Cell Organelles

Eukaryotes - have (most) DNA stored in the nucleus, an organelle that is bounded by a double membrane.

Prokaryotes - characterized by having:

• no nucleus

• DNA in an unbound region called the nucleoid

• No membrane-bound organelles

All cells have ribosomes. Both types of cells contain cytoplasm bound by the plasma membrane. Cells are the organization of life.

Cells →Tissues → Organs→ Organisms.

Cells have 3 main jobs:

• make energy

  • need energy for all activities

  • need to clean up waste produced while making energy

• make proteins

  • proteins do all the work in a cell, so we need lots of them

• make more cells

  • for growth

  • to replace damaged or diseased cells

Organelles - functioning parts of a cell that do the work of keeping cells alive (tiny organs)

Cytoplasm - Semi-fluid substance that fills the interior of the cell and suspends organelles

How do cells power our bodies?

Cell membrane:

• Function:

  • separates cell from outside

  • controls what enters or leaves cell

  • recognizes signals from other cells (communication between cells)

• Structure

  • double layer of lipids

    • phospholipid bilayer

  • receptor molecules

    • proteins that receive signals

Vacuoles & Vesicles:

• Function:

  • moving material around cell

  • storage

• Structure:

  • membrane sac

Lysosomes:

• Function:

  • digest food

    • used to make energy

  • clean up and recycle

    • digest broken organelles

• Structure:

  • membrane sac of digestive enzymes

Mitochondria

• Function

  • make ATP energy from sugar via cellular respiration

    • sugar + O2 → ATP

    • fuels the work of life

• Structure

  • double membrane

• In both animal and plant cells

• They have a smooth outer membrane and an inner membrane folded into cristae

• The inner membrane creates two compartments: intermembrane space and mitochondrial matrix

• Cristae presents a large surface area for enzymes that synthesize ATP

Make Proteins

Nucleus:

• Function:

  • control center of the cell

  • protects DNA

    • instructions for building proteins

• Structure:

  • nuclear membrane

    • double membrane (lipid bilayer)

  • nucleolus (contained in the nucleus)

    • ribosome factory

  • chromosomes

    • condensed DNA

• Nucleolus: located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis makes ribosomes

• Nuclear membrane: a double membrane; separating it from the cytoplasm — each membrane consists of a lipid bilayer

• Nuclear pores: regulate the entry and exit of molecules (proteins)

Ribosomes:

• Function:

  • protein synthesis

  • read instructions to build proteins from DNA

• Structure:

  • complexes of rRNA and protein

  • some free in cytoplasm

  • some attached to ER

Endoplasmic Reticulum (ER):

• Function:

  • site of protein synthesis (Rough ER)

    • helps complete the proteins after ribosome builds them

  • makes membrane lipids (Smooth ER)

• Structure:

  • rough ER

    • ribosomes attached

  • smooth ER

    • no ribosomes

Golgi Apparatus:

• Function:

  • finishes, sorts, labels, & ships proteins

    • like UPS headquarters

      • shipping and receiving department

  • ships proteins in vesicles

    • “UPS trucks”

• Structure:

  • membrane sacs

Cell Membrane Structure

Semipermeable: a material or membrane that allows some substances to pass through, but not all things

Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions.

Most of the lipids and some proteins in a membrane can shift about laterally. As temperatures cool, membranes switch from a fluid state to a solid state, this phase change is based on lipid composition.

Cholesterol: regulates membrane fluidity - at warm temperatures (such as 37 C), restrains movement of phospholipids. At cool temperatures it maintains fluidity by preventing tight packing.

Integral proteins: penetrate the hydrophobic interior of the lipid bilayer (sometimes called embedded proteins)

Integral proteins that span the membrane are called transmembrane proteins (membrane spanning proteins in the POGIL)

Peripheral (surface) proteins are loosely bound to the surface of the membrane

Six major functions of membrane proteins

1. Transport

2. Enzymatic activity

3. Signal transduction

4. Cell-cell recognition

5. Intercellular joining

6. Attachment to the cytoskeleton and extracellular matrix (ECM)

Cell to cell recognition is possible by membranes binding to each other, the carbohydrates serve as identification markers. Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins)

Passive transport: the movement of a substance across a membrane with no energy investment (ex. diffusion or osmosis)

Diffusion: the net movement of molecules from an area of high concentration to an area of low concentration (down the concentration gradient)

Individual molecules are moving randomly, and will continue to do so through dynamic equilibrium

Cell Membrane Transport (passive)

Osmosis: the diffusion of free water across a selectively permeable membrane.

Low concentration → High concentration

Isotonic solution: solute concentration is the same as inside the cell; no net water movement across the plasma membrane

Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: solute concentration is less than that inside the cell; cell gains water

Osmoregulation: the control of solute concentration and water balance, is a necessary adaptation for life in such environments.

Membrane transport using proteins (and other mechanisms)

Facilitated diffusion: Passive transport aided by proteins (no energy, still high to low)

• in facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane

• Channel proteins: provide corridors that allow a specific molecule or ion to cross the membrane

• Channel proteins include

  • aquaporins, for facilitated diffusion of water

  • ion channels that open or close in response to a stimulus (gated channels)

    • needs another molecule to bond to the protein to change the shape to open up the channel

• Carrier proteins: undergo a subtle change in shape that translocates the solute-binding site across the membrane

  • the shape change may be triggered by binding and release of the transported molecule

  • no net energy input is required

Active transport: moves substances against their concentration gradients (low concentration to high). This requires energy, usually in the form of ATP.

• Sodium-potassium pump

Membrane potential is the voltage across a membrane. Voltage is created by difference in the distribution of positive and negative ions across a membrane — this is needed to make things happen. This can create an electrochemical gradient - difference in concentration of both chemicals and charges across a membrane.

Electrogenic pump: a transport protein that generates voltage across a membrane. Ex: proton pump

Cotransport: occurs when active transport of a solute indirectly drives transport of other solutes

• plant cells use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

Exocytosis: transport vesicles migrate to the membrane, fuse with it, and release their contents.

Endocytosis: the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane

There are three types of endocytosis:

1. Phagocytosis (“cellular eating”)

2. Pinocytosis (“cellular drinking”)

3. Receptor-mediated endocytosis (need ‘keys’ to start endocytosis)

Scientific Theories of Cells

Hypothesis: a proposed explanation for a set of observations that can be tested.

Scientific Theory: an in-depth explanation of an observed phenomenon, that is supported with repeated testing and evidence

Cell Theory:

1. All living things are made of cells

2. Cells are the basic unit of structure and function to life

3. All new cells come from pre-existing cells

Endosymbiotic Theory: an explanation of the evolution of membrane bound organelles in eukaryotic cells. Some modern organelles such as the mitochondria were once prokaryotic microbes.

How did eukaryotic cells evolve?

1. infoldings of the cell membrane gave rise to the endomembrane system and the formation of the endoplasmic reticulum and nucleus

2. Ancestral eukaryotic cell engulfs (endocytosis) but does not consume aerobic bacteria that evolves into a mitochondria. A similar process occurred with photosynthetic bacteria that evolved into chloroplasts

What evidence supports endosymbiotic theory?

• mitochondria and chloroplasts are the same size as prokaryotic cells and divide by binary fission

• Mitochondria and chloroplasts have their own DNA which is circular, not linear

• Evidence of prokaryotic life 3.5 billion years ago. Evidence of eukaryotic life 1.5 billion years ago