Cell Structure and Function

light microscopes

used to study stained or living cells

electron microscopes

used to study detailed structures of a cell that cannot be easily seen or observed by light microscopy

prokaryotic cell

• smaller than eukaryotic & simpler

  • ex: bacteria & archea

• inside of the cell is filled with cytoplasm

  • genetic material in a prokaryote = one continuous, circular DNA molecule that is found free in the cell in the nucleoid

• cell wall composed of peptidoglycans that surrounds plasma membrane (lipid layer)

• small ribosomes

• some bacteria may also have flagella (used for mobility)

• no membrane-bound organelles

eukaryotic cell

• ex: fungi, protists, plants, and animals

• have many organelles

plasma membrane

• outer envelope of the cell

  • made up of mostly phospholipids and proteins.

• regulates the movement of substances into and out of the cell (semipermeable)

• many proteins are associated with the cell membrane

  • some loosely associated with the lipid bilayer (peripheral proteins) - located on the inner/outer surface of the membrane

  • others firmly bound to plasma membrane (integral proteins) - amphiatic

    • some extend all the way through the membrane (transmembrane proteins)

• fluid-mosaic model: arrangement of phospholipids & proteins

  • adhesion proteins

  • receptor proteins

  • transport proteins

  • channel proteins

  • cell surface markers

  • carbohydrate side chains found only on the outer surface

adhesion proteins

form junctions between adjacent cells

receptor proteins

serve as docking sites for arrivals at the cell (ex: hormones)

transport proteins

form pumps that use ATP for active transport of solutes across membrane

channel proteins

form channels that selectively allow passage of certain ions/molecules

cell surface markers

exposed on extracellular surface and play a role in cell recognition and adhesion (ex: glycoproteins)

carbohydrate side chains

found only on the outer surface of the plasma membrane

nucleus

• largest organelle in the cell

• directs what goes on in the cell

• responsible for the cell’s ability to reproduce (DNA - organized into chromosomes)

• nucleolus: where rRNA is made & ribosomes are assembled

ribosomes

• sites of protein synthesis

• manufacture all the proteins required by the cell or secreted by the cell

• round structures composed of two subunits, the large subunit and the small subunit

• composed of ribosomal RNA (rRNA) and proteins

  • can be either free floating in the cell or attached to ER

Endoplasmic Reticulum (ER)

• continuous channel that extends into many regions of the cytoplasm

• provides mechanical support and transportation

• rough ER compartmentalizes the cell

• smooth ER = lacks ribosomes

  • makes lipids, hormones, and steroids and breaks down toxic chemicals

Golgi Complex

• after the ribosomes on rough ER have synthesized proteins, Golgi complex modifies, processes, and sorts the products

• packaging and distribution centers for materials destined to be sent out of the cell

  • package the final products in little sacs called vesicles, which carry products to the plasma membrane

Mitochondria

• power stations responsible for converting energy from organic molecules into useful energy for the cell

  • most common energy molecule in the cell is ATP

• inner portion:

  • forms folds known as cristae

  • separates from innermost area (the matrix) from the inter-membrane space

• outer portion:

  • separates the inter-membrane space from the cytoplasm

Lysosomes

• sacs that carry digestive enzymes

  • used to break down old, worn-out organelles, debris, or large ingested particles

• essential during apoptosis (programmed cell death)

Vacuoles

• fluid-filled sacs that store water, food, wastes, salts, or pigments

• multiple functions in plant cells (larger as well)

Peroxisomes

• detoxify various substances, producing hydrogen peroxide as a byproduct

• enzymes that break down hydrogen peroxide into oxygen and water

Cytoskeleton

• determines the shape of this network of protein fibers

• most important = microtubules & microfilaments

  • microtubules: participate in cellular division & movement

  • microfilaments: movement (actin monomers that can grow & shrink)

Cilia & Flagella

• locomotive properties in single-celled organisms

• beating motion of cilia and flagella structure allows it to move

Cell Wall (plant cells)

• made up of cellulose

• rigid layer for support of cells outside of plasma membrane

Chloroplasts

• contain chlorophyll

  • gives plants green color

Central Vacuole (plant cells)

• large vacuole

• structural support

Centrioles (animal cells)

• located in the cytoplasm of animal cells near the nuclear envelope

• organize microtubules that serve as the cell's skeletal system

• help determine the locations of the nucleus and other organelles within the cell

Ability of molecules to move across the cell membrane depends on

1. the semipermeability of the plasma membrane

2. the size and charge of particles that want to get through

Aquaporins

water-specific channels

Passive Transport (Simple & Facilitated Diffusion)

• diffusion:

  • high concentration to low concentration - moves along concentration gradient

• simple diffusion:

  • hydrophobic molecules

  • just drifts through because it’s small & non-polar

• facilitated diffusion:

  • hydrophilic molecules usually

  • require the help of a channel-type protein

• diffusion always = passive transport (no outside energy)

Osmosis

• still passive transport BUT the membrane is not permeable for solutes

• PLANTS: cell wall protects against osmotic changes if it loses water — can expand & squeeze tightly against the cell wall if it takes in water

  • cell membrane can shrink away from the wall [plasmolysis]

Isotonic solution (osmosis)

solute concentration is the same inside and outside

Hypertonic solution (osmosis)

more total dissolved solutes than the cell

Hypotonic solution (osmosis)

less total dissolved solutes than the cell

Water potential (Ψ)

• measure of potential energy in water

• describes the eagerness of water to flow from an area of high water potential to an area of low water potential

• affected by: pressure potential (Ψp) and solute potential (Ψs)

  • adding a solute lowers the water potential of a solution — water to be less likely to leave this solution & more likely to flow into it

  • more solute molecules present, the more negative the solute potential is

Active Transport

• movement against the natural flow

• some proteins in the plasma membrane are powered by ATP

• ex: sodium-potassium pump

Sodium-potassium pump

• ushers out three sodium ions (Na+) and brings in two potassium ions (K+) across the cell membrane

• pump depends on ATP to get ions across that would otherwise remain in regions of higher concentration

Primary Active Transport

occurs when ATP is directly utilized to transport something

Secondary Active Transport

occurs when something is actively transported using the energy captured from the movement of another substance flowing down its concentration gradient

Endocytosis

• when the particles that want to enter a cell are just too large, the cell uses a portion of the cell membrane to engulf the substance

  • forms a pocket, pinches in, and eventually forms either a vacuole or a vesicle

• Pinocytosis: the cell ingests liquids

• Phagocytosis: the cell takes in solids

• Receptor-mediated endocytosis: involves cell surface receptors that work in tandem with endocytic pits that are lined with a protein called clathrin

  • when a particle, or ligand, binds to one of these receptors, the ligand is brought into the cell by the invagination, or “folding in” of the cell membrane

  • vesicle then forms around the incoming ligand and carries it into the cell’s interior

Bulk flow

one-way movement of fluids brought about by pressure.

• Ex: movement of blood through a blood vessel

Dialysis

diffusion of solutes across a selectively permeable membrane

Exocytosis

cell ejects waste products or specific secretion products

• fusion of a vesicle with the plasma membrane, which then expels the contents into the extracellular space

• reverse endocytosis