A tissue is a group of connected cells that have a similar function within an organism. More complex organisms such as jellyfish, coral, and sea anemones have a tissue level of organization. For example, jellyfish have tissues that have separate protective, digestive, and sensory functions.
Even more complex organisms, such as the roundworm shown in Figure above, while also having differentiated cells and tissues, have an organ level of development. An organ is a group of tissues that has a specific function or group of functions. Organs can be as primitive as the brain of a flatworm (a group of nerve cells), as large as the stem of a sequoia (up to 90 meters, or 300 feet, in height), or as complex as a human liver.
The most complex organisms (such as mammals, trees, and flowers) have organ systems. An organ system is a group of organs that act together to carry out complex related functions, with each organ focusing on a part of the task. An example is the human digestive system in which the mouth ingests food, the stomach crushes and liquifies it, the pancreas and gall bladder make and release digestive enzymes, and the intestines absorb nutrients into the blood.
Lesson Summary
The plasma membrane is a selectively permeable lipid bilayer that contains mostly lipids and proteins. These lipids and proteins are involved in many cellular processes.
The gel-like material within the cell that holds the organelles is called cytoplasm. The cytosol, which is the watery substance that does not contain organelles, is made up of 80% to 90% water.
The cytoskeleton has many functions. It helps to maintain cell shape, it holds organelles in place, and for some cells, it enables cell movement. The cytoskeleton also plays important roles in both the intracellular movement of substances and in cell division. Three main kinds of cytoskeleton fibers are microtubules, intermediate filaments, and microfilaments.
Cilia are extensions of the cell membrane that contain microtubules. Although both are used for movement, cilia are much shorter than flagella. Cilia cover the surface of some single-celled animals, such as paramecium, but cover only one side of cells in some multicellular organisms.
There are three features that plant cells have that animal cells do not have: a cell wall, a large central vacuole, and plastids.
Mitochondria use energy from organic compounds to make ATP.
Ribosomes are exported from the nucleolus, where they are made, to the cytoplasm.
The Golgi apparatus is a large organelle that is usually made up of five to eight cup-shaped, membrane-covered discs called cisternae. It modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell.
Individual organisms from a colonial organism or biofilm can, if separated, survive on their own, while cells from a multicellular organism (e.g., liver cells) cannot.
A tissue is a group of connected cells that have a similar function within an organism. An organ is a group of tissues that has a specific function or group of functions, and an organ system is a group of organs that act together to perform complex related functions, with each organ focusing on a part of the task.
Summary Animations
The following web site is an interactive representation of a plant and animal cell, with their various organelles.
http://www.cellsalive.com/cells/cell_model.htm
The following animation is a detailed example of the functions of the specific parts of the cell.
http://www.johnkyrk.com/er.html
The following site is a virtual cell where various organelles can be observed.
http://www.ibiblio.org/virtualcell/tour/cell/cell.htm
Department of Biological Sciences, Carnegie Mellon University
http://telstar.ote.cmu.edu/biology/
Review Questions
What are the main components of a plasma membrane?
What does the fluid mosaic model describe?
What is the difference between cytoplasm and cytosol?
What type of molecule is common to all three parts of the cytoskeleton?
Name the three main parts of the cytoskeleton.
What structures do plant cells have that animal cells do not have?
Identify two functions of plastids in plant cells.
What is the main difference between rough endoplasmic reticulum and smooth endoplasmic reticulum?
List five organelles eukaryotes have that prokaryotes do not have.
What is a cell feature that distinguishes a colonial organism from a multicellular organism?
What is the difference between a cell and a tissue?
Identify two functions of the nucleus.
Identify the reason why mitochondria are called "power plants" of the cell.
If muscle cells become more active than they usually are, they will grow more mitochondria. Explain why this happens.
Further Reading / Supplemental Links
N. J. Butterfield (2000). Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology 26 (3): 386–404.
The Bacterial Cytoskeleton. Shih YL, Rothfield L. Microbiol Mol Biol Rev. 2006 Sep;70(3):729-54.
http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16959967
http://en.wikipedia.org
Vocabulary
chloroplast
The organelle of photosynthesis; captures light energy from the sun and uses it with water and carbon dioxide to make food (sugar) for the plant.
cilia (cilium)
Made up of extensions of the cell membrane that contain microtubules; involved in movement.
cell wall
A rigid layer that is found outside the cell membrane and surrounds the cell; provides structural support and protection.
cytoplasm
The gel-like material within the cell that holds the organelles.
cytoskeleton
A cellular "scaffolding" or "skeleton" that crisscrosses the cytoplasm; helps to maintain cell shape, it holds organelles in place, and for some cells, it enables cell movement.
endoplasmic reticulum (ER)
A network of phospholipid membranes that form hollow tubes, flattened sheets, and round sacs; involved in transport of molecules, such as proteins, and the synthesis of proteins and lipids.
flagella (flagellum)
Long, thin structures that stick out from the cell membrane; help single-celled organisms move or swim towards food.
Fluid Mosaic Model
Model of the structure of cell membranes; proposes that integral membrane proteins are embedded in the phospholipid bilayer; some of these proteins extend all the way through the bilayer, and some only partially across it; also proposes that the membrane behaves like a fluid, rather than a solid.
gene
A short segment of DNA that contains information to encode an RNA molecule or a protein strand.
gene expression
The process by which the information in a gene is "decoded" by various cell molecules to produce a functional gene product, such as a protein molecule or an RNA molecule.
Golgi apparatus
A large organelle that is usually made up of five to eight cup-shaped, membrane-covered discs called cisternae; modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell.
integral membrane proteins
Proteins that are permanently embedded within the plasma membrane; involved in channeling or transporting molecules across the membrane or acting as cell receptors.
intermediate filaments
Filaments that organize the inside structure of the cell by holding organelles and providing strength.
lipid bilayer
A double layer of closely-packed lipid molecules; the cell membrane is a phospholipid bilayer.
lysosome
A vesicle that contains powerful digestive enzymes.
membrane prote
in
A protein molecule that is attached to, or associated with the membrane of a cell or an organelle.
microfilament
Filament made of two thin actin chains that are twisted around one another; organizes cell shape; positions organelles in cytoplasm; involved in cell-to-cell and cell-to-matrix junctions.
microtubules
Hollow cylinders that make up the thickest of the cytoskeleton structures; made of the protein tubulin, with two subunits, alpha and beta tubulin; involved in organelle and vesicle movement; form mitotic spindles during cell division; involved in cell motility (in cilia and flagella).
mitochondria (mitochondrion)
Membrane-enclosed organelles that are found in most eukaryotic cells; called the "power plants" of the cell because they use energy from organic compounds to make ATP.
multicellular organisms
Organisms that are made up of more than one type of cell; have specialized cells that are grouped together to carry out specialized functions.
nucleus
The membrane-enclosed organelle found in most eukaryotic cells; contains the genetic material (DNA).
organ
A group of tissues that has a specific function or group of functions.
organ system
A group of organs that acts together to carry out complex related functions, with each organ focusing on a part of the task.
peripheral membrane proteins
Proteins that are only temporarily associated with the membrane; can be easily removed, which allows them to be involved in cell signaling.
peroxisomes
Vesicles that use oxygen to break down toxic substances in the cell.
phospholipid
A lipid made up of up of a polar, phosphorus-containing head, and two long fatty acid, non-polar "tails." The head of the molecule is hydrophilic (water-loving), and the tail is hydrophobic (water-fearing).
plasma membrane
Phospholipid bilayer that separates the internal environment of the cell from the outside environment.
ribosomes
Organelles made of protein and ribosomal RNA (rRNA); where protein synthesis occurs.
selective permeability
The ability to allow only certain molecules in or out of the cell; characteristic of the cell membrane; also called the cell membrane.
spontaneous generation
The belief that living organisms grow directly from decaying organic substances.
tissue
A group of connected cells that has a similar function within an organism.
transport vesicle
A vesicle that is able to move molecules between locations inside the cell.
vacuole
Membrane-bound organelles that can have secretory, excretory, and storage functions; plant cells have a large central vacuole.
vesicle
A small, spherical compartment that is separated from the cytosol by at least one lipid bilayer.
Points to Consider
How do you think small molecules, or even water, get through the cell membrane?
Is it possible that proteins help in this transport process?
What type of proteins would help with transport?
Lesson 3.3: Cell Transport and Homeostasis
Lesson Objectives
Identify two ways that molecules and ions cross the plasma membrane.
Distinguish between diffusion and osmosis.
Identify the role of ion channels in facilitated diffusion.
Compare passive and active transport.
Identify the connection between vesicles and active transport.
Compare endocytosis and exocytosis.
Outline the process of cell communication.
Introduction
Probably the most important feature of a cell’s phospholipid membranes is that they are selectively permeable. A membrane that is selectively permeable has control over what molecules or ions can enter or leave the cell, as shown in Figure below. The permeability of a membrane is dependent on the organization and characteristics of the membrane lipids and proteins. In this way, cell membranes help maintain a state of homeostasis within cells (and tissues, organs, and organ systems) so that an organism can stay alive and healthy.
Figure 3.27
A selectively permeable membrane allows certain molecules through, but not others.
Transport Across Membranes
The molecular make-up of the phospholipid bilayer limits the types of molecules that can pass through it. For example, hydrophobic (water-hating) molecules, such as carbon dioxide (CO2) and oxygen (O2), can easily pass through the lipid bilayer, but ions such as calcium (Ca2+) and polar molecules such as water (H2O) cannot. The hydrophobic interior of the phospholipid does not allow ions or polar molecules through because they are hydrophilic, or water loving. In addition, large molecules such as sugars and proteins are too big to pass through the bilayer. Transport proteins within the membrane allow these molecules to cross the membrane into or out of the cell. This way, polar molecules avoid contact with the nonpolar interior of the membrane, and large molecules are moved through large pores.
Every cell is contained within a membrane punctuated with transport proteins that act as channels or pumps to let in or force out certain molecules. The purpose of the transport proteins is to protect the cell's internal environment and to keep its balance of salts, nutrients, and proteins within a range that keeps the cell and the organism alive.
There are three main ways that molecules can pass through a phospholipid membrane. The first way requires no energy input by the cell and is called passive transport. The second way requires that the cell uses energy to pull in or pump out certain molecules and ions and is called active transport. The third way is through vesicle transport, in which large molecules are moved across the membrane in bubble-like sacks that are made from pieces of the membrane.
Passive Transport
Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell. The three main kinds of passive transport are diffusion, osmosis, and facilitated diffusion.
Diffusion
Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. The difference in the concentrations of the molecules in the two areas is called the concentration gradient. Diffusion will continue until this gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient." The end result of diffusion is an equal concentration, or equilibrium, of molecules on both sides of the membrane.
If a molecule can pass freely through a cell membrane, it will cross the membrane by diffusion (Figure below).
Figure 3.28
Molecules move from an area of high concentration to an area of lower concentration until an equilibrium is met. The molecules continue to cross the membrane at equilibrium, but at equal rates in both directions.
Osmosis
Imagine you have a cup that has 100ml water, and you add 15g of table sugar to the water. The sugar dissolves and the mixture that is now in the cup is made up of a solute (the sugar), that is dissolved in the solvent (the water). The mixture of a solute in a solvent is called a solution.
Imagine now that you have a second cup with 100ml of water, and you add 45 grams of table sugar to the water. Just like the first cup, the sugar is the solute, and the water is the solvent. But now you have two mixtures of different solute concentrations. In comparing two solutions of unequal solute concentration, the solution with the higher solute concentration is hypertonic, and the solution with the lower concentration is hypotonic. Solutions of equal solute concentration are isotonic. The first sugar solution is hypotonic to the second solution. The second sugar solution is hypertonic to the first.
You now add the two solutions to a beaker that has been divided
by a selectively permeable membrane. The pores in the membrane are too small for the sugar molecules to pass through, but are big enough for the water molecules to pass through. The hypertonic solution is on one side of the membrane and the hypotonic solution on the other. The hypertonic solution has a lower water concentration than the hypotonic solution, so a concentration gradient of water now exists across the membrane. Water molecules will move from the side of higher water concentration to the side of lower concentration until both solutions are isotonic.
Osmosis is the diffusion of water molecules across a selectively permeable membrane from an area of higher concentration to an area of lower concentration. Water moves into and out of cells by osmosis. If a cell is in a hypertonic solution, the solution has a lower water concentration than the cell cytosol does, and water moves out of the cell until both solutions are isotonic. Cells placed in a hypotonic solution will take in water across their membrane until both the external solution and the cytosol are isotonic.
A cell that does not have a rigid cell wall (such as a red blood cell), will swell and lyse (burst) when placed in a hypotonic solution. Cells with a cell wall will swell when placed in a hypotonic solution, but once the cell is turgid (firm), the tough cell wall prevents any more water from entering the cell. When placed in a hypertonic solution, a cell without a cell wall will lose water to the environment, shrivel, and probably die. In a hypertonic solution, a cell with a cell wall will lose water too. The plasma membrane pulls away from the cell wall as it shrivels. The cell becomes plasmolyzed. Animal cells tend to do best in an isotonic environment, plant cells tend to do best in a hypotonic environment. This is demonstrated in Figure below.
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