Cell Structure and Function

Topics Covered ( Cell Structure, Cell Membranes)

• Cell Structure: Subcellular Components
• Cell Structure and Function
• Cell Size
• Plasma Membranes
• Membrane Permeability
• Membrane Transport

• Facilitated Diffusion
• Tonicity and Osmoregulation
• Mechanisms of Transport
• Cell Compartmentalization
• Origins of Cell Compartmentalization

Cell Structure: Function dictates Form

All Cells Have:

• Plasma membrane
• Semifluid substance called cytosol (cytoplasm is cytosol and organelles)
• Chromosomes (DNA)
• Ribosomes (make proteins)

General Definitions:

• chromosome - a single DNA molecule associated with proteins
• chromatin – all of the DNA and proteins of a cell
• gene –a section of DNA that codes for a trait

Cell Size:

• The surface area of the cell membrane must be able to accomodate the metabolic needs, based of the volume, of the cytoplasm
• Lower surface area to volume ratio increases efficiency by eliminating waste and procuring nutrients faster

Prokaryote:

• Nucleoid: contains most of prokaryotic DNA (which is circular)
• Membrane, Cell wall, Capsule
• Metabolism can be anaerobic or aerobic
• Mainly unicellular

Eukaryote: Cell with internal compartments made by membranes, nucleus, membrane bound organelles.

Compartmentalization: increases the efficiency of many subcellular processes by concentrating the required components to a confined space within the cell. Where a specific condition is required to facilitate a given subcellular process, this may be locally contained so as not to disrupt the function of other subcellular compartments.

• Origin:
  • Although prokaryotes generally lack internal membrane bound organelles, they have internal regions with specialized structures and functions.
  • Dogma that eukaryotes evolved from prokaryotes is supported by the endosymbiosis theory which states:
    • Chloroplasts and Mitochondria have:
      • 2 membranes (1 itself and another from the host that engulfed it, and another)
      • Have their own circular DNA and Ribosomes
      • Reproduce like ancient bacteria. They are not produced by the cell itself but passed from the parent cell.

Mitochondria: sites of cellular respiration (uses oxygen to generate ATP)

	2 membranes Outer: smooth, Inner: convoluted and with internal foldings called cristae 2 spaces created by membranes Outer Space: intermembrane space Inner Space: mitochondrial matrix -> contains ribosomes, DNA, and enzymes

Chloroplast: sites of photosynthesis (found in plants and algae)

	Contain chlorophyll, enzymes, molecules, and helps sugar production 2 membranes with a space in between Thylakoid: flattened, interconnected sacs Stack of thylakoids -> granum Stroma: fluid outside thylakoids. Contain chloroplast DNA, ribosomes, and other enzymes Chloroplasts are a type of plastid

Nucleus: Information Central

• Contain most of the cell’s genes (others in mitochondria and chloroplast)
• Nuclear Envelope: encloses nucleus, separating it from cytoplasm. It is selectively permeable
  • Double membrane
  • Pores regulate exit and entry genetic material and macromolecules in and out of the cell.
• Nucleolus: synthesize ribosomal RNA from the directions given by DNA
  • Nucleolus combines proteins imported from the cytoplasm with rRNA made in the nucleolus for both large and small ribosome subunits

Ribosomes: Protein Factories

• Made of rRNA and protein
• Carry out protein synthesis by producing proteins with information from mRNA
• Found in 2 locations
  • Free ribosomes found in cytosol
    • Most proteins made here stay within the cytosol
  • Bound Ribosomes attached to Endoplasmic Reticulum. Proteins made here are meant for export out of the cell

Endomembrane System: The following components are either continuous, or connected by transport performed by vesicles

• Endoplasmic Reticulum: Directly connected with nuclear envelope
  • ER membrane separates the internal compartment of the ER (lumen) from the cytosol
  • Smooth ER: synthesize lipids, metabolize carbohydrates, detoxify drugs and poison
  • Rough ER: Secretory proteins are made by ribosomes on ER surface
    • The secretory proteins are then threaded into the interior of the Rough ER to be modified and transported
      • Many secretory proteins are glycoproteins (carbs that are attached to proteins in the ER by enzymes built into the ER membrane)
    • Secretory proteins then depart from the ER wrapped in membranes of vesicles that bud from transitional ER (exit site of proteins leaving rough ER
      • Vesicles in transit from one part of the to another are called transport vesicles
• Golgi Apparatus: Products of ER are modified and stored and then sent out to other destinations
  • Transport vesicles come here after leaving rough ER
    • Transport vesicle enters through the cis face of the Golgi stacks (recieving side that is located near the ER)
    • The trans face (dispatching side) gives rise to vesicles that pinch off and travel to other sites
• Lysosome: membranous sac of enzymes that digest (hydrolyze) macromolecules
  • These work optimally in an acidic environment
  • Some cells like protists, eat by engulfing another cell through phagocytosis
    • The food vacuole formed this way fuses with a lysosome which digests the food
  • Lysosomes also use their enzymes to recycle the cell’s own organic material in a process called autophagy
• Vacuoles: large vesicles derived from ER and Golgi
  • Food Vacuole: hold food
  • Contractile Vacuole: pump excess water out of cells of freshwater protist
  • Central Vacuole: hold organic compounds and water in plant cells

Summary: Relationships among organelles in the Endomembrane system

Cytoskeleton: network of protein fibers

1. Maintains cell shape
2. Controls position of organelles by anchoring them to the plasma membrane
3. Involved in cytoplasmic streaming
   1. Cytoplasm Streaming: cytoplasm moves within cell, moving nutrients and wastes through cell
4. Anchors the cell when interacting with extracellular elements

Microtubules: hollow tubes made of the protein tubulin that make up cilia, flagella and spindle fibers

Microfilaments: assembled from actin filaments and help support the shape of the cell

• Enable animals to form cleavage furrow during cell division
• Help amoeba to move by sending out pseudopods
• Help skeletal muscle to contract as they slide along myosin filaments

Extracellular Components:

Centrioles, centrosomes, and microtubule organizing centers and non membranous structures that lie outside the cell

• Organize spindle fibers and give rise to the spindle apparatus
• 2 centrioles oriented at right angles to each other make up of one centrosome
  • Each centriole consist of 9 triplets of microtubules arranged in a circle
• Plants lack centrosomes, but have microtubule organizing centers
• In animal cells, the MTOC is synonymous with a centrosome

Cell Wall: protects cell, maintains shape, prevents excessive water uptake

• Everything other than animal cells have this
• Plant cell wall is made of cellulose while fungi cell walls are made of chitin

Extracellular Matrix: found in animal cells, made of glycoproteins. Supports, moves, and regulates the cell.

Cell Junctions: neighboring cells in tissues, organs, organ systems often adhere, interact and communicate through direct physical contact facilitated by junctions

Cell Membrane

Phospholipids: primary lipid in the bilayer. These lipids are amphipathic, meaning they contain hydrophilic and hydrophobic regions (hydrophilic heads and hydrophobic tails)

Membranes are Selectively Permeable

Fluidity: Membranes must be fluid to work properly

• fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character
• Cooler temperature -> membrane switch from a fluid state to a solid state
• The temperature at which a membrane solidifies depends on the types of lipids
  • Unsaturated fatty acids: more fluid, saturated fatty acids: more solid

Cholesterol's effect:

• At warm temps (37°C), cholesterol restrains movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing membrane from becoming solid

Functions of Lipid Part of the Membrane:

• Serve as a physical boundary between intracellular and extracellular components
• Protects cell by regulating what goes into and out of the cell

Functions of Carb And Protein Part of the Membrane:

• Attachment to cytoskeleton and extracellular matrix
• Transport
• Enzymes
• Signal Transduction
• Intercellular joining
• Cell to Cell Recognition: cells recognize each other by binding to surface molecules on the extracellular side of the membrane. These molecules vary from different species and also vary from cell to cell in an organism.
  • Your immune system can recognize antigens (substance capable of stimulating immune response)
    • self – yours/don’t attack
    • foreign – not yours/may need to attack
• Receptors: allows cells to recognize each other from a distance (visit unit 4 Signal Transduction)
  • Only target cells have receptors for a particular hormone

Membrane Transport Mechanisms

Passive: Diffusion of a substance through a membrane without energy

• Substance diffuses from high concentral to low concentration because the concentration gradient serves a potential energy

Simple Diffusion:

• Through lipid bilayer
• Small uncharged particles and larger hydrophobic particles (lipids)

Osmosis: transport of water

• Happens through bilayer
  • Even though water is polar, there is a super high concentration of water outside the membrane, so it can slowly move inward
  • Aquaporins (channel protein)
    • Only water molecules can pass through these pores

Facilitated Diffusion:

• Aided by protein
  • Hydrophilic Protein Channel: used as a tunnel
  • Carrier Protein: Changes shape to move stuff from one side to another
• Allows polar and charged particles to move across

Active: Involves protein pumps and requires energy (ATP)

• move materials from low concentration to high concentration; move against concentration gradient

Ion Pumps maintain Membrane Potential

• Membrane Potential: voltage across a membrane.
  • Voltage: electrical potential energy; separation of opposite charges
  • Cytoplasmic side of the membrane is negative compared to the extracellular side due to unequal distribution of anion and cations on both sides. Thus, membrane potential favors the passive transport of anion out of the cell and cations into the cell.
• 2 chemical forces drive the diffusion of ions across a membrane: the ion’s concentration gradient, and membrane potential. This combination of forces acting on an ion is called the electrochemical gradient
  • The active transport of ions across the cell membrane causes an electrical gradient to build up across this membrane. The electrical chemical gradient is a source of potential energy that can drive the transport of other molecules.

Electrogenic Pump: a transport protein that generates voltage across a membrane

• • In animals, it is a sodium potassium pump, and in plants, fungi, and bacteria, it is a proton pump

Cotransport: membrane protein enables the “downhill” diffusion of one solute to drive the “uphill” transport of the other.

Pinocytosis: uptake of large dissolved particles (liquids)

Phagocytosis: engulfing of small cells or large particles by pseudopods

Receptor mediated endocytosis: enables cell to take up large quantities of very specific substances

Bulk Flow: general term for the overall movement of a fluid in one direction in an organism (such as human blood). It always moves from source (where it originates) to sink (where it is used).

Tonicity: ability of a surrounding solution to cause a cell to gain or lose water

Hypotonic: more solutes in the cell than solution. Water diffuses into the cell

• More water solution than cell
• Plant Cell: swells until wall opposes uptake
• This makes the cell turgid (which is normal for a plant)
  • • When the guard cells (cells that control the opening and closing of stomata) absorb water by osmosis, they become turgid and curve, allowing stoma to open due to the high pressure inside them
• Freshwater Protist Cell: have contractile vacuole to get rid of excess water
• Animal Cell: bad for animal cells because the cell will lyse, making it burst (cytolysis)

Isotonic: equal concentration of solutes inside cell and solution

• exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell
• Plant Cell: no net movement of water into the cell, so the cell becomes flaccid (limp) and the plant wilts because the guard cells around the stoma close
• Animal Cell: perfect for animals

Hypertonic: more solute in solution than cell. Water diffuses out of the cell

• Plant Cell: plasmolysis occurs meaning plant cell loses water resulting in the membrane pulling away from the cell wall (this is lethal)
• Animal Cell: dessicate

Osmoregulation: maintains water balance and allows organisms to control their internal solute

composition/water potential.

• Growth and homeostasis are maintained by the constant movement of molecules across membranes.

Water Potential and Solute Potential:

	A low water potential means that water has a low force driving it to move from one area to another. High water potential means that a solution has more free water molecules compared to a solution with low water potential.