Topic 1: Cell Physiology Lectures 1-5

What is the plasma (cell) membrane?

Surrounds cell surface + separates intracellular fluid from extracellular fluid.

What are cell organelles?

• performs specific functions + compartmentalize

• are membrane-bound (enclosed by at least one lipid bilayer) or non-membrane bound

What are the 2 regions in the interior of the cell?

1. nucleus: houses genetic material (considered organelle but separate category)

2. cytoplasm: region outside of nucleus

   1. organelles suspended in cytosol (gel-like fluid)

What are the 4 functions of plasma (cell) membranes?

1. physical barrier

   1. keeps organelles + proteins inside cell

   2. maintains difference in fluid composition between ECF/ICF

      1. used to drive membrane transport processes

   3. maintains homeostasis

2. cell-to-cell communication

   1. has receptors that bind specific signalling molecules arriving at cell surface

3. structural support

   1. specialized connections between plasma membranes or between membranes + extracellular materials

4. transport

   1. selectively permeable ➡ plasma membrane maintains unique composition of cytosol

What do biological membranes surround in cells?

They surround the cell (plasma/cell membrane) and most intracellular organelles.

What’s the basic structure of all biological membranes?

A double layer of lipid molecules with embedded proteins (phospholipid bilayer).

Are all biological membranes made the same + if so, what does that reflect?

• no, they differ in their ratios of lipids and proteins

• reflects different functions of specific cells or organelles

What elements do lipids primarily contain?

• carbon + hydrogen, with some oxygen + phosphates

  • depending on structure, oxygen + phosphates may lead to polar areas

What property do membrane lipids have?

Amphipathic: polar, hydrophilic head + non-polar, hydrophobic tails.

What are the main types of membrane lipids?

1. phospholipids

2. cholesterol

3. glycolipids

What are the components of the polar head group in a phospholipid?

Phosphate group attached to glycerol backbone + nitrogen-containing chemical group (R).

Why is the head of a phospholipid hydrophilic?

The phosphate + nitrogen-containing chemical group are charged or polar, making them water-loving + having uneven distribution of electrical charge (positive + negative ends)

What forms the non-polar region of a phospholipid?

2 fatty acid chains made of carbon + hydrogen.

What is the difference between saturated + unsaturated fatty acid chains in phospholipids?

• saturated: no double bonds

• unsaturated: double bonds ➡ kink or bend in chain

How do phospholipids arrange themselves in water?

Spontaneously form a bilayer.

In a phospholipid bilayer, how are the polar heads + non-polar tails oriented?

• polar heads: face aqueous environments (extracellular fluid + intracellular fluid)

• non-polar tails: face inward ➡ hydrophobic core isolated from aqueous environment

What cellular structures are formed by phospholipid bilayers?

Plasma membrane + organelle membranes.

What is cholesterol?

• main steroid found in cell membranes (3 6-C rings, 1 5-C ring + CH group)

• amphipathic due to polar hydroxyl group

• maintains proper membrane fluidity

  • one molecule of cholesterol for each molecule of phospholipid by inserting itself between phospholipids in bilayer for stabilization

Do organelle membranes contain the same amount of cholesterol as the plasma membrane?

No, organelle membranes contain very little cholesterol.

What are glycolipids?

• lipids with carbohydrate (CHO) chain attached

• found on outer leaflet of plasma membrane

• amphipathic

• forms glycocalyx: identification + interaction between cells

What are the 3 types of membrane proteins?

1. integral (intrinsic)

   1. amphipathic + partially span the membrane or are transmembrane proteins (either go on partway or fully into membrane)

2. peripheral (extrinsic)

   1. not amphipathic + located on the inner or outer surface of membrane

3. glycoproteins

   1. has attached carbohydrates located on extracellular surface of plasma membrane

   2. form part of glycocalyx

How are membrane proteins distributed in the membrane?

Distributed unequally, contributing to selective permeability.

What are desmosomes + the 3 proteins that form them?

• adhering junctions that anchor cells together in tissues subject to considerable stretching ➡ structural integrity

• 3 proteins

  • plaques: cytoplasmic structures that are anchoring points for cadherins

  • cadherins: link cells

  • intermediate filaments: anchor cytoplasmic surface of desmosomes to cell components ➡ structural support

What defines tight junctions in epithelial cells?

• no space between adjacent cells; functions form nearly impermeable barrier

  • keeps 2 sides of a cell functionally different so transport + absorption happen in correct places

• commonly found in epithelial tissue specialized for molecular transport

What are occludins + what’s their function in tight junctions?

• proteins that link adjacent cells together

  • forms nearly impermeable junctions + limits molecule movement between cells

How do tight junctions affect the movement of molecules and membrane proteins?

They block extracellular (paracellular) pathways, forcing molecules to pass through cells (transcellular pathway) + limiting movement of integral membrane proteins and lipids, preserving distinct cell surface functions.

How do tight junctions maintain functional differences between the apical + basolateral sides of epithelial cells?

• by restricting membrane proteins to either apical or basolateral surfaces

  • apical (lumen): faces outside environment for absorption or secretion

  • basolateral (blood): faces underlying tissue + blood vessels specialized for transport to tissue or blood

    • ex. sodium/glucose transporters only on apical membrane

What are gap/communicating junctions?

• gap junctions: transmembrane channels that link the cytoplasms of adjacent cells

  • connexons: protein that forms the gap junctions

• functional significance: electrically + metabolically couple cells ➡ communication between them

  • allows small molecules + ions to move between cells (small due to limited diameter of connexons (1.5 nm))

What are the defining characteristics of desmosomes, tight junctions + gap junctions?

• desmosomes: mechanical attachment between cells ➡ cells can resist pulling forces

• tight junctions: forms nearly impermeable barrier between cells ➡ molecules just pass through cells

• gap junctions: direct communication between cells ➡ passage of small molecules + ions

What is the main function of nucleus + its 4 components?

• main function: transmission of genetic material + storage of info for protein synthesis

• components

  • chromatin: DNA associated with proteins that condenses to form chromosomes during cell division

  • nuclear envelope: double-layered, porous membrane that surrounds nucleus

  • nuclear pores: openings in nuclear envelope that allow selective movement of molecules in + out of nucleus

  • nucleolus: structure inside nucleus responsible for ribosomal RNA synthesis

What are the 2 cells that don’t have one nucleus?

1. red blood cell: no nucleus or organelles

   1. to make more room for hemoglobin ➡ increases gas-carrying capability

2. skeletal muscle cell: multinucleate

   1. satisfies muscle’s high protein-making needs

What are ribosomes, its subunits + types of functional ribosomes?

• non-membrane bound (not surrounded by phospholipid bilayer)

• protein synthesis: reads info in mRNA + assembles amino acids in correct order to make protein

• 2 subunits: small + large

  • each composed of proteins + rRNA

  • they join prior to protein synthesis ➡ functional (can synthesize proteins) ribosomes

• 2 major types of functional ribosomes

  • free ribosomes in cytoplasm

  • ribosomes bound to ER

What is the ER + its 2 types?

• ER: fluid-filled membranous system

• 2 types

  • RER

    • granular: ribosomes bound to its cytosolic surface

    • flattened sacs

    • function: synthesis of proteins by bound ribosomes then post-translational modifications of proteins

      • modifications needed for protein to become fully functional

  • SER

    • agranular: no ribosomes

    • branched tubular structure

    • functions

      • synthesizes lipids (fatty acids, steroids)

      • stores Ca²⁺ (muscle cells, sarcoplasmic reticulum)

      • drug detoxification (liver cells)

What is the Golgi apparatus?

• post-translational modification, sorting + packaging of proteins made in RER

• process

  • transport vesicle containing proteins from RER fuses with Golgi apparatus

  • proteins move through cisternae (flattened, slightly-curved membrane bound sacs stacked in layers)

  • proteins are packaged into outgoing transport vesicles + may be:

    • secreted from cell

    • become integral membrane proteins or proteins of lysosomes, ER or Golgi

What are lysosomes?

• contains digestive enzymes (specifically hydrolytic enzymes) that break down polymers into smaller subunits

  • degrades both extracellular + intracellular debris

    • recycle cell organelles

    • destroy endocytosed bacteria + viruses

• functions optimally at acidic pH (~5.0)

• made in RER

How are lysosomes connected to the Golgi apparatus + RER?

• vesicle containing enzymes buds of RER, moves to + fuses with Golgi apparatus

• enzymes are further processed + sorted in Golgi

• vesicle that buds off from Golgi will become lysosome

  • newly-formed lysosome ➡ more acidic + enzymes become active

How does the presence of a lysosomal phospholipid bilayer help?

Allows lysosome to keep acidic pH while cytoplasm pH remains neutral.

How is lysosome’s acidic pH maintained?

Maintained by proton pumps that transfer H-ions from cytosol across membrane into lysosome.

What is exocytosis?

Method of removing undigested substances/waste from cell that can’t be broken down by lysosomal enzymes.

What are peroxisomes?

• contains oxidative enzymes that use O to remove H from molecules

• acts on fatty acids, alcohol + drugs

• abundant in liver + kidney

What is the toxic byproduct of peroxisomal reactions + how is it detoxified?

• hydrogen peroxide (H₂O₂)

• catalase: converts 2 H₂O₂ → 2 H₂O + O₂

  • body can better handle water + oxygen

What are the 2 organelles only surrounded by double phospholipid bilayers?

Mitochondria + nucleus.

What is mitochondria?

• powerhouse of cell that makes ATP

• structure:

  • double membrane

    • outer membrane: separates mitochondria from cytosol

    • inner membrane (cristae) sheets or tubules increase SA

  • matrix

• process: cellular respiration: biochemical energy from nutrients converted into ATP

The number of mitochondria depends on what?

• energy needs of cell

  • ex. skeletal muscle, heart muscle, liver + sperm: lots needed

  • ex. red blood cells: none needed

Does mitochondria have its own DNA + if so, where did it come from?

• double-stranded circular DNA

• evolved symbiotically as bacteria entering eukaryotic cells billions of years ago

What is a cytoskeleton?

• non-membrane bound network of string of proteins

• made of:

  • microfilament/actin filament (smallest)

    • protein subunit: G-actin

  • intermediate filament (medium)

    • protein subunit: several proteins (ex. Keratin)

  • microtubule (biggest)

    • protein subunit: tubulin

• general functions:

  • maintain cell shape

  • maintain organelle positions

  • mediate cell + organelle motility