Campbell Biology Chapter 6 Review: A Tour of the Cell

6.1 Tools for seeing and separating cell parts

pp. 94–97

On the AP exam? Not tested

The current course doesn't test microscope types or cell fractionation, so you won't be asked about them directly; they only show up as the setup for experiments about organelles (Topic 2.1) or cell size (Topic 2.2).

In the course: Topic 2.1 Cell Structure and Function, Topic 2.2 Cell Size (notes, videos and more questions)

Key points

Most cells are somewhere between 1 and 100 µm across, far too small to see with your eyes alone. A micrometer (µm) is a thousandth of a millimeter, and a nanometer (nm) is a thousandth of a micrometer.
Three things decide how useful a microscope image is: magnification (how much bigger it looks), resolution (how close two points can be and still look like two) and contrast (how well the parts stand out). Enlarging an image past its resolution limit just makes the blur bigger.
Light microscopes work on living cells, but standard ones can't separate details closer than about 200 nm. Stains, fluorescent tags and sharper optical methods boost contrast and focus, and newer super-resolution methods now pick out details tens of nanometers apart or less.
Electron microscopes image with electrons, whose wavelength is far shorter than light's, so they resolve detail down to a few nanometers. A scanning EM shows a sample's surface in 3-D, and a transmission EM shows thin slices of the inside. Standard preparation kills the cells and can add artifacts; fast-frozen samples (cryo-EM) can now reveal the shapes of single proteins.
Cell fractionation breaks cells open and spins the mixture in a centrifuge again and again, faster each time. Large, dense parts like nuclei pack into a pellet at low speed, while small parts like ribosomes only settle after very fast, long spins.
Microscopes show what a part looks like, and chemical tests on a purified fraction show what it does. Combining the two is how biologists matched structures to jobs, such as showing which organelle holds a particular set of enzymes.

Key terms (14)

micrometer (µm): A millionth of a meter, or a thousandth of a millimeter. Most cells measure somewhere from 1 to 100 µm.
nanometer (nm): A thousandth of a micrometer. Membranes, ribosomes and single proteins are sized in nanometers.
magnification: How many times larger an image looks than the real object.
resolution: The smallest gap between two points at which you can still tell them apart. Better resolution means finer detail, not just a bigger picture.
contrast: How strongly different parts of a sample stand apart in an image. Dyes and glowing tags increase it.
light microscope: A microscope that uses ordinary light and lenses to enlarge a specimen. It works on living cells but can't show detail much finer than about 0.2 µm.
electron microscope: A microscope that builds its image from a beam of electrons, which gives roughly a hundred times sharper detail than light allows.
scanning electron microscope (SEM): An electron microscope that images a specimen's outer surface, giving pictures with a strong 3-D look.
transmission electron microscope (TEM): An electron microscope that fires electrons through an ultra-thin, stained slice, so you see structures inside the cell.
fluorescence microscopy: Light microscopy in which chosen molecules carry tags that glow, showing exactly where those molecules are in the cell.
artifact: A feature in an image that comes from how the sample was prepared rather than from the living cell.
cell fractionation: Breaking cells apart and sorting their pieces by size and density so each group can be studied separately.
centrifuge: A machine that spins tubes very fast, so heavier particles are forced to the bottom.
pellet: The solid clump that collects at the bottom of a centrifuge tube. Larger, heavier parts form a pellet even at low speeds.

Check yourself: 6.1 Tools for seeing and separating cell parts

4 questions on 6.1 Tools for seeing and separating cell parts. Pick an answer to see if you got it, and why.

Question 1 of 4

A student looks at a stained pond protist with a light microscope, first at 400× and then at 1,500×. The second image is larger but shows no new detail. What best explains this?

Question 2 of 4

A researcher wants to watch where one particular protein travels inside a living cell over the next hour. Which method fits best?

Question 3 of 4

A standard light microscope can't separate points closer than about 0.2 µm (200 nm). Which structure could it show as a distinct object?

Question 4 of 4

0 of 4 answered

6.2 Prokaryotic and eukaryotic cells, and why cells stay small

pp. 98–102

On the AP exam? Yes

Topics 2.2, 2.9 and 2.10 cover this: be ready to calculate surface area-to-volume ratios and explain why compartments help; you won't need exact cell-size ranges.

In the course: Topic 2.1 Cell Structure and Function, Topic 2.2 Cell Size, Topic 2.9 Cell Compartmentalization, Topic 2.10 Origins of Cell Compartmentalization (notes, videos and more questions)

Key points

Every cell has a plasma membrane, a watery cytosol, DNA and ribosomes. Because all cells share these parts, they're evidence that all life traces back to a common ancestor.
Bacteria and archaea are prokaryotes. Their DNA sits in a nucleoid, a region with no membrane around it, and they generally lack membrane-bound organelles, though some have specialized internal regions. Protists, fungi, plants and animals are eukaryotes, with a nucleus and many organelles.
Eukaryotic cells are usually about 10–100 µm across, roughly ten times wider than a typical bacterium.
When a cell grows, its volume (which scales with length cubed) rises faster than its surface area (length squared), so its surface area-to-volume ratio drops. Past a certain size, the membrane can't move nutrients in and wastes out fast enough for the cell's needs.
That's why big organisms have more cells rather than bigger ones, and why cells that trade lots of material with their surroundings are long and thin, flattened or covered in folds and projections such as microvilli.
Internal membranes split a eukaryotic cell into compartments, each with its own conditions, so reactions that would clash can run at the same time. Many enzymes sit right in these membranes, and each kind of membrane has its own mix of lipids and proteins.
Plant and animal cells share most organelles. Plant cells add chloroplasts, a large central vacuole, a cell wall and plasmodesmata; animal cells have lysosomes and centrosomes with centrioles, and some have flagella.

Key terms (11)

prokaryotic cell: A cell with no nucleus and, in general, no membrane-bound organelles. Bacteria and archaea are built this way.
eukaryotic cell: A cell whose DNA is enclosed in a nucleus and which has many membrane-bound organelles. Protists, fungi, plants and animals are made of them.
plasma membrane: The thin lipid-and-protein boundary around every cell that controls what gets in and out.
cytosol: The jellylike fluid inside a cell in which organelles and other parts are suspended.
cytoplasm: Everything inside the plasma membrane. In a eukaryote, the word means just the part outside the nucleus.
nucleoid: The region of a prokaryotic cell where its DNA is concentrated. Unlike a nucleus, it has no membrane around it.
organelle: A structure inside a cell that does a particular job. In eukaryotes, many are wrapped in their own membrane.
chromosome: One DNA molecule, together with the proteins that coil and organize it, carrying many genes.
surface area-to-volume ratio: A cell's surface area divided by its volume. A higher ratio means more membrane for exchange per unit of cell contents, which is why cells stay small.
microvilli: Many tiny finger-shaped projections of the plasma membrane that greatly increase a cell's surface area.
compartmentalization: Dividing a cell into separate membrane-bound spaces, so different reactions can run under their own conditions without getting in each other's way.

Check yourself: 6.2 Prokaryotic and eukaryotic cells, and why cells stay small

4 questions on 6.2 Prokaryotic and eukaryotic cells, and why cells stay small. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

Two spherical cells have radii of 2 µm and 6 µm. For a sphere, surface area = 4πr² and volume = (4/3)πr³. How does the larger cell's surface area-to-volume ratio compare with the smaller cell's?

Question 2 of 4Calculator allowed

Which cell has the greatest surface area-to-volume ratio? (Sphere: SA = 4πr², V = (4/3)πr³. Cylinder: SA = 2πr² + 2πrh, V = πr²h.)

Question 3 of 4

A microbiologist examines an unknown single-celled organism with an electron microscope. Which observation would show that it's a prokaryote?

Question 4 of 4

Animal cells build fatty acids with enzymes in the cytosol, but they break fatty acids down with enzymes inside mitochondria and peroxisomes. What's the main advantage of this arrangement?

0 of 4 answered

6.3 The nucleus and ribosomes

pp. 102–104

On the AP exam? Yes

Topic 2.1 tests ribosomes (free versus bound, and the fact that every cell has them), and Topics 6.3–6.4 use the path from DNA to mRNA to ribosome; the nuclear lamina, pore complexes and nucleolus aren't tested by name.

In the course: Topic 2.1 Cell Structure and Function, Topic 6.3 Transcription and RNA Processing, Topic 6.4 Translation (notes, videos and more questions)

Key points

The nucleus holds most of a eukaryotic cell's DNA; mitochondria and chloroplasts keep a small amount of their own.
The nuclear envelope has two membranes. Its pores, each lined with a large protein complex, control traffic: mRNA and ribosome subunits head out, and proteins such as the enzymes that copy DNA come in. A mesh of protein filaments called the nuclear lamina lines the inner side and holds the nucleus in shape.
DNA is packed with proteins as chromatin. It's loosely spread out between divisions and coils much more tightly before a cell divides, which makes chromosomes easier to move. Coiling changes their shape, not their number.
The nucleolus is a dense spot in the nucleus where rRNA is made and joined with proteins brought in from the cytoplasm to build the large and small ribosome subunits. These are exported separately and pair up only when they start translating an mRNA in the cytoplasm.
Ribosomes, made of rRNA and protein with no membrane, read mRNA and join amino acids into a polypeptide; the rRNA itself catalyzes the bonds. All cells have ribosomes, another sign that all life shares a common ancestor.
Free ribosomes in the cytosol make proteins that work in the cytosol, plus some bound for the nucleus, mitochondria, chloroplasts and peroxisomes. Ribosomes bound to the ER make proteins that will be secreted, placed in membranes or sent to lysosomes. Free and bound ribosomes are identical; a signal in the growing protein decides where its ribosome works.
A cell that's churning out protein needs a steady supply of new ribosomes, so it tends to have plenty of them and a big nucleolus to keep making more.

Key terms (12)

nucleus: The membrane-wrapped compartment that holds most of a eukaryotic cell's DNA and controls which genes are used.
nuclear envelope: The double membrane around the nucleus. Its outer membrane connects to the ER.
nuclear pore: An opening through the nuclear envelope, lined by a protein complex that decides which large molecules pass in or out.
nuclear lamina: A net of protein filaments lining the inside of the nuclear envelope that supports the nucleus and helps organize its DNA.
chromatin: DNA together with the proteins it's wrapped around. It loosens to let genes be read and tightens before a cell divides.
nucleolus: A dense region inside the nucleus where rRNA is made and assembled with proteins into ribosome subunits.
ribosomal RNA (rRNA): The kind of RNA that, with proteins, makes up a ribosome. It does the actual work of linking amino acids.
messenger RNA (mRNA): A copy of a gene's instructions that leaves the nucleus and tells a ribosome which amino acids to join, in what order.
ribosome: A two-part machine of rRNA and protein, with no membrane, that builds polypeptides by reading mRNA. Every living cell has them.
free ribosome: A ribosome working in the cytosol. It usually makes proteins that stay in the cytosol.
bound ribosome: A ribosome attached to the rough ER or nuclear envelope. It makes proteins headed for secretion, membranes or lysosomes.
signal sequence: A short stretch at the start of a new protein that steers its ribosome to the ER, so the protein is fed into the endomembrane system.

Check yourself: 6.3 The nucleus and ribosomes

4 questions on 6.3 The nucleus and ribosomes. Pick an answer to see if you got it, and why.

Question 1 of 4

A drug stops all molecules from leaving the nucleus through its pores. Which molecule would build up inside the nucleus?

Question 2 of 4

Which statement about ribosomes is accurate?

Question 3 of 4

A plasma cell secretes large amounts of antibody into the blood. A developing red blood cell fills its own cytosol with hemoglobin. Which prediction about their ribosomes is best supported?

Question 4 of 4

Cells in the silk glands of a caterpillar make enormous amounts of silk protein, and they have unusually large nucleoli. What do the large nucleoli most likely reflect?

0 of 4 answered

6.4 The endomembrane system: build, ship and recycle

pp. 104–109

On the AP exam? Yes

This is the heart of Topic 2.1: know the route from rough ER to Golgi to vesicle to plasma membrane, and what lysosomes and vacuoles do. Exocytosis and phagocytosis return in Topic 2.5 and contractile vacuoles in 2.7; specific drugs, diseases and Golgi models won't be tested.

In the course: Topic 2.1 Cell Structure and Function, Topic 2.5 Membrane Transport, Topic 2.7 Tonicity and Osmoregulation (notes, videos and more questions)

Key points

The endomembrane system is the nuclear envelope, ER, Golgi complex, lysosomes, vacuoles, vesicles and plasma membrane. Its parts are joined directly or linked by vesicles that bud off one membrane and fuse with another. Mitochondria, chloroplasts and peroxisomes aren't part of it.
Rough ER is dotted with ribosomes. Proteins they make are fed into the ER's interior (the lumen), where they fold and often get sugar chains added, turning them into glycoproteins. Rough ER also builds new membrane, both its proteins and its phospholipids.
Smooth ER has no ribosomes. Depending on the cell, it makes lipids such as steroid hormones, helps break down drugs and toxins, processes carbohydrates and stores calcium ions that act as signals.
The Golgi complex is a stack of flattened sacs with a receiving (cis) side facing the ER and a shipping (trans) side. As products pass through, it trims and swaps sugars, builds some polysaccharides (including ones for plant cell walls), and sorts products by molecular tags into vesicles headed to the right place. Evidence suggests the sacs themselves mature from the cis side to the trans side.
Lysosomes are acidic sacs of hydrolytic enzymes. They digest material brought in by phagocytosis, recycle the cell's own worn-out parts (autophagy) and help carry out apoptosis. Their enzymes barely work at the cytosol's neutral pH, which protects the cell if a little leaks out.
Vacuoles are large vesicles with many jobs: food vacuoles, contractile vacuoles that bail extra water out of freshwater protists, and the plant central vacuole, which holds ions, water and defense chemicals and lets the cell grow mainly by taking in water.
A secreted protein follows this path: rough ER → transport vesicle → Golgi (cis to trans) → secretory vesicle → fusion with the plasma membrane (exocytosis). Membrane travels the same path and changes its makeup along the way.

Key terms (15)

endomembrane system: The set of linked membranes (nuclear envelope, ER, Golgi, lysosomes, vacuoles, vesicles and plasma membrane) that make, modify and move the cell's products.
endoplasmic reticulum (ER): A maze of membrane-bound channels and flattened sacs, continuous with the nuclear envelope, that makes proteins and lipids.
rough ER: ER covered in ribosomes. It makes proteins for secretion, membranes and lysosomes, and it builds new membrane.
smooth ER: ER with no ribosomes. It makes lipids, breaks down some toxins and drugs, and stores calcium ions.
ER lumen: The inside of the ER's tubes and sacs, where newly made proteins fold and get modified.
glycoprotein: A protein with sugar chains attached. Many secreted and membrane proteins are glycoproteins.
transport vesicle: A small membrane bubble that buds off one membrane and carries its cargo to another part of the cell.
Golgi complex: A stack of flattened membrane sacs that modifies, sorts and ships products arriving from the ER. Also called the Golgi apparatus.
cis and trans faces: The two ends of a Golgi stack. Cargo from the ER enters at the cis end and leaves, sorted and finished, from the trans end.
lysosome: An acidic membrane sac full of hydrolytic enzymes that break down food particles, worn-out organelles and other large molecules.
hydrolytic enzyme: An enzyme that breaks a bond by adding water. Lysosomes use them to take apart proteins, fats, sugars and nucleic acids.
phagocytosis: Engulfing a large particle, or even a whole cell, by wrapping the plasma membrane around it to form a vacuole.
autophagy: A cell's way of recycling itself: it wraps worn-out parts in membrane and sends them to lysosomes to be broken down.
central vacuole: The large, water-filled sac in a mature plant cell that stores ions and other compounds and keeps the cell firm.
contractile vacuole: A water-bailing sac in protists that live in fresh water: it fills with water that seeped into the cell, then empties it outside.

Check yourself: 6.4 The endomembrane system: build, ship and recycle

4 questions on 6.4 The endomembrane system: build, ship and recycle. Pick an answer to see if you got it, and why.

Question 1 of 4

Which cell would most likely have the most extensive smooth ER?

Question 2 of 4

A protein is normally made on the rough ER and secreted. A mutation deletes the short stretch at the start of the protein that sends its ribosome to the ER. Where will the protein most likely end up?

Question 3 of 4

A weak base builds up inside lysosomes and raises their internal pH from about 4.8 to about 6.8. What's the most likely effect?

Question 4 of 4

In the Golgi complex, enzymes headed for lysosomes receive a phosphate-containing tag that sorts them into the right vesicles. In a rare disease, the enzyme that adds this tag doesn't work. Which outcome would you predict?

0 of 4 answered

6.5 Mitochondria, chloroplasts and peroxisomes

pp. 109–112

On the AP exam? Yes

Topics 2.1 and 2.10 test mitochondria, chloroplasts and the evidence for endosymbiosis, and Topics 3.4–3.5 use their inner compartments; peroxisomes and the other kinds of plastids aren't tested.

In the course: Topic 2.1 Cell Structure and Function, Topic 2.9 Cell Compartmentalization, Topic 2.10 Origins of Cell Compartmentalization, Topic 3.4 Photosynthesis, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Mitochondria, found in nearly all eukaryotes including plants, carry out most of cellular respiration and turn the energy in sugars and fats into ATP. Chloroplasts, found in plants and algae, capture light energy and store it in sugars.
Endosymbiotic theory says mitochondria descend from oxygen-using bacteria taken in by an ancestral host cell, which today's evidence places close to the archaea. Chloroplasts came later, from a photosynthetic cyanobacterium taken in by a cell that already had mitochondria.
The evidence: both organelles have two membranes, their own circular DNA and bacteria-like ribosomes, and they reproduce by splitting in two. Their DNA is most similar to that of particular groups of bacteria.
A mitochondrion's smooth outer membrane and folded inner membrane create two compartments: the intermembrane space and the matrix. The folds (cristae) add surface area for the proteins that make ATP, and enzymes in the matrix handle other steps of respiration. Very active cells, like muscle cells, have more mitochondria.
A chloroplast has a two-membrane envelope plus an inner system of flattened sacs called thylakoids, often stacked into grana. The fluid around them, the stroma, holds the chloroplast's DNA, ribosomes and the enzymes that build sugars. Chloroplasts belong to a family of plant organelles called plastids.
In living cells, mitochondria and chloroplasts move, change shape, fuse and divide, and mitochondria often link up into branching networks.
Peroxisomes are single-membrane compartments that strip hydrogen from various molecules, breaking down fatty acids and some toxic compounds. This makes hydrogen peroxide, which the enzyme catalase quickly turns into water. Peroxisomes aren't part of the endomembrane system.

Key terms (14)

mitochondrion: A double-membrane organelle where most of cellular respiration happens, turning energy from food into ATP. Plural: mitochondria.
chloroplast: A double-membrane organelle in plants and algae that captures light energy and uses it to build sugars.
endosymbiotic theory: The well-supported idea that mitochondria and chloroplasts began as free-living prokaryotes that were taken in by a host cell and stayed.
endosymbiont: An organism that lives inside the cells of another organism.
cristae: The folds of a mitochondrion's inner membrane. They pack in more membrane, and so more of the proteins that make ATP.
mitochondrial matrix: The fluid inside a mitochondrion's inner membrane, holding enzymes for some steps of respiration plus the organelle's DNA and ribosomes.
intermembrane space: The thin space sandwiched between a mitochondrion's two membranes. Chloroplasts have a similar gap in their envelope.
thylakoid: A flattened membrane sac inside a chloroplast where light energy is captured.
granum: A stack of thylakoids inside a chloroplast. Plural: grana.
stroma: The fluid in a chloroplast surrounding the thylakoids, where sugars are built and the chloroplast's DNA and ribosomes are found.
chlorophyll: The green pigment in chloroplasts that absorbs light for photosynthesis.
plastid: A family of plant organelles that includes chloroplasts, colorless starch-storing plastids and pigment-holding plastids that color fruits and flowers.
peroxisome: A single-membrane compartment whose enzymes pull hydrogen off molecules, making hydrogen peroxide, which another of its enzymes then breaks down.
catalase: An enzyme that rapidly breaks toxic hydrogen peroxide into water and oxygen.

Check yourself: 6.5 Mitochondria, chloroplasts and peroxisomes

4 questions on 6.5 Mitochondria, chloroplasts and peroxisomes. Pick an answer to see if you got it, and why.

Question 1 of 4

Which observation gives the strongest support to the idea that mitochondria came from bacteria?

Question 2 of 4

Mitochondria from a hummingbird's flight muscle have far more densely packed cristae than mitochondria from its fat-storage cells. What's the best explanation?

Question 3 of 4

Where in a chloroplast are the enzymes that build sugars from carbon dioxide?

Question 4 of 4

An experimental drug kills bacteria by blocking their ribosomes. When it's tested on human cells grown in the lab, it also lowers ATP production, most of all in muscle cells. Which explanation is most likely?

0 of 4 answered

6.6 The cytoskeleton: support, tracks and movement

pp. 112–118

On the AP exam? Background

The cytoskeleton isn't its own topic in the current course, but spindle microtubules in mitosis (Topic 4.5) do come up; you won't need fiber diameters, the 9 + 2 pattern or how dynein bends a cilium.

In the course: Topic 2.1 Cell Structure and Function, Topic 2.10 Origins of Cell Compartmentalization, Topic 4.5 Cell Cycle (notes, videos and more questions)

Key points

The cytoskeleton is a web of protein fibers running through the cytoplasm. It holds the cell's shape, anchors organelles, and can be taken apart and rebuilt quickly when the cell changes shape or moves.
Many movements rely on motor proteins that use ATP to change shape and step along cytoskeleton fibers. They haul vesicles and organelles, bend cilia and flagella, and make muscle contract.
Microtubules, the thickest fibers, are hollow tubes made of tubulin. They resist squeezing, serve as tracks for vesicles and organelles, and form the spindle that separates chromosomes. In animal cells they grow out from a centrosome that holds a pair of centrioles; plant and fungal cells organize them without centrioles.
Eukaryotic cilia and flagella are built on microtubules, usually 9 pairs in a ring around 2 central ones, anchored by a basal body. Dynein motors pull on neighboring pairs, which are tied together, so the whole structure bends. Flagella are long and few with a wavelike beat; cilia are short and many with a back-and-forth stroke. Many cells also have one nonmoving primary cilium that senses signals. Bacterial flagella are a completely different, rotating structure.
Microfilaments, the thinnest fibers, are twisted chains of actin. They resist pulling, support microvilli and, together with myosin, drive muscle contraction, crawling movement with pseudopodia, cytoplasmic streaming and the ring that squeezes a dividing animal cell in half.
Intermediate filaments are medium-width, rope-like fibers made of several kinds of proteins, such as keratins. They're the most permanent part of the cytoskeleton: they bear tension, hold the nucleus in place and form the nuclear lamina.

Key terms (15)

cytoskeleton: A network of protein fibers throughout the cytoplasm that supports the cell, anchors organelles and helps it move.
motor protein: A protein that uses energy from ATP to move along a cytoskeleton fiber, pulling cargo or sliding fibers past each other.
microtubule: A hollow tube made of tubulin, the thickest cytoskeleton fiber. Microtubules act as tracks and form the spindle and the core of cilia and flagella.
tubulin: The protein subunit that microtubules are built from. Units can be added or removed, so microtubules grow and shrink.
centrosome: The main microtubule-organizing center of an animal cell, sitting beside the nucleus. Microtubules sprout outward from it.
centriole: One of a pair of short cylinders of microtubules inside an animal cell's centrosome.
cilium: A short, hairlike projection supported by microtubules. Many cilia beating together move a cell or sweep fluid past it. Plural: cilia.
flagellum: A long, whiplike projection that drives a cell through fluid. Eukaryotic flagella are built on microtubules. Plural: flagella.
basal body: The structure at the base of a cilium or flagellum that anchors its microtubules to the cell.
dynein: The motor protein in cilia and flagella that pulls neighboring microtubule pairs, making the structure bend.
microfilament: The thinnest cytoskeleton fiber, made of two twisted chains of actin. It bears tension and takes part in many kinds of movement.
actin: The protein that builds microfilaments.
myosin: A motor protein that walks along actin filaments. It powers muscle contraction and other kinds of cell movement.
cytoplasmic streaming: A steady flow of cytoplasm around the inside of a cell, driven by actin and myosin, that moves materials through large cells.
intermediate filament: A rope-like cytoskeleton fiber of medium width made of proteins such as keratins. It gives lasting strength and holds organelles in place.

Check yourself: 6.6 The cytoskeleton: support, tracks and movement

4 questions on 6.6 The cytoskeleton: support, tracks and movement. Pick an answer to see if you got it, and why.

Question 1 of 4

A drug prevents tubulin from assembling into new fibers. In treated cells, which process is most directly blocked?

Question 2 of 4

In a growing pollen tube, organelles flow steadily up and down the length of the cell. A drug that takes apart actin fibers, but leaves other fibers intact, stops this flow. Which conclusion is best supported?

Question 3 of 4

Hair and fingernails are made mostly of keratin proteins. Which statement about keratins is accurate?

Question 4 of 4

Pigment cells in a frog's skin change the frog's color by moving pigment granules along microtubules. When these cells are drained of ATP, the granules stop moving, even though the microtubules stay intact. Why?

0 of 4 answered

6.7 Cell walls, the matrix outside cells, and cell junctions

pp. 118–122

On the AP exam? Background

Only parts of this are tested: cell walls as support against bursting (Topic 2.4) and plasmodesmata and gap junctions as routes for signals (Topic 4.1). The extracellular matrix, integrins, wall layers and the other animal junctions won't be tested by name.

In the course: Topic 2.1 Cell Structure and Function, Topic 2.4 Membrane Permeability, Topic 4.1 Cell Communication (notes, videos and more questions)

Key points

Many cells secrete materials that sit outside the plasma membrane. These help protect cells, hold tissues together and pass signals into the cell.
A plant cell wall is a mesh of tough cellulose fibers held in a gel of other carbohydrates and some protein. A young cell makes a thin, flexible primary wall; a sticky, pectin-rich layer (the middle lamella) glues neighboring cells together; and many mature cells add a thick secondary wall inside the primary one. Wood is mostly secondary walls.
The wall lets a plant cell take in water until it's firm without bursting, and it holds the plant up. Microtubules just under the membrane guide the enzymes that lay down cellulose, so they steer the direction a cell expands.
Animal cells have no wall but secrete an extracellular matrix (ECM) of glycoproteins, such as collagen, and proteoglycans, which are mostly carbohydrate. Fibronectin links the ECM to integrins, membrane proteins that connect to the cytoskeleton inside, so changes outside the cell can alter its shape, movement and even gene expression.
Plasmodesmata are channels through plant cell walls. Each is lined by plasma membrane shared by the two neighbors and filled with their joined cytosol, so water and small dissolved molecules move freely between them, and some proteins and RNA can pass too.
Animal cells have three main kinds of junctions: tight junctions stitch neighbors so closely that liquid can't seep through the gaps; desmosomes act like spot welds, held in place by intermediate filaments inside each cell; and gap junctions are protein channels that let ions and small molecules pass straight from one cell to the next.
No part of a cell works alone. A cell's abilities come from its nucleus, ribosomes, membranes, cytoskeleton and energy organelles all working together.

Key terms (14)

cell wall: A tough layer outside the plasma membrane of plant, fungal and many prokaryotic cells that supports the cell and keeps it from bursting.
cellulose: A glucose polymer that forms the strong fibers of plant cell walls.
primary cell wall: The first wall a plant cell builds. It stays stretchy enough to let the cell enlarge.
middle lamella: A sticky, pectin-filled layer between the walls of neighboring plant cells that glues them to each other.
secondary cell wall: A thick, strong wall that some plant cells add inside the primary wall once they stop growing.
extracellular matrix (ECM): The mesh of proteins and carbohydrates that animal cells secrete around themselves for support, attachment and signaling.
collagen: The most common protein in the animal ECM. It forms strong fibers that give tissues strength.
proteoglycan: A protein with many carbohydrate chains attached, so it's mostly sugar. Proteoglycans form a gel-like web in the ECM.
fibronectin: An ECM glycoprotein that binds integrins, helping attach cells to the matrix around them.
integrin: A membrane protein that grips the ECM outside and links to the cytoskeleton inside, carrying signals across the membrane.
plasmodesmata: Tiny membrane-lined openings in plant cell walls that connect the cytosol of one cell to the next. Singular: plasmodesma.
tight junction: A band of proteins that stitches neighboring animal cells tightly to each other, so liquid can't seep through the gaps between them.
desmosome: A spot-weld-like junction that bolts two animal cells to each other, with intermediate filaments inside each cell holding it in place. It keeps tissues from tearing.
gap junction: A channel of proteins between neighboring animal cells that lets ions and small molecules pass straight from one cytoplasm to the other.

Check yourself: 6.7 Cell walls, the matrix outside cells, and cell junctions

4 questions on 6.7 Cell walls, the matrix outside cells, and cell junctions. Pick an answer to see if you got it, and why.

Question 1 of 4

Insulin-making cells in the pancreas sit in clusters and release insulin in pulses that rise and fall together. When the channels that directly link neighboring cells' cytoplasm are blocked, the cells fall out of step. Which structures are most likely involved?

Question 2 of 4

A dye added to the fluid on one side of a sheet of epithelial cells normally stays on that side. A toxin that breaks down one type of junction protein lets the dye leak between the cells, yet the cells stay attached to each other and still pass small ions to their neighbors. Which junction did the toxin most likely disrupt?

Question 3 of 4

A researcher injects two fluorescent markers into one leaf cell. Both are charged, so neither can cross a plasma membrane. The small marker spreads into neighboring cells within minutes, but a much larger one stays in the injected cell. What's the best explanation?

Question 4 of 4

A plant cell and an animal cell are both placed in pure water. The animal cell swells and bursts, but the plant cell only swells until it's firm. Why?

0 of 4 answered