Cell

(Nerve Cell)

All living organisms, whether made up of one cell or many cells, have some degree of organization. A cell is the smallest unit that can perform all life's processes. Some organisms, such as bacteria, are made up of one cell and are called unicellular (YOON-uh-SEL-yoo-luhr) organisms. Other organisms, such as humans or trees, are made up of multiple cells and are called multicellular (MUHL-ti-SEL-yoo-luhr) organisms. Complex multicellular organisms have the level of organization shown in Figure 1-2. In the highest level, the organism is made up of organ systems, or groups of specialized parts that carry out a certain function in the organism. For example, an owl's nervous system is made up of a brain, sense organs, nerve cells, and other parts that sense and respond to the owl's surroundings.

Organ systems are made up of organs. Organs are structures that carry out specialized jobs within an organ system. An owl's ear is an organ that allows the owl to hear. All organs are made up of tissues. Tissues are groups of cells that have similar abilities and that allow the organ to function. For example, nervous tissue in the ear allows the ear to detect sound. Tissues are made up of cells. A cell must be covered by a membrane, contain all genetic information necessary for replication, and be able to carry out all cell functions.

Within each cell are organelles. Organelles are tiny structures that carry out functions necessary for the cell to stay alive. Organelles contain biological molecules, the chemical compounds that provide physical structure and that bring about movement, energy use, and other cellular functions. All biological molecules are made up of atoms. Atoms are the simplest particle of an element that retains all the properties of a certain element.

Word Roots and Origins cell from the Latin, cella meaning "small room," or "hut"

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www.scilinks.org Topic: Characteristics of Life Keyword: HM60257

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Maintained by the National Science Teachers Association

Response to Stimuli

Another characteristic of life is that an organism can respond to a stimulus—a physical or chemical change in the internal or external environment. For example, an owl dilates its pupils to keep the level of light entering the eye constant. Organisms must be able to respond and react to changes in their environment to stay alive.

Homeostasis

All living things, from single cells to entire organisms, have mechanisms that allow them to maintain stable internal conditions. Without these mechanisms, organisms can die. For example, a cell's water content is closely controlled by the taking in or releasing of water. A cell that takes in too much water will rupture and die. A cell that doesn't get enough water will also shrivel and die.

Homeostasis (HOH-mee-OH-STAY-sis) is the maintenance of a stable level of internal conditions even though environmental conditions are constantly changing. Organisms have regulatory systems that maintain internal conditions, such as temperature, water content, and uptake of nutrients by the cell. In fact, multi-cellular organisms usually have more than one way of maintaining important aspects of their internal environment. For example, an owl's temperature is maintained at about 40°C (104°F). To keep a constant temperature, an owl's cells burn fuel to produce body heat. In addition, an owl's feathers can fluff up in cold weather. In this way, they trap an insulating layer of air next to the bird's body to maintain its body temperature.

Metabolism

Living organisms use energy to power all the life processes, such as repair, movement, and growth. This energy use depends on metabolism (muh-TAB-uh-Llz-uhm). Metabolism is the sum of all the chemical reactions that take in and transform energy and materials from the environment. For example, plants, algae, and some bacteria use the sun's energy to generate sugar molecules during a process called photosynthesis. Some organisms depend on obtaining food energy from other organisms. For instance, an owl's metabolism allows the owl to extract and modify the chemicals trapped in its nightly prey and use them as energy to fuel activities and growth.

Growth and Development

All living things grow and increase in size. Some nonliving things, such as crystals or icicles, grow by accumulating more of the same material of which they are made. In contrast, the growth of living things results from the division and enlargement of cells. Cell division is the formation of two new cells from an existing cell, as shown in Figure 1-3. In unicellular organisms, the primary change that occurs following cell division is cell enlargement. In multi-cellular life, however, organisms mature through cell division, cell enlargement, and development.

Development is the process by which an organism becomes a mature adult. Development involves cell division and cell differentiation, or specialization. As a result of development, an adult organism is composed of many cells specialized for different functions, such as carrying oxygen in the blood or hearing. In fact, the human body is composed of trillions of specialized cells, all of which originated from a single cell, the fertilized egg.

Materials 500 mL beakers (3), wax pen, tap water, thermometer, ice, hot water, goldfish, small dip net, watch or clock with a second hand

Observing Homeostasis

Materials 500 mL beakers (3), wax pen, tap water, thermometer, ice, hot water, goldfish, small dip net, watch or clock with a second hand

Procedure

1. Use a wax pen to label three 500 mL beakers as follows: 27°C (80°F), 20°C (68°F), 10°C (50°F). Put 250 mL of tap water in each beaker. Use hot water or ice to adjust the temperature of the water in each beaker to match the temperature on the label.

2. Put the goldfish in the beaker of 27°C water. Record the number of times the gills move in 1 minute.

3. Move the goldfish to the beaker of 20°C water. Repeat observations. Move the goldfish to the beaker of 10°C. Repeat observations.

Analysis What happens to the rate at which gills move when the temperature changes? Why? How do gills help fish maintain homeostasis?

figure 1-3

This unicellular organism, Escherichia coli, inhabits the human intestines. E. coli reproduces by means of cell division, during which the original cell splits into two identical offspring cells.

figure 1-3

This unicellular organism, Escherichia coli, inhabits the human intestines. E. coli reproduces by means of cell division, during which the original cell splits into two identical offspring cells.

Reproduction

All organisms produce new organisms like themselves in a process called reproduction. Reproduction, unlike other characteristics, is not essential to the survival of an individual organism. However, because no organism lives forever, reproduction is essential for the continuation of a species. Glass frogs, as shown in Figure 1-4, lay many eggs in their lifetime. However, only a few of the frogs' offspring reach adulthood and successfully reproduce.

During reproduction, organisms transmit hereditary information to their offspring. Hereditary information is encoded in a large molecule called deoxyribonucleic acid, or DNA. A short segment of DNA that contains the instructions for a single trait of an organism is called a gene. DNA is like a large library. It contains all the books—genes—that the cell will ever need for making all the structures and chemicals necessary for life.

Hereditary information is transferred to offspring during two kinds of reproduction. In sexual reproduction, hereditary information recombines from two organisms of the same species. The resulting offspring are similar but not identical to their parents. For example, a male frog's sperm can fertilize a female's egg and form a single fertilized egg cell. The fertilized egg then develops into a new frog.

In asexual reproduction, hereditary information from different organisms is not combined; thus the original organism and the new organism are genetically the same. A bacterium, for example, reproduces asexually when it splits into two identical cells.

Change Through Time

Although individual organisms experience many changes during their lifetime, their basic genetic characteristics do not change. However, populations of living organisms evolve or change through time. The ability of populations of organisms to change over time is important for survival in a changing world. This factor is also important in explaining the diversity of life-forms we see on Earth today.

figure 1-4

Like many animal species, this glass frog, Centrolenella sp., produces and lays a large number of eggs. However, a high percentage of these eggs die. In contrast, the offspring of animals that give birth to just a few live offspring typically have a high rate of survival.

figure 1-4

Like many animal species, this glass frog, Centrolenella sp., produces and lays a large number of eggs. However, a high percentage of these eggs die. In contrast, the offspring of animals that give birth to just a few live offspring typically have a high rate of survival.

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