Cell structures

Why would a cell's size affect how efficiently it can exchange substances with its surroundings?

As a cell becomes larger, its volume increases faster than its surface area, reducing the surface-area-to-volume ratio. This makes exchange less efficient.

If you were shown an unfamiliar cell in an electron micrograph, what features could help you determine whether it is prokaryotic or eukaryotic?

Look for a nucleus and membrane-bound organelles. A eukaryote has a nucleus and many membrane-bound organelles, while a prokaryote lacks a nucleus and has very few organelles.

A cell is approximately 2 µm in diameter and contains no nucleus or membrane-bound organelles. What type of cell is it likely to be?

A prokaryotic cell, because its size is within the typical prokaryotic range and it lacks a nucleus and membrane-bound organelles.

What determines whether a cell needs a large amount of rough ER?

How much protein the cell needs to synthesise for secretion, insertion into membranes, or delivery to lysosomes.

Why would a cell that secretes large quantities of enzymes contain extensive rough ER?

because this organelle contains the ribosomes that synthesize protein molecules, fold them into their correct 3D shapes, and transport them toward the Golgi apparatus for export.

A membrane system in a cell is covered in tiny dark dots. What does this suggest, and why?

It is likely rough ER because the dark dots are ribosomes attached to its surface.

Why is the RER often located close to the nucleus?

The RER is continuous with the nuclear envelope, allowing newly synthesised proteins to enter the RER efficiently.

A mutation prevents ribosomes from attaching to the ER. Which cellular processes would be directly affected?

The synthesis of proteins destined for secretion, insertion into membranes or lysosomes would be affected.

What determines whether a cell requires extensive smooth ER rather than extensive rough ER?

If its main job is making fats, steroids, or breaking down toxins

Why would a cell producing large amounts of steroid hormones contain extensive smooth ER?

Smooth ER synthesises lipids, including steroids.

A cell needs to rapidly produce large quantities of phospholipids for new cell membranes. Which organelle would you expect to be particularly well developed?

Smooth ER.

Why must proteins produced by the RER be transported to the Golgi apparatus?

The Golgi modifies, sorts and packages the proteins into vesicles for secretion, delivery to organelles or formation of lysosomes.

A cell has extensive RER but very little Golgi apparatus. What process would likely be affected?

The modification, sorting and packaging of proteins produced by the RER.

You see a stack of flattened curved membrane sacs surrounded by many small vesicles. What structure is this likely to be?

Golgi apparatus.

Why are vesicles commonly seen around the Golgi apparatus?

Vesicles transport proteins to and from the Golgi and carry modified proteins to their destinations.

How can the Golgi apparatus contribute to the formation of lysosomes?

It packages hydrolytic enzymes into vesicles that form lysosomes.

A protein produced by the RER needs a carbohydrate added to it before being secreted. Which organelle is involved?

The Golgi apparatus, which modifies proteins, including adding carbohydrates to form glycoproteins

What determines how many mitochondria a cell is likely to contain?

Its energy requirements. Cells with high energy demands require more ATP and therefore tend to have more mitochondria.

Why does a mitochondrion have a folded inner membrane?

The folds, called cristae, increase the surface area available for aerobic respiration and ATP production

An unfamiliar organelle has a double membrane and a highly folded inner membrane. What is it likely to be, and what is the strongest identifying feature?

A mitochondrion. The folded inner membrane forming cristae is the strongest clue.

Why would a muscle cell require many mitochondria?

Muscle contraction requires ATP, so a muscle cell needs a high rate of aerobic respiration and therefore many mitochondria.

What would happen to ATP production if the inner mitochondrial membrane had far less surface area?

Less surface area would be available for aerobic respiration, reducing the potential rate of ATP production.

Why can mitochondria produce some of their own proteins?

They contain their own DNA and ribosomes.

What determines whether a ribosome is free in the cytoplasm or attached to the RER?

The destination of the protein being synthesised. Free ribosomes make proteins used inside the cell, while RER-bound ribosomes make proteins for secretion, membranes or lysosomes.

A cell needs to produce large quantities of proteins for secretion. Where would you expect many ribosomes to be located?

Attached to the rough ER.

Why are ribosomes described as non-membrane-bound organelles?

They are not surrounded by a membrane.

Why would a cell that needs to produce many proteins have a prominent nucleolus?

The nucleolus produces rRNA and assembles ribosomal subunits, which are required to form ribosomes for protein synthesis.

You see a dense, dark region inside the nucleus that is not surrounded by a membrane. What is it likely to be?

The nucleolus.

What would happen to protein synthesis if the nucleolus could no longer produce rRNA?

Ribosomal subunits could not be produced properly, reducing the number of functional ribosomes and therefore reducing protein synthesis.

Why are lysosomes surrounded by a membrane?

They contain hydrolytic enzymes, so the membrane separates these digestive enzymes from the rest of the cytoplasm.

Why do lysosomal enzymes work best at an acidic pH?

Their hydrolytic enzymes are adapted to function at acidic pH.

A cell has to destroy old organelles and recycle their components. Which organelle would be particularly important?

Lysosomes.

Why are microtubules important during cell division?

They form spindle fibres that separate chromosomes.

What determines the shape and support provided by the cytoskeleton?

Structures such as microtubules provide support and help maintain cell shape.

A drug prevents microtubules from forming. Which cellular processes could be affected?

Chromosome separation during cell division, movement of organelles and vesicles, and formation/function of cilia and flagella.

What is the relationship between centrioles and spindle fibres?

Centrioles organise the microtubules that form spindle fibres; the centrioles themselves do not become the spindle fibres.

You see two short cylindrical structures at right angles to each other in an animal cell. What are they likely to be?

A pair of centrioles.

Why would an absorptive cell benefit from having many microvilli?

Microvilli increase the surface area of the cell membrane, allowing faster absorption.

A cell lining the small intestine has numerous short projections from its surface. What advantage do these projections provide?

They increase surface area, allowing faster absorption.

How could you distinguish microvilli from cilia in an unfamiliar image?

Microvilli are shorter and more numerous and contain actin microfilaments. Cilia are longer and contain microtubules arranged in a 9 + 2 pattern.

Why would microvilli be unsuitable for moving mucus across the trachea?

Microvilli do not move. Cilia beat in a coordinated wave-like motion to move substances.

A cell needs to move mucus containing pathogens towards the throat. Which structure would you expect it to have?

Cilia.

What feature would allow you to identify a cilium in cross-section?

A 9 + 2 arrangement of microtubules: nine outer doublets surrounding two central microtubules.

Why would a photosynthetic cell contain many chloroplasts?

Chloroplasts are the site of photosynthesis, so more chloroplasts provide greater capacity for converting light energy into chemical energy.

Why are grana important for photosynthesis?

Grana contain chlorophyll and are the site of the light-dependent stage of photosynthesis.

An organelle has a double membrane, stacks of flattened discs and its own ribosomes and DNA. What is it likely to be?

A chloroplast.

What is the advantage of having many thylakoids stacked into grana?

They provide a large surface area containing chlorophyll for the light-dependent stage of photosynthesis.

What would happen to the light-dependent stage if the chloroplast had very few grana?

There would be less surface area containing chlorophyll, potentially reducing the capacity for the light-dependent reactions.

Why can chloroplasts produce some of their own proteins?

They contain their own DNA and ribosomes.

Why is the tonoplast important for maintaining a plant cell's rigidity?

It controls the movement of substances into and out of the vacuole, helping regulate water balance and maintain turgor pressure.

A plant cell becomes flaccid. Which structure's function is particularly relevant to this change?

The vacuole and tonoplast, because maintaining water content and turgor pressure depends on them.

How could you identify the tonoplast in an unfamiliar plant cell image?

It is the thin membrane surrounding the large central vacuole, inside the cell surface membrane.

Why is the vacuole usually the largest structure in a mature plant cell?

The large central vacuole occupies most of the cell and stores cell sap while helping maintain turgor pressure.

What determines the composition of cell sap?

It is a solution containing substances such as water, mineral ions, sugars, amino acids, pigments and waste products.

Why does water entering the vacuole help keep a plant cell rigid?

It increases the volume of cell sap and turgor pressure, pushing against the cell wall and helping maintain rigidity.

What is the most important structural difference between prokaryotic and eukaryotic cells?

Eukaryotic cells have a nucleus containing their DNA, whereas prokaryotic DNA lies free in the cytoplasm.

A cell contains circular DNA, 70S ribosomes and no nucleus. What type of cell is it likely to be?

A prokaryotic cell

Why can a bacterium be much smaller than a typical eukaryotic cell?

Prokaryotic cells have a simpler internal organisation and lack the nucleus and many membrane-bound organelles found in eukaryotes.

A bacterium loses its plasmid. Would its main chromosome necessarily be lost?

No. A plasmid is a separate small circle of DNA from the main circular bacterial chromosome.

What would be the advantage of pili to a bacterium?

They allow attachment to other cells or surfaces and can be involved in sexual reproduction

What is the function of a bacterial capsule?

It provides additional protection.

Why would a bacterium with a flagellum have an advantage in a changing environment?

The flagellum allows locomotion, enabling the bacterium to move.

A cell has a cell wall made of murein. What does this suggest?

It is a bacterium/prokaryote.

Why would identifying the cell wall material help distinguish different types of cells?

Different groups have different cell-wall materials: bacteria have murein, plants have cellulose and lignin, and fungi have chitin.