Cell Structure and Functions

Cell Structure and Functions

• Cells are the basic units of life
• Cells have different structures and functions
• Cells have major parts
• Each part has unique functions

Major Parts of the Cell

• Plasma Membrane: Outer covering that separates a cell's interior from its surroundings.
• Cytoplasm: Jelly-like substance inside the cell, containing other cellular components.
• Nucleus: Serves as the cell's control center, directing all its activities.

Plasma Membrane Details

• Encloses and protects organelles
• Protects from harm from foreign materials
• Made up of phospholipids forming a bilayer, cholesterol, proteins
• Controls the exchange of essential components
• Obtains chemical messages from other cells.

Components of Plasma Membrane

• Phospholipids: Lipids composed of glycerol, fatty acids, and phosphate-linked head groups.
• Cholesterol: A lipid with four fused carbon rings, located in the membrane's core.
• Proteins: Help move large molecules or aid in cell recognition. Peripheral proteins are attached on the surface; integral proteins are embedded completely through the membrane.

Cytoplasm Details

• Entire region of a cell between the plasma membrane and the nuclear envelope.
• Mainly water, proteins, and salts.
• Organelles are suspended within the cytosol, a gel-like solution within cytoplasm.
• Cytosol makes up about 70% of cellular content and contains both organic and inorganic compounds.
• Contains enzymes that break down waste and enable metabolic reactions.
• Allows for cellular expansion and growth.
• Together with the cytoskeleton, determines cell shape.
• Accommodates movement for some cell types.

Nucleus Details

• Discovered by Robert Brown in 1833.
• Most vital part of the cell, dubbed the "control center.”
• Directs all cell activities, determining the cell's appearance and function.
• Enclosed by a double layer of nuclear membrane to keep it distinct from other cellular components.
• Nucleolus is the site of ribosome synthesis.
• Contains chromosomes with tightly-wrapped and coiled DNA.

Endomembrane System

• Group of membranes and organelles in eukaryotic cells, working together to modify, package, and transport lipids and proteins.
• Organelles are commonly linked by structure.
• Examples include endoplasmic reticulum, ribosomes, vacuoles, Golgi apparatus, lysosomes, peroxisomes, and centrosomes.

Endoplasmic Reticulum (ER)

• Collection of interconnected membranous sacs and tubules
• Modifies proteins and synthesizes lipids
• Rough ER has ribosomes; smooth ER does not
• Rough ER makes membrane proteins and proteins for export out of the cell
• Smooth ER incorporates proteins into cisternae, transports proteins across cytoplasm, synthesizes fatty acids and phospholipids, and detoxifies hydrophobic chemicals.

Ribosomes

• Involved in protein synthesis.
• Present in both prokaryotic and eukaryotic cells.
• Abundant in developing muscle cells, skin, and hair.

Vacuoles

• Fluid-filled vesicles enclosed by a membrane.
• Has a selectively permeable membrane that allows water passage while retaining smaller molecules.
• Stores chemicals in the cell.
• Ability to break down large molecules makes it comparable to lysosomes in animal cells.

Lysosomes

• Serve as digestion slots for cellular materials that are due for expiration or are no longer useful.
• The cell's reprocessing area, breaking down foreign substances to recycle raw materials.
• Dubbed “suicide bags” capable of self-destruction to save other organelles from being poisoned.
• Involved in autophagy, the natural process of organelle destruction
• Also involved in phagocytosis, where cells take in food, and digesting food and wastes (example: macrophages digest bacteria)

Peroxisomes

• Small, round organelles enclosed by a single membrane, resembling lysosomes
• Responsible for self-damage and breaking down fatty acids, detoxifying many poisons
• Shields cells from damage caused by reactive oxygen species (ROS) molecules

Golgi Apparatus

• Stacks of flattened sacs called cisternae.
• Have a shipping (cis face) and receiving (trans face) side.
• Receive proteins from the ER.
• Packages and modifies those proteins for cells. Proteins package into vesicles to be transported out of the cell.

Centrosome

• Composed of two centrioles or cylindrical structures with a central cavity.
• Surrounded by microtubules arranged side by side.
• Part of the assembly of spindle fibers at cell division and contributes to the successful outcome of mitosis.
• Found in all eukaryotic cells; assist in arranging microtubules (hollow proteins) for cell division
• Enable said microtubules to form part of the cell's cytoskeleton, maintaining shape and stabilizing cell structure.

Mitochondria

• Oval-shaped organelles found in eukaryotic cells.
• Considered the “powerhouses” of the cell.
• Most plentiful in cells requiring significant amounts of energy (e.g., liver and muscle cells).
• Has two membranes:
  • Outer membrane: Selectively permeable.
  • Inner membrane: Folds inward (cristae) to increase surface area for cellular metabolism. Contains ribosomes and the mitochondria's DNA. Two enclosed spaces are formed: intermembrane space and matrix
• Sites of cellular respiration, a metabolic process using oxygen to generate ATP extracting energy from sugars, fats, and other fuels.

Chloroplast

• Double membrane-bound organelles found in plants and algae.
• Sites of photosynthesis, converting solar energy into chemical energy in organic compounds like sugars from carbon dioxide and water.
• Three membranes: outer, inner, and thylakoid system.
• Outer membrane is semi-porous.
• Intermembrane space is between the outer and inner membranes.
• Inner membrane regulates passage of substances, and fatty acids and other substances are synthesized within the membrane.
• Stroma: protein-rich, aqueous fluid inside the inner membrane outside the thylakoid space. It contains Chloroplast DNA, ribosomes, thylakoid systems, starch granules and other proteins.
• Thylakoid System: suspended within the stroma, collection of membranous sacs called thylakoids arranged in stacks called grana. Membranes are sites of light reactions of photosynthesis; chlorophyll is found in the thylakoid membranes.

Adenosine Triphosphate (ATP)

• Major energy currency of the cell in most energy-consuming activities.
• When a phosphate group of ATP is removed by hydrolysis, a substantial amount of free energy is released.