Eukaryotic cell structure & function

Plasma (cell) membrane

Boundary of cell, Fragile, flexible, & thin, See Fig. 6.8

Controls what enters & leaves cell

Phospholipid bilayer (~50%) with proteins embedded in bilayer (~50%)

Cytoplasm with organelles

Ribosomes - small particles

Large & small subunits

Composed of RNA & protein

Free in cytoplasm or attached to internal membranes, See Fig. 6.11

Site of protein synthesis in cell

Nucleus - control center of cell

Nuclear envelope - double membrane which separates nucleus from cytoplasm, See Fig. 6.10

Pores - allow communication between nucleus and cytoplasm

Proteins & RNA move through pores; Pores can change in number

Nucleus contains DNA

Chromosomes - DNA complexed with proteins

Chromatin - long thin threads which condense to form chromosomes

Nucleolus - regions on chromosomes that synthesize ribosomal RNA

Endomembrane system

Interconnected membranes of eukaryotic cell

Includes several organelles Nuclear envelope, Endoplasmic reticulum, Golgi bodies, Vesicles & vacuoles, (Plasma membrane)

Membranes are very fluid - phospholipid bilayer

If two membranes touch, they fuse together

Portions of membrane can break off as vesicles (small membrane bound structures)

Endoplasmic Reticulum (ER),. See Fig. 6.12

Extensive system of channels & cisternae formed by internal membranes

Organizes cellular activities by:

providing routes for movement of molecules
forming subcompartments - dynamic changes produce other organelles
providing sites for enzyme & ribosome attachment

Two types of ER

Rough ER - has ribosomes attached

Makes proteins that will be secreted outside the cell
Forms membrane

Smooth ER - may have a variety of enzymes attached

synthesizes lipids, carbohydrates
detoxifies substances (liver)

Golgi body (apparatus)

Flattened stacks of membranes, See Fig. 6.13

Package, modify, & distribute molecules synthesized elsewhere

Components made in ER (proteins & lipids) are modified (by adding polysaccharides)

More are found in cells secreting large amounts of compounds

Vesicles move to Golgi and fuse

Vesicles budded off Golgi

Move to plasma membrane - empty contents to outside
Add membrane to plasma membrane
Form other organelles - lysosomes

Lysosomes

Vesicles that contain an array of hydrolytic enzymes

Digest other cells, worn-out cell components, See Fig. 6.14

Phagocytes - wbc that engulf foreign cells (bacteria) and hydrolyze them with the many lysosomes they contain

Endomembrane system, Review Fig. 6.16

Vacuoles & Vesicles

Membrane bound structures

Central vacuole in plant cells stores molecules, wastes

Contractile vacuole in protozoa pumps out water

Food vacuoles contain material being digested

Peroxisomes, See Fig. 6.19

Membrane bound organelle, not formed from endomembrane system

Transfer hydrogen to oxygen to form hydrogen peroxide (H₂O₂) in a variety of reactions

Degrade H₂O₂ (toxic) & protect rest of cell

Main Energy transforming organelles

Mitochondria & Chloroplasts

Mitochondria - found in nearly all eukaryotic cells. Site of cellular respiration

Chloroplasts - found in plant & algal cells. Site of photosynthesis

Both contain ribosomes and DNA

Mitochondrion- powerhouse of cell

Surrounded by two membranes, See Fig. 6.17

Inner membrane is convoluted, infoldings called cristae

Matrix - fluid in inner membrane

Breaks down organic molecules, releases energy, forms ATP

Chloroplast

Surrounded by two membranes, See Fig. 6.18

Internal membrane arranged in flattened sacs - thylakoids

Grana - stacks of thylakoids

Stroma - fluid inside chloroplast

Converts light energy to chemical energy (photosynthesis)

Cytoskeleton - Functions

Maintains shape of cells that lack cell walls

Causes movement of cytoplasm & organelles

Provides sites of attachment of enzymes & ribosomes, organizes cell (like ER)

Cytoskeleton - composition

All components are proteins, See Table 6.1

Microfilaments - smallest (7nm), involved in movement

Intermediate filaments - (8-12 nm), provide structural support, shape

Microtubules - largest (20-25 nm), cell shape, guide movements of organelles, chromosomes

Microfilaments & Motility, See Figs. 6.26 & 6.27

Eukaryotic flagella & cilia

Beat and move cell or fluid, See Figs. 6.23, 6.24, 6.25

Flagella - longer and 1-2 per cell

Cilia - shorter, many per cell

Both composed of microtubules,

9 double + 2 single microtubules (9+2 structure)

Cell walls

Provide shape and rigidity to cell, See Fig. 6.28

Fibers (cellulose) embedded in matrix

Plants, Fungi, Algae

Extracellular matrix

Support, movement, adhesion, development, See Fig. 6.29

Glycoproteins (proteins + carbohydrate)

Animals

Prokaryote vs Eukaryote

Structure	Prokaryote	Eukaryote
Cell membrane	+	+
Cell Wall	+(most)	+(plants, fungi, algae) -(animals, protozoa)
Ribosomes	+	+
Internal membrane	Little	Great deal, ER, Golgi, etc.
Other organelles	Not generally	Many
Genetic information	1 DNA, in nuclear region	>1 chromosomes, in true nucleus
Movement	No cytoplasmic streaming, bacterial flagella	Cytoplasmic streaming, microfilaments, microtubules, 9+2 flagella & cilia