AOL 1 : Cell Organization and Membranes

Compare and contrast the organization of prokaryotic/eukaryotic cells and plant/animal cells.

• Prokaryotic vs. eukaryotic cells

  • Similarities

    • Both are surrounded by a plasma membrane made of a phospholipid bilayer.

    • Both contain cytoplasm, ribosomes for protein synthesis, and DNA as genetic material.

  • Prokaryotes (bacteria and archaea)

    • Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

    • They generally have no membrane-bound organelles.

    • They are usually small (about 1–10 µm) and typically have a single circular chromosome. Many also have a cell wall.

  • Eukaryotes (animals, plants, fungi, protists)

    • Their DNA is enclosed within a membrane-bound nucleus.

    • They contain many membrane-bound organelles that compartmentalize the cell, so different chemical reactions can occur in separate, specialized environments.

    • They are usually larger (about 10–100 µm) and have multiple linear chromosomes.

• Plant vs. animal cells (both eukaryotic)

  • Shared features

    • Both have a nucleus, mitochondria, ER, Golgi apparatus, and ribosomes.

  • Features found only in plant cells

    • A rigid cell wall made of cellulose outside the plasma membrane, which provides structural support.

    • Chloroplasts, which carry out photosynthesis.

    • A large central vacuole, which stores water and ions and maintains turgor pressure.

  • Features typical of animal cells

    • Lysosomes are prominent in animal cells, and plant cells rely more on the vacuole for similar digestive functions.

    • Animal cells have no cell wall and no chloroplasts, and they have small vacuoles rather than a large central one.

Define the terms organelle, cytoplasm, and cytosol.

• Organelle

  • An organelle is a specialized structure within a cell that performs a specific function.

  • In eukaryotes, most organelles are enclosed by their own membrane (for example, the nucleus, mitochondria, and ER).

  • Organelles allow compartmentalization: incompatible reactions can happen at the same time in different locations, and the conditions in each compartment (such as pH) can be tailored to its job.

• Cytoplasm

  • The cytoplasm is everything inside the plasma membrane except the nucleus (in eukaryotes).

  • It includes the cytosol, all of the organelles, and the cytoskeleton.

  • In prokaryotes, the cytoplasm is the entire interior of the cell, including the nucleoid region.

• Cytosol

  • The cytosol is the fluid, gel-like portion of the cytoplasm that surrounds the organelles.

  • It is mostly water, with dissolved ions, small molecules, and proteins.

  • Many metabolic reactions occur here, such as glycolysis and the first stages of protein synthesis on free ribosomes.

• How they relate

  • Cytosol is a component of cytoplasm, and cytoplasm is the cytosol plus the organelles and cytoskeleton.

Describe the function of common eukaryotic organelles: mitochondria, chloroplast, nucleus, endoplasmic reticulum, Golgi apparatus, lysosome, and vacuole.

• Nucleus

  • It stores the cell’s DNA and is the site of DNA replication and transcription (making RNA).

  • It is surrounded by a double membrane called the nuclear envelope, which has nuclear pores that regulate traffic of molecules such as mRNA and proteins.

  • The nucleolus within the nucleus is where ribosomal RNA is made and ribosome subunits are assembled.

• Mitochondria

  • They are the main site of aerobic cellular respiration, which produces most of the cell’s ATP.

  • They have a double membrane. The inner membrane is folded into cristae, which increase the surface area for the electron transport chain and ATP synthase.

  • They contain their own DNA and ribosomes.

• Chloroplasts (plants and algae)

  • They carry out photosynthesis, converting light energy, CO₂, and water into sugar and O₂.

  • They have an inner stack of membranes called thylakoids (grouped into grana) that hold chlorophyll and carry out the light reactions. The surrounding fluid, the stroma, is where the Calvin cycle builds sugar.

• Endoplasmic reticulum (ER)

  • The rough ER is studded with ribosomes. It synthesizes proteins that are secreted, inserted into membranes, or sent to certain organelles, and it helps fold them.

  • The smooth ER has no ribosomes. It synthesizes lipids (including phospholipids and steroids), metabolizes carbohydrates, detoxifies drugs and poisons, and stores Ca²⁺.

• Golgi apparatus

  • It receives proteins and lipids from the ER in vesicles, then modifies (for example, by adding carbohydrate groups), sorts, and packages them.

  • It ships them in vesicles to their destinations: the plasma membrane, lysosomes, or secretion outside the cell.

• Lysosome

  • It is a membrane-bound sac of hydrolytic enzymes that work best at acidic pH (about 5).

  • It digests macromolecules, worn-out organelles, and material taken in by the cell, such as food particles and bacteria.

• Vacuole

  • It is a membrane-bound sac used for storage of water, ions, nutrients, and waste.

  • In plants, the large central vacuole maintains turgor pressure against the cell wall (keeping the plant rigid) and can also perform lysosome-like digestion.

Identify the structures of different lipids: triglycerides, fatty acids, phospholipids, and cholesterol.

• General property of lipids

  • Lipids are mostly nonpolar (hydrophobic) molecules, so they dissolve poorly in water.

• Fatty acid

  • It has a hydrocarbon chain (typically 12–24 carbons) with a carboxyl group (–COOH) at one end.

  • The long chain is hydrophobic, while the carboxyl “head” is polar and can be charged.

• Triglyceride (fat or oil)

  • It consists of one glycerol (a three-carbon alcohol) bonded to three fatty acids through ester linkages.

  • Each ester bond forms by dehydration (condensation) synthesis, releasing one water molecule per bond.

  • Triglycerides are entirely nonpolar and are used for long-term energy storage.

• Phospholipid

  • It consists of a glycerol backbone bonded to two fatty acids and one phosphate group (often linked to another small polar group such as choline).

  • It is amphipathic: the phosphate-containing head is hydrophilic, and the two fatty acid tails are hydrophobic.

  • It is the main structural component of cell membranes.

• Cholesterol

  • It is a steroid, built on four fused carbon rings rather than fatty acid chains.

  • It has a short hydrocarbon tail and a small polar hydroxyl (–OH) group, which makes it weakly amphipathic.

  • It inserts between phospholipids in animal cell membranes and helps regulate membrane fluidity. It is also the precursor of steroid hormones.

Differentiate between saturated and unsaturated fatty acids.

• Saturated fatty acids

  • Every carbon in the chain is bonded to the maximum number of hydrogen atoms, so there are only single C–C bonds.

  • The chains are straight, so the molecules pack tightly together.

  • Tight packing allows strong van der Waals (London dispersion) interactions, so saturated fats are typically solid at room temperature (for example, butter and lard).

• Unsaturated fatty acids

  • They contain one or more C=C double bonds, which mean fewer hydrogens per carbon.

  • Monounsaturated fatty acids have one double bond, and polyunsaturated fatty acids have more than one.

  • A cis double bond introduces a kink (bend) in the chain, which prevents tight packing.

  • Weaker interactions between kinked chains make unsaturated fats typically liquid at room temperature (for example, olive oil).

• Why it matters for membranes

  • Kinked unsaturated tails keep phospholipids spaced apart, which increases membrane fluidity. Straight saturated tails pack closely and decrease fluidity.

Describe the structure of the plasma membrane and the different ways that proteins can be associated with the lipid bilayer.

• Overall structure: the fluid mosaic model

  • The membrane is a phospholipid bilayer in which the hydrophilic heads face outward toward the watery cytosol and extracellular fluid, and the hydrophobic tails face inward, away from water.

  • It is fluid because lipids and many proteins can move laterally within the layer.

  • It is a mosaic because it contains many different proteins embedded in or attached to the lipid bilayer.

  • Cholesterol is interspersed among the phospholipids in animal cells. Carbohydrates are attached to some lipids (glycolipids) and proteins (glycoproteins) on the outer surface.

  • The membrane is selectively permeable, since the hydrophobic core blocks most polar molecules and ions.

• Ways proteins associate with the bilayer

  1. Integral membrane proteins

     • They are embedded in the bilayer and have hydrophobic regions that interact with the lipid tails.

     • Transmembrane proteins are a type of integral protein that span the entire membrane, with portions exposed on both sides. They often act as channels, transporters, or receptors.

     • Their membrane-spanning segments are frequently α-helices made of hydrophobic amino acids.

  2. Lipid-anchored proteins

     • They are covalently attached to a lipid that inserts into one layer of the membrane, while the protein itself sits outside the bilayer.

  3. Peripheral membrane proteins

     • They are not embedded in the hydrophobic core.

     • They attach noncovalently (through ionic bonds and hydrogen bonds) to the polar heads of lipids or to other membrane proteins, and they are easily removed without disrupting the bilayer.

Predict the primary function of a cell based on the molecules and organelles it contains.

• General strategy

  • Identify the most abundant molecules and organelles, then ask which cellular processes depend on them. Structure reflects function.

• Examples of reasoning

  • Many mitochondria suggest a high demand for ATP, as in muscle cells or sperm cells, which need a lot of energy for movement.

  • Chloroplasts and chlorophyll indicate a cell that carries out photosynthesis, such as a leaf mesophyll cell.

  • Extensive rough ER and Golgi apparatus suggest the cell synthesizes and secretes proteins, such as an antibody-secreting immune cell or a pancreatic enzyme-secreting cell.

  • Extensive smooth ER suggests lipid or steroid synthesis or detoxification, as in liver cells and cells producing steroid hormones.

  • Many lysosomes suggest active digestion of engulfed material, as in white blood cells that engulf bacteria.

  • A large central vacuole suggests storage and turgor support, as in plant cells.

  • Abundant triglycerides (lipid droplets) suggest energy storage, as in fat cells.

  • Abundant cytoskeletal proteins such as actin and myosin suggest movement or contraction.

  • A rigid cell wall suggests structural support and protection.

Predict the relative abundance of organelles in different cells based on their functions.

• Principle

  • Cells invest more in organelles that support their main job, so the abundance of an organelle scales with how much of its function the cell needs.

• Examples

  • Mitochondria

    • They are very abundant in cardiac and skeletal muscle cells and neurons (high ATP demand), and less abundant in cells with low energy needs.

  • Rough ER and Golgi apparatus

    • They are abundant in secretory cells, such as pancreatic cells that secrete digestive enzymes and insulin, or plasma cells that secrete antibodies.

  • Smooth ER

    • It is abundant in liver cells (detoxification and lipid metabolism) and in cells that make steroid hormones.

  • Lysosomes

    • They are abundant in macrophages and other cells that digest large amounts of material.

  • Chloroplasts

    • They are abundant in photosynthetic cells exposed to light, such as leaf cells, and absent in root cells.

  • Nucleus

    • Most cells have one, but some are specialized to lack it (for example, mature red blood cells), and some have several (for example, skeletal muscle fibers).

  • Vacuoles

    • They are large in mature plant cells and small in animal cells.

Discuss how visual representations (models) of biological systems can be both useful and misleading.

• How models are useful

  • They simplify complex systems so we can focus on key components and relationships.

  • They make invisible or tiny structures (molecules, organelles) visible and easier to understand.

  • They help us generate and test predictions and communicate ideas, for example how a membrane’s structure explains its permeability.

  • They show relationships and organization, such as how organelles work together to make and secrete a protein.

• How models can be misleading

  • They are simplified, so they omit details and can suggest the omitted details don’t matter.

  • They are often two-dimensional and static, while real cells are three-dimensional, crowded, and dynamic. For example, a textbook cell shows neatly spaced organelles with empty cytosol, whereas real cytosol is packed with molecules.

  • Sizes and spacing are often not to scale, and colors are arbitrary.

  • A single “typical” cell can imply that all cells look the same, when cells vary enormously in shape, size, and organelle content.

  • They can suggest structures are rigid and fixed when membranes and proteins are constantly moving.

• Takeaway

  • Models are tools for thinking, not exact copies of reality. Evaluate what a model is meant to show, what it leaves out, and what its limitations are.

Predict the structure of a phospholipid membrane in hydrophilic vs. hydrophobic environments.

• Key idea

  • Phospholipids are amphipathic, so they arrange themselves to keep the hydrophobic parts away from water and the hydrophilic parts toward water.

• In a hydrophilic (aqueous) environment

  • Hydrophobic tails cluster together to avoid water, because water molecules would otherwise be forced into ordered arrangements around them (the hydrophobic effect).

  • The polar heads face the water and the tails face inward.

  • Possible structures

    • A bilayer, in which two layers have their tails facing each other and heads facing the water on both sides. This is the arrangement in cell membranes.

    • A micelle, a single-layer sphere with heads outward and tails inward, which forms with phospholipids or fatty acids that have a single tail.

    • A liposome, a bilayer that closes into a sphere with water both inside and outside. This closing avoids exposed hydrophobic edges.

  • The structure forms spontaneously and is held together mainly by the hydrophobic effect and van der Waals interactions.

• In a hydrophobic (nonpolar, such as oil) environment

  • The arrangement is reversed: the hydrophobic tails face outward toward the nonpolar environment and the hydrophilic heads cluster together inward, shielded from the oil.

  • This forms an inverted (reverse) micelle, with a polar core.

• At an oil–water interface

  • Phospholipids form a monolayer, with heads in the water and tails in the oil or air.

Evaluate how changing temperature, phospholipid chain length, and phospholipid saturation affects membrane fluidity.

• Core principle

  • Membrane fluidity depends on how tightly the phospholipid tails pack together. Tighter packing and stronger van der Waals interactions mean a less fluid membrane, and looser packing means a more fluid one.

• Temperature

  • Higher temperature gives the molecules more kinetic energy, so they move more and the membrane becomes more fluid.

  • Lower temperature reduces motion and packs the lipids more tightly, so the membrane becomes less fluid and can eventually become gel-like or solid. A membrane that is too rigid cannot function properly (for example, proteins cannot move or transport).

• Phospholipid chain length

  • Longer fatty acid tails have more surface area to interact with neighboring tails, so there are more van der Waals interactions and less fluidity.

  • Shorter tails have fewer interactions, so there is more fluidity.

• Phospholipid saturation

  • Saturated tails are straight and pack tightly, so they decrease fluidity.

  • Unsaturated tails have cis double bonds that create kinks, which prevent tight packing and increase fluidity.

• Cholesterol as a buffer

  • At high temperatures, it restrains phospholipid movement and reduces fluidity.

  • At low temperatures, it prevents tight packing and prevents the membrane from solidifying.

• Biological examples

  • Organisms in cold environments often have membranes with more unsaturated and shorter-chain lipids to stay fluid.

  • Cells can adjust their lipid composition to maintain fluidity as temperature changes.