Parts and Functions of the Human Cell

Also called the plasma membrane, this thin, flexible boundary separates the cell from its surroundings. It’s made of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates — imagine a fluid sea of lipids with islands of proteins.

Functions:

• Selective barrier: Controls entry and exit of substances (semipermeable).
• Communication: Receptors on the membrane detect hormones, neurotransmitters, and signals.
• Structural support: Provides shape and protection.
• Transport: Via diffusion, osmosis, active transport, and vesicular transport (endocytosis/exocytosis).

The cytoplasm is the jelly-like substance that fills the cell and holds the organelles. It’s mostly water, salts, and proteins. It’s where countless metabolic reactions occur — think of it as the workshop floor of the cell.

Contains:

• Cytosol: The fluid part.
• Organelles: The specialized “buildings” performing distinct tasks.
• Inclusions: Stored nutrients like glycogen, lipid droplets, and pigments.

The nucleus is the control center, housing the cell’s DNA, the master blueprint of life. It’s surrounded by a nuclear envelope with pores that regulate traffic between nucleus and cytoplasm.

Inside the nucleus:

• Chromatin: DNA + histone proteins, which condense into chromosomes during division.
• Nucleolus: The factory for ribosome production.

Functions:

• Controls cell activities by directing protein synthesis.
• Stores genetic material.
• Regulates cell growth, repair, and reproduction.

The mitochondria are double-membraned organelles producing ATP, the energy currency of the cell, through aerobic respiration.

Functions:

• ATP synthesis (via oxidative phosphorylation).
• Regulation of apoptosis (via cytochrome c release).
• Calcium storage and metabolism.

There are two types:

• Rough ER (RER): Studded with ribosomes. It synthesizes proteins for export or membrane use.
• Smooth ER (SER): Lacks ribosomes. It synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs (especially in liver cells).

The Golgi complex modifies, packages, and dispatches proteins made in the RER. Think of it as the post office sorting and shipping cellular products to their correct destinations.

Functions:

• Adds carbohydrate tags (glycosylation).
• Packages enzymes and hormones into vesicles.
• Forms lysosomes and secretory vesicles.

These small vesicles are filled with digestive enzymes (like proteases and lipases) that break down worn-out organelles, bacteria, or cellular debris.

Functions:

• Intracellular digestion and waste removal.
• Autophagy (self-cleaning of damaged organelles).

They contain oxidative enzymes that neutralize toxins and break down fatty acids.

Functions:

• Detoxify hydrogen peroxide via catalase.
• Metabolize long-chain fatty acids.
• Synthesize bile acids in liver cells.

Tiny but mighty, ribosomes are made of rRNA and proteins. They can float freely in cytoplasm or attach to the RER.

Functions:

• Translate genetic code (mRNA) into proteins.
• Free ribosomes → make intracellular proteins.
• RER-bound ribosomes → make proteins for secretion or membrane use.

A network of protein filaments providing shape, movement, and internal organization.

Main components:

• Microtubules: Maintain shape, form cilia/flagella, aid in chromosome movement.
• Microfilaments (actin): Enable cell movement and muscle contraction.
• Intermediate filaments: Provide tensile strength and stability.

These small structures near the nucleus organize microtubules during cell division.

Function:

• Form the mitotic spindle that separates chromosomes during mitosis.

• Cilia: Short, numerous projections that move substances across the cell surface (e.g., mucus in the respiratory tract).
• Flagella: Long, whip-like tail that propels the cell (e.g., sperm tail).