CELLS SUMMARY

 

Cells

Are the building blocks of organisms.

(Av.size: 20 micrometers)

Small due to:

  • Cell membrane considerations
  • Nucleus to cytoplasm ratio
  • Supply to demand ratio

Electron microscope

Uses a beam of electrons.

Advantage- has a: Disadvantage- specimen must be:
  • Shorter wavelength

  • Greater resolution

  • Dead

  • Dehydrated

Organelles

  • Are membrane bound structures.
  • Have specialised functions to perform.
  • Some do not have a membrane surrounding them.
  • chloroplast & permanent vacuoles are only found in plant cells.

Nucleus

  • Controls all cell activities
  • Contains genes
  • Contains code for protein synthesis
  • Involved in production of Ribosome’s & RNA (essential for cell division)

Structure

  • Contains- nucleic acids (DNA&RNA)
  • Double membrane = Nuclear envelope
  • Encrusted with ribosome’s
  • Covered in pores
  • Continuous with RER

RER

  • Protein isolation & transport

Structure of RER

  • Consists of interconnecting flattened tubules (cisternae) stacked together.
  • Membrane is encrusted with ribosome’s (Polysome configuration).

SER

  • Steroid synthesis
  • Lipid synthesis
  • Lipid & steroid transportation
  • Storage of Ca ions

Structure

  • No ribosomes
  • Rarely form cisternae
  • Membrane distinctly more tubular & smooth

ER in general

  • Increases in surface area for chemical reactions
  • Provides a pathway for transporting materials through the cell
  • Collects & stores materials made by the cell.

Ribosomes

2 types

  • 70s prokaryotes (+ chloroplasts and mitochondria)
  • 80s eukaryotes

structure

Consists of small & large subunit.

Golgi Apparatus

  • A stack of flattened cavities
  • Forms lysosomes
  • Produces enzymes for secretion
  • Protein and carbohydrate combine to form glycoprotein

Vesicles

Contain proteins for:

  • Secretion
  • To become part of plasma membrane
  • To become functions of enzymes

Lysosomes

  • Contain digestive hydrolytic enzyme
  • Fuse with the target, enzymes breakdown the target, products are absorbed by the cell
  • Secretes their enzymes outside the cell to breakdown other cells
  • Digests stuff taken in from the environment by the cell
  • Digests parts of cells e.g.: worn out organelles (autolysis)

Mitochondria

  • Synthesis of ATP
  • Biosynthesis
  • Found in all eukaryotes except mature red blood cells.
  • Number depends on activity of cell.
  • High metabolically active ones- have large numbers.
  • Low ones- have small numbers.

Contains

70s ribosome’s, DNA circlet, Matrix- fluid of mitochondria, double membrane & Cristae which is an inner folded membrane containing stalked particles.

Chloroplast

  • Site of photosynthesis

Contains

  • Lamellae, DNA circlet, double membrane, Stroma- fluid of chloroplast, starch grains, granum, thylakoid & chlorophyll.

Cell wall

  • Contains cellulose & hemi cellulose.
  • Are fully permeable & strong.

Structure

  • X- weave made from interwoven fibres.
  • Consists of straight chains of beta-glucose, forms micro fibrils & macro fibrils

Centrioles

  • Forms the spindle during cell division

Structure

  • 2 cylinders of protein microtubules arranged at 90 degrees
  • Not membrane bound

Differences between Prokaryotic & Eukaryotic cells

Feature

Prokaryote

Eukaryote

Size

Small about 0.5 micrometers

Up to 40 micrometers

Genetic material

Circular DNA (in cytoplasm)

DNA in form of linear chromosomes ( in nucleus)

Organelles

Few present, none membrane bound

Many organelles:

  • Double membranes e.g.: nucleus, mitochondria & chloroplasts

  • Single membrane e.g.: GA, ER & lysosomes

Cell walls

Rigid formed from glycoproteins (mainly murein)

  • Fungi: rigid, formed from polysaccharide, chitin.
  • Plant: rigid, formed from polysaccharides. E.g.: cellulose.
  • Animals no cell wall

Ribosome’s

70s

80s

Bacterial cells also contain flagellum, plasmid and capsule.

  • Cells form specialised cells, which form tissues.
  • Tissues- are cells of one type, which carry out one function. E.g.: muscle, nerves
  • Organ- is a structure made up of different tissues performing certain tasks.

Epithelial cells of small intestine

  • Microvilli increase surface area for absorption.
  • Mitochondria synthesises ATP for active transport

Palisade mesophyll cell

  • Elongated to absorb light
  • Contains many chloroplasts for photosynthesis

Differential centrifugation

Used to obtain a sample of isolated organelles.

  • Homogenise sample of cells
    • Conditions
    • Ice cold- to stop biological processes
    • Isotonic solution- to prevent osmotic damage
  • Add solution to a centrifuge & spin at a low speed
    • Densest organelles spin down first. E.g.: nucleus
  • Place supernatant back into centrifuge & spin at a higher speed
    • Next organelle spins down e.g.: mitochondria
  • Repeat & spin & higher speed
    • e.g.: RER, SER, GA.

 

Cell transport

 

Plasma membranes

  • Consists of 40% lipids & 60% protein.

  • The polar nature of phospholipids explains membrane assembly. 

  • Phospholipid heads are hydrophilic.

  • Fatty acid tails are hydrophobic.

Fluid mosaic model

  • Fluid- phospholipids move around the medium.

  • Mosaic- phospholipids are not attached to each other/arranged in sequence.

Movement in + out of cells

1. Diffusion (passive)

  • Small, gas mols pass between phospholipid mols in membrane. (H-L)

  • Water is a special case (osmosis)

2. Facilitated diffusion (passive)

  • Movement of lipid soluble, small & gas mols from a (H-L) conc. through intrinsic membrane proteins

  • Fatty acid tails creates a hydrophobic barrier to entry.

  • 2 types of f-d proteins

  • Pore (channel) proteins

  • Can be gated by chemicals / a change in voltage is required to open the protein pore.

  • Carrier proteins

  • Mols. undergo a conformational shape change.

  • Small mols. that cannot pass through the phospholipid bilayer, glucose, charged mols., ions. Na, K move by facilitated diffusion.

3. Osmosis (passive)

  • Movement of water mols from a less to more (-) WP via a selectively permeable.

  • Pure (distilled) water has the highest WP = 0 & has a greater average KE of water mols.

  • Water + solute has a (-) WP & has a less average KE of water mols.

  • WP is the ability of water mols to move. (Kpa)

Osmosis & plant cells

  • WP = OP + PP 

  • Cell WP = cytoplasm’s OP + wall PP

  • When plant fully turgid WP=0

Passive transport in general

  • Uses KE of mols., or ions, themselves as the motive power to move these materials… so direction of movement depends upon concentration & / electrical (charge) gradients.

  • E.g.: diffusion, osmosis, facilitated diffusion

4. Active transport (active)

  • Uses energy from ATP to move molecules / ions against unfavourable concentration & / electrical gradients. (L-H)

  • Movement is against conc. gradient

  • Requires the hydrolysis of ATP

  • E.g.: Na, K pumping by nerve cell membranes.

5. Bulk transport (active)

1. Endocytosis

  • Into cells (active process)

  • Plasma membrane forms a vesicle around substance & vesicles taken into cell.

  • Phagocytosis = cells + solid particles e.g.: macrophages

  • Pinocytosis = cells + dissolved molecules

2. Exocytosis

  • Out of cell (active process)

  • Materials formed by the cell are packaged in secretary vesicles, which fuse with the plasma membrane to release their contents.

  • E.g.: secreted proteins (digestive enzymes & hormones)

Transport

ATP required

High to low

Proteins involved

Simple diffusion

N

Y

N

Facilitated diffusion

N

Y

Y

Active Transport

Y

N

Y

 

 

 

 

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