the solution.

When the concentration of the solution inside and outside the cell is the same, the cell has reached equilibrium, and the solution is called an isotonic solution. Cells like to be balanced. When they are balanced, water flows in and out of the cell at an equal rate. Cells are happy.

Here’s how that might work in real life:

Ned is body surfing at Tybee after a big storm. He gets pulled under and inhales a lot of water. The cells lining the lungs are not permeable to salt. Salt cannot pass through them. However, these are permeable to water.

The average salinity (salt content) of the ocean is 35%. The average salinity of our blood is just under 1%.

This is a hypertonic situation because the concentration of salt in the water outside the lung cells (in the lung cavity) is much higher than the concentration of salt inside the lungs cell. In order to reach equilibrium, water will leave the cells and enter the lung cavity. This causes the lung cavity to fill with fluid and the lungs cannot work properly. You can end up suffocating because you cannot take a deep enough breath to get the oxygen you need nor can you exhale and get rid of the carbon dioxide that is poisoning your body.

HO

P.S. When your body cells lose water, you become severely dehydrated. People who drink seawater when they worry about becoming dehydrated actually speed up dehydration.

ACTIVE TRANSPORT

Some molecules or particles of food are too big to pass through the cell membrane through mitosis. Similarly, some waste materials are too large to pass out of the cell through osmosis. In these instances, the cell actually has to open a part of the cell membrane. This requires the cell to use some of its stored energy, so it is no longer passive transport. When the cell has to use some of its energy to move things in and out of the cell, it is called active transport.

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The prefixes exo and ecto mean out of or outer (like exit.) Exo=ExitThe prefix endo means to enter or inner. Endo=Enter

1. When the cell takes a particle food into the cell it is called ENDOcytosis.

5. When a cell releases large particles of waste from the cell, it is called EXOcytosis.

2. A food vacuole forms.

3. Lysosomes join with the food vacuole and release their digestive enzymes into the vacuole. Digestion occurs.

4. Nutrients are released into the cytoplasm. The leftover is waste.

CELLULAR RESPIRATION

So, once the glucose has been created, how then do cells get the energy from the glucose so that they can use it? The cell releases the energy from the glucose in a process known as cellular respiration. Through this process, oxygen enters a cell and reacts with the glucose to create a high energy substance known as ATP. Even plants need oxygen to break down the glucose and release the ATP.

Think of in the same way you might think of the engine of a car. Gasoline is a source of stored energy, but gasoline can do nothing on its own. It needs something to release the energy stored in it. In the case of gasoline, the agent that releases the energy is a spark. The spark ignites the gasoline and the energy within is released. In cellular respiration, the “spark” that releases the energy from glucose is oxygen.

As a result of cellular respiration, waste products are produced in the cell. These waste products easily dissolve in water and are therefore eliminated from the cell through osmosis.

Cell ReproductionMost cells reproduce through asexual reproduction. Asexual reproduction is the term used to refer to reproduction where only one parent contributes genetic information to the “daughter” cells. Because the new daughter cells receive genetic info from only one parent, they are genetically identical to the parent. They are clones.

There are several different types of asexual reproduction:

In budding, an organism will develop a growth or “bud” that eventually separates from the parent organism. The new “daughter” is genetically identical to the parent. This is common in corals.

Fragmentation occurs when a segment of an organism is torn from the parent. With some organisms like starfish, that segment can develop into a new adult starfish. It is genetically identical to the parent.

Mitosis and binary fission are very much alike. They both involve a cell creating a copy of its genetic material before splitting into two new daughter cells. Binary fission occurs primarily in unicellular organisms while mitosis generally occurs in cells that form tissues in multicellular organisms.

Asexual reproduction

Mitosis is the most common form of cell reproduction in complex, multicellular organisms like animals and plants. Mitosis generally occurs when the larger organism is growing, repairing damaged tissues, or replacing dead cells.

Binary Fission is the term generally used to refer to cell division in unicellular organisms like bacteria and protists (which we will talk about later.)

During mitosis and binary fission, a cell literally divides in half and creates two new cells. The two new cells are exact clones of the original “parent” cell. Before doing this, however, the cell must first create a copy of all its genetic information so that each of the new cells can have it.

The genetic information must somehow be passed on to the new cells. But if the chromosomes were simple to divide in half, each new cell would only have half the chromosomes of the parent cell. SO, before the cell divides, it first creates a copy of the chromosomes inside the cell.

The new pairs of chromosomes, known as chromatids, line up in the middle of the cell as it gets ready to divide.

As the cell divides, the chromatids split in half. Each half has a full set of chromosomes, just like the parent cell.

The cell continues to divide…….

Until it finally splits apart. Now there are two new cells. Each cell has a full set of chromosmes (diploid), and is an exact copy of the parent cell.

How are these two processes different?

Meiosis, or sexual reproduction is more complicated. Meiosis is a type cell division with only one purpose in mind: the creation of sex cells (gametes) such as sperm and eggs. We’ll talk more about meiosis during our genetics unit.

The cell makes a copy of its chromosomes just like during mitosis.

These new paired chromosomes line up in the center of the cell but NOT like they do in mitosis.

But here is where it starts getting different. Before splitting, the paired chromosomes swap a bit of DNA (genetic material) with each other. We’ll talk about why this is important when we study genetics.

Now the cell divides into two new cells, but these cells are NOT identical to the parent because some of the genetic material was rearranged.

And the cells aren’t done. They divide again, only this time the chromosomes were not copied before dividing. Each of the four new cells now has only have the chromosomes of the parent cell. They are haploid cells. Each cell is either a male or a female sex cell (in animals that would be a sperm or an egg).