Cell Structure Notes

Cell Structure

Learning Goals

• Explain the structure and importance of the plasma membrane.
• Identify the parts of a cell and describe their functions.
• Explain how different parts of the cell work together.
• Describe DNA and the nucleus.
• Define transcription and translation.

Parts of a Cell

The cell can be subdivided into 3 parts:

1. Plasma (cell) membrane
2. Cytoplasm
   • Cytosol
   • Organelles
3. Nucleus
   • Chromosomes
   • Genes

Prototypical Human Cell

• The image shown is a prototypical example of a cell containing the primary organelles and internal structures.

The Plasma Membrane

• The plasma membrane is a flexible yet sturdy barrier that surrounds and contains the cytoplasm of the cell.
• The cell membrane is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.

Lipid Bilayer

• The main component of the plasma membrane is a phospholipid bilayer.
• Phospholipids have a hydrophilic head and two hydrophobic tails.
• In water, they spontaneously arrange into a bilayer with the hydrophobic regions in the middle and the hydrophilic heads facing the water inside and outside the cell.

Membrane Proteins

Two main types of membrane proteins are:

• Integral (also called transmembrane) proteins
• Peripheral proteins

Functions of Membrane Proteins

• Membrane proteins can serve a variety of functions.
• The different proteins help determine many of the functions of the cell membrane.
• Different cell types have different membrane proteins to fit their function.

Membrane Fluidity

• Membranes are fluid structures because most of the membrane lipids and many of the membrane proteins move easily in the bilayer.
• Membrane lipids and proteins are mobile in their own half of the bilayer.
• Cholesterol serves to stabilize the membrane and reduce membrane fluidity.

Membrane Permeability

• Plasma membranes are selectively permeable.
• The lipid bilayer is always permeable to small, nonpolar, uncharged molecules.
• Transmembrane proteins that act as channels or transporters increase the permeability of the membrane.
• Macromolecules are only able to pass through the plasma membrane by vesicular transport.

The Cytoplasm

• The cytoplasm has 2 components:
  • Cytosol - also known as the intracellular fluid portion of the cytoplasm
  • Organelles - the specialized structures that have specific shapes and perform specific functions

Cytoskeleton

• The cytoskeleton of the cell provides structure and support for the cell but is also dynamic and can break down and reassemble.
• The cytoskeleton are made of three types of protein fibers:
  1. Microfilaments - thin
  2. Intermediate filaments - medium
  3. Microtubules - thick
• The cytoskeleton plays an important role in maintaining cell shape and structure, promoting cellular movement, and aiding cell division.

Centrosome/Centrioles

• Each cell has a centrosome made of two centrioles
• Functions in organizing the cytoskeleton

Cilia and Flagella

• Extensions of the plasma membrane that are organized by microtubules.
  • Cilia: move fluid over the cell's surface. They act like “little brushes”.
  • Flagella: move the cell through a fluid. In humans, they are only found in sperm cells.

Ribosomes

• Ribosomes are found in every type of cell on the planet.
• They function in protein synthesis
• Information in our DNA is transcribed to RNA, which is used by the ribosome to build a protein.
• A large subunit and a small subunit comprise ribosomes.
• During protein synthesis, ribosomes assemble amino acids into proteins.

Endoplasmic Reticulum (ER)

• The ER is continuous with the nuclear membrane of the nucleus
• Two distinct sections:
  • Rough ER
  • Smooth ER

Smooth Endoplasmic Reticulum

• The Smooth ER has many functions:
  • Synthesizes fatty acids and steroids
  • Inactivates or detoxes drugs
  • Carbohydrate metabolism
  • Calcium ion storage (important for cell communication and muscle contraction)

Rough Endoplasmic Reticulum

• The Rough ER synthesizes proteins, glycoproteins, and phospholipids
• The Ribosomes on the Rough ER synthesize proteins that will be exported to the Golgi and then exported to the plasma membrane.
• Secretory proteins (e.g., hormones) and membrane proteins (e.g., receptors) are made here.

Golgi Complex

• The cis face receives proteins from the Rough ER.
• Contents are modified and packaged as they move through the Golgi complex.
• Once they reach the trans face, they are packaged into vesicles and transported to their destination.

Lysosomes

• Contain digestive enzymes to recycle cell components.
• Can fuse with other vesicles to release the enzymes and digest the components of the vesicle.

Peroxisomes

• Peroxisomes are structures that are similar in shape to lysosomes but are smaller and contain enzymes that use oxygen to oxidize (break down) organic substances
• Related to metabolism and often physically associated with a mitochondrion.
• They are membrane-bound organelles that contain an abundance of enzymes for detoxifying harmful substances and lipid metabolism.

Mitochondria

• Site of cellular respiration
• Produces ATP to fuel cellular activities.
• The mitochondria are the energy-conversion factories of the cell.
• A mitochondrion is composed of two separate lipid bilayer membranes.
• Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency.

The Nucleus

• The nucleus contains the hereditary units of the cell, called genes
• Genes are arranged along chromosomes
• Chromosomes are long strands of DNA

Transcription and Translation

• Transcription: information in DNA is copied into an mRNA molecule, which can leave the nucleus
• Translation: The mRNA is read by the ribosome to build a protein
• What makes cell types different from one another is which genes in the DNA are transcribed and translated in a given cell. A skin cell produces a different collection of proteins than a muscle cell.