Lesson 7: Cell Theory, Structure, Function and Type
Video Lesson:
Lesson Objective:
By the end of this lesson, you will be able to:
- explain how living things are composed of cells
- identify unicellular organisms from multicellular organism.
- describe cell theory,
- name scientists who played a role in the discovery of the cell theory.
- identify the main structure of a cell,
- state that the cytoplasm of all cells contains most organelles like the nucleus, ribosomes, and rough endoplasmic reticulum,
- describe the function of the structure of a cell, and
- list the main structure of a cell.
- describe the difference between prokaryotes and eukaryotic cells
- give examples of the two types of cell.
Brainstorming Questions
- Why do we study about cell?
- From yoour grade seven, how cell can be considered as the basis of life?
- What are the main parts of cell?
Welcome to Unit 3! In this unit, you’ll dive into the fascinating world of cells— the fundamental building blocks of life. We’ll explore what makes cells so special, how they function, and why they’re crucial to all living organisms. Get ready to uncover the secrets behind cell structure and function, and learn about the various types of cells that make up the diverse array of life on Earth.
Key Terms
- Cell: is the smallest and the basic unit of living things. All living things are composed of cells.
- Unicellular organisms: living things consist of a single cell
- Multicellular organisms: groups of living things composed of many cells
- Organelles: small structures found in plant and animal cells. Some examples of organelles are: nucleus, mitochondria, vesicles, and Endoplasmic reticulum.
- Cytoplasm: is a thick fluid-filled region in the cell containing cell organelles. It is also the site where several chemical reactions such as protein synthesis take place.
- Nucleus: is the largest organelle which contains DNA and control all activity of the cell. Ribosomes: are small cellular structures that are found in large numbers in all cells. The function of ribosomes is to synthesize proteins
- Rough endoplasmic reticulum: contains the protein synthesizing machinery called ribosome. Vesicles: are small stack-like collections of the membranous sac. They store and transport and ship materials in the cell.
- Mitochondria: is the sites of respiration to produce energy.
- Lysosomes: contain digestive enzymes which can be used to digest bacteria or break down unwanted substance in the cell.
- Chloroplast: are the site for photosynthesis, which contain a green pigment called chlorophyll that can trap the sunlight energy.
What is a Cell?
Cell is the smallest and the basic unit of living things. All living things are composed of cells. Some of these living things consist of a single cell and are called unicellular organisms. For example, paramecium is a unicellular organism. Most of the unicellular organisms are invisible with naked eyes, but there are some exceptions like chicken egg cells which can be seen with naked eyes.
The other groups of living things such as plants and animals including humans which consist of many cells are called multicellular organisms. The body of multicellular organisms is composed of many cells of specialized types which work cooperatively. For example, in the case of human body, everything you do, every action and every thought are possible because of the processes that occur at the cellular level. Each activity of our body is a result of the cooperative work of muscle cells for movement, nerve cells for gathering information and brain cell for interpretation of message to action.
Most cells are very small and are, therefore, visible only with a microscope but an ostrich egg is a single cell with about 6 inches in diameter and weighing about 1.36 kg; nerve cells in your body can stretch several feet long, and nerve cells in giant squid can be more than 30 feet long.
Cell Theory
In 1665, the English scientist Robert Hooke was the first to observe cells and coined the term “cell” after examining thin slices of cork from plant material. He was intrigued by the regular, pore-like structures he saw, which resembled tiny, empty boxes surrounded by walls. Hooke had discovered and described cells without realizing that they are the fundamental units of all living things.
Following Hooke’s discovery, the Dutch microbiologist Anton van Leeuwenhoek (1632–1723) made significant advancements in microscopy. By building numerous microscopes with magnifications up to 300X, Leeuwenhoek observed protozoa for the first time in 1674 and, several years later, bacteria. He isolated these “very little animalcules” from various sources, including rainwater, pond and well water, and the human mouth and intestine. Leeuwenhoek is acknowledged as the father of microbiology for his discovery of microorganisms like protists and bacteria.
More than 150 years later, German scientists Matthias Schleiden and Theodor Schwann laid the groundwork for what we now call cell theory. In 1838, Matthias Schleiden, a botanist, proposed that all plants are made up of cells. In 1839, Theodor Schwann, a zoologist, extended this idea to animals, establishing that all animals are composed of cells. Together, they established cells as the basic unit of structure and function in living organisms.
In 1856, the German pathologist Rudolf Virchow added to this understanding by asserting that cells arise only by the division of existing cells.
These discoveries collectively led to the formulation of cell theory, which states:
- Living organisms are composed of one or more cells.
- Cells are the smallest unit of life.
- Cells arise only by the division of pre-existing cells.
Cells vary significantly in size and shape but share certain common features:
- Every living cell is surrounded by a membrane that separates the cell’s contents from its external environment.
- Cells contain genetic material that stores all the instructions needed for the cell’s activities. Many of these activities are chemical reactions catalyzed by enzymes produced within the cell.
- Cells have their own energy release systems that power all the cell’s activities, making them the smallest living structures capable of surviving independently.
Cell Structure and Function
Cells are composed of various structures, each serving specific functions. The organization of cells can differ greatly between organisms. In multicellular organisms, some cells share similar shapes and functions, while others have distinct shapes and specialized functions. For example, in the human body, nerve cells differ significantly in shape and function from muscle cells.
Despite differences, plant and animal cells share several common structures:
- Nucleus
- Cytoplasm
- Cell (Plasma) Membrane
Additionally, the cytoplasm contains numerous tiny structures called organelles, each with specific functions. Some organelles have membranes, while others do not. An organelle is a distinct structure within a cell having a specific function. Some of these organelles have a membrane, while others without membrane.
Cell Wall
- Structure: The cell wall is a rigid layer composed mainly of cellulose, hemicellulose, and pectin, located outside the cell membrane.
- Function: It provides structural support, protection, and helps maintain cell shape. It also prevents excessive water uptake.
- Found in Plant cells only
Cell Membrane (Plasma Membrane)
- Structure: The cell membrane is a phospholipid bilayer with embedded proteins.
- Function: It regulates the movement of substances in and out of the cell, provides protection, and supports cell communication.
- Found in both plant and animal cells
Cell membrane is semi- permeable (selectively permeable) because it chooses what can go in and out of the cell.
Cytoplasm
- Structure: The cytoplasm is a gel-like substance composed of cytosol (fluid), organelles, and inclusions.
- Function: It is the site of many cellular processes, including metabolism and transport of molecules within the cell.
- Found in both plant and animal cells
Nucleus
- Structure: The nucleus is a large, membrane-bound organelle containing the cell’s genetic material (DNA) organized into chromosomes. It has a nuclear envelope with pores.
- Function: It controls the cell’s activities, including growth, metabolism, and reproduction. The nucleus also contains the nucleolus, where ribosome synthesis occurs.
- Found in both plant and animal cells
Nucleolus
- Structure: The nucleolus is a dense region within the nucleus.
- Function: It is responsible for producing and assembling ribosome components.
- Found in both plant and animal cells
Most cells contain one nucleus but there are interesting exceptions. For example, both the mature red blood cells of mammals and the sieve tube elements of the phloem of flowering plants are without a nucleus. Both lose their nucleus as they mature. The individual cylindrical fibers of voluntary muscle consist of a multinucleate sack. Fungal mycelia also contain multinucleate cytoplasm
Ribosomes
- Structure: Ribosomes are small, spherical structures composed of RNA and proteins. They can be found free-floating in the cytoplasm or attached to the rough endoplasmic reticulum.
- Function: They are the sites of protein synthesis.
- Found in both plant and animal cells
Endoplasmic Reticulum (ER)
- Structure: The ER is a network of membranous tubules and sacs. It comes in two forms:
- Rough ER: Studded with ribosomes.
- Smooth ER: Lacks ribosomes.
- Function:
- Rough ER: Synthesizes and transports proteins.
- Smooth ER: Synthesizes lipids, detoxifies drugs and poisons, and stores calcium ions.
- Found in both plant and animal cells
Golgi Apparatus
- Structure: The Golgi apparatus is a series of flattened, membrane-bound sacs (cisternae).
- Function: It modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.
- Found in both plant and animal cells
Golgi apparatus is an organelle named by its discoverer (Italian scientist Camillo Golgi),who first described this structure in 1898. You can think of the Golgi apparatus as a detailing facility that receives ship newly manufactured cars (proteins), puts on the finishing touches, stores the completed cars, and then ships them out when needed. Products made in the ER reach the Golgi apparatus in transport vesicles.
Mitochondria
- Structure: Mitochondria are relatively large and double-membraned organelles with their own DNA. The inner membrane is folded into structures called cristae.
- Function: They are the powerhouses of the cell, generating ATP through cellular respiration and that is why they are called “the power house of the cell“
- Found in both plant and animal cells.
Lysosomes
- Structure: Lysosomes are small, membrane-bound vesicles containing digestive enzymes.
- Function: They break down waste materials, cellular debris, and foreign invaders such as bacteria.
- Found in both plant and animal cells
Lysosomes are involved in the breakdown of the contents of ‘food’ vacuoles. For example, harmful bacteria. Any foreign matter or food particles are taken up into these vacuoles is then broken down by the digestive enzymes. This occurs when lysosomes fuse with the vacuole. Lysosomes also destroy damaged organelles in this way. Phagocytosis came from two Greek words phagein which means to eat, and kytos is vessel referring here to the cell.
Peroxisomes
- Structure: Peroxisomes are small, membrane-bound organelles containing enzymes.
- Function: They break down fatty acids and detoxify harmful substances.
- Found in both plant and animal cells
Vacuoles
- Structure: Vacuoles are large, membrane-bound sacs, often occupying most of the cell’s volume in mature plant cells. Animal cells may have small vacuoles.
- Function: They store nutrients, waste products, and other materials. They also help maintain turgor pressure, which provides structural support. Vacuoles perform a variety of functions in different kinds of cells.
- Food vacuole found in some animals to digest food and in the case of some unicellular organisms there is a contractile vacuole which is function is to expel excess water. For example, amoebas and many other unicellular eukaryotes have food vacuole in order to eat smaller organisms or food particles, by a process called phagocytosis. Many unicellular protists living in fresh water have contractile vacuoles that pump excess water out of the cell, thereby keeping a suitable concentration of ions and molecules inside the cell.
Chloroplasts
- Structure: Chloroplasts are double-membrane organelles containing chlorophyll and other pigments. They have their own DNA and internal membrane structures called thylakoids, stacked into grana.
- Function: They are the sites of photosynthesis, converting light energy into chemical energy stored in glucose.
- Found in Plant cells only
Plasmodesmata
- Structure: Plasmodesmata are channels that pass through the cell walls, connecting the cytoplasm of adjacent cells.
- Function: They facilitate the transport of materials and communication between cells.
- Found in Plant cells only
Cytoskeleton
- Structure: The cytoskeleton is a network of protein filaments and tubules, including microfilaments, intermediate filaments, and microtubules.
- Function: It provides structural support, maintains cell shape, and facilitates cell movement and intracellular transport. (Found in both plant and animal cells)
Centrosomes (without Centrioles)
- Structure: Plant cells have centrosomes, but they lack centrioles found in animal cells.
- Function: They play a role in organizing microtubules and cell division.
Centrioles and Microtubules
- Structure: The cytoplasm of cells contains small cylindrical fibers called microtubules. Animal cells have structures called centrioles, composed of microtubules, that exist in pairs.
- Function: Microtubules have various roles, including maintaining cell shape, facilitating intracellular transport, and moving chromosomes during cell division. Centrioles anchor microtubules and help organize them during cell division, ensuring proper chromosome separation.
Cilia and Flagella
Cilia and flagella are the most common organelles for locomotion in unicellular organisms.
- Structure: Cilia are short, hair-like projections, while flagella are long, whip-like structures. Both are composed of microtubules and covered by the cell membrane.
- Function: They are involved in cell movement and the movement of fluids around cells. Cilia can also create currents in the surrounding fluid, such as in the respiratory tract where they help move mucus. Cilia can also play a role in sensing environmental stimuli. Flagella enable entire cells to move, typically seen in sperm cells. Cilia are usually numerous and shorter, while cells with flagella typically have one or a few.
Types of Cells
Based on cell structure and cellular organization, cells are grouped into two types. These are Prokaryotic cells and Eukaryotic cells. Prokaryotic cells are usually much smaller than eukaryotic cells. They have a much simpler structure and are thought to be the first cells to have evolved. Prokaryotic cells such as bacterial cells don’t have membrane-bound organelles or a nucleus. Eukaryotes such as plants and animals cells, in contrast, have membrane-bound organelles and a nucleus.
Prokaryotic and Eukaryotic Cells
The term Eukaryotic is derived from the Greek eu, true, and karyon, kernel, referring to the nucleus. Thus Eukaryotic cells are cells with a “true nucleus”. Similarly, the term Prokaryotic is derived from the Greek pro means before and karyon means nucleus; “before nucleus”, reflecting the earlier evolution of prokaryotic cells. Eukaryotic cells are usually between 10 and 100 micro meters (mm) wide, while prokaryotes are between 1 and 10 m The term Eukaryotic is derived from the Greek eu, true, and karyon, kernel, referring to the nucleus. Thus Eukaryotic cells are cells with a “true nucleus”.
Similarly, the term Prokaryotic is derived from the Greek pro means before and karyon means nucleus; “before nucleus”, reflecting the earlier evolution of prokaryotic cells. Eukaryotic cells are usually between 10 and 100 micro meters (mm) wide, while prokaryotes are between 1 to 10 mm in size.
Prokaryotic cells
Prokaryotes are different from eukaryotic cells because of the absence of a nucleus and other membrane-bounded organelles. These Prokaryotic cells are generally much smaller than eukaryotic cells. Prokaryotic organisms are unicellular. The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration.
Eukaryotic cells
Unlike prokaryotes, eukaryotes have a nucleus and other membrane- bound organelles. For example, each organ in an animal’s body is specialized to perform a particular role, each organelle in a eukaryotic cell has a distinctive structure and function. Prokaryotic organisms are unicellular but eukaryotic organisms are usually multicellular.
| Structure | Eukaryotic cells | Prokaryotic cells |
| Organelles | Membrane-bound organelles (for example, nucleus, ER) | No Membrane-bound organelles |
| Ribosomes | relatively large | relatively small |
| Chromosomes | DNA arranged in long strands, asso- ciated with proteins | DNA present, not associated with pro- teins, circular plasmids may also be present |
| Cell wall | always present in plant cells, made of cellulose, never present in animal cells | always present, primarily made of Peptido- glycan |
| Cilia and flagella | sometimes present | some have flagella, but these have a differ- ent structure from those in eukaryotic cells |
Animal and plant cells
Animal and plant cells have certain features in common like cell membrane, nucleus, cytoplasm, mitochondria, and Golgi apparatus. The animal and plant cell structures have also some differences. For example, cell wall, chloroplast and large permanent vacuoles are found in plant cells while centriole and lysosome are found only in animal cells.
In addition, plant cells are usually larger and are more easily visible under alight microscope than animal cells. The plant vacuole is surrounded by a membrane and contains fluid. The fluid in the vacuole is a solution of pigments, enzymes, sugars and other organic compounds (including some waste products), mineral salts, oxygen and carbon dioxide. Vacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards). Chloroplasts are found in the green parts of the plant, mainly in the leaves.
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (provides structure and protection) | Absent |
| Cell Membrane | Present | Present |
| Nucleus | Present | Present |
| Chloroplasts | Present (in green parts, mainly leaves) | Absent |
| Vacuole | Large permanent vacuole (surrounded by a membrane, contains fluid with pigments, enzymes, sugars, organic compounds, mineral salts, oxygen, and carbon dioxide) | Small temporary vacuoles may be present |
| Mitochondria | Present | Present |
| Rough and Smooth Endoplasmic Reticulum | Present | Present |
| Golgi Bodies | Present | Present |
| Ribosomes | Present | Present |
| Lysosome | Absent | Present (contains digestive enzymes) |