Cell biology

Question 1

Cell theory

Answer 1

1. All cells come from pre-existing cells
2. All organisms are composed of cells
3. Cells are the smallest unit of life

Question 2

Structure of a typical bacterial cell

Answer 2

1. Small and unicellular
2. Peptidoglycan cell wall
3. 70s ribosomes
4. Circular DNA
5. Lack of membrane-bound organelles

Question 3

Structure and function of rough endoplasmic reticulum

Answer 3

S:
- Consists of a three-dimensional network of interconnecting flattened membrane-bound sacs called cisternae.
- Has bound ribosomes attached to the cytoplasmic side of the rER membrane

F:
1. Folding polypeptides into its native three-dimensional conformation
2. Chemical modification of protein (glycosylation)
3. Transport of protein

Question 4

Structure and function of smooth endoplasmic reticulum

Answer 4

S:
Consists of a three-dimensional network of interconnecting, membrane-bound tubules called cisternae.

F:
1. Lipid synthesis
2. Detoxification
3. Calcium storage
4. Carbohydrate metabolism

Question 5

Structure and function of
Golgi body

Answer 5

S:
Consists of stacks of flattened, curved, membrane-bound sacs called cisternae

F:
1. Chemical modification of proteins and lipids from rER and sER respectively (glycosylation and phosphorylation)
2. Temporary storage
3. Repacking proteins into secretory vesicles to be secreted out of cells
4. Formation of lysosomes

Question 6

Structure and function of mitochondria

Answer 6

S:
1. Cylindrical/ rod-shaped organelle bounded by a double membrane.
2. Inner membrane of mitochondria is extensively folded to form cristae. This increases the surface area of the inner membrane, allowing proteins and enzymes to be embedded such as electron carriers and stalked particles containing ATP synthase
3. Between the outer and inner membrane exists an intermembrane space, consisting of a semi-fluid mitochondrial matrix, containing Krebs cycle enzymes, 70s ribosomes, circular DNA and glycogen granules

F:
Main site of cellular respiration where glucose is metabolised to synthesize adenosine triphosphate (ATP) ATP is the main energy source of cellular processes

Question 7

Structure and function of ribosomes

Answer 7

S:
Comprises of a small and a large subunit, each made up of one ribosomal ribonucleic acid and one ribosomal protein each

F:
Main site of protein synthesis, where they translate messenger RNA into a polypeptide chain

Question 8

Structure and function of lysosomes

Answer 8

S:
Small, spherical vesicle containing hydrolytic enzymes

F:
1. Digests food and foreign particles during phagocytosis
2. Removal of unwanted/ damaged organelles during autophagy

Question 9

Structure and function of chloroplasts

Answer 9

S:
1. Cylindrical organelle bounded by a double membrane
2. Inner membrane gives rise to thylakoids and lamellae. Thylakoids are stacked to form grana. This increases the surface area of the inner membrane. This allows proteins and enzymes to be embedded such as electron carriers and stalked particles containing ATP synthase.
3. The interior of the chloroplast is filled with a gel-like matrix called the stroma, which contains 70s ribosomes, Calvin cycle enzymes, circular DNA and scratch grains

F:
Main site of photosynthesis where trapped sunlight is used to synthesize organic compounds such as sugars from carbon dioxide and water.

Question 10

Structure and function of cell surface membrane

Answer 10

S:
- Main components are phospholipids, proteins, glycoproteins, glycolipids and cholesterol
- Fluid Mosaic Model:
Fluid is used to describe how the phospholipids and embedded proteins are able to move freely within the phospholipid bilayer
Mosaic is used to describe how there are proteins scattered and embedded throughout the phospholipid bilayer

F:
1. Provides a boundary that separates cell contents from the external environment
2. Compartmentalisation to prevent indiscriminate mixing of cellular contents and specialization of cell function
3. Partially permeable
4. Signal transduction due to presence of receptor proteins
5. Intercellular communication

Question 11

Structure and function of nuclear envelope

Answer 11

S:
Made up of a double membrane

Question 12

Structure and function of centrioles

Answer 12

S:
Consists of a pair of rod-like structures, positioned at right angles to each other. Composed of 9 sets of triplet microtubules arranged in a ring.

F:
Function during cell division where centriole is replicated, both migrate to opposing poles of the animal cell, to organise microtubules into spindle fibres. This helps to align the chromosomes during metaphase and pull the chromosomes apart during anaphase

Question 13

Structure and function of nucleus

Answer 13

S:
Consists of a nuclear envelope with nuclear pores, nucleolus and nucleoplasm and chromatin

F:
1. Contains the genetic material of the organism in the form of DNA
2. Site of synthesis of rRNA
3. Site of DNA replication
4. Genes in chromatin responsible for synthesis of rRNA, mRNA and tRNA
5.Directs protein synthesis

Question 14

Structure and function of nucleolus

Answer 14

F:
Contains the instructions in the DNA for the synthesis of RNA

Question 15

Fluid mosaic model

Answer 15

Fluid is used to describe:
- Phospholipids and proteins are able to move freely within the phospholipid bilayer
- Phospholipids move laterally within a layer frequently but rarely flip-flop across the membrane as it is unlikely that the hydrophillic heads will through the hydrophobic core of the membrane

Mosaic is used to describe:
- Protein molecules are embedded and scattered among the phospholipids. these protein molecules include channel proteins, receptors, and glycoproteins

Question 16

Main components of cell surface membrane

Answer 16

1. Phospholipids
2. Proteins
3. Glycoproteins and glycolipids
4. Cholesterol

Question 17

Roles of phospholipids

Answer 17

S:
1. Phospholipids are amphipathic. it consists of a hydrophillic phosphate. group and 2 hydrophobic fatty acid chains/ tails
2. They are arranged as a bilayer in the csm
3. The hydrophillic phosphate heads face the aq medium
4. The hydrophobic fatty acid chains/ tails face each other in the middle of the bilayer to form the hydrophobic core

F:
1. The arrangement of the phospholipids in the form of a bilayer allows it to form a barrier in the aqueous environment

Question 18

Structure of proteins

Answer 18

Extrinsic/Peripheral proteins:
1. External extrinsic proteins are attached to fibers of extracellular matrix to keep the cell in place
2. Internal extrinsic proteins are attached to the cytoskeleton to maintain the cell shape and motility

Intrinsic/Integral proteins
1. Intrinsic proteins are embedded in the hydrophobic core of phospholipid bilayer
2. They may be unilateral or transmembrane
3. They contain both hydrophobic and hydrophillic regions
4. The hydrophobic regions form hydrophobic interactions with the hydrophobic fatty acid tails of the phospholipid bilayer
5. The hydrophillic regions form hydrogen bonds with the aqueous medium and hydrophillic phosphate heads of the phospholipid molecules

Question 19

Roles of glycoproteins and glycolipids

Answer 19

S:
- Glycoproteins are proteins with addition of short carbohydrate chains
- Glycolipids are lipids/phospholipids with addition of short carbohydrate chains
- they are found on the outer surface of the membrane of all eukaryotic cells

F:
1. Main function is for cell-cell recognition where cells are able to recognise and bind to one another
2. Cell-cell adhesion. carb chains of two adjacent cells may bind with one another -> enables cells to form a tissue
3. Act as receptor sites for signal molecules to bind which allows for cell signalling

Question 20

Roles of cholesterol

Answer 20

S:
- found only in the membrane of animal cells
- cholesterols are interspersed among the bilayer, hindering the close packing of the phospholipids

F:
1. helps to regulate fluidity of cell membranes in animals cells
2. Maintains mechanical stability of csm
3. Prevents leakage of small polar molecules

Question 21

Transport proteins

Answer 21

• Channel proteins have hydrophilic pores which allow movement of hydrophilic molecules such as charged ions and polar molecules through the hydrophobic core of the phospholipid bilayer
• Carrier proteins have specific binding sites for specific solutes to bind to. once solute binds to the carrier protein, it undergoes a change in conformation. the carrier protein constantly flips between two states, opening to opposite sides of the membrane alternatively
  down/ against concentration gradient using ATP
  Eg. glucose

Question 22

Tonics :0

Answer 22

Hypotonic: surrounding solution lower solute conc. -> net movement of water into cell

Isotonic: smae

Hypertonic: surrounding solution higher solute conc. -> net water movement out of cell

Question 23

Factors affecting membrane fluidity

Answer 23

1. Temperature
   - at low temp, molecules have low KE, and thus the membrane is less fluid
   - at high temp, molecules have higher KE, thus they vibrate faster and the membrane is more fluid. Phospholipids may move apart frequently, creating gaps in the membrane, increasing its permeability
2. Liquid composition
   - unsaturated fatty acid chains have kinks at sites of double bonds. this prevents fatty acid chains from packing closely together, causing the membrane to be more fluid.
   - more unsaturated fatty acid chains -> more fluid the membrane
3. Presence of cholesterol
   - at high temp, cholesterol prevents phospholipids from moving too far apart by forming hydrophobic interactions with the fatty acid tails
   - At low temp, cholesterol prevents phospholipids from packing too close together. this prevents cell freeze

Question 24

Functions of csm

Answer 24

1. Boundary (csm)
   - the csm forms a boundary that separates the cell contents from the external environment
2. Compartmentalisation (internal)
   - Internal cell membranes enable compartmentalisation of cell contents and prevent indiscriminate mixing
   -> this allows for specialisation of cell function by concentrating specific substances
3. Partially permeable barrier
   - hydrophobic tails repel hydrophilic molecules such as charged ions and polar substances
   - allows the passage of hydrophobic molecules such as non-polar molecules and relatively small molecules
4. Signal transduction
   - receptor proteins have sites of attachments for signal molecules to bind which allow for cell signalling
5. Intercellular communication
   - cells are able to recognise each other (i.e. cell recognition) through glycolipids and glycoproteins present in the csm.

Question 25

Simple diffusion

Answer 25

Net movement of substances down a concentration gradient, until a dynamic equilibrium is reached

Question 26

Facilitated diffusion

Answer 26

Net movement of substances down a concentration gradient, with the help of specific transport proteins, until a dynamic equilibrium is reached

Question 27

Osmosis

Answer 27

Net movement of water molecules down a water potential gradient, from a region of less negative wp to more negative wp, across a partially permeable membrane, until a dynamic equilibrium is reached

Question 28

Active transport

Answer 28

The movement of substances against a concentration gradient, using energy in the form of ATP.

Movement is highly selective and usually unidirectional. Involves specific transmembrane carrier proteins called ‘pumps’ which use ATP to change its conformation. Conformational change translocates the solute attached to the protein across the membrane

Question 29

Endocytosis

Answer 29

Process of uptake of substances by invagination of the cell surface membrane or extension of the pseudopodia.

Small area of csm buds off to form a vesicle containing the substance

Phagocytosis:
- uptake of large, solid materials
- this process is specific as phagocytes recognise the molecules before engulfing them
- cell extends pseudopodia to engulf the bacteria
- a phagocytic vesicle is formed with the bacterium enclosed
- lysosome membrane fuses with membrane of phagocytic vesicle
- hydrolytic enzymes in lysosome digest the contents into soluble products. these soluble products diffuse into the cytoplasm for cell use
- membrane of vesicle fuses with csm, releasing any undigested materials out of the cell by exocytosis

Pinocytosis:
- uptake of droplets of extracellular fluids via tiny vesicles
- this process is unspecific as all solutes dissolved in the droplets are taken in
- carried out by invagination of the csm

Question 30

Exocytosis

Answer 30

Materials are enclosed in a vesicle -> secretory vesicle move along cytoskeleton towards csm -> membrane of vesicle fuses with csm -> contents are then discharged out from the cell by exocytosis