Cell Biology, Membrane Transport & Cell Division — Revision Sheet 04

A comprehensive set of 117 flashcards reviewing cell architecture, organelle functions, membrane dynamics and transport mechanisms, the eukaryotic cell cycle, mitosis, and meiosis.

What is a prokaryotic cell like in terms of DNA location, membrane, organelles and size?

Its DNA is in a nucleoid, not inside a nucleus. It has a plasma membrane but no membrane-bound organelles. Typical size: $0.5-5\,\mu\text{m}$.

What are the cytoskeleton and genome like in a prokaryotic cell?

It has primitive protein filaments. It usually has one circular chromosome and may also contain plasmids.

What is a eukaryotic cell like in terms of DNA location, membrane-bound organelles and size?

Its DNA is enclosed in a nucleus with a double membrane. It has membrane-bound organelles such as ER, Golgi, mitochondria and lysosomes. Typical size: $10-100\,\mu\text{m}$.

What are the cytoskeleton and genome like in a eukaryotic cell?

It has microtubules, microfilaments and intermediate filaments. It has multiple linear chromosomes packaged with histones as chromatin.

What is the outer boundary, plastid status and vacuole pattern of an animal cell?

Its outer boundary is the plasma membrane, with no cell wall. It has no chloroplasts. Vacuoles are small and temporary, if present.

What are the outer boundary, plastids and vacuole like in a plant cell?

It has a plasma membrane plus a cellulose cell wall. It has chloroplasts for photosynthesis and a large central vacuole for water storage and turgor.

How do animal and plant cells differ in centrosomes and cell connections?

Animal cells have a centrosome with a pair of centrioles ($9+0$ triplets) and tight junctions, desmosomes and gap junctions. Plant cells have a centrosome without centrioles and plasmodesmata.

What do all living cells share?

Plasma membrane, cytosol, chromosomes (DNA) and ribosomes.

What is the nucleus?

A membrane-enclosed genetic center. Its double-membrane nuclear envelope has pores; it contains most DNA as chromatin (DNA + histones).

What does the nucleolus do?

It synthesizes rRNA and assembles ribosomal subunits.

What do free ribosomes make?

They make proteins used in the cytosol.

What do bound ribosomes on rough ER or the nuclear envelope make?

They make proteins for membranes, organelles or secretion.

What is the endomembrane system?

A group including the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles/vacuoles and plasma membrane. They communicate by membrane continuity or transport vesicles.

What is the route of a secreted protein through the cell?

Nucleus (transcription) $\rightarrow$ mRNA $\rightarrow$ bound ribosome / rough ER (translation) $\rightarrow$ transport vesicle $\rightarrow$ Golgi (cis-to-trans modification) $\rightarrow$ secretory vesicle $\rightarrow$ plasma membrane (exocytosis).

What is smooth ER, and what does it do?

ER without ribosomes. It synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons and stores $Ca^{2+}$.

What is rough ER, and what does it do?

ER covered with ribosomes. It synthesizes secretory proteins (often glycoproteins), makes membrane phospholipids and packages proteins into vesicles.

What is the Golgi apparatus?

It receives ER vesicles at its cis face and ships vesicles from its trans face. It modifies carbohydrate chains, makes some macromolecules (such as pectins), sorts and targets products.

What are lysosomes?

Acidic hydrolytic sacs (about $\text{pH } 5$) in animal cells. They digest engulfed material during phagocytosis and recycle damaged organelles during autophagy.

What are vacuoles, and what are their types/functions?

Food vacuoles form during phagocytosis; contractile vacuoles expel excess water; the plant central vacuole stores water, ions, pigments and defensive compounds and supports cell expansion.

What are mitochondria?

Organelles for cellular respiration. They have an outer membrane and a folded inner membrane (cristae) surrounding the matrix.

What are chloroplasts?

Organelles for photosynthesis. They contain chlorophyll, stroma and thylakoids stacked into grana.

What is the endosymbiont theory, and what evidence supports it?

Mitochondria and chloroplasts arose from prokaryotes engulfed by ancestral eukaryotes. Evidence: double membranes, circular DNA, bacterial-like ribosomes and binary fission.

What are peroxisomes?

Organelles that oxidize substrates using $O_2$, forming $H_2O_2$. Catalase converts $H_2O_2$ into water.

What are microtubules: structure and functions?

Hollow $\alpha/\beta$-tubulin tubes about $25\,\text{nm}$ wide. They help maintain cell shape, transport organelles, form the mitotic spindle and build cilia/flagella ($9+2$).

What are microfilaments: structure and functions?

Two actin strands about $7\,\text{nm}$ wide. They support tension and microvilli, work with myosin in muscle contraction, form pseudopodia and support cytoplasmic streaming.

What are intermediate filaments: structure and functions?

Fibrous cables about $8-12\,\text{nm}$ wide, such as keratins. They provide structural strength, anchor organelles and form the nuclear lamina.

What is the fluid mosaic model of the plasma membrane?

The membrane is a flexible, amphipathic phospholipid bilayer with proteins embedded in or associated with it. Components move within the membrane, creating a fluid mosaic.

How are phospholipids arranged in the fluid mosaic model?

Hydrophilic phosphate heads face the watery environments; hydrophobic fatty-acid tails point inward.

How do unsaturated tails and cholesterol affect membrane fluidity?

Kinks in unsaturated tails prevent tight packing and increase fluidity at low temperatures. Cholesterol buffers fluidity: it restrains movement when warm and prevents tight packing when cool.

What are integral, transmembrane and peripheral membrane proteins?

Integral proteins penetrate the hydrophobic interior; transmembrane proteins span the bilayer; peripheral proteins associate loosely with the membrane surface or integral proteins.

What do membrane proteins do?

They support transport, enzymatic activity, signal transduction, cell recognition, intercellular joining and attachment to cytoskeleton/ECM.

What is selective permeability?

The membrane allows some substances to cross more easily than others, depending on their size, polarity and charge.

Which substances pass through the bilayer easily?

Nonpolar/hydrophobic substances, such as $O_2$, $CO_2$ and hydrocarbon chains, pass easily.

How do small polar molecules such as water cross the membrane?

They cross slowly; transport proteins can assist.

Which substances are blocked by the hydrophobic core and need transport proteins?

Ions ($Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$) and large polar molecules such as glucose and sucrose.

What is the basic direction of passive versus active transport?

Passive transport moves down a gradient (high $\rightarrow$ low). Active transport moves against a gradient (low $\rightarrow$ high) and requires energy.

What is simple diffusion? Include direction, protein use and ATP use.

Net movement of nonpolar solutes directly through the bilayer down their concentration gradient (high $\rightarrow$ low), toward equilibrium. No transport protein and no ATP.

What is facilitated diffusion? Include direction, protein use and ATP use.

Passive movement of polar or charged substances down their gradient (high $\rightarrow$ low) through a channel or carrier protein. It uses a protein but does NOT use ATP.

What is osmosis? Include water direction, proteins and ATP.

Free water moves from lower solute concentration (higher free water) toward higher solute concentration (lower free water) across a selectively permeable membrane. Aquaporins may assist; no ATP.

What is primary active transport?

A pump moves a substance against its gradient (low $\rightarrow$ high) using ATP hydrolysis directly. It uses a transport protein and ATP.

What is cotransport / secondary active transport?

One solute moves downhill and drives another solute uphill through a cotransporter. It uses an ion gradient established by primary active transport, rather than directly hydrolyzing ATP at that cotransporter.

What is bulk transport?

Vesicles move material into or out of the cell using vesicles and membrane machinery. It requires energy.

What is the Na⁺/K⁺ pump, and what does it move per ATP?

It uses one ATP to pump $3\,\text{Na}^+$ out and $2\,\text{K}^+$ in. It maintains high $\text{K}^+$ and low $\text{Na}^+$ inside and creates an electrogenic gradient (membrane potential).

What is cotransport, and give the source example?

Downhill diffusion of $\text{Na}^+$ or $\text{H}^+$ (previously pumped out) drives uphill transport of another solute. Example: a plant sucrose–$\text{H}^+$ cotransporter.

What is exocytosis?

Secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

What is endocytosis, and what are its main forms?

The plasma membrane forms vesicles to bring material into the cell. Phagocytosis is cellular eating; pinocytosis is cellular drinking; receptor-mediated endocytosis uses specific ligands binding receptors in coated pits (e.g., LDL cholesterol uptake).

What is the key exam trap about facilitated diffusion?

It uses a transport protein but does NOT use ATP. It is passive because the substance moves down its gradient.

What happens to an animal cell in a hypertonic outside solution?

The outside has higher solute concentration; water leaves the cell and it shrivels.

What happens to a plant cell in a hypertonic outside solution?

Water leaves the cell; plasmolysis occurs.

What happens to an animal cell in a hypotonic outside solution?

The outside has lower solute concentration; water enters and the cell may lyse.

What happens to a plant cell in a hypotonic outside solution?

Water enters and the cell becomes turgid, its ideal state.

What happens to animal and plant cells in an isotonic outside solution?

Solute concentration is equal; there is no net water movement. Animal cell: normal. Plant cell: flaccid.

What is a genome?

The complete set of a cell’s DNA. Prokaryotes usually have one circular chromosome in the nucleoid plus plasmids; eukaryotes have multiple linear chromosomes packaged as chromatin.

What is a diploid cell, and what is a human somatic cell?

A diploid ($2n$) somatic cell has two chromosome sets, one maternal and one paternal. Human somatic cells have $2n = 46$ chromosomes; gametes are haploid ($n = 23$).

What happens during G₁?

The cell grows and makes proteins and organelles.

What happens during S phase?

DNA replicates. Each duplicated chromosome has two identical sister chromatids joined by cohesins; they are most closely attached at the centromere.

What happens during G₂?

The cell grows further, duplicates centrosomes and makes proteins needed for mitosis.

What happens during M phase?

Mitosis divides the nuclear genetic material; cytokinesis divides the cytoplasm.

What is interphase, and how much of the cell cycle does it take?

Interphase consists of $G_1 + S + G_2$ and takes approximately $90\%$ of the cell cycle.

What is mitosis?

The division of nuclear genetic material that distributes identical chromosome sets to two daughter nuclei.

What are the five stages of mitosis in order?

Prophase $\rightarrow$ prometaphase $\rightarrow$ metaphase $\rightarrow$ anaphase $\rightarrow$ telophase.

What happens during prophase, and what is seen under a microscope?

Chromatin condenses into visible duplicated chromosomes; nucleoli disappear; the spindle begins forming; centrosomes move apart. Microscope clue: condensed chromosomes appear while the nuclear boundary is still visible.

What happens during prometaphase, and what is seen under a microscope?

The nuclear envelope fragments; spindle microtubules enter; kinetochores form at centromeres and attach to kinetochore microtubules. Microscope clue: nuclear envelope gone/breaking and chromosomes scattered.

What happens during metaphase, and what is seen under a microscope?

Chromosomes align at the metaphase plate; kinetochores attach to microtubules from opposite poles. Microscope clue: one clear line of chromosomes at the cell equator.

What happens during anaphase, and what is seen under a microscope?

Separase cleaves cohesins; sister chromatids separate into daughter chromosomes and move centromere-first to opposite poles. Microscope clue: two groups pulling apart, often V- or U- shaped.

What happens during telophase, and what is seen under a microscope?

Two nuclei reform; nucleoli reappear; chromosomes decondense; the spindle depolymerizes. Microscope clue: two chromosome clusters/nuclei at opposite ends.

What is cytokinesis?

Division of the cytoplasm into separate daughter cells, usually after mitosis.

What is a mitotic spindle?

Microtubules and associated proteins that move chromosomes during cell division.

What is a centrosome?

A microtubule-organizing centre that duplicates during interphase.

What are centrioles?

$9+0$ triplet cylinders found in animal centrosomes. They are not required for spindle formation.

What are asters?

Radial arrays of microtubules around the centrosomes.

What do kinetochore and nonkinetochore microtubules do?

Kinetochore microtubules move chromosomes. Nonkinetochore microtubules overlap and push spindle poles apart during anaphase.

How does cytokinesis occur in an animal cell?

An actin–myosin contractile ring tightens like a drawstring and forms a cleavage furrow.

How does cytokinesis occur in a plant cell?

Golgi-derived vesicles join at the centre to form a cell plate. It expands outward and fuses with the plasma membrane, forming a new cell wall.

What are cell-cycle checkpoints?

Control points where stop/go signals regulate whether the cell progresses to the next stage.

What happens at the G₁ restriction checkpoint?

With a go-ahead signal, the cell proceeds; without it, the cell enters $G_0$. Many mature nerve and muscle cells remain in $G_0$; liver cells can return to $G_1$ in response to growth factors.

What does the G₂ checkpoint check?

It checks that DNA replication is complete and correct before mitosis.

What does the M (spindle assembly) checkpoint check?

Anaphase waits until all kinetochores attach correctly to microtubules from both poles. This helps prevent aneuploidy.

What are Cdks, and how are they controlled?

Cyclin-dependent kinases (Cdks) are protein kinases present at relatively constant levels, but they are inactive unless bound to cyclins. Cyclin levels rise during $S/G_2$ and fall during $M$.

What is MPF, and what does it do?

MPF is a cyclin–Cdk complex that triggers entry into mitosis by phosphorylating nuclear lamina, chromosome-condensing and spindle-forming proteins. During anaphase, it promotes destruction of its cyclin and switches off.

What is meiosis, and what is its overall sequence?

One round of DNA replication followed by two consecutive divisions. Meiosis I separates homologous chromosomes ($2n \rightarrow n$); meiosis II separates sister chromatids. The result is four genetically distinct haploid cells.

What are the four main stages of meiosis, in order?

Prophase I $\rightarrow$ metaphase I $\rightarrow$ anaphase I $\rightarrow$ meiosis II (which includes prophase II, metaphase II, anaphase II and telophase II).

What happens during prophase I?

Homologous chromosomes pair (synapsis); crossing-over occurs between nonsister chromatids.

What happens during metaphase I?

Homologous chromosome pairs (tetrads) align at the cell equator.

What happens during anaphase I?

Homologous chromosomes separate; sister chromatids remain joined.

What happens during meiosis II?

No DNA replication occurs between divisions. Sister chromatids separate in anaphase II, similarly to mitosis. The process ends with four haploid cells.

What is synapsis?

Homologous chromosomes pair gene-by-gene; the synaptonemal complex holds them together.

What is crossing-over, and between which chromatids does it occur?

Nonsister chromatids of homologous chromosomes break at corresponding points and exchange DNA segments during prophase I.

What is a chiasma?

An X-shaped region marking crossing-over and holding homologous chromosomes together.

What are the key differences between meiosis I and meiosis II?

Meiosis I: homologues pair and crossing-over occurs; tetrads align; homologous chromosomes separate while sister chromatids stay joined. Meiosis II: no DNA replication between divisions; sister chromatids separate; ends with haploid cells.

What are the five main differences between mitosis and meiosis?

DNA replication: before mitosis vs before meiosis I only. Divisions: one vs two. Synapsis/crossing-over: absent vs present in prophase I between nonsister chromatids. Products: two diploid genetically identical cells vs four haploid genetically distinct cells. Role: growth/repair/asexual reproduction vs gamete or spore production and genetic variation.

What is an aquaporin?

A channel protein that helps water diffuse rapidly across a membrane.

What is autophagy?

Lysosomal recycling of damaged organelles.

What is a centromere?

The region where sister chromatids are most closely joined.

What is an electrogenic pump?

A transport protein that generates voltage across a membrane.

What is a kinetochore?

A protein structure at the centromere that attaches to spindle microtubules.

What is the metaphase plate?

The imaginary plane midway between spindle poles where chromosomes align.

What is plasmodesmata?

Channels through plant cell walls that connect adjacent cytoplasms.

What is plasmolysis?

The plasma membrane pulls away from the cell wall after water loss.

What is separase?

An enzyme that cleaves cohesins to start sister-chromatid separation.