Cell structure and organization | Biology Form One Notes New Syllabus
The cell is the basic functional unit of life.
Task
Search from library and reliable internet sources the information about cell.
In 1665, an English scientist Robert Hooke revolutionized our understanding of life by designing a microscope and examining cork from a tree’s bark.
He discovered that the cork consisted of numerous box-like structures that are closely packed together, resembling a honeycomb. These structures were named ‘cells’.
Since then, it has been revealed that living things are made up of one or more cells, and these are fundamental units of life. All essential life processes occur within the cell.
The remarkable observations and conclusions made by Hooke, along with other scientists, paved the way for the development of the cell theory.
This widely accepted theory indicates the relationship between cells and living things.
Components of the cell theory
(a) All living things are composed of one or more cells.
(b) Cells are the basic unit of structure and functions of living things.
(c) All cells are produced from other cells.
(d) Cells contain inheritable information which controls their activities.
(e) All cells are basically similar in chemical composition.
(f) All life processes take place in the cells.
Scientists can use the concept of cell theory to learn about nutrition, growth, reproduction, respiration, movement, sensitivity, and excretion.
Activity: Exploring Prokaryotic and Eukaryotic Cells under the microscope
Materials: Microscope, prepared slides of prokaryotic cells and eukaryotic cells or charts showing prokaryotic cells and eukaryotic cells, notebook, and pencil or pen.
Procedure:
Prepare the microscope with the appropriate magnification power for the slides.
Mount the slide of prokaryotic cell on the light microscope's stage. Hold it in place with the stage clips.
Observe the structure of the prokaryotic cell under the microscope.
Replace the slide with a prepared slide of eukaryotic cell and focus on different areas of the slide. Observe the structure of the eukaryotic cell.
Record the observations.
Describe your observations.
Question: Based on your observations, explain the similarities and differences between prokaryotic cell and eukaryotic cell.
Types of Cells
1. Prokaryotic Cells
A prokaryote is a microorganism whose usually single cell lacks a nucleus or other membrane-bound organelles.
Organisms made of these cells are called prokaryotes and are single-celled, such as bacteria.
Characteristics of Prokaryotic Cells
They do not have a true nucleus.
They lack membrane-bound organelles, such as mitochondria.
They reproduce via binary fission and sometimes conjugation.
They are mostly smaller than eukaryotic cells.
Some have flagella for movement or pili for attachment.
They possess circular Deoxyribonucleic acid (DNA).
They can be rod-shaped, spherical, spiral, comma-shaped, or spirochaete-shaped.
2. Eukaryotic Cells
Eukaryotic cells are cells that contain a nucleus and other membrane-bound organelles.
These cells are found in organisms known as eukaryotes, which include both animals and plants.
Characteristics of Eukaryotic Cells
They have a membrane-bound nucleus.
They are generally larger and more complex than prokaryotic cells.
They contain specialized membrane-bound organelles, such as mitochondria and chloroplasts.
They have linear DNA.
Comparison between prokaryotic cells and eukaryotic cells
Feature | Prokaryotic Cells | Eukaryotic Cells |
Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
Organelles | Lacks membrane-bound organelles | Contains membrane-bound organelles |
DNA Shape | Circular | Linear |
Size | Generally smaller | Generally larger and more complex |
Examples | Bacteria, blue-green bacteria | Animal cells, plant cells |
The animal cell
An animal cell is a type of eukaryotic cell that lacks a cell wall and has a true membrane-bound nucleus.
A generalised structure of an animal cell
Activity: Observing animal cells
Materials:
Prepared slides of a cheek cell, a microscope, ICT tools, notebook, and a pen or pencil
Procedure:
Prepare the microscope with the appropriate magnification power.
Mount the prepared slide on the stage of the light microscope.
Use the microscope to focus on different areas of the slide.
Use a low-power objective lens to observe the prepared slide.
Draw what you have observed.
Question:
Describe the structure of human cheek cells.
Functions of different parts of an animal cell
Cell membrane
The cell membrane is a thin layer that encloses the whole cell. The cell membrane is made up of two layers.
The cell membrane is made of lipids and protein molecules.
This membrane is flexible and semi-permeable.
Semi-permeable means that it allows certain substances to pass in or out of the cell.
The flexibility of the animal cell membrane and the lack of a cell wall result in an irregular shape.
Cytoplasm
This is a jelly-like substance made up of water and dissolved chemical substances.
The cytoplasm is the site for many chemical reactions in the cell.
Cell organelles, such as the vacuoles, nucleus, and mitochondria, are suspended in the cytoplasm.
Nucleus
This is a spherical organelle suspended in the cytoplasm.
The nucleus consists of a nucleolus and fluid called nucleoplasm.
It is surrounded by a membrane called the nuclear membrane.
Functions of the nucleus
Determines the chemical processes that take place in the cell
Controls the functions of the cell
Determines the cell's size, shape, and functions
Determines the hereditary characteristics of the cell
Cell Vacuoles
Cell vacuoles are fluid-filled spaces bound by a membrane.
Animal cells have small and temporary vacuoles, mainly used to secrete and excrete wastes from the cell.
Mitochondria
Mitochondria (singular: mitochondrion) are oval-shaped organelles that have two membranes.
The outer membrane is smooth
The inner membrane has folds called cristae (singular: crista)
The matrix is the internal space enclosed by the inner membrane
Functions of mitochondria
Mitochondria produce energy for the cell, which is why they are sometimes referred to as the powerhouses of cells.
The Plant Cell
Similar to the animal cell, the plant cell has a cell membrane, cytoplasm, nucleus, and mitochondria.
These organelles function in the same way as those of an animal cell. However, plant cells have additional structures which serve specific roles in plants.
Additional structures in a plant cell
Cell Wall
This is an outer covering made of cellulose that surrounds the cell membrane.
The cell wall is fully permeable.
It allows the passage of water and minerals.
The cell wall protects and supports the cell.
The cell wall gives the plant cell a definite shape.
Chloroplasts
Chloroplasts are oval organelles that contain green pigments in plants.
This pigment is called chlorophyll.
It is important in photosynthesis, the process by which green plants make their own food.
Chlorophyll absorbs light energy needed for photosynthesis.
Cell Vacuole
Plant cells have a large and permanent vacuole that usually occupies the central part of the cell.
This vacuole contains sap and is surrounded by a membrane called tonoplast.
The cell sap is dilute and exerts pressure outwards against the cell wall.
This pressure, called turgor pressure, helps plant cells to maintain their shape.
Activity: Observing the plant cell
Materials:
Onion, knife, forceps, mounting needle, microscope, microscope slide, coverslip, notebook, and a pen or pencil
Procedure
Separate a fleshy leaf from an onion bulb.
Cut a small square of the leaf.
Safety precaution
Take precaution when using sharp objects such as knife, forceps, and mounting needle.
Using forceps, peel off the epidermis from the inner surface of the square.
Put it on a microscope slide, and add a drop of water.
Using forceps, a mounting needle, or sharp pencil point, carefully lower a coverslip over the epidermis specimen.
Use a low-power objective lens to examine the specimen under the microscope.
Question
Describe the structure of a plant cell.
Based on your observations on Activities explain the similarities and differences between plant and animal cells.
Similarities and differences between animal and plant cells
Similarities
Animal and plant cells are similar in that both have a cell membrane, cytoplasm, a nucleus, cell vacuoles and mitochondria.
Differences
Table: Differences between plant and animal cells
Plant cell | Animal cell |
1. Has a cell wall | 1. Has no cell wall |
2. Has chloroplast | 2. Lacks chloroplast |
3. Has a large and permanent central vacuole | 3. Has small and temporary vacuoles |
4. It is regular in shape | 4. It is irregular in shape |
5. Nucleus is located at the periphery | 5. Nucleus is centrally positioned |
6. Stores food in the form of starch | 6. Stores food in the form of glycogen |
Exercise
What are the differences between prokaryotic and eukaryotic cells?
Mention the structures that are found in plant but not in animal cells.
Draw a diagram of an animal cell and label the parts which perform the following functions.
(a) Energy production
(b) Control all the functions of the cell
(c) Suspend organelles
(d) Allow passage of some substances
Cell differentiation
An organism that is made up of one cell is called a unicellular organism. Examples of such organisms are Amoeba sp., Paramecium sp., and bacteria.
In such organisms, only one cell carries out all the life processes, such as respiration, reproduction, and excretion.
Organisms made up of more than one cell are called multicellular organisms.
A multicellular organism consists of a few to millions of cells.
These cells have different functions and have features that make them better suited to carry out these functions. This is called cell differentiation.
Cell differentiation is the process in which a cell changes from one form to another and becomes more specialised to perform specific functions.
A group of cells that perform the same function form a tissue.
Examples of animal tissues are: bone, muscle, and blood.
Examples of plant tissues are: xylem and phloem.
An organ consists of different tissues that work together to perform a certain function.
Animal organs include: the heart, liver, stomach, and brain.
Plant organs include: the stem, flowers, fruits, leaves, and roots.
An organ system is made up of organs that work together to perform a certain function.
Examples of systems are: respiratory system, digestive system, reproductive system, hormonal system, skeletal system, and blood circulatory system.
For example, the blood circulatory system transports blood to all parts of the body.
Blood circulatory system consists of the heart, blood vessels and blood itself.
Most multicellular organisms are made up of different organ systems working together.
Therefore, there is special organisation from the cell to tissue, tissue to organ, organ to organ system to organism.
Importance of Cell Differentiation
Cell differentiation leads to division of labour.
Division of labour among the cells means specific cells performing specific functions. This helps the body to carry out all life processes at the same time and more efficiently.
Specialised Animal Cells
The following are examples of specialised animal cells:
White Blood Cells
White blood cells are also called leucocytes.
They protect the body against illness and diseases.
The cells can change their shapes so as to engulf and destroy harmful microorganisms.
Some cells contain digestive enzymes which destroy the microorganisms.
Red Blood Cells
Red blood cells are also called erythrocytes.
They lack nuclei and are bi-concave in shape.
This provides a large surface area for transportation of oxygen from the lungs to various parts of the body.
They also contain haemoglobin which carries oxygen to different parts of the body.
Structure of a red blood cell:
Sperm Cells
The sperm cell fertilizes the female egg during reproduction.
The sperm cell has a head and a tail.
The tail enables the sperm cell to swim to the egg.
Examples of Specialized Plant Cell:
Root Hair Cell
Function:
Root hair cells absorb water and mineral salts from the soil.
Their elongated shape increases the surface area for absorption, making them highly efficient in helping the plant take in nutrients.
Activity: Observing Root Hair Structure in Plants
Materials Needed:
Fresh root sample
Microscope
Microscope slide
Coverslip
Water
Dropper or pipette
Stain (optional)
Pen or pencil
Procedure:
Obtain a fresh root sample from a young plant. Ensure the root is intact and has visible root hairs.
Gently rinse the root under running water to remove any soil particles.
Take a microscope slide and place a small droplet of water on it.
Carefully place the root sample on the water droplet, ensuring the root hairs are facing down.
If using a stain, add a drop or two of the stain to the water on the slide.
Question
Based on your observation, describe the structure of the root hair.
Guard Cells
Guard cells surround the stomata.
They control the opening and closing of stomata (singular is stoma).
The inner walls of guard cells are thicker than the outer walls. This makes them expand irregularly.
When the guard cells expand, the stoma opens. When they contract, the stoma closes.
Stomata are tiny pores used for gaseous exchange and loss of excess water.
Xylem Vessels
Xylem vessels are made up of hollow dead cells with walls made of lignin.
Lignin is a tough rigid material that makes up the wall of xylem vessels.
The cells are connected to form xylem vessels.
Roles of Xylem vessels
These vessels transport water and minerals from the root to the leaves.
They also provide support to the plant.
Question
What have you learned from the simulations?
Activity: Observing xylem in plant stems
Materials:
Fresh herbaceous plant stem, microscope, microscope slide, coverslip, dropper or pipette, sharp knife, notebook, and a pen or pencil.
Procedure:
Select a fresh herbaceous plant with a thin stem. Herbaceous plants are non-woody plants with soft stems that are easier to work on.
Carefully cut a thin cross-section of the stem using a sharp knife. The cross-section should be about 1–2 mm thick.
Place the cross-section of the stem on a clean glass slide.
Add a few drops of water to the cross-section to keep it moist and prevent it from drying out.
Carefully place a coverslip over the cross-section, ensuring it covers the entire area of the specimen.
Gently press down on the coverslip to remove any air bubbles and ensure the slide is well-prepared for observation.
Place the prepared slide on the stage of the microscope.
Start with the lowest magnification power and adjust the focus knobs.
Activity: Observing cell structure and differentiation in plants
Materials:
ICT tools, notebook, and pen or pencil.
Procedure:
Visit reliable online sources and select the appropriate simulations/video that describe cell structure and differentiation in plants.
Use the selected simulations/video to learn the concept of cell structure and differentiation in plants.
to bring the cross-section into focus.
Gradually increase the magnification power to higher levels to see the finer details of the xylem tissue.
Observe the cross-section under the microscope. Look for elongated cells with thick walls, which represent the xylem tissue. You may see them arranged in bundles.
Palisade Cells
Palisade cells found in plant leaves contain large amounts of chloroplasts.
Chloroplasts are the sites for photosynthesis.
They contain chlorophyll which traps sunlight energy during photosynthesis.
Revision exercise 3
1. One of the following is NOT a component of the cell theory: (a) All living things are composed of cells. (b) All cells are produced from other cells. (c) All cells are basically similar in chemical composition. (d) All life processes take place outside the cells.
2. One of the following is NOT a characteristic of prokaryotic cells. (a) They have no nuclear membrane. (b) They reproduce by binary fission. (c) They have circular DNA. (d) They have mitochondria.
3. Which of the following is the functional unit of life? (a) Tissue (b) Cell (c) Organ (d) System
4. Which of the following organism has differentiated cells? (a) Bacteria (b) Amoeba (c) Plants (d) Paramecium
5. Write TRUE for correct statements and FALSE for incorrect statements in the spaces provided.
6. (a) A tissue comprises different organs that perform the same function. _________
7. (b) Chloroplasts and mitochondria perform similar functions. _________
8. (c) Palisade cell is one of the specialised cells in plants. _________
9. (d) A cell membrane is similar in function to a cell wall. _________
10. Name the parts of the cell described in each of the following statements.
(a) The semi-permeable membrane that encloses the cytoplasm of a cell.
(b) A large cellular organelle that contains hereditary information.
(c) The site for respiration in the cell.
(d) A jelly-like mixture consisting mostly of water, located between the cell membrane and the nucleus.
(e) A part that protects, supports, and gives shape to plant cells.
Differentiate:
(a) Prokaryotic from eukaryotic cells
(b) Organ from tissue
Compare the structures of a plant and an animal cell as seen under the light microscope.
11. Explain what will happen if the following are removed from the plant or animal cell.
(a) Nucleus
(b) Cell wall
(c) Chloroplast
(d) Mitochondria
12. Using examples of plant and animal cells, explain the meaning of cell differentiation.
13. Describe the functions of any three specialised cells in plants.
