Eukaryotic Cells Lesson: Parts and Their Functions

Lesson Overview

• Organelles: The Cell's Internal Parts
• Nucleus: DNA Storage and Control Center
• Nucleolus: The Ribosome Factory
• Cell Membrane: The Protective Barrier
• Mitochondria: The Cell's Powerhouse
• Lysosomes: The Cell's Recycling Center
• Plant Cell Specialties: Cell Wall, Chloroplasts, and Vacuole
• Prokaryotes vs. Eukaryotes: Key Differences

Eukaryotic cells are complex cells that contain a nucleus and many other specialized structures. These cells make up all animals, plants, fungi, and protists – meaning everything from a human to a mushroom to an amoeba is made of eukaryotic cells.

In this lesson, we'll explore the key parts of a eukaryotic cell and how each part functions, with examples and memory tricks to help you remember them.

Organelles: The Cell's Internal Parts

Organelles are the different parts inside a cell, analogous to organs in our body. Each organelle has a unique function that helps the cell survive, much like how the heart or lungs have special jobs in a body. Most organelles in eukaryotic cells are enclosed by membranes. Here are some of the main organelles and their roles:

• Nucleus: Holds the cell's DNA and controls activities (the "control center" of the cell).
  
• Mitochondria: Produces energy for the cell (the "powerhouse" of the cell).
  
• Endoplasmic Reticulum (ER): A network that helps process and transport proteins and lipids. (Rough ER has ribosomes making proteins; Smooth ER makes lipids.)
  
• Golgi Apparatus: Packages and ships proteins and other molecules (the cell's post office).
  
• Ribosomes: Tiny structures that assemble proteins. They can float in the cytoplasm or attach to the Rough ER.
  
• Lysosomes: Organelles with digestive enzymes to break down waste (the cell's cleanup crew).
  
• Chloroplasts: (In plant cells) Use sunlight to make food (sugars) by photosynthesis.
  
• Vacuoles: Sacs that store water, nutrients, or waste. (Plant cells have a large central vacuole.)

Take This Quiz:

Eukaryotic Cells Quiz

Nucleus: DNA Storage and Control Center

The nucleus is a large, round organelle usually found near the center of the cell. It's surrounded by its own membrane (the nuclear envelope) and it stores the cell's genetic material, DNA. Because it houses DNA, the nucleus acts like the control center or "brain" of the cell: DNA contains instructions for all the cell's activities, telling the cell when to grow, divide, or make proteins.

• Inside the nucleus, DNA is organized into chromosomes (long strands of DNA). These carry genes that determine the cell's traits and direct its functions.
  
• The nuclear envelope protects this genetic material by keeping it separate from the rest of the cell.
  
• Only eukaryotic cells have a true nucleus. In simpler cells (prokaryotes like bacteria), there is no nucleus – their DNA floats in the cell's interior. (A handy phrase: "Pro means no nucleus; Eu means do have a nucleus.")

Since the nucleus stores DNA, it's correct to say the nucleus is the cell's "DNA storage" site. No other organelle contains the cell's chromosomes. 

Nucleolus: The Ribosome Factory

Inside the nucleus, there is often a darker round region called the nucleolus. This structure isn't surrounded by a membrane, but it's a crucial part of the nucleus. The nucleolus is like a factory for making ribosomes.

• The nucleolus produces and assembles components of ribosomes (the structures that build proteins). It makes ribosomal RNA and puts together the subunits of ribosomes, which then leave the nucleus to do their job in the cytoplasm.
  
• You can think of the nucleolus as a little workshop inside the nucleus, producing tools (ribosome parts) that the cell needs to build proteins.
  
• It's easy to confuse the nucleolus with the nucleus since their names sound similar. Remember that the nucleolus is a small part inside the nucleus. One way to keep it straight is to notice the "-olus" ending, indicating a smaller component (a sub-unit inside the nucleus) with a specific job.

Cell Membrane: The Protective Barrier

Every cell is wrapped in a cell membrane (also called the plasma membrane). This thin, flexible layer acts as a selectively permeable barrier between the inside of the cell and its environment:

• It controls what goes into and out of the cell. Important nutrients can enter and waste products can exit, but the membrane blocks many unwanted substances. By regulating this exchange, the cell membrane helps maintain a stable internal environment.
  
• All cells (plants, animals, and even bacteria) have a cell membrane that protects them. Note: Plant cells, fungal cells, and bacteria also have a rigid cell wall outside the membrane (discussed below), but animal cells do not. The cell wall provides extra support, but the underlying cell membrane is still present in those cells.

Mitochondria: The Cell's Powerhouse

Mitochondria are rod-shaped organelles found in the cytoplasm of nearly all eukaryotic cells. They are often called the "powerhouses" of the cell because their main job is to produce energy:

Inside mitochondria, food molecules like sugars are broken down through a process called cellular respiration. This releases energy, which is stored in molecules of ATP (adenosine triphosphate). ATP is the usable energy currency that powers the cell's activities.

Plant cells have mitochondria too. Even though plants make their own food in chloroplasts, they still use mitochondria to convert that food into usable energy (ATP).

Lysosomes: The Cell's Recycling Center

Lysosomes are small, spherical organelles filled with enzymes that digest and break down materials. They function as the cell's waste disposal and recycling center:

• Lysosomes contain digestive enzymes that break down unwanted materials in the cell, such as worn-out organelles, food particles, or invading bacteria. They digest these materials into smaller components.
  
• By breaking down waste and recycling parts, lysosomes keep the cell clean and healthy.
  
• Lysosomes are common in animal cells. (Plant cells have other ways to handle waste, often using their vacuoles for digestion and storage of waste.)

Plant Cell Specialties: Cell Wall, Chloroplasts, and Vacuole

Plant cells are eukaryotes that have a few extra structures not found in animal cells:

• a rigid outer cell wall,
  
• green disc-like chloroplasts,
  
• and a large central vacuole.

These structures are key to identifying a plant cell and understanding its functions:

• Cell Wall: A sturdy layer outside the cell membrane, made of cellulose. It provides support and protection, giving plant cells a fixed shape. (Animal cells do not have a cell wall.)
  
• Chloroplasts: Green organelles that capture sunlight to perform photosynthesis – making glucose (food) for the plant. Chloroplasts contain chlorophyll (the green pigment that absorbs light). Cells that photosynthesize (plants and some algae) have chloroplasts, while animal cells do not.
  
• Central Vacuole: A very large, fluid-filled sac that stores water, nutrients, and waste. When it's full of water, it helps the cell stay firm and rigid. (This is why plants stand upright when watered - their vacuoles are full and press against the cell wall.)

Aside from these special parts, plant cells have all the common organelles that animal cells have (nucleus, mitochondria, etc.). For example, a plant cell uses chloroplasts to make food (sugar), and then uses its mitochondria to convert that food into energy.

How to identify a plant cell: If you see a cell with a clear cell wall and green chloroplasts, you're likely looking at a plant cell. In contrast, an animal cell will have no cell wall (just a flexible membrane) and no chloroplasts.

Prokaryotes vs. Eukaryotes: Key Differences

Eukaryotic cells are often compared to prokaryotic cells to highlight their complexity. Understanding how they differ helps clarify what makes eukaryotic cells unique in structure and function.

Take This Quiz:

Prokaryotic and Eukaryotic Cells