Week 3 - Organelles of the Cell (RAH Y1 - Cell Biology)

What is the cell membrane?

A thin, flexible, selectively permeable boundary around the cell.

What is the cell membrane made of?

A phospholipid bilayer with embedded proteins.

What does selectively permeable mean?

It allows some substances to cross while restricting others.

What is a phospholipid?

A molecule with a hydrophilic head and two hydrophobic fatty acid tails.

What does hydrophilic mean?

Attracted to water.

What does hydrophobic mean?

Repelled by water.

What does amphipathic mean?

Having both hydrophilic and hydrophobic regions.

What is the role of cholesterol in membranes?

It helps maintain membrane stability and fluidity.

What are glycoproteins?

Proteins with carbohydrate chains attached; they help recognition and adhesion.

What are glycolipids?

Lipids with carbohydrate chains that help identify cells.

What are integral membrane proteins?

Proteins embedded in the membrane, including channels, carriers and receptors.

What are peripheral membrane proteins?

Proteins loosely attached to the membrane surface.

What is the fluid mosaic model?

A flexible membrane of moving lipids and proteins with a patchwork-like structure.

What are three functions of membrane proteins?

Transport substances, receive signals and catalyse reactions.

What is passive transport?

Movement across a membrane without energy supplied by the cell.

What is active transport?

Movement using energy, often against a concentration gradient.

What is diffusion?

Net movement of particles from high to low concentration.

What is simple diffusion?

Passive movement directly through the membrane, e.g. oxygen and carbon dioxide.

What is osmosis?

Passive movement of water across a selectively permeable membrane towards higher solute concentration.

What are aquaporins?

Channel proteins that allow water to cross membranes rapidly.

What is facilitated diffusion?

Passive movement down a concentration gradient through channel or carrier proteins.

What do channel proteins do?

Provide pores for specific ions or molecules to cross membranes.

What do carrier proteins do?

Bind substances and change shape to move them across the membrane.

What are gated ion channels?

Channels that open or close in response to particular signals.

What do GLUT transporters do?

Move glucose across membranes by facilitated diffusion.

Why does active transport require energy?

It moves substances against their concentration gradient.

What is ATP hydrolysis?

The breakdown of ATP to release usable energy.

What is the sodium-potassium pump?

An ATP-powered pump that moves 3 Na⁺ out and 2 K⁺ into the cell per cycle.

Why is the sodium-potassium pump important?

It maintains sodium and potassium gradients and helps maintain membrane potential.

What is the role of calcium pumps?

They help keep cytoplasmic calcium concentration low by moving Ca²⁺ away.

What is secondary active transport?

Transport powered by an existing ion gradient rather than direct ATP use by that transporter.

What is uniport?

Transport of one type of substance.

What is symport?

Transport of two different substances in the same direction.

What is antiport?

Transport of two different substances in opposite directions.

What is a vesicle?

A small membrane-bound sac that transports or stores substances.

Why do cells use vesicles for large molecules?

Large molecules often cannot cross the membrane through ordinary diffusion or transport proteins.

What is endocytosis?

Energy-requiring uptake of materials into a cell using membrane-bound vesicles.

What happens during endocytosis?

The membrane folds around material, then pinches off to form an internal vesicle.

What are the three main types of endocytosis?

Phagocytosis, pinocytosis and receptor-mediated endocytosis.

What is phagocytosis?

Cell eating: engulfment of large particles or whole cells.

Which cells can perform phagocytosis?

Specialised cells such as macrophages and neutrophils.

What is a phagosome?

A vesicle containing material engulfed during phagocytosis.

What happens when a phagosome fuses with a lysosome?

Enzymes help break down the engulfed material.

What is pinocytosis?

Cell drinking: uptake of extracellular fluid and dissolved substances.

How does pinocytosis differ from phagocytosis?

Pinocytosis takes in fluid; phagocytosis engulfs large particles or whole cells.

What is receptor-mediated endocytosis?

Specific substances bind membrane receptors before being taken into vesicles.

Why is receptor-mediated endocytosis useful?

It enables selective uptake, even when a substance is at low extracellular concentration.

How are iron and transferrin involved in endocytosis?

Transferrin carrying iron binds its receptor, is taken into a vesicle, and iron is released inside.

What happens to receptors after receptor-mediated endocytosis?

They can be recycled to the cell surface.

What is exocytosis?

Release of materials outside the cell when a vesicle fuses with the plasma membrane.

What are the steps of exocytosis?

A vesicle travels to the membrane, fuses with it and releases its contents outside.

What is the role of the Golgi apparatus in exocytosis?

It packages materials into vesicles for transport and secretion.

What are two functions of exocytosis?

Secretion of substances and replacement of membrane lost during endocytosis.

How do endocytosis and exocytosis differ?

Endocytosis brings materials in; exocytosis releases materials out.

What is surface area-to-volume ratio (SA:V)?

Surface area divided by volume.

Why is SA:V important for cells?

It affects how efficiently substances can be exchanged relative to cell volume.

Why do small cells generally exchange materials efficiently?

They usually have a higher surface area-to-volume ratio.

What happens to SA:V as a cell gets larger?

It generally decreases because volume grows faster than surface area.

How do villi help the small intestine?

They increase surface area for absorption of digested nutrients.

What is the surface area of a cube with side x?

SA = 6x².

What is the volume of a cube with side x?

V = x³.

What is the SA:V ratio of a cube with side x?

SA:V = 6/x.

What is the surface area of a sphere with radius r?

SA = 4πr².

What is the volume of a sphere with radius r?

V = (4/3)πr³.

What is the SA:V ratio of a sphere with radius r?

SA:V = 3/r.

How do you calculate an SA:V ratio?

Calculate surface area and volume, then divide surface area by volume.

What are the SA:V units if dimensions are in µm?

µm⁻¹, because area is in µm² and volume is in µm³.

What is the surface area of a cylinder with two hemispherical ends?

SA = 2πrL + 4πr².

What is the volume of a cylinder with two hemispherical ends?

V = πr²L + (4/3)πr³.

In rod-shaped cell formulas, what does L represent?

The length of the cylindrical middle section only.

Why must L exclude the hemispherical ends?

The formulas account for the ends separately, so including them in L would count them twice.

What is a cell?

The basic structural and functional unit of life.

What is an organelle?

A specialised cell structure that performs a particular function.

What is a membrane-bound organelle?

An organelle enclosed by a membrane.

What is a prokaryotic cell?

A cell without a membrane-bound nucleus or membrane-bound organelles.

What is a eukaryotic cell?

A cell with a membrane-bound nucleus and membrane-bound organelles.

Where is DNA located in a prokaryotic cell?

In the nucleoid region; it is not enclosed by a nuclear envelope.

How does prokaryotic DNA usually differ from eukaryotic nuclear DNA?

Prokaryotic DNA is usually circular; eukaryotic nuclear DNA is arranged in linear chromosomes.

What is a plasmid?

A small additional DNA molecule commonly found in bacteria.

What is the main structural material in bacterial cell walls?

Peptidoglycan.

What are the main functions of cell division?

Growth, tissue repair, cell replacement and reproduction.

What is the cell cycle?

The process by which a cell grows, duplicates its DNA and divides.

What is binary fission?

Prokaryotic cell division that produces two daughter cells.

Where does DNA replication begin in binary fission?

At a specific DNA site called the origin of replication.

What is a septum in binary fission?

A dividing partition formed as the cell membrane grows inward.

What is cytokinesis?

The physical division of a cell's cytoplasm into daughter cells.

Why can bacterial infections spread rapidly?

Many bacteria reproduce quickly under suitable conditions, sometimes dividing in about 20 minutes.

What is mitosis?

Nuclear division that normally produces two genetically identical daughter nuclei.

What are the four main stages of mitosis?

Prophase, metaphase, anaphase and telophase.

What happens during prophase?

Chromatin condenses into chromosomes and the mitotic spindle begins to form.

What happens during metaphase?

Chromosomes align at the cell equator.

What happens during anaphase?

Sister chromatids separate and move towards opposite poles.

What happens during telophase?

Chromosomes reach opposite poles and new nuclear envelopes form.

What happens during cytokinesis in animal cells?

A contractile ring creates a cleavage furrow that separates the cell.

What is the usual outcome of mitosis?

Two daughter cells with the same chromosome number as the parent.

What is meiosis?

A specialised cell division involving two nuclear divisions that produces haploid cells.

What is the usual outcome of meiosis in the standard model?

Four genetically varied haploid daughter cells.

How many nuclear divisions occur in meiosis?

Two: meiosis I and meiosis II.

When does DNA replicate during meiosis?

Once, before meiosis I.

What is a haploid cell?

A cell containing one set of chromosomes (n).