AP Biology 4.1–4.3 — Cells

Quiz review: studying cells and microscopes, prokaryotic cells and surface area-to-volume ratio, eukaryotic organelles, plant vs. animal cells, and endosymbiotic theory.

What is a cell?

The smallest unit of life.

What is the order of biological organization from smallest to largest?

Cell → tissue → organ → organ system → organism.

Give an example of biological organization from cell to organism.

Muscle cell → muscle tissue → heart → circulatory system → human.

What are the two broad categories of cells?

Prokaryotic cells and eukaryotic cells.

Which organisms are prokaryotes?

Bacteria and archaea.

Which organisms are eukaryotes?

Animals, plants, fungi, and protists.

How does a light microscope form an image?

Visible light passes through the specimen and lenses.

Can a light microscope view living cells?

Yes.

What is the usual magnification of a light microscope?

Around 400×.

What magnification can a light microscope reach with oil immersion?

Around 1,000×.

What is the approximate resolution of a light microscope?

About 200 nm.

Why are stains often used with light microscopes?

Cells are often transparent, so stains improve contrast; staining usually kills the cells.

What is magnification?

How much larger an object appears.

What is resolving power or resolution?

The ability to distinguish two nearby structures as separate objects; higher resolution means more detail and clarity.

Why are magnification and resolution not the same?

Making an image larger does not necessarily make its details clearer.

How does a compound microscope orient an image?

It inverts the image: up appears down and right appears left.

How does a microscope image move when the slide moves?

The image appears to move in the opposite direction of the slide.

Light microscope vs. electron microscope

Light microscopes use visible light, can view living cells, and have lower magnification/resolution; electron microscopes use electrons, require dead specimens, and have much higher magnification/resolution.

How much magnification can an electron microscope reach?

About 100,000×.

Why can electron microscopes not view living cells?

Electrons work best in a vacuum, and specimen preparation kills the cells.

What does SEM stand for, and what does it show?

Scanning electron microscope; electrons scan the surface to show detailed external features. Remember S = surface.

What does TEM stand for, and what does it show?

Transmission electron microscope; electrons pass through the specimen to show detailed internal structures. Remember T = through.

What are the three principles of unified cell theory?

All living things contain one or more cells; the cell is the basic unit of life; new cells arise from existing cells.

What did Robert Hooke contribute to cell biology?

In 1665, he examined cork and coined the term “cell.”

What did Antony van Leeuwenhoek contribute to cell biology?

He observed living single-celled organisms called “animalcules,” and later observed bacteria and protozoa.

What did Matthias Schleiden and Theodor Schwann contribute?

They helped propose unified cell theory.

What did Rudolf Virchow contribute?

He later made important contributions to cell theory, including the idea that cells come from existing cells.

What are the basic traits of prokaryotic cells?

They are generally unicellular and lack a nucleus and other membrane-bound organelles; their DNA is in a nucleoid.

What four structures do all cells share?

Plasma membrane, cytoplasm, DNA, and ribosomes.

What is the function of the plasma membrane?

It separates the cell interior from the environment and controls what enters and leaves.

What is cytoplasm?

The material inside the plasma membrane; in eukaryotes it includes cytosol, organelles, cytoskeleton, and chemicals outside the nucleus.

What is the function of DNA?

It stores genetic information.

What is the function of ribosomes?

They synthesize proteins.

How is prokaryotic DNA organized?

Usually as one circular chromosome located in the nucleoid, with no nucleus.

What are plasmids?

Small circular DNA molecules in some bacteria that may carry useful genes, such as antibiotic-resistance genes.

What are the functions of a prokaryotic cell wall?

Protection, shape, and helping prevent dehydration.

What substance is found in most bacterial cell walls?

Peptidoglycan.

What is a bacterial capsule, and what does it do?

A usually polysaccharide layer that helps the bacterium attach to surfaces.

What is the function of bacterial flagella?

Movement or locomotion; flagella help bacteria swim.

What is the function of pili?

Exchange of genetic material during conjugation; pili pass DNA.

What is the function of fimbriae?

They help bacteria attach to host cells.

Distinguish flagella, pili, and fimbriae.

Flagella = movement; pili = DNA exchange; fimbriae = attachment.

What is the typical size range of prokaryotic cells?

About 0.1–5 μm.

What is the typical size range of eukaryotic cells?

About 10–100 μm.

Why are cells generally small?

Small cells have a higher surface-area-to-volume ratio, allowing more efficient nutrient uptake and waste removal.

How do surface area and volume scale as a cell grows?

Surface area increases with r², while volume increases with r³.

What happens to SA:V ratio and exchange efficiency as cell size increases?

Cell size ↑ → SA:V ratio ↓ → exchange efficiency ↓.

Compare the SA:V ratios of a small cube and a larger cube.

A small cube may have 6:1 while a larger cube has 3:1; the smaller cube exchanges substances more efficiently.

How can cells overcome limits caused by a low SA:V ratio?

Stay small, divide, become flat/thin/elongated, fold membranes, or develop specialized structures and organelles.

What are the defining features of eukaryotic cells?

A membrane-bound nucleus, membrane-bound organelles, and multiple linear chromosomes.

What is compartmentalization?

Organelles create specialized areas where different cellular processes can occur under suitable conditions.

What is the plasma membrane made of?

A phospholipid bilayer with embedded proteins.

What substances does the plasma membrane regulate?

Organic molecules, ions, water, oxygen, wastes, and other materials entering or leaving the cell.

What are microvilli, and what is their function?

Fingerlike plasma-membrane projections that increase surface area and absorption.

Where are microvilli commonly found?

On cells lining the small intestine.

How do microvilli connect to SA:V ratio?

They increase surface area without greatly increasing volume, providing more area for exchange.

What is cytosol?

The gel-like fluid portion of the cell.

Cytoplasm vs. cytosol

Cytosol is only the fluid; cytoplasm includes the cytosol plus organelles, cytoskeleton, and chemicals between the plasma membrane and nuclear envelope.

What are major functions of the nucleus?

It houses DNA and directs ribosome synthesis and protein production.

What is the nuclear envelope?

A double membrane surrounding the nucleus.

What do nuclear pores do?

They control movement between the nucleus and cytoplasm.

What is nucleoplasm?

The semi-solid fluid inside the nucleus.

What is chromatin?

DNA plus proteins; it is the more unwound form of chromosomes when a cell is not dividing.

How do prokaryotic and eukaryotic chromosomes differ?

Prokaryotes usually have one circular chromosome; eukaryotes have multiple linear chromosomes.

What is the function of the nucleolus?

It assembles ribosomal subunits, which leave the nucleus through nuclear pores.

Nucleus vs. nucleolus

The nucleus stores DNA and controls cell activities; the nucleolus assembles ribosomal subunits.

What would happen if the nucleolus stopped working?

Ribosome production would be disrupted, so protein synthesis would decrease.

What are ribosomes made of?

RNA and proteins arranged into a large subunit and a small subunit.

How do ribosomes make proteins?

They read mRNA and assemble amino acids into proteins.

Are ribosomes found in both prokaryotes and eukaryotes?

Yes; ribosomes are not membrane-bound organelles.

What is the main function of mitochondria?

Cellular respiration: converting chemical energy from nutrients into ATP, the cell’s main short-term energy carrier.

What are the main structures of a mitochondrion?

Outer membrane, inner membrane, cristae (inner-membrane folds), and matrix (space inside the inner membrane).

What is the function of mitochondrial cristae?

They increase inner-membrane surface area; ATP synthesis occurs on the inner membrane.

What unusual genetic features do mitochondria have?

They contain their own DNA and ribosomes.

What are the main functions of peroxisomes?

Break down fatty acids and amino acids and detoxify certain poisons.

How do peroxisomes protect cells from hydrogen peroxide?

Their enzymes break H₂O₂ into water and oxygen.

What do vesicles and vacuoles have in common?

Both are membrane-bound storage or transport sacs.

Vesicles vs. vacuoles

Vesicles are smaller and can fuse with the plasma membrane or other membrane systems; vacuoles are generally larger.

Which major structures do both plant and animal cells share?

Nucleus, plasma membrane, cytoplasm, ribosomes, mitochondria, and peroxisomes.

Which structures are characteristic of plant cells?

Cell wall, chloroplasts, plastids, and a large central vacuole; plant cells usually lack a centrosome and most lack lysosomes.

Which structures are characteristic of animal cells?

A centrosome and lysosomes; animal cells lack cell walls, chloroplasts, plastids, and a large central vacuole.

What is a centrosome?

An animal-cell microtubule-organizing center involved in cell division.

What is the structure of a centrosome?

It contains two perpendicular centrioles; each centriole contains nine triplets of microtubules.

What is the function of lysosomes?

Acidic digestive organelles that break down proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles.

How do lysosomes demonstrate compartmentalization?

They keep digestive enzymes in an acidic compartment without making the entire cytoplasm acidic.

Where is the plant cell wall located, and what does it do?

Outside the plasma membrane; it provides protection, structural support, and shape.

What are plant cell walls made of?

Primarily cellulose.

Plant cell wall vs. bacterial cell wall

Plant cell walls are mainly cellulose; most bacterial cell walls contain peptidoglycan.

What is the main function of chloroplasts?

Photosynthesis.

What is the overall equation for photosynthesis?

CO₂ + H₂O + light → glucose + O₂.

What unusual genetic features do chloroplasts have?

They contain their own DNA and ribosomes.

What are the major structures of a chloroplast?

Outer and inner membranes, thylakoids, grana, stroma, and chlorophyll.

What is a thylakoid?

A membrane sac inside a chloroplast.

Granum vs. grana

A granum is one stack of thylakoids; grana are multiple stacks.

What is chloroplast stroma?

The fluid surrounding the grana.

What is chlorophyll?

The green pigment that captures light energy for photosynthesis.