Subcellular Components (College Board AP® Biology): Study Guide

Cell structure & function

• The structure and function of subcellular components and organelles contribute to the function of cells

Ribosomes

• Ribosomes are found in all cells, both prokaryotic and eukaryotic

  • This supports the view that all life forms share common ancestry

• Ribosomes are the site of protein synthesis; they bind to messenger RNA (mRNA) and synthesize proteins according to mRNA sequences

• Within cells ribosomes are either:

  • free in the cytoplasm (all cells)

  • bound to the endoplasmic reticulum (ER) to form rough ER (only eukaryotic cells)

• They are non-membrane structures, composed of protein and ribosomal RNA (rRNA)

Endomembrane system

• The endomembrane system consists of a group of membrane-bound organelles and subcellular components

• The components of the endomembrane system work together to modify, package, and transport polysaccharides, lipids, and proteins within the cell

• The endomembrane system includes:

  • endoplasmic reticulum (ER)

  • Golgi complex

  • lysosomes

  • vacuoles

  • transport vesicles

Endoplasmic reticulum

• The endoplasmic reticulum (ER) is a series of interconnected, membrane-bound sacs in the cytoplasm of eukaryotic cells

• The ER provides mechanical support by helping cells maintain shape, as well as playing a role in intracellular transport

• There are two types of ER:

  • Rough endoplasmic reticulum (RER)

    • The surface is studded with ribosomes

    • Formed from folds of membrane that are continuous with the nuclear envelope

    • Helps to compartmentalize the cell and is involved with protein synthesis

  • Smooth endoplasmic reticulum (SER)

    • Does not have ribosomes on the surface; its function is distinct from the RER

    • SER is involved in the detoxification of cells and lipid synthesis

The Golgi complex

• The Golgi complex is a membrane-bound structure that consists of a series of flattened membrane sacs called cisternae

• Functions of the Golgi include:

  • correctly folding and chemically modifying newly synthesized cellular products transferred from the ER

  • packaging proteins for trafficking into vesicles which transport them around the cell

Lysosomes

• Lysosomes are membrane-enclosed sacs which contain hydrolytic enzymes

• They have three main roles:

  • breaking down cellular waste materials, e.g. worn-out organelles

  • destruction of pathogens

  • programmed cell death, known as apoptosis

Vacuoles

• A vacuole is a membrane-bound sac with contents that are chemically different to those of the cytoplasm

• Vacuoles play different roles in different cell types

  • In plants cells permanent vacuoles store water, helping to maintain water balance and keeping the cell turgid by exerting pressure on the cell wall

  • In animal cells smaller, temporary vacuoles may store metabolites or transport substances

Mitochondria

• Mitochondria (singular mitochondrion) are membrane-bound organelles that provide compartments for the metabolic reactions of aerobic respiration within eukaryotic cells

• A double membrane surrounds each mitochondrion

  • The outer membrane is smooth

  • The inner membrane is highly folded to form cristae

• The folds of the inner mitochondrial membrane provide a large surface area that enables ATP to be synthesized efficiently during respiration

Chloroplasts

• Chloroplasts are found in green plants and photosynthetic algae

• They are also surrounded by a double membrane and are the site of photosynthesis

  • Membrane-bound compartments called thylakoids stack to form structures called grana

  • Thylakoid membranes contain chlorophyll, which absorbs light energy for the reactions of photosynthesis