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2.1 Cell Structure and Function

Keywords

English Term 中文翻译 Definition & Explanation
Ribosome 核糖体 A non-membrane-bound structure made of rRNA and protein; the site of protein synthesis.
Endomembrane System 内膜系统 A network of membrane-bound organelles that work together to synthesize, modify, package, and transport cellular products.
Rough ER 粗面内质网 A network of interconnected membranous sacs studded with ribosomes; plays a key role in protein synthesis and compartmentalization.
Smooth ER 滑面内质网 A network of interconnected membranous tubules lacking ribosomes; functions in lipid synthesis and detoxification.
Golgi Complex 高尔基体 A series of flattened membrane sacs that correctly fold, chemically modify, and package newly synthesized cellular products.
Lysosome 溶酶体 A membrane-enclosed sac containing hydrolytic enzymes used to digest materials and initiate apoptosis.
Vacuole 液泡 A membrane-bound sac used for storage; large central vacuoles in plants maintain turgor pressure.
Mitochondrion 线粒体 The organelle where aerobic cellular respiration occurs; has a highly folded inner membrane to maximize ATP production.
Chloroplast 叶绿体 A double-membrane organelle found in plants and algae; the site of photosynthesis.

1. The Universal Organelle: Ribosomes

Ribosomes are unique subcellular structures comprised of ribosomal RNA (rRNA) and proteins. Their primary function is to synthesize proteins according to messenger RNA (mRNA) sequences.

Core Concept: Evidence of Common Ancestry

Unlike most organelles we will discuss, ribosomes are non-membrane-bound. Because they do not have a membrane, they are found in cells across all forms of life (both prokaryotes and eukaryotes). The universal presence of ribosomes is a key piece of evidence reflecting the common ancestry of all known life.

2. The Endomembrane System: The Cellular Factory

The endomembrane system consists of a group of membrane-bound organelles that work seamlessly together to synthesize, modify, package, and transport large molecules (polysaccharides, lipids, and proteins) intercellularly. It includes the nuclear envelope, endoplasmic reticulum (ER), Golgi complex, lysosomes, vacuoles, transport vesicles, and the plasma membrane.

A. The Endoplasmic Reticulum (ER)

The ER provides mechanical support by helping cells maintain their shape and plays a massive role in intracellular transport.

  • Rough ER: It is associated with membrane-bound ribosomes, giving it a "rough" appearance. It allows for the compartmentalization of the cell and directly helps carry out protein synthesis (specifically for proteins destined to be secreted or embedded in membranes).
  • Smooth ER: It lacks ribosomes. Its primary functions include the detoxification of cells (e.g., breaking down drugs or poisons) and lipid synthesis.

AP Exam Exclusion Statement: Smooth ER

Knowledge of the specific functions of the smooth ER in specialized cells (e.g., sarcoplasmic reticulum in muscle cells) is beyond the scope of the AP Exam.

B. The Golgi Complex

The Golgi is the "shipping and receiving center" of the cell. It consists of a series of flattened membrane sacs. Proteins arriving from the Rough ER in transport vesicles enter the Golgi to be:

  1. Correctly folded and chemically modified (e.g., adding sugar chains to create glycoproteins).
  2. Packaged for trafficking (sending products to their final destinations, either inside or outside the cell).

AP Exam Exclusion Statement: Golgi Details

Knowledge of the role of the Golgi in the synthesis of specific phospholipids and the packaging of specific enzymes for lysosomes, peroxisomes, and secretory vesicles is beyond the scope of the AP Exam.

C. Lysosomes and Vacuoles

  • Lysosomes: Membrane-enclosed sacs containing hydrolytic enzymes (which use hydrolysis to break bonds). They act as the cell's "stomach" and recycling center, digesting unneeded materials. They also play a crucial role in programmed cell death (apoptosis).
  • Vacuoles: Membrane-bound sacs serving various roles:
    • In Plant Cells: A specialized large central vacuole stores water and nutrients, taking up most of the cell's volume to maintain turgor pressure (keeping the plant upright).
    • In Animal Cells: Vacuoles are much smaller in size, more plentiful, and simply store cellular materials.
(Placeholder: A diagram tracing a secreted protein: manufactured in the Rough ER -> transported via vesicle -> modified in the Golgi -> transported via vesicle -> secreted out of the plasma membrane.)

3. The Energy Organelles: Mitochondria and Chloroplasts

Both mitochondria and chloroplasts specialize in energy transformation and notably contain a double membrane (an outer membrane and an inner membrane), which aligns with the Endosymbiotic Theory.

A. Mitochondria

Mitochondria are the sites of aerobic cellular respiration. The double membrane provides separate compartments for different metabolic reactions.

  • Outer Membrane: Smooth and highly permeable.
  • Inner Membrane: Highly convoluted (forming folds called cristae).
    • Biological Significance: The folding drastically increases the surface area of the inner membrane, allowing more space for electron transport chains and ATP synthase, which enables ATP to be synthesized much more efficiently.

B. Chloroplasts

Chloroplasts are specialized organelles found only in plants and photosynthetic algae. Like mitochondria, they contain a double membrane. They capture light energy and serve as the specific location for photosynthesis.

(Placeholder: Side-by-side diagrams of a mitochondrion (highlighting the highly folded inner cristae) and a chloroplast (highlighting the stroma and thylakoids).)

Quiz

Campbell Biology Chapter 6 Practice Test: A Tour of the Cell

Click the link above to practice related multiple-choice questions (opens in a new tab).