Questions: Cell: The Unit of Life

Q1 State the cell theory and name the scientists who built it.

Answer: The cell theory says that all living organisms are made of cells and the products of cells, and that all cells arise from pre-existing cells. Schleiden and Schwann gave the first two points, and Virchow added the third in 1855 with the rule Omnis cellula-e cellula. Modern biology adds that the cell is the basic unit of structure and function in every organism.

Hinglish: Cell theory kehti hai ki sab organisms cells aur cells ke products se bane hain, aur har cell pehle se maujood cell se banta hai. Schleiden aur Schwann ne pehle do points diye, aur Virchow ne 1855 mein Omnis cellula-e cellula ka rule jodkar theory ko final shape di.

Q2 How does a prokaryotic cell differ from a eukaryotic cell?

Answer: A prokaryotic cell has no membrane-bound nucleus; its genetic material is naked circular DNA lying in the cytoplasm. It has no membrane-bound organelles except ribosomes, its ribosomes are 70S, and it usually has a cell wall. A eukaryotic cell has a nuclear envelope around its DNA, membrane-bound organelles such as mitochondria and ER, 80S ribosomes and chromosomes organised with histones.

Hinglish: Prokaryotic cell mein membrane-bound nucleus nahi hota aur DNA naked circular hota hai; organelles bhi nahi hote siwaye 70S ribosomes ke. Eukaryotic cell mein nuclear envelope, membrane-bound organelles aur 80S ribosomes hote hain.

Q3 Describe the cell envelope of a bacterium.

Answer: The envelope has three tightly bound layers. The outermost is the glycocalyx, which may be a loose slime layer or a thick capsule. Below it is the cell wall, which gives shape and prevents the cell from bursting. Below the wall is the plasma membrane, which is selectively permeable. The three layers act as one protective unit, and Gram staining separates bacteria on the basis of this envelope.

Hinglish: Bacterial cell envelope teen layers ka hota hai: bahar glycocalyx (slime layer ya capsule), phir cell wall jo shape deti hai, phir plasma membrane. Teeno milkar ek protective unit banate hain, aur Gram staining isi envelope ke aadhar par bacteria ko do groups mein baantti hai.

Q4 What is a mesosome and what does it do?

Answer: A mesosome is a special infolding of the plasma membrane seen in prokaryotes. It may take the form of vesicles, tubules or lamellae. It increases the surface area of the membrane, and it takes part in cell wall formation, DNA replication and the distribution of DNA to daughter cells. It also helps in respiration and secretion.

Hinglish: Mesosome prokaryotic cell mein plasma membrane ka infolding hota hai, vesicles, tubules ya lamellae ke roop mein. Yeh membrane ka surface area badhata hai aur cell wall formation, DNA replication aur DNA distribution mein help karta hai.

Q5 Explain the fluid mosaic model of the plasma membrane.

Answer: Singer and Nicolson proposed the model in 1972. The membrane is a bilayer of phospholipids with polar heads facing out and hydrophobic tails facing in, with cholesterol in the bilayer. Protein molecules are scattered through or sit on the lipid, and carbohydrates are attached to proteins and lipids on the outer surface. The lipid is quasi-fluid, so proteins can move sideways within the bilayer, and that movement is the fluidity of the membrane. The scattered proteins make the mosaic pattern.

Hinglish: Fluid mosaic model Singer aur Nicolson ne 1972 mein diya. Membrane phospholipid bilayer hai jisme polar heads bahar aur hydrophobic tails andar hote hain, saath mein cholesterol. Proteins lipid mein bikhre hote hain ya uspar baithte hain, aur carbohydrates bahar attached rehte hain. Lipids quasi-fluid hain isliye proteins laterally move kar sakte hain, aur isi ko fluidity kehte hain; bikhre proteins mosaic pattern banate hain.

Q6 What are the three routes by which molecules cross the plasma membrane?

Answer: Neutral solutes cross by simple diffusion along their concentration gradient without energy. Water crosses by diffusion, and this is called osmosis. Polar molecules need carrier proteins to help them across, which is facilitated transport. Ions or molecules moved against their concentration gradient need energy in the form of ATP, and this is active transport, as in the sodium-potassium pump.

Hinglish: Neutral solutes simple diffusion se gradient ke saath jaate hain, paani ka diffusion osmosis kehlata hai, polar molecules ko carrier protein chahiye hota hai, aur gradient ke against jaane wale ions/molecules ke liye ATP lagta hai, jise active transport kehte hain.

Q7 Name the members of the endomembrane system and give one function of each.

Answer: The endomembrane system includes the endoplasmic reticulum, the Golgi complex, lysosomes and vacuoles. The ER makes and transports proteins and lipids; rough ER carries ribosomes and smooth ER makes lipids. The Golgi modifies and packages materials and makes glycoproteins and glycolipids. Lysosomes carry hydrolytic enzymes for digestion. Vacuoles store water, sap and waste, and the plant vacuole occupies most of the cell volume.

Hinglish: Endomembrane system mein ER, Golgi complex, lysosomes aur vacuoles aate hain. ER proteins aur lipids banata hai, Golgi unhe modify aur package karta hai, lysosomes digestion karte hain, aur vacuoles paani aur waste store karti hain.

Q8 Compare mitochondria and chloroplasts.

Answer: Both are double-membrane bound organelles with their own circular DNA and 70S ribosomes, and both divide by fission. Mitochondria have cristae formed by folds of the inner membrane, and they carry out aerobic respiration to make ATP, so they are the powerhouses. Chloroplasts have thylakoids stacked into grana inside a stroma, and they trap light for photosynthesis. Mitochondria occur in plant and animal cells, while chloroplasts occur in plant cells and euglenoids.

Hinglish: Dono double membrane wale organelles hain jinme apna circular DNA aur 70S ribosomes hote hain. Mitochondria ki inner membrane cristae banati hai aur woh ATP banate hain; chloroplast mein thylakoids aur grana hote hain aur woh photosynthesis karta hai.

Q9 Describe the structure of the nucleus.

Answer: The nucleus is bounded by a nuclear envelope of two parallel membranes with a perinuclear space of 10 to 50 nanometres. The envelope is interrupted by nuclear pores, through which RNA and proteins move between the nucleus and the cytoplasm. Inside is the nucleoplasm with chromatin and one or more nucleoli. Chromatin is made of DNA, histones, non-histone proteins and RNA. The outer nuclear membrane continues with the endoplasmic reticulum and bears ribosomes.

Hinglish: Nucleus do parallel membranes ki nuclear envelope se bandha hota hai jinme perinuclear space hota hai. Nuclear pores RNA aur proteins ko aane-jaane dete hain. Andar nucleoplasm mein chromatin aur nucleolus hote hain; chromatin DNA, histones, non-histone proteins aur RNA se banta hai.

Q10 How are chromosomes classified by the position of the centromere?

Answer: Four types are named. A metacentric chromosome has the centromere in the middle and two equal arms. A sub-metacentric chromosome has the centromere slightly off the middle, giving one shorter and one longer arm. An acrocentric chromosome has the centromere close to an end, so that one arm is very short and the other very long. A telocentric chromosome has the centromere at the end.

Hinglish: Centromere ki position se chaar type hote hain: metacentric (beech mein, barabar arms), sub-metacentric (thoda off-centre), acrocentric (end ke paas, ek bahut chhota arm) aur telocentric (end par).

That completes the ten written answers. The chapter quiz below checks the cell theory, the membrane model, the endomembrane system, the vacuole and the chromosome types.