Basic Cell Concepts
1. Who discovered cells, and how?
Cells were discovered by Robert Hooke in 1665. He observed thin slices of cork under a primitive microscope and noticed that they were composed of tiny, box-like compartments that reminded him of the small rooms in a monastery, which he called "cells".
2. Why is the cell called the structural and functional unit of life?
The cell is called the structural and functional unit of life because:
- All living organisms are composed of cells
- Cells are the smallest entities that can perform all life processes
- All metabolic reactions necessary for life occur within cells
- New cells arise only from pre-existing cells
- The structure of cells determines their specific functions
3. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
If the organization of a cell is destroyed, the cell will not be able to perform its normal functions properly. This disruption can lead to:
- Breakdown of metabolic processes
- Inability to maintain homeostasis
- Loss of cellular integrity
- Ultimately, cell death
4. Why is the plasma membrane called a selectively permeable membrane?
The plasma membrane is called selectively permeable because it allows only certain substances to pass through while restricting others. It can:
- Allow small molecules like oxygen, carbon dioxide, and water to pass freely
- Control the movement of ions and larger molecules
- Use transport proteins for specific substances
- Maintain the internal environment of the cell
Cell Structure
1. Can you name the two organelles we have studied that contain their own genetic material?
The two organelles that contain their own genetic material are:
- Mitochondria - Often called the powerhouse of the cell
- Chloroplasts - Found in plant cells and responsible for photosynthesis
2. Why are lysosomes known as suicide bags?
Lysosomes are called "suicide bags" because:
- They contain powerful digestive enzymes that can break down all types of biological polymers
- When the cell is damaged or dying, lysosomes may burst and release their enzymes
- These enzymes then digest the cell's own components, leading to cell death
- This process is called autolysis
3. Where are proteins synthesised inside the cell?
Proteins are synthesized on ribosomes, which can be found:
- Freely floating in the cytoplasm
- Attached to the rough endoplasmic reticulum (RER)
4. Which organelle is known as the powerhouse of the cell? Why?
The mitochondria is known as the powerhouse of the cell because:
- It produces ATP (adenosine triphosphate) through cellular respiration
- ATP is the main energy currency of the cell
- Mitochondria break down nutrients to release energy
- This energy is used to power various cellular activities
5. Where do the lipids and proteins constituting the cell membrane get synthesised?
Lipids and proteins for the cell membrane are synthesized in:
- Lipids: Smooth Endoplasmic Reticulum (SER)
- Proteins: Rough Endoplasmic Reticulum (RER) and ribosomes
After synthesis, these components are transported to the Golgi apparatus for modification and sorting before being incorporated into the cell membrane.
6. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
| Feature |
Prokaryotic Cell |
Eukaryotic Cell |
| Size |
Generally small (1-10 μm) |
Generally large (5-100 μm) |
| Nuclear region |
Poorly defined and known as nucleoid |
Well defined and surrounded by a nuclear membrane |
| Chromosome |
Single |
More than one chromosome |
| Membrane-bound cell organelles |
Absent |
Present |
Cell Processes
1. How do substances like CO₂ and water move in and out of the cell? Discuss.
CO₂ and water move in and out of cells through different processes:
Carbon dioxide (CO₂):
- Moves by simple diffusion
- Moves from areas of higher concentration to lower concentration
- Being a small nonpolar molecule, it can easily pass through the lipid bilayer
Water:
- Moves primarily by osmosis (a special type of diffusion)
- Moves from areas of higher water concentration to lower water concentration
- Can also move through specialized water channels called aquaporins
2. What is osmosis?
Osmosis is the movement of water molecules from a region of higher water concentration (or lower solute concentration) to a region of lower water concentration (or higher solute concentration) through a selectively permeable membrane.
3. How does an Amoeba obtain its food?
Amoeba obtains its food through a process called phagocytosis:
- It extends pseudopodia (false feet) around the food particle
- The pseudopodia surround and engulf the food particle, forming a food vacuole
- Digestive enzymes are secreted into the food vacuole
- Nutrients are absorbed into the cytoplasm
- Undigested material is expelled from the cell
4. What would happen if the plasma membrane ruptures or breaks down?
If the plasma membrane ruptures or breaks down:
- The cell would lose its structural integrity
- Cellular contents would leak out
- The cell would not be able to control what enters or exits
- Homeostasis would be disrupted
- Ultimately, the cell would die
5. What would happen to the life of a cell if there was no Golgi apparatus?
Without the Golgi apparatus:
- Proteins and lipids would not be processed, modified, and sorted properly
- Lysosomes would not be formed
- Cell membrane components would not be replaced
- Secretion of substances would be impaired
- The cell would eventually die due to improper functioning
6. Carry out the following osmosis experiment and answer the questions:
Experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now:
- (a) Keep cup A empty
- (b) Put one teaspoon sugar in cup B
- (c) Put one teaspoon salt in cup C
- (d) Put one teaspoon sugar in the boiled potato cup D
Keep these for two hours. Then observe the four potato cups.
(i) Explain why water gathers in the hollowed portion of B and C.
Water gathers in the hollowed portion of B and C because of osmosis. The sugar and salt solutions inside these cups have higher solute concentration than the water in the trough. This creates a concentration gradient that causes water to move from the trough (higher water concentration) into the potato cups (lower water concentration) through the selectively permeable membrane of the potato cells.
(ii) Why is potato A necessary for this experiment?
Potato A serves as a control in this experiment. It helps to demonstrate that without any solute inside the cup, there is no significant movement of water. This confirms that the water movement observed in other cups is indeed due to the presence of solutes and not due to other factors.
(iii) Explain why water does not gather in the hollowed out portions of A and D.
Water does not gather in:
- Cup A: Because there is no solute inside to create a concentration gradient for osmosis.
- Cup D (boiled potato): Because boiling destroys the cell membranes and kills the cells. The selectively permeable membrane is damaged, so osmosis cannot occur effectively.
7. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
- Growth and repair: Mitosis - produces two identical daughter cells with the same number of chromosomes as the parent cell.
- Formation of gametes: Meiosis - produces four genetically different daughter cells with half the number of chromosomes (haploid).
Cell Comparison
1. Make a comparison and write down ways in which plant cells are different from animal cells.
| Feature |
Plant Cell |
Animal Cell |
| Cell Wall |
Present (made of cellulose) |
Absent |
| Chloroplast |
Present (for photosynthesis) |
Absent |
| Vacuole |
Large central vacuole |
Small and numerous vacuoles |
| Shape |
Generally rectangular or fixed shape |
Generally round or irregular shape |
| Centrioles |
Absent |
Present |
| Storage |
Stores starch |
Stores glycogen |
| Plasmodesmata |
Present |
Absent |
2. How is a prokaryotic cell different from a eukaryotic cell?
| Feature |
Prokaryotic Cell |
Eukaryotic Cell |
| Nucleus |
Absent (has nucleoid) |
Present with nuclear membrane |
| DNA |
Circular, naked DNA |
Linear DNA with proteins |
| Membrane-bound organelles |
Absent |
Present (mitochondria, ER, Golgi, etc.) |
| Ribosomes |
70S |
80S (in cytoplasm) |
| Cell division |
Binary fission |
Mitosis or meiosis |
| Size |
Small (1-10 μm) |
Large (5-100 μm) |
| Examples |
Bacteria, Archaea |
Plants, Animals, Fungi, Protists |