Eukaryotic Cells Common Cellular Structures

Cell Structure

Define a eukaryotic cell.

A cell containing a nucleus and other membrane-bound organelles.

Which groups of organisms contain eukaryotic cells?

Animals, plants, fungi and protoctists.

What structures are found in a typical animal cell?

Cell-surface membrane, cytoplasm, nucleus, mitochondria, ribosomes, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, lysosomes and centrioles.

Define cytoplasm.

A jelly-like liquid that makes up the bulk of the cell and contains the organelles.

What is the protoplasm?

The cytoplasm and nucleus combined.

Define an organelle.

A specialised structure within a cell that performs a particular function.

Define ultrastructure.

The detailed organisation of a cell that can be observed using an electron microscope.

Why can the ultrastructure of cells be seen using an electron microscope?

Electron microscopes have much greater resolution than light microscopes, allowing much smaller structures to be distinguished.

What is the function of the cell-surface membrane?

It forms the outer boundary of the cell and controls the movement of substances into and out of the cell.

What are intracellular membranes?

Membranes found inside a cell.

Why are intracellular membranes important?

They compartmentalise the cell, localise enzymes and keep different biological molecules and reactions separate.

How can membranes increase the efficiency of reactions in cells?

They localise enzymes and reactants in particular areas, allowing reactions to occur efficiently.

Define the nucleus.

A large membrane-bound organelle containing DNA and RNA that controls the activities of the cell.

What is usually the largest organelle in a eukaryotic cell?

The nucleus.

Approximately how large is the nucleus?

About 1–20 µm.

What shape is the nucleus usually?

Spherical.

What surrounds the nucleus?

A double nuclear membrane called the nuclear envelope.

Define the nuclear envelope.

A double membrane surrounding the nucleus.

What are nuclear pores?

Holes in the nuclear envelope that allow substances to pass between the nucleus and cytoplasm.

Why are nuclear pores important?

They allow chemical substances such as RNA and proteins to move between the nucleus and cytoplasm.

What two main types of substance are found inside the nucleus?

Nucleic acids and proteins.

Which nucleic acids are found in the nucleus?

DNA and RNA.

What is chromatin?

The granular combination of DNA bonded to protein found in the nucleus when the cell is not actively dividing.

When is DNA present as chromatin?

When the cell is not actively dividing.

Define the nucleolus.

An extra-dense region inside the nucleus made mainly of DNA and protein.

What is the main function of the nucleolus?

It is involved in the production of ribosomes.

What additional role may the nucleolus have?

It may help control cell growth and division.

What is the overall function of the nucleus?

It contains genetic information and controls cell activities.

Define a mitochondrion.

A rod-like organelle surrounded by a double membrane and containing the enzymes needed for aerobic respiration.

What is the plural of mitochondrion?

Mitochondria.

Approximately how large are mitochondria?

About 1 µm wide and up to 10 µm long.

What is the main function of mitochondria?

They are the site of aerobic respiration and ATP production.

Why are mitochondria described as the powerhouses of the cell?

They release energy from respiratory substrates during aerobic respiration to produce ATP.

Which cells tend to contain many mitochondria?

Cells with high energy demands.

Why do metabolically active cells contain many mitochondria?

They require large amounts of ATP, so they need more sites of aerobic respiration.

Give examples of cells that contain many mitochondria.

Muscle cells and liver cells.

Why do muscle cells contain many mitochondria?

They require large amounts of ATP for muscle contraction.

What membranes surround a mitochondrion?

An outer membrane and an inner membrane.

What are cristae?

Folds of the inner mitochondrial membrane.

What is the advantage of cristae?

They increase the surface area for enzymes and proteins involved in aerobic respiration.

How is mitochondrial structure related to its function?

The highly folded inner membrane forms cristae, providing a large surface area for reactions involved in aerobic respiration.

Define the mitochondrial matrix.

The fluid-filled interior of a mitochondrion surrounded by the inner membrane.

What does the mitochondrial matrix contain?

Enzymes involved in aerobic respiration.

Do mitochondria contain their own genetic material?

Yes, mitochondria contain their own DNA.

Why is mitochondrial DNA significant?

It allows mitochondria to produce some of their own proteins and provides evidence for the endosymbiotic theory.

Why can mitochondria produce some of their own proteins?

They contain their own DNA and 70S ribosomes.

What evidence suggests mitochondria originated from prokaryotic cells?

They contain their own DNA and 70S ribosomes, similar to prokaryotes.

Define the endosymbiotic theory.

The theory that mitochondria and chloroplasts originated as free-living prokaryotic organisms that began living symbiotically inside other cells.

Define a centriole.

A bundle of microtubules found near the nucleus that is involved in cell division.

Where are centrioles usually found?

In pairs near the nucleus.

What is the structure of a centriole?

A bundle of nine sets of microtubules.

Approximately how large is a centriole?

About 0.5 µm long and 0.2 µm wide.

What is the function of centrioles?

They organise the spindle of microtubules during cell division.

What happens to centrioles during cell division?

The pair moves apart and helps form the spindle.

Define the spindle.

A set of overlapping protein microtubules involved in moving chromosomes during mitosis and meiosis.

What is the function of the spindle?

It moves chromosomes to opposite ends of the cell during cell division.

Define a ribosome.

A small organelle made of ribosomal RNA and protein that is the site of protein synthesis.

What are ribosomes made from?

Ribosomal RNA (rRNA) and proteins.

What is the function of ribosomes?

Protein synthesis.

What are the two main types of ribosome?

80S ribosomes and 70S ribosomes.

Where are 80S ribosomes found?

In the cytoplasm of eukaryotic cells.

What subunits make up an 80S ribosome?

A large 60S subunit and a small 40S subunit.

Where are 70S ribosomes found in eukaryotic cells?

Inside mitochondria and chloroplasts.

Where else are 70S ribosomes found?

In prokaryotic cells such as bacteria and cyanobacteria.

What subunits make up a 70S ribosome?

A large 50S subunit and a small 30S subunit.

What does the S in 80S and 70S represent?

Svedberg units, which describe how quickly particles sediment during centrifugation.

Why does 60S + 40S not equal 100S?

Svedberg units depend on sedimentation rate, size and shape, so their values are not directly additive.

What is the approximate RNA to protein ratio in 80S ribosomes?

About 1:1.

What is the approximate RNA to protein ratio in 70S ribosomes?

About 2:1.

Why are 70S ribosomes in mitochondria important evidence for the endosymbiotic theory?

70S ribosomes are characteristic of prokaryotes, supporting the idea that mitochondria evolved from free-living prokaryotic cells.

When are mitochondrial and chloroplast 70S ribosomes reproduced?

Independently when the cell divides.

Define a lysosome.

A membrane-bound organelle containing digestive enzymes used to break down materials inside cells.

What type of enzymes do lysosomes contain?

Hydrolytic digestive enzymes.

What is the function of lysosomes?

They digest food, destroy microorganisms and break down worn-out or damaged cell components.

Why are lysosomal enzymes enclosed by a membrane?

To prevent the digestive enzymes from damaging other parts of the cell.

How are lysosomes involved in intracellular digestion?

They fuse with vesicles containing material and release digestive enzymes to break the material down.

How can lysosomes help destroy bacteria?

A lysosome can fuse with a vesicle containing bacteria and release enzymes that digest them.

How are lysosomes involved in recycling cell components?

They digest worn-out or obsolete organelles so their components can be reused or removed.

What can happen when an entire cell becomes old, damaged or unnecessary?

Lysosomes may rupture and release enzymes that digest the cell.

Define apoptosis.

Programmed cell death involving the controlled destruction of worn-out, damaged or unnecessary cells.

How are lysosomes involved in apoptosis?

Their enzymes can be released to break down the contents of the cell.

Why is apoptosis important?

It removes old, damaged, diseased or unnecessary cells and helps maintain healthy tissues.

Give an example of apoptosis during development.

Removal of the webbing between the fingers and toes of a developing fetus.

How can failure of apoptosis contribute to cancer?

Damaged cells may fail to die and can continue dividing uncontrollably.

What can happen if apoptosis occurs excessively?

Too many cells may be destroyed, causing tissue damage.

Why must apoptosis be carefully controlled?

Too little apoptosis can allow abnormal cells to survive, while too much can destroy healthy cells.

What is the rough endoplasmic reticulum (RER)?

A network of membrane-bound flattened sacs with ribosomes attached to its surface.

Why does rough endoplasmic reticulum appear rough?

Ribosomes are attached to its outer surface.

What is the main function of rough endoplasmic reticulum?

It synthesises, processes and transports proteins.

Which proteins are commonly made on ribosomes attached to the RER?

Proteins that will be secreted from the cell or used in membranes and lysosomes.

What is the smooth endoplasmic reticulum (SER)?

A network of membrane-bound tubules that does not have ribosomes attached.

Why does smooth endoplasmic reticulum appear smooth?

It has no ribosomes attached to its surface.

What is the main function of smooth endoplasmic reticulum?

It is involved in the synthesis and processing of lipids.

What is the main structural difference between RER and SER?

RER has ribosomes attached, whereas SER does not.