Download Unit A - apel slice

Lesson 1
What Are Cells?
Fast Fact
---see picture
Nature's Kaleidoscope The beautiful designs on this page may look like those in a
kaleidoscope, but these designs occur in nature. This photograph of tiny, singlecelled organisms called diatoms was taken through a microscope. In the
Investigate, you'll observe other cells through a microscope.
Observing Cells
Materials
 dropper
 paper towels
 red food coloring
 microscope
 microscope slide
 colored pencils
 slice of onion
 prepared slide of animal skin
 coverslip
Procedure
CAUTION: Food coloring stains. Avoid getting it on your clothing.
1. Use the dropper to place one drop of food coloring in the center of the slide. Break
the onion slice, and pull off a piece of onion skin. Put the onion skin in the drop of
food coloring. Gently lower the coverslip at an angle so that it spreads the food
coloring. Use a paper towel to remove any excess food coloring.
2. Observe the onion skin cells under the microscope. Use the colored pencils to record
your observations in a drawing.
3. Observe the prepared slide of the animal skin cells. Use colored pencils to record
your observations in another drawing.
Draw Conclusions
1. Compare the onion skin cells and the animal skin cells.
2. Inquiry Skill When scientists observe, they use their senses to learn about objects
and events. In the center of most cells are structures that direct how the cells
function. Look for these structures. Based on what you observe, how many
directing structures do you think each cell has?
Investigate Further
In what ways do you think all plant cells are alike? Plan and conduct a simple
experiment to test your ideas.
31
Reading in Science
VOCABULARY
cell p.32
microscopic p. 32
organism p.32
cell membrane p.34
nucleus p. 34
cytoplasm p. 34
protist p. 36
SCIENCE CONCEPTS
 how living things are made of cells
 why different cells have different jobs
READING FOCUS SKILL
MAIN IDEA AND DETAILS Once you understand cells, look for details about each
cell type.
Main Idea
detail
detail
detail
Cells
You've probably seen cork stoppers in jars and cork bulletin boards. But did you know
that cork comes from the bark of an oak tree? In 1665, Robert Hooke, an English
scientist, observed a layer of cork through a microscope. He saw the same kind of
structures you observed in the onion skin. Because the structures he saw looked
like tiny rooms, Hooke named them cells.
The microscope you used in the Investigate enabled you to observe and compare cells
from an animal and a plant. A cell is the basic unit of structure and function in
living things. Most cells are microscopic—they can be seen only with a
microscope. Hooke's description of cells encouraged scientists to learn more about
them. Using microscopes, they found that all cells share some characteristics and
that different cells do different jobs. They also know that all organisms are made up
of cells. An organism is any living thing that maintains vital life processes.
---see picture
Robert Hooke's seventeenth-century microscope looks much different from a
modern microscope. But both can show cells in a layer of cork.
32
You may see hundreds of different organisms each day. Each of these is made up of
cells. Some simple organisms are just a single cell. But most plants and animals are
made up of huge numbers of cells.
Plants and animals have different types of cells, each with its own job. For example,
your body has digestive cells, which help you break down food, and nerve cells,
which help you sense and respond to your surroundings. Different types of cells
work together to carry out functions that keep an organism alive. To carry out its
own functions, each cell has structures called organelles, which help keep the cell
alive.
MAIN IDEA AND DETAILS How do cells keep organisms alive and healthy?
Insta-Lab
A Cell Model
Carefully use a plastic knife to cut a peeled hard-boiled egg in half. Compare it to the
animal skin cells you observed in the Investigate. Why is an egg a good model for
animal cells?
---see pictures
The single cell of this amoeba carries out all the functions the organism needs to
stay alive.
The outer cells of this plant's leaf help keep the plant from losing too much water.
Inner cells of the leaf make food for the plant. Each plant cell also carries out
life functions for itself.
A salamander's skin cells don't look like a plant's outer cells, but—like a plant's
outer cells—they help keep the organism from drying out. As in the plant,
each cell carries out life functions for itself.
33
Plant and Animal Cells
All cells have similar structures and organelles. Every cell is surrounded by a thin
covering called the cell membrane. This structure protects the cell, holds its
contents together, and controls what goes in and out of the cell. Most organelles are
also surrounded by membranes. Each type of organelle has a specific function that
helps the cell.
Most cells have a nucleus. The nucleus directs all of a cell's activities, including
reproduction. Inside the nucleus are chromosomes (KROH•m u h•so h mz),
threadlike structures that contain information about the characteristics of the cell.
When a cell reproduces, the nucleus divides and each new cell gets identical
chromosomes.
Between the cell membrane and the nucleus is a jellylike material called cytoplasm
(syt•oh•plaz•uhm). Cytoplasm contains chemicals that help keep a cell healthy.
Several kinds of organelles are suspended in the cytoplasm. Mitochondria are the
"powerhouses" of a cell. They release energy from nutrients. Vacuoles store
nutrients, water, or waste materials in plant cells. Smaller vesicles in animal cells
have similar functions.
Plant cells have structures not found in animal cells. A thick cell wall helps support a
plant cell. The cell wall lies outside the cell membrane. In the cytoplasm of many
plant cells are chloroplasts. Chloroplasts make food for plant cells.
* MAIN IDEA AND DETAILS What purposes does the cell membrane serve?
Science Up Close
Comparing Plant and Animal Cells
Plant and animal cells have certain structures in common. They have a nucleus,
cell membrane, cytoplasm, and organelles. Study the differences between plant
and animal cells.
---see diagram
Plant cells have different sizes, shapes, and functions, but most have the same
organelles.
34
---see diagram
Like plant cells, animal cells have different sizes, shapes, and functions. They also
have most of the same organelles. How is an animal cell different from a plant
cell?
Cell Structures
Organelle
Nucleus
Function
Kind of Cell
directs a cell's activities
inside nucleus; contains
information about cell
holds a cell together and
separates it from its
surroundings
plant and animal
plant
Cytoplasm
supports, and protects a plantcell
a jellylike substance
containing chemicals that
help the cell stay healthy
Chioroplast
makes food for the cell
plant
Vacuole
stores food, water, or wastes
plant
Vesicle
stores food, water, or wastes
animal
Mitochondria
release energy from nutrients
plant and animal
Chromosome
Cell membrane
Cell wall
plant and animal
plant and animal
plant and animal
For more links and activities, go to www.hspscience.com
35
Pond water contains many single-celled organisms.
Single-Celled Organisms
What do you think of when you hear the word bacteria? Many people think of "germs."
But not all bacteria are harmful. Many of these single-celled organisms are helpful.
Some enrich the soil by breaking down dead plants and animals. Others help
animals digest food. Still others help make food! Cheese and yogurt are foods that
form when certain bacteria mix with milk.
Bacterial cells are different from plant and animal cells. Like plant cells, bacterial cells
are surrounded by cell walls. But bacteria do not have a nucleus or membranebound organelles. Instead, their chromosomes and other materials are in the
cytoplasm.
Another kind of single-celled organism makes up most of the group called protists. A
protist (pRoHT•ist) is a simple organism, usually a single cell, with a nucleus and
organelles. Some protists have cell walls and chloroplasts. These protists are
plantlike. Other protists have no cell walls or chloroplasts and are animal-like.
The diatoms shown at the beginning of this lesson are plantlike—they have chloroplasts
and make their own food. They are part of a group of protists called algae (AL•jee).
Algae live mostly in water and produce a lot of Earth's oxygen. They also produce
a lot of food for ocean life.
Animal-like protists are called protozoa. They get food by "eating" other small
organisms, such as algae and bacteria.
MAIN IDEA AND DETAILS How do bacterial cells differ from plant and animal
cells?
36
Reading Review
1. MAIN IDEA AND DETAILS Draw and complete the graphic organizer.
---see chart
2. SUMMARIZE Use your graphic organizer to write a lesson summary.
3. DRAW CONCLUSIONS You're looking at a slide through a microscope, and you
see cells with cell walls but without chloroplasts. What kind of cells are you
probably looking at? How do you know?
4. VOCABULARY Use lesson vocabulary and other lesson terms to make a crossword
puzzle.
Test Prep
5. Critical Thinking Suppose you observe cells through a microscope. How might you
infer whether the cells are animal cells?
6. What is the function of a cell's nucleus?
A. to make food
B. to direct the cell's activities
C. to protect the cell
D. to keep the cell healthy
Writing
Expository Writing
Imagine a cell as being a city. Write a description of how each part of the cell helps
carry out a task important for the maintenance of the city.
Math
Extend a Pattern
Imagine that a single cell divides into two cells every 15 minutes. If each of these cells
also divides in two, and so on, how long will it take for a single cell to produce 500
cells?
Language Arts
Word Parts
The word cytoplasm contains the word parts cyto- and -plasm. Use a dictionary to find
the meanings of the parts. How do those meanings relate to the meaning of the
whole word cytoplasm?
For more links and activities, go to www.hspscience.com
37