Cell Structure & Function

The cell is the basic unit of life. Every living organism -- from the bacteria in yogurt to the yak in the Himalayas -- is made of cells. Some organisms are single-celled (like bacteria), while humans are made of trillions of cells working together.

Plant Cell vs. Animal Cell

Both types share many organelles, but there are key differences:

| Feature | Plant Cell | Animal Cell | |---------|-----------|-------------| | Cell wall | Present (cellulose) | Absent | | Chloroplast | Present | Absent | | Vacuole | One large central vacuole | Small or absent | | Shape | Fixed, rectangular | Irregular | | Centrioles | Usually absent | Present |

Key Organelles and Their Functions

Cell Membrane: Controls what enters and leaves the cell. Present in ALL cells. It is selectively permeable -- allows some substances through but not others.

Nucleus: The "brain" of the cell. Contains DNA (genetic material) and controls cell activities.

Mitochondria: The "powerhouses" that produce energy through cellular respiration. More active cells (like muscle cells) have more mitochondria.

Ribosomes: Tiny structures that make proteins, following instructions from DNA.

Endoplasmic Reticulum (ER): A network of membranes. Rough ER (with ribosomes) makes proteins. Smooth ER makes lipids.

Golgi Body: Packages and distributes proteins and lipids -- like the "post office" of the cell.

Chloroplast: Found only in plant cells. Contains chlorophyll for photosynthesis -- converting sunlight into food.

Cell Wall: Found in plant cells. Provides extra support and protection outside the cell membrane.

Diffusion and Osmosis

Diffusion: Movement of particles from a region of higher concentration to lower concentration. Example: Perfume spreading across a room.

Osmosis: Diffusion of water through a selectively permeable membrane from a region of higher water concentration to lower water concentration.

Example: When you soak dry chickpeas in water, they swell up because water enters the cells by osmosis.

Example: If you put a fresh vegetable in salty water, it becomes limp because water leaves the cells by osmosis.

Nepal Connection: Rice paddies in the Terai depend on osmosis -- water must enter the rice plant's root cells to nourish the growing plant. Understanding cell biology helps improve agricultural practices across Nepal.

Key Takeaways

• Cells are the basic unit of life; plant and animal cells have important differences
• Each organelle has a specific function (nucleus = control, mitochondria = energy)
• The cell membrane controls what enters and exits
• Diffusion and osmosis move substances without using energy