What Is a Cell and How Does It Work?
What Is a Cell and How Does It Work?
A cell is the smallest living unit of the human body. Cells take in nutrients, produce energy, make proteins, remove waste, respond to signals, and carry genetic information. About 30 trillion human cells work together to form tissues, organs, and organ systems that keep the body alive. (Hatton et al., 2023)
- What Is a Cell and Why Is It Called the Basic Unit of Life?
- What Are the Three Main Parts of a Human Cell?
- What Are Cell Organelles and What Do They Do?
- How Does a Cell Stay Alive and Carry Out Its Functions?
- How Does DNA Control What a Cell Does?
- Are All Cells the Same?
- Why Do Human Cells Have Different Shapes and Functions?
- How Do Cells Work Together to Form Tissues, Organs, and Organ Systems?
What Is a Cell and Why Is It Called the Basic Unit of Life?
Cells are called the basic units of life because all living things are made of one or more cells. A cell is also the smallest structure that can carry out the basic processes needed for life.
A single-celled organism, such as a bacterium, can live and reproduce on its own. Human cells are different. Most are specialized and depend on other cells, tissues, and organs for oxygen, nutrients, protection, and waste removal.
Everything the body does depends on cell activity. Thinking involves nerve cells. Movement depends on muscle cells. Oxygen is carried by red blood cells. Digestion depends on cells that produce enzymes and absorb nutrients.
How Small Are Human Cells?
Most human cells are far too small to see without a microscope. Scientists measure them in micrometers. One micrometer is one-thousandth of a millimeter.
A typical animal cell may be about 10 to 30 micrometers across. However, human cells vary greatly in size. A red blood cell is about 7 to 8 micrometers wide, while a human egg cell is about 120 to 150 micrometers wide. Some nerve cells have extensions that can reach more than 3 feet, or about 1 meter, in length.
For comparison, an average strand of human hair is about 70 micrometers wide. Several human cells could fit side by side across its width.
Light microscopes can show whole cells and some larger structures. Electron microscopes provide much finer detail and can reveal smaller structures inside cells.
How Many Cells Are in the Human Body?
The exact number depends on a person’s size, age, and sex. A major scientific estimate published in 2023 calculated about 36 trillion cells in a typical adult male, 28 trillion in a typical adult female, and 17 trillion in a child.
For a simple general estimate, the adult human body contains about 30 trillion human cells. Red blood cells make up most of this number. (Hatton et al., 2023)
Many cells are continually replaced. Skin cells are shed, blood cells are removed from circulation, and cells lining the intestines are renewed. Other cells, including many nerve cells, may remain for decades.
What Are the Three Main Parts of a Human Cell?
A typical human cell has three main parts: the cell membrane, the cytoplasm, and the nucleus. However, some specialized cells are different. Mature red blood cells, for example, do not have a nucleus.
Human Cell Parts at a Glance
| Cell Part | Main Function | Simple Comparison |
|---|---|---|
| Cell membrane | Controls movement into and out of the cell | Security gate |
| Cytoplasm | Contains the cell’s fluid and internal structures | Factory floor |
| Nucleus | Stores most of the cell’s DNA | Control center |
What Does the Cell Membrane Do?
The cell membrane is a thin, flexible barrier surrounding the cell. It is mainly made of two layers of fatty molecules called phospholipids, along with many proteins.
The membrane controls which materials enter or leave the cell. Oxygen and carbon dioxide can pass through it, while substances such as glucose, salts, and amino acids usually move through special channels or carrier proteins.
The cell membrane also contains receptors. These receptors allow the cell to detect hormones, nutrients, immune signals, and messages from nearby cells.
What Happens in the Cytoplasm?
The cytoplasm fills the area between the cell membrane and the nucleus. It includes a water-based fluid called cytosol and the structures suspended within it.
Many important chemical reactions occur in the cytoplasm. Nutrients are broken down, proteins are made, and materials are moved from one part of the cell to another. (OpenStax)
What Does the Nucleus Do?
The nucleus contains most of the cell’s DNA, which is organized into chromosomes. A double membrane called the nuclear envelope separates the nucleus from the cytoplasm.
Small openings in this membrane allow RNA, proteins, and other materials to move between the nucleus and the rest of the cell. The nucleus helps control cell activity by regulating which genetic instructions are used.
Do All Human Cells Have a Nucleus?
Most human cells have a nucleus, but there are important exceptions.
Mature red blood cells lose their nucleus and most other organelles as they develop. This creates more room for hemoglobin, the protein that carries oxygen. Platelets also have no nucleus because they are small cell fragments rather than complete cells.
What Are Cell Organelles and What Do They Do?
Organelles are specialized structures that perform particular jobs inside a cell. They divide the cell’s work into separate areas, making its activities more organized and efficient.
What Do Mitochondria Do?
Mitochondria produce most of the chemical energy used by human cells. They use energy released from nutrients to make adenosine triphosphate, or ATP.
ATP powers activities such as muscle contraction, movement of materials, protein production, and cell repair. Cells that need large amounts of energy, including heart and muscle cells, usually contain many mitochondria.
What Do Ribosomes Do?
Ribosomes are small structures that make proteins. They read instructions carried by messenger RNA and join amino acids together in the correct order.
Some ribosomes float freely in the cytoplasm. Others are attached to the rough endoplasmic reticulum. The proteins they produce may become enzymes, hormones, receptors, antibodies, or parts of cell structures.
What Do the Endoplasmic Reticulum and Golgi Apparatus Do?
The endoplasmic reticulum, or ER, is a network of folded membranes.
Rough ER has ribosomes attached to its surface. It helps produce, fold, and prepare proteins that will be used in cell membranes or sent outside the cell.
Smooth ER has no attached ribosomes. It makes lipids, stores calcium, and helps some cells process drugs and harmful chemicals.
The Golgi apparatus receives proteins and lipids from the ER. It modifies, sorts, and packages them into small membrane sacs called vesicles. The vesicles then carry the materials to other parts of the cell or to the cell membrane. (National Institute of General Medical Sciences)
What Do Lysosomes Do?
Lysosomes contain enzymes that break down large molecules, damaged cell parts, and unwanted material.
They act as a recycling system by turning old cell components into smaller molecules that may be reused. In some immune cells, lysosomes also help destroy bacteria and other material taken into the cell.
What Does the Cytoskeleton Do?
The cytoskeleton is a changing network of protein fibers throughout the cell. It helps the cell keep its shape and supports the cell membrane.
It also holds organelles in position, helps cells move, separates chromosomes during cell division, and provides internal tracks along which materials can travel.
How Does a Cell Stay Alive and Carry Out Its Functions?
A cell must constantly manage energy, nutrients, water, salts, signals, and waste.
The cell membrane brings in needed materials and removes others. Enzymes break nutrients into smaller molecules. Mitochondria use energy from those nutrients to make ATP. Ribosomes and other organelles use that energy to build proteins and maintain the cell.
Cells also communicate with their surroundings. Receptors on the cell membrane detect hormones and other chemical signals. These messages may tell the cell to grow, divide, release a substance, change its activity, or stop working.
Cells have repair systems that correct many forms of damage. When a cell is badly damaged, no longer needed, or unsafe, it may undergo apoptosis. Apoptosis is a controlled form of cell death that allows the body to remove cells without releasing their contents into nearby tissue. (National Cancer Institute)
How Does DNA Control What a Cell Does?
DNA contains the genetic information used to build and maintain the body. Sections of DNA called genes provide instructions for making proteins or functional RNA molecules.
When a cell needs a particular protein, it copies the needed DNA information into messenger RNA, or mRNA. The mRNA leaves the nucleus and travels to a ribosome. The ribosome reads the message and joins amino acids together to make the protein. (National Human Genome Research Institute)
Proteins carry out much of the cell’s work. They help form cell structures, speed up chemical reactions, transport materials, receive signals, and control cell growth.
DNA does not act alone. Signals from other cells, nutrients, hormones, age, and environmental conditions can affect which genes are active. By turning different genes on or off, cells can change their behavior without changing most of their DNA.
Are All Cells the Same?
No. All cells share several basic features, including a cell membrane, cytoplasm, genetic material, and ribosomes. However, their size, structure, and internal organization vary.
Human and plant cells are eukaryotic cells, meaning their DNA is enclosed within a nucleus. Bacteria are prokaryotic cells and do not have a membrane-bound nucleus.
What Is the Difference Between Human and Plant Cells?
Human and plant cells both have a nucleus, mitochondria, ribosomes, an endoplasmic reticulum, and a Golgi apparatus.
Plant cells also usually have:
- A rigid cell wall outside the cell membrane
- A large central vacuole that stores water and other substances
- Chloroplasts in cells that carry out photosynthesis
Not every plant cell has chloroplasts. For example, many cells in plant roots do not receive light and do not perform photosynthesis.
Human cells do not have cell walls or chloroplasts. Their flexible membranes allow them to form many different shapes and move in ways that most plant cells cannot.
What Is the Difference Between Human and Bacterial Cells?
Human cells are generally larger and contain a nucleus and membrane-bound organelles.
Bacterial cells are usually much smaller. Their DNA is located in an area called the nucleoid rather than inside a nucleus. Bacteria have ribosomes, but they do not have mitochondria, a Golgi apparatus, or other membrane-bound organelles found in human cells.
Many bacteria also have a cell wall that helps protect the cell and maintain its shape.
Why Do Human Cells Have Different Shapes and Functions?
Most nucleated human cells contain the same basic set of DNA instructions. However, they do not use all those instructions at the same time.
During development, cells undergo differentiation. Different groups of genes become active in different cells. This causes cells to produce different proteins and develop structures suited to particular jobs.
A muscle cell activates genes needed for contraction. A nerve cell activates genes needed for communication. A skin cell produces proteins that help form a protective barrier.
What Are Some Examples of Specialized Human Cells?
- Red blood cells: These cells are flexible, biconcave disks. Their shape increases the surface area available for gas exchange and helps them squeeze through narrow capillaries. Mature red blood cells have no nucleus, leaving more room for oxygen-carrying hemoglobin.
- Nerve cells: Neurons have branching extensions called dendrites and a longer extension called an axon. These structures allow them to receive and send electrical and chemical signals.
- Muscle cells: Muscle cells contain proteins that slide past one another to produce force and movement. Many muscle cells are long because they must contract along a particular direction.
- Skin cells: Keratinocytes form most of the outer layer of the skin. They produce keratin and form a tightly connected barrier that helps protect the body from injury, water loss, and microorganisms.
How Do Cells Work Together to Form Tissues, Organs, and Organ Systems?
Human cells do not work in isolation. They communicate, attach to one another, and cooperate in organized levels.
- Cells: The smallest living units of the body.
- Tissues: Groups of cells and surrounding material working together. Examples include muscle tissue, nervous tissue, epithelial tissue, and connective tissue.
- Organs: Structures made from two or more tissue types. The heart, lungs, stomach, skin, and kidneys are organs.
- Organ systems: Groups of organs that work together. The digestive system breaks down and absorbs food, while the circulatory system carries oxygen and nutrients.
- Organism: All organ systems working together as one living human body.
This organization allows trillions of specialized cells to work together. The activity of each cell contributes to the function of the tissues, organs, and organ systems that keep the body alive.
Written by Chris Morais, MSc, MPhil, PhD — Making complex biology simple
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
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