2.1.1 Cell Structure (Foundations in Biology)

Question 1

What are prokaryotic organisms?

Answer 1

• prokaryotic cells
• single-celled organisms
• no nucleus present
• e.g. bacteria

Question 2

What are eukaryotic organisms?

Answer 2

• eukaryotic cells
• all animal, plant, fungal and protoctist cells

They have:

• a nucleus surrounded by a nuclear envelope, containing DNA organised and wound into linear chromosomes
• contains nucleolus
• cytoplasm
• cytoskeleton
• plasma membrane
• membrane-bound organelles
• small vesicles
• ribosomes

Question 3

Draw and label a plant cell

Answer 3

Question 4

Draw and label an animal cell

Answer 4

Question 5

What are organelles?

Answer 5

• within every cell, there are various membrane-bound compartments, known as organelles
• each with a specific function

Question 6

Describe the structure and function of plasma (cell surface membrane)

Answer 6

Structure:

• membrane found on the surface of animal cells and just inside the cell wall of plant cells and prokaryotic cells
• made of mainly lipids and protein

Function:

• regulates the movement of substances into and out of the cell
• also has receptor molecules, allowing it to respond to chemicals like hormones

Question 7

Describe the structure and function of the cell wall

Answer 7

Structure:

• rigid structure that surrounds plant cells and fungi
• mainly made of bundles of cellulose fibres in plant cells
• in fungi, cell walls are made of chitin

Function:

• supports plant cells
• prevents cells from bursting when turgid
• permeable and allows solutions to pass through

Question 8

Describe the structure and function of the nucleus, nuclear envelope and nucleolus

Answer 8

Structure:

• nucleus is surrounded by a double membrane, the nuclear envelope, containing nuclear pores that allow molecules to move in and out of the nucleus
• the nucleolus is an area within the nucleus that produces ribosomes
• contains RNA and does not have a membrane around it
• chromatin is genetic material, consisting of DNA wound around histone proteins
• chromatin coils and condenses to form chromosomes, only visible when cells divide

Function:

• nuclear envelope separates contents of nucleus from the rest of the cell
• some regions of the outer and inner nuclear membranes fuse together to allow some dissolved substances and ribosomes to pass through
• pores enable larger substances, such as mRNA, to leave nucleus and some steroid hormones to enter from cytoplasm
• nucleolus is where ribosomes are made
• nucleus controls the cell’s activities
• stores the organism’s genome in DNA
• transmits genetic information
• provides instruction for protein synthesis

Question 9

Describe the structure and function of lysosomes

Answer 9

Structure: ​

• a round organelle surrounded by a membrane, with no clear internal structure
• formed from the Golgi apparatus
• contains powerful hydrolytic (digestive) enzymes
• abundant in phagocytic cells such as, neutrophils and macrophages

Function:

• lysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell
• engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse
• break down pathogens ingested by phagocytic cells
• play an important role in apoptosis

Question 10

Describe the structure and function of ribosomes

Answer 10

Structure:

• a very small organelle (20nm) that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum
• made up of ribosomal RNA
• made in the nucleolus, as two separate subunits, which pass through the nuclear envelope into the cytoplasm and then combine
• not surrounded by a membrane

Function:

• ribosomes bound to the exterior of the RER are mainly for synthesising proteins that will be exported outside the cell
• ribosomes free in the cytoplasm are primarily the site of assembly of proteins that will be used inside the cell

Question 11

Describe the structure and function of rough endoplasmic reticulum (RER)

Answer 11

Structure:

• a system of membranes containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane
• surface is covered with ribosomes

Function:

• the intracellular transport system: the cisternae form channels for transporting substances from one area of a cell to another
• provides a large surface area for ribosomes, which assemble amino acids into proteins
• these proteins then actively pass through the membrane into the cisternae and are transported to the Golgi apparatus for modification and packaging

Question 12

Describe the structure and function of the smooth endoplasmic reticulum (SER)

Answer 12

Structure:

• system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membtane
• similar to rough endoplasmic reticulum, but with no ribosomes

Function:

• contains enzymes that catalyse reactions involved with lipid metabolism such as:
• synthesis of cholesterol
• synthesis of lipids/phospholipids needed by the cell
• synthesis of steroid hormones
• it is involved with absorption, synthesis and transport of lipids from the gut

Question 13

Describe the structure and function of the vesicle

Answer 13

Structure:

• a small fluid-filled sac in the cytoplasm, surrounded by a membrane

Function:

• transports substances in and out of the cell (via plasma membrane) and between organelles

Question 14

Describe the structure and function of the golgi apparatus

Answer 14

Structure:

• a group of fluid-filled, membrane-bound, flattened sacs
• vesicles are often seen at the edges of the sacs

Function: ​

• proteins are modified by:
• adding sugar molecules to make glycoproteins
• adding lipids molecules to make lipoproteins
• being folded into their 3D shape
• processes and packages new lipids and proteins into vesicles that are pinched off then are:
• stored in the cell
• moved to the plasma membrane, to be incorporated or exported
• makes lysosomes

Question 15

Describe the structure and function of the mitochondrion

Answer 15

Structure:

• usually oval shaped 2-5µm long
• have a double membrane
• inner membrane is folded to form structures called cristae
• inside is the matrix, containing enzymes involved in respiration
• contains a small amount of DNA: mitochondrial (mt)DNA

Function:

• site of aerobic respiration where ATP is produced
• they are self-replicating, so more can be made if the cell’s energy needs to increase
• they can also produce their own enzymes
• found in very large numbers in cells that are very active and require a lot of energy

Question 16

Describe the structure and function of chloroplasts

Answer 16

Structure:

• large organelle, 4-10µm long
• surrounded by a double membrane and membranes inside called thylakoid membranes, which contain chlorophyll
• stacked up in some parts of the chloroplast to form grana
• grana are linked by lamellae
• fluid-filled matrix is called the stroma
• chloroplasts contain loops of DNA and starch grains
• abundant in leaf cells, particularly palisade and mesophyll layer

Function:

• site of photosynthesis
• first stage of photosynthesis, where light energy is trapped by chlorophyll and used to make ATP, occurs in the grana
• second stage occurs in stroma

Question 17

Describe the structure and function of the centriole

Answer 17

Structure:

• a component of cytoskeleton present in most eukaryotic cells, except flowering plants and most fungi
• consists of two bundles of microtubules at right angles to each other
• made of tubulin protein subunits and arranged to form a cylinder

Function:

• involved with the separation of chromosomes during cell division:
• before a cell divides, the spindle, made of threads of tubulin, forms from the centrioles
• chromosomes attach to the middle part of the spindle and motor proteins walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell
• involved in the formation of cilia and undulipodia (flagella):
• before the cilia form, the centrioles multiply and line up beneath the cell surface membrane
• microtubules then sprout outwards from each centriole, forming a cilium or flagella

Question 18

Describe the structure and function of cilia

Answer 18

Structure:

• small, hair-like structures found on the surface membrane of some animal cells
• formed from centrioles
• they have an outer membrane and a ring of nine pairs of protein microtubules inside, with two microtubules in the middle

Function:

• microtubules allow cilia to move
• moves substances along the cell surface

Question 19

Describe the structure and function of the flagellum

Answer 19

Structure:

• like cilia but longer
• stick out from the cell surface and are surrounded by plasma membrane
• same 9+2 formation as cilia

Function:

• microtubules contract to make flagellum move
• like outboard motors to propel cells forward

Question 20

Describe the structure and function of the vacuole

Answer 20

Structure:

• surrounded by a membrane called the tonoplast and contains fluid

Function:

• only plant cells have a large permanent vacuole
• filled with water and solutes and maintains cell stability because it pushes against the cell wall, making the cell turgid
• this helps to support the plant if all cells are turgid

Question 21

Describe how organelles are involved in protein production

Answer 21

1. gene has coded instructions for a protein is transcribed into mRNA
2. Many mRNA are made and they pass out of the pores in the nuclear envelope to the ribosomes
3. at the ribosomes, instructions are translated and protein molecules are assembled
4. the molecules pass into the cisternae of the RER and along these hollow sacs
5. vesicles with these protein molecules are pinched off from the RER and pass, via microtubules and motor proteins to the Golgi apparatus
6. these vesicles fuse with the Golgi apparatus, where the molecules are modified for release
7. inside vesicles pinched off from the Golgi apparatus, these molecules pass to plasma membrane
8. the vesicles and plasma membrane fuse and the insulin is released to the outside of the cell

Question 22

What is the cytoskeleton?

Answer 22

Structure:

• a network of protein structures within the cytoplasm consisting of:
• rod-like microfilaments made of subunits of the protein actin; they are polymers of actin and each microfilament is about 7nm in diameter
• intermediates filaments about 10nm in diameter
• straight, cylindrical microtubules, made of protein subunits called tubulin, about 18-30nm in diameter
• the cytoskeletal motor proteins, myosins, kinesins and dyneins, are molecular motors
• they are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source

Function:

• the protein microfilaments within the cytoplasm give support and mechanical strength, keep the cell’s shape stable and allow cell movement
• microtubules also provide shape and support to cells and help substances and organelles to move through the cytoplasm within a cell
• they form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of the cell to another
• they form the spindle before a cell divides. these spindle threads enable chromosomes to be moved within the cell
• microtubules also make up the cilia, flagella and centrioles
• intermediate filaments are made of a variety of proteins. they:
• anchor the nucleus within the cytoplasm
• extend between cells in some tissues, between special junctions, enabling cell-cell signalling and allowing cells to adhere to a basement membrane, therefore stabilising tissues

Question 23

What are the four main functions of the cytoskeleton?

Answer 23

• Mechanical Strength: the microtubules and microfilaments support the cell’s organelles, keeping them in position
• Structure: They help to strengthen the cell and maintain its shape
• Aiding transport: they’re responsible for the movement of materials within the cell. they help the cell to divide (cytokinesis)
• Enabling movement: proteins of the cytoskeleton can also cause the cell to move. E.g. movement of cilia and flagella are caused by the cytoskeletal protein filaments that run through them

Question 24

Give the characteristics of prokaryotic cells

Answer 24

• extremely small cells (less than 2 um)
• DNA is circular
• no nucleus, DNA free in cytoplasm
• cell wall made of a peptidoglycan not cellulose
• few organelles and no membrane-bound organelles
• flagella made of protein flagellin
• smaller ribosomes
• have a much less well-developed cytoskeleton with no centrioles
• e.g. E.coli bacterium

some also have:

• protective waxy capsule surrounding cell wall
• small loops of DNA, called plasmids, as well as the main large loop of DNA
• flagella
• pili

Question 25

Describe the DNA in prokaryotic cells

Answer 25

• structure of the DNA is fundamentally the same as in eukaryotes. but it is packaged differently
• they generally have one molecule of DNA, a chromosome, which is supercoiled to make it more compact
• the genes on the chromosome are often grouped into operons, meaning a number of genes are switched on or off at the same time

Question 26

Compare ribosomes in prokaryotes and eukaryotes

Answer 26

• ribosomes in prokaryotic cells are smaller than those in eukaryotic cells
• their relative size is determined by the rate at which they settle, or form a sediment, in solution
• larger eukaryotic ribosomes are designated 80S and smalled prokaryotic ribosomes, 70S
• larger 80S ribosomes are involved in the formation of more complex proteins

Question 27

Describe the cell wall in prokaryotic cells

Answer 27

• made from peptidoglycan, also known as murein
• it is a complex polymer formed from amino acids and sugars

Question 28

Describe flagella in prokaryotic cells

Answer 28

• it is thinner than those in eukaryotic cells
• does not have the 9+2 arrangement
• the energy to rotate the filament that forms the flagellum is supplied from the process of chemiosmosis. not from ATP as in eukaryotic cells
• the flagellum is attached to the cell membrane by a basal body and rotated by a molecular motor

Question 29

Give the characteristics of eukaryotic cells

Answer 29

• larger cells (10-100um)
• DNA is linear
• Nucleus present, DNA inside it
• no cell wall in animals, cellulose cell wall in plants and chitin cell wall in fungi
• many organelles
• flagella made of microtubule proteins in 9+2 formation
• larger ribosomes

Question 30

How do prokaryotes cells divide?

Answer 30

• binary fission
• do not have linear chromosomes, so cannot carry out mitosis
• however, before division, DNA is copied so that each new cells receives a large loop of DNA

Question 31

What is resolution?

Answer 31

• the ability of an optical instrument to see or produce an image that shows fine detail clearly
• how detailed the image is
• how well a microscope distinguishes between two points that are close together

Question 32

What is magnification?

Answer 32

• how much bigger the image appears compared to the specimen

Question 33

How to calculate magnification

Answer 33

• magnification = image size / object size

Question 34

Describe the light (optical) microscope

Answer 34

• light microscopes use visible light (between 400-700nm)
• they have a lower resolution than electron microscopes
• maximum resolution of about 0.2 µm
• used to look at whole cells or tissues
• maximum useful magnification is around x 1500
• relatively cheap
• easy to use
• able to study whole organisms

Question 35

Describer the laser scanning confocal microscope

Answer 35

• Laser scanning confocal microscope use laser beams to scan a specimen, which is usually tagged with a fluorescent dye
• causes the dye to fluoresce
• light is then focused through a pinhole onto a detector
• the detector is hooked to a computer, generating an image, which can be 3D
• produces a much clearer image than light microscope because the pinhole means out of focus light is blocked
• can be used to look at objects at different depths in thick specimens
• high resolution and high contrast images

Question 36

Describe electron microscopes

Answer 36

• they use electrons to form an image
• higher resolution than light microscopes

Transmission electron microscope (TEM):

• use electromagnets to focus a beam of electrons, then transmitted through specimen
• denser parts absorb more electrons, which make it darker in the image
• provides high resolution images, but can only be used on thin specimens

- Scanning electron microscope (SEM):

• scan a beam of electrons across the specimen
• this knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image
• show the surface of specimen and can be 3D
• lower resolution image than TEM

Question 37

What is the ultrastructure of a cell?

Answer 37

• features of a cell that can be seen using an electron microscope

Question 38

What are the maximum resolutions of the three microscopes?

Answer 38

Light: 0.2 µm

TEM: 0.0002 µm

SEM: 0.002 µm

Question 39

What are the maximum magnifications of the three microscopes?

Answer 39

Light: x 1500

TEM: more than x 1,000,000

SEM: less than x 500,000

Question 40

What is differential staining?

Answer 40

• stains are coloured chemicals which bind to molecules in or on the specimen
• some bind to specific cell structures, which stains each structure differently to allow easy identification with one preparation

Question 41

How does differential staining work?

What makes certain stains bind to certain molecules?

Answer 41

• crystal violet and methylene blue are positively charged dyes, attracted to negatively charged materials in the cytoplasm, leading to staining of cell components
• dyes such as nigrosin and Congo red are negatively charged and repelled by the negatively charged cytosol
• they stay outside cells, leaving cells unstained, making them stand out against a stained background
• a negative stain technique

Question 42

Describe the Gram stain technique

Answer 42

• separates bacteria into two groups: Gram-positive bacteria and Gram-negative bacteria
• crystal violet is applied to a bacterial specimen, then iodine, which fixes the dye, then slide is washed with alcohol
• Gram-positive bacteria retains the crystal violet stain and will appear blue or purple under a microscope
• Gram-negative bacteria have thinner cells walls and therefore lose the stain
• they are stained with safranin dye, which is called a counterstain
• these appear reds
• Gram-positive bacteria are susceptible to penicillin, Gram-negative are not

Question 43

Difference between gram-positive and gram-negative bacteria

Answer 43

• Gram-positive bacteria have a thick peptidoglycan layer and no outer lipid membrane whilst Gram-negative bacteria have a thin peptidoglycan layer and have an outer lipid membrane

Question 44

How does penicillin affect G+ and/or G- bacteria?

Answer 44

• penicillin affects the production of peptidoglycan
• Peptidoglycan molecules form strong links that give the bacterial cell strength as well as preventing leakage from the cytoplasm
• Gram-positive bacteria have a peptidoglycan layer on the outside of the cell wall
• Gram-negative bacteria have peptidoglycan between membranes
• Penicillin works best on gram-positive bacteria by inhibiting peptidoglycan production, making the cells leaky and fragile
• The cells burst open and are much easier for the immune system to break down, which helps the sick person heal more quickly
• Human cells do not contain peptidoglycan, so penicillin specifically targets bacterial cells

Question 45

Give examples of different types of differential staining

Answer 45

• acetic orcein binds to DNA and stains chromosomes dark red
• eosin stains cytoplasm
• Sudan red stains lipids
• iodine in potassium iodide solution stains the cellulose in plant walls yellow and starch granules blue/black

Question 46

Describe the Acid-fast technique of differential staining

Answer 46

• used to differentiate species of Mycobacterium from other bacteria
• lipid solvent carries the carbolfuchsin dye into cells
• cells are then washed with dilute acid-alcohol solution
• mycobacterium are not affected by the acid-alcohol and retain the stain, which is bright red
• other bacteria lose the stain and are exposed to a methylene blue stain