DTW GED PREP

CELLS, TISSUES, AND ORGANS

All living things—from the smallest bacteria to the largest animals—are made up of cells. Cells are the basic building blocks of life. Everything that happens in an organism—breathing, growing, moving, or even thinking—begins with the work of cells. Scientists explain the connection between cells and life through a foundational idea in biology called the cell theory. This theory has three main principles:

•  All living things are made of one or more cells.
•  Cells are the basic units of structure and function in living things.
• All new cells come from pre-existing cells.

These ideas might seem obvious today, but until the invention of the microscope in the late 1500s, people didn’t know that cells even existed. Around 1590, scientists developed the first microscopes, which allowed them to observe tiny structures that were previously invisible. Over the next 200 years, new discoveries helped scientists better understand the role of cells. By the early 1800s, they had gathered enough evidence to form the cell theory, which remains one of the cornerstones of modern biology.

Specialized Cells and Tissues

In multicellular organisms like humans, not all cells are the same. Each type of cell has a specific structure that allows it to perform a unique function. These are called specialized cells. For example:

• Muscle cells are long and flexible so they can contract and allow movement.
• Nerve cells (or neurons) have long branches that help them send messages across the body.
• Red blood cells are disc-shaped and smooth so they can carry oxygen through narrow blood vessels.
• White blood cells change shape to help fight infections.
• Skin cells are flat and tightly packed to create a protective barrier.

The structure of each type of cell is designed to help it do its job well. This is an important biological concept: structure is related to function.

A labeled illustration of various human cell types, highlighting their shapes and explaining how their structure helps them function.

When groups of specialized cells work together to perform the same task, they form tissues. There are four main types of tissues in the human body:

• Connective tissue connects and supports other tissues. Examples include bone, cartilage, and blood.
• Epithelial tissue covers the surface of the body and lines internal organs. Skin is made of epithelial tissue.
• Muscle tissue helps the body move by contracting.
• Nervous tissue carries signals between the brain and the rest of the body.

Illustration of the four major tissue types with labeled examples.

Levels of Organization

The human body is organized into several levels, from the simplest to the most complex:

• Cells – the basic units of life.
•  Tissues – groups of similar cells that perform a shared function.
• Organs – structures made of different tissues working together to perform specific tasks.
• Organ systems – groups of organs that work together to carry out major body functions.

For example, the cardiovascular system includes the heart, blood, and blood vessels. The heart is an organ made of cardiac muscle tissue, which is made of specialized muscle cells that contract rhythmically to pump blood. Each level of organization builds on the one before it, creating a highly coordinated and functional body.

A “levels of organization” pyramid or flowchart, showing cells → tissues → organs → organ systems → organism.

Cell Functions and Components

Although different cells in the body have specialized jobs, all cells carry out certain basic life functions. Two of the most important are:

• Metabolism: Metabolism is the name for all the chemical reactions that happen inside cells. These reactions do two main things; Break down food molecules to release energy and use that energy to build substances the cell needs, such as proteins. Without metabolism, cells wouldn’t be able to function or survive.
• Reproduction: Cells reproduce through a process called cell division. In this process, one cell splits to form two new cells. These new cells are calle

daughter cells, and they are identical to the original parent cell.

Cell Components

Just as the human body is made of organs, each cell is made up of organelles—small structures inside the cell, each with its own function. For example:

• The nucleus holds the cell’s DNA and controls its activities.
• Mitochondria produce energy for the cell.
• Ribosomes help build proteins.

The cell membrane surrounds the cell and controls what enters and exits.These parts all work together to keep the cell alive and functioning. Another key player in the cell is the enzyme. Enzymes are proteins that speed up chemical reactions. Without enzymes, cells would not be able to carry out life-sustaining reactions quickly enough.

A labeled diagram of a typical animal cell, showing major organelles and their functions.

Cell Division

The Cell Cycle

Cell division happens in three main stages:

1. The cell copies its DNA.

2. The nucleus splits into two, each containing identical DNA.

3. The cell divides, forming two identical daughter cells.

Mitosis

The second stage—when the nucleus divides—is called mitosis. Mitosis has four main phases:

• Prophase – DNA coils into chromosomes, and the nuclear membrane disappears.
• Metaphase – Chromosomes line up in the center of the cell.
• Anaphase – Chromosomes are pulled apart to opposite ends.
• Telophase – New nuclear membranes form, and the cell begins to split.
• 
  After mitosis, the result is two genetically identical cells, each with a full set of DNA.

A step-by-step illustration of the four phases of mitosis.

Meiosis: Cell Division for Reproduction

Meiosis is a special kind of cell division used to make reproductive cells—egg and sperm. These cells must have half the number of chromosomes as normal body cells so that when they combine, the resulting cell has the correct number.

Meiosis occurs in two stages: Meiosis I and Meiosis II. Together, they reduce the DNA by half and create four unique reproductive cells.

Meiosis I :

• Prophase I – Chromosomes condense, and matching pairs exchange segments (crossing over).
• Metaphase I – Chromosome pairs line up.
• Anaphase I – One chromosome from each pair moves to opposite sides.
• Telophase I – The cell divides into two.

Meiosis II:

• Prophase II – The two cells prepare to divide again (no DNA is copied).
• Metaphase II – Chromosomes line up again.
• Anaphase II – Chromosomes split into chromatids.
• Telophase II – The cells divide once more, creating four cells.

Each resulting cell has half the DNA of a normal body cell and is genetically unique.

A two-part diagram showing Meiosis I and Meiosis II, with key events at each stage and comparison to mitosis.