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C6 - Cell Organelles and Parts

I can differentiate and describe the structure and function of cell organelles and parts

Introduction

Your body is made up of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves—and you—alive? To answer these questions, you need to know more about the structures that make up cells.

Overview of Cell Structures

In some ways, a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and other organelles. Figure below shows the structures inside a typical eukaryotic cell, in this case the cell of an animal. Refer to the figure as you read about the structures below. You can also explore the structures of an interactive animal cell at this link: http://www.cellsalive.com/cells/cell_model.htm.

Animal Cell. This animal cell consists of cytoplasm enclosed within a plasma membrane. The cytoplasm contains many different organelles.

The Plasma Membrane

The plasma membrane forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It protects and supports the cell and also controls everything that enters and leaves the cell. It allows only certain substances to pass through, while keeping others in or out. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability. To understand how the plasma membrane controls what crosses into or out of the cell, you need to know its composition.

The plasma membrane is discussed at http://www.youtube.com/watch?v=-aSfoB8Cmic (6:16). The cell wall (see below) is also discussed in this video.

The Phospholipid Bilayer

The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in Figure below, each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell. Molecules that are hydrophobic can easily pass through the plasma membrane, if they are small enough, because they are water-hating like the interior of the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane—at least not without help—because they are water-loving like the exterior of the membrane.

Phospholipid Bilayer. The phospholipid bilayer consists of two layers of phospholipids (left), with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. A single phospholipid molecule is depicted on the right.

Other Molecules in the Plasma Membrane

The plasma membrane also contains other molecules, primarily other lipids and proteins. The green molecules in Figure above, for example, are the lipid cholesterol. Molecules of cholesterol help the plasma membrane keep its shape. Many of the proteins in the plasma membrane assist other substances in crossing the membrane.

Extensions of the Plasma Membrane

The plasma membrane may have extensions, such as whip-like flagella or brush-like cilia. In single-celled organisms, like those shown in Figure below and Figure below, the membrane extensions may help the organisms move. In multicellular organisms, the extensions have other functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose.

Flagella

Cilia. Cilia and flagella are extensions of the plasma membrane of many cells.

Cytoplasm and Cytoskeleton

The cytoplasm consists of everything inside the plasma membrane of the cell. It includes the watery, gel-like material called cytosol, as well as various structures. The water in the cytoplasm makes up about two thirds of the cell’s weight and gives the cell many of its properties.

Functions of the Cytoplasm

The cytoplasm has several important functions, including

  1. suspending cell organelles
  2. pushing against the plasma membrane to help the cell keep its shape
  3. providing a site for many of the biochemical reactions of the cell

Cytoskeleton

Crisscrossing the cytoplasm is a structure called the cytoskeleton, which consists of thread-like filaments and tubules. You can see these filaments and tubules in the cells in Figure below. As its name suggests, the cytoskeleton is like a cellular “skeleton.” It helps the cell maintain its shape and also holds cell organelles in place within the cytoplasm.

Cytoskeleton. The cytoskeleton gives the cell an internal structure, like the frame of a house. In this photograph, filaments and tubules of the cytoskeleton are green and red, respectively. The blue dots are cell nuclei.

The cytoskeleton is discussed in the following video http://www.youtube.com/watch?v=5rqbmLiSkpk&feature=related (4:50).

The Nucleus and Other Organelles

Eukaryotic cells contain a nucleus and several other types of organelles. These structures are involved in many vital cell functions.

The Nucleus

The nucleus is the largest organelle in a eukaryotic cell and is often considered to be the cell’s control center. This is because the nucleus controls which proteins the cell makes. The nucleus of a eukaryotic cell contains most of the cell’s DNA, which makes up chromosomes and is encoded with genetic instructions for making proteins.

Mitochondria

The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cell. This is why mitochondria are sometimes referred to as the power plants of the cell. They use energy from organic compounds such as glucose to make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy. Scientists think that mitochondria were once free-living organisms because they contain their own DNA. They theorize that ancient prokaryotes infected (or were engulfed by) larger prokaryotic cells, and the two organisms evolved a symbiotic relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the prokaryotes became permanent guests of the larger cells, as organelles inside them. This theory is called the endosymbiotic theory, and it is widely accepted by biologists today

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. There are two types of endoplasmic reticulum: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in Figure below.

  • RER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins.
  • SER looks smooth because it does not have ribosomes. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER. SER also makes lipids, stores substances, and plays other roles.

Ribosomes

Ribosomes are small organelles where proteins are made. They contain the nucleic acid RNA, which assembles and joins amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm as well as on the RER.

Golgi Apparatus

The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is shown in Figure below. The Golgi apparatus is somewhat like a post office. It receives items (proteins from the ER), packages and labels them, and then sends them on to their destinations (to different parts of the cell or to the cell membrane for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell. At the link below, you can watch an animation showing how the Golgi apparatus does all these jobs. http://www.johnkyrk.com/golgiAlone.html

This drawing includes the nucleus, RER, SER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.

Vesicles and Vacuoles

Both vesicles and vacuoles are sac-like organelles that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus (see Figure above) store and transport protein and lipid molecules. Some vesicles are used as chambers for biochemical reactions. Other vesicles include:

  • Lysosomes, which use enzymes to break down foreign matter and dead cells.
  • Peroxisomes, which use oxygen to break down poisons.

Centrioles

Centrioles are organelles involved in cell division. They help organize the chromosomes before cell division so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells and are located near the nucleus (see Figure above).

Special Structures in Plant Cells

Plant cells have several structures that are not found in animal cells, including a cell wall, a large central vacuole, and organelles called plastids. You can see each of these structures in Figure below. You can also view them in an interactive plant cell at the link below. http://www.cellsalive.com/cells/cell_model.htm

Plant Cell. In addition to the organelles and other structures found inside animal cells, plant cells also have a cell wall, a large central vacuole, and plastids such as chloroplasts. Can you find each of these structures in the figure?

Cell Wall

The cell wall is a rigid layer that surrounds the plasma membrane of a plant cell. It supports and protects the cell. Tiny holes, or pores, in the cell wall allow water, nutrients, and other substances to move into and out of the cell. The cell wall is made up mainly of complex carbohydrates, including cellulose.

Central Vacuole

Most mature plant cells have a large central vacuole. This vacuole can make up as much as 90% of the cell’s volume. The central vacuole has a number of functions, including storing substances such as water, enzymes, and salts. It also helps plant tissues, such as stems and leaves, stay rigid and hold their shape. It even helps give flowers, like the ones in Figure below, their beautiful colors.

These flowers are red because of red pigment molecules in the central vacuoles of their cells. The bright colors are an important adaptation. They help the flowers attract pollinators such as hummingbirds so the plants can reproduce.

Plastids

Plastids are organelles in plant cells that carry out a variety of different functions. The main types of plastids and their functions are described below.

  • Chloroplasts are plastids that contain the green pigment chlorophyll. They capture light energy from the sun and use it to make food. A chloroplast is shown in Figure above.
  • Chromoplasts are plastids that make and store other pigments. The red pigment that colors the flower petals in Figure above was made by chromoplasts.
  • Leucoplasts are plastids that store substances such as starch or make small molecules such as amino acids.

Like mitochondria, plastids contain their own DNA. Therefore, according to endosymbiotic theory, plastids may also have evolved from ancient, free-living prokaryotes that invaded larger prokaryotic cells. If so, they allowed early eukaryotes to make food and produce oxygen.


Lesson Summary

  • The plasma membrane is a phospholipid bilayer that supports and protects a cell and controls what enters and leaves it.
  • The cytoplasm consists of everything inside the plasma membrane, including watery cytosol and organelles. The cytoplasm suspends the organelles and does other jobs. The cytoskeleton crisscrosses the cytoplasm and gives the cell an internal framework.
  • The nucleus is the largest organelle in a eukaryotic cell and contains most of the cell’s DNA. Other organelles in eukaryotic cells include the mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus, vesicles, vacuoles, and centrioles (in animal cells only). Each type of organelle has important functions in the cell.
  • Plant cells have special structures that are not found in animal cells, including a cell wall, a large central vacuole, and organelles called plastids.

Lesson Review Questions

Recall

1. Describe the composition of the plasma membrane.

2. List functions of the cytoplasm and cytoskeleton.

3. What is the role of the nucleus of a eukaryotic cell?

4. List three structures that are found in plant cells but not in animal cells.

Think Critically

5. Explain why hydrophobic (“water-hating”) molecules can easily cross the plasma membrane, while hydrophilic (“water-loving”) molecules cannot.

6. What is endosymbiotic theory? How does it explain the presence of certain organelles in eukaryotic cells?

7. Explain how the following organelles ensure that a cell has the proteins it needs: nucleus, rough and smooth ER, vesicles, and Golgi apparatus.