Lesson #2: CELL MEMBRANE

The cell membrane (also called the plasma membrane) is the outermost boundary in animal cells but is located inside the cell wall in plant, fungal, and bacterial cells.

Structure and Chemical Composition

  • The cell membrane is made of lipids, proteins, and carbohydrates.
  • It follows the fluid-mosaic model, proposed in 1972 by Singer and Nicolson.

Fluid-Mosaic Model

  • The membrane is flexible, like a fluid.
  • It is made of a phospholipid bilayer (two layers of lipids).
  • Proteins are embedded in this lipid bilayer, acting as channels and carriers.
  • Carbohydrates attach to proteins (glycoproteins) or lipids (glycolipids), helping in cell recognition and signaling.
  • The fluidity of the membrane allows it to change shape, repair itself, and allow movement of proteins.

Functions of the Cell Membrane

  1. Selective Permeability – It controls what enters and leaves the cell, allowing some molecules to pass while blocking others.
  2. Communication – Membrane proteins help cells recognize each other and communicate through chemical signals.
  3. Transport of Materials – It allows diffusion, osmosis, and active transport of nutrients and waste.
  4. Protection – It provides a barrier against harmful substances.
  5. Membrane-Bound Organelles – In eukaryotic cells, organelles like mitochondria, Golgi apparatus, and endoplasmic reticulum have their own membranes, helping them function separately.

CYTOPLASM

The cytoplasm is the semi-fluid substance inside the cell membrane, filling the space between the nucleus and the cell membrane.

Composition of Cytoplasm

  • 80% Water
  • Proteins and enzymes (which help in chemical reactions)
  • Salts, ions, and organic molecules
  • Suspended organelles (like mitochondria, ribosomes, and Golgi bodies)

Functions of the Cytoplasm

  1. Provides Support – It keeps the organelles in place and prevents them from collapsing.
  2. Medium for Chemical Reactions – Many essential biochemical processes, like glycolysis (breaking down glucose for energy), occur in the cytoplasm.
  3. Transport of Materials – It allows the movement of molecules and nutrients inside the cell.
  4. Stores Substances – The cytoplasm can store nutrients, waste, and chemical building blocks.

Cytosol vs. Cytoplasm

  • Cytoplasm = Cytosol + Organelles
  • Cytosol is the liquid part of the cytoplasm, containing dissolved substances but excluding membrane-bound organelles.

Real-Life Example of Cytoplasm Function

Imagine a fish tank:

  • The cytoplasm is like the water in the tank.
  • The fish, plants, and decorations represent the organelles.
  • Just as water allows fish to swim, the cytoplasm provides a medium for organelles to function properly.

Conclusion

  • The cell wall provides strength and protection in some organisms.
  • The cell membrane is a flexible barrier that controls what enters and exits the cell.
  • The cytoplasm acts as the cell’s internal environment, supporting organelles and carrying out essential metabolic functions.

NUCLEUS

The nucleus is known as the “control center” of the cell because it directs all cellular activities, especially protein synthesis. It contains genetic material (DNA), which carries instructions for the cell’s functions and characteristics.

Roles of the Nucleus

  1. Control Center – It regulates the cell’s activities by controlling protein production.
  2. Heredity – It contains DNA, which passes genetic traits from parents to offspring.
  3. Cell Division – It plays a crucial role in cell division by ensuring that DNA is correctly replicated and distributed.

Position in the Cell

  • Animal Cells: Located in the center of the cell.
  • Plant Cells: Pushed to the side due to the large central vacuole.

Structure of the Nucleus

1) Nuclear Envelope

  • The nucleus is enclosed by a double membrane called the nuclear envelope.
  • It contains nuclear pores, which allow the movement of materials like RNA and proteins between the nucleus and the cytoplasm.
  • Example: Think of the nuclear envelope as the security gate of a factory that controls what enters and exits.

2) Nucleoplasm

  • It is the jelly-like fluid inside the nucleus that contains essential substances for nuclear functions.

i) Nucleolus

  • A dark, round body within the nucleoplasm.
  • Function: It is the site of ribosome assembly.
  • Example: The nucleolus is like a workshop where machines (ribosomes) are built for protein production.

ii) Chromatin

  • It consists of fine, thread-like structures made of DNA and proteins.
  • During cell division, chromatin condenses to form chromosomes.
  • Example: Chromatin is like a long string of instructions that get neatly packed into books (chromosomes) when needed.