The Cell

Why are cells so small? Why can’t they be as huge as an hippo?

What limits cell size? Surface to volume ratio as cell gets bigger its volume increases faster than its surface area smaller objects have greater ratio of surface area to volume What cell organelle governs this? Alka-seltzer Demo As cell gets larger, volume increases cubically, but surface area only increases by the square. The volume of the cell is demanding… it needs exchange. The surface area is the exchange system… as cell gets larger, the surface area cannot keep up with demand. Instead of getting bigger, cell divides -- mitosis. Why is a huge single-celled creature not possible? s:v 6:1 1.2:1 2005-2006 6:1

Limits to cell size Metabolic requirements set upper limit in large cell, cannot move material in & out of cell fast enough to support life aa aa What process is this? CH NH3 aa CHO O2 CH CHO CO2 CHO CO2 CO2 What process is this? diffusion What’s the solution? cell divides make a multi-celled creature = lots of little cell, rather than one BIG cell aa NH3 O2 NH3 O2 CHO NH3 aa CO2 CH aa CH O2 aa O2 2005-2006 What’s the solution?

How to get bigger? Become multi-cellular (cell divides) aa aa aa O2 aa But what challenges do you have to solve now? CO2 CO2 O2 NH3 aa NH3 aa CO2 NH3 O2 CO2 CO2 CH CHO CO2 NH3 Larger organisms do not generally have larger cells than smaller organisms — simply more cells What’s challenges do you have to solve now? how to bathe all cells in fluid that brings nutrients to each & removes wastes from each aa O2 NH3 NH3 CO2 CO2 CO2 CHO aa NH3 NH3 NH3 CH CHO CO2 CO2 O2 aa aa CH 2005-2006

Cell characteristics All cells: surrounded by a plasma membrane have cytosol semi-fluid substance within the membrane cytoplasm = cytosol + organelles contain chromosomes which have genes in the form of DNA have ribosomes tiny “organelles” that make proteins using instructions contained in genes

Prokaryote bacteria cells Eukaryote animal cells Types of cells - no organelles - organelles Eukaryote animal cells Eukaryote plant cells

Types of cells Prokaryotic vs. eukaryotic cells Prokaryotic cell DNA in nucleoid region, without a membrane separating it from rest of cell Cell wall present in all (type differs) Eukaryotic cell chromosomes in nucleus, membrane-enclosed organelle Cell walls present in fungi and plants only More complex Membrane bound organelles present

The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the eukaryotic cell. 2005-2006

Why organelles? mitochondria chloroplast Golgi ER Specialized structures specialized functions cilia or flagella for locomotion Containers partition cell into compartments create different local environments separate pH, or concentration of materials distinct & incompatible functions lysosome & its digestive enzymes Membranes as sites for chemical reactions unique combinations of lipids & proteins embedded enzymes & reaction centers chloroplasts & mitochondria chloroplast Golgi Why organelles? There are several reasons why cells evolved organelles. First, organelles can perform specialized functions. Second, membrane bound organelles can act as containers, separating parts of the cell from other parts of the cell. Third, the membranes of organelles can act as sites for chemical reactions. Organelles as specialized structures An example of the first type of organelle is cilia, these short filaments act as "paddles" to help some cells move. Organelles as Containers Nothing ever invented by man is as complex as a living cell. At any one time hundreds of incompatible chemical reactions may be occurring in a cell. If the cell contained a uniform mixture of all the chemicals it would not be able to survive. Organelles surrounded by membranes act as individual compartments for these chemical reactions. An example of the second type of organelle is the lysosome. This structure contains digestive enzymes, these enzymes if allowed to float free in the cell would kill it. Organelle membranes as sites for chemical reactions An example of the third type of organelle is the chloroplast. The molecules that conduct the light reactions of photosynthesis are found embedded in the membranes of the chloroplast. ER

Cells gotta work to live! What jobs do cells have to do? make proteins proteins control every cell function make energy for daily life for growth make more cells growth repair renewal

Building Proteins Organelles involved nucleus ribosomes endoplasmic reticulum (ER) Golgi apparatus vesicles The Protein Assembly Line Golgi apparatus nucleus ribosome ER vesicles

Synthesizing proteins cytoplasm cisternal space mRNA ribosome membrane of endoplasmic reticulum polypeptide signal sequence ribosome

Nucleolus Function ribosome production build ribosome subunits from rRNA & proteins exit through nuclear pores to cytoplasm & combine to form functional ribosomes small subunit large subunit ribosome rRNA & proteins nucleolus

Types of Ribosomes Free ribosomes Bound ribosomes suspended in cytosol synthesize proteins that function in cytosol Bound ribosomes attached to endoplasmic reticulum synthesize proteins for export or for membranes membrane proteins

Which cells have lot of rough ER? Rough ER function Finalize protein formation and prepare for export out of cell (protein folding) protein secreting cells will have lots packaged into transport vesicles to golgi Which cells have lot of rough ER? Which cells have a lot of ER? protein production cells like pancreas = production of digestive enzymes (rough endoplasmic reticulum from a cell of exocrine pancreas (88000X))

Which cells have lots of Golgi? Golgi Apparatus Function finishes, sorts, tags & ships cell products like “UPS shipping department” ships products in vesicles membrane sacs “UPS trucks” transport vesicles secretory vesicles Which cells have lots of Golgi? Cells specialized for secretion? endocrine glands: produce hormones pituitary, pancreas, adrenal, testes, ovaries exocrine glands: produce digestive enzymes & other products pancreas, mammary glands, sweat glands

Making proteins Putting it together… cytoplasm nucleus cell membrane transport vesicle Golgi apparatus smooth ER rough ER nuclear pore nucleus ribosome cell membrane protein secreted cytoplasm

Smooth ER function Membrane production Many metabolic processes synthesis synthesize lipids oils, phospholipids, steroids & sex hormones hydrolysis hydrolyze glycogen into glucose in liver detoxify drugs & poisons ex. alcohol & barbiturates

Where old organelles go to die! Lysosomes Function little “stomach” of the cell digests macromolecules “clean up crew” of the cell cleans up broken down organelles Structure vesicles of digestive enzymes synthesized by rER, transferred to Golgi only in animal cells

Cellular digestion Lysosomes fuse with food vacuoles polymers digested into monomers pass to cytosol to become nutrients of cell vacuole lyso– = breaking things apart –some = body

When cells need to die… Lysosomes can be used to kill cells when they are supposed to be destroyed some cells have to die for proper development in an organism apoptosis “auto-destruct” process lysosomes break open & kill cell ex: tadpole tail gets re-absorbed when it turns into a frog ex: loss of webbing between your fingers during fetal development Feedback mechanism There are sensors in the cell that monitor growth. They trigger self-destruct when they sense processes. Brown spots on leaves too. Virus infected plant cell auto-destructs and even cells around it to wall off virus.

syndactyly Fetal development 6 weeks 15 weeks

Making Energy Cells must convert incoming energy to forms that they can use for work mitochondria: from glucose to ATP chloroplasts: from sunlight to ATP & carbohydrates ATP = active energy carbohydrates = stored energy ATP ATP +

Mitochondria & Chloroplasts Important to see the similarities transform energy generate ATP double membranes = 2 membranes semi-autonomous organelles move, change shape, divide internal ribosomes, DNA & enzymes

Mitochondria Function cellular respiration generate ATP from breakdown of sugars, fats & other fuels in the presence of oxygen break down larger molecules into smaller to generate energy = catabolism generate energy in presence of O2 = aerobic respiration

Mitochondria Almost all eukaryotic cells have mitochondria there may be 1 very large mitochondrion or 100s to 1000s of individual mitochondria number of mitochondria is correlated with aerobic metabolic activity more activity = more energy needed = more mitochondria What cells would have a lot of mitochondria? active cells: • muscle cells • nerve cells

Chloroplasts Chloroplasts are plant organelles class of plant structures = plastids amyloplasts store starch in roots & tubers chromoplasts store pigments for fruits & flowers chloroplasts store chlorophyll & function in photosynthesis in leaves, other green structures of plants & in eukaryotic algae

Who else divides like that? Chloroplasts Function photosynthesis generate ATP & synthesize sugars transform solar energy into chemical energy produce sugars from CO2 & H2O Semi-autonomous moving, changing shape & dividing can reproduce by pinching in two Who else divides like that? bacteria!

Mitochondria & chloroplasts are different Organelles not part of endomembrane system Grow & reproduce semi-autonomous organelles Proteins primarily from free ribosomes in cytosol & a few from their own ribosomes Own circular chromosome directs synthesis of proteins produced by own internal ribosomes ribosomes like bacterial ribosomes Who else has a circular chromosome not bound within a nucleus? bacteria

Food & water storage plant cells animal cells food vacuoles central vacuole animal cells contractile vacuole

Vacuoles & vesicles Function little “transfer ships” Food vacuoles phagocytosis, fuse with lysosomes Contractile vacuoles in freshwater protists, pump excess H2O out of cell Central vacuoles in many mature plant cells

Vacuoles in plants Functions storage stockpiling proteins or inorganic ions depositing metabolic byproducts storing pigments storing defensive compounds against herbivores selective membrane control what comes in or goes out