Answer key

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Chapter: 03. The Cell

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Explain why the cell is considered the basic unit of structure and function in all living organisms.

The cell is considered the basic unit of structure because all living organisms, from the smallest bacteria to the largest trees and animals, are fundamentally composed of one or more cells. Just like bricks build a house, cells are the building blocks of life. It is the basic unit of function because all essential life processes, such as obtaining energy (metabolism), growing, responding to stimuli, and reproducing, occur within individual cells. No smaller part of an organism can perform all these life-sustaining activities independently.

Describe the role and characteristics of vacuoles, particularly highlighting their differences in plant and animal cells.

Vacuoles are clear, fluid-filled spaces within the cytoplasm, but their presence and role differ significantly between plant and animal cells. In mature plant cells, there is typically a single, very large central vacuole that can occupy a substantial portion of the cell volume. This vacuole is filled with cell sap and is crucial for maintaining turgor pressure against the cell wall, which provides rigidity and structural support to the plant. It also serves as a storage site for water, nutrients, and waste. In contrast, animal cells usually lack a large, permanent vacuole; they might have a few small, temporary vacuoles, like food vacuoles for digestion or contractile vacuoles for water regulation in protozoans, but these are not for structural support.

Describe the structure and functions of the cell membrane.

The cell membrane, also known as the plasma membrane, is a thin, flexible outer boundary that encloses the cytoplasm of a cell. Its structure allows it to perform several vital functions. Firstly, it forms the clear boundary of the cell, separating its internal contents from the external environment and also distinguishing one cell from another. Secondly, it plays a crucial role in maintaining the cell’s shape. Most importantly, the cell membrane is selectively permeable, meaning it controls which substances, like oxygen, water, and nutrients, are allowed to enter or leave the cell, ensuring only necessary materials pass through while waste products are removed.

List and explain three reasons why cell division is essential in living organisms.

Cell division is a fundamental process in all living organisms, crucial for their survival and development. Firstly, it is essential for growth, as new cells are continuously produced, allowing an organism to increase in size from a single cell (or a smaller form) to its mature size. Secondly, cell division facilitates the repair of wounded tissues; when an organism sustains an injury, new cells are generated to replace the damaged ones and close the wound. Thirdly, it is vital for the replacement of old and worn-out cells; cells have a limited lifespan, and cell division ensures that dead or dysfunctional cells are constantly replaced by healthy new ones, maintaining the proper functioning of tissues and organs.

Explain why cells exhibit a great variation in their shapes and give examples of how shape relates to function in different cell types.

Cells exhibit a great variation in their shapes because their specific shape is directly adapted to the particular function they perform. This optimization allows them to carry out their roles more efficiently within an organism. For instance, nerve cells are long and branched to effectively transmit electrical signals over long distances. Muscle cells are spindle-shaped, allowing them to contract and relax for movement. White blood cells are amoeboid (irregularly shaped) so they can change shape and move through tissues to engulf foreign invaders. In plants, cylindrical xylem vessels form continuous tubes for water transport, while kidney-shaped guard cells regulate gas exchange through stomata.

Describe the structure and main functions of the cell wall in plant cells.

The cell wall is a distinctive, non-living outer layer found exclusively in plant cells, located outside the cell membrane. It is primarily composed of cellulose, a strong carbohydrate. Structurally, it is rigid and provides a fixed shape to the plant cell. Its main functions include providing mechanical support and rigidity, which helps plants stand upright and gives strength to their tissues. Furthermore, the cell wall acts as a protective layer, safeguarding the internal organelles of the cell from physical damage and excessive water uptake. Unlike the cell membrane, it is permeable, allowing water and gases to pass through freely.

Describe the key points of the Cell Theory, and mention the scientists credited with its explanation.

The Cell Theory is a fundamental concept in biology that explains the nature of living organisms. It was explained by two scientists, Schleiden and Schwann. The theory states two main points: firstly, that cells are the structural and functional units of all living organisms, meaning they are the fundamental building blocks and perform all life processes. Secondly, it states that new cells arise only from the division of pre-existing cells, emphasizing that life comes from pre-existing life, not spontaneously.

Name the three types of plastids and explain their respective functions in plant cells.

Plastids are specialized organelles found exclusively in plant cells, playing vital roles in storage and photosynthesis. The three types are:

Chloroplasts: These are green-colored due to the presence of chlorophyll and are the sites of photosynthesis, where light energy is converted into food (sugars). They are often called the “kitchen of the cell.”

Chromoplasts: These are colored plastids containing pigments that give red, yellow, and orange hues to various flowers and fruits, attracting pollinators and seed dispersers.

Leucoplasts: These are colorless plastids primarily responsible for storing different types of food, such as starch, proteins, and fats, in various parts of the plant.

Differentiate between unicellular and multicellular organisms, providing examples for each.

Unicellular organisms are living beings composed of a single cell, where that one cell carries out all necessary life processes such as nutrition, respiration, and reproduction. Examples include bacteria, Amoeba, and Chlamydomonas. In contrast, multicellular organisms are made up of many cells, often millions or billions, with different cells specializing in different functions. These specialized cells group together to form tissues, organs, and organ systems, leading to a more complex organization. Humans, plants like Neem trees, and animals like fish are examples of multicellular organisms.

Describe the structure and functions of the nucleus, often called the “control centre of the cell.”

The nucleus is a prominent, typically spherical or oval organelle found within eukaryotic cells, and it acts as the “control centre.” It is enclosed by a double-layered nuclear envelope, which has nuclear pores regulating the movement of substances. Inside, it contains nucleoplasm, a fluid, and a nucleolus, which is involved in ribosome formation. Most importantly, it houses chromatin material (composed of DNA and proteins) that condenses into chromosomes during cell division. The nucleus controls all cellular activities by directing protein synthesis and regulating gene expression. It also stores and transmits hereditary information, ensuring offspring inherit characteristics from their parents, and plays a crucial role in cell division.

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