Cell Structure and Function

Cell Theory

Organelles: tiny, specialized structures performing specific cellular functions.

• All organisms are composed of one or more cells.

• Cells are the basic living unit of structure and function in organisms.

• All cells come only from other cells.

Cell size (SA: V)

• Plant and animal cells’ size range: 10µm - 100µm

• SA: V (surface to volume) ratio: the reason for the small size of cells.

  • As the cell increases in volume, the proportionate amount of surface area decreases.

• Surface area = the ability to get materials in and out of cells.

  • Nutrients enter, and wastes exit.

• Volume = the needs of cells.

  • Large cells require more nutrients and produce more waste than small cells.

• Small cells have a much greater SA: V than large cells → likely have adequate surface area for exchanging wastes for nutrients.

• Large cells increase the SA: V by dividing into two cells, maintaining the same volume but increasing the surface area.

Eukaryotic cells

• Have nuclei and membrane-bound organelles that contain genetic materials and control cellular activities.

• Have a more complex structure than prokaryotic cells.

Outer Boundaries of Animal and Plant Cells

• Plasma (cell) membrane: surrounds all cells.

  • A phospholipid bilayer in which proteins are embedded.

  • Functions: define the cell boundary; regulate the entrance and exit of molecules in and out of the cytoplasm.

• Cytoplasm: fluid component that organelles sit in, containing dissolved molecules and ions.

  • Allows transport from one side of the cell to the other.

  • Reservoir for water, nutrients, and wastes.

• Cell wall: thick walls of cellulose layers outside of the membrane, sometimes reinforced with lignin.

  • Highly porous, non-selective.

  • Functions: provides strength & rigidity; prevents plant cells from exploding from internal water pressure in the large central vacuole.

Nucleus

• Function: storage of genetic information; synthesis of DNA and RNA.

  • Every cell in an organism has exactly the same genetic information, BUT each cell has certain genes that are turned on and others that are turned off.

  • Activated genes specify the amino acid sequence when a protein is created.

• Chromatin: The loose, unbound form of DNA inside the nucleus.

  • Condenses into chromosomes only right before cell division

  • Usually in loose form

• Nucleolus (plural: nucleoli): a darker region inside the nucleus, where ribosomal RNA (rRNA) is produced.

• Nuclear membrane (envelope): the DOUBLE layer that surrounds the nucleus.

  • Separates the nucleus from the rest of the cell.

  • Protects the DNA from damage.

• Nuclear pores: allow proteins into the nucleus and ribosomal subunits out.

Ribosomes

• Composed of two subunits, one large and one small.

• Function: protein synthesis

• Can be found free in the cytoplasm or attached to the endoplasmic reticulum.

Endoplasmic Reticulum

• Composed of channels and flattened vesicles.

• Physically continuous with the nuclear envelope.

• Rough ER is studded with ribosomes.

  • Rough ER makes proteins → Those proteins are processed and modified → Vesicles full of those modified proteins then head to the Golgi.

• Smooth ER does NOT have ribosomes.

  • Function: various; lipid synthesis in some cells.

  • Also forms vesicles, which then head to the Golgi.

Golgi apparatus

• Composed of a stack of 3-20 curved “saccules” (flattened spheres) made of membrane.

• One side points toward the ER, and the other side points toward the plasma membrane.

• Vesicles are often seen on the side of the Golgi.

• Function: receives protein and also lipid-filled vesicles that bud from the smooth ER.

• Proteins are modified as they pass through the Golgi.

  • Ex: Glycoproteins have their sugar chains modified.

  • At the far end, molecules are repackaged in the secretory vesicles.

  • Those vesicles move to the plasma membrane, where the contents leave the cell.

Lysosome

• Structure: membrane-bound vesicles produced by the Golgi that contain hydrolytic digestive enzymes.

• Function: Fuse with other vesicles with organic molecules → Vesicle contents are digested by the enzymes.

  • Ex: Inside white blood cells, lysosomes fuse with vesicles containing pathogens, which are then digested.

  • Autodigestion: parts of the cell can be digested by its own enzymes.

Vacuole

• Composed of a large membranous sac (basically a huge vesicle).

• Smaller in animal cells than in plant cells.

• Plant cells have a large central vacuole, so full of fluid that it gives added support to the cell.

• Function: storage of substances

Energy-related organelles

• Includes chloroplasts and mitochondria.

• Major function: convert energy into a form that can be used by the cell.

• Chloroplasts: turn solar energy into carbohydrates.

• Mitochondria: turn carbohydrates into ATP molecules.

Chloroplasts

• Perform photosynthesis: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

• Color: green due to the pigment called chlorophyll.

• Structure:

  • Stroma: outer membrane with a space inside.

  • Thylakoids: internal membranes that look like flattened stacks.

• Thylakoid membranes of grana: chlorophyll is located here, where photosynthesis occurs.

• Have their own DNA in the stroma.

Mitochondria

• Perform cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP

• All eukaryotic cells contain mitochondria (ie, plant and animal cells).

• Structure:

  • Matrix: an outer membrane with a space inside.

  • Cristae: internal membranes with folds that increase the surface area for cellular respiration.

• ALSO have their own DNA in the matrix.

Cytoskeleton

• Structure: a network of interconnected filaments and tubules that extend from the nucleus to the plasma membrane.

• Function: maintain cell shape and cause the cell and its organelles to move.

• Dynamic meaning: the elements undergo rapid assembly and disassembly.
  → Ex: Formation then disassembly of spindle fibres during mitosis and meiosis.

• Include flagella, cilia, and centrioles.

Endosymbiont hypothesis

• Mitochondria and chloroplasts are derived from prokaryotes that were engulfed by larger eukaryotic cells.

  • Mitochondria were once aerobic heterotrophic bacteria.

  • Chloroplasts were once cyanobacteria.

• The host (eukaryotic) cell benefited from the ability to utilize oxygen and synthesize food from these two prokaryotes.
  → These two were taken up and not destroyed.

(*) EVIDENCE:

• Similar to their hypothetical bacteria in size and structure.

• Bounded by a double-layer membrane: the outer one was derived from the engulfing vesicles; the inner one was from the prokaryotes’ plasma membranes.

• Contain a limited amount of genetic information; divide by splitting (binary fission); and have DNA in a circular loop, which is similar to that of the prokaryotes.

• Have their own ribosomes, which resemble those of prokaryotes, and produce some proteins.

• Their RNA base sequence of ribosomes is similar to that of prokaryotes, solidifying their prokaryotic origins.