Observing and Comparing Cells

Once students figure out how evidence in the fossil record is used to support the claim that life has evolved, they then focus on the phenomena of cellular structures across three different modern organisms, looking for similarities and differences. They connect their comparisons with their analysis of evolutionary relationships, constructing an explanation for how similarities in different structures and DNA support the theory of evolution.

• As life has evolved over history, it has become more complex and diverse, resulting in the many millions of species alive today.
• Scientists believe that all life is related, with some species more closely related than others.
• A cell is the smallest unit of life. All living things are made up of cells, either a single cell or many different cells. Cells are alive because they use energy to carry out life functions, which include growing, developing, and getting energy from food.
• Plant cells have chloroplasts, which contain a green pigment called chlorophyll. This pigment gives plant cells their green color. It also absorbs sunlight, which begins a process of turning carbon dioxide and water into a kind of sugar called glucose, which plants use for energy, growth, and development.
• In cells called prokaryotes, DNA floats loosely in the cytoplasm, while in eukaryotic cells, DNA is stored in a structure called the nucleus.
• Animals, plants, fungi, and protists are all eukaryotic organisms alive today. Eukaryotes have a nucleus and other membrane-bound organelles to carry out the cell’s life functions. Organelles are small, specialized parts that carry out specific functions in a cell.
• Scientists look for similarities and differences among plants, animals, and fungi to determine their evolutionary relationships.
• Fungi are more closely related to animals than they are to plants.
  
  

• How are cells related to living things?
• How is the structure of all cells similar?
• How are eukaryotes different from prokaryotes?
• What organelles do all plant, animal, fungi, and protist cells have in common?
• How are plant cells different from any other kind of cell?
• Why did scientists think for many years that fungi were more closely related to plants than to animals?
• What evidence told scientists that fungi are actually more closely related to animals than to plants?
• How can DNA help to support evolutionary relationships?
  
  

Misconception: In natural selection, organisms “try” to adapt so they can get what they need. Mushrooms and other fungi are a type of plant.

Fact: The genetic variations that lead to natural selection occur randomly. Natural selection also depends on the environment because the organisms with genes best suited to their environment will survive and reproduce. Fungi are a kingdom of their own, in addition to the animal and plant kingdoms.

animal : a kingdom of eukaryotic organisms that eat other organisms for energy, breathe oxygen, and undergo growth and reproduction

cell : the smallest unit of life; makes up single-celled and multi-celled organisms; surrounded by a cell membrane and filled with cytoplasm

eukaryotes :  single or multi-celled organisms that have a nucleus and membrane-bound organelles

fungi : a kingdom of eukaryotic organisms that decompose and absorb nutrients from the organic material they grow in

nucleus : a membrane-bound organelle that holds an organism’s genetic material (DNA)

organelle : a small, specialized part that carries out specific functions in a cell

plant : a kingdom of eukaryotic organisms; includes organisms that contain chloroplasts in their cells to capture energy from sunlight for growth and development

Yeast in Space

In 2011, the space shuttle Atlantis blasted off from Earth. In addition to astronauts, it also carried samples of baker’s yeast. This is the same yeast that bakers add to bread to make it rise.

Baker’s yeast is alive. It is a kind of fungus. Like all living things, baker’s yeast is made up of cells. A cell is the smallest unit of life. Cells are alive because they use energy to carry out life functions. Life functions include growing, developing, and getting energy from food. Like all matter, cells are made up of molecules. Living things take in new molecules from air, food, and liquids. These molecules get into the cells. They help them function. Energy is required because it powers all of these actions.

Some organisms, including baker’s yeast, are made up of just one cell. Other organisms, including plants, snakes, and humans, are made up of hundreds, thousands, or millions of cells.

Relationship between Humans and Yeast

Atlantis carried samples of baker’s yeast to deliver them to the International Space Station. The space station is a moving science laboratory that orbits Earth. It is a low-gravity environment. Scientists on the space station wanted to study the yeast cells in low gravity. They care about this because there are many similarities between yeast cells and human cells. Scientists want to learn how the health of humans who spend a lot of time in low-gravity environments might be affected.

Similarities Among Cells

Yeast cells and human cells are similar because they are related to one another. Before we explore that idea, however, we need to first begin with the basic structure of all cells. Cells come in a variety of shapes and sizes, but all cells are surrounded by a cell membrane. The cell membrane is a protective membrane that surrounds a cell. It selects what molecules can enter and exit the cell. All cells are also filled with cytoplasm. Cytoplasm is a jelly-like liquid that fills a cell and holds its structures.

And finally, all cells have genetic information stored in DNA. DNA stands for deoxyribonucleic (pronounced dee-ox-ee-rye- bo-new-clay-ick) acid. It holds the instructions for making proteins. Proteins are large, complex molecules. They play many important roles in the body.

Plant, Animal, or Fungi?

Picture a plant, a fungus, and an animal. You will likely imagine three very different organisms. These differences begin at the level of the cell. Even though the cells of plants, animals, and fungi share many of the same structures, there are also important differences.The differences between plants, fungi, and animals begin with their cells.

To begin, plant cells have cell walls. The cell wall is a stiff, shell-like structure. It surrounds the cell membrane of plant cells and helps plants stand up. Plant cell walls are made of cellulose. Cellulose is a type of sugar molecule.

Fungi cells also have a cell wall. However, their cell wall is made of chitin (pronounced KITE- in). Chitin is the same molecule that makes crab and lobster shells hard. The cell wall is a hard covering that waterproofs fungi cells. It gives them their rigid, threadlike shape.

Animal cells don’t have cell walls. Instead, animals often have skeletons to help support them. Insects have exoskeletons. Because animal cells are not confined by a rigid cell wall, they have evolved more kinds and shapes of cells. This includes those that allow them to move.

Relationships Among Cells

Scientists can compare the similarities and differences among plant, animal, and fungi cells to find out how closely related they are. For example, for many years, scientists thought that fungi were a kind of plant. Think about the external structures of a fungus like a mushroom and a plant. It makes sense that they would seem so closely related. Neither can move and both grow in the ground. This is different from animals. Animals move around to access food, find mates, and avoid predators.

Many multi-celled fungi also grow threadlike structures called hyphae. Hyphae can fuse together into a network of tissues called mycelium. Tissues are made up of cells that live close together and do the same job. Mycelia look similar to plant roots. However, in the late 1990s, scientists made a startling discovery. They realized that fungi are actually much more closely related to animals than they are to plants.

Genetic Comparisons

It took a long time to figure out the evolutionary relationship between animals, plants, and fungi. This discovery required modern technology. Scientists needed to be able to compare the genetic information of plants, animals, and fungi. Remember that every living thing has its genetic information stored in DNA. The exact sequence of an organism’s DNA is unique to it. No other living thing has the exact identical DNA sequence. But all organisms within a species have very similar DNA. A species is a group of genetically similar organisms. For example, humans are a species. Snakes are a species. Baker’s yeast are also a species.

All people have 99.9 percent of their genetic makeup in common. This means that all of the differences among all of the people in the world comes from just 0.1 percent of our DNA. Scientists compare the DNA sequence of different species to infer evolutionary relationships. The more similar the DNA is between two species, the more closely related they are.

In the 1990s, scientists were able to compare the genes of different animal and fungal species. They found that around 1.1 billion years ago, plants evolved into a different species from animals and fungi. At some later point, animals and fungi separated on the evolutionary tree. Scientists still don’t know when this animal-fungi split happened. The chitin in both fungi cell walls and crab shells is also evidence that animals and fungi share a common ancestor and then evolved into different groups after plants evolved into their own group.

Studying Yeast for Medicine

The similarities between human genes and yeast genes led scientists to conduct experiments on yeast cells to better understand human cells. For example, studying yeast cells helped scientists better understand the connections between genes and proteins.

Additionally, at least 20 percent of human genes known to be involved in disease are also found in yeast. There are many ways scientists can use these similarities to help improve human health. These ways include testing new drugs on yeast cells to see how effective the drugs are before trying them on people. This is also why yeast cells are currently orbiting Earth on the International Space Station as scientists monitor the effects of low gravity.

For the hands-on activity of this lesson, students figure out differences in cellular phenomena in plants, animals, and fungus. Students use a microscope to gather data and compare these three different kinds of cells to investigate the relationship between the structure and function of different organisms’ cells.