Unit 2 Cell Structure and Function

How do ribosomes reflect the common ancestry of all known life?

Its fundamental structure, composition (rRNA + protein), and function (protein synthesis) are conserved across all Bacteria, Archaea, and Eukarya, meaning it evolved early and was passed down, with slight modifications, to all life from a Last Universal Common Ancestor (LUCA), essentially acting as a universal molecular machine.

Subcellular components ___ to all___

universal; cells

• All living cells contain a genome and ribosomes, reflecting the common ancestry of all known life

Role of the ribosome

Synthesis protein according to mRNA sequence and the instructions that are encoded in that mRNA sequence originate from the genome of the cell

• not membrane-enclosed structure

• consist of two subunits THAT ARE NOT MEMBRANE-ENCLOSED

• made of divisional RNA(rRNA) and proteins

Endoplasmic reticulum

• Network of membrane tubes within the cytoplasm of eukaryotic cells

• Two forms of ER

1. Rough ER

   • Has ribosomes attached to its membrane

   • compartmentalizes the cell (rough ER is associated with packaging the newly synthesized proteins made by attached ribosomes for possible export from the cell)

     • provides mechanical support

     • plays a role in intracellular transport

     • Rough ER carries out protein synthesis on ribosomes that are bound to its membrane

1. Smooth ER

• Does NOT have ribosomes attached

• Functions include detoxification and lipid synthesis

Golgi complex

• series of flattened membrane-bound sacs found in eukaryotic cells

• Involved in the correct folding and chemical modification of newly synthesized proteins and packaging for protein trafficking

  

Mitochondria

• Has a double membrane

• The outer membrane is smooth, and the inner membrane is highly convoluted, forming folds called cristae, allowing more ATP to be made

• Functions in the production of ATP energy that eukaryotic cells can use for cell work

• The Krebs cycle (citric acid cycle) reactions occur in the MATRIX of the mitochondria

• ELECTRONS transport and ATP SYNTHESIS occur in the INNER mitochondrial membrane

lysosome

• Membrane-enclosed sacs found in some eukaryotic cells that contain hydrolytic enzymes

• Hydrolytic enzymes can be used to digest a variety of materials, such as damaged cell parts or macromolecules

  • intracellular digestion

  • recycling of organic materials

  • programmed cell death (apoptosis)

Vacuole

• Membrane-bound sacs found in eukaryotic cells

• Play a variety of roles, ranging from storage of water and other macromolecules to the release of waste from a cell, macromolecules, and cellular waste products

• In plants, vacuoles aid in retention of water for turgor pressure

Chloroplast

• Found in eukaryotic cells such as photosynthetic algae and plants

• Double outer membrane

• Specialized for capturing energy from the sun and producing sugar for the organism

Chloroplast compartments (Thylakoid)

• Highly folded membrane compartments that are organized in stacks called grana

• Membranes contain chlorophyll pigments that comprise the photosystems, and electron transport proteins can be found between the photosystems, embedded in the thylakoid membrane

• Light-dependent reactions occur here

• Folding of these internal membranes increases the efficiency of these reactions

Chloroplast compartments (Stroma)

• Fluid between the inner chloroplast membrane and outside thylakoids

• The carbon fixation (Calvin Cycle) reactions occur here

Turgor pressure!

Internal cellular force, usually caused by water pushing up against the plasma membrane and cell wall

Why are cells typically small?

• Smaller cells have a HIGHER SA:V RATIO and more EFFICIENT exchange of materials with the environment.

• Cells with higher volume will decrease SA, making it difficult to meet the demand for internal resources and remove enough waste, RESTRICTING cell size and shape.

How is the surface area to volume ration calculated?

• SURFACE AREA EQUATION: 6s²

• VOLUME EQUATION: s³

• SA TO VOLUME RATIO: SA/V

• As the cells got LARGER, the SA to V ratio DECREASES.

Cells are typically small, but what happens when the cell gets larger?

• Moving materials (nutrients and waste) in and out of cells gets more difficult the larger a cell is.

• Since there was a higher concentration of bleach outside the cubes, bleach was able to move across the surface of each cube

  

96%, 66%, 39%

What are some examples of structural modifications of cells that increase surface area?

• Microvilli, which are the tiny folds in the intestinal cells

• Villi, which are finger-like projections on the tissue

How does the surface area to volume ration effect the rate of heat exchange with the environment?

• A higher surface area to volume ratio leads to a faster rate of heat exchange, meaning quicker heat loss or gain, while a lower ratio results in slower heat exchange

• Allowing for better heat retention, which explains why small organisms lose heat rapidly but large ones retain it more effectively, influencing adaptations to different climates

How are specialized structures and strategies used by cells and organisms for the efficient exchange of molecules with the environment?

• Using specialized structures like villi, microvilli, and stomata, it increases surface area and reduces diffusion distance, alongside strategies such as active transport and compartmentalization

Effects of surface area to volume ratios on the exchange of materials

• The surface area of the plasma membrane must be large enough to adequately exchange materials.

• Smaller cells typically have a higher surface area-to-volume ratio and more efficient exchange of materials with the environment

• As cells increase in volume, the relative surface area decreases, and the demand for internal resources increases

  • O2, nutrients, heat, wastes, CO2 (increases)

Membrane folding increases surface area (in root hairs)!

• Root hairs on the surface of plant roots increase the surface area of the root, allowing for increased absorption of water and nutrients

Membrane folding increases surface area (intestines)!

• After chemical digestion occurs in the digestive tract and macromolecules are broken down into monomers, the monomers need to be transported across the surface of the small intestine and into the bloodstream so they can be transported to cells

• The outer lining of the small intestine is highly folded, containing finger-like projections called villi

• The surface of each villi has additional microscopical projections called microvilli, which further increase the surface area available for absorption of nutrients.

• If a condition arises that leads to the loss of this folding, these cells would not be as efficient in absorbing nutrients for the organism

Membrane folding increases surface area (heat exchange)!

• As organisms increase in size, their surface area to volume ratio decreases, affecting properties like the rate of heat exchange with the environment

• The flattened shape of the ear allows the elephant to dissipate more thermal energy as blood flows closer to the surface

  

Membrane folding increases surface area (leaf)!

• Organisms have evolved highly efficient strategies to obtain nutrients and eliminate wastes

• Cells and organisms use specialized exchange surfaces, such as stomatal openings of leaves, to obtain molecules from and release molecules into the surrounding environment

• When stomata are open, CO2 can enter the leaf and O2 and H2) can be released into the atmosphere

Cells have membranes that allow them to establish an internal environment

• Cell membranes provide a boundary between the interior of the cell and the outside environment

• Allows the cell membranes to control the transport of materials in and out of the cell

Phospholipids have both hydrophilic and hydrophobic regions

• Phospholipids are amphipathic

  • Hydrophilic phosphate head is polar

  • Hydrophobic fatty acid tail is nonpolar

• Phospholipids spontaneously form a bilayer in an aqueous environment

  • Tails are located inside the bilayer

  • Heads are exposed to the aqueous outside and aqueous inside environments

Embedded proteins can be hydrophilic or hydrophobic

• Peripheral proteins (2)

  • Loosely bound to the surface of the membrane

  • Hydrophilic with charged and polar side groups

• Integral proteins (3)

  • span the membrane

  • Hydrophilic with charged and polar side groups

  • Hydrophobic with non-polar side groups penetrate the hydrophobic interior of the bilayer

    • Example: Transmembrane proteins

Embedded proteins play various roles in maintaining the internal environment of the cells

• membrane protein functions

  • 1. Transport

  • 2. Cell-cell recognition

  • 3. Enzymatic activity

  • 4. Signal Transduction

  • 5. Intercellular joining

  • 6. Attachment for extracellular matrix or cytoskeleton

Fluid Mosaic Model

• Structured as a mosaic of protein molecules in a fluid bilayer of phospholipid

• The structure is not static and is held together primarily by hydrophobic interactions, which are weaker than covalent bonds

• Most lipids and some proteins can shift and flow along the surface of the membrane or across the bilayer

Fluid Mosaic Model (steroids)

• Cholesterol, a type of steroid, is randomly distributed and wedged between phospholipids in the cell membrane of eukaryotic cells

• Cholesterol regulates bilayer fluidity under different environmental conditions

Fluid Mosaic Model (carbohydrates)

• Diversity and location of the (molecules) carbohydrates and lipids enable them to function as markers

  • Glycoproteins- one or more carbohydrates attached to a membrane protein

  • Glycolipids- lipids with one or more carbohydrates attached

Can hydrophilic substances move freely across the membrane

• Hydrophilic substances, such as large polar molecules and ions, can NOT freely move across the membrane

• Hydrophilic substances move through transport proteins

  • Channel Proteins- A hydrophilic tunnel spanning the membrane that allows specific target molecules to pass through

  • Carrier Proteins- Span the membrane and change shape to move a target molecule from one side of the membrane to the other

• Small polar molecules, like H2O, can pass directly through the membrane in minimal amounts

The cell wall is a structural boundary and permeable barrier

• As a structural boundary:

  • Protects and maintains the shape of the cell

  • Prevents against cellular rupture when internal water pressure is high

  • Helps plants stand up against the force of gravity

• As a permeable barrier

  • Plasmodesmata- small holes between plant cells that allow the transfer of nutrients, waste, and ions

• Animal cells DO NOT have cell walls

Cell walls are composed of complex carbohydrates

• Cell Wall- comprised of complex carbohydrates

  • Plants- Cellulose

    • Polysaccharide

  • Fungi- Chitin

    • Polysaccharide

  • Prokaryotes- peptidoglycan

    • Polymer consisting of sugar and amino acids

Selectively permeable membranes allow for the formation of concentration gradients

• Concentration gradient

  • A concentration gradient is when a solute is more concentrated in one area than another

  • A membrane separates two

    

    different concentrations of molecules

Passive transport is the net movement of molecules from high to low concentration

• Net movement of molecules from high concentration to low without metabolic energy, such as ATP, is needed

• Plays a primary role in the import of materials and the export of wastes

• Diffusion- movement of molecules from high concentration to low concentration

  • Small non-polar molecules pass freely (N2, O2, CO2)

• Facilitated Diffusion- movement of molecules from high concentration to low concentration through transport proteins

  • Allows for hydrophilic molecules and ions to pass through membranes

Active transport requires energy

• Active transport requires the direct input of energy (such as ATP) to molecules from regions of low concentration to regions of high concentration

Endocytosis requires energy to move large molecules into the cell

• In endocytosis, the cell uses energy to take in macromolecules and particulate matter by forming new vesicles derived from the plasma membrane

  • Phagocytosis- a cell takes in large particles

• Pinocytosis- a cell takes in extracellular fluid containing dissolved substances

• Receptor-mediated endocytosis: receptor proteins on the cell membranes are used to capture specific target molecules

Exocytosis requires energy to move large molecules out of the cell

• In exocytosis, internal vesicles use energy to fuse with the plasma membrane and secrete large macromolecules out of the cell

  • Proteins such as signaling proteins

  • Hormones

  • Waste

Membrane proteins are necessary for facilitated diffusion

• Facilitate Diffusion- movement of molecules from high concentration to low concentration through transport proteins

  • Large and small polar molecules

  • Large quantities of water can pass through aquaporins

  • Charged ions, including Na+ and K+ require channel proteins

What does the cell membrane allow?

Membranes may become polarized by the movement of ions

• The cell membrane allows for the formation of gradients

  • Electrochemical gradient

    • Type of concentration gradient

    • Membrane potential- electrical potential difference (voltage) across a membrane

• Membranes may become polarized by the movement of ions across the membrane

How does Na+/K+ ATPase contribute to the membrane potential

• Na+/K+ ATPase (Na+/K+ Pump) contributes to the maintenance of the membrane potential

  • 3 Na+ Pumped

  • 2 K+ Pumped

How is water moved by osmosis?

• Osmosis is the diffusion of free water across a selectively permeable membrane

  • Large quantities of water move via aquaporins

• Osmolarity is the total solute concentration in a solution

  • Water has high solvency abilities

  • Solute is the substance being dissolved

  • A solvent is a substance that dissolves a solute

  • Solution is a uniform mixture of one or more solutes dissolved in a solvent

    • (solvent + solute = solution)

What is tonicity, and what are its effects?

• Tonicity is the measurement of the relative concentrations of solute between two solutions (inside and outside of the cell)

• Internal cellular environments can be hypotonic, hypertonic, or isotonic to external environments.

  • Hypertonic

    • More Solute and less solvent

• Isotonic

  • Equal concentration of solute and solvent

• Hypotonic

  • Less Solute and more solvent

What are tonicity effects on a cell’s physiology

• Water moves by osmosis into the area with a higher solute concentration

  • Water concentration and solute concentration are inversely related

  • Water would diffuse out of a hypertonic environment

  • Solutes diffuse along their own concentration gradients, from the hypertonic environment into the hypotonic environment

• When a cell is in an isotonic environment, a dynamic equilibrium exists with equal amounts of water moving in and out of the cell at equal rates

  • No net movement of water takes place

How does osmoregulatory mechanisms contribute to survival?

• In plant cells, osmoregulation maintains water balance and allows control of internal solute composition/water potential

  • Environmental Hypertonicity

    • Less cellular solute and more cellular war

    • Plasmolysis

  • Isotonic Solution

    • Equal Solute and Water

    • Flaccid

  • Environmental Hypotonicity

    • More cellular solute and less cellular water

    • Turgid

How do osmoregulatory mechanisms contribute to survival (plants)?

• The cell wall helps maintain homeostasis for the plant in environmental hypotonicity

  • Osmotic pressure is high outside of the plant cell due to environmental hypotonicity

  • Water flows into the plant vacuoles via osmosis, causing the vacuoles to expand and press against the cell wall

  • The cell wall expands until it begins to exert pressure back on the cell; this pressure is called turgor pressure

  • Turgidity is the optimum state for plant cells

How do osmoregulatory mechanisms contribute to survival (animals)?

• In animal cells, osmoregulation maintains water balance and allows control of internal solute composition/water potential

  • Environmental Hypertonicity

    • Less cellular solute and more cellular water

    • Shriveled

  • Isotonic Solution

    • Equal solute and water

    • Normal

  • Environmental Hypotonicity

    • More cellular solute and less cellular water

    • Lysed

What are the components of an effective graph?

• Title

• Labeled axes with units

• Scaling— uniform intervals

• Identifiable lines or bars

• Trend lines

What are the different types of graphs?

• Line Graph

  • Trends or progress OVERTIME for multiple groups or treatments

• X Y Graph

  • Scatterplot

  • Compare/Determine linear relationship between two variables

• Histograms

  • How the data is spread out across equal-sized ranges

  • Relationship between two OR MORE variables

• Bar Graph

  • Compare multiple groups or treatments to each other

• Box and Whisker Plot

  • Show VARIABILITY in a sample

  • Compare distributions in relation to mean

• Dual Y

  • Illustrate relationship between TWO DEPENDENT variables

When should graphs be used?

When you are given collected data.

• The graph type used is based on the type of data collected.

How does water potential impact the movement of water?

Water moves by osmosis (MORE to LESS water potential)

• Water potential shows how likely water is to move.

• It depends on two things:

  • Pressure potential (pushing force)

  • Solute potential (how much stuff is dissolved in the water)

So, water moves based on pressure and how the solution is.

• Values of water potential can be +, -, or 0

• More negative the water potential, the more likely water will move into the area.

  • EXAMPLE: If the water potential inside the cell is –3 bars and outside the cell is –6 bars, water will move out of the cell. This is because water moves from higher water potential (less negative) to lower water potential (more negative), so it moves toward the outside of the cell.

How does solute potential impact the movement of water?

In an open system, the pressure potential is ZERO, so water potential = solute potential

• Solute Potential = -iCRT

  • i = ionization constant

  • C = molar Concentration

    • Molarity (M) = moles of solute/volume of solution

  • R = pressure constant (L*Bars)/(mol*K)

  • T = temperature in KELVIN (°C + 273 = Kelvin)

• Addition of solutes is equal to a more NEGATIVE solute potential

How do organisms maintain water balance?

Water will move from the soil into the roots since the soil has higher water potential than the roots.

What is water potential of pure water?

• Has a value of zero (0) in an OPEN CONTAINER because there is no solute and no pressure.

What allows organism to control their internal solute composition and water potential?

OSMOREGULATION

• Increasing the amount of solute in water will cause

  • An increase in solute potential

  • A decrease in water potential

• Increasing water potential will cause

  • An increase in pressure potential

• Decreasing pressure potential will cause

  • A decrease in water potential

What is active transport?

Movement of molecules or ions against the concentration gradient.

• Ions move from LOW to HIGH concentration.

• Active transport needs energy (like ATP) to move ions and molecules across the cell membrane.

  • This energy is used to build and maintain concentration differences (electrochemical gradients) across the membrane.

Why are membrane proteins necessary for active transport?

The membrane blocks most substances, so proteins are needed to physically move molecules across using energy.

• Allows Na+ and K+ to pass through at certain binding sites.

• Serves as a “gate” or “channel.”

  • REMEMBER: Phospholipid bilayer:

    • HYDROPHOBIC tails (water-hating)

    • HYDROPHILIC heads (water-loving) — INTERACTS with the ions which are HYDROPHOBIC

      • These TWO clash, so a MEMBRANE PROTEIN is NECESSARY

How does the Na+/K+ pump and ATPase maintain membrane potential?

The Na+/K+ pump and ATPase balance sodium and potassium levels on each side of the membrane.