Cell Facts rev 9-11

Structure & Function of Cells 1

Cell Facts rev 9-11

Cells are the basic structural and functional units of life

• The activity of an organism depends on the activity of its cells

• The biochemical activities of a cell are dictated by its organelles

• The continuity of life has a cellular basis- – all cells come from pre-existing cells and – the survival of life is contingent on the work of

cells


• All cells are surrounded by an outer membrane called the cell membrane or plasma membrane (PM).

• Cells are classified as either eukaryotes or prokaryotes• Eukaryotes have: • A plasma membrane

– A nucleus• contains genetic information and directs all

functions of the cell. – Cytoplasm: composed of a gel-like fluid called

cytosol.– Organelles:


• Prokaryotes have no nucleus or true organelles– have a plasma membrane surrounded by a

rigid cell wall.– genetic material not necessarily enclosed

within nucleus

Cell Structure Reflects Cell Function

All cells carry out defined activities to maintain life:

-Gather raw materials -- Make macromolecules

-Excrete wastes -- Grow and reproduce


• Structural differences between cells generally reflect functional differences– Muscle cells

– Nerve cells:

– Some cells have microvilli to increase surface area

• All raw materials, energy, waste and information can enter or leave the cell only by moving across the plasma (cell) membrane


The Structure of the Plasma Membrane • Constructed of 2 layers of Phospholipids

(called a lipid bilayer) cholesterol, and some proteins– Phospholipids: are lipids with a polar head

and nonpolar tail– Cholesterol: makes the membrane more rigid– Proteins: are embedded in the phospholipid

bilayer and provide means of transporting molecules and information across the membrane; some proteins anchor the cell’s internal support network


• The plasma membrane creates a barrier between the external cell environment and the internal cell environment.

• By regulating the environment, the PM keeps the materials needed for growth and reproduction inside the cell and determines what and when other materials should be allowed to enter the cell.

• Molecules cross the plasma membrane in 3 major ways: passive transport (diffusion and osmosis), active transport, and endocytosis or exocytosis.


• Passive Transport: transports a molecule without requiring the cell to use any energy. It relies on the mechanism of diffusion.– Diffusion—movement of molecules from one region

to another as the result of random motion.– If there are more molecules in one region than

in another, then, simply by chance, molecules move away or diffuse from the area of high concentration towards the area of low concentration.

– So, diffusion requires that there be a difference in concentration, called a concentration gradient, between 2 areas.


Diffusion: Passive Transport

• Passive transport: no energy required

• Diffusion: movement from area of high concentration to low

Figure 3.6

Cell Facts & Tissues-BIO 006

Osmosis is the diffusion of water across a selectively permeable membrane. – Water moves from the region of higher

concentration (lower concentration of solutes) to the area of lower concentration (higher concentration of solutes)

• All substances do not readily diffuse into and out of living cells because the plasma membrane is selectively permeable– The plasma membrane is highly permeable to

water but not to all ions or molecules.

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Three Forms of Passive Transport

Passive transport always uses the concentration gradient and thus relies on diffusion without the using any additional energy.

• Diffusion through the lipid bilayer:– This structure allows some molecules to move

through the PM and prevents others.


• Diffusion through channels– channels are constructed of proteins that

span the entire lipid bilayer.– size, shape, and electrical charge of the

channel determine which molecules are able to pass through.

– Some channels are open all the time (i.e. water channels).

– Some channels are gated


• Facilitated Transport or Facilitated Diffusion– The molecule attaches to a transport or

carrier membrane protein in order to pass through the channel.

– Once the molecule is attached, a change in the protein’s shape or orientation occurs

– this change actually transfers the molecule to the other side of the membrane where it is released.

– Once the molecule is released, the protein returns to its original form.


Active Transport

• requires energy to move substances through the PM against the concentration gradient. – Energy source is ATP

• allows a cell to get essential molecules even when their concentration outside the cell is low and to get rid of molecules which it doesn’t need.

• Proteins which actively transport molecules across the PM are called pumps.


• One of the most important PM pumps is the sodium-potassium pump. This pump uses energy from ATP to transport sodium out of the cell and potassium into the cell.

Naout of cell K into cell

• Active transport requires energy to move substances from an area of lower concentration to an area of higher concentration.


Endocytosis and Exocytosis

• Some molecules are too big to be transported by active or passive transport.

• Cells use endocytosis and/or exocytosis to move large molecules

• Endocytosis moves materials into the cell• Exocytosis moves materials out of the cells


Active Transport: The Sodium/Potassium Pump

• Sodium/potassium pump expels unwanted ions, keeps needed ones, maintains cell volume

• ATP used to expel three sodium ions for every two potassium ions brought into the cell

• To increase cell volume = increase water in cytoplasm by decreasing the pump and allowing more sodium inside cell

• To decrease cell volume = less water in cytoplasm by increasing the pump and expelling more sodium ions


Tonicity of ECF

• Tonicity is the relative concentration of solutes in 2 liquids

• Because water can diffuse across the PM easily, the ability of the cell to control its volume is dependent upon the tonicity of the ECF.

• Isotonic: ECF has the same solute concentration as the ICF (intracellular fluid). The body’s regulatory mechanisms ensure that the ECF solute concentration remains relatively constant.

• Hypertonic: When the ECF concentration is higher than the intracellular fluid, water diffuses out of the cell and the cell shrinks. This will impair normal function and lead to cell death.

Hypertonic When the fluid outside the cells has a greater concentration of dissolved substance in it than the fluid inside the cells , it is called hypertonic. The outside water concentration (98.5%) is lower than the concentration of water inside the cells (99.1%). By osmosis water diffuses from high to low concentration. This results in the water moving out of the cells faster than the water enters. The cells shrink.

Cell fluid is 99.1% water

and 0.9% NaCl

Fluid outside cell contains 98.5% water and 1.5% NaCl

Water

• Hypotonic: When the ECF concentration is lower than the intracellular fluid (fluid outside the cells has a lower concentration of dissolved substance in it than the fluid inside the cells), water diffuses into the cell and the cell swells and may burst. This will impair normal function and lead to cell death.


Hypotonic

Cell fluid is 99.1% water and

0.9% NaCl

Cell

Fluid outside cell contains 100% water and 0% NaCl

Water

Water

Hemolysis or cell swells and bursts


Variations in Tonicity-a Summary• Isotonic: extracellular and intracellular

concentration equal• Hypotonic: extracellular concentration less than

intracellular• Hypertonic: extracellular concentration more

than intracellular


Structure and Function of the NucleusFunctions:• Information center of the cell• Contains the genetic information of the cell• Controls the cell

Structural features:• Outer surface is a double-layered nuclear membrane• Nuclear membrane has nuclear pores which permit some

small proteins and RNA through but not DNA• Nucleolus: RNA and proteins which make up ribosomes

are made. The components of the ribosomes pass through the nuclear pores to the cytoplasm for final assembly into ribosomes.


Ribosomes and Endoplasmic Reticulum (ER)

Ribosomes: responsible for protein assembly. • Float freely in cytoplasm or are attached to the ER.

– Ribosomes attached to the ER release proteins into the folds of the ER

– Free floating ribosomes produce proteins for use by the cell


Endoplasmic reticulum (ER) (and the ribosomes) manufacture proteins and chemical compounds produced by the cell.

• Is a folded membranous system• Some areas of the ER’s surface are dotted with

ribosomes and are called Rough ER. – Areas without ribosomes are called Smooth

ER.• Rough ER: protein synthesis and release (to

smooth ER);• Smooth ER: also synthesizes lipids; packages

proteins and lipids for transfer to the Golgi Apparatus.


Golgi Apparatus

• Stack of membranous sacs which receive substances from ER (usually proteins), refines and packages them

Figure 3.17 (1/2)


Vesicles

• Are membrane-bound spheres that enclose something within the cell. – Ship and store cellular products– Secretory vesicles

• Contain products that need to be exported from the cell

• Endocytotic vesicles• Enclose bacteria and raw materials from the

extracellular environment and bring these into the cell by endocytosis.


– Peroxisomes and lysosomes• Contain powerful enzymes that must be

stored within the vesicle so they don’t damage the rest of the cell

• Peroxisomes have enzymes that destroy toxic wastes

• Lysosomes contain digestive enzymes that digest bacteria; dissolve and remove cellular debris


Mitochondria: Provide Energy to the Cell

• “power plant”• Has a double membrane;

the inner membrane contains enzymes that break down foods

• Liberate energy used to create ATP

• Manufactures ATP as it is needed by the cell.

Figure 3.19a


Krebs cycle (also called the Citric Acid Cycle) (takes place in the mitochondria)– Releases high-energy electrons– Produces two ATP and carbon dioxide

• Electron transport system • energy rich electrons are transferred from 1 protein

carrier molecule to another so that energy from the electrons can be released

• Energy from electrons is used to produce ATP• Produces water and carbon dioxide as a waste by-

product

Cytoskeleton: has microtubules & microfilaments to help form the framework for the plasma membrane; also supports and anchors the other structures within the cell


Cell Structures for Support and Movement: Cilia, Flagella, Centrioles•Cilia and flagella provide movement.•Centrioles: used in cell division


Energy Storage

• Cells store energy.• Can be stored as:

– Fat—long term energy– Glycogen (carbohydrate storage)—short term

energy• most cells rely on glycogen rather than on

fat because the energy in the chemical bonds of glycogen can produce ATP more quickly


Cells Use and Transform Matter and Energy: Two Basic Metabolic Pathways

Metabolism is the sum of all the chemical reactions used by an organism.

• 2 basic types of metabolic pathways:Anabolism: Making of larger molecules– requires enzymes to perform chemical

reaction– usually requires energy (ATP)– Used in building up cell components– Used in storing energy


• Catabolism: – Breaking down of molecules; used in breaking

down nutrients/recycling cell components – requires enzymes to perform chemical reaction– Releases stored energy from the cell– Follows the Citric Acid or Kreb’s cycle to release

energy


Additional Energy Sources

Most of the body’s energy reserves are stored as fats (78%) and proteins (21%), glycogen (1%)

• When we take in too many calories, some of this energy replenishes the body’s stores of glycogen. The rest is converted to fat and stored in fat tissue

• Fats are stored as triglycerides; fats have twice the energy of carbohydrates

• Proteins have the same energy as carbohydrates


Anerobic Metabolism

• For small periods of time, a small amount of ATP can be made without oxygen being used.

• However, lactic acid is formed as a waste by-product.– This causes a burning sensation in your muscles and

can also cause muscle cramps. – Muscle fatigue sets in more quickly.– When oxygen is available, the lactic acid will be

metabolized – Anerobic metabolism produces only 2 ATP molecules.


• Aerobic Metabolism– Requires oxygen– Yields 36 ATP; produces carbon dioxide as a

by-product• After exercise, oxygen metabolizes the lactic

acid (from anerobic metabolism) and replenishes energy stores

• The term oxygen debt describes the continued increased metabolism that must occur in a cell to remove the excess acid that accumulates during prolonged exercise.

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Cell Facts rev 9-11