AP Biology Unit 2: Cell Structure and Function and Cell Transport

Comprehensive practice flashcards for AP Biology Unit 2, covering cell structure, organelles, compartmentalization, endosymbiosis, membrane models, transport mechanisms, and water potential calculations.

What are the four fundamental principles of cells outlined in the notes?

1. All organisms are made of cells.
2. The cell is the simplest collection of matter that can live.
3. Cell structure is correlated to cellular function.
4. All cells are related by their descent from earlier cells.

What are the key structural distinctions between prokaryotic and eukaryotic cells?

Prokaryotic cells have no nucleus (DNA is in an unbound region called the nucleoid), have no membrane-bound organelles, and include bacteria and Archaea.

Eukaryotic cells have a nucleus containing DNA, possess membrane-bound organelles, are usually larger, have cell size limited by metabolic activities, and include plants, animals, and fungi.

How do scanning electron microscopes (SEMs) differ from transmission electron microscopes (TEMs)?

SEMs focus a beam of electrons onto the surface of a specimen to produce images that look 3-D. TEMs focus a beam of electrons through a specimen and are used mainly to study the internal structure of cells.

What are the four basic features shared by all cells?

1. Plasma membrane (a selectively permeable double layer of phospholipids)
2. Cytosol (a semifluid substance that includes cytoplasm and organelles)
3. Chromosomes (carry genes)
4. Ribosomes (make proteins)

Which subcellular components are found exclusively in plant cells or animal cells when comparing the two?

Found in plant cells but not animal cells: Chloroplasts, central vacuole and tonoplast, cell wall, and plasmodesmata.

Found in animal cells but not plant cells: Lysosomes, centrioles, and flagella (present only in some plant sperm).

What are the structural features and primary functions of the nucleus, nuclear envelope, and nucleolus?

The nucleus contains most of the cell's genes (DNA). The nuclear envelope encloses the nucleus, separating it from the cytoplasm with a double membrane, each consisting of a lipid bilayer. The nucleolus is located within the nucleus and is the site of ribosomal RNA ($\text{rRNA}$) synthesis.

What are ribosomes composed of, and what are their two cellular locations?

Ribosomes are complexes made of ribosomal RNA ($\text{rRNA}$) and protein. They carry out protein synthesis in two locations: free ribosomes in the cytosol, and bound ribosomes on the outside of the endoplasmic reticulum or the nuclear envelope.

How do the structural and metabolic functions of the smooth ER differ from the rough ER?

Smooth ER lacks ribosomes; it synthesizes lipids (membrane components), metabolizes carbohydrates, stores calcium, and detoxifies poison.

Rough ER has bound ribosomes and produces proteins and membranes.

What are the cisternae and the main functions of the Golgi apparatus?

The Golgi apparatus consists of flattened membranous sacs called cisternae. Its functions include modifying products of the ER, manufacturing certain macromolecules, and sorting and packaging materials into transport vesicles between its receiving cis face and shipping trans face.

What is protein glycosylation in the Golgi apparatus, and what are its three forms?

Glycosylation is the enzyme-mediated attachment of a carbohydrate to a functional group on a protein to determine its function (folding, stability) and targeting. The three types are N-linked (attached to nitrogen), O-linked (attached to a hydroxyl group, $\text{-OH}$), and C-linked (attached to carbon; rare).

What is the function of lysosomes, and what is autophagy?

Lysosomes are membranous sacs of hydrolytic enzymes that hydrolyze proteins, fats, polysaccharides, and nucleic acids. Autophagy is the process where lysosomes use these hydrolytic enzymes to recycle the cell's own organelles and macromolecules.

What are the specialized functions of food vacuoles, contractile vacuoles, and central vacuoles?

Food vacuoles are formed by phagocytosis for digestion. Contractile vacuoles are found in many freshwater protists to pump excess water out of the cell. Central vacuoles are found in many mature plant cells to hold organic compounds and water.

How is the mitochondrion structured to maximize chemical energy conversion?

It features a smooth outer membrane and an inner membrane folded into cristae, creating two compartments: the intermembrane space and the mitochondrial matrix. The folding of cristae creates more surface area for enzymes that synthesize ATP.

What are the internal structural components of a chloroplast?

Chloroplasts contain chlorophyll along with enzymes that function in photosynthesis. Their structure includes thylakoids (membranous sacs), grana (stacks of thylakoids), and stroma (the internal fluid).

Based on this diagram of plastid differentiation, what are the distinct roles of etioplasts, chromoplasts, chloroplasts, leucoplasts, amyloplasts, elaioplasts, and proteinoplasts?

• Etioplast: Crystalline prolamellar bodies, precursor of chlorophyll
• Chromoplast: Pigment synthesis and storage
• Chloroplast: Photosynthesis, lipid synthesis
• Leucoplast: Synthesis of fatty acids and amino acids
• Amyloplast: Storage of starch and sugars
• Elaioplast: Storage of lipids, oils
• Proteinoplast: Sites of enzyme (protein) activity

What metabolic reaction characterizes peroxisomes?

Peroxisomes are specialized single-membrane oxidative metabolic compartments that undergo oxidation reactions, producing hydrogen peroxide ($\text{H}_2\text{O}_2$) and converting it to water ($\text{H}_2\text{O}$), while aiding in the breakdown of lipids.

According to the table shown, compare the structure, diameter, protein subunits, and main functions of microtubules, microfilaments, and intermediate filaments.

• Microtubules: Hollow tubes; $25\,\text{nm}$ diameter with $15\text{-nm}$ lumen; tubulin ($\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$ dimers); maintain cell shape (compression-resisting girder), motility (cilia/flagella), chromosome movements in cell division, organelle movements.
• Microfilaments: Two intertwined strands of actin; $7\,\text{nm}$ diameter; actin subunits; maintain cell shape (tension-bearing), muscle contraction, cytoplasmic streaming, amoeboid movement, animal cell division.
• Intermediate Filaments: Fibrous proteins coiled into cables; $8\text{--}12\,\text{nm}$ diameter; proteins such as keratins; maintain cell shape (tension-bearing), anchorage of nucleus and organelles, formation of nuclear lamina.

What is the function of dynein in cilia and flagella?

Dynein is a motor protein that drives the bending movements of a cilium or flagellum by interacting with the microtubules sheathed within the plasma membrane.

What is cytoplasmic streaming, and what forces drive it in plant cells?

Cytoplasmic streaming is the circular flow of cytoplasm within cells that speeds the distribution of materials. In plant cells, it is driven by actin-myosin interactions and sol-gel transformations.

Which cellular structures constitute the endomembrane system?

The nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane (which are either continuous or connected via transport vesicles).

What are the advantages and disadvantages of cellular compartmentalization?

Advantages:

• Efficiency: Different microenvironments can exist in each membrane region for more effective work.
• Multi-functionality: Many different activities can occur simultaneously in different compartments.
• Increased surface area: Folded/double membranes provide increased area for reactions.

Disadvantages:

• Requires larger cell size, which decreases the surface area-to-volume ratio and increases diffusion time.
• Creates membrane barriers that block the free movement of materials.
• Requires active transport consuming additional $\text{ATP}$.

What evidence shown in the diagram supports the endosymbiont theory for the origin of mitochondria and chloroplasts?

Mitochondria and chloroplasts evolved from prokaryotes engulfed by ancestral eukaryotic host cells. Evidence includes: being enveloped by a double membrane, possessing their own circular DNA molecules, containing free ribosomes, and growing/reproducing somewhat independently in cells.

Based on this surface area-to-volume comparison, why does a cell's volume growth constrain its maximum size?

As cell dimensions increase (e.g., from a side length of $1$ to $5$), volume increases proportionately more ($125\times$) than surface area ($25\times$), causing the surface-to-volume ratio to drop from $6$ to $1.2$. Dividing the same volume into smaller compartments restores a high ratio ($6$), preserving efficient exchange of materials.

What four cell types are cited as adaptations for maximizing surface area-to-volume ($\text{SA:V}$) ratios?

1. Root hair cells in plants
2. Guard cells in plants
3. Gut epithelial cells (with microvilli)
4. Lung epithelial cells

How does cholesterol maintain membrane fluidity across varying temperatures in animal cells?

At warm temperatures (such as $37\,^{\circ}\text{C}$), cholesterol restrains phospholipid movement. At cool temperatures, it prevents tight packing of phospholipids, inhibiting the membrane from switching into a solid state.

What is the structural difference between peripheral and integral membrane proteins?

Peripheral proteins are not embedded in the lipid bilayer; integral proteins penetrate the hydrophobic core of the bilayer, and those that span the entire membrane are called transmembrane proteins.

What are the structural roles and functions of tight junctions, desmosomes, and gap junctions depicted here?

• Tight junctions: Press neighboring cell membranes together to prevent leakage of extracellular fluid across cell layers.
• Desmosomes (anchoring junctions): Fasten cells together into strong sheets using intermediate filaments.
• Gap junctions (communicating junctions): Provide cytoplasmic channels between adjacent cells for direct communication and transport.

What are the layers of a plant cell wall and the function of plasmodesmata?

Plant cell walls consist of a relatively thin and flexible primary cell wall, a middle lamella between adjacent cells, and in some cells a secondary cell wall between the plasma membrane and primary wall. Plasmodesmata are perforating channels that permit water, small solutes, proteins, and RNA to move between adjacent plant cells.

What are the components of an electrochemical gradient across a cell membrane?

An electrochemical gradient drives the diffusion of ions across a membrane and includes two forces: a chemical force (the ion's concentration gradient) and an electrical force (the effect of membrane potential/voltage on the ion's movement).

How does an electrogenic pump function, and what is the primary electrogenic pump in plants, fungi, and bacteria?

An electrogenic pump is a transport protein that generates voltage across a membrane. The main electrogenic pump in plants, fungi, and bacteria is the proton pump, which expels $H^+$ using $\text{ATP}$.

How does the sodium-potassium ($\text{Na}^+/\text{K}^+$) pump maintain ion gradients in animal cells?

Energized by the transfer of a phosphate group from $\text{ATP}$ hydrolysis, the pump moves $\text{Na}^+$ out of the cytoplasm and $\text{K}^+$ into the cell against their gradients, maintaining a high intracellular $\text{K}^+$ and low intracellular $\text{Na}^+$ concentration.

Identify and explain the cellular transport mechanisms shown in this summary chart, including their representative cell examples.

• Phagocytosis ('cell eating'): Engulfs large particles/bacteria into a phagosome; example: macrophage/neutrophil destroying harmful materials.
• Receptor-mediated endocytosis: Ingestion of specific ligands via receptor-coated vesicles; example: hepatocyte taking in LDL cholesterol.
• Pinocytosis ('cell drinking'): Ingestion of droplets of extracellular fluid and dissolved solutes; example: intestinal or kidney epithelial cells.
• Exocytosis ('releasing out'): Secretory vesicles fuse with the plasma membrane to expel substances; example: pancreatic beta cells secreting insulin or neurons releasing neurotransmitters.

What morphological states do animal and plant cells exhibit when placed in hypotonic, isotonic, and hypertonic solutions as shown here?

• Hypotonic solution: Animal cells become lysed (burst); plant cells become turgid (normal state due to turgor pressure).
• Isotonic solution: Animal cells are normal; plant cells become flaccid.
• Hypertonic solution: Animal cells become shriveled; plant cells become plasmolyzed as the plasma membrane pulls away from the cell wall.

What are the formulas and variables used to calculate water potential ($\Psi$) and solute potential ($\Psi_S$)?

• Water potential: \Psi = \n\Psi_P + \Psi_S, where $\Psi_P$ is pressure potential ($\Psi_P = 0$ in an open container, and $\Psi_P = 1$ for a turgid plant cell).
• Solute potential: $\Psi_S = -iCRT$, where:
  • $i$ = ionization constant ($1.0$ for sucrose and glucose; $2$ for $\text{NaCl}$)
  • $C$ = molar concentration
  • $R$ = pressure constant ($0.0831\,\text{liter bars/mole K}$
  • $T$ = temperature in Kelvin ($273 + ^{\circ}\text{C}$)