Cells: Organization and Communication (February 11, 2025)

Cell are the Building Blocks of Life

• Cells are highly organized and dynamic

  • The human body contains trillions of cells.

  • Cells differ in both shape and size.

• The study of cells is called cytology.

  

All Cells Have Similar Characteristics

• Current cell theory holds:

  • All living things are composed of cells.

  • All cells arise from preexisting cells through cell division.

  • Cells contain hereditary material, which they pass to daughter cells during cell division.

  • The chemical composition of all cells is quite similar.

  • The metabolic processes associated with life occur within cells.

The Cell is a Highly Organized Structure That Has Three Basic Parts

• A barrier called the plasma membrane (or cell membrane)

  • Plant cells and bacteria have a cell wall next to their plasma membrane.

• An area where the cell’s genetic material is stored.

  • A nucleus is animal and plant cells.

  • A nucleoid in bacterial cells.

• A fluid called cytosol.

  • Found between plasma membrane and nucleus.

  • Filled with organelles, each with a function vital to the life of the cell.

    

Early Life Forms were Prokaryotic

• Millions of years ago, prokaryotic cells adapted to the extreme environments of early Earth.

• Today, they survive as bacteria and Archaebacteria

• Prokaryotic cells have no internal membrane-bound organelles.

  • Although they have genetic material, they have no nucleus

    • Their genetic material is confined to the nucleoid region in their cytoplasm

  • They contain ribosomes

    • These permit prokaryotic cells to produce the proteins they require for life.

Eukaryotic Cells Have a Nucleus and Membrane-Bound Organelles

• Eukaryotic cells are believed to have evolved from prokaryotic ancestors.

  • Endosymbiosis theory proposes that some early prokaryotic cells engulfed (absorbed) other smaller, energy-producing prokaryotic cells.

  • The engulfed prokaryotic cells survived inside their “host cells.”

  • Both co-evolved symbiotically.

• Plant, animal, and fungal cells are eukaryotic.

  

Plant Cells

• Plant cells have most of the same organelles as animal cells.

• Plant cells also have additional organelles that are not found in animal cells.

  • The cell wall lies next to the plasma membrane to provide structural stability to the cell.

  • The central vacuole maintains cell pressure (turgor)

    • The central vacuole consists of water and nutrients.

  • Chloroplasts produce sugars through the process of photosynthesis.

    • The sugars contain energy which is used by the plants as nutrients.

    • The sugars (with their energy) also are passed on to the organisms that eat the plants.

    • Many believe that chloroplasts originated as bacteria that were “adopted” (engulfed) by the primitive ancestors of plant cells through endosymbiosis.

The Cell Membrane Separates the Cell From the Extracellular Fluid

• The cell membrane is composed of two layers of phospholipids, which are interspersed with proteins, lipids, cholesterol, and sugars.

• Phospholipids are arranged in a double layer, or bilayer.

  • The hydrophilic, water-loving, polar heads of the phospholipid molecules are oriented toward the aqueous environment both inside and outside the cell.

  • The hydrophobic, water-fearing, non-polar lipid portions of the phospholipid molecules are sandwiches in the center of the bilayer.

• Proteins and lipids associated with the cell membrane have sugars attached to their external surface.

  • They are called glycoproteins and glycolipids.

Cell Membrane Structure

The Glycocalyx

• The glycoproteins and glycolipids form a layer on the cell membrane called the glycocalyx.

• The glycocalyx is unique and defines cells as belonging to a specific organism.

  • Both blood type and tissue type are defined by the specific structures on the glycocalyx.

• Each person’s white blood cells carry a group of identifying proteins called the human leukocyte antigens (HLAs).

  • These serve as markers indicating that our cells belong to us.

  • HLA is used to match tissues before organ transplants.

  • HLA is inherited — close tissue matches often occur within immediate family.

The Cell Membrane Has a Fluid Mosaic Structure

• The basic structure of the plasma membrane is a continuously swirling fluid with consistency similar to olive oil.

• Cholesterol molecules separate phospholipid fatty avid tails, which contributes to the fluid nature of the plasma membrane.

• The membrane proteins also are in constant motion, floating within the phospholipid bilayer.

• Membrane fluidity enables membranes to de-form and change their overall shape during cell locomotion.

Movement Across the Membrane

• The membrane is a semipermeable barrier

• It provides a means for

  • Nutrients to enter the cell

  • Waste products to exist the cell.

• Movement across the membrane occurs in two ways

  • Passive transport

  • Active transport

Passive Transport

• Passive transport does not require the cell to expend energy to move molecules across membranes down their concentration gradients.

  • Includes diffusion, facilitated diffusion, and filtration.

• Diffusion is the movement of a substance towards the area of its lower concentration,

• Diffusion of water across a semipermeable membrane is termed osmosis.

  • Water moves in a direction across the cell membrane that equalizes solute concentrations on each side of the membrane.

  • Locations with higher solute concentrations have lower water concentrations.

    • “Pull” water towards them.

  • Water moves down its concentration gradient.

    

Osmosis

• Usually, the extracellular fluid is isotonic to the cells.

  • Water flows equally into and out of the cell.

• A hypotonic solution may cause a cell to burst.

  • Has water with a lower concentration of solutes than the cytosol.

• A hypertonic solution may cause a cell to shrivel up.

  • Has water with a higher concentration of solutes than the cytosol.

Facilitated Diffusion

• The cell does not require to expend energy to transport molecules.

• Uses integral membrane proteins (transporter proteins) to move molecules across the cell membrane.

  • Molecule to be transported binds to transporter protein on one side of bilayer.

  • Protein “passes” (transports) molecule to other side of membrane.

• Solutes are transported across the membrane down their concentration gradients.

  • From area of high solute concentration to area of low solute concentration.

  • This is the main method by which glucose is moved into cells.

  

Active Transport

• Cell must expend energy to transport molecules or ions across cell membrane.

  • Using energy derived from the hydrolysis (breakdown) of ATP into ADP

• Molecules or ions are transported across the membrane against their concentration gradients.

  • From area of low solute concentration to area of high solute concentration.

• Integral membrane proteins (transporter proteins, or “pumps”) move molecules or ions across the cell membrane.

  • Molecule or ion to be transported binds to transporter protein on one side of bilayer (on the side where there is a lower concentration of the material).

  • Using energy transporter protein “pumps” molecule or ion to other side of membrane (to side where there is a higher concentration of the material)

Active Transporter Pumps Often Have Reciprocal Functions

• Pumping one molecule or ion into the cell while simultaneously removing a second from the cell.

  • The sodium/potassium ATPase pumps two K⁺ into the cell while pumping three Na⁺ out of the cell.

Active Transport Concentrates Molecules an Ions Inside and Outside of Cells

• Active transport accounts for

  • Establishing potassium and sodium ion concentrations inside and outside of the cell membranes of nerve cells in preparation for nerve impulse transmission.

  • The almost complete uptake of digested nutrients from the small intestine blood by the kidneys.

  • The collecting of iodine in thyroid gland cells.

Endocytosis and Exocytosis

• Active transport also can move molecules into or out of the cell, in bulk.

• In endocytosis

  • Extracellular molecules and particles are taken into the cell via vesicle formation.

• In exocytosis

  • Secretory products or waste products are removed form the cell.

The Components of a Cell are Called Organelles

• Each organelle plays a role in regulating the life and death of cells.

Cytoskeleton

• The cytosol is a highly organized “chemical soup” complete with a support structure called the cytoskeleton.

• The cytoskeleton provides

  • Shape and structural support for the cell.

  • A scaffold for suspending and moving organelles within the cell.

• The cytoskeleton continuously changes shape.

  • Forming and breaking down and reforming.

  • Giving cells a plasticity, or fluidity, that allows them to change shape or move organelles quickly.

The Cytoskeleton is Composed of Three Types of Filamentous Proteins

• All three types of cytoskeleton contribute to cell shape — but each also has its own specific function.

  • Microfilaments

    • Long filaments constructed of actin protein subunits.

    • Responsible for cellular locomotion and muscle contractions.

    • Establish the basic shape and strength of the cell.

  • Intermediate filaments

    • Strong cables of protein subunits.

      • Protein type depends on type of intermediate filament.

    • Stronger than microfilaments - protect cell from mechanical stresses

  • Microtubules

    • Long tiny tubules made of tubulin protein subunits

    • Are instrumental in chromosome movement during cell division.

    • Also used as tracks for organelle movement.

Flagella and Cilia

• Flagella are single, long, whip-like structures that propel the cell forward

  • The only human cell that moves by flagellum is the sperm.

• Cilia are shorter extensions that look like hairs or eyelashes.

  • Cilia line the upper respiratory tract, moving mucus upward and sweeping out debris and pathogens.

  • Cilia also line the fallopian tubes, moving the egg from the ovary to the uterus.

The Endoplasmic Reticulum

• Endoplasmic reticulum or ER (literally “within fluid network”)

• The membranes of the ER are directly connected to the double membrane surrounding the cell nucleus.

  

Humans Have Two Types of Endoplasmic Reticulum

• Rough endoplasmic reticulum (RER)

  • Processing and sorting area for proteins synthesized by the ribosomes that stud its outer membrane.

  • Ribosomes are small nonmembrane-bound organelles composed of protein and ribosomal RNA that function as protein factories.

    • Synthesize proteins that may be incorporated into other organelles or into the plasma membrane.

• Smooth endoplasmic reticulum (SER)

  • Responsible for the synthesis of fatty acid and steroid hormones.

  • SER has no attached ribosomes.

• Both SER and RER produce vesicles filled with product ready for the next step in processing.

  • Vesicles usually move substances from to the cell membrane for exocytosis.

  • Or to the Golgi complex for further packaging.

The Golgi Complex is Involved with Processing Proteins and Fatty Acids

• The Golgi complex, or Golgi apparatus, is found near the end of the SER that is farthest from the nucleus.

• Resembles a stack of pancakes called saccules.

  • Saccules are slightly curved, with concave and convex faces.

    • Concave portions face the ER; convex portions face plasma membrane

    • Vesicles are found at the edges of these saccules.

    • Vesicles that leave the Golgi complex migrate all over the cell.

Lysosomes are Chemical Packages that Contain Hydrolytic Enzymes

• Produced by the Golgi complex.

• When a lysosome (lyse means to break open or break apart) fuses with an endocytic vesicle, it pours its contents into the vesicle.

  • The hydrolytic enzymes break down the vesicle’s contents.

  • Phagocytosed bacteria are routinely destroyed in the body by lysosomal activity.

The Nucleus Contains a Cell’s Genetic Library

• Usually the largest organelle in a eukaryotic cell.

• It is covered by two phospholipid bilayers, called the nuclear envelope.

  • The envelope is punctuated by nuclear pores, which also molecules to enter and exit the nucleus.

  • The small size of the nuclear pores prevents the genetic material (DNA) from leaving the nucleus.

Mitochondria Covert Nutrients into Usable Energy in the Form of ATP

• Have a smooth outer membrane and a folded inner membrane (cristae)

• Mitochondria require oxygen, and produce carbon dioxide in their endless production of ATP.

  • Cellular respiration

Cellular Respiration Occurs in Four Steps

Cell Communication

• Cell communicate with one another to function as a tissue

  • Tissues send signals throughout the organ for the organ to function properly.

  • Organs in a particular system communicate to carry out the system’s process.

• The signals sent from cell to cell include information

  • The timing of cell divisions, the health of adjacent cells, and the status of the external environment.

• Cells communicate with one another via chemical messengers or through physical contact.

• Cell signaling occurs via three routes

  • Each route differs in the speed and distance of the signal transmission.

Cell Signaling

• Circulating hormones can be released into the bloodstream, potentially reaching every cell.

  • Much hormonal communication is long distance, carrying information to distant cells that will alter their functioning.

  • Target cells are often remote from the secreting cells.

• Paracrine (local hormones) can be released to affect only cells in the vicinity.

  • Mostly used when quick responses are required

  • Cells that are infected with a virus may secrete the paracrine interferon.

    • Interferon alerts the surrounding cells, warning them of the viral invasion, helping them to fight it.

  • Neurons use paracrine to stimulate nearby nerve, muscle, or glandular cells by releasing short-lived chemicals called neurotransmitters.

    • Neurons must respond instantly to information; therefore they secrete neurotransmitters directly into the space between cells.

Cells Can Interact with other Cells Directly Through Cell-to-Cell Junctions

• Cell-to-cell junctions occur in tissues like your skin and heart muscle tissue, where cells are in direct contact with one another.

  • For example, skin cells are knit tightly together, so any change in one cell will be immediately passed to the next.

• Gape junctions are used for instantaneous communication.

  • Occur across very short distances and are extremely specific.

  • Immediate and short-lived.

  • Important between heart cells.