UWCSEA IB Biology 2025 - A2.2 - Cell structure

A2.2.2— How do you calculate actual size of specimens on a microscope using image size and magnification? What is a scale bar? How can it be used to work out actual sizes within images?

Actual size = Image size / Magnification; A scale bar is a reference for size; a scale Bar:A visual reference with known dimensions on images. it helps determine actual sizes within images; by comparing the scale bar to the image, actual sizes can be calculated;

A2.2.7—What processes of life take place in unicellular organisms?

All the life processes: Metabolism: Definition: Chemical reactions within cells which are enzyme-based Response to Stimuli: Definition: Reacting to changes in the external environment. Homeostasis: Definition: Maintenance of constant internal conditions. Movement: Definition: Control over place and position. Growth: Definition: Increase in cell size or number. Reproduction: Definition: Production of offspring, sexual or asexual. Excretion: Definition: Removal of metabolic waste products. Nutrition: Definition: Intake or production of nutrients.

A2.2.8—What is the differences in eukaryotic cell structure between animals, fungi and plants?

Animal cells have no cell wall, no large permanent vacuoles, no chloroplasts, but do have centrioles and sperm cells have flagella Plant cells can have chloroplasts, do have a cell wall made of cellulose, permanent vacuoles, but do not have centrioles or flagella and cilia Fungi have a cell wall made of chitin, and small vacuoles, but do not have chloroplasts, centrioles, cilia or flagella

A2.2.10—Skill: identify animal cells under the light microscope

Animal cells have only a cell membrane and nucleus with cytoplasm under the light microscope

A2.2.9—What are atypical cell structures in eukaryotes? Explain in terms of aseptate fungal hyphae, skeletal muscle, red blood cells and phloem sieve tube elements

Aseptate Fungal Hyphae: Lack of septa (cross walls) in fungal hyphae. Allows continuous cytoplasmic and organelle flow; so no clear boundary between cells and nuclei dotted throughout; Skeletal Muscle Cells: Long, multinucleated cells with no clear boundary between; Red Blood Cells: Small, biconcave cells without a nucleus; Maximizes surface area for oxygen transport, flexibility; Phloem Sieve Tube Elements: Tubular cells in phloem for nutrient transport; Lack of nucleus and organelles enhances nutrient flow.

HL ONLY - A2.2.13—What is cell differentiation and how does it lead to specialized tissues in multicellular organisms?

Cell differentiation is the process leading to specialised tissues in multicellular organisms; this results from different genes being switched on in different cells; this is called gene expression; causing genes to be transcribed and protein to be produced; different patterns of gene expression are often triggered by environmental changes;

HL ONLY - A2.2.14—How is the evolution of multicellularity thought to have occurred?

Multicellularity evolved repeatedly; fungi, eukaryotic algae, plants, and animals are multicellular; Advantages:Allows for a larger body size; facilitates cell specialisation, where different cells perform different functions; allowing emergence of whole system interactions; enabling complex life to form;

A2.2.1—What is the basic structural unit of all living organisms?

The basic structural unit of all living organisms is the cell; Cell Characteristics: Microscopic, membrane-bound structure; Contains genetic material (DNA/RNA); Carries out life processes;

HL ONLY - A2.2.12—How can endosymbiosis be used to explain the origins of Eukaryotic cells?

Eukaryotes evolved from a common ancestor with a nucleus; mitochondria and chloroplasts (in plant cells) originated through endosymbiosis; where larger prokaryotic cells engulfed smaller cells; without digesting them; leading to a symbiotic relationship; Evidence:Presence of 70S ribosomes in mitochondria and chloroplasts; which are only found in prokaryotic cells; Naked circular DNA found in mitochondria and chloroplast; and the ability of mitochondria and chloroplasts to replicate inside cells;

A2.2.10 Skill: Identify microvilli

Finger like projections on animal cells

A2.2.10—What does smooth endoplasmic reticulum look like on electron micrographs?

Tubules without ribosomes

A2.2.4—What structures are common to cells in all living organisms? What is the reason for each structure?

Typical cells have DNA as genetic material; and a cytoplasm composed mainly of water; where enzyme-based reactions occur; which is enclosed by a plasma membrane composed of lipids;

A2.2.3—What developments have taken place in microscopy? How are electron microscopy, freeze fracture, cryogenic electron microscopy, and the use of fluorescent stains and immunofluorescence in light microscopy used?

New higher resolution microscopes were developed after light microscopes; resolution is being able to tell two close together points apart; Electron Microscopes: Higher-resolution imaging using electrons; allows small cell organelles like ribosomes to be seen, not just nuclei; Freeze Fracture; samples are frozen, cracked and then viewed under electron microscopes; Cryogenic Electron Microscopy; samples are frozen and electron beams are analysed by a computer to build up a 3d model; Fluorescent Stains and Immunofluorescence: Stains attach to cell components and then can be used to track movement of molecules;

A2.2.2—What are the parts of a microscope? How do you focus one?

Parts of a microscope include the eyepiece, objective lenses, stage, condenser, and light source. To focus, use the coarse adjustment for initial focusing and the fine adjustment for detailed focusing; Microscope Parts:Eyepiece: Contains the lens viewed through. Objective Lenses: Magnify the specimen. Stage: Supports the specimen. Condenser: Focuses light on the specimen. Light Source: Illuminates the specimen.

A2.2.10—Skill: identify plant cells under the light microscope

Plant cells have a cell wall and may have chloroplasts and a large permanent vacuole; usually rectangular or square, but can be circular as well;

A2.2.6—What is the structure of a Eukaryote cell? What parts do they have and what are the roles of these parts?

Plasma Membrane: Made of proteins and phospholipids, separates cell interior, controls entry and exit of substances Cytoplasm: Water-based fluid where enzyme-catalysed metabolic reactions take place; Mitochondria: Double-membrane organelles that produce ATP in aerobic respiration, which is the energy for cellular processes; 80S Ribosomes: Sites for translation (mRNA used to make protein), larger and higher mass than prokaryotic ribosomes; Facilitates protein synthesis in eukaryotic cells. Nucleus: Contains DNA with histone proteins, nucleolus produces ribosomes. Smooth Endoplasmic Reticulum: Produces and stores lipids, including steroids. Rough Endoplasmic Reticulum: Has ribosomes, produces proteins (protein synthesis) for external use. Golgi Apparatus: Processes, modifies and packages proteins, releasing them in vesicles. Vesicle: Small sac that transports and releases cellular substances. Vacuole: Maintains osmotic balance, stores substances, may have hydrolytic breakdown (digestive) functions. Cytoskeleton: System of microtubules and microfilaments that supports organelles, maintaining cell structure.

A2.2.5—What is the structure of a prokaryote cell?

Simple cell structure without compartmentalisation and without a nucleus; Prokaryotic means before nucleus; they contain small ribosomes (70s); loose DNA in a circular chromosome; in nucleoid region; cell wall made of peptidoglycan; Plasmids, which are small, circular DNA pieces which can transfer to other bacteria;

A2.2.10—What does rough endoplasmic reticulum look like on electron micrographs?

Small dots (ribosomes) on tubule networks