Eukaryotic Cell Structures and Functions

The cell/cytoplasmic membrane is a typical bilayer of phospholipids and proteins. It acts as a selectively permeable barrier, regulating transport and facilitating signal transduction.

Exam Tip: If a question asks which part of the eukaryotic cell is in direct contact with the outside environment, the answer is the glycocalyx.

The Genetic Center: Nucleus and Nucleolus

The nucleus is the most prominent organelle and serves as the genetic center of the cell, acting as a repository for DNA and the site of RNA synthesis. It is enclosed by a nuclear envelope, which consists of two parallel membranes perforated with pores.

Within the nucleus lies the nucleolus, a dark, dense area responsible for ribosomal RNA (rRNA) synthesis and the assembly of ribosomes. During mitosis, the nucleus undergoes significant changes, including the pairing of chromosomes and the formation of spindle fibers to ensure proper DNA distribution to daughter cells.

The Synthesis and Transport System

Eukaryotic cells utilize a sophisticated "assembly line" to produce and move proteins and lipids.

Endoplasmic Reticulum (ER) and Ribosomes

The endoplasmic reticulum is a continuous network of membranes that facilitates the transport of materials. It exists in two forms:

1. Rough Endoplasmic Reticulum (RER): Studded with ribosomes, giving it a "rough" appearance. The RER is the site of protein synthesis; proteins are synthesized by ribosomes and then shunted into the ER for packaging and transport.
2. Smooth Endoplasmic Reticulum (SER): A tubular network lacking ribosomes. It functions in nutrient processing and the synthesis and storage of lipids.

Ribosomes themselves are composed of rRNA and proteins and are the actual sites of protein synthesis.

Golgi Apparatus and Vesicles

Once proteins are processed in the ER, they are sent via transport vesicles to the Golgi apparatus. The Golgi consists of stacks of flattened sacs called cisternae. Its primary role is to modify, store, and package proteins into condensing vesicles for secretion or delivery to other organelles.

Lysosomes and Vacuoles

For digestion and storage, the cell uses specialized sacs:

• Lysosomes: Vesicles originating from the Golgi that contain enzymes for intracellular digestion of food particles and protection against microbes.
• Vacuoles: Membrane-bound sacs used for temporary storage, transport, and regulation of osmotic pressure. For instance, "food vacuoles" (phagosomes) result from the merging of a lysosome with a vacuole containing ingested material.

Energy Production: Mitochondria and Chloroplasts

Energy is the lifeblood of the cell, and eukaryotes use specialized organelles to convert nutrients and light into usable forms.

• Mitochondria: Known as the powerhouses of the cell, they produce ATP through the Krebs cycle and oxidative phosphorylation. They possess an outer membrane and an inner membrane folded into structures called cristae, which increase surface area for aerobic respiration. Interestingly, mitochondria contain their own circular DNA and prokaryotic-like ribosomes.
• Chloroplasts: Found in algae and plants, these organelles convert sunlight into chemical energy via photosynthesis. They contain an outer membrane and an inner membrane folded into sacs called thylakoids, which are stacked into grana.

Structural Support: The Cytoskeleton

The cell is not a shapeless bag of liquid; it is supported by the cytoskeleton, a flexible framework of proteins that provides structural support, anchors organelles, and enables movement. The cytoskeleton is composed of two primary types of filaments:

1. Microtubules: Larger, hollow tubes (approx. 25 nm in diameter) that are essential for movement and structural integrity.
2. Microfilaments: Smaller, thinner filaments (approx. 8 nm) composed of actin that assist in cell shape and movement.

Summary Table of Organelle Functions

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