Life Science: Cells

Life Science: Cells

All living organisms are made up of cells, but do you know what a cell is? In this lesson, you can learn about the three main structures that all cells contain and about the two major types of cells.

Cell Theory

A cell is often called the basic unit of life. To explain the main characteristics of cells, scientists have developed the cell theory.

Cell theory states three things:

• All organisms are made up of at least one cell.

• A cell is the smallest unit that can carry out all the functions necessary for life

• All cells arise from pre-existing cells.

Multiple Choice

Which of the following is NOT part of cell theory?

• All organisms are made up of many cells

• All organisms are made up of at least one cell

• Cells are the basic units of living things

• Cells always come from pre-existing cells

Cell Structure

• While there are many different kinds of cells, all cells have three main parts:

  Plasma membrane, DNA and Cytoplasm

Multiple Choice

All cells have which of the following structures?

• Plasma membrane, DNA, organelles

• Plasma membrane, DNA, nucleus

• Plasma membrane, DNA, cytoplasm

• Plasma membrane, nucleus, cytoplasm

Plasma Membrane

The plasma membrane or cell membrane is the outer portion of a cell. It's important for separating and protecting internal cellular structures from the surrounding environment. This allows a cell to have internal conditions that are very different from its surroundings.

This membrane is also important for allowing only certain substances to enter or leave the cell. This is called selective permeability. For example, nutrients have to be able to get into a cell so that it can survive. Waste substances must also be able to leave the cell.

Plasma Membrane

In some cells, the plasma membrane is not the very outermost cellular structure. It is the cell wall instead. Cell walls are external to the plasma membrane and provide additional support and protection. They can be found in plants, fungi, and bacteria.

DNA

DNA is short for deoxyribonucleic acid. It is the molecule that makes up genes, which carry important instructions for keeping a cell alive. A gene is the unit of heredity which is transferred from parents to offspring.

Cytoplasm

Everything else inside a cell is called the cytoplasm. It's made up of a jelly-like water-based fluid called cytosol. There are other non-fluid substances found in the cytoplasm, but they vary between types of cells.

Multiple Choice

Which of the following structures is selectively permeable?

• cell membrane

• DNA

• cytosol

• cytoplasm

Category of Cells

There are two main categories of cells: prokaryotic cells and eukaryotic cells.

Bacteria are very ancient and simple organisms. One individual is composed of only a single prokaryotic cell. These cells are very small and do not have any structures separating their DNA from the cytosol. In fact, they do not have any membrane-bound structures other than the plasma membrane. Additionally, the DNA of prokaryotic cells is arranged in a circle.

Eukaryotic vs. Prokaryotic

Eukaryotic means 'true nucleus.' A nucleus is a membranous structure surrounding the cell's DNA. Prokaryotes do not have a nucleus (the term prokaryotic means 'before a nucleus').

Eukaryotic vs. Prokaryotic

A eukaryotic cell has many membrane-bound structures called organelles in its cytoplasm. There are many different kinds of organelles, and each one is specialized to carry out a specific and unique function. For example, the mitochondrion is an organelle that breaks down nutrients to make energy. One more difference between eukaryotic and prokaryotic cells is that the DNA in eukaryotes is linear.

Multiple Choice

Which of the following statements is NOT true?

• Prokaryotic cells are much simpler than eukaryotic cells.

• Prokaryotic cells have a nucleus.

• Eukaryotic cells are larger than prokaryotic cells.

• Eukaryotic cells have a nucleus.

Multiple Choice

Human cells can be classified as which of the following?

• Neither prokaryotic nor eukaryotic cells

• Prokaryotic cells

• Both prokaryotic and eukaryotic cells

• Eukaryotic cells