A-level Biology Cells Paper 1

ribosomes structure

• small cytoplasmic granules found in all cells

• made of two ribosomal RNA (rRNA) and protein

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  Two types:

• 80S (found in eukaryotic cells, ≈ 25nm diameter)

• 70S (found in prokaryotic cells ,mitochondria and chloroplasts, slightly smaller)

ribosomes function

• site of protein synthesis

• ribosomes translate mRNA into polypeptide chains (proteins)

  • can occur freely in cytoplasm or attached to rough endoplasmic reticulum

lysosomes structure

• small round organelles (vesicles), bound by a single membrane

• formed by Golgi Apparatus vesicles

• contains hydrolytic digestive enzymes such as lysozymes

lysosomes function

• break down material ingested by phagocytic cells (e.g. white blood cells, bacteria )

• digest worn-out organelles + recycle useful chemicals

• release enzymes to the outside of the cell ( exocytosis) to destroy material around the cell

• performs autolysis (break down of cells)

cell wall structure

• found in all plant cells → always outside the cell surface membrane

• composed mainly of cellulose micro fibrils → provides strength

• middle lamella: thin layer marking the boundary bet. adjacent cells → cementing them together

  • in algae : either made of cellulose/glycoproteins (or both)

  • in fungi: made of chitin ( a polysaccharide), glycan, glycoprotein

cell wall function

• provides mechanical strength to prevent the cell bursting under osmotic pressure

• maintains cell shape

• contributes to overall strength of a plant as a whole

• allows water to pass along it → helps movement of water through the plant

Endoplasmic Reticulum (ER) structure

• extensive network of membrane bound flattened sacs (cisternae)

• continuous with the outer membrane of the nucleus

  There are two types:

  • Rough Endoplasmic Reticulum: Has ribosomes attached to it’s surface

  • Smooth Endoplasmic Reticulum: Lacks ribosomes, more tubular in appearance

Rough Endoplasmic Reticulum Function

• synthesis proteins and glycoproteins

• Provides a pathway for transport for proteins throughout the cell

• proteins are packaged into vesicles which transports them to the Golgi apparatus

Smooth Endoplasmic Reticulum Function

• synthesises, stores and transports lipids

• synthesises, stores and transports carbohydrates

Mitochondria Structure

• double membrane

  • Outer membrane- controls entry and exit of materials

  • Inner membrane- tightly folded into cristae → increases surface area for enzyme attachment

• Cristae- extensions of inner membrane, site of electron transport chain and ATP synthesis

• Matrix - contains enzymes for respiration, mitochondrial DNA, ribosomes and proteins

Mitochondria Function

• Site of aerobic respiration and ATP production

• Krebs cycle + link reaction occur in the matrix

• oxidative phosphorylation and ATP synthesis occur on the cristae

• provides energy for active processes (e.g. active transport, muscle contraction, synthesis of molecules)

Chloroplasts Structure

• Double membrane - controls entry and exit of substances

• Grana - stacks of thylakoids (contain chlorophyll), site of light dependent stage of photosynthesis

• Thylakoids - disc- like structures; some are connected by intergranal lamellae

• Stoma- fluid filled matrix ; contains enzymes, sugars, organic molecules, starch grains, lipid droplets, ribosomes and circular DNA

• Circular DNA & Ribosomes - allow chloroplasts to produce proteins + enzymes needed for photosynthesis

Chloroplasts Function

• Grana (thylakoids) : site of light absorption, ATP production and photolysis of water

• stroma - site of light dependent ( calvin cycle ) - produces sugars

• chloroplasts can synthesise some proteins needed for photosynthesis independently

microtubules structure

• tiny hollow, cylindrical tubes made up of the protein tubulin

• part of the cytoskeleton, distributed throughout the cytoplasm

• can rapidly assemble + disassemble to change cell shape or transport materials

• form larger structures such as centrioles, cilia and flagella

microtubules function

• provide mechanical support to maintain cell shape

• forms tracks for the movement of organelles, vesicles + other subs in cell 

• form the spindle fibres during cell division, which have chromosomes 

• form the structural basis of cilia + flagella - enabling cell movement or fluid movement across surfaces

vacuole structure

• membrane bound organelle (surrounded by tonoplasts)

• contains cell sap: a watery solution of sugars, salts pigments and waste

• usually large in plant cells, smaller or absent in animal cells

vacuole function

• storage: stores water, ions, nutrients and waste products

• maintains turgor pressure: pushes cytoplasm against cell wall, keeping the cell rigid

• support: contributes to structural support in plants

• isolation: sequesters harmful substances

• pigmentation: pigments in vacuole can occur petals/ fruits

• lysosomal role: may contain enzymes for breaking down macromolecules

golgi apparatus structure

• series of flattened, membrane bound sacs called cisternae

• has a cis face ( receives vesicles from the ER) and a trans face ( ships vesicles out)

• often found in the ER ( Endoplasmic reticulum)

• surrounded by vesicles for transport 

golgi apparatus function

• modifies proteins and lipids received from the ER ( adding sugars to make glycoproteins)

• sorts and packages molecules into vesicles for transport inside or outside the cell 

• produces lysosomes and other secretory vesicles 

• secretes substances from the cell via. exocytosis 

nucleus structure

• membrane bound organelle present in the eukaryotic cells 

• surrounded by the nuclear envelope there is:

       > a double membrane

       > contains nuclear pores mRNA, tRNA and ribosome sub unit           transport 

• contains chromatin

        > DNA + histone proteins 

        > during cell division, condenses into chromosomes 

• contains the nucleolus 

       > dense region inside the nucleus 

       > site of ribosomal rRNA (RNA) and ribosome assembly 

nucleus function

• contains cell activities:

     > the nucleus acts as the control centre of the cell

     > directs proteins synthesis by controlling transcription of genes            into mRNA

• storage of genetic material:

       > DNA stores all the instructions for all cellular proteins

       > maintains genetic integrity across all cell divisions 

• ribosome production 

       > nucleolus synthesises rRNA + assembles ribosomal sub -units,   which are exported to the cytoplasm

• Gene expression regulation

      > determines which genes are active, inactive in a cell at a                   given time   

• cell division co-ordination

> chromatin condenses into chromosomes, ensuring accurate DNA

   replication + segregation 

additional notes for A* (nucleus structure)

• nuclear pores allow selective exchange

       > mRNA + tRNA exit nucleus

       > Enzymes + nucleotides enter nucleus

• nuclear envelope continuity 

       > inner membrane supports chromatin 

       > outer membrane continuous with rough ER, linking protein                synthesis + nuclear regulation 

• chromatin ( two types)

       > euchromatin → active, loosely packed DNA

       > heterochromatin → inactive, densely packed DNA