AP Biology Unit 2: Cells

Free Unit Notes

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I. Cell Structure and Function (Topic 2.1)

  A. Shared cell structures

1. All cells have a plasma membrane, cytosol, genetic material, and ribosomes. The membrane separates the cell from its surroundings, and the cytosol is the fluid where many reactions occur.

2. Prokaryotic cells lack a membrane-bound nucleus and typically lack membrane-bound organelles. Their DNA is in a nucleoid region. Bacteria and archaea are prokaryotes.

3. Eukaryotic cells have a nucleus and membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes.

  B. Nucleus and ribosomes

1. The nucleus holds most eukaryotic DNA. Nuclear pores control exchange across its double-membrane envelope.

2. Ribosomes contain ribosomal RNA (rRNA) and proteins. They read messenger RNA (mRNA) to assemble polypeptides and are not surrounded by a membrane.

3. Ribosomes occur in all forms of cellular life, supporting common ancestry. Free ribosomes commonly make cytosolic proteins; ER-bound ribosomes make proteins entering the endomembrane system.

  C. Endomembrane system

1. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, vesicles, and plasma membrane. These structures cooperate to process and move cellular materials.

2. The endoplasmic reticulum (ER) is a membrane network that supports cell organization and transports materials internally.
   a. Rough ER has attached ribosomes. Proteins entering it are folded and begin processing for secretion, membranes, or certain organelles.
   b. Smooth ER makes lipids, helps detoxify chemicals, and stores calcium in some cells.

3. The Golgi apparatus is a stack of flattened membrane sacs. It modifies, sorts, and packages proteins and lipids for delivery.

4. Lysosomes contain digestive enzymes in an acidic compartment. They break down macromolecules and worn-out cell parts and can help with programmed cell death, or apoptosis.

5. Vacuoles store water, nutrients, and other materials. A plant cell’s large central vacuole helps maintain pressure against the cell wall; animal cells can have smaller storage compartments.

6. A secreted protein is made at rough ER, travels in a vesicle to the Golgi, and leaves in a secretory vesicle that fuses with the plasma membrane. Golgi damage can disrupt delivery even if protein synthesis continues.

  D. Energy-converting organelles and support structures

1. Mitochondria carry out major stages of aerobic respiration. Their double membrane separates reaction spaces; folds called cristae increase inner-membrane area for ATP production.

2. Chloroplasts perform photosynthesis in plants and algae. A double membrane surrounds them, and internal thylakoid membranes capture light energy. The surrounding stroma contains enzymes that help build sugars.

3. The cytoskeleton is a network of protein filaments that supports cell shape, moves materials, and helps with cell division and movement.

4. A cell wall lies outside the plasma membrane and provides support and protection. Animal cells lack cell walls.

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II. Cell Size (Topic 2.2)

  A. Surface area and volume

1. Surface area determines the space available for exchange; volume reflects the amount of cell contents requiring nutrients and producing wastes.

2. As a cell grows without changing shape, volume increases faster than surface area. Its surface-area-to-volume ratio (SA:V) decreases, making exchange less adequate for its needs.

3. Small cells usually have more exchange area per unit volume and shorter diffusion distances. Thin shapes, membrane folds, microvilli, and root hairs can improve exchange.

4. Smaller organisms generally exchange heat faster per unit mass because they have more surface area relative to volume. Among comparable animals, smaller bodies generally have higher metabolic rates per unit mass.

  B. Calculating surface-area-to-volume ratios

1. Calculate SA:V by dividing total surface area by total volume. Use consistent length units; surface area uses squared units and volume uses cubed units.

2. For a cube with side s: SA = 6s²; V = s³; SA/V = 6/s. Doubling s multiplies area by 4 and volume by 8, so the ratio is halved.

3. For a sphere with radius r: SA = 4πr²; V = (4/3)πr³; SA/V = 3/r.

4. For a rectangular solid: SA = 2lw + 2lh + 2wh; V = lwh. Here l, w, and h are length, width, and height.

5. For a closed cylinder: SA = 2πr² + 2πrh; V = πr²h. Here r is radius and h is height.

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III. Plasma Membrane (Topic 2.3)

  A. Phospholipid bilayer

1. Phospholipids are amphipathic: they have a water-attracting, polar head and water-repelling, nonpolar fatty-acid tails.

2. In the phospholipid bilayer, heads face the watery cytosol and external fluid. Tails face inward toward one another, creating a hydrophobic barrier.

  B. Fluid mosaic model

1. The fluid mosaic model describes a flexible bilayer containing many different components. Lipids and many proteins move sideways within it.

2. Membrane proteins serve as channels, carriers, receptors, enzymes, and anchors. Regions touching lipid tails are usually hydrophobic; regions facing water or lining channels are usually hydrophilic.

3. Glycoproteins are proteins with attached carbohydrate chains; glycolipids are lipids with attached carbohydrate chains. Their external carbohydrate chains help cells recognize and interact with one another.

4. Unsaturated fatty-acid tails have bends that reduce tight packing and generally increase fluidity. Saturated tails pack more tightly.

5. Cholesterol helps stabilize animal cell membranes. It limits excessive movement at high temperatures and reduces tight packing at low temperatures.

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IV. Membrane Permeability (Topic 2.4)

  A. Selective permeability

1. Selective permeability means a membrane allows some substances to cross more easily than others. Passage depends on size, polarity, charge, and available transport proteins.

2. Small nonpolar molecules, such as O₂, CO₂, and N₂, pass readily through the bilayer by simple diffusion.

3. Small uncharged polar molecules, including water and ammonia, can cross in limited amounts. Water often crosses much faster through aquaporin channels.

4. Ions and large polar molecules cross the hydrophobic core poorly. Ions usually require channels or carriers; glucose typically requires a carrier.

  B. Cell walls and exchange

1. Cell walls support plants, fungi, and many bacteria and archaea. They resist expansion and help prevent osmotic lysis, or bursting after excessive water entry.

2. Walls allow many small substances through but can restrict some larger materials. The plasma membrane beneath the wall provides more selective control of exchange.

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V. Membrane Transport (Topic 2.5)

  A. Gradients and passive transport

1. A concentration gradient is a difference in a substance’s concentration between regions. Selective membranes allow different concentrations to persist across them.

2. Diffusion is net movement from higher to lower concentration due to random particle motion. Passive transport requires no direct input of cellular energy.

3. At dynamic equilibrium, particles still move both ways, but equal opposing rates produce no net movement.

4. An ion’s electrochemical gradient combines its concentration difference with the electrical attraction or repulsion caused by charge differences across the membrane.

5. Diffusion generally becomes faster with a steeper gradient, more exchange area, a shorter distance, or greater permeability. Higher temperature usually speeds molecular movement within normal biological conditions.

  B. Energy-dependent and bulk transport

1. Active transport uses energy to move substances, often against their concentration or electrochemical gradients. It helps maintain internal conditions that differ from the surroundings.

2. Endocytosis brings material into a cell when the plasma membrane folds inward and forms a vesicle.
   a. Phagocytosis takes in large particles; pinocytosis takes in extracellular fluid.
   b. Receptor-mediated endocytosis selectively takes in molecules that bind to membrane receptors.

3. Exocytosis releases material when a vesicle fuses with the plasma membrane. Cells use it to secrete products such as proteins.

4. Endocytosis and exocytosis require cellular energy. Their cargo moves in vesicles rather than through the lipid core or a transport protein.

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VI. Facilitated Diffusion (Topic 2.6)

  A. Channels and carriers

1. Facilitated diffusion is passive movement through a transport protein down a concentration or electrochemical gradient. It does not directly use ATP.

2. Channel proteins provide hydrophilic passageways for particular substances. Some channels open or close in response to signals.

3. Aquaporins are channels that allow large amounts of water to cross membranes rapidly.

4. Carrier proteins bind specific substances and change shape to move them across. A carrier moving glucose down its gradient performs facilitated diffusion.

  B. Limits and disruptions

1. Carrier-mediated transport can reach a maximum rate when all available carriers are occupied. A plateau in a transport graph can indicate limited carrier capacity.

2. Blocking a carrier can reduce transport even when a favorable gradient remains. Adding carriers can increase capacity if other conditions allow.

3. Passive transport can continue after ATP production stops while a gradient remains. It may slow later as energy-dependent pumps stop maintaining that gradient.

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VII. Tonicity and Osmoregulation (Topic 2.7)

  A. Osmosis and tonicity

1. Osmosis is net water movement across a selectively permeable membrane from higher water potential to lower water potential.

2. When pressure is equal, water generally moves toward the side with more dissolved solute that cannot cross the membrane.

3. Tonicity describes how an external solution affects cell volume. It depends mainly on solutes that do not readily cross the membrane.

4. A hypotonic solution has fewer nonpenetrating solutes than the cell. Water enters; an animal cell swells and may lyse, while a plant cell becomes firm, or turgid.

5. A hypertonic solution has more nonpenetrating solutes than the cell. Water leaves; an animal cell shrinks, and a plant cell may undergo plasmolysis, when its membrane pulls away from its wall.

6. An isotonic solution causes no sustained net change in cell volume. Animal cells maintain their volume; plant cells are usually flaccid, with little pressure against the wall.

  B. Water potential

1. Water potential (Ψ) predicts net water movement. Water moves from higher Ψ to lower Ψ; for example, from −2 bar toward −5 bar.

2. Ψ = Ψs + Ψp, where Ψs is solute potential and Ψp is pressure potential. Pure water at atmospheric pressure has Ψ = 0.

3. Adding solute lowers Ψs, making it more negative. Positive pressure raises Ψp; tension can make Ψp negative. An open solution at atmospheric pressure has Ψp = 0.

4. For dilute solutions, Ψs = −iCRT.
   a. i is the ionization factor: approximately 1 for sucrose and 2 for NaCl when it dissociates fully.
   b. C is molar concentration in mol/L; R = 0.0831 L·bar/(mol·K); T is temperature in kelvin, calculated as °C + 273.
   c. These units give Ψs in bar. Keep pressure units consistent when combining potentials.

5. A 0.20 M sucrose solution at 298 K has Ψs = −(1)(0.20)(0.0831)(298) ≈ −4.95 bar. At atmospheric pressure, its total Ψ is also −4.95 bar.

6. Turgor pressure is the pressure of a plant cell’s contents against its wall. It can stop net water entry when the cell’s total water potential equals its surroundings.

  C. Maintaining water balance and interpreting evidence

1. Osmoregulation is control of water and solute balance. It protects cell volume and allows normal metabolism; some freshwater protists expel excess water with contractile vacuoles.

2. In tissue experiments, percent change in mass = [(final mass − initial mass) / initial mass] × 100%. Positive values indicate net gain; negative values indicate net loss.

3. Plot external solute concentration on the x-axis and percent mass change on the y-axis, with units and an appropriate scale. The zero-change crossing estimates the concentration causing no net mass change under the test conditions.

4. Change one factor at a time and keep factors such as tissue size, temperature, and exposure time consistent. Use repeated samples to assess variation.

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VIII. Mechanisms of Transport (Topic 2.8)

  A. Primary active transport

1. Primary active transport directly couples transport to an energy source, commonly ATP hydrolysis. Membrane pumps can move ions against their electrochemical gradients.

2. The sodium–potassium pump uses one ATP to move three Na⁺ out and two K⁺ into the cell per cycle. It helps maintain ion gradients and the membrane potential, or voltage across the membrane.

3. Proton pumps use energy to move H⁺ across membranes. The resulting gradient stores energy for other processes.

  B. Secondary active transport

1. Secondary active transport uses one substance moving down its gradient to drive another substance against its gradient. This coupling is called cotransport.

2. A cotransporter may not use ATP directly, but the system depends on energy used earlier to establish the driving gradient.

3. If ATP supply falls, pumps slow and gradients gradually weaken. Secondary transport and water balance can then be disrupted.

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IX. Cell Compartmentalization (Topic 2.9)

  A. Specialized reaction spaces

1. Compartmentalization separates cellular processes into specialized regions. Membranes control which substances enter and leave each region.

2. Compartments maintain conditions suited to particular reactions and reduce interference between processes. Lysosomes, for example, keep digestive enzymes in an acidic space separate from the cytosol.

3. Internal membranes also increase the surface area available for reactions. Mitochondrial cristae and chloroplast thylakoids provide surfaces for energy-converting proteins.

  B. Coordination and membrane integrity

1. Compartments exchange materials through regulated transport and vesicles. Their coordinated work allows the cell to function as a whole.

2. Mitochondrial inner membranes and thylakoid membranes separate regions with different H⁺ concentrations. These gradients help power ATP synthesis.

3. If a membrane leaks H⁺, the gradient can weaken and ATP production can decrease even when the energy-converting proteins remain present.

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X. Origins of Cell Compartmentalization (Topic 2.10)

  A. Comparing cellular organization

1. Eukaryotic internal membranes divide cells into specialized compartments. Prokaryotes typically lack membrane-bound organelles but still have specialized internal regions and functions.

  B. Endosymbiotic theory

1. Endosymbiotic theory explains that mitochondria and chloroplasts descended from bacteria that lived inside ancestral host cells.

2. Bacteria that performed aerobic respiration gave rise to mitochondria. A later incorporation of photosynthetic bacteria gave rise to chloroplasts in the ancestors of plants and algae.

3. Several lines of evidence support this explanation.
   a. Both organelles have their own DNA, commonly circular, and bacterial-like ribosomes.
   b. They divide in a process resembling bacterial binary fission and have double membranes.
   c. Comparisons of genes and molecular structures connect them to bacterial lineages.

4. These organelles still depend on proteins encoded by nuclear DNA. Endosymbiosis explains their ancestry rather than the origin of every membrane-bound organelle.

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