Lecture 4 Cell Structure and Function Part 1

Cell Study

• Cells are usually too small to be seen by the naked eye, making microscopes and biochemistry essential tools for studying them.

Microscopy

• Microscopes are used to visualize cells.
• In a light microscope (LM), visible light is passed through a specimen and then through glass lenses.
• Lenses refract (bend) the light to magnify the image.

Prokaryotic and Eukaryotic Cells

• There are two types of cells: prokaryotic and eukaryotic.
• Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells.
• Protists, fungi, animals, and plants all consist of eukaryotic cells.
• Basic features of all cells:
  • Plasma membrane
  • Semifluid substance called cytosol
  • Chromosomes (carry genes)
  • Ribosomes (make proteins)
• Prokaryotic cells:
  • No nucleus
  • DNA in an unbound region called the nucleoid
  • No membrane-bound organelles
  • Cytoplasm bound by the plasma membrane
• Eukaryotic cells:
  • DNA in a nucleus that is bounded by a double membrane
  • Membrane-bound organelles
  • Cytoplasm in the region between the plasma membrane and nucleus
  • Generally larger than prokaryotic cells

Cell Size and Metabolic Requirements

• Metabolic requirements set upper limits on the size of cells.
• The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell.
• The surface area to volume ratio of a cell is critical.
• As a cell increases in size, its volume grows proportionately more than its surface area.

Eukaryotic Cell Structure

• A eukaryotic cell has internal membranes that divide the cell into compartments (organelles).
• The cell's compartments provide different local environments so that incompatible processes can occur in a single cell.
• The basic fabric of biological membranes is a double layer of phospholipids and other lipids.
• Plant and animal cells have most of the same organelles.

Nucleus

• The nucleus contains most of the cell's genes and is usually the most conspicuous organelle.
• The nuclear envelope encloses the nucleus, separating it from the cytoplasm.
• The nuclear envelope is a double membrane; each membrane consists of a lipid bilayer.
• Pores, lined with a structure called a pore complex, regulate the entry and exit of molecules from the nucleus.
• The nuclear side of the envelope is lined by the nuclear lamina, which is composed of proteins and maintains the shape of the nucleus.
• There is evidence for a nuclear matrix, a framework of protein fibers throughout the interior of the nucleus.

Chromosomes and Ribosomes

• In the nucleus, DNA is organized into discrete units called chromosomes.
• Each chromosome contains one DNA molecule associated with proteins, called chromatin.
• Chromatin condenses to form discrete chromosomes as a cell prepares to divide.
• The nucleolus, located within the nucleus, is the site of ribosomal RNA (rRNA) synthesis.
• Ribosomes are complexes made of ribosomal RNA and protein.
• Ribosomes build proteins in two locations:
  • In the cytosol (free ribosomes)
  • On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)

Endomembrane System

• The endomembrane system consists of:
  • Nuclear envelope
  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Vacuoles
  • Plasma membrane
• These components are either continuous or connected via transfer by vesicles.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells.
• The ER membrane is continuous with the nuclear envelope.
• There are two distinct regions of ER:
  • Smooth ER, which lacks ribosomes
  • Rough ER, whose surface is studded with ribosomes
• Functions of Smooth ER:
  • Synthesizes lipids
  • Detoxifies drugs and poisons
  • Stores calcium ions
• Functions of Rough ER:
  • Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates)
  • Distributes transport vesicles, secretory proteins surrounded by membranes
  • Is a membrane factory for the cell

Golgi Apparatus

• The Golgi apparatus consists of flattened membranous sacs called cisternae.
• The Golgi apparatus:
  • Modifies products of the ER
  • Manufactures certain macromolecules
  • Sorts and packages materials into transport vesicles

Lysosomes

• A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules.
• Lysosomal enzymes work best in the acidic environment inside the lysosome.
• Hydrolytic enzymes and lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing.
• Some lysosomes probably arise by budding from the trans face of the Golgi apparatus.

Vacuoles

• Vacuoles are large vesicles derived from the ER and Golgi apparatus.

• Vacuoles perform a variety of functions in different kinds of cells.

• Examples of vacuole functions:

  • Food vacuoles are formed by phagocytosis
  • Contractile vacuoles, found in many freshwater protists, pump excess water out of cells
  • Central vacuoles, found in many mature plant cells, contain a solution called sap
  • The central vacuole plays a major role in the growth of plant cells### The Endomembrane System

• The endomembrane system consists of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• These components are either continuous or connected via transfer by vesicles.

The Endoplasmic Reticulum (ER)

• The ER accounts for more than half of the total membrane in many eukaryotic cells.
• The ER membrane is continuous with the nuclear envelope.
• There are two distinct regions of ER:
  • Smooth ER, which lacks ribosomes.
  • Rough ER, whose surface is studded with ribosomes.

Functions of Smooth ER

• Synthesizes lipids.
• Detoxifies drugs and poisons.
• Stores calcium ions.

Functions of Rough ER

• Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates).
• Distributes transport vesicles, secretory proteins surrounded by membranes.
• Is a membrane factory for the cell.

The Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.
• Modifies products of the ER.
• Manufactures certain macromolecules.
• Sorts and packages materials into transport vesicles.

Lysosomes

• A membranous sac of hydrolytic enzymes that can digest macromolecules.
• Lysosomal enzymes work best in the acidic environment inside the lysosome.
• Hydrolytic enzymes and lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing.
• Some lysosomes probably arise by budding from the trans face of the Golgi apparatus.

Functions of Lysosomes

• Digest macromolecules.
• Engulf and digest another cell by phagocytosis, forming a food vacuole.
• Use enzymes to recycle the cell's own organelles and macromolecules, a process called autophagy.

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.
• Perform a variety of functions in different kinds of cells.
• Types of vacuoles include:
  • Food vacuoles, formed by phagocytosis.
  • Contractile vacuoles, found in many freshwater protists, pump excess water out of cells.
  • Central vacuoles, found in many mature plant cells, contain a solution called sap, and play a major role in the growth of plant cells.

Cellular Structure and Function

• The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell.
• The surface area to volume ratio of a cell is critical.
• As a cell increases in size, its volume grows proportionately more than its surface area.

The Nucleus

• Contains most of the cell's genes and is usually the most conspicuous organelle.
• The nuclear envelope encloses the nucleus, separating it from the cytoplasm.
• The nuclear envelope is a double membrane, each consisting of a lipid bilayer.
• Pores, lined with a structure called a pore complex, regulate the entry and exit of molecules from the nucleus.
• The nuclear side of the envelope is lined by the nuclear lamina, which is composed of proteins and maintains the shape of the nucleus.

Ribosomes

• Complexes made of ribosomal RNA and protein.
• Build proteins in two locations:
  • In the cytosol (free ribosomes).
  • On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes).