2.1 Cell structure

What are the distinguishing features of eukaryotic cells?

• cytoplasm containing membrane-bound organelles

• so DNA enclosed in a nucleus

Describe the general structure of eukaryotic cells

• cell surface membrane

• mitochondrion

• nucleus

• ribosomes

• roughendoplasmic reitculum

• smooth endoplasmic reticulum

• golgi apparatus

• lysosome

• plant only: chloroplast, cell wall, cell vacuole

Describe the structure of the cell-surface membrane

• hydrophilic phosphate heads

  • point to / are attracted to water

• hydrophobic fatty acid tails

  • point away from / are repelled from water

• form a phospholipid bilayer

• protein may span the width of the bilayer, or only partially

Describe the function of the cell-surface membrane

• selectively permeable → enables control of passage of substances in / out of the cell

• molecules / receptors / antigens on surface → allow cell recognition / signalling

Describe the structure of the nucleus

Nuclear envelope:

• double membrane

• has nuclear pores

Nucleoplasm

Nucleolus (dense region)

Protein / histone bound linear DNA

• chromatin = condensed

• chromosome = highly condensed

Describe the function of the nucleus

• holds / stores genetic information which codes for polypeptides

• site of DNA replication

• site of transcription, producing mRNA

• nucleolus makes ribosomes / rRNA

Describe the structure of a ribosome

• made of ribosomal RNA and protein (2 subunits)

• not a membrane bound organelle

Describe the function of a ribosome

site of protein synthesis (translation)

Describe the structure of rER and sER

• system of membranes

• rER has ribosomes

Describe the function of rER

• ribosome on surface synthesise proteins

• proteins processed / folded / transported inside rER

Describe the function of sER

• synthesises and processes lipids

• e.g. cholesterol and steroid hormones

Describe the structure of Golgi apparatus and Golgi vesicles

• apparatus: flattened membrane sacs

• vesicle: small membrane sac

Describe the function of Golgi apparatus

• modifies protein, e.g. adds carbohydrates to produce glycoproteins

• modifies lipids, e.g. add carbohydrates to make glycolipids

• packages proteins / lipids into Golgi vesicles

• produces lysosomes (a type of Golgi vesicle)

Describe the function of Golgi vesicles

• transports proteins . lipids to their required destination

• e.g. moves to and fuses with cell-surface membrane

Describe the structure of lysosomes

• membrane

• hydrolytic enzymes

Describe the function of lysosomes

• release hydrolytic enzymes (lysozymes)

• to break down / hydrolyse pathogens or worn-out cell components

Describe the structure of mitichondria

• outer membrane

• cristae - inner membrane fold

• matrix, containing

  • small 70s ribosomes

  • circular DNA

Describe the function of mitochondria

• site of aerobic respiration

• to produce ATP for energy release

• e.g. for protein synthesis / vesicle movement / active transport

Describe the structure of chloroplasts in plants and algae

• double membrane

• stroma, containing:

  • thylakoid membrane

  • small / 70s mribosomes

  • circular DNA

  • starch granules / lipid droplets

• lamella- thylakoid linking grana

• grana - stacks of thylakoid

Describe the function of chloroplasts in plants and algae

• absorbs light energy for photosynthesis

• to produce organic substances e.g. carbohydrates / lipids

Describe the structure of the cell wall in plants, algae and fungi

● Composed mainly of cellulose (a polysaccharide) in plants / algae

● Composed of chitin (a nitrogen-containing polysaccharide) in fungi

Describe the function of the cell wall in plants, algae and fungi

● Provides mechanical strength to cell

● So prevents cell changing shape or bursting under pressure due to osmosis

Describe the structure of the cell vacuole in plants

• tonoplast membrane

• cell sap

Describe the function of the cell vacuole in plants

● Maintains turgor pressure in cell (stopping plant wilting)

● Contains cell sap → stores sugars, amino acids, pigments and any waste chemicals

Name the three groups eukaryotic cells are organised into in complex multicellular organisms

• tissues

• organs

• organ systems

Describe a tissue

Group of specialised cells with a similar structure working together to perform a specific function, often with the same origin.

Describe an organ

Aggregations of tissues performing specific functions.

Describe an organ system

Group of organs working together to perform specific functions

Describe how you can apply your knowledge of cell features / organelles to explain adaptations of eukaryotic cells

General answer format:

● [Named cell] has many [named organelle, eg. ribosomes]

● To [link organelle function to cell function eg. increase rate of protein synthesis, making many antibodies]

What are the distinguishing features of prokaryotic cells?

● Cytoplasm lacking membrane-bound organelles

● So genetic material not enclosed in a nucleus

Describe the general structure of prokaryotic cells

Always present:

• cell-surface membrane

• cell wall - contain murein, a glycoprotein

• cytoplasm

• small ribosomes

• circular DNA

  • free in cytoplasm

  • not associated with proteins

Sometimes present

• capsule

• plasmids - small rings of DNA

• flagella

Compare and contrast the structure of eukaryotic and prokaryotic cells

membrane bound organelles: E has them, P does not

nucleus: E has one, P does not

shape of DNA: E has long, straight and linear with associated histones, P has short and circular with no associated histones

ribosomes: E has larger 80s in cytoplasm, P only has smaller 70s

cell walls: E only in plants, algae, and fungal cells and contains cellulose or chitin, P always has a cell wall containing murein

plasmids/ capsule: E never has these, P sometimes does

flagella: both E and P sometimes have this

size: E is larger, P is much smaller

Explain why viruses are described as acellular and non-living

● Acellular - not made of cells, no cell membrane / cytoplasm / organelles

● Non-living - have no metabolism, cannot independently move / respire / replicate / excrete

Describe the general structure of a virus particle

1. Nucleic acids surrounded by a capsid

(protein coat)

2. Attachment proteins allow attachment

to specific host cells

3. No cytoplasm, ribosomes, cell wall,

cell-surface membrane etc.

4. Some also surrounded by a lipid

envelope eg. HIV

Describe the difference between magnification and resolution

● Magnification = number of times greater image is than size of the real (actual) object

○ Magnification = size of image / size of real object

● Resolution = minimum distance apart 2 objects can be to be distinguished as separate objects

describe the principles and limitations of an optical microscope

• light focused using glass lenses

• light passes through specimen, different structure absorb different amount & wavelengths of light

• generates a 2D image of a cross-section

• low resolution due to long wavelength of visible light

• can’t see internal structure of organelles or ribosomes

• specimen = thin

• low magnification (x 1500)

• can view living organisms

• simple preparation

• can show colour

describe the principles and limitations of a transmission electron microscope

• electrons focused using electromagnets

• electrons pass through specimen, denser parts absorb more and appear darker

• generates a 2D image of a cross-section

• very high resolution due to short wavelength of electrons

• can see internal structures of organelles and ribosomes

• specimen = very thin

• high magnification (x 1,000,000)

• can only view dead / dehydrated specimens as uses a vacuum

• complex preparation so artefacts often present

• does not show colour

describe the principles and limitations of a scanning electron microscope

• electrons focused using electromagnets

• electrons deflected / bounce off specimen surface

• generates a 3D image of surface

• high resolution due to short wavelength of electrons

• can’t see internal structures

• specimen does not need to be thin

• high magnification (x1,000,000)

• can only view dead / dehydrated specimens as it uses a vacuum

• complex preparation so artefacts are often present

• does not show colour

List the steps in calculation involving magnification, real size, and image size

1. note formula . rearrange if necessary

2. convert units if necessary - image and actual size must be in the same unit

3. calculate answer and check units required or if standard form etc. is required

Describe hoe the size of an object viewed with an optical microscope can be measured

1. Line up scale of eyepiece with scale of stage micrometre

2. Calibrate eyepiece graticule - use stage micrometre to calculate size of divisions on eyepiece graticule

3. Take micrometre away and use graticule to measure how many divisions make up an object

4. calculate size of object by multiplying number of divisions by size of division

5. Recalibrate eyepiece graticule at different magnification

Describe the principles of cell fractionation and ultracentrifugation as used to separate cell components

1. homogenise tissue / use a blender

2. Place in a cool, isotonic, buffered solution

3. filter homogenate

4. ultracentrifugation - separates organelles in order of density / mass

Explain homogenising samples in cell fractionation

disrupts cell membrane, breaking open cells and releasing contents / organelles

Explain using a cold, isotonic, buffered solution in cell fractionation

• cold to reduce enzyme activity → so organelles are not broken down / damaged

• isotonic so water doesn’t move in or out of organelles by osmosis → so they don’t burst

• buffered to keep pH constant → so enzymes don’t denature

Explain why the homogenate is filtered in cell ultracentrifugation

removes large, unwanted debris e.g. whole cells, connective tissue

Explain what ultracentrifugation does as a step in cell fractionation

• centrifuge homogenate in a tube at a high speed

• remoe pellet of heaviest organelle and respin supernatant at a higher speed

• repeat at increasing speeds until separated out, each time pellet is made of lighter organelles

• nuclei → chloroplasts / mitochondria → lysosomes → ER → ribosomes