Cell structure

Chapter Outline Cell theory Properties common to all cells Cell size and shape – why are cells so small? Prokaryotic cells Eukaryotic cells Organelles and structure in all eukaryotic cell Organelles in plant cells but not animal Cell junctions

History of Cell Theory mid 1600s – Anton van Leeuwenhoek Improved microscope, observed many living cells mid 1600s – Robert Hooke Observed many cells including cork cells 1850 – Rudolf Virchow Proposed that all cells come from existing cells

Cell Theory All organisms consist of 1 or more cells. Cell is the smallest unit of life. All cells come from pre-existing cells.

Observing Cells (4.1) Light microscope Can observe living cells in true color Magnification of up to ~1000x Resolution ~ 0.2 microns – 0.5 microns

Observing Cells (4.1) Electron Microscopes Preparation needed kills the cells Images are black and white – may be colorized Magnifcation up to ~100,000 Transmission electron microscope (TEM) 2-D image Scanning electron microscope (SEM) 3-D image

Cell Structure All Cells have: an outermost plasma membrane genetic material in the form of DNA cytoplasm with ribosomes

Cell Structure All Cells have: an outermost plasma membrane Structure – phospholipid bilayer with embedded proteins Function – isolates cell contents, controls what gets in and out of the cell, receives signals

Cell Structure All Cells have: genetic material in the form of DNA Eukaryotes – DNA is within a membrane (nucleus) Prokaryotes – no membrane around the DNA (DNA region called nucleoid)

Cell Structure All Cells have: cytoplasm with ribosomes Cytoplasm – fluid area inside outer plasma membrane and outside DNA region Ribosome – site of protein synthesis

Why Are Cells So Small? (4.2) Cells need sufficient surface area to allow adequate transport of nutrients in and wastes out. As cell volume increases, so does the need for the transporting of nutrients and wastes.

Why Are Cells So Small? However, as cell volume increases the surface area of the cell does not expand as quickly. If the cell’s volume gets too large it cannot transport enough wastes out or nutrients in. Thus, surface area limits cell volume/size.

Why Are Cells So Small? Strategies for increasing surface area, so cell can be larger: “ Frilly” edged……. Long and narrow….. Round cells will always be small.

Prokaryotic Cell Structure Prokaryotic Cells are smaller and simpler in structure than eukaryotic cells. Typical prokaryotic cell is __________ Prokaryotic cells do NOT have: Nucleus Membrane bound organelles

Prokaryotic Cell Structure Structures Plasma membrane Cell wall Cytoplasm with ribosomes Nucleoid Capsule* Flagella* and pili* *present in some, but not all prokaryotic cells

Eukaryotic Cells Structures in all eukaryotic cells Nucleus Ribosomes Endomembrane System Endoplasmic reticulum – smooth and rough Golgi apparatus Vesicles Mitochondria Cytoskeleton

CYTOSKELETON MITOCHONDRION CENTRIOLES LYSOSOME GOLGI BODY SMOOTH ER ROUGH ER RIBOSOMES NUCLEUS PLASMA MEMBRANE Fig. 4-15b, p.59

Nucleus (4.5) Function – isolates the cell’s genetic material, DNA DNA directs/controls the activities of the cell DNA determines which types of RNA are made The RNA leaves the nucleus and directs the synthesis of proteins in the cytoplasm

Nucleus Structure Nuclear envelope Two Phospholipid bilayers with protein lined pores Each pore is a ring of 8 proteins with an opening in the center of the ring Nucleoplasm – fluid of the nucleus

Nuclear pore bilayer facing cytoplasm Nuclear envelope bilayer facing nucleoplasm Fig. 4-17, p.61

Nucleus DNA is arranged in chromosomes Chromosome – fiber of DNA and the proteins attached to the DNA Chromatin – all of the cell’s DNA and the associated proteins

Nucleus Structure, continued Nucleolus Area of condensed DNA Where ribosomal subunits are made Subunits exit the nucleus via nuclear pores

Endomembrane System (4.6 – 4.9) Series of organelles responsible for: Modifying protein chains into their final form Synthesizing of lipids Packaging of fully modified proteins and lipids into vesicles for export or use in the cell

Endomembrane System Endoplasmic Reticulum (ER) Continuous with the outer membrane of the nuclear envelope Two forms - smooth and rough Transport vesicles Golgi apparatus

Endoplasmic Reticulum Rough Endoplasmic Reticulum (RER) Network of flattened membrane sacs create a “maze” Ribosomes attached to the outside of the RER make it appear rough

Endoplasmic Reticulum Function RER Where proteins are modified and packaged in transport vesicles for transport to the Golgi body

Endomembrane System Smooth ER (SER) Tubular membrane structure Continuous with RER No ribosomes attached Function SER Synthesis of lipids (fatty acids, phospholipids, sterols..)

Endomembrane System Additional functions of the SER In muscle cells, the SER stores calcium ions and releases them during muscle contractions In liver cells, the SER detoxifies medications and alcohol

Golgi Apparatus Golgi Apparatus Stack of flattened membrane sacs Function Golgi apparatus Completes the processing substances received from the ER Sorts, tags and packages fully processed proteins and lipids in vesicles

Golgi Apparatus Golgi apparatus receives transport vesicles from the ER on one side of the organelle Vesicle binds to the first layer of the Golgi and its contents enter the Golgi

Golgi Apparatus The proteins and lipids are modified as they pass through layers of the Golgi Molecular tags are added to the fully modified substances These tags allow the substances to be sorted and packaged appropriately. Tags also indicate where the substance is to be shipped.

Transport Vesicles Transport Vesicles Vesicle = small membrane bound sac Transport modified proteins and lipids from the ER to the Golgi apparatus (and from Golgi to final destination)

Endomembrane System Putting it all together DNA directs RNA synthesis  RNA exits nucleus through a nuclear pore  ribosome  protein is made  proteins with proper code enter RER  proteins are modified in RER and lipids are made in SER  vesicles containing the proteins and lipids bud off from the ER

Endomembrane System Putting it all together  ER vesicles merge with Golgi body  proteins and lipids enter Golgi  each is fully modified as it passes through layers of Golgi  modified products are tagged, sorted and bud off in Golgi vesicles  …

Endomembrane System Putting it all together  Golgi vesicles either merge with the plasma membrane and release their contents OR remain in the cell and serve a purpose

Vesicles Vesicles - small membrane bound sacs Examples Golgi and ER transport vesicles Peroxisome Where fatty acids are metabolized Where hydrogen peroxide is detoxified Lysosome

Lysosomes (4.10) The lysosome is an example of an organelle made at the Golgi apparatus. Golgi packages digestive enzymes in a vesicle. The vesicle remains in the cell and: Digests unwanted or damaged cell parts Merges with food vacuoles and digest the contents Figure 4.10A

Lysosomes (4.11) Tay-Sachs disease occurs when the lysosome is missing the enzyme needed to digest a lipid found in nerve cells. As a result the lipid accumulates and nerve cells are damaged as the lysosome swells with undigested lipid.

Mitochondria (4.15) Function – synthesis of ATP 3 major pathways involved in ATP production Glycolysis Krebs Cycle Electron transport system (ETS)

Mitochondria Structure: ~1-5 microns Outer membrane Inner membrane - Highly folded Folds called cristae Intermembrane space (or outer compartment) Matrix DNA and ribosomes in matrix

Mitochondria (4.15) Function – synthesis of ATP 3 major pathways involved in ATP production Glycolysis - cytoplasm Krebs Cycle - matrix Electron transport system (ETS) - intermembrane space

Vacuoles (4.12) Vacuoles are membrane sacs that are generally larger than vesicles. Examples: Food vacuole - formed when protists bring food into the cell by endocytosis Contractile vacuole – collect and pump excess water out of some freshwater protists Central vacuole – covered later

Cytoskeleton (4.16, 4.17) Function gives cells internal organization, shape, and ability to move Structure Interconnected system of microtubules, microfilaments, and intermediate filaments (animal only) All are proteins

Microfilaments Thinnest cytoskeletal elements (rodlike) Composed of the globular protein actin Enable cells to change shape and move

Cytoskeleton Intermediate filaments Present only in animal cells of certain tissues Fibrous proteins join to form a rope-like structure Provide internal structure Anchor organelles in place.

Cytoskeleton Microtubules – long hollow tubes made of tubulin proteins (globular) Anchor organelles and act as tracks for organelle movement Move chromosomes around during cell division Used to make cilia and flagella

Cilia and flagella (structures for cell motility) Move whole cells or materials across the cell surface Microtubules wrapped in an extension of the plasma membrane (9 + 2 arrangement of MT)

Plant Cell Structures Structures found in plant, but not animal cells Chloroplasts Central vacuole Other plastids/vacuoles – chromoplast, amyloplast Cell wall

Chloroplasts (4.14) Function – site of photosynthesis Structure 2 outer membranes Thylakoid membrane system Stacked membrane sacs called granum Chlorophyll in granum Stroma Fluid part of chloroplast

Plastids/Vacuoles in Plants Chromoplasts – contain colored pigments Pigments called carotenoids Amyloplasts – store starch

Central Vacuole Function – storage area for water, sugars, ions, amino acids, and wastes Some central vacuoles serve specialized functions in plant cells. May contain poisons to protect against predators

Central Vacuole Structure Large membrane bound sac Occupies the majority of the volume of the plant cell Increases cell’s surface area for transport of substances  cells can be larger

Cell surfaces protect, support, and join cells Cells interact with their environments and each other via their surfaces Many cells are protected by more than the plasma membrane

Cell Wall Function – provides structure and protection Never found in animal cells Present in plant, bacterial, fungus, and some protists Structure Wraps around the plasma membrane Made of cellulose and other polysaccharides Connect by plasmodesmata (channels through the walls)

Vacuole Walls of two adjacent plant cells Plasmodesmata Layers of one plant cell wall Cytoplasm Plasma membrane

Origin of Mitochondria and Chloroplasts Both organelles are believed to have once been free-living bacteria that were engulfed by a larger cell.

Proposed Origin of Mitochondria and Chloroplasts Evidence: Each have their own DNA Their ribosomes resemble bacterial ribosomes Each can divide on its own Mitochondria are same size as bacteria Each have more than one membrane

Cell Junctions (4.18) Plasma membrane proteins connect neighboring cells - called cell junctions Plant cells – plasmodesmata provide channels between cells

Cell Junctions (4.18) 3 types of cell junctions in animal cells Tight junctions Adchoring junctions Gap junctions

Cell Junctions Tight junctions – membrane proteins seal neighboring cells so that water soluble substances cannot cross between them See between stomach cells

Cell Junctions Anchoring junctions – cytoskeleton fibers join cells in tissues that need to stretch See between heart, skin, and muscle cells Gap junctions – membrane proteins on neighboring cells link to form channels This links the cytoplasm of adjoining cells

Gap junction Anchoring junction Tight junction

Cell structure

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Cell structure