The Parts of a Cell
In many ways, living cells are like factories that produce goods. They take in raw materials, use them to build products such as proteins, package the products, and transport them to different parts of the cell or to other cells. They also eliminate waste and reproduce. The different jobs are performed by structures within the cell called organelles. The cells of animals and plants share most of the same kinds of organelles and other cell parts.
Animal and Plant Cells
Just as a factory needs a manager, a cell needs an organelle to direct the cell processes. The nucleus is the distinct central organelle that contains the cell’s genetic material in the form of deoxyribonucleic acid (DNA). DNA is an organic molecule that stores the instructions for making proteins and other molecules needed by the cell. Two thin membranes make up the nuclear envelope that surrounds the nucleus.
The cell is generally divided into two parts-the nucleus and the cytoplasm that surrounds the nucleus. Cytoplasm is a jelly like substance made of water, amino acids, carbohydrates, fats, and nucleic acids. A variety of organelles are located in the cytoplasm.
Ribosomes are organelles that use amino acids to assemble proteins according to instructions in DNA. A ribosome is made up of ribonucleic acid (RNA) and protein. Ribosomes are formed within a structure known as the nucleolus, which is in the nucleus of the cell. Unlike most other organelles, ribosomes are not surrounded by membranes.
The cell contains a system of membranes and sacs known as the endoplasmic reticulum, or ER. The endoplasmic reticulum acts like a highway along which molecules can move from one part of the cell to another.
The part of the ER that is involved in the production of proteins has ribosomes along its surface. This type of ER, known as rough ER, 1s common in cells that make large amounts of proteins. Smooth ER, which does not have ribosomes, is involved in regulating processes in cells.
After products are made in a factory, they must be organized, boxed, and shipped. Similarly, proteins produced in rough ER are passed along to an organelle called the Golgi apparatus, which consists of a stack of membranes. Its job is to modify and package proteins and other molecules so they can either be stored in the cell or sent outside of the cell. In some glands, the Golgi apparatus packages and releases hormones.
Some factories have their own generators that produce the electricity they need. Cells also have energy generators called mitochondria, which convert energy stored in organic molecules into compounds the cell can use. This happens through a process called cellular respiration. The greater the energy needs of a cell, the more mitochondria that cell will have.
Factories and cells also need cleanup crews. Lysosomes are small organelles filled with enzymes, proteins that speed up the rate of a chemical reaction. The enzymes enable the lysosomes to digest, or break down, organic molecules such as carbohydrates, lipids, and proteins.
A factory needs a place to store materials and waste products. Similarly many cells have sac like organelles called vacuoles that store materials for the cell. In some cells, vacuoles store water, salts, carbohydrates, and proteins. Vacuoles may also digest food and store and dispose of waste.
The cell membrane is the soft, flexible barrier that surrounds the cell and holds it together. A cell must be able to take in nutrients and dispose of wastes. The cell membrane controls how these substances pass into and out of a cell.
Distinguishing Between Cells
Plant cells have many of the same parts animal cells have, including the nucleus, cytoplasm, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, vacuoles, and the cell membrane. It is, however, possible to differentiate, or recognize the differences, between plant and animal cells. For example, animal cells either do not contain vacuoles or have several small vacuoles, but plants usually have one large vacuole. The pressure of the liquid in this vacuole helps support the plant. Plant cells also contain parts that animal cells do not have, namely chloroplasts and a cell wall.
In addition to mitochondria, plant cells contain chloroplasts, which are organelles in which light energy is used to produce food for the cell through a process called photosynthesis. This food is then broken down to release chemical energy that is used to perform the functions of the cell.
Another structure associated with plant cells is the cell wall, which is a rigid outer layer that supports the cell and protects it from harm. Plant cell walls are made of a carbohydrate called cellulose. Pores in the cell wall allow materials to pass into and out of the cell.
Two Main Types of Cells
There are two main types of cells. A eukaryotic cell has a membrane-bound nucleus and many organelles enclosed in their own membranes. Plant and animal cells, along with fungi, algae, and some single-celled organisms, have eukaryotic cells. A prokaryotic cell has a cell membrane, cell wall, and cytoplasm but does not have a nucleus or membrane-bound organelles. Prokaryotes include bacteria and archaebacteria. All prokaryotes are single-celled organisms.
Transport Across the Cell Membrane
Cell membranes are a selectively permeable membrane. This means that some substances can move through it, and others cannot. It is important for materials necessary for the cell’s function to stay within the cell, for nutrients to move into the cell, and for waste products to move out.
Recall that diffusion is the movement of molecules from an area of high concentration to an area of low concentration. Diffusion is one way that molecules enter and leave a cell. Suppose, for example, that the concentration of a substance is different on either side of the cell membrane. If the substance can cross the cell membrane, its molecules will move toward the area where it is less concentrated.
Perhaps the most important substance that passes through the cell membrane is water. Water molecules pass through a selectively permeable membrane by a type of diffusion known as osmosis. During osmosis, water moves from a place of higher concentration of water to a place of lower concentration of water-either into or out of the cell.
If the concentration of water outside the cell is lower than the concentration inside the cytoplasm, water diffuses out of the cell, causing it to shrivel. If the concentration of water inside the cytoplasm is lower than the concentration outside the cell, water diffuses into the cell, causing it to swell.
Some substances need to move into or out of a cell, even though their concentration is higher in the direction they are moving. Therefore the cell must be able to move materials by another process. This is accomplished by active transport. During active transport, the cell moves molecules from an area of lower concentration to an area of higher concentration. Because this movement opposes the natural flow to areas of lower concentration, carrier proteins in the cell membrane pick up and transport materials. This work requires energy.