|Nucleus, Mitochondria, Chloroplasts
|CCSTD Science Grade 7 1.c., d.|
As you learned in our last Instruction, most cells are eukaryotic cells.
Cells are composed of three major regions:
Plant cells have something extra -- a cell wall that goes all the way around the outside of the cell membrane. But for animals (like us), it's the membrane that holds the cell together and provides a semi-permeable barrier to the cell’s surroundings.
The Cell Membrane
The cell membrane is fluid. It has tiny openings that let things in and out. If you look at the diagram of the cell membrane above, you will see some important components. The bulk of the membrane is made of molecules that have a region that repels water (shown as the blue layer) and a region that is attracted to water, shown as the orange balls.
If the entire membrane were made of these molecules (which we call phospholipids) then nothing at all would pass through and the cell would not be able to communicate with any other cell in the body. It is essential to the organism that cells are able to communicate with other cells.
The membrane also contains some proteins shown in purple, green and red, that serve to allow for the passage of only some molecules and ions across the membrane. Some of the proteins have a very special function. They respond to the presence of hormones sent by other cells in the body by changing so much that they cause changes inside the cell. The cell membrane is very good at controlling the environment inside the cell and responding to changes from outside the cell.
The nucleus is the "brain" of the cell. Its job is to control the cell's structure, function, growth and division. It is usually located in the center of the cell, where it is enclosed in a membrane called the nuclear envelope – which like the cell membrane is also a semi-permeable barrier. The nuclear envelope is a double membrane as shown in the diagram.
The nucleus communicates with the rest of the cell through the nuclear pores. The nucleus is like a vault. It holds the cell's DNA -- which contains the genetic instructions for the cell's activities. In the diagram there is a structure within the nucleus called the nucleolus. This structure is the place where ribosomes are made. Ribosomes are very important structures upon which proteins are formed within the cell. In the diagram above, you can see the ribosomes that have already been sent out of the nucleus and are on the endoplasmic reticulum, one of the places within the cell where proteins are made.
One way the nucleus directs activities within the cell is by sending instructions through its envelope to the various organelles in the cell's cytoplasm. There are two main types of organelles -- organelles that produce protein and organelles that transfer chemical energy present in food molecules such as glucose into molecules called ATP (short for adenosine triphosphate). The chemical energy present in ATP can then be used by the cell for such activities as movement and building large molecules.
Organelles that participate in the production of proteins include:
(Lysosomes are also thought to participate in breaking down damaged proteins or proteins no longer needed by the cell.)
Organelles in which proteins are formed are very important because the proteins they produce are essential for life. We have talked about the genetic material that is stored in the nucleus. Genetic material contains the coding for the manufacture of the molecules that perform most of the tasks of each cell. Proteins are generally very large and complex molecules, some of which are able to respond to the needs of the cell by changing the rate at which they work. Here are some tasks of proteins:
Organelles That Transfer Energy
The two most important energy-producing organelles are mitochrondria and chloroplasts. Mitrochondria liberate energy for the work that cells must do, while chloroplasts capture the energy of the sun for photosynthesis.
Both plant and animal cells contain mitrochondria -- but only plant cells contain chloroplasts.
Think of mitochondria as a cell's tiny powerhouses.
A cell can contain thousands of mitochondria, depending on the work it has to do. If it needs lots of energy, it will have more mitochondria -- because it's mitochondria that are able to capture chemical energy into forms that enable us to live. When we eat, our intestines break down food down into small molecules with the aid of enzymes. Those molecules are absorbed by cells.
To go into a little detail, it actually takes place like this:
Inside the mitochondria there's a fluid called the matrix, which is composed of water and proteins. The proteins in this matrix take the breakdown products of food molecules and combine them with oxygen. As a result of this process, molecules of ATP are formed. The ATP has stored chemical energy which can be used by the cell for movement, for synthesis and so on.
Chloroplasts are a lot like mitochondria. The difference is that while mitochondria take food and harness its chemical energy, chloroplasts absorb energy from the Sun and use this to synthesize food from water and carbon dioxide. They do this through a process called photosynthesis, which you have studied in previous Lessons, bBut we should probably review it again:
Life wouldn't be possible without photosynthesis. And photosynthesis wouldn't be possible without chloroplasts and chlorophyll.
Experiments for Home and Classroom
The basic genetic material in all cells is DNA. In this activity, students learn how to extract DNA from inside a person's cheek and isolate it in a test tube. This exercise is like the DNA identification activities you see on forensics TV shows like CSI. Click here.
In this classroom activity called "Perspective: Powers of Ten," students are invited to look very closely at an object like a leaf to see right down to the cells and the cell parts it's made up of. A field microscope and some basic drawing ability are required. Click here.