1. Chapter 3: Cell Structure and Function

2. Overview
Cell Theory
Cell Size
Function of Organelles

3. Cell Theory All organisms are composed of one or more cells
Cells are the basic living unit of structure and function in organisms
All cells come only from other cells

4. Cell Size Surface Area to Volume Ratio
Surface area affects the ability of nutrients to get into the cell and wastes to get out
Large cells need more nutrients and produce more wastes than small cells
Small cells have more surface area per volume than large cells
-Refer to figures on p. 48 of textbook
Why are cells so small? And why are we multicellular?
Consider the surface area to volume ratio:
Nutrients need to enter the cell and wastes need to exit the cell at it’s surface
The amount of surface affects the ability to get material in and out of the cell
Large cells require more nutrients and produce more waste than a small cell : the volume represents the need of the cell
Small cells have more surface area per volume than do large cells
Small cells (not large cells) are likely to have an adequate surface area for exchanging wastes for nutrients
Cell division restores the amount of surface area needed for adequate exchange of materials
Cells that specialize in absorption have modifications to greatly increase surface area to volume ration  microvili
Cells need a surface area that can adequately exchange material with the environment. Surface-area-to-volume considerations require the cell to remain small.
(As the cube gets bigger, the volume increases much more rapidly than the surface area, because the volume increases as the cube of the linear dimension, but the surface area increases as the square. This relationship applies, not just to cubes, but to spheres and any other fixed shape) See calculations on page 48
Small cell: 1mm tall= volume 1mm3 (heightxwidthxdepth)
= surface area is 6 mm2 (6 sides)
= ratio 6:1
Larger cell: 2mm tall = volume 8mm3 (heightxwidthxdepth)
= surface area is 24 mm2 (6 sides x (2x2))
= ratio 3:1
Small cell has a greater surface area to volume ratio more adequate surface area for exchanging wastes and nutrients

5. Eukaryotic Organelles
What defines a eukaryotic cell?
Has a nucleus
How do plant cells differ from animal cells?
Cell wall
Primary cell wall Cellulose = strength
Secondary cell wall  Lignin = even stronger
Chloroplasts
Chlorophyll  Photosynthesis
Refer to figures on p. 50 and 51 of textbook

6. The Outer Boundaries Plasma membrane Composition:
Phospholipid bilayer with embedded proteins
Function:
Defines cell boundary
Regulates entrance and exit of molecules
Cytoplasm
Semifluid medium that contains organelles
Surrounds the nucleus inside the cell membrane
Animal and plant cells are surrounded by a plasma membrane that consists of a phospholipid bilayer in which protein molecules are imbedded.
Plasma membrane is the living boundary that separates the living contents from the non-living surrounding environment. Regulates the entrance and exit of molecules into and out of the cytoplasm
Inside the cell, the nucleus is surrounded by the cytoplasm, a semifluid medium that contains organelles

7. The Nucleus Nucleoplasm Chromatin (DNA and proteins)
Composition:
Nuclear envelope (Double membrane with pores)
Nucleoplasm
Chromatin (DNA and proteins)
Nucleoli (Nucleolus)
Composition: Concentrated area of chromatin, RNA and proteins
Function: Ribosomal subunit formation
Function:
Storage of genetic information
Synthesis of DNA and RNA
The nucleus is of primary importance because it stores the genetic material DNA, which governs the characteristics of the cell and its metabolic functioning.
When you look at the nucleus, even under an electron microscope, you cannot see DNA molecules, you can see Chromatin (which consists of DNA and proteins)
Chromatin looks grainy, but actually is a thread-like material that undergoes coiling to form rod-like structures called Chromosomes, just before the cell divides. Chromatin is immersed in a semifluid medium called the nucleoplasm
Some regions look darker than the rest of the chromatin, these are nucleoli (nucleolus) where ribosomal RNA is produced.
The nucleus is separated from the cytoplasm by a double membrane known as the nuclear envelope, which is continuous with the endoplasmic reticulum. The nuclear envelope has nuclear pores to permit passage of proteins into the nucleus and ribosomal sub-units out of the nucleus.

8. View Fig 3.4, p. 52 in your textbook

9. Ribosomes Protein and rRNA Two subunits (large and small)
Composition:
Protein and rRNA
Two subunits (large and small)
Very small organelles
Occur in the cytoplasm:
- Singly
- Groups (polyribosomes)
Attached to the
Endoplasmic reticulum
Function:
Protein synthesis
Ribosomes are composed of two subunits, one large and one small. Each subunit has its own mix of proteins and rRNA. Protein synthesis occurs at the ribosomes. Ribosomes can be found within the cytoplasm, either singly or in groups called polyribosomes. Ribosomes can also be found attached to the endoplasmic reticulum. Proteins synthesized at ribosomes attached to the endoplasmic reticulum have a different fate: either secreted from the cell or become a part of the external surface.

10. The Endomembrane System
Consists of:
Nuclear envelope
Endoplasmic
Reticulum
Golgi Apparatus
Lysosomes
Transport Vesicles
Refer to Fig 3.6 p. 54
This system compartmentalizes the cell so that particular enzymatic reactions are restricted to specific regions. Organelles that make up the endomembrane system are connected either directly or by transport vesicles.

11. The Endoplasmic Reticulum
Composition:
Membranous, flattened channels, tubular canals
Function:
Synthesis and/or modification of proteins
+ other substances
Distribution via vesicle formation
The Rough ER
Composition: studded with ribosomes
Function: protein synthesis
The Smooth ER
Composition: having no ribosomes
Function: lipid synthesis and forms vesicles for transport
Refer to Fig 3.5 p. 53
The ER is a complicated system of membranous channels and saccules (flattened vesicles) and is physically continuous with the outer membrane of the nuclear envelope.
Rough ER is studded with ribosomes on the side of the membrane that faces the cytoplasm. Here proteins are synthesized and enter the ER interior, where processing and modification begin. Most proteins are modified by the addition of a sugar chain, which makes them a glycoprotein.
Smooth ER, which is continuous with the rough ER does not have attached ribosomes. Smooth ER synthesizes the phospholipids that occur in membranes and has various other functions depending on the type of cell. The smooth ER also forms vessicles in which proteins are transported to the Golgi Apparatus.

12. The Golgi Apparatus Processing Packaging Secretion Distribution of
Composition:
Stack of membranous saccules
Function:
Processing
Packaging
Secretion
Distribution of
proteins and lipids
Formation of lysosomes
The Golgi apparatus consists of a stack of slightly curves saccules whose appearance can be compared to a stack of pancakes.
In animal cells, one side of the sack (the inner face) is directed toward the ER and the other side (the outer face) is directed toward the plasma membrane.
Vesicles can frequently be seen at the edge of the saccules.
The Golgi apparatus receives proteins and lipid filled vesicles from the smooth ER, these move trough the Golgi from the inner face to the outer face. During the passage through the Golgi, glycoproteins have their sugar chains modified before they are repacked in secretory vesicles. Secretory vesicles proceed to the plasma membrane where they discharge their contents = secretion
Summary: Golgi is involved in processing, packaging and secretion
The Golgi is also involved in the formation of lysosomes, vesicles that contain proteins and remain within the cell.

13. Lysosomes Contains hydrolytic digestive enzymes
Composition:
Membranous vesicle produced by Golgi apparatus
Contains hydrolytic digestive enzymes
Function:
Intracellular digestion
Macromolecules
Bacteria
Cell contents
Lysosomes are membrane bounds vesicles produced by the Golgi apparatus. They contain hydrolytic digestive enzymes.
Sometime macromolecules are brought into the cell by vesicle formation at the plasma membrane. When lysosomes fuse with such a vesicle, its contents are digested into simpler sub-unites by the lysosomal enzymes.
Some white blood cells defend the body by engulfing pathogens by vesicle formation, when lysosomes fuse with these vesicles, the bacteria are digested.
Lysosomes can also be involved in autodigestion (parts of the cell = normal rejuvenation)
Lysosomes contain enzymes for digesting all sorts of molecules

14. Vacuoles Large membranous sac (vacuoles are larger than vesicles)
Composition:
Large membranous sac
(vacuoles are larger than vesicles)
Both animals and plant have them, but much bigger in plants
(in plants: large, central, filled with water, sugar, salt, pigments, toxins  support, color and protection)
Function:
Store substances
A vacuole is a large membranous sac. A vesicle is smaller than a vacuole.
Animal cells have vacuoles, but they are much more prominent in plant cells
Plants have a large central vacuole filled with watery fluid that gives added support to the cell
Vacuoles store substances
Plant vacuoles contain water, sugar, salts, pigments (responsible for colors) and toxic molecules (protection)
In protozoans, the vacuole is specialized to rid the cell of excess water and digest nutrients

15. Peroxisomes Contain specific enzymes
Composition:
Membranous vesicles
Contain specific enzymes
Function:
Various metabolic tasks
Result in the production of hydrogen peroxide molecules
Catalase breaks into water and oxygen
Peroxisomes are similar to lysosomes, are membrane-bounded vesicles that enclose enzymes
The enzymes in peroxisomes are synthesized by cytoplasmic ribosomes and transported into a peroxisome by carrier proteins. Peroxisomes contain enzymes whose action results in hydrogen peroxide (H2O2) . Hydrogen peroxide is a toxic molecule, so it is immediately broken down to water and oxygen by another perioxisomal enzyme, catalase.
The enzyme in a perioxisome depends on the function of the cell, they are especially prevalent in cells that break down fats.
In plants, perioxisomes oxidize fatty acid molecules into sugar that is needed by the growing plant

16. Energy-Related Organelles
ATP
CO2 + H2O
Useable energy for cells
Carbohydrate
Chloroplasts and mitochondria are the two eukaryotic membranous organelles that specialize in converting energy to a form that can be used by the cell.
Chloroplasts use solar energy to synthesize carbohydrates, and carbohydrate derived products are broken down in the mitochondria to produce ATP molecules. When cells use ATP as an energy source, energy dissapates as heat, thus life could not exist without a constant input of solar energy.
Only plants, algae and cyanobacteria are capable of photosynthesis:
Solar energy+ carbon dioxide+water  carbohydrate + oxygen
Solar is the ultimate source of energy for cells, nearly all organisms rely directly or indirectly on carbohydrates produced by photosynthesizers.
All organisms carry on cellular respiration: chemical energy of carbohydrates is produced to ATP. All organisms, except bacteria, carry out cellular respiration in the mitochondria:
Carbohydrate + oxygen  carbon dioxide + water+ energy (ATP)
ATP synthetic reactions, active transport, all energy-requiring processes in the cell
Mitochondria
Chloroplast
Cellular Respiration:
carbohydrate + oxygen 
Solar energy + carbon dioxide +water
Photosynthesis:
Solar energy + carbon dioxide +water  carbohydrate + oxygen

17. Chloroplasts (only in plant cells)
Composition:
Membranous grana bounded by two membranes
Stroma (fluid filled space with DNA, ribosomes, enzymes)
Thylakoids (flattened sacs)  stacked = grana
 contains chlorophyll
Function:
Photosynthesis
Plant and algal cells contain chloroplasts, the organelles that allow them to produce their own food. They belong to the group of organelles called plastids. Chloroplasts are green due to the pigment chlorophyll.
Chloroplast is bound by two membranes that enclose a fluid-filled space called the stroma.
A membrane system within the stroma is organized into interconnected flattened sacs called thylakoids. Thylakoids are stacked into structures called grana, there can be hundreds of grana in a single chloroplast.
Chlorophyll is located in the thylakoid membranes of grana, captures the solar energy needed to enable the chloroplast to produce carbohydrates. The stroma also contains DNA, ribosomes, and enzymes that synthesize carbohydrates from carbon dioxide and water.

18. Mitochondria (produce ATP) Composition: Inner membrane folded= Cristae
Double membrane
Inner membrane folded= Cristae
Increases surface area for ATP production
Inner fluid filled space = Matrix
Contains DNA, ribosomes, enzymes to break down carbohydrates
Function:
Cellular Respiration
(produce ATP)
All eukaryotic cells contain mitochondria.
Mitochondria are bound by a double membrane. The inner fluid filled space is called the matrix. The matrix contains DNA, ribosomes and enzymes that break down carbohydrate products, releasing energy to be used for ATP production.
The inner membrane is folded and called the cristae, which provides greater surface area to accommodate the production of ATP

19. The Cytoskeleton
Composition:
Microtubules
Help evenly distribute chromosomes during cell division
Maintain shape and help organelles move around
Intermediate filaments
Help support nuclear envelope and plasma membrane
Lends mechanical strength
Actin filaments
Structural role
Formation of Pseudopods
Movement of cells and organelles (Myosin)
Function:
Shape of the cell
Movement of cell parts
The cytoskeleton is a network of interconnected filaments and tubules that extend from the nucleus to the plasma membrane. The cytoplasm is highly organized, it contains actin filaments, microtubules, and intermediate filaments.
Compare to bones and muscles of an animal. Like these, fibers of the cytoskeleton maintain cell shape and cause the cell and its organelles to move.
The cytoskeleton is dynamic: assembly occurs when monomers join a fiber and disassembly occurs when monomer leave a fiber, this occurs in seconds to minutes.
Microtubules: small, hollow cylinders, made of globular protein called tubulin. When microtubules assemble, tubulin molecules come together as dimers, the dimers arrange in rows.  13 rows of tubulin dimers surrounding an empty central core.
Microtubule assembly is under the control of the microtubule organizing centre= centrosome, which lies near the nucleus
During cell division, microtubules help evenly distribute chromosomes. When not dividing they help maintain shape and act as a track along which organelles move.
Intermediate filaments: intermediate size between actin and microtubules. Rope-like assemblies of fiberous polypeptides that support the nuclear envelope and the plasma membrane. Gives mechanical strength to skin cells. Assemble and dissasemble in the same manner as actin and mircotubules.
Actin Filaments: Long, extremely thin fibers, occur in bundles or mesh-like networks. The actin filament contains two chains of globular actin monomers twisted about one another in a helical manner.
Form a structural role by forming a dense, complex web under the plasma membrane, where they are anchored to special proteins
Account for the formation of pseudopods, extensions that allow certain cells to move in an amoeboid fashion
Involved in the movement of cells and organelles by interacting with a motor molecule called myosin.

20. Centrioles (only in animal cells)
Composition:
9+0 pattern of microtubule triplets
Function:
Formation of basal bodies
organization of cilia + flagella
Microtubule formation
Refer to Fig 3.14 p. 60
Centrioles are short cylinders with a 9+0 pattern of microtubule triplets.
In animals a centrisome contains two centrioles lying at right angles to each other.
The centrosome is the major microtubule organizing centre for the cell and centrioles may be invoolved in the process of microtubule assembly and dissasembly.
Before animal cells divide, the centrioles replicate, each pair becomes part of a separate centrosome. During cell division the centrosomes move apart and function to help organize the mitotic spindle
Centrioles are believed to give rise to basal bodies that direct the organization of microtubules withing cilia and flagella

21. Cilia and Flagella Composition: Functions:
9+2 pattern of microtubule doublets, around 2 central microtubules
Membrane bounded cylinders enclosing a matrix
Cilia are much shorter than flagella
Functions:
Movement of the cell
Cilia and flagella are hair-like projections that can move like a whip, or stiffly like an oar.
Cells that have these organelles are capable of movement.
Ex. Unicellular paramecium move with cilia, sperm cells move with flagella, respiratory tract is lined with cilia to help remove debris.
In eukaryotic cell, cilia are much shorter than flagella, but are similarly contructed
Membrane bounded cylinders enclosing a matrix area.  9 microtubule doublets are arranged in a circle around two central microtubules. Cilia and flagella move when the microtubule doublets slide past one another.
They have a basal body lying in the cytoplasm at the base which initiates the polymerization of the 9 outer doublets of a cilium or flagella.

22. Q U
E S T I O N S ?