Unit 1: An overview of cells - Subjective Questions

Major discoveries:

• Robert Hooke (1665): Observed thin slices of cork using a compound microscope and described the small compartments as cells.
• Antonie van Leeuwenhoek: Observed living cells, including bacteria, protozoa, and sperm cells, using improved single-lens microscopes.
• Robert Brown (1831): Identified the nucleus in plant cells.
• Matthias Schleiden (1838): Proposed that all plants are made of cells.
• Theodor Schwann (1839): Extended the idea to animals and stated that all living organisms are composed of cells.
• Rudolf Virchow (1855): Proposed that new cells arise from pre-existing cells.

These discoveries formed the basis of the modern cell theory.

Modern cell theory states that:

• All living organisms are composed of one or more cells.
• The cell is the basic structural and functional unit of life.
• All cells arise from pre-existing cells through cell division.
• Cells contain hereditary information that is passed from parent cells to daughter cells.
• Energy flow and metabolism occur within cells.
• Cells share basic chemical components and fundamental biochemical processes.

The theory explains the unity of life while also allowing for diversity in cell structure and function.

Common properties of living cells include:

• Organization: Cells possess an organized structure enclosed by a plasma membrane.
• Metabolism: They obtain and use energy through chemical reactions.
• Growth: Cells increase in size and synthesize new cellular materials.
• Reproduction: They produce new cells by cell division.
• Response to stimuli: Cells detect and respond to changes in their environment.
• Homeostasis: They maintain relatively stable internal conditions.
• Heredity: Genetic information is stored in DNA and transmitted to daughter cells.
• Adaptation and evolution: Cellular characteristics can change over generations.

These properties enable cells to survive, reproduce, and perform specialized functions.

Unity of cells:

All cells have several common features, including a plasma membrane, cytoplasm, genetic material, and ribosomes. They also use similar molecules such as DNA, RNA, proteins, carbohydrates, lipids, and ATP.

Diversity of cells:

Cells differ in size, shape, internal organization, metabolism, and function. For example:

• A bacterium is a small prokaryotic cell without a membrane-bound nucleus.
• A neuron has a long extension specialized for transmitting signals.
• A red blood cell is adapted for oxygen transport.
• A muscle cell contains abundant contractile proteins.
• A plant cell has a cell wall, chloroplasts, and a large central vacuole.

Thus, cells share a common basic plan but are structurally and functionally specialized.

Light microscopy uses visible light and glass lenses to magnify and resolve cellular structures.

• Magnification enlarges the apparent size of an object.
• Resolution is the ability to distinguish two closely spaced points as separate.
• Contrast increases the difference between the specimen and its background.
• The objective lens forms the primary image, while the eyepiece magnifies it further.

• Total magnification is calculated as:

Light microscopes can be used to observe living cells and stained specimens. However, their resolution is limited compared with electron microscopes.

Comparison of microscopy methods:

• Bright-field microscopy: Uses transmitted visible light. It is suitable for stained specimens but provides limited contrast in living, unstained cells.
• Phase-contrast microscopy: Converts differences in light phase into differences in brightness. It is useful for observing living, unstained cells.
• Fluorescence microscopy: Uses fluorescent molecules that emit light after excitation. It allows specific molecules or organelles to be localized.
• Electron microscopy: Uses a beam of electrons and has much greater resolving power than light microscopy. Transmission electron microscopy reveals internal ultrastructure, whereas scanning electron microscopy shows surface details.

Electron microscopy usually requires fixed, dehydrated specimens and therefore does not allow routine observation of living cells.

Both types possess a plasma membrane, cytoplasm, ribosomes, and genetic material.

Major components of a prokaryotic cell:

• Capsule or slime layer: Provides protection and helps the cell attach to surfaces.
• Cell wall: Maintains shape and prevents bursting due to osmotic pressure.
• Plasma membrane: Regulates transport and contains enzymes for energy production.
• Cytoplasm: Contains enzymes, metabolites, ribosomes, and genetic material.
• Nucleoid: Region containing the main circular DNA molecule.
• Plasmids: Small circular DNA molecules that may carry useful genes, such as antibiotic-resistance genes.
• Ribosomes: Synthesize proteins.
• Flagella: Facilitate movement.
• Pili or fimbriae: Help in attachment and, in some bacteria, DNA transfer.

Prokaryotic cells lack a membrane-bound nucleus and other membrane-bound organelles.

The plasma membrane is a thin, flexible boundary surrounding the cell. It follows the fluid mosaic model.

• It consists mainly of a phospholipid bilayer.
• Hydrophilic phosphate heads face the aqueous environments, while hydrophobic fatty acid tails face inward.
• Proteins embedded in the bilayer function as channels, carriers, receptors, enzymes, and attachment sites.
• Carbohydrate chains attached to lipids and proteins participate in cell recognition.
• Cholesterol helps regulate membrane fluidity and stability in animal cells.

The membrane is selectively permeable. It controls the movement of substances, receives signals, supports cell recognition, and helps maintain the cell's internal environment.

Major animal-cell organelles:

• Nucleus: Stores DNA and controls gene expression.
• Nucleolus: Produces ribosomal RNA and assembles ribosomal subunits.
• Ribosomes: Synthesize proteins.
• Rough endoplasmic reticulum: Produces and transports proteins destined for membranes or secretion.
• Smooth endoplasmic reticulum: Synthesizes lipids, participates in detoxification, and stores calcium ions.
• Golgi apparatus: Modifies, sorts, and packages proteins and lipids.
• Mitochondria: Generate ATP through cellular respiration.
• Lysosomes: Digest macromolecules and worn-out cell components.
• Peroxisomes: Break down fatty acids and detoxify harmful compounds.
• Cytoskeleton: Provides shape, movement, and intracellular transport.
• Centrosome: Organizes microtubules and assists in cell division.

Major plant-cell structures:

• Cell wall: A rigid cellulose layer that provides support and protection.
• Plasma membrane: Regulates movement of substances into and out of the cell.
• Nucleus: Contains genetic material and controls cellular activities.
• Chloroplasts: Carry out photosynthesis using chlorophyll.
• Mitochondria: Produce ATP through cellular respiration.
• Central vacuole: Stores water, ions, pigments, and waste products; it also maintains turgor pressure.
• Endoplasmic reticulum: Synthesizes and transports proteins and lipids.
• Golgi apparatus: Modifies and distributes cellular products.
• Ribosomes: Synthesize proteins.
• Plasmodesmata: Channels that connect neighboring plant cells.

Plant cells have chloroplasts, a cell wall, and a large central vacuole, which distinguish them from typical animal cells.

Both contain a plasma membrane, nucleus, cytoplasm, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, and cytoskeleton.

Cell structure is closely adapted to its function.

• Red blood cells: Their biconcave shape increases surface area for gas exchange, and the absence of a nucleus provides more space for hemoglobin.
• Neurons: Long axons and branched dendrites allow them to receive and transmit electrical signals over distances.
• Muscle cells: Numerous mitochondria supply ATP for contraction, while specialized protein filaments generate force.
• Root hair cells: Long extensions increase surface area for absorption of water and minerals.
• Palisade cells: Numerous chloroplasts enable efficient photosynthesis.
• Sperm cells: A flagellum provides movement, and mitochondria supply energy for swimming.

These examples demonstrate that differences in cellular architecture support specialized biological functions.

Cells remain relatively small because exchange with the environment occurs across the plasma membrane, while metabolic demands depend largely on the cytoplasmic volume. As a cell grows, volume increases faster than surface area, reducing the efficiency of exchange.

For a spherical cell with radius :

Therefore:

As increases, the surface-area-to-volume ratio decreases. Small cells therefore exchange nutrients, gases, and wastes more efficiently. Cells may also maintain efficiency through flattened or elongated shapes, folding of membranes, or compartmentalization.

Major cellular components:

• Water: The main solvent and medium for biochemical reactions.
• Proteins: Perform structural, enzymatic, transport, signaling, and defensive functions.
• Lipids: Form membranes and serve as energy stores and signaling molecules.
• Carbohydrates: Provide energy, contribute to cell walls, and participate in recognition.
• Nucleic acids: DNA stores hereditary information, while RNA participates in gene expression and protein synthesis.
• Ions and minerals: Support osmotic balance, electrical signaling, enzyme activity, and structural functions.

These components are organized into membranes, organelles, and molecular systems. Their coordinated activities allow the cell to grow, communicate, obtain energy, reproduce, and maintain homeostasis.

Viruses are infectious agents composed of genetic material enclosed in a protein coat. Some viruses also possess a lipid envelope derived from the host-cell membrane.

Basic structure:

• Genome: Contains either DNA or RNA, but not both as the primary genetic material.
• Capsid: A protein coat that protects the genome and helps deliver it into host cells.
• Envelope: A lipid membrane present in some viruses; it may contain viral glycoproteins for attachment.
• Enzymes: Some viruses carry enzymes needed for replication.

Viruses are considered acellular because they lack cytoplasm, ribosomes, a plasma membrane of cellular origin, and independent metabolism. They cannot reproduce on their own and must use the machinery of a host cell.

General stages of viral multiplication:

1. Attachment: Viral surface proteins bind to specific receptors on the host cell.
2. Entry: The virus or its genome enters the host cell.
3. Uncoating: The capsid is removed, releasing the viral genome.
4. Genome replication: The viral nucleic acid is copied using host or viral enzymes.
5. Protein synthesis: Host ribosomes produce viral proteins.
6. Assembly: Newly synthesized genomes and proteins form complete virus particles.
7. Release: Virions leave the cell by lysis, exocytosis, or budding.

The exact process depends on whether the virus contains DNA or RNA and whether it is enveloped. Viral replication can damage or destroy host cells.

Viroids are very small infectious agents composed only of a short, circular, single-stranded RNA molecule. They do not possess a capsid, envelope, protein-coding genes, or proteins.

Viroids cause diseases mainly in plants by interfering with gene regulation and normal cellular processes.

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by an ancestral eukaryotic cell.

Evidence includes:

• Both organelles possess circular DNA, similar to bacterial chromosomes.
• They contain 70S-like ribosomes.
• They reproduce by a process resembling binary fission.
• They have double membranes, consistent with an engulfment event.
• Their size and genetic features resemble those of bacteria.
• Molecular comparisons link mitochondria to aerobic bacteria and chloroplasts to cyanobacteria.

The engulfed cells provided useful metabolic abilities, such as aerobic respiration or photosynthesis, while receiving protection and nutrients from the host cell. Over time, many of their genes moved to the host nucleus.

Cellular compartmentalization is the division of the cytoplasm into specialized regions, particularly membrane-bound organelles.

• The nucleus separates genetic material from the cytoplasm and regulates gene expression.
• Mitochondria provide sites for controlled energy production.
• The endoplasmic reticulum synthesizes and transports proteins and lipids.
• The Golgi apparatus modifies and sorts cellular products.
• Lysosomes provide an acidic environment for digestion without exposing the entire cytoplasm to digestive enzymes.
• Chloroplasts isolate photosynthetic reactions in plant cells.

Compartmentalization increases efficiency, allows incompatible reactions to occur simultaneously, creates specialized conditions, and supports greater cellular complexity.