Week 11 / Cell Structure Lecture

Question 1

What are the major structural components of the plasma membrane (cell membrane)? [4]

Answer 1

Phospholipid bilayer

Cholesterol

Proteins (integral and peripheral)

Attached carbohydrates (glycolipids and glycoproteins)

Question 2

What are the main functions of the plasma membrane? [4]

Answer 2

Acts as a barrier between the inside and outside of the cell

Controls the entry of materials (transport)

Receives chemical and mechanical signals

Transmits signals between intracellular and extracellular spaces

Question 3

How are phospholipids organized in the plasma membrane?

Answer 3

Phospholipids are amphipathic.

• They form a bilayer with nonpolar fatty acid chains in the middle.
• The polar regions are oriented toward the surfaces of the membrane, attracted to the polar water molecules in the extracellular fluid and cytosol.

Question 4

What are the characteristics of the phospholipid structure in the plasma membrane?

Answer 4

• Fatty acid tails are hydrophobic.
• The phosphate group head is hydrophilic.

Question 5

Where can cholesterol be found?[1]

What is the role of cholesterol in the plasma membrane? [2]

Answer 5

• The extracellular plasma membrane contains cholesterol, while intracellular membranes have very little.
• Cholesterol associates with specific phospholipids and proteins to form clusters.
• These clusters help pinch off portions of the membrane to form vesicles that transport contents to intracellular organelles.

Question 6

How do cholesterol molecules interact with the plasma membrane?
[3]

Answer 6

• Cholesterol molecules are weakly amphipathic.
• The —OH group is the only polar region, forming hydrogen bonds with the polar heads of phospholipids and glycolipids.
• The stiff steroid rings and hydrocarbon tail are nonpolar, fitting among the fatty acid tails of phospholipids and glycolipids.

Question 7

How does cholesterol affect membrane fluidity in different temperatures?

Answer 7

• At warm temperatures, cholesterol restrains phospholipid movement, reducing fluidity.
• At cold temperatures, cholesterol maintains fluidity by preventing tight packing of phospholipids.

Question 8

What is the structure of glycolipids ? [1]
Where are they found and in what way are they organized?[2]

Answer 8

• The carbohydrate groups form a polar “head,” while the fatty acid “tails” are nonpolar.
• Glycolipids are found only in the membrane layer facing the extracellular fluid, contributing to the asymmetry of the bilayer.

Question 9

What are the characteristics of integral membrane proteins?
(what do they associate with , what do they cross , other names when they span the entire membrane?)[3]?

What are the functions of integral membrane proteins (what do they form , transmit)? [1]

Answer 9

• Integral proteins are closely associated with membrane lipids and cannot be extracted without disrupting the lipid bilayer. They are amphipathic.
• Most integral proteins span the entire membrane and are called transmembrane proteins.
• Many transmembrane proteins cross the lipid bilayer multiple times.
• Some form channels for ions or water, while others transmit chemical signals or anchor extracellular and intracellular protein filaments to the plasma membrane.

Question 10

What are the characteristics of peripheral membrane proteins?

Where are they located ?

Answer 10

• Peripheral membrane proteins are not amphipathic and do not associate with the nonpolar regions of the membrane lipids.
• They are located at the membrane surface, where they bind to the polar regions of integral membrane proteins.

Question 11

what are the roles of glycocalyx in the plasma membrane? [1]

What do the carbohydrate parts of the glycolipids and glycoprotein form?[1]

What are many membrane proteins ?
What are attached to “ proteins “ and where are attached things facing
?[1]

Answer 11

• The glycocalyx acts like a molecular “signature,” enabling cells to recognize one another.
• The carbohydrate portions of glycolipids and glycoproteins form a sugary coat called the glycocalyx.
• Many membrane proteins are glycoproteins, which have carbohydrate groups attached to their ends that protrude into the extracellular fluid.

Question 12

“What are the main classifications and types of plasma membrane proteins, and what are their key functions?”

Answer 12

The two main classifications of membrane proteins (by position and by function)

The types of proteins under each classification (e.g., integral, peripheral, anchoring, recognition, enzymes, receptor, carrier, and channel proteins)

The specific functions of each protein type (e.g., structural components, attaching membranes, identifying cells, catalyzing reactions, binding ligands, moving solutes, etc.)

Question 13

What are the types of cell junctions

What are integrins and what are their roles the role of integrins in the plasma membrane?

Answer 13

• Cells are physically joined by junctions: desmosomes, tight junctions, and gap junctions.
• Integrins are transmembrane proteins that bind to specific proteins in the extracellular matrix and link them to membrane proteins on adjacent cells.

Question 14

What are the characteristics of desmosomes (what are they , where are they located , what do they serve as)?

what are the functions of desmosomes in cell adhesion?

what are the functions of cadherins?

Answer 14

• Desmosomes are characterized by dense plaques of protein on the cytoplasmic surface of the plasma membrane, serving as anchoring points for cadherins.

-Desmosomes hold adjacent cells firmly together, particularly in areas subjected to considerable stretching, such as the skin.

• Cadherins extend from the cell into the extracellular space, linking and binding with cadherins from adjacent cells.

Question 15

What are the characteristics and functions of tight junctions in cell membranes?
(how do they form)
Whats the difference between desmosomes and tight junctions (shape)?

Answer 15

• Tight junctions form when the extracellular surfaces of two adjacent plasma membranes join together, leaving no extracellular space between them.
• Unlike desmosomes, which are limited to a disk-shaped area, tight junctions occur in a band around the entire circumference of the cell.

Question 16

What are the characteristics and functions of gap junctions in cell communication?

Answer 16

• Gap junctions consist of protein channels that link the cytosols of adjacent cells.
• Connexins from the two membranes join to form small, protein-lined channels.
• The diameter of about 1.5 nm limits passage to small molecules and ions, such as Na⁺ and K⁺, while excluding large proteins.
• An example of gap junctions is found in muscle cells of the heart.

Question 17

What are the primary functions of cell membranes? [4]

Answer 17

1. Regulate the passage of substances into and out of cells and between organelles and cytosol.
2. Detect chemical messengers arriving at the cell surface.
3. Link adjacent cells together through membrane junctions.
4. Anchor cells to the extracellular matrix.

Question 17

What is cytosol ? [1]
What are the characteristics of cytosol ? [1]

What are the functions of cytosol in the cytoplasm? [2]

Answer 17

• Cytosol is the fluid portion of the cytoplasm that surrounds organelles, constituting about 55% of the total cell volume.
• It is composed of 75-90% water, along with various dissolved and suspended components (such as ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste products).
• In some cells, organic molecules can aggregate into masses for storage, which may appear and disappear at different times in the cell’s life.
• Cytosol is the site of many chemical reactions essential for the cell’s existence.

Question 18

What are the key organelles found in the cytoplasm of a cell?[7]

Answer 18

• Cytoskeleton
• Flagella, cilia, and centrioles
• Endoplasmic reticulum
• Golgi apparatus
• Mitochondrion
• Nucleus, nucleolus, and nuclear envelope
• Vesicles (e.g., lysosome)

Question 19

What are the functions (3) and components (3) of the cytoskeleton in a cell?

Answer 19

• Functions:
  
  • Maintains the shape of the cell
  • Positions organelles
  • Changes cell shape
• Components:
  
  • Microfilaments
  • Intermediate filaments
  • Microtubules

Question 20

What are the properties and behaviors of the different components of the cytoskeleton?

Answer 20

• Microfilaments (Actin filaments) and Microtubules:
  
  • Can be rapidly assembled and disassembled, allowing the cell to alter its cytoskeletal framework according to changing requirements.
• Intermediate Filaments:
  
  • Once assembled, they are less readily disassembled, providing more stability to the cell structure.

Question 21

What are the structure (3)and function (1)of the centrosome in a cell?

Answer 21

• Structure:
  
  • Consists of two centrioles arranged perpendicular to each other.
  • Each centriole is composed of microtubules arranged in 9 clusters of 3 (triplets).
  • Contains pericentriolar material, which is composed of tubulin.
• Function:
  
  • The centrosome is responsible for organizing the mitotic spindle, which moves chromosomes to the ends of the cell during cell division.

Question 22

What are the characteristics (1) and functions (1) of cilia in cells?

What are the characteristics (1) and functions (1) flagella in cells?

Answer 22

• Cilia:
  
  • Specialized for motion and often found in groups.
  • In the respiratory system, they move mucus, helping to clear debris and pathogens.
• Flagella:
  • A single tail-like structure, as seen in sperm.
  • Propels sperm forward in the reproductive tract.

Question 23

What are the characteristics and functions of ribosomes in a cell
(where its made
, function ,
structure,
,composition,
types ) ?

Answer 23

• Location:
  
  • Made within the nucleus (specifically in the nucleolus).
• Function:
  
  • Sites of protein synthesis, either on the endoplasmic reticulum (E.R.) or freely within the cytoplasm.
• Composition:
  
  • Consist of ribosomal RNA (rRNA) and proteins.
• Structure:
  
  • Contain large and small subunits.
• Types:
  
  • Can be attached to the endoplasmic reticulum or free in the cytosol.

Question 24

What are the structure, functions, and types of the endoplasmic reticulum (E.R.) in a cell?

(structure,
, function ,
types
[name both types , functions(1,4) and characteristics (1,1)] ) ?

Answer 24

• Structure:
  
  • The endoplasmic reticulum is a network of folded membranes.
• Functions:
  
  • Synthesis and intracellular transport of substances.
• Types of E.R.:
  
  • Rough E.R.:
    
    • Studded with ribosomes, serving as the site of protein synthesis.
  • Smooth E.R.:
    
    • Lacks ribosomes and has several functions:
      
      • Lipid synthesis.
      • Release of glucose into the bloodstream from liver cells.
      • Drug detoxification, particularly in liver cells.
      • Storage and release of Ca²⁺ in muscle cells (where it is known as the sarcoplasmic reticulum or SR).

Question 25

What are the structure (1) and functions(4) of the Golgi complex in a cell?

Answer 25

• Structure:
  
  • Composed of flattened membranes (cisterns) with bulging edges, resembling stacks of pita bread.
• Functions:
  
  • Modifies proteins to form glycoproteins and lipoproteins, which:
    
    • Become part of plasma membranes.
    • Are stored in lysosomes.
    • Are exported from the cell by exocytosis.

Question 26

What are the structure (3) and functions (2)of lysosomes in a cell?

What disease occurs with a lack of lysosomal enzyme?

Answer 26

• Structure:
  
  • Spherical or oval-shaped organelles surrounded by a single membrane.
  • A typical cell may contain several hundred lysosomes.
  • Contain a variety of digestive enzymes.
• Functions:
  
  • Assist in the final processes of digestion within cells.
  • Carry out autophagy (destruction of worn-out parts of the cell) and facilitate the death of old cells (autolysis).
  • Tay-Sachs disease is a hereditary disorder caused by the absence of a lysosomal enzyme, leading to nerve destruction.

Question 27

What are the structure (1) and functions (3) of peroxisomes in a cell?

Answer 27

• Structure:
  
  • Peroxisomes are moderately dense oval bodies enclosed by a single membrane.
• Function:
  
  • Consume molecular oxygen and undergo reactions that remove hydrogen from organic molecules, including lipids, alcohol, and potentially toxic ingested substances.
  • Play a crucial role in detoxification and are abundant in liver cells.

Question 28

What are the structure and functions of mitochondria in a cell?

Answer 28

• Structure:
  
  • Mitochondria are sausage-shaped organelles with many folded membranes known as cristae, surrounded by a liquid matrix containing enzymes.
  • They contain some DNA and ribosomes, allowing them to synthesize some proteins.
• Function:
  
  • Mitochondria are primarily responsible for producing ATP through cellular respiration, serving as the powerhouse of the cell.

Question 29

What are the functions of mitochondria in a cell?

Answer 29

• Function:
  
  • Major site of ATP production, oxygen utilization, and carbon dioxide formation.
  • Contains enzymes that are active in the Krebs cycle and oxidative phosphorylation.
• Location:
  
  • Mitochondria are abundant in cells that require a high amount of ATP, such as muscle, liver, and kidney cells.

Question 30

What are the structure and components of the nucleus in a cell?

Answer 30

• Structure:
  
  • The nucleus is a round or oval structure surrounded by a nuclear envelope that contains nuclear pores.
• Components:
  
  • Contains a nucleolus, which is responsible for making ribosomes that pass into the cytoplasm through the nuclear pores.
  • Within the nucleus, DNA, in association with proteins, forms a fine network of threads known as chromatin.

Question 31

What are the key features and functions of the nucleus, particularly focusing on the nucleolus?

Answer 31

• Key Features:
  
  • The most prominent structure in the nucleus is the nucleolus, a densely staining filamentous region without a membrane.
  • The nucleolus is associated with specific regions of DNA that contain the genes for forming the types of RNA found in ribosomes.
• Functions:
  
  • Stores genetic material (DNA) arranged in 46 chromosomes, which collectively form the human genome containing approximately 30,000 genes.
  • The DNA in the nucleus contains information for directing protein synthesis both in the cell and in new cells formed through cell reproduction.