Chapter 2: Basic Biological Principles Lesson 2.2 ...

Chapter 2: Basic Biological Principles Lesson 2.2: Structural and Functional Relationships at Biological Levels of Organization

Cells may be small in size, but they are extremely important to life. Like all other living things, you are made of cells. Cells are the basis of life, all organisms are made up of one or more cells. Cells of all living organisms have many of the same structures and carry out the same basic life processes. Knowing the structures of cells and the processes they carry out is necessary to understanding life itself. You will learn more about these amazing building blocks of life, like the dividing cell pictured above, when you read this chapter.

Lesson Objectives ? Describe the diversity of cell shapes, and explain why cells are so small. ? Identify the parts that all cells have in common. ? Describe the structure and function of the plasma membrane. ? Outline the form and function of the nucleus and other organelles. ? Compare and contrast prokaryotic and eukaryotic cells. ? Explain how cells are organized in living things. Describe the relationship between structure and function at various levels of biological organization.

Vocabulary ? endoplasmic reticulum endosymbiosis extracellular Golgi apparatus intracellular mitochondria (mitochondrion, singular) multicellular nucleus organ ? organelle organism organ system ? plasma membrane ? ribosome tissue unicellular

36

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 and how they function.

DIVERSITY OF CELLS

Today, we know that all living cells have certain things in common. For example, all cells share functions such as obtaining and using energy, responding to the environment, and reproducing. The function a cell must carry out influences its physical features and its internal organization. We also know that different types of cells--even within the same organism--may have their own unique functions as well. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary in size as well as shape, but all cells are very small. In fact, most cells are much smaller than the period at the end of this sentence. If cells have such an important role in living organisms, why are they so small? Even the largest organisms have microscopic cells. What limits cell size?

Cell Size The answer to these questions is clear once you know how a cell functions. To carry out life

processes, a cell must be able to quickly pass substances into and out of the cell. For example, it must be able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. It is this need to pass substances across the surface that limits how large a cell can be. Look at the two cubes in Figure 2.7. As this figure shows, a larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells; a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen and produces more wastes. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the harder it will be for the cell to get rid of its wastes and take in necessary substances. This is what limits the size of the cell.

Figure 2.7 Surface Area to Volume Comparison. A larger cube has a smaller surface area (SA) to volume (V) ratio than a smaller cube. This holds true for cells and limits how large they can be.

37

Cell Shape Cells with different functions often have different shapes. The cells pictured in Figure 2.8 are

just a few examples of the many different shapes that cells may have. Each type of cell in the figure has a shape that helps it do its job. For example, the job of the nerve cell is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail-like projections on the algae cells? Algae live in water, and their tails help them swim. Pollen grains have spikes that help them stick to insects such as bees. How do you think the spikes help the pollen grains do their job? (Hint: Insects pollinate flowers.)

Figure 2.8 As these pictures show, cells come in many different shapes. Clockwise from the upper left photo are a nerve cell, red blood cells, bacteria, pollen grains, and algae. How are the shapes of these cells related to their functions?

PARTS OF THE CELL COMMON TO ALL ORGANISMS

Although cells are diverse, all cells have certain parts in common. The parts include a plasma membrane, cytoplasm, ribosomes, and DNA.

1. The plasma membrane (also called the cell membrane) is a thin coat of phospholipid and protein molecule bilayer that surrounds a cell . It forms the physical boundary between the cell and its environment, so you can think of it as the ``skin" of the cell. It controls the movement of materials in and out of the cell through either active or passive transport mechanisms.

2. Cytoplasm refers to all of the cellular material inside the plasma membrane. Cytoplasm is made up of a watery substance called cytosol and contains other cell structures such as ribosomes.

3. Ribosomes are cellular structures either in the cytoplasm or attached to the rough endoplasmic reticulum composed of RNA (ribonucleic acid) and proteins where proteins are made in eukaryotic and prokaryotic cells.

4. DNA is a nucleic acid molecule found in cells. It contains the genetic instructions that cells need to make proteins, carry out live processes, and pass on inheritable characteristics.

These parts are common to all cells, from organisms as different as bacteria and human beings. How did all known organisms come to have such similar cells? The similarities show that all life on Earth has a common evolutionary history.

38

The Plasma Membrane The plasma membrane forms a barrier between the cytoplasm inside the cell (intracellular) and

the environment outside the cell (extracellular). It protects and supports the cell, controls everything that enters and leaves the cell, and recognizes chemical signals. 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 semi-permeability. To understand how the plasma membrane controls what crosses into or out of the cell, you need to know its composition. The cell membrane consists of two layers of phospholipids with proteins embedded within these layers. The surface of the cell contains molecules which recognize other molecules which may attach to or enter the cell, see Figure 2.9. The plasma membrane is discussed at .

Figure 2.9 Plasma membrane or cell membrane is often referred to as a lipid bilayer.

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 2.10, 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.

Figure 2.10 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

yellow molecules in Figure 2.9, for example, are the lipid cholesterol. Molecules of cholesterol help the plasma membrane keep its shape. Many of the proteins, the blue molecules in Figure 2.9, in the plasma membrane assist other substances in crossing the membrane. Glycoproteins and surface carbohydrates

39

serve as cell receptors, points of attachment, for other cells, infectious bacteria, viruses, toxins, hormones, and many other molecules.

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 2.11 and Figure 2.12, 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.

Figure 2.11 Flagella on bacteria cells aid in cell movement.

Figure 2.12 Cilia are extensions of the plasma membrane of many cells.

Cytoplasm The cytoplasm consists of the fluid, the cytoskeleton, and all the membrane-bound organelles

except the nucleus. The part of the cytoplasm that contains molecules and small particles, like ribosomes is called the cytosol. 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 threadlike

filaments and tubules. You can see these filaments and tubules in the cells in Figure 2.13. 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 of cells that do have organelles. Some unicellular organisms do not have organelles, they do not have a cytoskeleton. The cytoskeleton is discussed in the following video .

Figure 2.13 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.

40

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download