Cell Structure (OCR A Level Biology): Flashcards

Any three of: cell membranes, ribosomes, endoplasmic reticulum, lysosomes.

It uses electromagnets to transmit a beam of electrons through a thin specimen. Denser parts of the specimen absorb more electrons, so they appear darker on the final image.

It passes a beam of electrons across the surface of the specimen and detects the rate at which the electrons bounce back.

They are high-resolution and two-dimensional, showing the internal structures within cells and organelles.

They are three-dimensional and show the surface of the specimen (but have a lower resolution than a TEM).

Because the specimen must be viewed in a vacuum.

Electron microscopes have a higher resolution than light microscopes, allowing smaller structures to be distinguished.

Specimens viewed with an electron microscope must be placed in a vacuum, which means that living specimens cannot be observed.

True

False — a TEM has the higher resolution; an SEM has a lower maximum resolution.

How many times larger an image appears compared with the actual size of the object.

A small glass disc with an engraved scale placed in the eyepiece of a microscope to measure the size of objects in the field of view.

A microscope slide with an engraved scale of known dimensions, used to calibrate an eyepiece graticule.

The use of two or more different stains so that different cellular components or tissue types within a specimen show up in contrasting colours.

So that light can pass through the specimen, allowing its structures to be seen.

Use a pipette to add a few drops of the sample to a clean slide, then gently lower a coverslip onto it, pressing down to remove air bubbles.

To remove air bubbles, which would otherwise obscure the image.

It is easier to locate the specimen in the field of view, and it helps prevent damage to the lens or coverslip if the stage has been raised too high.

Many tissues are transparent, so stains add colour and contrast, making the structures within the specimen visible.

Different tissues absorb different dyes depending on their chemical nature. The dye absorbs some wavelengths of light and reflects others, so the structures that take up the dye stand out.

A graticule has no fixed units. Calibrating it against the known scale of a stage micrometer works out how many micrometres each graticule division represents, so it can then be used as a ruler.

Changing the magnification changes the real distance that each graticule division represents, so a new calibration is needed.

Toluidine blue (turns cells blue) and phloroglucinol (turns cells red/pink).

So they absorb electrons. Heavy-metal stains (e.g. osmium tetroxide) are used; because electrons have no colour, stained structures show up in black and shades of grey.

An eyepiece graticule must be calibrated against a stage micrometer before it can be used to measure the real size of objects.

False — chloroplasts show up green, their natural colour, so they do not need staining.

True

A clear line drawing that records only the structures actually seen when a specimen is viewed under the microscope.

A low-magnification drawing showing the arrangement of tissues, with no individual cells drawn.

Any three of:\n\n- Use a sharp HB pencil\n- Use clear, single lines with no shading\n- Give the drawing a title and record the magnification\n- Draw structures in proper proportion\n- Make the drawing large (use as much of the space as possible)\n- Use a ruler for label lines

With a ruler; they should not cross or have arrowheads, should connect directly to the structure being labelled, and should be kept to one side of the drawing.

A title, and the magnification at which the observations were made.

A plan drawing is made at low magnification and shows the arrangement of tissues with no individual cells. A cellular drawing is made at high magnification and shows individual cells in detail.

So that the drawing is an accurate, objective record of the real specimen.

Individual cells are never shown in a plan drawing, which instead shows the arrangement of tissues at low magnification.

False — individual cells are never drawn in a plan diagram; it shows only the arrangement of tissues.

False — biological drawings use clear, single lines with no shading.

A high-magnification drawing that shows individual cells in detail.

How many times larger an image is than the actual (real) size of the object.

The ability to distinguish two separate points as two separate objects.

magnification = size of image ÷ size of real object

actual size = size of image ÷ magnification

No — magnification is a ratio, so it has no units.

actual size = image size ÷ magnification

= 30 mm ÷ 3000 = 0.01 mm

= 10 µm

They must all be in the same units, so unit conversions are often needed first.

Multiply by the relevant factor. There are 1000 µm in 1 mm, so multiply by 1000.

1000 nm = 1 µm

By multiplying the magnification of the eyepiece lens by the magnification of the objective lens.

Because electrons have a much shorter wavelength than visible light, so structures can be closer together before the diffracted beams overlap and still be distinguished.

It is limited by the wavelength of light — points closer together than about half the wavelength cannot be distinguished as separate.

A TEM has the higher resolution, and therefore a higher useful magnification, than an SEM.

All measurements must be in the same units before the magnification formula is used.

The resolution of a light microscope is limited by the wavelength of light.

True

False — it is limited by the wavelength of light.

It stores the cell's genetic material (DNA, held as chromatin) and controls the cell's activities, including protein synthesis.

It makes ribosomes (it is the site of ribosome production).

It is the double membrane surrounding the nucleus; its pores allow substances such as mRNA and ribosomes to pass between the nucleus and the cytoplasm.

Its surface is covered in ribosomes; it folds, processes and transports the proteins made by them.

It produces, processes and stores lipids, carbohydrates and steroids.

It modifies and packages proteins and lipids into vesicles, ready for transport or secretion.

They are the site of translation (protein synthesis).

They are the site of aerobic respiration, producing ATP.

They contain hydrolytic enzymes that break down waste materials, worn-out organelles and pathogens.

They are the site of photosynthesis in plant cells.

It is partially permeable and controls the movement of substances into and out of the cell.

They organise the spindle fibres (microtubules) during cell division.

It provides structural support and maintains the shape of the cell.

Hair-like projections, made of microtubules, that move substances across the cell surface.

A longer microtubule-based projection that moves the whole cell (e.g. propelling a sperm cell).

Folds of the cell surface membrane that increase its surface area to speed up the exchange of substances.

A cellulose cell wall, chloroplasts, a large permanent vacuole, and plasmodesmata.

The folds of the inner membrane of a mitochondrion, which increase the surface area for the reactions of aerobic respiration.

Stacks of thylakoids inside a chloroplast, where the light-dependent stage of photosynthesis takes place.

The fluid-filled interior of a chloroplast, where the light-independent stage (Calvin cycle) of photosynthesis takes place.

Narrow threads of cytoplasm that pass through plant cell walls, connecting the cytoplasm of neighbouring cells.

The large permanent vacuole of a plant cell is surrounded by a selectively permeable membrane called the tonoplast.

Eukaryotic cells contain 80S ribosomes, whereas prokaryotes contain smaller 70S ribosomes.

Cellulose.

True

False — mitochondria are the site of aerobic respiration; photosynthesis occurs in chloroplasts.

An image of a specimen taken through a light (optical) microscope.

An image of a specimen taken using an electron microscope (TEM or SEM).

A cellulose cell wall, chloroplasts, and a large permanent vacuole.

Centrioles and microvilli.

A plant cell has a cell wall and a large permanent vacuole; an animal cell has neither (and may show centrioles or microvilli).

Because electron microscopes have a much greater magnification and resolution than light (optical) microscopes.

The presence of a cell wall, chloroplasts and a large permanent vacuole identifies a cell as a plant cell.

True

False — centrioles are found only in animal cells.

False — an electron microscope has greater resolution.

In the nucleus, by transcription — an mRNA copy of the DNA is made.

It makes the ribosomes needed for protein synthesis.

It attaches to a ribosome (often on the rough ER), where translation joins amino acids into a polypeptide.

They are folded and processed to produce proteins.

Inside vesicles, which pinch off the RER and fuse with the Golgi apparatus.

It modifies the proteins and packages them into vesicles, ready for secretion.

A vesicle carries it to the cell surface membrane, fuses with it, and releases the protein out of the cell by exocytosis.

Nucleus (transcription)\n\n→ ribosome / rough ER (translation and folding)\n\n→ vesicle\n\n→ Golgi apparatus (modification and packaging)\n\n→ vesicle\n\n→ cell surface membrane (exocytosis)

Because it needs many ribosomes, RER, Golgi apparatus, vesicles and mitochondria to produce, process, package and secrete large amounts of protein (and to supply the ATP required).

The process in which a vesicle fuses with the cell surface membrane to release its contents out of the cell.

Proteins are carried between the RER and the Golgi apparatus inside membrane-bound sacs called vesicles.

A vesicle fuses with the cell surface membrane to release proteins from the cell by exocytosis.

True — the nucleolus inside the nucleus makes the ribosomes used in protein synthesis.

False — translation happens at a ribosome in the cytoplasm or on the rough ER, not in the nucleus.

A network of protein fibres (microfilaments and microtubules) within the cytoplasm that supports the cell, moves its contents and enables cell movement.

Solid protein strands, made mainly of actin, that can cause cell movement and move organelles within the cell.

Hollow (tubular) protein strands, made mainly of tubulin, along which organelles are moved using energy from ATP.

Microfilaments and microtubules.

• Provides mechanical strength and support (maintains cell shape and holds organelles in place)

• Aids transport within the cell (organelles move along its 'tracks')

• Enables cell movement (via cilia and flagella)

Its network of protein fibres acts like 'scaffolding' that maintains the cell's shape and holds organelles in position.

It forms 'tracks' along which organelles (e.g. vesicles) and chromosomes can be moved.

Through cilia and flagella, which contain microtubules that move them.

Microfilaments are made mainly of the protein actin, whereas microtubules are made mainly of tubulin.

False — a third type, intermediate filaments, is also present (though you only need to know microfilaments and microtubules).

True

By moving against each other, which can move some organelles and cause some cell movement.

A cell with no nucleus and no membrane-bound organelles, whose DNA is a single circular molecule free in the cytoplasm (e.g. a bacterium).

A cell that has a nucleus and membrane-bound organelles (e.g. animal, plant and fungal cells).

A small loop of DNA, separate from the main circular DNA, that can be passed between prokaryotes (e.g. carrying antibiotic-resistance genes).

An outer protective layer around some prokaryotes that helps protect them from drying out and from the host's immune system.

Any three of:

• Prokaryotes have no nucleus (DNA free in the cytoplasm); eukaryotes have a nucleus

• Prokaryotes have no membrane-bound organelles; eukaryotes do

• Prokaryotes have smaller 70S ribosomes; eukaryotes have larger 80S ribosomes

• Prokaryotic DNA is circular and not associated with proteins; eukaryotic DNA is linear and wound around histones

• Prokaryotic cell walls contain murein

As a single circular molecule, free in the cytoplasm and not associated with proteins.

Eukaryotic DNA is linear and wound around histone proteins, and is enclosed within a nucleus.

Prokaryotes have smaller 70S ribosomes; eukaryotes have larger 80S ribosomes.

Murein (a glycoprotein, also called peptidoglycan).

It rotates like a propeller to enable the cell to move.

Because size is not a structural feature. A structural difference must refer to structures that are present or absent (e.g. a nucleus or membrane-bound organelles).

Prokaryotic cells contain smaller 70S ribosomes, whereas eukaryotic cells contain larger 80S ribosomes.

A plasmid is a small loop of DNA in a prokaryote that can carry genes such as those for antibiotic resistance.

Any three of:

• Both are surrounded by a cell surface (plasma) membrane

• Both have cytoplasm

• Both contain ribosomes (though of different sizes)

• Both contain DNA as their genetic material

False — prokaryotic cells lack membrane-bound organelles.

False — these features are not present in all prokaryotes.