Membranes of the Eukaryotic Cell

Biology

Definition of a cell:

basic structural and functional unit of life

the smallest units that display the characteristics of life, i.e. reproduction, metabolism, response to stimuli

General Subdivisions of a Cell

• Plasma Membrane = selectively permeable boundary between the cell and the environment

• Nucleus = regulatory center of the cell

• Cytoplasm = everything between the plasma membrane and the nucleus (fluid + organelles)

Plasma Membrane1. Structure = phospholipd bilayer with

proteins embedded in, and attached to, the inner (intracellular) and outer (extracellular) surfaces

2. Function

a. Selectively permeable barrier: controls what enters and leaves the cell

b. Phospholipids are liquid at body temperature, so proteins float around in the membrane

-functions as a Fluid Mosaic

Plasma Membrane

Plasma Membrane (2)2. Function – continued

c. Main responsibility: ensure the composition of extracellular fluid is not the same as the composition of the intracellular fluid

d. Water-soluble substances (salts, nutrients) cross membrane with aid of protein channels, which are selective about what can pass through

e. Lipids can pass directly through bilayer by diffusion (the random walk of molecules)

f. Attachment site of cytoskeleton, the internal support of the cell

Plasma Membrane (3)

3. Glycocalyx = protein and carbohydrate coat covering the extracellular surface of the plasma membrane

a. Allows attachment to other cells

b. Allows the cell to interact with the environment

c. Gives each person’s cell a distinctive surface;

allows your body to recognize foreign tissues

as different from your own (i.e., blood type, transplant rejection)

Plasma Membrane

General Subdivisions of a Cell

• Plasma Membrane = selectively permeable boundary between the cell and the environment

• Nucleus = regulatory center of the cell

• Cytoplasm = everything between the plasma membrane and the nuclear compartment

Cell Structure Overview

Nucleus

1. Nuclear Envelope (membrane) a. Phospholipid bilayer with nuclear pores

b. Controls what enters/leaves the nucleus

-- things only go in or out by passing through

protein channels, which are selective

c. Encloses all the chromosomes

Nucleus2. Chromatin = all the chromosomes, which are

long strands of the molecule DNA

-- DNA regulates all cell activities, yet never leaves the nucleus; how is this possible?

produces RNA, short messenger molecules

that exit through nuclear pores

RNA carries instructions out into the cytoplasm

Nucleus3. Nucleolus – site of ribosome synthesis

a. compartment in the nucleus where ribsomes

are assembled b. ribosomes are then moved out into

cytoplasm through nuclear pores c. ribosomes and RNA work together outside

the nucleus, to build all the proteins in the cell

Nuclear Compartment

Ribosomes

Ribosomes = site of protein synthesis

--assembled in the nucleolus

--exported into the cytoplasm

a. Free – unbound in the fluid cytoplasm, produce proteins for use in the cell

b. Bound – attached to the endoplasmic reticulum (ER), produce proteins for export, or for the plasma membrane

Cytoplasm + Organelles

1. Fluid portion = “cytoplasm”a. Water with dissolved salts, nutrients and

enzymes (= proteins that do a particular job)b. Site of many metabolic reactions

2. Cytoskeleton = network of protein fibers extending throughout the fluid cytoplasm

a. Support and structure for the cellb. Very dynamic, always remodeling itselfc. Critical for cells to divide and copy themselves

Cell Structure

Organelles1. Separate compartments within the cytoplasm

formed by membranes

2. Mitochondrion = “thread granule”, major source of cell’s energy

a. energy is taken from sugar, stored in molecule called ATP (adenosine triphosphate)b. requires oxygen to make this exchange

(aerobic metabolism)c. contained within double membrane

Mitochondrion

Organelles (2)3. The Cytomembrane System = system of

tubes and chambers formed by membranes

a. extensively distributed throughout the fluid

cytoplasm

b. involved in synthesis, modification,

processing & packaging of cellular lipids

and proteins

Cell Structure

Cytomembrane System1. Endoplasmic Reticulum (ER) = “within the

cytoplasm network”, a system of tubes and sacs formed by membranes (an enclosed space)

a) Rough = with bound ribosomes--modifies proteins produced by the ribosomes

b) Smooth = without bound ribosomes

-doesn’t modify proteins-functions in lipid synthesis, drug

detoxification, carbohydrate metabolism

Cytomembrane System (2)2. Golgi Apparatus = series of flattened sacs formed

by membranes, functions in final protein processing prior to use by the cell

a. proteins get shuttled from the ER to one end

of the Golgi

b. in each sac, different modifications are made

(proteins get individually tailored)

c. proteins get sorted and shipped off to their

destination (like the post office of the cell)

Cytomembrane System (2)3. Vesicles = small membrane-bound

structures that transport proteins and lipids around the cell a. little transporters that shuttle their

contents from one organelle to anotherb. when they contact the appropriate organelle, they fuse with its outer membrane and dump their contents insidec. same for plasma membrane, allowing the export of materials from the cell (exocytosis)

Vesicles move from ER to Golgi

Cytomembrane System (2)4. Lysosome = membrane-bound vesicle that

contains digestive enzymes or toxic chemicals

a. merges with vesicles containing food

particles, invading bacteria

b. harsh chemicals and enzymes degrade the food or bacteria, without harming rest of cell