The Cell: Structure, Function, and Types (Unit 2, Chapter 4)

The Cell

Introduction

Cells are the fundamental units of life, forming the basis of all living organisms. This section explores the structure and function of cells, distinguishing between prokaryotic and eukaryotic types, and describes the components and organelles found within eukaryotic cells.

Objectives

• Distinguish between prokaryotic and eukaryotic cells by visual comparison.

• List the structures of every eukaryotic cell.

• Provide reasons why cells are so small.

• Identify for each organelle: Function, Location, Structure.

• Describe the structural differences between plant cells and animal cells.

• Compare the structural differences between prokaryotic and eukaryotic cells.

Components of Every Eukaryotic Cell

Plasma Membrane

The plasma membrane is a selective barrier that separates the contents of the cell from its external environment. It regulates the movement of substances in and out of the cell.

• Structure: Phospholipid bilayer with embedded proteins.

• Function: Selective permeability, protection, communication.

• Location: Surrounds the entire cell.

Nucleus

The nucleus is the control center of eukaryotic cells, containing most of the cell's genetic material (DNA).

• Structure: Enclosed by a double membrane called the nuclear envelope, which contains pores.

• Nucleolus: Region within the nucleus that synthesizes ribosomes.

• Function: Storage and protection of DNA, regulation of gene expression.

• Location: Central region of the cell.

Cytoplasm

The cytoplasm includes all the material between the plasma membrane and the nucleus. It is a semifluid substance where organelles are suspended and move around.

• Function: Site of many metabolic reactions, supports organelles.

• Location: Fills the cell interior.

Why Are Cells So Small?

Cell Size and Efficiency

Cells are small because smaller cells are more efficient at exchanging materials with their environment. This efficiency is due to the surface area-to-volume ratio.

• Key Point: Cells have only one nucleus, which limits their size.

• Key Point: Smaller objects have a greater ratio of surface area relative to their volume, facilitating faster exchange of materials.

Formula:

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

As cell size increases, the ratio decreases, making transport less efficient.

Cytoplasmic Organelles

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis. They can be found in two locations:

• Free ribosomes: Located in the cytoplasm, unattached to other structures.

• Bound ribosomes: Attached to the walls of the endoplasmic reticulum (ER).

• Function: Protein synthesis (translation of mRNA into polypeptides).

Endomembrane Network

Overview

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins. These organelles are either physically connected or communicate via vesicles.

• Nuclear Envelope: Surrounds the nucleus, regulates entry and exit of materials.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids.

• Lysosomes: Digestive organelles containing enzymes.

• Plasma Membrane: Interface for cell communication and transport.

Additional info: The endomembrane system is essential for compartmentalization and efficient cellular function in eukaryotes.