AP Bio Unit 2: Cell Structure and Function

Basic features of all cells

• Plasma Membrane: regulates what moves in and out of the cell

• Semifluid substance called cytosol

• Ribosomes (make proteins)

• Chromosomes (carry genes)

Prokaryote Cells

• No nucleus

• DNA in an unbound region called the nucleoid

• No membrane-bound organelles

• Cytoplasm is bound to the plasma membrane

Eukaryote Cells

• DNA in the nucleus that is bound by a double membrane

• Membrane-bound organelles

• Cytoplasm in the region between the plasma membrane and the nucleus

• Eukaryote cells are generally much larger than prokaryote cells

Plasma membrane

selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell

• the boundary that separates the living cell from its surroundings

Rough ER

• covered in ribosomes (rough)

• makes proteins that will be exported into the membrane or other organelles

• proteins get made here and then get shipped to golgi for modification

Smooth ER

• network of membranes (no-ribosomes = smooth)

• Function:

  • makes lipids

  • Metabolizes carbohydrates

  • detoxifies drugs and poisons (especially in liver cells)

  • stores calcium ions (vital for muscle contraction)

Golgi Apparatus

• Stack of flattened membranes (like pancakes) called cisternae

• Function:

  • modifies products of ER

  • manufactures certain macromolecules

  • sorts and packages materials into transport vesicles

Nucleus

• stores DNA

• double membrane (nuclear envelope)

• nuclear pores allow RNA and proteins to move in and out of the nucleus

Nucleolus

• Makes rRNA and assembles ribosome subunits

SA to V ratio

• Larger cell = larger metabolic reactions

• If the V is too large, diffusion can’t occur at a fast enough rate to keep up with SA, so the cell generally stops growing

  • SA can’t keep up with V increasing, so the cell uses “compartments”

Chromatin

• an unravelled chromosome

Animal cells DO NOT have

• central vacuole

• chloroplasts

• cell wall (made of cellulose)

Plant cells DO NOT have

• centrioles

  • pair of centrioles = centrosome

Ribosomes

• make proteins and carry out their synthesis in 2 locations:

  • In the cytosol (free ribosomes)

  • On the outside of the ER or the nuclear envelope (bound ribosomes)

lysosome

It is a membranous sac of hydrolytic enzymes (uses water to break down larger molecules into smaller ones)

• They can digest any type of macromolecule

phagocytosis

(cell-eating) Engulfs something from outside.

• The lysosome fuses with the food vacuole and digests the molecules

Ex. 

1) cell engulfs an outside particle

2) forms a vesicle (phagosome)

3) lysosome fuses with it

4) digestion occurs

autophagy

(self-eating) Recycles organelles

• cell breaks down its own damaged organelles and recycles them

Ex. 

1) A worn-out organelle gets wrapped in a membrane (autophagosome)

2) lysosome fuses with it

3) digestion/recycling occurs

vacuoles

large vesicles derived from the ER and Golgi (function: stores)

• Vesicle = small bubble

• Vacuole = big bubble

Food Vacuoles

formed by phagocytosis

Contractile Vacuoles

found in many freshwater protists (single-celled eukaryotic organisms), pump excess water out of cells

Central Vacuoles

Found in many mature plant cells, hold organic compounds and water

Ex. 

• The bladder stores Urine, and the kidneys help us pee

  • If humans are dehydrated, our pee is more yellow

  • If humans are hydrated, our pee is clear

    • Kidneys balance our salt/water ratio

Endomembrane

complex system that plays a role in the cell’s compartmental organization

Mitochondria

makes ATP

Chloroplasts

sites of photosynthesis in plant cells

Mitochondria and Chloroplasts?

• used to be bacteria

  • enveloped by a double membrane

  • contain free ribosomes and circular DNA

  • Grow and reproduce somewhat independently in cells

• Leads to the endosymbiotic theory

Endosymbiotic theory

Suggests that an early ancestor of eukaryotes engulfed an oxygen-using non-photosynthetic prokaryotic cell

• evolved into the mitochondria and chloroplasts

symbiosis

living together

Nuclear membrane

has pores letting things go in/out

exocytosis

flushing protein out of the cell

lysosomal pathway

ER, Golgi, Lysosome

secretory pathway

ER, Golgi, Plasma Membrane

OR

Rough ER, Golgi, Transport Vesicle, Plasma Membrane

• two types:

  • constitutive secretion: continuous release

  • regulated secretion: release only when signaled

Peroxisomes

produce hydrogen peroxide (toxic) through oxidation and convert it to water/oxygen by losing hydrogen

cytoskeleton

embedded in cytoplasm

• Organizes the cell’s structures and activities

microtubles

(bigger)

• Shapes the cell

• Guides movement of organelles (vesicles)

• Separates chromosomes during cell division

• control cilia and flagella

  • Sperm is made of flagella, which is made up of microtubules

microfilaments

(smaller)

• muscle cell = actin and myosin

• cytoplasmic streaming (moves chloroplast)

• how an amoeba moves

centrosome

in animal cells, microtubules grow out of the centrosome near the nucleus

centrioles

in animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring

Basal body

anchors the cilium or flagellum

dynein

motor protein which drives the bending movements of a cilium or flagellum

Microfilaments

solid rods about 7nm in diameter, built as a twisted double chain of actin subunits

• contain the protein myosin

pseudopodia

(cellular extensions) allow cells to crawl along a surface

cytoplasmic streaming

circular flow of cytoplasm within cells, driven by actin-myosin interactions

Intermediate filaments

range in diameter from 8 to 12 nm, larger than microfilaments but smaller than microtubules

• more permanent cytoskeleton fixtures than the other two classes

  • support cells shape and fix organelles in place

Extracellular components

anything outside the plasma membrane

cell wall of plants

protects the plant cell, maintains its shape, and prevents excessive uptake of water

• made of cellulose fibers

• cell wall pushes water out to ensure more doesn’t come in

  • primary cell wall: thin and flexible

  • middle lamella: thin layer between primary walls of adjacent cells

  • secondary cell wall (in some cells): between the plasma membrane and the primary cell wall

Plasmodesmata

A junction that runs from one plant cell to the next and allows water and small solutes to move

• only in plant cells

  • are channels that make holes in plant cell walls

Cell junctions

neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact

Tight Junctions, Desmosomes, Gap Junctions

• tight junction

  • plugs the gap junctions (no leaking)

• desmosomes

  • glue that sticks cels together

• gap junctions

  • communication junctions that provide cytoplasmic channels between adjacent cells

    • exactly same as plasmodesmata but for animal cells

selective permeability

allowing some substances to cross it more easily than others

amphiphatic

molecules containing hydrophobic and hydrophillic regions

fluid mosaic model

The membrane is a. mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

cholesterol

steroid that has four rings and is non-polar

glycoprotein

sugar attached to protein

integral/transmembrane proteins

run through the membrane

• hydrophobic regions consist of one or more stretches of non-polar amino acids, often coiled into alpha helices (secondary structure)

peripheral proteins

don’t run through the membrane (only on the outside)

How are membranes held together?

mainly by weak hydrophobic interactions

phospholipid movement

• can only move left/right to open/close spaces

• Rarely, the flip-flop of phospholipids occurs due to polarity

  • The polar head doesn’t want to go through a non-polar region

• Some proteins can move sideways

Can the membrane become solid?

Even though it’s a fluid mosaic, the membrane gets so cold that it becomes solid at times

• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

  • membranes must be fluid to work properly

What are cholesterols difference effects on membrane fluidity?

• warm/normal temp = bad (viscous membrane/saturated fatty acid tails packed together)

• cool/low temp = good (double bonds prevent viscousness, maintaining fluidity)

  • Cholesterol is also present in plants

  • Membranes are able to change lipid compositions in response to temperature changes

Membrane Proteins

determine most of the membrane’s functions

What happens if molecules can’t fit through phospholipid bilayer?

goes through a protein channel

Cell-surface proteins

HIV must bind to the immune cell surface protein CD4 and a co-receptor CCR5 to infect a cell

• We now have medicine/gene surgery to get rid of CCR5 co-receptor, so HIV can’t bind to it

glycolipids

sugar attached to lipid

cell-to-cell recognition

Cells recognize each other by binding to molecules (often containing carbohydrates) on the extracellular surface of the plasma membrane

• carbohydrates on the extracellular side of the plasma membrane very among species, individuals, and even cell types

How are the asymmetrical distribution of proteins, lipids, and carbohydrates determined?

determined when the membrane was built by the ER and Golgi

What are the molecules that can cross easiest through the plasma membrane?

small non-polar molecules

For what molecules is it the hardest to cross through the plasma membrane?

large hydrophilic(polar) molecules, including ions

• sugars are polar so they don’t cross the membrane easily

Channel proteins

A type of transport protein that has a hydrophilic channel that certain molecules or ions can use

• Aquaporins greatly facilitate the passage of water molecules

Carrier proteins

A type of transport protein that binds to molecules and changes their shape to shuttle them across the membrane

Diffusion

the tendency for molecules to spread out evenly into the available space

• high to low concentration

• even at equilibrium, movement doesn’t stop

osmosis

diffusion of water across a selectively permeable membrane

• water moves towards the higher solute concentration side (hypertonic solution) until solute concentration is even

concentration gradient

the region along which the density of a chemical substance increases/decreases

passive transport

No energy is required by the cell

• high to low concentration

Hypotonic

higher solute concentration inside the cell; water moves in

• animal cells don’t like it (will explode)

• plant cells like it

  • water moves in the cell

Isotonic

solute concentration is equal in and out of the cell; water moves in and out

• animal cells like this

• plant cells need more water (start wilting)

Hypertonic

higher concentration of solute outside the cell; water moves out

• animal cells don’t like this (shrivel up)

• plant cells don’t like this (plasmolyzed: plasma membrane shrivels)

tonicity

the ability of a surrounding solution to cause a cell to gain or lose water

osmoregulation

The control of solute concentrations and water balance is a necessary adaptation for life in such environments

• ex. Eukaryote Paramecium is hypertonic to its pond water environment, has a contractile vacuole that pumps out water

  • every cell osmoregulates the water/salt balance

What happens to bacteria and archaea that live in hypersaline (excessively salty) environments?

they have a cellular mechanism that balances the internal and external solute concentrations

Water balance of cell walls in plants

Cell walls help maintain water balance

• A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

• A plant cell in an isotonic solution has no net movement of water into the cell; the cell becomes flaccid (limp)

• A plant cell in a hypertonic solution loses water, and the membrane pulls away from the cell wall, causing the plant to wilt

  • a potentially lethal effect called plasmolysis

    • plasmo (cell-membrane) lysis (pulls away)

facilitated diffusion

Transport proteins speed the passive movement of molecules across the plasma membrane

• Transport proteins include channel proteins and carrier proteins

• Requires no ATP

• type of simple diffusion

Ion channels

facilitate the transport of ions

• Some ion channels, called gated channels, open or close in response to a stimulus

  • ex. in nerve cells, ion channels open in response to electrical stimulus

Active transport

• low to high concentration 

• requires ATP

• Allows cells to maintain concentration gradients that differ from their surroundings

  • ex. an animal cell has much higher potassium and much lower sodium concentration compared to its surroundings

    • controlled by sodium potassium pump

Membrane potential

• also the same as charge difference and voltage difference

  • It’s the voltage across a membrane

• voltage is created by differences in distribution of positive and negative ions across a membrane

electrochemical gradient

drives the diffusion of ions across a membrane

• a chemical force (ion concentration gradient)

• an electrical force (the effect of the membrane potential on the ion’s movement)

proton pump

actively transports hydrogen ions out of the cell

electrogenic pump

a transport protein that generates a voltage across a membrane

• The sodium-potassium pump is the major electrogenic pump of animal cells

Cotransport

occurs in plant cell membranes when active transport of a solute indirectly drives transport of other substances

bulk transport

large molecules such as polysaccharides and proteins, cross the membrane in bulk via vesicles

• requires energy

exocytosis

transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell

• many secretory cells use exocytosis to export their products

endocytosis

the cell takes in macromolecules by forming vesicles from the plasma membrane

• There are three types of endocytosis

  • Phagocytosis (cell-eating)

  • Pinocytosis (cell-drinking)

  • Receptor-mediated endocytosis

    • binding of specific solutes to receptors triggers vesicle formation

hypercholesterolemia

above normal levels of cholesterol

• have missing or defective LDL receptor proteins

• cholesterol is wanted in cells not in the blood stream