Free preview · Class 9 Biology
The cell
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Comprehensive Chapter Summary 1. Introduction to the Cell All living organisms are made up of cells. The cell is considered the basic structural and functional unit of life. Just as a building is made up of bricks, living organisms are made up of cells. A brick is a basic unit of a building, while a cell is the basic unit of a living organism. Some organisms consist of only one cell, while others are made up of millions or even trillions of cells. Examples Bacteria are generally unicellular organisms. Amoeba is a unicellular organism. Humans are multicellular organisms. Plants are multicellular organisms. Animals are multicellular organisms. A single cell can perform many activities necessary for life, including: Obtaining nutrients Producing energy Removing waste Growing Responding to the environment Reproducing In multicellular organisms, cells become specialized to perform particular functions. For example: Red blood cells transport oxygen. Nerve cells transmit messages. Muscle cells help in movement. Root cells absorb water and minerals. Leaf cells perform photosynthesis. Thus, cells are not simply small structures. They are highly organized systems capable of carrying out the processes of life. 2. Discovery of the Cell The discovery of the cell was an important event in the development of biology. In 1665, an English scientist named Robert Hooke examined a thin slice of cork using a microscope. He observed many small, box-like structures. These structures reminded him of the small rooms occupied by monks. He therefore called them “cells.” It is important to understand that Hooke observed the dead cell walls of cork rather than living cells. Later, scientists developed better microscopes and observed living cells in greater detail. Important scientists Robert Hooke Observed cork under a microscope. Introduced the term cell in 1665. Antonie van Leeuwenhoek Observed living cells and microorganisms. Used improved microscopes. Matthias Schleiden Studied plants. Concluded that plants are made up of cells. Theodor Schwann Studied animals. Concluded that animals are made up of cells. Their observations helped scientists develop the cell theory. 3. Cell Theory The cell theory explains the importance of cells in living organisms. The classical cell theory has three major statements: 1. All living organisms are made up of one or more cells. For example, bacteria consist of a single cell, whereas humans consist of many cells. 2. The cell is the basic structural and functional unit of life. This means that the body of an organism is organized from cells and that important life processes occur within cells. 3. All cells arise from pre-existing cells. A new cell develops from an already existing cell through cell division. Modern Cell Theory Modern cell theory also recognizes that: Cells contain hereditary information. Genetic information is passed from one cell to another during cell division. Cells have a similar basic chemical composition. Energy flow occurs within cells. 4. Types of Cells Cells can broadly be divided into two major types: Prokaryotic cells Eukaryotic cells The main difference between them is the organization of their genetic material and the presence or absence of a true nucleus. 5. Prokaryotic Cells The word prokaryotic means “before nucleus.” Prokaryotic cells are generally small and simple compared with eukaryotic cells. They do not have a true membrane-bound nucleus. Instead, their genetic material is present in a region called the nucleoid. Examples The most common examples of prokaryotic organisms are: Bacteria Archaea Characteristics of Prokaryotic Cells Prokaryotic cells generally: Are small in size. Have no true nucleus. Have DNA located in the nucleoid region. Do not contain membrane-bound organelles. Have ribosomes. Usually have a cell membrane. Many have a cell wall. May have structures such as flagella. 6. Structure of a Prokaryotic Cell A typical bacterial cell may contain several structures. Cell wall The cell wall provides: Support Protection Shape It helps prevent the cell from bursting under certain conditions. Cell membrane The cell membrane controls the movement of substances into and out of the cell. Cytoplasm The cytoplasm is the internal region where many chemical reactions take place. Nucleoid The nucleoid contains the main bacterial DNA. Unlike the nucleus of a eukaryotic cell, the nucleoid is not surrounded by a nuclear membrane. Ribosomes Ribosomes are responsible for protein synthesis. Flagellum Some bacteria possess a flagellum, which helps them move. 7. Eukaryotic Cells The word eukaryotic means “true nucleus.” Eukaryotic cells are generally larger and more complex than prokaryotic cells. They contain a well-defined nucleus surrounded by a nuclear membrane. Examples Eukaryotic organisms include: Animals Plants Fungi Protists Human cells are eukaryotic cells. 8. Characteristics of Eukaryotic Cells Eukaryotic cells: Have a true nucleus. Have membrane-bound organelles. Are generally larger than prokaryotic cells. Have complex internal organization. Contain DNA inside the nucleus. Have cytoplasm. Have a cell membrane. Contain ribosomes. 10. Main Parts of a Eukaryotic Cell A typical eukaryotic cell contains: Cell membrane Cytoplasm Nucleus Mitochondria Ribosomes Endoplasmic reticulum Golgi apparatus Lysosomes Vacuoles Plant cells additionally contain: Cell wall Chloroplasts Large central vacuole Each organelle performs a specific function. 11. Cell Membrane The cell membrane, also called the plasma membrane, surrounds the cell. It separates the internal contents of the cell from its external environment. Functions of the Cell Membrane The cell membrane: Protects the cell. Maintains the internal environment. Controls movement of substances. Allows useful substances to enter. Helps remove waste substances. Helps the cell communicate with its environment. The cell membrane is described as selectively permeable because it allows some substances to pass more easily than others. 12. Cytoplasm The cytoplasm is the material present inside the cell membrane but outside the nucleus. It consists mainly of: Cytosol Cell organelles Various dissolved substances Many important chemical reactions occur in the cytoplasm. For example, some stages of cellular respiration occur in the cytoplasm. The cytoplasm also provides a medium in which organelles can remain suspended. 13. Nucleus The nucleus is one of the most important organelles of a eukaryotic cell. It is often called the control center of the cell because it contains genetic information and regulates many cellular activities. Structure of the Nucleus The nucleus generally consists of: Nuclear envelope Nucleoplasm Chromatin Nucleolus Nuclear Envelope The nuclear envelope surrounds the nucleus. It separates the genetic material from the cytoplasm. Chromatin Chromatin consists mainly of: DNA Proteins During cell division, chromatin becomes highly condensed and forms chromosomes. Nucleolus The nucleolus is a dense region inside the nucleus. It is involved in the production of components needed to form ribosomes. 14. Functions of the Nucleus The nucleus: Contains genetic information. Controls many cellular activities. Regulates gene expression. Plays an important role in cell division. Helps control growth and development. Because of these functions, the nucleus is commonly described as the control center of the cell. 15. Mitochondria Mitochondria are important organelles found in eukaryotic cells. A single mitochondrion is called a mitochondrion. They are commonly called the “powerhouses of the cell.” This is because mitochondria are major sites of aerobic cellular respiration, during which energy from nutrients is used to produce ATP. Structure of Mitochondria Mitochondria have: An outer membrane An inner membrane Intermembrane space Matrix The inner membrane has many folds called cristae. The folds increase the surface area available for important reactions involved in energy production. Matrix The internal region enclosed by the inner membrane is called the matrix. ATP ATP stands for adenosine triphosphate. ATP is often called the energy currency of the cell because cells use it to power many activities. Functions of Mitochondria Mitochondria: Carry out important steps of aerobic respiration. Help produce ATP. Provide energy for cellular activities. Easy exam statement Mitochondria are called the powerhouse of the cell because they produce ATP during cellular respiration. 16. Ribosomes Ribosomes are small structures involved in protein synthesis. They are found in both: Prokaryotic cells Eukaryotic cells Unlike mitochondria and the nucleus, ribosomes are not surrounded by a membrane. Ribosomes may be found: Free in the cytoplasm Attached to rough endoplasmic reticulum Function Their major function is: Protein synthesis Proteins are essential for: Growth Repair Enzymes Cell structure Transport Many other biological functions 17. Endoplasmic Reticulum The endoplasmic reticulum, or ER, is a network of membranes inside eukaryotic cells. There are two major types: Rough endoplasmic reticulum Smooth endoplasmic reticulum Rough Endoplasmic Reticulum Rough ER has ribosomes attached to its surface. Function It is involved mainly in: Protein synthesis Processing of proteins Transport of proteins It is particularly important in cells that produce large amounts of proteins. Smooth Endoplasmic Reticulum Smooth ER does not have ribosomes attached to its surface. Functions It is involved in: Lipid synthesis Metabolism of certain substances Detoxification in some cells Calcium storage in specialized cells 18. Golgi Apparatus The Golgi apparatus is an organelle made up of flattened membrane-bound sacs. It receives materials from the endoplasmic reticulum and modifies, sorts, and packages them. Functions The Golgi apparatus: Modifies proteins and lipids. Sorts cellular materials. Packages substances. Helps transport substances. Contributes to the formation of lysosomes. Easy way to remember Golgi apparatus = Packaging and distribution center 19. Lysosomes Lysosomes are membrane-bound organelles containing digestive enzymes. They are involved in the breakdown of: Waste materials Cellular debris Old cell components Certain foreign materials Lysosomes therefore help maintain the cleanliness and organization of cells. Easy way to remember Lysosome = Cellular digestive system 20. Vacuoles A vacuole is a membrane-bound structure used for storage. Vacuoles can contain: Water Food materials Ions Waste products Other substances In plant cells Plant cells generally have a large central vacuole. It helps: Store water and dissolved substances. Maintain internal pressure. Support the plant cell. The pressure generated by water inside the vacuole contributes to turgor. 21. Chloroplasts Chloroplasts are specialized organelles found mainly in plant cells and some other photosynthetic organisms. They are the main sites of photosynthesis in plant cells. Chlorophyll Chloroplasts contain the green pigment chlorophyll. Chlorophyll absorbs light energy required for photosynthesis. Function The major function of chloroplasts is: Photosynthesis During photosynthesis, plants use light energy to help convert carbon dioxide and water into carbohydrates, with oxygen released as a by-product. Easy way to remember Chloroplast = Photosynthesis 22. Cell Wall The cell wall is a rigid structure outside the cell membrane. Plant cell walls are mainly composed of cellulose. Functions The cell wall: Provides support. Maintains cell shape. Protects the cell. Helps prevent excessive expansion when water enters. Animal cells do not have a cell wall. 23. Plant Cell A typical plant cell contains: Cell wall Cell membrane Cytoplasm Nucleus Chloroplasts Mitochondria Ribosomes Endoplasmic reticulum Golgi apparatus Large central vacuole Special features of plant cells Plant cells have: Cell wall Chloroplasts Large central vacuole These structures help plant cells perform their specialized functions. 24. Animal Cell A typical animal cell contains: Cell membrane Cytoplasm Nucleus Mitochondria Ribosomes Endoplasmic reticulum Golgi apparatus Lysosomes Small vacuoles Animal cells do not normally have: Cell wall Chloroplasts.