Chapter 5: Homeostasis and TransportLesson 1: Cell Structures Involved in Cell Transport

Describe the structure and function of the plasma membrane in relation to cell transport.

Identify the types of membrane proteins involved in cell transport.

Identify the roles of the cytoplasm and cytoskeleton in cell transport.

Identify the role of the endomembrane system in eukaryotic organisms in cell transport.

Discuss special transport structure of plant cells.

Outline the role of cell transport in homeostasis.

Lesson Objectives

Barrier between cytoplasm inside and environment outside the cell Protects, supports the cell Controls what goes in and what goes out

◦ Selective permeability allows only certain substances to pass through Water most important substance; most essential

molecules for life are soluble Inorganic ions: sodium (Na+), potassium (K+), calcium

(Ca2+), and chlorine (Cl-) Gases: oxygen helps release energy that powers cellular

reactions Hormones: transmit messages

FROM OUTSIDE TO INSIDEThe Plasma Membrane

Consists of two layers of phospholipids◦ hydrophobic, or water-hating, interior (tails)◦ hydrophilic, or water-loving, exterior (heads)

Proteins embedded within have a variety of functions

Hydrophobic molecules are nonpolar, easily cross; if small enough

Hydrophilic molecules are polar, need help to cross; usually through embedded proteins

The Phospholipid Bilayer

NOT INVOLVED IN TRANSPORT◦ Peripheral proteins loosely bound to membrane

surface or to part of a integral protein INVOLVED IN TRANSPORT

◦ Integral proteins transmembrane protein; extend into and usually span interior of membrane

Proteins Embedded in the Plasma Membrane

Transport proteins ions, polar molecules Types of transport proteins:

◦ Channel proteins hydrophilic channel Water passes through aquaporin channel proteins Ions pass through ion channels Other ions pass through gated channels; open and

close in response to a stimulus◦ Carrier proteins hold molecules and change shape

as they pass them through Sodium-potassium pump

Integral proteins types…

Contains enzymes, salts, organelles, and a variety of organic molecules◦ Enzymes help dissolve cellular waste for export◦ Salts are conductors of electricity

Movement generated through churning of cytoplasmic streaming speeds up distribution of nutrients, proteins, and organelles within cell

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter4/animation_-_cytoplasmic_streaming.html

Cytoplasm’s Role in Intracellular Transport

Consists of three different threadlike structures: microfilaments, intermediate filaments, microtubules◦ Microfilaments and microtubules both components of

intracellular transport Microfilaments composed of protein called actin; act like

tracks within cells for myosin molecules Microtubules tracks for vesicle intracellular transport

Role of Cytoskeleton in the Cytoplasm

Separates and compartmentalizes interrelated functions of variety of organelles◦ Equates to a division of labor

Organelles involved:◦ Nuclear envelope, smooth endoplasmic reticulum,

rough ER, Golgi, lysosomes, endosomes, vesicles, vacuoles, and plasma membrane

Some through direct connection; others through vesicular transport◦ Vesicles bud off of sending organelles; fuse with

receiving organelles

The Endomembrane System in Eukaryotic Cells

Helps make and transport proteins and lipids Connected to the pores of the nuclear envelope Two types of endoplasmic reticulum:

◦ Rough endoplasmic reticulum (RER)◦ Smooth endoplasmic reticulum (SER).

Rough Endoplasmic Reticulum (RER)◦ Studded with ribosomes; involved in protein synthesis, the production and

transport of new membrane, and the modification and transport of newly formed proteins within the cell

◦ Proteins synthesized on the RER are transported to other locations through vesicles formed in the SER.

Smooth Endoplasmic Reticulum (SER)◦ Not studded with ribosomes◦ Contains enzymes for lipid biosynthesis (change to phospholipids and steroids)◦ Forms transition vesicles that travel along microtubular tracks in the cytoplasm

Transport molecules made in the RER to the Golgi apparatus

The Endoplasmic Reticulum (ER)

Processes proteins and prepares them for use both inside and outside the cell

Receives proteins from the ER that have been transported in vesicles, packages and labels them

Sends them on to their next destinations in another set of vesicles

Also involved in the transport of lipids around the cell

http://www.johnkyrk.com/golgiAlone.html

The Golgi Apparatus

Sac-like organelles Store and transport large molecules Pinch off ER and Golgi

◦ Transport to plasma membrane; fuse with it◦ Going out = exocytosis

Pinch inward at plasma membrane◦ Transport to destinations inside the cell◦ Going in = endocytosis

Vesicles

Lysosomes bud off of the Golgi apparatus; infused with hydrolytic enzymes◦ Digestive vesicles of the cell; contain enzymes called hydrolases

Digest proteins, nucleic acids, lipids, and complex sugars. ◦ Vesicles enter the cell through endocytosis; sent to lysosomes contents

processed Break down and disarm many potentially pathogenic and foreign materials; expel

them outside the cell through exocytosis Endosomes formed during endocytosis allow materials from outside

the cell to enter the cell◦ Formed when the cell’s plasma membrane folds inward to surround

macromolecules, encircles them, brings them into the cell by pinching off the membrane at their point of entry

Vesicles larger than 100 nanometers in size are referred to as vacuoles.

Lysosomes and Endosomes

Structure not found in animal cells, cell wall◦ Rigid layer that surrounds the plasma membrane◦ Supports and protects the cell

Tiny holes, or pores, in the cell wall called plasmodesmata (singular, plasmodesma) ◦ Form open channels through which strands of cytosol

connect between adjacent cells◦ Allow water, nutrients, and other substances to move

into and out of the cells

Special Transport Structure in Plant Cells

Cell to function normally=> stable state must be maintained inside the cell◦ Example: concentration of salts, nutrients, and other substances

must be kept within a certain range Process of maintaining stable conditions inside a cell (or an

entire organism) is homeostasis◦ Homeostasis requires constant adjustments, because conditions are

always changing both inside and outside the cell◦ Structures and processes described in this lesson and the next

lesson play important roles in homeostasis and are consider homeostatic mechanisms

◦ By moving substances into and out of cells, they keep conditions within normal ranges and maintain homeostatic regulation inside the cells and the organism as a whole

If homeostatic mechanisms fail to maintain homeostatic regulation disease or death of a cell or organism may follow.

Homeostasis and Cell Function

A major role of the plasma membrane is transporting substances into and out of the cell.

The plasma membrane is selectively permeable, allowing only certain substances to pass through.

Proteins embedded within the plasma membrane help to move hydrophilic, polar molecules into the cell.

The cytoplasm is the internal medium for cell transport. Vesicle aids in the import and export of macromolecules. Plant cells have a special transport structures that are not

found in animal cells, called plasmodesmata. Cell transport helps cells maintain homeostasis by keeping

conditions within normal ranges inside all of an organism’s cells.

If homeostatic mechanisms fail, homeostatic regulation may fluctuate and disease or death of a cell or organism can follow.

Lesson Summary