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AQA GCSE Biology Notes

1.4.4 Functions of Cell Structures

In this section, we explore the intricate functions of cell structures in plant, animal, and bacterial cells. These structures, each with a unique role, are pivotal for the cell’s survival, growth, and efficient operation.

Cell Wall

Plant Cells

  • Composition: Primarily made of cellulose, a complex carbohydrate.
  • Functions:
    • Structural Support: Provides rigidity, maintaining the cell's shape.
    • Water Regulation: Prevents the cell from absorbing too much water.
    • Protection: Shields against mechanical stress and pathogenic invasion.

Bacterial Cells

  • Composition: Composed of peptidoglycan, a polymer distinct from cellulose.
  • Functions:
    • Shape Maintenance: Gives the cell its characteristic shape.
    • Environmental Barrier: Protects against external threats and osmotic pressure.

Cell Membrane

All Cells

  • Structure: Consists of a phospholipid bilayer with embedded proteins and cholesterol.
  • Functions:
    • Selective Permeability: Regulates the entry and exit of substances.
    • Signal Reception: Contains receptors for cellular communication.
    • Interaction with Environment: Enables cell to respond to external changes.

Nucleus

Plant and Animal Cells

  • Structure: Enclosed by a nuclear envelope, containing DNA and nucleolus.
  • Functions:
    • Genetic Storage: Houses chromosomes with genetic information.
    • Regulation of Gene Expression: Controls cellular activities by regulating DNA transcription.
    • Ribosome Production: The nucleolus within the nucleus produces ribosomes.
Labelled structure of the nucleus

Image courtesy of BruceBlaus.

Cytoplasm

All Cells

  • Composition: A gel-like substance composed mainly of water, salts, and proteins.
  • Functions:
    • Metabolic Reactions: Hosts numerous biochemical processes.
    • Medium for Molecular Movement: Facilitates the transport of molecules within the cell.
    • Structural Support: Gives cells their shape and keeps organelles in place.

Chloroplasts

Plant Cells

  • Structure: Contains a double membrane, thylakoids, and chlorophyll.
  • Functions:
    • Photosynthesis: Converts solar energy into glucose, a vital energy source.
    • Starch Storage: Stores glucose in the form of starch.
    • Oxygen Production: Releases oxygen as a byproduct of photosynthesis.
Labelled Structure of chloroplast

Image courtesy of brgfx on freepik

Ribosomes

All Cells

  • Structure: Made of rRNA and proteins; found in cytoplasm and on the endoplasmic reticulum.
  • Functions:
    • Protein Synthesis: Translates mRNA into amino acid chains (proteins).
    • Enzyme Production: Produces enzymes essential for various cellular processes.

Mitochondria

Plant and Animal Cells

  • Structure: Double membrane-bound, with inner folds (cristae) and a matrix.
  • Functions:
    • ATP Production: Generates ATP via cellular respiration.
    • Regulation of Cellular Metabolism: Influences metabolic activities.
    • Apoptosis Induction: Plays a role in programmed cell death.
Illustration of mitochondria and ribosomes

Image courtesy of julos on freepik

Vacuoles

Plant Cells

  • Large Central Vacuole: Occupies a significant portion of the cell’s volume.
  • Functions:
    • Storage: Holds water, nutrients, and waste products.
    • Turgor Pressure Maintenance: Contributes to cell rigidity.
    • pH and Ion Balance: Regulates the cell’s internal pH and ion balance.

Animal Cells

  • Smaller Vacuoles: Perform similar functions on a smaller scale.

Bacterial Cell Specific Structures

Circular DNA and Plasmids

  • Structure: Circular DNA is not enclosed in a nucleus; plasmids are small DNA loops.
  • Functions:
    • Genetic Information: Carries genes essential for survival and reproduction.
    • Adaptation and Survival: Plasmids often contain genes for antibiotic resistance.
Diagram showing bacterium with its chromosomal DNA and several plasmids

Image courtesy of Spaully

Integration of Cell Structure Functions

The harmonious functioning of these structures is essential for the cell’s vitality. Mitochondria and chloroplasts manage energy, ribosomes synthesise proteins, the nucleus and circular DNA oversee genetic control, and cell walls and membranes maintain structural integrity and interaction with the environment. The vacuoles in plant and animal cells are crucial for storage and waste management. This intricate interplay underscores the complexity of cellular life and forms the foundation for more advanced biological concepts. Understanding these roles provides insight into cell biology, essential for comprehending broader biological processes.

FAQ

The cytoskeleton is a network of protein filaments and tubules that extends throughout the cytoplasm of all eukaryotic cells. It includes three main types of filaments: microfilaments (actin filaments), intermediate filaments, and microtubules. The cytoskeleton serves multiple essential functions: it provides mechanical support and maintains cell shape; it enables cell motility, including muscle contraction, cell division, and intracellular transport of organelles and vesicles; and it plays a role in cell signalling pathways. Microfilaments facilitate cell movement and are involved in muscle contraction and cell division. Intermediate filaments provide tensile strength for the cell, helping it withstand stress. Microtubules are responsible for separating chromosomes during cell division and are the tracks along which organelles move within the cell. The dynamic nature of the cytoskeleton is crucial for cellular responses to stimuli and for the adaptation to changing environments.

Peroxisomes are small, membrane-bound organelles present in virtually all eukaryotic cells. They contain enzymes that catalyse various biochemical reactions, most importantly the breakdown of very long chain fatty acids through β-oxidation. Peroxisomes also play a key role in the detoxification of harmful substances, including the breakdown of hydrogen peroxide – a byproduct of cellular metabolism that can be damaging to cells – into water and oxygen. In addition, peroxisomes are involved in the synthesis of certain important biomolecules, such as bile acids and plasmalogens (phospholipids essential for the normal function of the brain and lungs). The absence or malfunction of peroxisomes can lead to severe metabolic disorders, highlighting their importance in cell function and overall organism health.

The endoplasmic reticulum (ER) is a network of membranous tubules and sacs called cisternae, playing a central role in the synthesis, folding, modification, and transport of proteins and lipids. There are two types: rough ER, studded with ribosomes, and smooth ER, which lacks ribosomes. The rough ER is primarily involved in protein synthesis; newly synthesised proteins enter the ER lumen where they are folded and modified, for instance through glycosylation. These proteins are then transported to other parts of the cell, often in vesicles that bud from the ER. The smooth ER, on the other hand, is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. The ER thus serves as a manufacturing and packaging system, playing a critical role in the maintenance of cellular function and health.

Lysosomes are small, spherical organelles found in animal cells that contain a variety of enzymes capable of breaking down all types of biological polymers – proteins, nucleic acids, carbohydrates, and lipids. These enzymes are active at a much lower pH than the cytoplasm, and thus lysosomes maintain an acidic environment for optimal enzyme activity. Lysosomes play several critical roles: they digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria; they break down complex molecules into simpler compounds, which are then either recycled within the cell or expelled as waste. This process is vital for cellular housekeeping and turnover. If cell debris accumulates, it can disrupt cellular functions and lead to diseases. Moreover, in multicellular organisms, lysosomes help in tissue remodelling during development and contribute to the immune response by destroying pathogens engulfed by white blood cells.

The Golgi apparatus, often referred to as the cell’s post office, is a series of flattened, stacked pouches called cisternae. It primarily functions in modifying, sorting, and packaging proteins and lipids for secretion or use within the cell. Proteins and lipids arrive from the endoplasmic reticulum in transport vesicles; once in the Golgi apparatus, they may undergo further modifications, such as glycosylation or the addition of other functional groups. The Golgi apparatus also produces lysosomes. The final products are then packaged into vesicles that bud from the edges of the Golgi cisternae. These vesicles may either fuse with the cell membrane to release their contents outside the cell or transport their contents to other locations within the cell. This organelle is crucial for correctly processing and targeting proteins and lipids, ensuring they reach their proper destinations, which is vital for the cell's functionality and overall health.

Practice Questions

Describe the role of mitochondria in animal cells and explain how this is vital for the overall functioning of the cell.

Mitochondria are often referred to as the powerhouses of the cell due to their role in producing adenosine triphosphate (ATP), the cell's main energy currency. They achieve this through a process called cellular respiration, where glucose and oxygen are converted into ATP, water, and carbon dioxide. This ATP is essential for various cellular processes, including muscle contraction, nerve impulse propagation, and biochemical syntheses. Without mitochondria efficiently producing ATP, cells would lack the energy required for these vital processes, leading to impaired functioning of the entire organism. Additionally, mitochondria play a role in cellular signalling, differentiation, and death (apoptosis), further underlining their importance in overall cell health and functionality.

Explain how the structure of chloroplasts in plant cells is related to their function in photosynthesis.

Chloroplasts in plant cells are specifically structured to optimise photosynthesis, the process by which plants convert light energy into chemical energy. They have a double membrane that encloses a fluid-filled space, the stroma, and a system of interconnected sacs called thylakoids, which contain chlorophyll. The chlorophyll molecules, located in the thylakoid membranes, capture light energy and initiate the light-dependent reactions of photosynthesis. The stroma then hosts the light-independent reactions, where the energy from light reactions is used to synthesise glucose from carbon dioxide and water. This efficient arrangement of chloroplasts ensures that light energy is effectively converted into a usable form, essential for the plant's growth and energy needs.

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