Cell Biology Quiz

Cells: The Working Units of Life

• Cells are the fundamental units of life.
• All organisms are composed of cells.
• All cells come from preexisting cells.
• Modern cells evolved from a common ancestor.
• Cell theory is a unifying theory in biology.

Cell Size

• Most cells are small due to a high surface area-to-volume ratio.
• As cell volume increases, chemical activity also increases, along with the need for resources and waste removal.
• Large organisms consist of many small cells.
• This maintains efficient material exchange
• Examples: Diameter (µm), surface area (µm²), volume (µm³), surface area-to-volume ratio

Microscopy

• Microscopes are used to see cells.
• Magnification: Increases apparent size.
• Resolution: The clarity of a magnified object (minimum distance two objects can be apart and still be seen as two distinct objects).
• Light microscopes use glass lenses and light; resolution = 0.2 µm
• Electron microscopes use electromagnets to focus an electron beam; resolution = 0.2 nm

Prokaryotic Cells

• Prokaryotic cells have no membrane-enclosed internal compartments.
• These cells are simple.
• Enclosed by a cell membrane.
• DNA located in a region called the nucleoid.
• Cytoplasm: Rest of the cell contents.
• Ribosomes: Sites of protein synthesis.
• Most prokaryotes have a rigid cell wall outside the cell membrane.
• Bacterial cell walls contain peptidoglycan, some have additional outer membrane.
• Some bacteria have a slimy capsule (polysaccharides).
• Bacteria that carry out photosynthesis have an internal membrane system for photosynthesis.
• Cytoskeleton maintain cell shape; and involved in cell division and cell movement.
• Examples of organelles in prokaryotic cells include capsule, cell wall, plasma membrane, ribosomes, nucleoid, cytoplasm, flagella and pili.

Eukaryotic Cells

• Eukaryotic cells have membrane-enclosed organelles for various functions.
• Eukaryotic cells are about 10 times larger than prokaryotes.
• Organelles were first studied using light microscopy, then electron microscopy.
• Stains targeted to specific molecules helped determine chemical composition of organelles. Cell fractionation separates organelles by size or density for chemical analyses.
• Examples of organelles in eukaryotic cells include ribosomes, nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, mitochondria and chloroplasts.

Nucleus

• The nucleus is a major organelle.
• Contains most of the cell's DNA.
• Site of DNA replication.
• Site where gene transcription is turned on or off.
• Assembly of ribosomes begins within nucleolus.
• The nucleus is surrounded by a nuclear envelope (double membrane).
• Nuclear pores control movement of molecules across the envelope.
• The outer membrane is continuous with the endoplasmic reticulum.

Endomembrane System

• An interconnected system of membrane-enclosed compartments.
• Tiny, membrane-bound vesicles shuttle substances between the various components.
• In living cells, membranes and the materials they contain are in constant motion.
• Examples of components include rough endoplasmic reticulum and smooth endoplasmic reticulum, golgi apparatus, and lysosomes.

Endoplasmic Reticulum

• Rough endoplasmic reticulum (RER): Ribosomes are attached, proteins enter RER lumen and modified, and transported to other regions in vesicles. These vesicles pinch off from the RER.
• Smooth endoplasmic reticulum (SER): No ribosomes, chemically modifies small molecules such as drugs and pesticides, site of glycogen degradation in animal cells, synthesis of lipids and steroids, stores calcium ions.

Ribosomes

• Sites of protein synthesis.
• Similar structure in prokaryotes and eukaryotes.
• Consist of two subunits (Ribosomal RNA [rRNA] and more than 50 different protein molecules).

Golgi Apparatus

• Flattened sacs (cisternae) and small vesicles.
• Receives proteins from the RER.
• Modifies, concentrates, packages, and sorts proteins.

Lysosomes

• Contain digestive enzymes that hydrolyze macromolecules into monomers.
• Sites for hydrolysis of materials taken into the cell by phagocytosis.
• Discarded components exit the cell by exocytosis.
• Primary lysosomes originate from the Golgi apparatus.

Mitochondria

• Transform energy in fuel molecules (such as glucose) to energy-rich ATP (cellular respiration).
• Cells with high energy needs have many mitochondria.
• Mitochondria can reproduce and divide independently of the nucleus.
• Mitochondria have two membranes: inner membrane folds inward to form cristae, a large surface area for proteins involved in cellular respiration. The mitochondrial matrix contains DNA and ribosomes for making proteins needed for cellular respiration.

Cytoskeleton

• Supports and maintains cell shape.
• Holds organelles in position.
• Moves organelles.
• Involved in cytoplasmic streaming.
• Interacts with extracellular structures to hold the cell in place.
• Composed of three types of filaments (microfilaments, intermediate filaments, and microtubules)

Microfilaments

• Help a cell or parts of a cell to move.
• Determine cell shape.
• Made from the protein actin.
• Actin polymerizes to form long helical chains (reversible).
• Have distinct ends (plus and minus).

Intermediate Filaments

• Tough, ropelike protein structures.
• Anchor cell structures in place.
• Resist tension.
• 50 distinct molecular classes, in 6 classes.

Microtubules

• Long, hollow cylinders.
• Form a rigid internal skeleton.
• Act as the framework of motor proteins.
• Made from dimers of protein tubulin.
• Can rapidly change length by adding or losing dimers at plus or minus ends.
• Important in cilia and flagella (with a 9+2 array)

Cilia and Flagella

• Cilia and eukaryotic flagella are made of microtubules in a 9+2 array.
• Cilia (short, hundreds on one cell); move stiffly to propel the cell or move fluid over a cell.
• Flagella (longer, usually one or two present); movement is snakelike.
• Microtubules serve as tracks for motor proteins, which move vesicles or organelles from one part of a cell to another, such as kinesin and dynein.

Extracellular Matrix

• Extracellular structures are secreted to the outside of the cell membrane.
• In eukaryotes, extracellular structures consist of fibrous macromolecules embedded in a gel-like medium.
• Many animal cells are surrounded by an extracellular matrix composed of fibrous proteins such as collagen, gel-like proteoglycans, and other proteins.
• Holds cells together in tissues.
• Contributes to properties of bone, cartilage, and skin.
• Filters materials passing between different tissues.
• Orients cell movements in development and tissue repair.
• Plays a role in chemical signaling.

Evolution of Eukaryotic Cells

• Evidence of the first eukaryotes appears in the fossil record about 2.7 billion years ago.
• The advent of compartmentalization and evolution of eukaryotic cells was a major event in the history of life.
• Evidence suggests that the nucleus and endomembrane system may have originated from the inward folds of the cell membrane in prokaryotes (endosymbiosis).
• Enclosed compartments would be advantageous because chemicals could be concentrated and chemical reactions would proceed more efficiently.
• Some organelles arose by symbiosis (endosymbiosis).

Test your knowledge on key concepts in cell biology, including the structures and functions of eukaryotic and prokaryotic cells. This quiz covers topics such as the nucleus, chromatin, and differences between Gram-positive and Gram-negative bacteria. Challenge yourself with matching organelles to their functions and understanding cellular processes.