7-3 Cell Boundaries
All cells are surrounded by a thin, flexible barrierknown as the cell membrane. Many cells alsoproduce a strong supporting layer around themembrane known as a cell wall.
Cell Membrane
•The cell membrane regulates what enters and leavesthe cell and also provides protection and support.
•The composition of nearly all cell membranes is adouble-layered sheet called a lipid bilayer.
•Protein molecules are embedded in the lipid bilayer.Some form channels andpumps that help to movematerial across the cellmembrane.
Cell Wall
•The main function of the cell wall is to provide supportand protection for the cell.
•Made from fibers of carbohydrate and protein.
•Plant cell walls are composed mostly of cellulose, atough carbohydrate fiber.
•Present in many organisms, including plants, algae,fungi, and manyprokaryotes.
Diffusion Through Cell Boundaries
*One of the most important functions of the cellmembrane is to regulate the movement ofdissolved molecules from the liquid on one sideof the membrane to the liquid on the other side.
•Solution = mixture of two or more substances.
•The substances dissolved in the solution arecalled solutes.
•The concentration of a solution isthe mass of solute in a given volume ofsolution, or mass/volume.
Solution concentration
The first solution is twice as concentrated as thesecond solution.
Diffusion
•In a solution, particles move constantly. Theycollide with one another and tend to spreadout randomly.
•As a result, the particles tend to move from anarea where they are more concentrated to anarea where they are less concentrated, aprocess known as diffusion.
•When the concentration of the solute is thesame throughout a system, the system hasreached equilibrium.
Suppose a substance is present in unequal concentrations oneither side of a cell membrane. The particles will tend to movetoward the area where it is less concentrated until substance onboth sides of the cell membrane will be the same.
Diffusion
•Because diffusion depends upon randomparticle movements, substances diffuse acrossmembranes without requiring the cell to useenergy. NO ENERGY NEEDED!!!
Osmosis – The diffusion of water through aselectively permeable membrane.
•If a substance is able to diffuse across a membrane,the membrane is said to be permeable to it.
•Most biological membranes are selectivelypermeable, meaning that some substances canpass across them and others
•A membrane is impermeable to substances thatcannot pass across it.
•Most biological membranes are selectivelypermeable, meaning that some substances canpass across them and others cannot.
There are more sugar molecules on the left side of themembrane than on the right side. That means that theconcentration of water is lower on the left than it is onthe right. The membrane is permeable to water but notto sugar. This means that water can cross themembrane in both directions, but sugar cannot. As aresult, there is a net movement of water from the areaof high concentration to the area of low concentration.
The Effects of Osmosis on Cells
Isotonic – “same strength”
Hypertonic – “abovestrength”
Hypotonic – “belowstrength
Facilitated Diffusion
Cell membranes have proteinchannels that act as carriers,making it easy for certainmolecules to cross.
•These cell membranechannels facilitate, orhelp, the diffusion ofglucose across themembrane. (Forexample in red bloodcells)
•The molecules willmove from highconcentration to lowconcentration.
•No energy needed.
Active Transport
•Cells sometimes must move materials in theopposite direction – against a concentrationdifference.
•(low concentration -> high concentration)
• This requires energy!!!
Active Transport
Endocytosis – the process of taking material into the cell bymeans of infoldings, or pockets, of the cell membrane.
Phagocytosis – “cell eating” Extensions of cytoplasm surrounda particle and package it within a food vacuole. The cell thenengulfs it.
Pinocytosis – Tiny pockets form along the cell membrane, fillwith liquid, and pinch off to form vacuoles within the cell.
Active Transport
Many cells also release large amounts of materialfrom the cell through a process known asexocytosis.
The membrane of the vacuole surrounding thematerial fuses with the cell membrane, forcing thecontents out of the cell.
7-4 The Diversity of Cellular Life
•All living things are composed of cells
•They use the same basic chemistry.
•The follow the same genetic code, and evencontain the same kinds of organelles.
BUT….. There’s such great diversity of life!!!
Their differences arise from the ways in which cellsare specialized to perform certain tasks and theways in which cells associate with one another toform multicellular organisms.
Unicelluar Organisms – single-celled organism
•Sometimes, a cell is the organism.
•They grow, respond to the environment, transformenergy, and reproduce.
•They are found everywhere!!!
Volvox
amoeba
Yeast
Multicellular Organisms – Organisms madeup of may cells.
•Cell Specialization – The process in which cells developin different ways to perform different tasks.
•Red blood cells are specialized to transport oxygen.
•Muscle cells contract and relax to move part of the body.
•Pancreatic cells produce compounds such as insulin thatthe body needs
Multicellular Organisms
•Specialized plant cells, known as guard cells, regulateexchanges of carbon dioxide, oxygen, water vapor,and other gases.
•Guard cells monitor the plant’s internal conditions,changing their shape according to those conditions.
Levels of Organization
•The levels of organization in a multicellularorganism are individual cells, tissues, organs,and organ systems.
•A tissue is a group of similar cells that perform aparticular function
•Many groups of tissues work together as anorgan.
•A group of organs that work together toperform a specific function is called an organsystem.
•The organization of the body’s cells intotissues, organs, and organ systems creates adivision of labor among those cells that makesmulticellular life possible.
