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

Subcellular components & organelles

• Eukaryotic and prokaryotic cells contain many subcellular structures and organelles

• Eukaryotic cells have a more complex ultrastructure than prokaryotic cells

• The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles, which provide compartmentalization and organize cellular products

Ribosomes

• Ribosomes are found in all cells, prokaryotic and eukaryotic

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

  • Either freely in the cytoplasm (of all cells)

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

• Ribosomes are the site of protein synthesis; they bind to mRNA

• They consist of a large and a small subunit composed of protein and ribosomal RNA (rRNA)

  • Protein provides structure to the ribosome

  • rRNA facilitates the binding of mRNA and tRNA and catalyzes the formation of peptide bonds between amino acids

Endoplasmic reticulum

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

• There are two types of ER:

  • Rough Endoplasmic Reticulum (RER)

    • The surface is studded with ribosomes

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

    • Helps to compartmentalize the cell

    • Its role is to process proteins made by the ribosomes

  • Smooth Endoplasmic Reticulum (ER)

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

    • ER is involved in the production, processing and storage of lipids, carbohydrates and steroids as well as detoxification

The Golgi complex

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

• Flattened sacs of the membrane are similar to the smooth endoplasmic reticulum

  • The interior part of each cisterna, the lumen, holds the necessary enzymes for the Golgi to function

  • The Golgi modifies proteins and lipids before packaging them into Golgi vesicles

  • Vesicles then transport the proteins and lipids to their required destinations

• Proteins that go through the Golgi apparatus go through a number of processes referred to as protein trafficking:

  • Exported (e.g., hormones such as insulin)

  • Put into lysosomes (such as hydrolytic enzymes)

  • Delivered to membrane-bound organelles

• The Golgi complex is responsible for glycosylation and other chemical modifications of proteins

  • During the process of glycosylation, sugar molecules are attached to proteins and lipids which determines protein function or targeting

Mitochondria

• Mitochondria are the site of chemical respiration within eukaryotic cells

• A double membrane surrounds each mitochondrion

  • The inner membrane is highly folded to form cristae

  • The outer membrane is smooth

• The double membrane allows proton gradients to form across the membranes for the production of ATP

• The internal part of the mitochondrion (the matrix) contains enzymes needed for chemical respiration

• Small circular pieces of DNA, mitochondrial DNA (mtDNA) and ribosomes are also found in the matrix (needed for replication of mitochondria)

Chloroplasts

• Chloroplasts are found in green plants and photosynthetic algae

• Chloroplasts are larger than mitochondria and are also surrounded by a double membrane

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

  • They resemble a stack of pancakes

• Grana are joined together by lamellae (thin and flat thylakoid membranes)

• Chloroplasts are the site of photosynthesis:

  • The light-dependent stage takes place in the thylakoids

  • The light-independent stage (Calvin cycle) takes place in the stroma

  • Chloroplasts also contain small circular pieces of DNA and ribosomes used to synthesize the proteins needed in chloroplast replication and photosynthesis

Lysosomes

• Lysosomes are membrane-enclosed sacs which contain hydrolytic enzymes

• They have three main roles:

  • To break down waste materials such as worn-out organelles

  • Roles within cells of the immune system to destroy pathogens

  • In apoptosis

Vacuoles

• A vacuole is a membrane-bound sac, most often found in plant cells

• The vacuole contents are chemically different to that of the cytoplasm

• Vacuoles have several distinct roles:

  • As a cellular storage of water, helping to maintain water balance in the plant

  • To keep the cell turgid, filling with water and exerting pressure on the cell wall, giving the plant's tissues structural strength

  • Some vacuoles contain colored pigments that give the plant its characteristic color

  • Other plant species store bitter/astringent compounds in the vacuole that deter insects/other animals

  • They can often be a place where waste products are sequestered (temporarily stored before excretion from the cell)

• A vacuole can sometimes take up most of the inner volume (around 80%) of a plant cell

• Vacuoles in animal cells are not permanent and are small

  • Their role in animals is as a temporary store of metabolites or for the transport of substances