CELLS: Structures and Functions

vacuole It is a fluid-filled organelle that stores enzymes and metabolic waste. Plant cell vacuoles can be large, centrally located & filled with water or other liquids. tonoplast – the membrane enclosing a vacuole in a plant cell

plant cell tonoplasts

A large plant cell vacuole, filled with water, produces turgor pressure & gives the cell some support.

PLASTIDS Example: chloroplast Plant cells have organelles surrounded by two membranes and contain DNA. Some store starch or fats, while others contain pigments that absorb visible light. Example: chloroplast

ribosome An organelle that organizes the synthesis of proteins. Ribosomes are numerous in the cell. They are attached to the endoplasmic reticulum, or found unattached in the cytoplasm.

chloroplast (plant cells only) It stores sun’s energy in energy-rich food molecules (chemical energy in organic compounds) through photosynthesis. (It is referred to as a plastid – It’s where carbohydrates are made)

Plant cells with many chloroplasts

cytoplasm The area of the cell between the cell membrane & the nucleus. The cytoplasm contains organelles and cytosol. Cytosol is a gelatin-like aqueous solution of dissolved salts, minerals, and organic compounds.

nucleolus This is the organelle in which ribosomes are synthesized and partially assembled before they pass out of the nucleus

mitochondrion It is the location of cellular respiration. It transfers energy from food (organic compounds) to make high energy molecules (ATP) for cell use.

nucleus The nucleus controls most cell processes and contains stored hereditary information in DNA; & site of RNA synthesis.

chromatin It appears as granular material visible in the nucleus. It is composed of fine DNA strands and protein which condenses into chromosomes for cell division.

It is part of the cytomembrane system. It forms an intracellular highway to transport molecules throughout the cell. It chemically modifies proteins to prepare them for export, and assembles cell membrane components.

ALSO: Endoplasmic reticulum synthesizes steroids, regulates calcium levels in muscle cells, breaks down toxic substances in liver cells. RER – rough endoplasmic reticulum (has attached ribosomes) SER – smooth endoplasmic reticulum (has no ribosomes)

lysosomes Lysosomes are the cell’s garbage disposal system. They breakdown the products of ingested materials. They contains enzymes to digest molecules, old organelles, and foreign substances, such as bacteria & viruses (rare in plant cells).

microtubules & microfilaments They provide support, and movement of organelles in the cell. Major parts of the cell’s “cytoskeleton.” (Help maintain cell shape)

Cytoskeleton filaments

microtubules Microtubules maintain cell shape & serve as tracks along which organelles are moved. They separate chromosomes, form centrioles, and form cilia & flagella. Microtubule “9+ 2 ” arrangement in flagella and cilia = 9 pairs of microtubules in a circle with 2 single in center.

Microtubule Origination Centers There are areas within the cell that can rapidly form microtubules. These origination centers occur throughout the cell. Centrosomes are microtubule origination centers that produce the centrioles, as well as the spindle apparatus for cell division.

Centrioles have 9 + 0 arrangement with none in center. Centrioles play a role in animal cell division.

centrioles centrioles

Cilia & Flagella

microfilaments Microfilaments aid cell movement & muscle cells contraction. They are narrower than microtubules & form extensive networks in some cells. They also provide a tough, flexible framework that supports the cell. (Motor Proteins – force organelles to move along the microfilaments and microtubles.)

Golgi apparatus It processes and packages substances produced by the cell so they can be exported from the cell. It is part of the “cytomembrane system” & forms vesicles that eventually fuse with the plasma membrane. It forms lysosomes.

cytomembrane system It consists of ribosomes, endoplasmic reticulum, vesicles, Golgi apparatus, and the cell membrane. It is responsible for moving proteins from their point of synthesis to the cell exterior.

1) Ribosomes, attached to the E.R. makes proteins. 2) The proteins are modified in the E.R. 3) Vesicles (little sacs) carrying the proteins move from the E.R. to the Golgi apparatus, & fuse with it. 4) The proteins are modified inside the Golgi. 5) Vesicles carrying the proteins move from the Golgi to the cell membrane, and fuse with it. 6) The proteins exit the cell in a process called exocytosis.

Cell Wall (plant cells only) It supports and protects a plant cell. It is composed of long chains of cellulose. Pores in the cell wall allow ions and molecules to enter & leave cell

Animal cells do not have chloroplasts, or cell walls Animal cells do not have chloroplasts, or cell walls. They do have centrioles, which plant cells lack.

cell membrane It controls movement of materials into and out of the cell (contributes to homeostasis of the cell). It is composed of a phospholipid bilayer, proteins, carbohydrates & cholesterol.

The phospholipids are arranged with their hydrophobic (nonpolar) tails facing each other. The hydrophilic (polar) heads are arranged toward the water in the cytoplasm, or on the outside of the cell.

phospholipid

Phospholipids in a bilayer

Fluid-Mosaic Model The cell membrane consists of a continuous, fluid, double layer of phospholipids. Proteins either are embedded in the bilayer or associated with either the cytoplasmic (inside the cell) or extracellular face (toward the outside of the cell). Carbohydrates are linked to the proteins (glycoproteins) or lipids (glycolipids) only on the extracellular side.

Cholesterol, in varying amounts depending on the cell type, lies within the membrane serving to stabilize it. It prevents the fatty acid tails from sticking together.