Unit 3 - Cell Membranes

What is a cell?

A really really small compartment that can carry out functions so that chemical reactions can happen in a specific way for life

Why do living things need to be made out of cells?

• Need small spaces for high enzyme/substrate concentrations to get chemical reactions to happen at a speed compatible with life so we need small compartments each with its own environment

• Need different enzymes in different places to make things happen in the right places, so we need many different tiny compartments

• Also need a way to signal when to make enzymes or when to activate them so need small separate compartments that can be separately responsive to signals

• Need the machinery to make the enzymes, messengers, receptors , etc. provided by cells

What is a membrane?

A solid sheet of phospholipids protecting/encasing a cell and its organelles

What is the function of a membrane?

• Act as a barrier to separate the cell from everything else and create a compartment

• Control what moves in and out

• Communicate with other cells

• allows cells to maintain homeostasis

Phospholipids

The main barrier component in cell membranes is the phospholipid bilayer

• Head

  • Nitrogen group

  • Phosphate group

  • glycerol

  • hydrophilic (polar)

• Tail

  • Lipids (fatty acids, double bond)

  • hydrophobic (non-polar)

Osmosis

diffusion of water (high to low)

Diffusion

When the net movement of a substance travels down its concentration gradient.

• moves from high to low concentration due to random motion of molecules - SPREADING OUT

• net movement = overall movement

Types of Protein Channels

• open

• gated

• shape changing

• active transport

Simple diffusion

through the phospholipid bilayer – driven by the kinetic energy of the molecules or atoms themselves

• can’t be charged/polar

• can’t be big

• passive transport (no energy needed)

• O₂, CO_2, water

Can also be through an open channel (for molecules that always need to get in but are too big or are charged and can’t get through the phospholipid bilayer (ex: glucose: big and polar))

• passive transport (no energy needed)

• ex: sodium ions through sodium channels

Facilitated Diffusion

Diffusion through proteins located in the cell membrane to help things pass through.

• also doesn’t require energy

• goes with the concentration gradient just like simple diffusion (high to low)

Gated Channels

Opens/closes based on the messengers that bind to protein receptors. Unless a messenger is sent and binds to the receptor, will not allow anything through. (high to low, facilitated diffusion) (ex: sodium)

Why do receptors have to be globular?

Receptors have to be globular proteins so that their shape is only specific to one message and can only bind to that one specific message. And, once bonded to that message, can change shape so that it causes a chain of shape changes within the cell in order to carry out whatever function the messenger carried. If they weren’t the correct intricate shape, the cell should receive incorrect messages as anything could bind to it such as substrates. Gated channels within the cell membrane would also be open at incorrect times is the receptors weren’t the correct shape to receive their messages.

Shape Change/Carrier Proteins

The molecule binds to a pocket in the carrier proteins, causing it to change shape. Then, the molecule is released inside the cell. (high to low, facilitated diffusion, with the concentration gradient, happens all the time) (ex: amino acids)

Active Transport

Goes against the concentration gradient (low to high). A phosphate binds to the protein (phosphorylated), causing it to change shape and throw the molecules out. The shape change causes the phosphate to fall off, returning the protein to its original shape.

Co-Transport

Combination of active transport and facilitated diffusion

• cells pump ions which then bind to molecules to be transported, the ion/molecule combination then diffuses through a facilitated channel.

  

Aquaporin

Protein channels along some cell membrane in order to allow water in larger quantities.

Marker Proteins

identifies the cell for the immune system

Attachment proteins

• Cell to cell attachment

• Attachment to the cytoskeleton

• Attachment to the extra-cellular matrix proteins

• form tissues and organs

Equilibrium

When the net movement is 0.

Hyper-tonic

More solute, less water. Water will move in. (ex: distilled water to pure water, roots of a plant to saturated soil)

Hypo-tonic

Think po for low. Lower solute concentration, water will move out. (ex: distilled water to salt water)

Isotonic

Equal concentration of solutes.

Cholesterol in the Membrane

• keep it from being too fluid or solid

• keeps the phospholipids from sticking together

• helps anchor proteins in the membrane

Electrochemical gradient

• From outside to inside a muscle cell

• Sodium into a nerve cell and down the axon

• if it’s more positive outside, and more negative inside, it will be attracted causing it to move faster, and vice versa.

Things that affect the rate of diffusion

• surface area/volume ratio

• concentration difference

• electrical gradient

• size of molecule

• # of protein channels

• size of pores

• temperature (rate at which molecules are moving)

Why do elephants have big flat ears?

Elephants have big flat ear in order for the high surface area to volume concentration, which allows hot molecules to diffuse out quickly, keeping the elephants cool in their hot climate.

Why do babies have higher heart rates compared to adults?

Since they’re smaller, they have a larger surface area to volume ratio meaning they diffuse heat out faster. In order to stay warm, they have to do cellular respiration faster to generate more heat, which requires more oxygen. This requires them to breathe and pump blood around faster.

Endocytosis

• engulfing large particles with part of the cell membrane and bringing them into the cell in a vesicle (examples: fatty acids and cholesterol)

  • Pinocytosis (liquids) - just turning over the cell membrane (removing the cell membrane)

  • Phagocytosis (solids) - uses receptors too (same as receptor-mediated, just for bigger things)

  • Receptor-mediated endocytosis - solids

Exocytosis

(secretion) – wrapping molecules in a membrane vesicle inside the cell – having it fuse with the cell membrane and dump the contents outside the cell (with or against conc. gradient) – big stuff like proteins

• mucus is exocytozized in the trachea, proteins and messengers are also exocytosed