Cell Structure

The Cell

ESSENTIAL to the study of biology

Simplest form of life Every organism’s basic

unit of structure and function

Named by Robert Hooke in 1665 after observing cork cells (cell walls) under microscope.

Prokaryotic vs. Eukaryotic Cells

Prokaryote “before” “nucleus”/ NO

NUCLEUS/few organelles Bacteria DNA is concentrated in

nucleoid (non membrane-bound)

Eukaryote “true” “nucleus” / many

membranous organelles Protists, plants, fungi,

animals Nucleus with nuclear

membrane holds DNA

Why so small?

Metabolism requires that cells stay small As a cell grows, its

volume grows proportionately more than its surface area

Cells need a high surface area to volume ratio to exchange materials with their environment through plasma membrane.

Compartmental Organization of Cells

Compartments (ORGANELLES) provide different local environments (pH, etc.)Incompatible but equally important

processes can occur next to each other in different “rooms”

Cytoskeleton & Related Organelles

Cytoskeleton Maintains shape of cell Responsible for

movement of cell and movement of organelles within cell

Made of three types of protein fibers:

Microtubules, microfilaments, & intermediate filaments

Cytoskeleton & Related Organelles

Microtubules Hollow tubes Made up of A- and B-

tubulin Responsible for:

• Cell motility• cilia/flagella

• Chromosome movements (mitosis)

• centrioles

• Movement of organelles

• Maintenance of cell shape

Cytoskeleton & Related Organelles

Components of Cytoskeleton:Microtubules – 25 nm diameterIntermediate Filaments – 8 – 12 nm

diameterMicrofilaments – 7 nm diameter

Cytoskeleton & Related Organelles

Intermediate Filaments Made up of fibrous

proteins Made up of keratin Responsible for:

• Structural support• Maintenance of cell

shape• Anchors nucleus &

certain organelles

Cytoskeleton & Related Organelles

MicrofilamentsMade up of 2 intertwined strands of

actinResponsible for:

• Muscle contraction• Cytoplasmic streaming• Cell motility (pseudopodia)• Cell division (cleavage furrow)• Maintenance of/changes in cell shape

Centrioles Only found in animal

cells Visible only during cell

division 9+0 arrangement of

microtubules May give rise to cilia &

flagella May be involved in

formation of spindle fibers in animal cells, but not plants!

Flagella and Cilia

Structures for cell motility

Flagella (long & few in #)

Cilia (short & numerous)

9 + 2 internal structure

Basal body has 9+0

structure like centrioles

dynein

microtubule

Figure 4.25Page 73

Cellular Organelles

Nucleus: “control center” of the cell Surrounded by a nuclear

envelope Contains DNA Nucleolus: site of ribosome

synthesis

Cellular Organelles

Ribosomes Site of protein assembly Free and bound ribosomes

• Free: float through cytoplasm (make proteins for use inside that cell)

• Bound: attached to Rough ER (make proteins to be transported out of the cell)

Cellular Organelles

Endoplasmic Reticulum:Made up of membranous tubules and

cisternae (sacs)Smooth ER: NO ribosomes attached

• Synthesis and transport of lipids• Controls glucose glycogen conversion in

liver & muscles• Detoxification of drugs and other poisons• Sarcoplasmic reticulum (muscle ER) stores

calcium needed in muscle contraction.Rough ER: ribosomes attached

• Synthesis & transport of proteins

Endomembrane System

Smooth and Rough ER

Endomembrane System

Golgi Apparatus: Products of the

Endoplasmic Reticulum are modified and stored here

Modifies & packages proteins

Endomembrane System

Lysosomes: Used by cells to

digest macromolecules

Sac of hydrolytic enzymes

Apoptosis: • Programmed cell

death Usually found only

in animal cells

Endomembrane System Vacuoles:

Food vacuoles (storage)

Contractile vacuoles (pump extra water out of cells in freshwater protists)

Central vacuole (plant cells)

• Stores organic compounds, inorganic ions (K+, Cl-), and water

• Surrounded by tonoplast

Endomembrane System

Peroxisomes:Contain enzymes that transfer

hydrogen from various substances to oxygen, producing H2O2 as a byproduct

Various functions:• Break fatty acids down into smaller

molecules for cellular respiration• Detoxify alcohol in liver

Energy-related organelles Mitochondria

Site of cellular respiration (Energy from the breakdown of organic molecules is used to phosphorylate ADP to produce ATP)

“powerhouse of the cell”

More metabolic activity = more mitochondria

Energy-related organelles

Mitochondrial Structure: Outer membrane Inner membrane:

• Cristae = large surface area makes more efficient at producing energy

Intermembrane space

Mitochondrial matrix

Energy-related organelles

Chloroplasts: Found in plants and

eukaryotic algae Site of photosynthesis Contain the green

pigment chlorophyll

Energy-related organelles Chloroplast

Structure Thylakoids

• Grana = stacks of thylakoids

• (Light Dependent Phase)

Stroma• Fluid outside the

thylakoids• (Calvin Cycle)

Cellular Organelles

Cell Wall Found only in plant cells Protects the cell Maintains

cell shape Prevents excessive uptake

of water Holds plant up against

gravity Primary Cell Wall-thin;

cellulose Secondary Cell Wall-

thicker; found in woody plants

Cellular Organelles

Extracellular Matrix: Found in animal cells Made up of

glycoproteins (collagen) & proteoglycans

• Proteins + carbohydrates

Provides support and anchorage for cells

Differs from one cell type to another

Intercellular Junctions

Neighboring cells are connected to one another

Plant cells: Plasmodesmata:

• Channels in the cell wall through which strands of cytoplasm pass through and connect the living contents of adjacent cells

Intercellular Junctions (Animal Cells)

Tight junctions-membrane proteins interlock

Desmosomes, (anchoring junction)-intermediate filaments “sew” membranes together

Gap junctions- channels align allowing materials to flow between cells

Intercellular Junctions

Tight junctions: Membranes of

neighboring cells are fused

Form a continuous “belt” around a cell

Example: junction between epidermis of the skin

Intercellular Junctions Desmosomes

Anchoring junctions

Act as rivets Muscle cells

are held together by desmosomes.

What happens when a muscle is torn?

Intercellular Junctions

Gap junctions Communicating

junctions Cytoplasmic

channels between adjacent cells

Salts, sugars, AAs, etc. can pass through

Membrane Structure & Function

Functions of the Cell Membrane

Isolates the cytoplasm from the external environment

Regulates the flow of materials into and out of the cell

Communicates with other cells

The Fluid Mosaic Model Currently accepted model of

the cell membrane Proposed by Singer and

Nicolson in 1972 Phospholipid bilayer

Hydrophilic “head” – exposed to the outside

Hydrophobic “tail” – hides inside

Membrane proteins are randomly dispersed in phospholipid bilayer

Fluid Mosaic Model

Fluidity of the Membrane The lipids and

proteins can drift throughout the membrane

Membrane is NOT stiff/rigid

Cholesterol makes the membrane stronger by limiting the movement of phospholipids

Membrane as a Mosaic Lipid bilayer has

membrane proteins embedded in it

Integral proteins Go through the

membrane (both sides)

Peripheral proteins attached to the

surface of the membrane

Selective Permeability of the Cell Membrane

The cell membrane can “choose” what enters and exits a cell

“Gatekeeper of the Cell”

Passive Transport

Def’n: Diffusion of a substance that does

NOT require the input of energy by the cell

3 types of passive transport:DiffusionOsmosisFacilitated diffusion

Diffusion

Movement of molecules from high to low concentration until equilibrium is reached.

Passive Transport= no energy required

What substances may diffuse across membrane? Nonpolar (non-charged) molecules; small polar molecules

Diffusion Each substance

diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances

Diffusion

Does all movement stop once equilibrium is reached?? NO!! Equal rates in all

directions

Osmosis Def’n:

The passive transport of water across a selectively permeable membrane

Hyper-, hypo-, iso- tonic• RELATIVE TERMS!!

• Always referring to solute concentration

Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic)

Osmosis in Plant and Animal Cells Animal Cells:

Cell crenate/shrinks Cytolysis

• Occurs when a cell is in a hypotonic solution

• Water goes from solution into cell

Plant Cells: Turgid- vacuole

swells Flaccid-vacuole

shrinks (plasmolysis)

Facilitated Diffusion

Def’n:The diffusion of large molecules

across the cell membrane using transport proteins

Glucose; ions Does NOT require an input of energy

Solute is still moving down its concentration gradient

Facilitated Diffusion

Transport proteins are specific for their solutes

Transport proteins can become saturated

Some are gated channels: Chemical or electrical

stimulus causes them to open

Example

Which direction will sucrose move?

Which direction will glucose move?

Which direction will fructose move?

Pop Quiz – Passive Transport1. What is passive transport across

the cell membrane?

2. Describe 3 types of passive transport discussed yesterday.

3. If a blood cell is placed in a hypotonic solution what would be the result?

4. What type of solution is preferred in plant cells.

Active Transport

Def’n:The pumping of solutes against their

gradientsRequires an input of energy by the cellUsed so cells can “stockpile” extra

supplies (storage of any substance in a very high concentration; i.e. Iodine stored in thyroid; glycogen stored in liver

Sodium/Potassium Pump

Electrogenic Pumps

Voltage across membranes = stored energy that can be used for cellular work

Sodium-Potassium Pump:3 Na+ OUT of the cell for every 2 K+

pumped inNet transfer of one positive charge from

cytoplasm to extracellular fluid Very important for transferring signals

between nerve cells

Sodium-Potassium Pump

Cotransport

Substance that has been pumped across a membrane can do work as it “leaks” back by diffusion

Another substance “hitches a ride”

Endocytosis & Exocytosis

Def’n:The movement of large molecules

(polysaccharides, proteins, etc.) across the membrane using vesicles

Endocytosis = cell takes in macromolecules

Exocytosis = cell secretes macromolecules

Endocytosis Cell takes in macromolecules

by forming vesicles made from the plasma membrane

Phagocytosis = “cell eating” Large molecules

Pinocytosis = “cell drinking” Small molecules & liquids

Receptor-mediated endocytosis = seeks out specific molecules

Exocytosis

The cell secretes macromolecules by the fusion of vesicles with the plasma membrane

Used to release hormones, chemical signals, etc.

Pop Quiz 2

1. What is Active Transport across the cell membrane?

2. What does it mean when molecules are moving up the concentration gradient?

3. Describe (in detail) how the sodium potassium pump works in nerve cells.

4. What is the difference between phagocytosis and pinocytosis?